MECHANISMS OF RELEASE OF URANIUM AND THORIUM SERIES RADIONUCLIDES FROM A SUITE OF NATURAL MINERALS by ELIZABETH C. GARVER Thesis Submitted to the Graduate School of Wayne State University, Detroit, Michigan in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE 2003 MAJOR: GEOLOGY Approved by: _________________________________ Advisor Date ACKNOWLEDGEMENTS I would like to extend my gratitude and appreciation to the members of my defense committee for their time and willingness to serve: Dr. Jeff Howard, Associate Professor of Geology, Dr. Edmond van Hees, Lecturer and Dr. Mark Baskaran, Associate Professor of Geology. I would also like to sincerely thank Dave Lowrie, Academic Services Officer for the Department of Geology, for helping with many aspects of this work. Not only did Dave locate all the samples used in the analysis, but he also gave instruction in sample preparation, assisted in maintaining the laboratory equipment and offered useful mineralogical knowledge, all with a cheerful attitude. Many thanks for his help over the past two years. I will be forever grateful to my friend and coworker, Sarah Trimble, for all of her help and support in completing this degree. Sarah not only performed the uranium and thorium analysis for many of the samples, but countless times offered her laboratory expertise and assistance when help was needed. Most importantly, she faithfully gave the support and encouragement of a true friend. Thanks also to Vinoth Mani for his assistance and help in the lab with changing samples Many thanks to my advisor, Dr. Mark Baskaran, for providing me with this opportunity to further my education and knowledge. I appreciate the patience and understanding he has given to me over the past two years, and am grateful for much of the advice he has offered to me, both academically and personally. ii Finally, I would like to thank the Wayne State University College of Science for providing me with partial financial support in the form of a graduate teaching assistantship and a graduate research assistantship. iii TABLE OF CONTENTS Page ACKNOWLEDGMENTS………………………………………………………. ii LIST OF TABLES……………………………………………………………... viii LIST OF FIGURES…………………………………………………………….. x CHAPTER I. INTRODUCTION…………………………………………… 1 Previous Work………………………………………………… 7 Objectives of this Study………………………………………. 16 II. METHODS AND MATERIALS…………………………… 18 Mineral Selection……………………………………………… 18 Sample Preparation……………………………………………. 19 Radon Emanation in Gas……………………………………… 21 Radon Extraction to Column…………………………... 23 Radon Transfer to Scintillation Cell…………………… 26 Counting of Sample……………………………………. 26 Calibration and Determination of Background of counting cells……………………………………….. 28 Calculation of Emanated 222Rn Concentration………… 29 Leaching and Recoil of U-Th Series Nuclides Experiment……. 31 Measurements of 212Pb, 228Ac, 210Pb and 234Th………… 32 Measurements of 224Ra, 226Ra and 228Ra……………….. 35 Polonium Plating……………………………………….. 36 iv TABLE OF CONTENTS (continued) CHAPTER II. Page METHODS AND MATERIALS (continued) Determination of the Rate of Emanation of 222 Rn into Solution…………………………………….. 37 Determination of U and Th……………………………. 37 Measurement by Gamma-ray Spectrometry…………………... 39 Gamma-ray Spectrometer……………………………... 39 Calibration of the Gamma-ray Spectrometer………….. 42 Calculation of the Specific Activity of 226 Ra, 228Ra, 234Th and 210Pb…………………………... 43 Calculation of the Specific Activity of 212 Pb, 228Ac and 224Ra………………………………….. 45 Determination of Activity by Alpha Spectrometry…………….. 46 Calculation of the Specific Activity of 210Po…………... 46 Calculation of the Specific Activity of U and Th………. 48 Error Propagation………………………………………………. 50 III. RESULTS……………………………………………………… 51 Concentration of 238U and 232Th in bulk mineral samples……… 51 222 Rn Emanation………………………………………………... 51 Recoil and/or Leaching Results……………………………….... 53 Alpha Radiation Dose………………………………………….. 63 v CHAPTER III. Page RESULTS (continued) Dissolution of the Mineral Sample……………………………. 63 IV. DISCUSSION………………………………………………… 68 Radon Emanation Coefficient Variation……………………… 68 Emanation of 222Rn into gas…………………………… 68 Emanation of 222Rn into liquid………………………… 75 Activity Ratios of the U-Th series nuclides recoiled and/or leached into solution……………………………………. 79 Monazite……………………………………………….. 80 Zircon…………………………………………………... 80 Cerite…………………………………………………… 81 Thorite………………………………………………….. 81 Mechanisms of Release of U-Th series radionuclides into solution……………………………………………………. 82 Congruent versus Incongruent Dissolution……………. 83 Leaching of U-Th series nuclides……………………… 84 Monazite……………………………………………….. 85 Zircon………………………………………………….. 85 Cerite…………………………………………………... 86 Thorite…………………………………………………. 87 Activity Ratios of 234U/238U and 228Th/232Th………………….. 87 Comparison of Activity Ratios………………………………... 88 vi CHAPTER V. Page CONCLUSIONS……………………………………………. 92 Conclusions regarding the Radon Emanation Coefficient…………………………………………………….. 92 Conclusions regarding the Recoil and/or Leaching Rates…………………………………………………………... 93 General Conclusions and Recommendations for Future Work………………………………………………. 94 APPENDIX 1: DECAY PLOTS…………………………………………… 96 REFERENCES……………………………………………………………... 103 ABSTRACT…………………………………………………………………. 109 AUTOBIOGRAPHICAL STATEMENT…………………………………. 111 vii LIST OF TABLES TABLE Page 1. Mineral samples chosen for the experiments and their localities, chemical formulae and estimated ages………………………………………………… 20 2. Calculated cell efficiency for the scintillation cells used……………………. 30 3. Minerals used for the leaching and recoil experiments and their activities…. 33 4. Gamma-ray energies and branching ratios of the gamma-emitting isotopes measured……………………………………………………………………… 40 5. Chemical procedure and counting method used for each isotope measured……………………………………………………………………… 41 6. Calculated dpm/cpm ratios for the gamma detector………………………….. 44 7. Background counts for the alpha-ray spectrometer detectors used for the measurement of the alpha-emitting radionuclides……………………………. 47 8. Concentrations of 238U and 232Th (parent isotopes) in the ground bulk mineral samples………………………………………………………………………… 52 9. Reproducibility of 222Rn measurements mean radon emanation rate and radon emanation coefficient values and the calculated coefficient of variation……… 54 10. Radon emanation coefficient values as a function of temperature, grain size and medium into which radon is released…………………………………………… 55 11. Concentrations of the U-Th series nuclides measured in the leachate…………. 56 12. Relative leaching rates of the 238U nuclides……………………………………. 58 13. Relative leaching rates of the 232Th nuclides…………………………………… 59 14. Calculated alpha-radiation dose mineral has been exposed to since formation… 65 viii TABLE Page 15. Percentage of dissolution of the mineral samples as determined by residual material in leachate…………………………………………………………….. 66 16. Percentage of dissolution of the mineral samples as determined by the ratio of parent nuclides 238U and 232Th in leachate vs. in bulk mineral sample………… 67 17. Comparison of REC values……………………………………………………. 70 ix LIST OF FIGURES FIGURE Page 1. 238 U decay series…………………………………………………………….. 2 2. 232 Th decay series……………………………………………………………. 3 3. 235 U decay series…………………………………………………………….. 4 4. Pyrex gas-washing bottle used for 222Rn measurements……………………. 22 5. Storm King and Associates radon extraction board…………………………. 24 6. Storm King and Associates radon transfer board……………………………. 25 7. Pylon AB-5 portable radiation monitor……………….……………………... 27 8. Flowchart of procedures implemented in leaching/recoil experiment………. 34 9. Flowchart for separation and purification procedures used for U and Th…… 38 10. Histogram representation of the ratios of various isotopic pairs in the leachate of unheated minerals………………………………………………………… 60 11. Histogram representation of the ratios of various isotopic pairs in the leachate of minerals heated to 200ºC………………………………………………… 61 12. Histogram representation of the ratios of various isotopic pairs in the leachate of the minerals heated to 600ºC……………………………………………… 62 13. Illustration of the potential fates of recoiled 222Rn atoms……………………. 69 x CHAPTER ONE INTRODUCTION For decades scientists have utilized the radioactive uranium and thorium that naturally occur in geological samples for many applications in earth and environmental science by studying their decay to stable lead via several radioactive daughter products. Three prominent decay series that occur in nature are: 206 238 U - Pb, 232Th - 208Pb and 235U - 207Pb. The isotopes in these series as well as their half- lives are given in Figures 1, 2 and 3. Radioactive disequilibrium between daughters and parents in the environment has provided a wealth of information on a wide range of topics, including the sources and fate of contaminants, the establishment of chronology in rocks and minerals, the determination of residence times of these nuclides in aqueous systems, the determination of residence and transport times of water masses, etc. These nuclides have been extensively used in the dating of hard rocks (sedimentary, metamorphic and igneous). In all three decay series, the numerous daughter nuclides have a wide range of geochemical affinities. For example, U has a long residence time in aqueous systems, on the order of ~5 x 105 yrs while Pb, Th, and Pa have relatively shorter residence times, on the order of 101-102 yrs in the open ocean. Because of its chemical nature, a significant portion of the radon which occurs in these chains can escape from rocks and minerals through recoil and fission tracks formed by decay within the mineral because of its chemical nature. Many other daughter nuclides in the U-Th series are released into the aqueous phase at different rates than their parent nuclides causing disequilibrium between the 2 238 U 4.47 x 109 yr 234 U 2.48 x 105yr 234 Pa 1.18 min 230 234 Th 24.1 days Th 7.52 x 104 yr 226 Ra 1.62 x 103 yr 222 Rn 3.82 days 218 Po 3.05 min 214 Po 1.6x10-4 s 214 Pb 26.8 min Po 138 days 210 Bi 19.7 min 214 210 Bi 5.01 days 210 Pb 22.3 yrs 206 Pb stable isotope Figure 1: 238U decay series isotopes and their half-lives. Isotopes in boxes represent those produced by emission of an alpha particle and those in diamonds by emission of a beta particle. 3 232 Th 1.40 x 1010 yr Parent 228 Th 1.91 yr 228 Ac 6.13 hr 228 Ra 5.75 yr 224 Ra 3.66 days 220 Rn 55.6 s 216 Po 0.15 s 212 Po 3.0 x10-7 s 212 Bi 60.6 min 208 212 Pb stable Pb 10.6 hr 208 Tl 3.05 min Figure 2: 232Th decay series isotopes and their half-lives. Isotopes in boxes represent those produced by emission of an alpha particle and those in diamonds by emission of a beta particle. 4 235 U 7.04 x 108 yr Parent 231 Pa 3.25 x 104 yr 227 Th 18.7 days 231 Th 25.5 hr 227 Ac 21.8 yr 223 Ra 11.4 days 219 Rn 3.96 s 215 Po 1.78 x 10-3 s 211 Bi 2.15 min 207 Pb stable 211 Pb 36.1 min 207 Tl 4.77 min Figure 3: 235U decay series isotopes and their half-lives. Isotopes in boxes represent those produced by emission of an alpha particle and those in diamonds by emission of a beta particle. 5 daughter and parent. Some commonly examined isotopic pairs include: 231 234 Th/238U, Pa/235U, 210Pb/226Ra, 234U/238U, 210Pb/226Ra, etc. Two major mechanisms have been proposed to explain the observed disequilbria in the U and Th series in groundwater systems. In the first mechanism, recoiled parent nuclides undergoing alpha decay, with recoil ranges of ~40 nm are stopped when they enter the aqueous phase that is in contact with the mineral. Since the parent nuclides, 232Th and 238U, are not recoil products of a nuclear disintegration, the release rates of daughter products and 238U (or 232Th) in the aqueous phase will be different. This will cause disequilibrium in the minerals and the solution surrounding the mineral (Kigoshi, 1971). The second mechanism involves radiation-enhanced leaching of nuclides. Each α-decay produces a trail of highly localized damage along its stopping path. The recoil tracks that intersect the surface provide conduit pathways for the mobility of radionuclides that have undergone decay. Over time these pathways become naturally annealed and the rate of this annealing process can be largely amplified when the minerals or rocks undergo heating events. During major volcanic activity, rocks exposed adjacent to the eruption site could be heated which could lead to the increased release of radon. Accelerated release of 222 Rn over extended periods of time could significantly alter the U-Pb chronology on those minerals. 206 It has been shown that the ages obtained by Pb/238U, 207Pb/235U and 208Pb/232Th on a rock or mineral yield discordant ages, and commonly this discordancy is attributed to differential mobilities of radiogenic Pb (Wetherill, 1963; Wasserburg, 1963). The noble gas radon occurs in all three decay series as 222Rn in the 238U series, 220Rn in the 232Th series and 219Rn in the 235U series. 6 Because the diffusion length of each isotope of radon depends on its half-life (L = (D/λ)1/2 where D is the diffusion coefficient for the material and λ is the decay constant of the radionuclide), the longest diffusion length exists for the one that has the longest half life (in this case, 222Rn). Thus, more 222Rn can diffuse out than 220Rn or 219 Rn and this potential higher release rate of 222 Rn will also cause discordancy, yielding lower ages obtained by the 206Pb/238U method. Understanding the emanation rate of 222 Rn is not only of significance to the scientific community, but is also of concern to public health, since its entry into homes can pose a serious threat. The concentrations of 222 Rn in subsurface, air and water samples have been utilized as tracers to investigate other geological, geochemical and geophysical problems. Some of the applications of 222Rn as a tracer include locating subsurface uranium deposits (Fleischer et al., 1972), detecting the long-distance migration of gases within the earth (Fleischer and Mogro-Campero, 1978), identifying hydrocarbon deposits in the subsurface (Fleischer and Turner, 1984) and even utilizing 222 Rn groundwater concentrations as a precursor to seismic activity (e.g. Igarashi and Wakita, 1990; Monnin and Seidel, 1992; Igarashi et al., 1995; etc.). Although extensive attempts have been made to determine the variations on the radon emanation coefficient (REC) on rocks and minerals, no systematic attempt has been made to determine variations imposed on the REC as a function of temperature for various minerals. Here we attempt to understand the radon releaserate differences caused by variations in both grain-size and temperature. Most of the radon released from mineral grains enters into the aqueous/air space by recoil. The extent of recoil is a measure of the internal structure of a 7 mineral, under the assumption that the uniformly. 238 U and its daughter products are distributed The internal damage within a mineral structure caused by radiation released during decay events could significantly alter the rate of diffusion of radon and thereby affect subsequent REC values. In other words, the changes in the REC values for a given mineral can provide valuable information about the internal structure (mainly damage caused by fission, recoil and alpha tracks) of that mineral. In this present work, changes in REC values for a suite of natural minerals were investigated and the implications to the potential effects on U-Pb ages are discussed. 1.2 Previous Work A vast amount of work has been done with the specific intent of understanding the mechanisms by which a radon atom emanates from a mineral grain (Summarized in Tanner, 1978; Semkow, 1990; Rama and Moore, 1984; Amin and Rama, 1986; etc.). As the radium atom in a natural sample undergoes alpha decay to produce the radon daughter, a specific amount of energy, known as its recoil energy, is imparted to the radon atom. This energy is expended in the form of kinetic energy to the radon daughter as well as thermal energy to the surrounding grain. Much of the pioneering work on this topic began by examining the behavior of this radon atom as it recoils in terms of its mobility based on two properties, its location in the grain and the medium into which it is recoiled. It has been widely observed that the amount of radon that emanates from a mineral grain is much greater than is expected based on the radium concentration and surface area of the mineral grain. This discovery led to two independent hypotheses: 1) There must be an inner network of 8 pore-space sufficient to allow the diffusion of radon from within the grain, and/or 2) The radium in the mineral must be preferentially distributed at the boundary of the grain. When examining radioactive minerals it is also of interest to understand what is happening to the internal structure of the mineral as more and more alpha decay events take place. Over time a mineral will become metamict, or amorphous, based on the damage produced from the large amount of exposure to radiation. In addition to radon emanation studies, the leaching and/or recoil rate of various materials has been the focus of much research as it also can lend much insight into structural changes taking place within the mineral. The following paragraphs will describe the previous work that has been done and discuss the progression of our understanding surrounding this field. In the time spanning from the late 1960’s to the late 1970’s, Allan B. Tanner of the United States Geological Survey completed a series of articles published as proceedings of symposiums giving us the first comprehensive review of the previous work that had been done concerning radon migration. His work paved the way for much of our current knowledge of many of the aspects in this field. The first discussion he covered is on the fate of the recoiling radon atom. When the alpha particle is emitted during the decay of radium, most of the energy is imparted on this particle, however the remaining energy is distributed to the radon atom and is on the order of 104 - 105 times that of typical chemical bond energies. For minerals of average density he cited the work of Quet et al., 1975, and states that the range of the recoiling atom is equal to 20 – 70 nm. Tanner presented four scenarios that may take place as a radium atom decays to radon (Figure 20, Chapter 4). First, if the radium 9 atom is located further inside the grain than the recoil range, it will not escape the grain but will remain embedded. Second if the radon atom is located near the edge of the grain at a distance less than the recoil range, the radon atom will escape the grain and embed itself in a neighboring grain. Third, if the void space between grains where a recoiling atom escapes is filled with water, the atom will escape its host grain, but its recoil energy will be absorbed due to the increased density of the medium it is traveling through and thus it will be stopped short of entering another grain. In this case the radon atom is then able to diffuse by any available path. The final scenario he described is if there is void space between neighboring grains that is filled with air. Depending on the distance between grains, the recoiling atom may travel through the void with enough energy to embed itself in a neighboring grain, or it may lose its energy short of that grain and again be allowed to diffuse. He therefore concluded the presence of water, or another liquid, would increase the probability that radon will ultimately escape the grain. He referred to this type of escape as the direct-recoil fraction and defined the minerals ability to have radon escape as emanating power. This value is also commonly referred to as the emanation coefficient or radon emanation fraction. Tanner also cited previous research and described what is called the indirectrecoil fraction. When a recoiling atom escapes its host grain and implants itself into a neighboring grain, it will form a pocket in that grain as a result of the combined thermal and kinetic energy imposed upon it. The depth of this pocket will be determined by the remaining energy of the recoiled atom and will be a function of the 10 recoil range in that solid. Any radon that is then able to diffuse out of this pocket is what is defined as the indirect-recoil fraction. The idea of pathways for radon loss being created by radiation damage to the mineral structure was also addressed. Tanner described the original investigation on this topic (Lambert et al., 1972; Lambert and Bristeau, 1973). The research indicated that this type of damage alone does not significantly increase the escape by diffusion. The final speculation was that any effect the damage had would be to create potential channels for fluid to enter, thereby increasing the emanation rate. In 1984, Rama and Moore examined the source of the large amount of unsupported radon that is found in groundwater. Rama and Moore (1984) hypothesized that the large amount of radon observed is due to its diffusion from within the solids into the groundwater, not completely from direct recoil out of the surface of the solid. They studied emanation from various grain sizes ranging from <74μm to 10 cm. It is expected that the emanation should increase with increasing surface area (decreasing grain size); however they found that the results were similar for various grain sizes. They suggested that the solids they studied (granites, sands and monazites) were permeated with a network of pore space on the order of nanometers which would provide a vast amount of internal surface area. The radon atoms were recoiled into the fluid in this pore space and subsequently diffused from the solids. They reasoned that the longer-lived non-gaseous isotopes generated by the decay of this radon would adsorb to the surface of the solid, providing the observed elevated concentration of radon. 11 Krishnaswami and Seidemann (1988) addressed this hypothesis. They argued based on two experimental observations: 1) 222 Rn and 220Rn leakage are both higher than would be expected for the surface area of samples examined; and 2) The leakage of the short-lived 220 Rn is nearly the same as that of 222 Rn, so the transport time scale in the grain must be short (< 1 minute). They conducted an experiment to compare the emanation of radon to the emanation of a different noble gas, argon, on two natural samples, granite and plagioclase. They found that the radon emanation rate was much higher than that of argon. This large difference was attributed to heterogeneous distribution of radium in the samples, with the preference for radium to be located at grain boundaries. They did concede that their results for these common rocks do not explain the high leakage of radon from U-Th bearing minerals, where radium is expected to be homogeneously distributed. Fleischer (1988) also addressed the topic of Rn release. He described the mechanisms by which an alpha-recoiled atom could be released into surrounding solids or fluids. The main discussion is the presence of water and its role in 222 Rn migration. He determined that water will increase the emanation rate in soils up to a certain moisture content, about 20-40 weight %, where the water present is sufficient to stop the recoiling atoms from entering adjacent grains and can leach recoil damage. A moisture content higher than 20-40 weight %, however, will start to become a diffusive barrier, causing a decrease in the emanation rate. Rama and Moore (1990) followed up their previous work by examining the radon emanation from single large crystals of apatite, monazite, uraninite and zircon. They found that apatite, monazite and uraninite have large emanation coefficients 12 while zircon is consistently lower. They also observed that when looking at large crystals, the emanation coefficient for 220Rn is much lower than that for 222Rn, but for smaller crystals, the coefficients are comparable. This difference was explained in terms of the time it would take a 220 Rn atom to travel out of the crystal after being recoiled into the pore space. Because of the 55.6 second half-life, decay would take place prior to their arrival at the grain boundary for larger grains. They concluded that recoil, not diffusion, controls the escape from the solid, and contended that the crystals themselves contain a large network of nanometer size pores that are connected to the surface. Kigoshi (1971) published some of the first results related to the leaching and/or recoil of nuclides from a material into a solution. When 1-10 μm diameter zircon crystals were placed in 0.3 N HNO3, he observed an increase in the amount of 234 Th in the aqueous phase with time. He also found excess aqueous phase. From these results, he concluded that the 234 234 U over 238 U in the U/238U disequilibrium observed in natural groundwater systems was caused not only by the preferential dissolution of the alpha decay product 234 U, but also by the contribution of 234 Th directly recoiled into the solution. His study further showed that the concentration of 234 Th observed in the solution was independent of how tightly the sample was packed, implying that these recoiled atoms readily diffuse. Fleischer (1982, 1988) continued this area of research by conducting nuclear track studies of the damage done by alpha-decay and its relation to leaching and observed isotopic disequilibrium between 234U and 238U. The importance of water in creating disequilibrium between 234 U and 238 U in natural systems was emphasized, 13 noting that the 234 U/238U ratio on the moon is unity (Rosholt and Tatsumoto, (1970, 1971). The second main area of importance discussed is the radiation damage from alpha-decay. Raabe et al. (1973) observed an enormous difference in the dissolution of Plutonium oxides depending on whether they contained found that those containing 239 Pu or 238 238 Pu. It was Pu dissolved ~200 times faster than those with 239 Pu. Based on kinetic isotopic effects which vary according to the square root of the mass ratio ( [Ma/Mb]1/2 – 1, where Ma =239 amu and Mb=238 amu), the expected difference is 0.21%. Because the discrepancy is so large (100,000 times), a different mechanism must be responsible. They noted that the difference in alpha decay rates between 238 Pu and 239Pu (200 to 1) provides a resolution to the discrepancy and suggested that dissolution is related to radiation damage caused by alpha-decay events. The disequilibrium in the 238 U and 232 Th series in solutions containing minerals rich in U and Th were discussed and used to infer the amount of time alpharecoil damage is retained in a mineral. Observations have been made to determine the 228 Th/232Th and 234 U/238U ratios present in solutions in which the following minerals were placed: monazite (Eyal, 1982; Eyal and Kaufman, 1982), uraninite and thorianite (Eyal and Fleischer, 1985 a,b), betafite (Eyal et al., 1986) and thoritemelanovite (Eyal et al., 1987). It was typically found that the enhancement in the 234 U/238U activity ratio in the leachate is ~10% and the 228 Th/232Th activity ratio is much larger, up to factors of 2 to 7 times. Eyal and Fleischer (1985 a,b) used the information obtained from these ratios and the mean life of expression to determine the annealing time for a mineral. 234 U to develop an 14 In a series of papers Olander and Eyal (1990 a,b,c) studied extensively the leaching systematics of a monazite sample as a follow up to the previously done work by Eyal and Kaufman (1982) and Eyal (1982) that examined the leaching behavior of U-Th nuclides for a leaching time scale up to 6.8 years. They studied the effects of heating the monazite samples at various temperatures to anneal the radiation damage and quantify its role in the release of U-Th nuclides. They found that monazite radiation damage is stable up to ~400°C when heated for several hours. This was consistent with annealing temperatures determined for other minerals (e.g., Huang et al., 1967; Ehlert et al., 1983; Vance and Metson, 1985; Lumpkin et al., 1986; Weber et al., 1986). Samples heated to 800°C indicated that the radiation damage had been removed. It was also found that point defect recovery processes occur under ambient conditions for monazite. Comparison of the calculation of the displacements per atom along with X-ray diffraction data showed that monazite retained its crystalline structure while other minerals that had undergone similar radiation damage had become completely amorphous (Lumpkin and Ewing, 1988). Olander and Eyal (1990 a)characterized the variation of the U and Th leaching rates with time and found that there is an initial period of very high leaching (~1% / hr for U and ~0.2 % / hr for Th) which quickly declined in a time period of ~ 22 days and remained roughly constant up to 6.8 years (~0.5% / yr for U and 0.2% / yr for Th). The very high initial leaching rate was explained by the enhanced leachability of those nuclides sitting in the alpha-recoil tracks. For example, the was found to be larger than the 232 228 Th concentration Th concentration because of its location in the tracks produced by the 228Ra decay. They also noted that a considerable amount of the 15 228 Th that is observed in the solution is the product of matrix dissolution of the mineral releasing otherwise insoluble 228Ra atoms which subsequently decay to 228Th while in the solution. They observed differences in the isotopic fractionation taking place between the U and Th nuclides over time, and found that in the long-term tests the leachability of 238 U approached that of 234 U, however 228 Th continued to leach more readily with time as compared to that of 232Th. This difference was attributed to two main factors. First, 234 U has a half-life that is similar in magnitude to the time needed for natural annealing to take place within the mineral. Because of this, many of the 234U atoms will be located in positions that are only as leachable as those of the 238 U atoms (essentially undamaged zones). Second, the 228Ra which is released from the mineral is insoluble in the solution they used which was a mixture of 0.1 M NaHCO3 and 0.1 M Na2CO3. Therefore, over time as the leaching solution is replaced by new solution in each leaching step, the monazite surface and will continue to supply 228 228 Ra will be retained on the Th to the solution via radioactive decay. In further work by Eyal and Olander (1990 d), it was determined that even though monazite resists metamictization, the radiation damage induced by alphadecay events has a significant effect on the dissolution of 232 Th structurally incorporated in the mineral. Its release, as well as that of 228Th, is enhanced based on effects to the three actinide dissolution-controlling processes, namely solid-state actinide diffusion, chemical conversion of the actinide ion to a water soluble species at the surface and matrix dissolution. 16 1.3 Objectives of this Study In order to better understand the effects induced by the heating of a mineral to its radon emanation coefficient and the leaching/recoil rates of U-Th series radionuclides, we have selected a suite of five minerals to examine: monazite, thorite, cerite and zircon. uraninite, All minerals were separated to several size fractions and subjected to various degrees of heating before being used for analysis. The radon emanation coefficients were determined and the concentrations of many of the long and short-lived isotopes contained in the U-Th decay series were measured in an attempt to answer the following specific questions: 1. Does the radon emanation coefficient drastically decrease if we anneal all the recoil and alpha tracks within the mineral grain? 2. How does the radon emanation coefficient for a mineral submersed in solution compare to the REC for that mineral in air for both annealed and unannealed samples? 3. Is there any variation in the REC for different minerals subjected to the same conditions based on differences in the minerals internal structures? 4. Does the dissolution of these minerals in 0.1 N HNO3 follow congruent or incongruent dissolution? 5. Do any variations exist in the leaching and recoil rates between the parent isotopes 232Th and 238U? 6. Do any variations exist in the leaching and recoil rates between the parent isotope 232Th and its daughter product 228Th? How does this 17 compare with 238U and 234 U in the U-series? How do leaching and recoil rates vary between various members of the same decay series? 7. Are there significant differences in the leaching and recoil rates of short lived radionuclides between annealed and unannealed minerals? 8. What is the relative importance of recoil and leaching for the various members of the decay chains? 9. Does the leaching/recoil rate of various members of the U- and Thseries depend on the alpha radiation dose? The methods used, experimental results and the subsequent implications to the above questions comprise the remainder of this thesis. 18 CHAPTER TWO MATERIALS AND METHODS 2.1 Introduction Numerous studies have evaluated the emanation of radon from mineralogical samples into both gas and aqueous phases (e.g., Kigoshi, 1971; Tanner, 1978; Rama and Moore, 1984; Amin and Rama, 1986). The primary focus of this study is to determine the effect of heating of a suite of minerals on the rate of emanation of 222Rn as well as on the rate of recoil and/or leaching of U-Th series radionuclides. Therefore, two independent experiments have been conducted on the same set of minerals. The first experiment investigated the variations on the emanation rates of 222 Rn at various temperatures from various sized grains. The experiment also was designed to determine the rate of release of 222Rn into a slightly acidic solution and to examine variations in the radon release rate after heating. In the second experiment, several short and long-lived isotopes in the U-Th series were also measured in an attempt to gain further insight into the potential changes in the mineral structure taking place, as well as the importance of recoil, when a mineral is heated. The procedures implemented in each experiment are given below. 2.2 Mineral Selection The samples used for analysis were acquired by David Lowrie of the Wayne State Geology department. The minerals for the present investigation were monazite, uraninite, zircon, thorite and cerite. The monazite sample was obtained from the New Mexico Bereao of Geology and Mineral Resources. The other samples were obtained 19 from the Wayne State University collection or Mr. Lowrie’s personal collection. The minerals were chosen both for their high levels of radioactivity, as well as their mineralogical nature. The minerals used as well as their chemical formulae, localities, estimated ages and mineralogical classifications are given in Table 1. The minerals uraninite and monazite were chosen for analysis based on their high levels of activity as well as to facilitate comparison with earlier work (Rama and Moore, 1990). In addition to their higher levels of radioactivity, the zircon, cerite and thorite samples were chosen on the basis of their relative insolubility. 2.3 Sample Preparation The minerals were prepared for analysis by the following method. First, the minerals were crushed to roughly 0.5-2 centimeter size particles using a chipmunk jaw crusher. A portion of the mineral was then further ground to a fine powder using a Siemens puck and mill grinder. The mineral monazite was the first to be examined and was subsequently dry sieved into the following five size fractions: <63 µm, 63250 µm, 250-500 µm, 500-1000 µm, and 1000-2000 µm. The emanation data obtained (See Table 9) for monazite for these five size fractions did not show any significant variation in the intermediate size fractions, and therefore all other minerals were separated into only two sizes, <63 µm and 1000-2000 µm. All five size fractions were sampled 4 to 8 times in order to establish the reproducibility of the results. After separation, each size fraction was subjected to varying degrees of heating in the laboratory furnace. The minerals were heated to Table 1: Mineral samples used for analysis Mineral Locality Specific Gravity* Crystal System Monazite Petaca District, New Mexico 4.6-5.4 Monoclinic (Ce, La, Nd, Th) PO4 1.4 ± 0.2 Rare Earth Phosphate Thorite Tory Hill, Ontario 5.3 Tetragonal (Th, U) SiO4 1.0 ± 0.2 Thorium Uranium Silicate Uraninite Fission Mine, Wilberforce, Ontario 7.5-9.7 Isometric UO2 1.0 ± 0.2 Uranium Oxide Zircon 1. Goias, Brazil 2. Tory Hill, Ontario 4.7 Tetragonal ZrSiO4 2.9 ± 0.2 Zirconium Silicate Cerite Boulder Co., Colorado 4.7-4.9 Hexagonal (Ce, Ca)9(Mg,Fe)Si7 (O, OH, F)28 1.0 ± 0.2 Silicate, Neosilicates * From Klein and Hurlbutt, Manual of Mineralogy (1977) Chemical Formula* Estimated Age (billion years) Mineralogical Group 21 room temperature (~25°C), 100°C, 200°C and 600°C. The samples were heated in porcelain crucibles with lids in a Thermolyne 30400 muffle furnace. The minerals that were heated to 100ºC and 200ºC were placed in the furnace when it was at room temperature and the furnace was then heated to the desired temperature. After 48 hours, the furnace was turned off, and the samples were allowed to cool in the furnace. An additional ~24 hours later the samples were removed from the furnace and transferred to the gas-washing bottle. The minerals that were heated to 600ºC were also placed in the furnace while it was at room temperature. The furnace was then turned on, set to 600ºC and the minerals were heated for ~6 hours. Again, the samples were allowed to first cool in the furnace before being removed and transferred to the gas-washing bottles. 2.4 Radon Emanation in Gas The minerals chosen for this part of the study were monazite, zircon, uraninite and thorite. 222 Rn measurement was done using the procedure of Mathieu et al.(1977). Sample sizes were chosen based on the measured concentrations of 226 Ra in the mineral and assuming a typical radon emanation rate. The sizes ranged from half of a gram to two grams. Each mineral sample was placed into a 500 ml gaswashing bottle, flushed with helium and sealed (Figure 4). The samples were then allowed to sit for a time period ranging from seven to twenty-three days in order to allow the emitted radon to accumulate to a measurable amount. There are three major steps in the measurement of 222Rn in any sample. 22 Figure 4: Pyrex 500 ml gas-washing bottle with valves enabling controlled opening and closing to extraction system. 23 They are: 1) Extraction of 222 Rn from the sample into a stainless column containing charcoal (called charcoal column); 2) Transfer of 222 Rn from the charcoal column into a scintillation counting cell; and 3) counting. The procedures used to achieve the extraction, transfer and subsequent counting are detailed below. 2.4.1 Radon Extraction to Column: The radon extraction board used was built by Storm King Associates, and is illustrated in Figure 5. To begin the procedure, the charcoal column was cooled below -10ºC using dry ice and connected to the extraction board by Swagelok connectors. The system and the column were evacuated and allowed to remain under vacuum for roughly ten minutes. At this point, the ‘out’ and ‘in’ valves were then closed and the gas-washing bottle was connected to the system. The valves were again opened and the tubes on the gas-washing bottle were evacuated. The system and the tubes were then brought to atmospheric pressure with helium. The sample was then opened to the system via the valves on the gas-washing bottle and the gas within the sample and the system were circulated through the system for fifteen minutes. During this circulation the radon in the sample was trapped on the cooled activated carbon in the column. Once circulation was complete, the column was disconnected from the extraction board and brought to room temperature in preparation for the transfer of the radon to the scintillation cell. The sample in the gas-washing bottle was again resealed for a future replicate measurement. 24 Figure 5: Radon extraction board (Storm King Associates). 25 Figure 6: Radon transfer board (Storm King Associates) and furnace. 26 2.4.2 Radon Transfer to Scintillation Cell The procedure begins by evacuating the transfer board shown in Figure 6 using both evacuation valves for several minutes. A scintillation cell is then connected to the board via the Swagelok quick-connect and allowed to evacuate. The column was then connected to the system via quick-connects and Tygon tubing and evacuated for one minute by opening the plug valve. After one minute, the plug valve was closed and pressure in the system brought to 15 inches of Hg vacuum by first moving the upper three-way valve to helium, and then moving it to the off position. The column was placed in the furnace at 450-470°C and the scintillation cell was allowed to fill with the 222 Rn gas along with He by moving the lower right three-way valve to fill. After five minutes helium was slowly let into the system by opening the plug valve and again moving the upper three-way valve to helium, allowing the system to come to atmospheric pressure. When this transfer of the gas was completed, the scintillation cell was disconnected from the system. The column was evacuated, removed from the furnace and allowed to cool. The scintillation cell then stood for at least two hours before being counted in order to allow equilibrium to be achieved between 222Rn and its daughter products. 2.4.3 Counting of Sample The sample in the scintillation cell was counted using the Pylon AB-5 Portable Radiation Monitor pictured in Figure 7. The cell was loaded on to the detector and the monitor was then turned on and allowed to sit for a few minutes to make sure any external photons did not affect the photomultiplier tube. The counting 27 Figure 7: Pylon AB-5 portable radiation monitor. A counting cell is also shown in the foreground. 28 was then carried out by pressing the START/STOP button and recording the exact time the counting began. The sample was counted for a period of time ranging from half an hour to ~3 hours depending on the activity. The time the counting ended was also recorded as well as the total counts. The calculation of the activity based on these counts is given in section 2.4.5. 2.4.4 Calibration and Determination of Background of Counting Cells Three counting cells (C1, C2 and C3) were used in the determination of 222Rn. The efficiency of each cell was determined by repeated measurements using two standard solutions of known activity. The first standard used was RGU-1 obtained from the International Atomic Energy Agency. Approximately 1 gram of this standard powder was chemically digested and transferred to a 500 ml gas-washing bottle in ~250 ml of distilled water. The certified concentration of uranium in RGU-1 is 400 ± 2.1 μg g –1 . The second standard used was the 226 Ra standard solution. Fifteen ml of this standard solution was put in a 500 ml gas-washing bottle with ~250 ml of distilled water. The 226 Ra activity of the second standard is 22.57 dpm ml-1. Both standards were used for each cell and the measurements were repeated until the value of cell efficiency for each cell was well established using either standard. The following equation was used to determine the cell efficiency: Ec = (Cs) / (As x ds) ( Eq 2.4.a) where Ec is the efficiency of the counting cell, Cs is the background subtracted net counts of the standard in counts per minute, As is the activity of the standard in dpm (disintegrations per minute), and ds is the factor to correct for the decay of 222Rn ( = e- 29 λt where λ is the decay constant for 222 Rn in days and t is the time elapsed in days between the median time the cell was filled with the sample and the median time the cell was counted). The determined cell efficiency for each cell used is given in Table 2. The background of each cell was determined by counting the cell immediately prior to its use for each sample. The background value was consistently less that 1% of the activity of the sample. 2.4.5 Calculation of Emanated 222 Rn Concentration The calculation of the specific activity of 222Rn activity was done as follows: A222 = (C222 – Bc) / (Ec x d222 x w ) (Eq 2.4.b) where A222 is the activity of 222Rn in dpm g-1 of dry sample, C222 is the count rate of the sample in cpm (counts per minute), Bc is the background count rate of the cell used for analysis in cpm, Ec is the cell efficiency as determined by Eq. 2.4.a, d222 is the decay correction factor applied for 222Rn ( = e-λt where λ is the decay constant for 222 Rn in days and t is the time elapsed between the median time the cell was filled and the median time the cell was counted) and ‘w’ is the weight of the sample in grams. The value obtained for the activity of 222 Rn was also represented in terms of the rate at which atoms escape the grain, known as the emanation rate. It is calculated as follows: Er = *A222 / ( w x *t x λ) (Eq 2.4.c) where Er is the emanation rate of the mineral sample ( in atoms g-1 min-1 ), *A222 is the activity of 222Rn in the sample in dpm, ‘w’ is the weight of the dry mineral 30 Table 2: Calculated cell efficiency for radon scintillation cells. Cell Standard Date Sampled Cell Efficiency Mean C1 RGU-U RGU-1 RGU-1 RGU-U RGU-U 5/13/2002 6/11/2002 6/18/2002 7/1/2002 7/9/2002 2.52 ± 0.68 2.51 ± 0.63 2.42 ± 0.78 2.44 ± 0.62 2.44 ± 0.51 2.47 ± 0.29 C2 RGU-U RGU-1 RGU-1 RGU-U 6/25/2002 7/9/2002 7/16/2002 7/23/2002 2.72 ± 0.82 2.34 ± 0.50 2.30 ± 0.75 2.69 ± 0.70 2.51 ± 0.35 C3 RGU-U RGU-1 RGU-1 RGU-U 5/24/2002 6/24/2002 7/1/2002 7/17/2002 2.32 ± 0.42 2.52 ± 0.95 2.69 ± 0.74 2.57 ± 0.60 2.52 ± 0.35 31 sample, *t is the time elapsed between when the mineral was sealed in the gaswashing bottle and when 222 Rn extraction was done (in days), and λ is the decay constant of 222Rn in days. The proportion of the released fraction to the total amount of radon produced is defined as the emanation coefficient and is expressed as the 222 Rn released divided by the total 226 Ra. The emanation coefficient is calculated by the following equation: Eco = (Er / A226) x 100 where Eco is the 222 (Eq 2.4.d) Rn emanation coefficient for the mineral sample in percent, Er is the emanation rate (in atoms g-1 min-1), A226 is the activity of 226 Ra in the sample in dpm g-1 and 100 is the factor used to represent the value as a percent. 2.5 Leaching and Recoil of U-Th Series Radionuclides Experiment This experiment was carried out using the minerals thorite, monazite, zircon and cerite that were heated and separated in the manner given in section 2.3. Each mineral was analyzed at the size fraction of less than 63 μm and at three degrees of heating: i) room temperature; ii) 200°C for 48 hours; and iii) 600°C for 6 hours. The sample weight chosen for each mineral was selected on the basis of 232 238 U and Th concentrations (as measured by gamma spectrometry) of a powdered sample so as to yield a measurable amount of recoiled nuclides in the solution phase, as was determined in the previous work by Kigoshi (1971). Each sample was placed in a 500 ml glass beaker and a volume of ~10 times the sample weight of 0.1 N HNO3 was added to the beaker. The details of the concentrations of 238U and 232Th, weight taken and volume of 0.1N HNO3 added are given in Table 3. The samples were allowed to 32 sit in the solution for approximately 20-25 days before analysis. At the end of this time period the solution was separated from the mineral by centrifugation and was subsequently divided into three aliquots for three different sets of analyses. The residual mineral was then put into a 500 ml gas-washing bottle in ~300 ml 0.1 N HNO3, flushed with helium and sealed for later radon analysis. The flowchart in Figure 8 illustrates the method used for each aliquot of the solution. These methods, as well as the isotopes that were measured are given in detail in sections 2.5.1 – 2.5.3. 2.5.1 Measurements of 212Pb, 228Ac, 210Pb and 234Th Approximately 35% of the solution separated from the mineral was used to determine the activities of the short-lived isotopes 212 Pb and 228 Ac, as well as the activities of 210Pb and 234Th via a ferric hydroxide precipitation. To achieve this, the sample solution was placed in a 250 ml glass beaker and 200-400 μl of FeCl3 was added, equivalent to 10-20 mg of Fe. About 1 gram of NH3Cl and 100 ml of distilled water were also added. The solution was then stirred continuously with a glass rod while ammonia was slowly added in order to bring the solution to a pH of roughly seven and facilitate the Fe(OH)3 precipitation. Once the precipitation was complete, the solution and precipitate were transferred to 50 ml centrifuge tubes and centrifuged for ten minutes. The supernatant was poured into a clean glass beaker and the precipitate was dissolved in 6 M HCl and transferred to a 10 ml gamma counting vial. The ferric hydroxide precipitation was then performed again on the supernatant to ensure quantitative removal of Pb, Ac, and Th from the solution. Any precipitate formed the second time was added to the 10 ml gamma counting vial and the sample 33 Table 3: Minerals used for leaching and recoil experiment Mineral Activity 238U (dpm g-1) Activity 232Th (dpm g-1) weight (g) Volume 0.1 N HNO3 (ml) Volume solution / weight solid (ml g-1) Monazite 1058 ± 22 24477 ± 266 30 200 6.7 Thorite 6263 ± 95 52119 ± 547 5 50 10 Cerite 2563 ± 30 859 ± 10 12 120 10 Zircon 3008 ± 28 273 ± 3 10 100 10 34 Mineral Sieved Heated at RT, 200, 600ºC (<63μm) Crushed Put in 0.1 N HNO3 Solution (20-25days) Solution separated and centrifuged 1 4 Fe(OH)3 precipitation 2 3 BaSO4 precipitation +0.01-0.02 g Fe + ~1 g NH3Cl Mineral in gas washing bottle + 1 ml saturated BaNO3 + 1 ml conc. H2SO4 Precipitate and centrifuge Precipitate Repeat Procedure Transfer ppt. to counting vial in 6 M HCl Flushed with helium and sealed for 18-24 days Dry solution and convert to HCl medium + 1 ml 209Po spike + 1 ml 20%Hydroxylamine Hydrochloride Plate on polished silver planchet for 1 hr. (Heat and stir continuously) Discard excess solution and transfer to counting vial Centrifuge/remove excess solution Rn Analysis Gamma count sample Polonium Plating Solution dried and converted to 9M HCl for U-Th analysis Alpha count planchet Figure 8: Flowchart for leaching experiments: 1) ~40% of sample solution used to determine 212Pb, 228 Ac, 210Pb & 234Th. 2) ~25% of solution used to determine 223Ra, 224Ra, 226Ra & 228Ra. 3) Residual mineral sample used to determine 222Rn. 4) ~35% of sample solution used to determine 210Po. 35 was ready for gamma counting. Approximately 20 minutes elapsed between the time of separation of the solid and solution and the time of the Fe(OH)3 precipitation. The sample was counted in the gamma detector in a time series fashion in order to follow the decay of 212 Pb and 228 Ac. The sample was first counted immediately, then once or twice within the first 24 hours, after 48 and 72 hours, and again after one week or more. The counting time lasted from ~15 to 30 minutes during the first 72 hours and then for longer times (600-1500 minutes) after one week. The activities of 210 Pb and 234 Th were determined directly by gamma measurement. The subsequent calculation of the specific activities of these isotopes is given in detail in section 2.6. 2.5.2 Measurements of 224 Ra, 226Ra and 228Ra Approximately 25% of the sample solution was used to obtain the activities of 226 Ra, 228 Ra and the short-lived 224 Ra by means of a BaSO4 precipitation. For this procedure, the aliquot of sample solution taken was placed in a 50 ml glass beaker and weighed. One ml of saturated barium nitrate was added, followed by 1 ml concentrated sulfuric acid. The precipitate readily formed and settled to the bottom of the beaker. This precipitate was allowed to settle for approximately 1 hour and then it was transferred to a 10 ml gamma counting vial and centrifuged. The supernatant was discarded and the sample was immediately counted. Approximately 90 minutes elapsed between the time the solution was separated and the precipitation was done to the time of the first counting. 36 A time series counting was also done on this precipitate in order to follow the decay of 224 Ra. Care was taken to be certain that 212 Pb and 224 Ra were in secular equilibrium since 224Ra was measured via its daughter product 212Pb in the series. The activities of 224 Ra, 226 Ra and 228 Ra are obtained directly by gamma counting. The calculations of these activities are given in section 2.6. 2.5.3 Polonium Plating The remaining solution (~35%) was used to determine the 210Po in the sample. This activity was found by plating the polonium onto a silver planchet and subsequently counting the planchet by alpha spectrometry (Flynn, 1968; Baskaran and Naidu, 1995). The procedure used is as follows. One ml of a 209 Po spike with a known activity of 8.09 ± 0.02 dpm ml-1 (this spike was calibrated with NIST-tracable standards) was added to the solution in a Teflon beaker, dried completely and converted to HCl medium by adding 5 ml of concentrated HCl and ~20 ml of distilled water. One ml of 20% hydroxylamine hydrochloride was added to this solution. A magnetic stirrer was placed in the solution and the beaker was covered with a Teflon lid. The solution was heated with the magnetic stirrer for approximately 5 minutes. A polished silver planchet (with one side taped) was then put into the solution, tape-side down and allowed to plate for one hour. The tape is to ensure that the plating takes place on only one side of the silver planchet. After one hour, the silver planchet was removed from the solution, rinsed with distilled water and allowed to air dry. This planchet was then placed in the alpha detector and counted for approximately 1-7 days, depending on the 210 Po 37 activity. The remaining solution was then dried and converted to 9M HCl medium in preparation for the chromatography column work to determine the concentrations of U and Th nuclides in the sample. 2.5.4 Determination of the Rate of Emanation of 222Rn into Solution As previously stated in Section 2.5, when the solution used for the recoil/leaching experiment was separated from the mineral, the mineral was transferred to a 500 ml gas-washing bottle in ~300 ml of 0.1 N HNO3. This bottle was then flushed with helium via the extraction system and sealed. The bottle was kept sealed for a period of time ranging from 10 to 25 days. The sample was then analyzed for radon emanation into the solution using the same extraction, transfer and counting procedures described in section 2.4 2.5.5 Determination of U and Th Radiochemical analysis of the sample for the determination of uranium and thorium isotopes was performed by Sarah Trimble at Wayne State University. Briefly, a known amount of 229Th and 232U was added to the sample aliquot and the U and Th were separated and purified using ion-exchange column (Trimble, 2003). The following isotopes were measured: 234 U, 238 U, 228 Th, 230 Th and 232 Th. The specific procedure is shown in the form of a flow chart in Figure 9 (modified from Trimble, 2003). The separated U and Th were electroplated as described in Trimble (2003). The electroplated U and Th planchets were assayed in a high-resolution, low background alpha spectrometer. Calculation of the determined activities is given in section 2.7. 38 Sample converted to 9 M HCl Dry; Add 5ml conc. HCl; Dry; Repeat; Add 6ml 9 M HCl 9 M HCl anion exchange resin column Th Convert to 8 M HNO3 Dry; Add 5ml of conc. HNO3; Dry; Repeat; Add 6ml of 8 M HNO3 8 M HNO3 anion exchange column Elute with 40ml warm H2O + 5 drops conc. HCl Discard effluent U 9 M HCl Effluent Elute with 40 ml warm H2O + 5 drops conc. HCl Convert to 8 M HNO3 8 M HNO3 anion exchange column Elute with 40ml warm H2O + 5 drops conc. HCl Discard effluent U Th Repeat 8 M HNO3 anion exchange column as above for second purification Th U Electroplating procedure Figure 9: 2003). Repeat 8 M HNO3 anion exchange column as above for second purification Electroplating procedure U-Th separation and purification procedure (Modified from Trimble, 39 Table 4. Gamma-ray energies and branching ratios for the measured gamma-emitting isotopes * Isotope Energy (keV) Branching Ratio (%) Pb 46.5 4.06 Th 63.0 3.5 352.0 36.7 609.3 46.9 228 338.6 12.4 228 911.1 27.2 224 238.6 43.0 210 234 226 Ra (via Pb) 214 226 Ra (via Bi) 214 Ra (via 228Ac) Ra (via 228Ac) Ra (via Pb) 212 * Table of Isotopes, Lederer, M.C. et al. 1978 39 2.6 Measurement by Gamma-ray Spectrometry: 2.6.1 Gamma Ray Spectrometer For all samples, measurement of the following long and short-lived isotopes was performed by gamma spectrometry at Wayne State University: 226 Ra, 228Ra and 228 234 Th, 210Pb, 212Pb, 224Ra, Ac. The system consisted of a Canberra high-purity germanium well detector coupled to a Canberra InSpector multi-channel analyzer. The manufacturer specified resolution for this detector is 1.8 keV (FwHM) at 122 keV and 2.5 keV (FwHM) at 1.33 MeV, and its relative efficiency is 14%. The gamma-ray energies used and branching ratios for each of these isotopes are given in Table 4. The parent activities of 238 U (via 234 Th) and 232 Th (via 228 Ac) were first determined for the unheated ground mineral samples by gamma measurement of a known amount of sample in a 10 ml counting vial. The vial was placed directly into the detector and counted for an appropriate length of time in order to determine these activities with relatively low error. The counting time varied from less than an hour to up to one day because of the highly variable concentrations of 238U and 232Th in these samples. The activities of the isotopes (212Pb, 228 Ac, 224 Ra, 226 Ra, and 210 Pb, 232 Th, 234 Th, 230 Th, 228 Th, 238 U, 234 U, 228 Ra) recoiled and/or leached by the solution were also determined by gamma spectrometry. Table 5 lists each isotope and the method by which it was measured. As mentioned in previous sections, the processing of both the ferrichydroxide and barium-sulfate precipitates ultimately leads to the transfer of sample to 10 ml gamma counting vials. For both precipitates these samples were immediately counted after processing for a length of time ranging from half an hour to several hours, 40 Table 4. Gamma-ray energies and branching ratios for the measured gamma-emitting isotopes * Isotope Energy (keV) Branching Ratio (%) Pb 46.5 4.06 Th 63.0 3.5 352.0 36.7 609.3 46.9 228 338.6 12.4 228 911.1 27.2 224 238.6 43.0 210 234 226 Ra (via Pb) 214 226 Ra (via Bi) 214 Ra (via 228Ac) Ra (via 228Ac) Ra (via Pb) 212 * Table of Isotopes, Lederer, M.C. et al. 1978 41 Table 5: Method used to determine each nuclide. Isotope Half-life Laboratory Method Measurement 210 Po 138 days Polonium Plating αspectrometry 210 Pb 22.3 yrs Fe(OH)3 Precipitate γspectrometry 212 Pb 10.6 hrs Fe(OH)3 Precipitate γspectrometry 228 Th 1.91 yrs Ion-exchange column αspectrometry 230 Th 7.52 x 104 yrs Ion-exchange column αspectrometry 232 Th 1.40 x 1010 yrs Ion-exchange column αspectrometry 234 Th Fe(OH)3 Precipitate γspectrometry 24.1 days 238 U 4.47 x 109 yrs Ion-exchange column αspectrometry 234 U 2.48 x 105 yrs Ion-exchange column αspectrometry 6.13 hrs Fe(OH)3 Precipitate γspectrometry 3.66 days BaSO4 Precipitate γspectrometry 228 Ac 224 Ra 226 Ra 1.62 x 103 yrs BaSO4 Precipitate γspectrometry 228 Ra 5.75 yrs BaSO4 Precipitate γspectrometry 42 depending on the activity of the short-lived isotopes precipitate) and 212 Pb and 228 Ac (Fe(OH)3 224 Ra (BaSO4 precipitate). The samples were then counted in a time series fashion to follow their decay, allowing extrapolation of their activities at the time the solution was separated from the mineral. In several samples, the activities of the short-lived nuclides were low and the parent-supported levels were relatively high (224Ra in the case of 212Pb, 228Ra in the case of 228Ac) and in those cases the initial activity of the short-lived nuclide was used to calculate the final activity. The samples were again counted after the activities of the short-lived isotopes had decayed away to determine the equilibrium activities of their longer lived daughters 226Ra and 228Ra (BaSO4 precipitate), and 234 Th and 210 Pb (Fe(OH)3 precipitate). From the raw counts for each of the gamma peaks, the specific activities were calculated and the details are given in sections 2.6.3 and 2.6.4. 2.6.2 Calibration of the Gamma-ray Spectrometer In order to calculate accurate values for the activities of the nuclides of interest, it is necessary to calibrate the gamma counting system. The gamma counting system used for the analyses in this study was calibrated using standard reference materials obtained from the International Atomic Energy Agency (IAEA). Standard reference materials with known activities (dpm) were put into counting vials at various volumes (1 to 10 ml in 1 ml increments) and were counted. The IAEA standards used in the calibration were: RGU-1 (238U standard in secular equilibrium with all daughter products, used for 210 Pb and 226 234 Th, Ra) and RGTH-1 (232Th standard in secular equilibrium with all daughter products, used for 228 Ra, 224 Ra, and 212 Pb). The standards were prepared for the calibration by oven drying at ~70°C for approximately 12 hours. A known amount of the 43 dried standard was then packed into a counting vial at the proportion of ~1g ml-1 and the total activity (dpm) of the standards was calculated using the following equation: As (dpm) = ws (g) x Nsλs (dpm µg-1) x Cs (µg g-1) (equation 2.6.a) where As is the total certified activity the nuclide, ws is the weight of the standard put in the vial, Nsλs is the conversion factor to express µg g-1 in dpm g-1 (this value is 0.746 dpm µg-1 for 238U and 0.2445 dpm µg-1 for 232Th), and Cs is the certified concentration of the standard as provided by the IAEA in µg g-1. The certified concentrations for the standards RGU-1 and RGTH-1 are 400 ± 2.1 µg g-1 and 800.2 ± 15.8 µg g-1, respectively. Each standard was counted in the gamma counting system and the cpm value for each radionuclide was calculated by dividing its background subtracted net counts by the time it was counted in minutes. The necessary factor of dpm/cpm needed for accurate calcluation of the specific activity of each isotope is then readily calculated. A table of the dpm/cpm factors used for the calculations in this study is provided in Table 6. 2.6.3 Calculation of the Specific Activity of 226Ra, 228Ra, 234Th, and 210Pb Using the dpm/cpm ratios given in Table 6, the specific activities of 226Ra, 228Ra, 234 Th and 210Pb released by the mineral into the solution phase, by recoil and/or leaching of the mineral, were calculated as follows: An = Nn x (dpm/cpm)n x dn x (Vt/Vs) x (1/w) (Eq 2.6.b) where An is the specific activity of the nuclide of interest in dpm g-1, Nn is the background subtracted net counts per minute of that nuclide as determined by the integrated area of its energy peak(s), (dpm/cpm)n is the appropriate ratio obtained from the calibration of the nuclide of interest at the specified counting geometry, dn is the 44 Table 6: Calculated dpm/cpm ratios* from the gamma-ray spectrometer calibration 238 U Geometry weight (ml) (g) 1 1.0059 2 2.0012 3 3.0293 4 3.9856 5 5.0011 6 5.979 7 7.0098 8 8.0039 9 8.9988 10 9.9997 234 Th weight (g) 1.0022 1.996 3.0061 4.0098 5.0093 6.0067 6.9945 8.0023 8.9979 9.9961 210 Pb 46.5 keV dpm/cpm 36.3 ± 0.5 38.3 ± 0.4 41.9 ± 0.5 48.0 ± 0.5 52.1 ± 0.5 62.8 ± 0.6 73.0 ± 0.7 84.4 ± 0.7 90.5 ± 0.8 103.1 ± 1.0 234 Th 63.0 keV dpm/cpm 42.9 ± 0.7 41.9 ± 0.5 47.3 ± 0.6 52.6 ± 0.6 53.0 ± 0.5 65.5 ± 0.7 77.5 ± 0.8 87.8 ± 0.7 94.7 ± 0.9 111.0 ± 1.3 212 Pb 238 keV dpm/cpm 4.5 ± 0.1 5.1 ± 0.1 5.2 ± 0.1 6.0 ± 0.1 7.6 ± 0.2 7.4 ± 0.1 8.1 ± 0.2 9.5 ± 0.2 10.7 ± 0.2 12.9 ± 0.3 226 Ra 352 keV dpm/cpm 8.90 ± 0.06 9.16 ± 0.06 10.57 ± 0.07 11.80 ± 0.07 12.41 ± 0.07 14.20 ± 0.08 16.23 ± 0.09 18.63 ± 0.10 19.90 ± 0.11 22.55 ± 0.13 226 Ra 609 keV dpm/cpm 24.88 ± 0.22 25.25 ± 0.19 27.52 ± 0.21 29.44 ± 0.21 30.75 ± 0.20 33.64 ± 0.22 37.75 ± 0.24 42.17 ± 0.25 44.71 ± 0.28 49.90 ± 0.33 228 Ra 338.6 keV dpm/cpm 31.0 ± 0.7 34.1 ± 0.8 35.3 ± 0.7 40.1 ± 0.8 49.7 ± 1.0 48.3 ± 1.1 53.8 ± 1.1 63.0 ± 1.3 70.9 ± 1.5 84.9 ± 1.8 * The IAEA standards used (with all daughter products in equilibrium): RGU-1 standard (400 ± 2 µg g-1) and RGTh-1 standard (800.2 ± 15.8 µg g-1); Conversion factors are 238U = 0.746 dpm µg-1 and 232Th = 0.2445 dpm µg-1 228 Ra 911 keV dpm/cpm 39.1 ± 0.9 43.8 ± 1.0 44.3 ± 0.9 50.3 ± 1.1 61.9 ± 1.3 60.0 ± 1.4 65.7 ± 1.4 75.8 ± 1.6 85.9 ± 1.8 101.9 ± 2.1 45 factor for decay correction of the nuclide of interest (= eλt where, λ is the decay constant for the nuclide (in days) and t is the time elapsed (in days) between separation of the solution from the mineral and the beginning of gamma counting), (Vt/Vs) is the ratio of the total volume of the solution from the sample to the volume used for a particular procedure (Fe(OH)3 or BaSO4 precipitation), and ‘w’ is the weight in grams of the ground mineral originally put into solution. 2.6.4 Calculation of the Specific Activity of 212 Pb, 228Ac and 224Ra The specific activities of the shorter-lived isotopes of 212Pb, 228Ac and 224Ra were calculated using the following equation: *As = *Ns x (dpm/cpm)*s x (*Vt/*Vs) x (1/w) (Eq 2.6.c) where *As is the specific activity of the nuclide of interest in dpm g-1, Ns is the background subtracted net counts per minute, (dpm/cpm)*s is the ratio given in Table 6, (*Vt/*Vs) is the ratio of the total volume of solution of the sample to the volume used for the procedure (Fe(OH)3 or BaSO4 precipitation), and ‘w’ is the weight of the ground mineral originally put into solution in grams. Because these isotopes are decaying very quickly (half-lives ranging from ~6 hours to ~4 days), the samples are counted multiple times in order to follow their decay. Since the decay we are following of these short- lived isotopes is exponential, the natural log of their activities can be plotted versus time to obtain a linear relationship. The equation of this line gives us the intercept at the time equal to zero (i.e. the time the solution was separated from the mineral), and we are therefore able to obtain the initial activities of these isotopes. The plots used to obtain these values are given in the next chapter. In those samples where 212 Pb and/or 228 Ac 46 levels were low compared to their parents (224Ra or 228Ra) in the Fe(OH)3 precipitate, the data from the first counting was used to calculate the final activity and the appropriate decay/ingrowth corrections were applied as follows: AFS = {(specific activity calculated by Eq 2.6.c) – [(224Ra or 228Ra)(1-e-λt1)]}*eλt2 where AFS is the final specific activity, ‘λ’ is the decay constant of 212Pb (or 228Ac) and t1 is the time elapsed from precipitation until the mid-counting and t2 is the time elapsed between time of mineral-solute separation and counting. 2.7 Determination of Activity by Alpha Spectrometry The activities of the following isotopes were determined by alpha spectrometry: 210 Po, 238 U, 234 U, 232 Th, 230 Th, 228 Th and 224 Ra. The samples were counted using an ORTEC Company 8-input Octete-PC alpha-ray spectrometer utilizing surface detectors coupled to an integrated pre-amplifier, amplifier and multi-channel analyzer. The samples were counted for varying amounts of time ranging from 2 to 14 days, depending on their activities. The background activity of each detector was determined and subsequently subtracted from the sample counts. The background of the detectors used for this subtraction are given in Table 7. 2.7.1 Calculation of the Specific Activity of 210 Po Each of the plated silver planchets was counted for 1-14 days, depending on its activity, in the alpha-ray detector. The counts for the 210 Po in the sample as well as for the 209Po that was added as a spike were determined by integration of the area 47 Table 7: Background of the alpha detectors used for the measurement of alpha-emitting radionuclides. BKG BKG BKG BKG BKG BKG BKG BKG BKG BKG BKG 232 230 229 228 224 238 235 234 232 210 Detector # for Th for Th for Th for Th for Ra for U for U for U for U for Po for 209Po (cph) (cph) (cph) (cph) (cph) (cph) (cph) (cph) (cph) (cph) (cph) 1 NM NM NM NM NM 0.024 0.03 0.027 0.0719 0.095 0.061 2 NM NM NM NM NM 0.0012 0.0024 0.0072 0.1082 0.048 0.028 3 0.827 1.648 2.505 1.414 1.815 0.868 1.026 1.138 1.812 NM NM 4 0.0906 0.1201 0.3119 0.6638 2.598 NM NM NM NM NM NM 5 0.0080 0.0053 0.0120 0.0053 0.0275 NM NM NM NM NM NM 6 0.0452 0.0204 0.0027 0.0177 0.0324 NM NM NM NM NM NM 7 0.0059 0.0088 0.053 0.0795 0.4742 0.0118 0.0353 0.0206 0.0353 NM NM 8 0.0059 0.0147 0.0295 0.0736 0.3505 0.0029 0.0177 0.0147 0.0265 NM NM * NM = background was not calculated for these regions 48 under their respective peaks representing ~ 200 keV. The specific activity of 210Po in the sample was then calculated using the following equation: A10 = (N10 x A09/N09) x (Vt/Vp) x d10 / w (Eq. 2.7.a) where A10 is the activity of 210Po (dpm g-1 dry solid) that was recoiled into and/or leached by 0.1 N HNO3, N10 is the background subtracted net counts of activity of the 210 Po, A09 is the known 209 Po spike added to the sample in dpm (8.09 ± 0.02 dpm), N09 is the background subtracted net counts of 209 Po, (Vt/Vp) is the ratio of the volume of the solution originally separated from the mineral to the volume of that solution used for polonium plating, d10 is the factor to correct for the decay of 210Po (= eλt, where λ is the decay constant for 210 Po in days and t is the time elapsed between separation of the solution from the mineral and the beginning of counting in the alpha detector), and ‘w’ is the weight of the dry sample used for analysis. The background counts for each detector were subtracted from the total counts for each sample as follows: Net counts = (for a sample) 2.7.2 Total counts – (BKG (counts per hour) x time (hr)) (for a sample) (from Table 6) Calculation of the Specific Activity of Uranium and Thorium The radiochemical processing of the samples for uranium and thorium by alpha spectrometry involves separation, subsequent purification of U and Th and separate electroplating. The uranium source consists of 238 U, 235 U, 234 U in the sample and the 232 U spike that was added to each sample. The thorium source consists of 228 Th, and the with its parent 229 Th spike that is added to each sample. 228 232 Th, 230 Th, 224 Ra grows toward equilibrium Th because of the finite time delay between electroplating of Th and alpha counting. Each planchet is placed into the alpha detector and counted for 3-14 days. 49 The same approximate number of channels was chosen for the peak integration area for all the peaks produced in the same spectrum. In general, the area chosen for the integration represented energy equal to ~200 keV. The uranium isotopes were calculated using the equation: Au = (Nu x Asp / Nsp) x (Vt/Vp) / w (Eq 2.7.b) where Au is the calculated specific activity of the uranium isotope of interest in dpm g-1, Nu is the background subtracted net counts of the uranium isotope of interest (238U or 234 U), Asp is the known activity of the 232U spike added to the sample in dpm (6.68 ± 0.02 dpm g-1), Nsp is the background subtracted net counts of 232 U, (Vt/Vp) is the ratio of the total volume of solution separated from the mineral to the volume used for the procedure, and ‘w’ is the weight of the sample taken for analysis. The thorium isotopes were calculated using the equation: Ath = (Nth x Asp / Nsp) x (Vt/Vp) / w (Eq 2.7.c) where Ath is the calculated specific activity of the thorium isotope of interest in dpm g-1, Nth is the background subtracted net counts of that isotope, Asp is the known activity of the 229Th spike added to the sample in dpm (9.90 dpm), Nsp is the background subtracted net counts of 229Th, (Vt/Vp) is the ratio of the total volume of solution separated from the mineral to the volume used for the procedure and ‘w’ is the weight of the sample taken for analysis. For both calculations, the background counts for each detector were subtracted from the total counts for each sample as follows: Net counts = (for a sample) In addition, 5.5% of applied. Total counts – (BKG (counts per hour) * time (hr)) (for a sample) (from Table 6) 224 Ra decay has energy close to 228 Th and hence a correction was 50 2.8 Error Propagation: The detailed methodology on the error propagation is the following. In the case of the radon emanation coefficient, the propagated error arises from the errors associated with counting including background, errors associated with the (dpm/cpm) conversion factor, and the error associated with the efficiency of the counting cell. For nuclides that were counted in the gamma spectrometer, the propagated errors arise from counting statistics including background and the errors associated with the (dpm/cpm) conversion factors. In the case of nuclides measured using the alpha spectrometer, the propagated errors arise from counting statistics including background, and the error associated with the activity of the internal spike. 51 CHAPTER THREE RESULTS Two experiments were conducted on the suite of natural minerals chosen for this investigation. The radon emanation coefficient for release into air was determined in the first experiment using the minerals monazite, uraninite, zircon and thorite. The leaching and/or recoil rates of various U-Th series radionuclides, as well as the radon emanation coefficient for release into solution, were examined in the second experiment. The minerals chosen for the second experiment were monazite, zircon, thorite and cerite. The following results illustrate the effect of the various degrees of heating of the mineralogical samples to their 222Rn emanation coefficients as well as their recoil and/or leaching abilities. 3.1 Concentration of 238U and 232Th in the minerals: The concentrations of 238 U (measured via 226 Ra) and 232 Th (measured via 228 Ac) are given in Table 8. 3.2 222 Rn Emanation The 222 Rn emanation rate was first determined for the mineral monazite at 25ºC to determine the reproducibility of the experimental results. Reproducibility measurements were conducted on five different size fractions of monazite and the coefficient of variation on the radon emanation rate was calculated. The size fraction used were <63 μm, 63-250 μm, 250-500 μm, 500-1000 μm and 1-2 mm. Each sample was analyzed 3 to 52 Table 8: Activities of 238U and 232Th in the ground bulk mineral samples as determined by gamma spectrometry. 232 238 Mineral Th (via Ac) dpm g-1 U (via 226Ra) dpm g-1 Monazite 24477 ± 266 1058 ± 22 Zircon 273 ± 3 3008 ± 28 Thorite 52119 ± 547 6263 ± 95 Cerite 859 ± 10 2563 ± 30 Uraninite 22604 ± 439 349740 ± 4150 228 53 4 times and the mean emanation rate was then determined for each size fraction. The mean emanation rate from 9.81 atoms g-1 m-1 for the size fraction 1-2 millimeters to 24.7 atoms g-1 m-1 for the size fraction <63 μm. These results and the coefficient of variation for each grain size are given in Table 9. The 222 Rn emanation rate remained the same (between 8.69-10.75 atoms g-1 m-1) for the mineral size ranges 250-500 μm, 500-1000 μm and 1-2 mm and only two sizes were chosen for further work: <63 μm and 1-2 mm. The mean emanation rate and mean emanation coefficient for all minerals analyzed are given in Table 10. For monazite, the mean emanation coefficient for release into gas ranged from 0.29 to 2.05 %, while for release into solution, the values ranged from 2.51 to 4.17 %. The corresponding values in zircon ranged from 0.46 to 1.04 % for gas and 0.58 to 0.64% for fluid. The REC values for thorite ranged from 1.34 to 5.38 % for gas and 14.9 to 23.7 % for liquid. The mineral cerite was analyzed only for release into fluid and the emanation coefficient ranged from 16.8 to 22.9 %, while uraninite was only analyzed for release into gas and the radon emanation rate ranged from 0.30 to 0.76 %. 3.2 Recoil and/or Leaching Results The concentrations of the isotopes 222 Rn, 228 Th, 230Th, 232 Th, 234 210 Po, 210 Pb, 212 Pb, 228 Ac, 228 Ra, 226 Ra, 224 Ra, Th, 234U, and 238U that were leached or recoiled into the 0.1 N HNO3 solution were determined in dpm g-1 (Table 11). The concentrations of the shortlived isotopes 212Pb, 224Ra and 228Ac that had been leached or recoiled into the solution at the time the solution was separated from the mineral were determined either by extrapolation from a plot following their decay or by using 54 Table 9: Reproducibility and coefficient of variations of Monazite sample at Room temperature Grain Size Number of 222Rn Emanation rate -1 -1 (µm) Observations (atoms g m )* (N) 222 Rn emanation rate on Range Coefficient of variation (%) < 63 4 21.69 ± 0.25 20.19 - 24.7 9.6 63-250 4 12.84 ± 0.18 10.36 - 17.02 22.5 250-500 4 10.75 ± 0.14 8.29 - 13.10 18.4 500-1000 4 8.69 ± 0.11 7.90 - 10.88 16.8 1000-2000 3 10.41 ± 0.16 9.81 - 11.18 6.7 *The calculation used to determine the error associated with the radon emanation rate is given in section 2.8 55 Table 10: Radon Emanation Rate and Emanation Coefficient (REC) as a function of temperature, grain size and medium entered. Mean Emanation Mean Emanation Sample Grain Size Temperature Time Rate Coefficient -1 -1 (μm) (˚C) (days) (atoms g m ) (%) Monazite <63 RT 7.05-10.99 21.69 ± 0.25 2.05 ± 0.03 RT/Fluid 16.94 44.1 ± 0.1 4.17 ± 0.09 100 8.00-12.90 9.49 ± 0.07 0.90 ± 0.02 200 9.01-12.98 11.70 ± 0.11 1.11 ± 0.02 200/Fluid 16.94 26.61 ± 0.09 2.51 ± 0.05 600 5.89-7.02 6.98 ± 0.10 0.66 ± 0.01 600/Fluid 16.933 43.6 ± 0.1 4.12 ± 0.09 1000-2000 RT 7.98-9.02 10.41 ± 0.16 0.98 ± 0.02 100 8.83-12.09 8.66 ± 0.06 0.82 ± 0.01 200 11.98-13.0 5.27 ± 0.07 0.50 ± 0.01 600 7.08-10.89 3.11 ± 0.06 0.29 ± 0.01 Zircon <63 RT 6.94-10.94 31.39 ± 0.21 1.04 ± 0.01 RT/Fluid 27.91 17.7 ± 0.07 0.588 ± 0.006 100 7.93 16.58 ± 0.17 0.55 ± 0.01 200 6.95-7.99 19.15 ± 0.19 0.64 ± 0.01 200/Fluid 28.00 19.17 ± 0.07 0.637 ± 0.006 600 8.91-11.96 14.05 ± 0.08 0.47 ± 0.004 600/Fluid 28.02 17.31 ± 0.07 0.575 ± 0.006 1000-2000 RT 6.94-13.08 14.13 ± 0.12 0.47 ± 0.01 600 7.08-10.89 13.77 ± 0.19 0.46 ± 0.01 Uraninite <63 RT 6.98-13.0 1842 ± 4 0.53 ± 0.01 100 6.94-10.94 1756 ± 4 0.50 ± 0.01 200 6.08-8.97 2668 ± 5 0.76 ± 0.01 600 9.93-14.97 1053 ± 2 0.30 ± 0.003 1000-2000 RT 6.99-13.01 1860 ± 5 0.53 ± 0.01 Thorite <63 RT 28.0 336.7 ± 1.0 5.38 ± 0.08 RT/Fluid 17.82 935 ± 2 14.9 ± 0.2 200 23.87 150.0 ± 0.6 2.39 ± 0.04 200/Fluid 20.12 1410 ± 4 22.5 ± 0.3 600 21.85 84.3 ± 0.4 1.34 ± 0.02 600/Fluid 17.83 1483 ± 4 23.7 ± 0.4 Cerite <63 RT/Fluid 9.93 430 ± 1 16.8 ± 0.2 200/Fluid 9.93 588 ± 1 22.9 ± 0.3 600/Fluid 9.91 483 ± 1 18.9 ± 0.3 The calculation used to determine the error associated with the radon emanation rate is given in section 2.8. The 226Ra activities are: Monazite (1058 ± 22), Zircon (3008 ± 28), Uraninite (349740 ± 4150), Thorite (6263 ± 95) and Cerite (2561 ± 30). 56 Table 11: Concentrations (dpm g -1) of nuclides leached and/or recoiled into solution. Sample Thorite 1 Thorite 4 Thorite 5 Cerite 1 Cerite 3 Cerite 5 Mon-1 Mon-3 Mon-4 Zircon 1 Zircon 2 Temp(ºC) RT 200 600 600 200 RT RT 200 deg 600 deg 25 200 600 Ac-228 23225±556 621±60 217±13 40±4 87±6 BD 1035±36 7362±128 2623±53 167±15 143±9 33±5 Pb-212 Th-234 26292±535 2847±291 2835±72 BD 49±5 BD 54±3 18±5 44±4 BD 43±4 BD 2581±52 110±10 3299±67 142±19 1908±38 96±14 87±3 61±8 68±2 84±5 29±1 74±4 Pb-210 Ra-228 3307±38 27988±686 83±5 358±10 BD 144±5 46±3 51±2 BD 77±2 BD 100±3 BD 3393±57 118±3 2367±40 145±4 1972±33 19±2 79±2 31±1 71±1 75±2 19.3±0.5 Ra-224 Ra-226 21083±432 4426±48 1282±32 50±2 274±7 24±1 57±2 173±3 51±2 226±3 67±2 222±4 2208±32 181±1 1428±29 127±1 1959±29 113±1 77±3 37.5±0.7 67±2 46.5±0.5 22.3±0.9 49.4±0.5 Rn-222 Po-210 3037±8 3.53±0.09 5149±13 21.8±0.3 4803±12 4.50±0.06 775±2 2.2±0.2 1059±2 0.91±0.02 869±2 1.21±0.02 3845±11 0.55±0.01 2285±8 0.78±0.02 3436±10 1.02±0.02 89.7±0.4 0.33±0.01 97.4±0.04 0.099±0.007 88±0.4 0.9±0.03 U-238 U-234 2.92±0.03 16±2 10.8±0.2 65±1 3.3±0.1 4.5±0.2 131±8 130±8 3.2±0.2 2.8±0.2 0.85±0.08 0.97±0.09 265±29 249±27 108±3 96±3 30±3 29±2 13.5±0.5 18.2±0.6 29±3 50±5 Th-228 Th-230 14.1±0.8 1.3±0.1 304±3 34.4±0.4 25.1±0.2 3.68±0.04 3.6±0.1 2.01±0.06 4.5±0.1 1.33±0.04 4.64±0.09 1.1±0.03 0.630±0.004 0.083±0.001 3.63±0.02 0.315±0.003 2.04±0.01 0.648±0.003 0.286±0.003 0.329±0.002 47.6±0.7 21.9±0.3 0.45±0.02 18.1±0.3 Th-232 1.78±0.09 109±1 14.58±0.01 0.315±0.008 0.314±0.007 0.284±0.005 0.711±0.003 1.462±0.006 0.826±0.004 36.3±0.5 7.3±0.1 Po-210/Pb210 Ac-228/Ra-228 0.00107±3E-5 0.26±0.02 0.83±0.03 1.7±0.2 Th-234/U-238 U-234/U-238 976±140 5.4±0.08 Pb-212/Ra-224 Ra-224/Ra-228 1.25±0.04 0.75±0.02 Ra-224/Th-228 Rn-222/Ra-226 1491±86 0.686±0.008 0.66±0.002 Zircon 3 26±3 47±5 1.5±0.1 0.047±0.005 0.79±0.08 1.12±0.08 - 0.31±0.01 1.37±0.06 0.14±0.04 0.99±0.09 0.89±0.09 1.1±0.2 0.4±0.002 0.94±0.14 1.3±0.006 0.89±0.03 3.2±0.01 0.96±0.07 4.5±0.6 1.35±0.07 2.9±0.3 1.7±0.2 2.21±0.08 0.18±0.02 3.6±0.1 1.90±0.08 0.95±0.06 1.13±0.07 0.87±0.09 0.66±0.03 0.64±0.6 0.67±0.03 1.17±0.03 0.65±0.01 2.31±0.07 0.60±0.02 0.97±0.02 0.99±0.02 1.13±0.05 0.98±0.04 1.02±0.04 0.94±0.03 1.31±0.07 1.16±0.06 15.9±0.7 4.48±0.08 11.4±0.5 4.69±0.06 14.5±0.5 3.91±0.07 3505±55 21.2±0.1 393±8 18.0±0.2 960±15 30.4±0.3 119±4 2.39±0.05 1.40±0.05 2.09±0.02 50±3 1.78±0.02 6.0±0.2 4.2±0.1 103±4 10.9±0.03 200±8 0.007±0.0002 0.007±0.0003 0.017±0.002 0.0032±0.0003 0.012±0.0005 3.11±0.08 1.33±0.3 2.1±0.02 2.0±0.1 1.7±0.3 The calculation used to determine the error associated with the radon emanation rate is given in section 2.8. 2.9±0.4 1.8±0.3 57 the first counting with corrections applied for both decay of the short-lived isotopes as well as their ingrowth from parent nuclides. The decay plots used for the initial activity at the time the mineral and solutions were separated are given in Appendix 1. The results are also given in terms of the activities of each nuclide in relation to its parent. This is known as the leaching rate [R] and is calculated as the activity of the isotope of interest divided by the activity of its parent isotope. For 226 234 Th, 234 U, 230 Th, Ra, 222Rn, 210Po and 210Pb, the parent isotope is 238U. For 228Ac, 228Ra, 224Ra and 212Pb, the parent isotope is 232 Th. The underlying assumption is that 238 U and 232 Th are in secular equilibrium with the corresponding decay series radionuclides in the mineral. The leaching rates for the minerals examined at the various temperatures are given in Tables 12 and 13. In addition, the daughter/parent ratios for all the nuclides measured in the 0.1 N HNO3 solution used in the leaching/recoil experiment are shown as histograms for each mineral at various temperatures. These results are given in Figures 10-12. 58 Table 12: Relative leaching rate [R] calculated as the activity of the nuclide in the leachate divided by its parent activity (238U) in the leachate Sample 234 Th 234 U 230 Th 226 Ra 222 Rn 210 Pb 210 Po Monazite 25ºC 0.0147 ± 0.002 0.941 ± 0.007 1.1E-05 ± 1E-6 0.024 ± 0.003 0.51 ± 0.06 BD 7.3E-05 ± 8E-6 200ºC 0.047 ± 0.006 0.888 ± 0.002 0.00001 ± 3E-6 0.043 ± 0.001 0.76 ± 0.02 0.039 ± 0.001 2.6E-4 ± 9E-6 600ºC 0.12 ± 0.02 0.94 ± 0.01 0.00037 ± 2E-5 0.145 ± 0.008 4.4 ± 0.2 0.18 ± 0.01 0.0013 ± 7E-5 25ºC 4.5 ± 0.6 1.35 ± 0.01 0.0244 ± 0.0009 2.8 ± 0.1 6.6 ± 0.2 1.4 ± 0.2 0.024 ± 0.001 200ºC 2.8 ± 0.3 1.71 ± 0.05 0.74 ± 0.07 1.6 ± 0.1 3.3 ± 0.3 1.1 ± 0.1 0.0034 ± 0.0004 600ºC 2.9 ± 0.3 1.81 ± 0.05 0.7 ± 0.07 1.9 ± 0.2 3.4 ± 0.4 2.9 ± 0.3 0.035 ± 0.004 25ºC BD 0.992 ± 0.008 1.3 ± 0.21 261 ± 25 1022 ± 96 BD 1.4 ± 0.1 200ºC BD 0.89 ± 0.05 0.42 ± 0.03 72 ± 5 335 ± 24 BD 0.29 ± 0.02 600ºC 0.14 ± 0.04 1.14 ± 0.1 0.015 ± 0.001 1.32 ± 0.08 5.9 ± 0.4 0.35 ± 0.03 0.017 ± 0.002 25ºC 181 ± 26 1.03 ± 0.06 0.08 ± 0.01 282 ± 29 193 ± 20 211 ± 22 0.22 ± 0.2 200ºC BD 1.02 ± 0.01 0.54 ± 0.01 0.79 ± 0.04 81 ± 2 1.31 ± 0.08 0.345 ± 0.009 600ºC BD 0.98 ± 0.03 0.8 ± 0.04 5.2 ± 0.3 1044 ± 45 BD 0.98 ± 0.04 Zircon Cerite Thorite 59 Table 13: Relative leaching rate [R] calculated as the activity of the nuclide in the leachate divided by its parent activity (232Th) in the leachate. Sample 228 Ac 228 Th 228 Ra 224 Ra 212 Pb Monazite 25ºC 1456 ± 51 0.886 ± 0.007 4772 ± 83 3105 ± 47 3630 ± 75 200ºC 5036 ± 90 2.48 ± 0.02 1619 ± 28 977 ± 20 2256 ± 47 600ºC 3176 ± 66 2.47 ± 0.02 2387 ± 42 2372 ± 37 2310 ± 47 25ºC 252 ± 22 0.982 ± 0.005 120 ± 3 117 ± 4 132 ± 5 200ºC 3.9 ± 0.2 1.31 ± 0.03 1.96 ± 0.04 1.83 ± 0.07 1.87 ± 0.06 600ºC 4.6 ± 0.07 0.062 ± 0.003 2.64 ± 0.08 3.1 ± 0.1 4 ± 0.2 25ºC BD 16.3 ± 0.4 354 ± 13 237 ± 8 152 ± 13 200ºC 276 ± 19 14.3 ± 0.5 246 ± 9 162 ± 7 141 ± 14 600ºC 127 ± 12 11.4 ± 0.4 161 ± 9 182 ± 8 172 ± 10 25ºC 13019 ± 753 7.9 ± 0.6 15688 ± 912 11818 ± 668 14738 ± 832 200ºC 5.7 ± 0.6 2.79 ± 0.04 3.3 ± 0.1 11.8 ± 0.3 26.0 ± 0.7 600ºC 14.8 ± 0.9 1.72 ± 0.01 9.9 ± 0.3 18.8 ± 0.5 3.4 ± 0.3 Zircon Cerite Thorite 60 Figure 10: Histogram representation of the ratios of various isotopic pairs recoiled into solution for unannealed (unheated) minerals. Activity ratios greater than 2.0 are not shown and can be seen in Table 11. 61 Figure 11: Histogram representation of the ratios of various isotopic pairs recoiled or leached into solution for minerals annealed at 200°C. Activity ratios greater than 2.0 are not shown and can be seen in Table 11. 62 Figure 12: Histogram representation of the ratios of various isotopic pairs recoiled or leached into solution from minerals annealed at 600° C. Activity ratios greater than 2.0 are not shown and can be seen in Table 11. 63 3.3 Radiation Dose The dose of alpha-decay radiation that each mineral has undergone since its formation can be readily calculated using the equation (Murakami et al., 1991): Dα = 8N1[e(a1t)-1] + 7N2[e(a2t)-1] + 6N3[e(a3t)-1] Equation 3.1 where Dα is the dose in alpha-decay events per milligram of sample, N1, N2, and N3 are the present numbers of 238U, 235U and 232Th in the sample in atom mg-1, a1, a2, and a3 are the decay constants for 238U, 235U and 232Th, respectively in years-1, and t is the age of the mineral. The value of N2 is taken as (1/139) N1 based on natural isotopic abundance. Using this equation and the measured amounts of N1 and N3 in the samples, as well as their estimated ages, the alpha-decay doses have been calculated for the samples and these results are given in Table 14. The radiation dose that the minerals have been exposed to since their formations ranged from 0.29 alpha-decay events mg-1 of sample for cerite to 33.8 events mg-1 for uraninite. 3.3 Dissolution of Mineral Sample The major goal of this investigation is to determine the recoil input of various nuclides in water; however, most of the particle-reactive nuclides that are delivered to the aqueous phase by recoil will get adsorbed onto mineral grains. Thus, we have used a dilute acid to prevent the particle-reactive nuclides from getting adsorbed onto the grains. In the process, some amount of U-Th series nuclides will also be leached in addition to some amount of possible congruent dissolution. The percent of the mineral sample that underwent dissolution while in the slightly acidic solution was determined in order to distinguish between the recoiled and/or leached fraction and the dissolved fraction 64 (congruent dissolution). This difference was determined by drying a known volume of sample solution and weighing the residual solid. The equation used is: D = (Rs / w) * (Vt / Vs) * 100 Equation 3.2 where D is the measure of dissolution of the mineral and is given in percent, Rs is the residual weight of the mineral in grams, w is the weight of the ground mineral put into solution in grams, (Vt/Vs) is the factor to calculate for the entire sample solution, and 100 is the factor to express dissolution as a percent. This value was determined for each of the minerals placed in solution and the values ranged from ~2-4 % for monazite and zircon to ~9-11 % for cerite and thorite (Table 15). The percentage of dissolution for each of the samples was also calculated based on the ratio of the parent nuclides 238U and 232 Th in the leachate to the parent nuclides in the mineral samples (Table 16). 65 Table 14: Alpha decay radiation dose mineral has been subjected to since formation. Sample Age 238 U 232 Th 238 235 U x108 atom mg-1 232 Th x1010 atom mg-1 Dose * x1011 event mg-1 109 years dpm g-1 dpm g-1 U x1010 atom mg-1 Zircon (Brazil) 2.90±0.2 3008 461.5 1.94 1.40 0.93 1.13 ± 0.12 Monazite 1.40±0.2 1058.5 26590 0.683 0.49 53.7 2.46 ± 0.36 Thorite 1.00±0.2 6263 27243 4.04 2.91 55.0 2.25 ± 0.48 Cerite 1.00±0.2 2563 913 1.65 1.19 1.84 0.29 ± 0.07 Uraninite 1.00±0.2 349741 26302 226 162 53.1 33.8 ± 7.6 * The dose was calculated as given I section 3.3 and equation 3.1. 66 Table 15: Percentage of dissolution in sample as calculated based on residual nuclides in leachate. Sample Temperature ºC % dissolved Thorite 25 200 600 25 200 600 25 200 600 25 200 600 9.89 9.48 9.07 9.49 9.77 11.48 3.73 3.90 2.17 3.75 2.48 2.96 Cerite Monazite Zircon 67 Table 16: Calculation of mineral dissolution in leaching/recoil experiment determined as the concentration of the parent nuclides 238U and 232Th in leachate compared to the concentration in the bulk mineral sample. Sample Zircon (Brazil) 238 232 U Th Temperature -1 -1 dpm g dpm g ºC Bulk sample Bulk sample 3008 ± 14 3008 ± 14 3008 ± 14 273 ± 3 273 ± 3 273 ± 3 238 232 238 232 U dpm g-1 leachate Th dpm g-1 leachate U (%) dissolved Th (%) dissolved 25 200 600 13.5±0.5 29±3 26±3 0.66±0.002 36.3±0.5 7.3±0.1 0.45 0.96 0.86 0.14 7.87 1.58 265±29 108±3 30±3 0.711±0.003 1.462±0.006 0.826±0.004 25.03 10.2 2.8 0.0027 0.0055 0.0031 Monazite 1058 ± 11 24477 ± 266 1058 ± 11 24477 ± 266 1058 ± 11 24477 ± 266 25 200 600 Thorite 6263 ± 47 52119 ± 547 6263 ± 47 52119 ± 547 6263 ± 47 52119 ± 547 25 200 600 2.92±0.03 1.78±0.09 10.8±0.2 109±1 3.3±0.1 14.58±0.01 0.047 0.17 0.053 0.0065 0.40 0.054 Cerite 2563 ± 15 2563 ± 15 2563 ± 15 600 200 25 131±8 0.315±0.007 3.2±0.2 0.314±0.007 0.85±0.08 0.284±0.005 5.1 0.12 0.033 0.035 0.034 0.031 859 ± 10 859 ± 10 859 ± 10 68 CHAPTER FOUR DISCUSSION 4.1 Radon Emanation Coefficient Variation The results obtained for the radon emanation coefficient (REC) of the samples analyzed varied depending on the specific mineral examined and the medium into which the radon atom entered. As discussed in chapter one, the recoiling 222Rn atom produced by the alpha-decay of 226 Ra in a mineral grain will ultimately reside in one of four different locations (Figure 13). A comparison of the REC results obtained in this investigation is given in Table 16. The specific aspects of these results and their implications are discussed below for each mineral. 4.1.1 Emanation of 222Rn into gas For the mineral monazite, there is a marked difference in the REC values depending on grain size. For the RECs determined at 25°, 200° and 600°C, the value of REC for the 1-2 mm size grain was ~50% of the REC for the <63 µm (Table 10). This difference suggests that surface area is important to the REC value for monazite. For the samples heated to 100°C, the REC values were similar for both grain sizes (within 10%). Similarly, the ratios of the REC at heated temperatures to the REC at room temperature remained fairly constant for 200 and 600°C heated samples for both grain sizes. The REC at 200°C is ~54% of the REC at room temperature and the REC at 600°C is ~ 30% of the REC at room temperature. For the size fraction <63 µm, the value of REC dropped 69 α α (d) α 226 Ra α (a) • • 222 Rn c) b) • • Figure 13: Potential fate of recoiled 222Rn atom: represents the 226Ra atom undergoing alpha decay to 222 222 produce Rn and • represents the recoiled Rn atom. a) The 226Ra atom is located deeper inside the grain than the recoil length and the 222Rn atom remains embedded in the host grain. b) The 222Rn atom is recoiled out of the host grain and gets embedded in an adjacent grain. c) The 222Rn atom enters a liquid medium and its recoil energy is absorbed, decreasing the recoil length and leaving the 222Rn atom free to diffuse in the fluid. d) The 222Rn atom travels through void space filled with gas retaining its energy and ultimately embedding itself in a nearby grain. (Modified from Tanner, 1978). 70 Table 17: Comparison of REC values for emanation into gas or liquid before and after heating the minerals and ratios of REC at various temperatures to values at 25ºC. ND: not determined. Monazite Zircon Thorite Uraninite Cerite REC % (Gas) 25ºC 100ºC 200ºC 600ºC 2.05 0.9 1.11 0.66 1.04 0.55 0.64 0.47 5.38 ND 2.39 1.34 0.53 0.5 0.76 0.3 ND ND ND ND Ratio (Gas) 100/25ºC 200/25ºC 600/25ºC 0.44 0.54 0.32 0.53 0.62 0.45 0.44 0.25 0.94 1.43 0.57 - REC %(Liquid) 25ºC 200ºC 600ºC 4.17 2.51 4.12 0.59 0.64 0.58 14.9 22.5 23.7 ND ND ND 16.8 22.9 18.9 Ratio (Liquid) 200/25ºC 600/25ºC 0.60 0.99 1.08 0.98 1.51 1.59 - 1.36 1.13 Ratio (Gas/Liquid) 25ºC 200ºC 600ºC 0.49 0.44 0.16 1.76 1 0.81 0.36 0.11 0.06 - - 71 when the mineral was heated to 100°C (~44% of value at room temperature for smaller grain size), but then rose slightly for samples heated to 200°C. The greatest reduction in the REC value was seen for the mineral heated to 600°C, with the value reduced by ~70% as compared to the unheated sample. The results obtained for the mineral zircon exhibited some similarities and some differences as compared to monazite. For REC values determined at room temperature, there again was a ~50% decrease for the 1-2 mm grain as compared to the <63µm grain. However, at 600°C, both grain sizes yielded the same REC (within 3%). The REC obtained for the sample heated to 100°C was also lower than the REC for the sample heated to 200°C. The ratios of the REC’s for the heated samples to the unheated samples are 53% for 100°C, 62% for 200°C and 45% for 600°C. These values are higher than those for the mineral monazite but show the same trend. The greatest reduction in the REC was also seen for the sample heated to 600°C, with the value reduced by ~55%. The REC for the unheated samples of the mineral uraninite at both size fractions yielded similar values, so the heating experiments were only carried out on the smaller size fraction. Unlike the other minerals whose REC values were highest for unheated samples, the highest REC value for the mineral uraninite was obtained for the sample heated to 200°C. This value was found to be 143% of the value obtained at room temperature. The ratio of the sample heated to 100°C to the unheated sample was ~94% and the ratio of the sample heated to 600°C to the unheated sample was ~57%. The greatest reduction in REC is seen between the 72 sample heated to 200°C and the sample heated to 600°C, approximately a 60% reduction. The REC for the mineral thorite was determined for samples subjected to only three temperatures, room temperature, 200°C and 600°C. These values were also determined for only one grain size, <63 µm. The highest REC value was obtained from the unheated sample and the lowest was for the sample heated to 600°C. The ratios of the REC’s of the heated samples to the unheated were ~45% for 200°C and ~25% for 600°C. These values are similar to those obtained for monazite and slightly lower than zircon. It is unknown whether thorite would have shown the same trend of decrease in REC at 100°C followed by increase at 200°C like monazite, zircon and uraninite, because a sample of the mineral was not heated to 100°C. The greatest reduction in the REC value was found between the unheated sample and the sample heated to 600°C and was approximately a 75% decrease. Based on the observations given in the above paragraphs, the following comparisons can be made: 1. The radon emanation coefficient is lowest for all minerals when they were heated to 600°C, and REC values reduced 55-75% as compared to the highest value. 2. Except for the mineral uraninite, the REC is highest for all minerals at room temperature. 3. For monazite, zircon and uraninite, the REC follows the same trend according to heating history: 25 > 200 > 100 > 600 °C (The REC for thorite was not determined at 100ºC). 73 4. The mineral monazite shows variation in the REC based on differing grain sizes and this variation remains constant with heating history differences, however the mineral zircon only has differences depending on grain size for unheated samples. 5. Zircon has the least amount of variation in the REC values at different degrees of heating. The major mechanisms that can explain release of radon from the various mineral grains are: a) diffusion of radon through the solid phase, in particular through a network of nanometer-sized interconnected pore spaces (e.g. Rama and Moore, 1984); b) direct alpha recoil from the decay of 226Ra resulting in the release of 222 Rn atoms from the outer 20 nm thickness (corresponding to the recoil range) of the grain; and c) release of 222Rn from the diffusion out of the recoil tracks in the mineral where the recoil track is created from the mineral surface inward. The main reason we see a decrease in the REC value when a mineral is heated is that permanent changes (or at least quasi-permanent over a time scale greater than the experimental time scale) take place affecting the internal tracks in the mineral. At 100°C, many of the recoil tracks within the mineral are altered leading to a decrease in the release of radon. When the minerals are heated to 200°C, the recoil tracks appear to become widened, leading to a freer pathway for the release of radon and resulting in the observed increase in the REC value. The specific changes taking place to cause these changes are not clear. Exceeding this temperature, the recoil tracks become completely annealed and therefore the radon atoms lose the major conduit of travel out of the grain. This is seen by the significant decrease in the REC 74 values for all minerals after heating to 600°C. As stated previously, we see radical reductions in the REC values compared at 600°C versus room temperature ranging from a 55% reduction for zircon all the way to a 75% reduction for thorite. In those minerals that are compatible with U and Th, it is reasonable to assume that the radium distribution within these minerals is homogenous. We speculate that in minerals such as these where radium is uniformly distributed (as opposed to the distribution in common rocks where most U and Th resides in accessory minerals, adsorbed to clay minerals or occluded in fine-grained cements and other coatings), a major portion (>50%) of radon is released via the conduit pathways created by alpha-decay. The tracks created by the decay of the alphaemitting radionuclides in a mineral result in a mosaic of channels along which the mineral can be altered to increase the fraction of diffusion and indirect recoil of U-Th series nuclides. Additionally, alpha-decay damage can also increase the susceptibility to dissolution, increase chemical diffusion, decrease mineral density, etc. (Murakami et al., 1991). Depending on the crystalline structure and bonding strength between atoms, the extent of radiation damage can cause the mineral to become amorphous, or metamict. The observation that the ratio of REC for heated samples to unheated samples remains constant for the various degrees of heating in the mineral monazite for both size fractions ( ~50% lower for the larger size fraction as calculated from data in Table 16) implies that, as expected, the grain size and therefore the surface area plays a role in the ability of radon to emanate. This observation only holds true for the mineral monazite. Because large variations in the REC are observed for all other 75 minerals based on the heating history and the emanating medium, the effect of grain size on the overall variation in REC is considered to be secondary to the other variables. 4.1.1 Emanation of 222Rn into liquid The REC values for 222 Rn emanation into 0.1 N HNO3 solution were determined for the minerals monazite, zircon, thorite and cerite at the <63 µm size fraction for three temperatures: room temperature, 200°C and 600°C. The mineral monazite has a different trend in REC values with differing degrees of heating when emanation was into liquid rather than gas. The highest REC value was obtained for the unheated mineral. The lowest value was determined for monazite heated to 200°C. The REC value for the monazite heated to 600°C was nearly identical to that of the unheated sample. The ratio of the REC for the sample heated to 200°C to the unheated sample is ~60% and for the sample heated to 600°C is 99%. For all temperatures, the value of REC into solution is at least twice the value of REC into gas. For the mineral zircon, the trend obtained for variation in REC into liquid was also different than that observed for emanation into gas. The highest REC value was obtained from the sample heated to 200°C and the lowest was for the sample heated to 600°C with the unheated sample intermediate to the others. There was very little variation in the values with differing heating histories, less than 10% difference, hence it seems that the heating of the mineral zircon does not affect its REC into solution. The value of REC into solution for the unheated zircon is ~50% of the 76 value into gas, while for samples heated to 200°C the value is constant. For the sample heated to 600°C, the value of REC into liquid is ~20% higher than the value into gas. The mineral thorite has exactly the opposite trend with heating for the REC into liquid as opposed to air. The highest value obtained was for the sample heated to 600°C and the lowest for the unheated mineral. Both heated samples (200°C and 600°C) had approximately the same REC into solution with less than a 5% variation. The values of the REC into liquid were much higher than those into gas. Because of the opposing trend with heating depending on the medium entered, the REC’s vary with temperature as follows: For unheated minerals the REC into liquid is ~3 times that into gas; for samples heated to 200°C the REC into liquid is ~10 times that into gas; and for samples heated to 600°C the REC into liquid is ~18 times that into gas. The REC of the mineral cerite was determined only for emanation into solution. The highest REC obtained was for the sample heated to 200°C and the lowest was for the unheated sample. There is not a lot of variation in the values, with the value for the sample heated to 600°C about 17% lower than that heated to 200°C, and the unheated sample ~27% lower than that heated to 200°C. This relatively small difference implies that, like zircon, heating history has a rather limited effect on the REC into solution for cerite. These observations in conjunction with those given in section 4.1.1 imply the following: 77 1. The radon emanation coefficient is distinctly higher for emanation into solution versus gas for monazite and thorite, but relatively constant for zircon. 2. The variation of heating history has a much more profound effect for the REC values in gas than it does for the REC values in solution for all minerals. The percentage decrease in the REC with heating ranges from 55-75% for emanation in gas, but only 10-40% for emanation into liquid. 3. For the mineral monazite, the REC in gas at room temperature and for samples heated at 200°C is ~50% less than the corresponding REC in solution. The REC in gas for samples heated to 600°C is 85% less than the corresponding REC in liquid, and the RECs in liquid are the same for room temperature and 600°C. 4. The trend of the REC value of the mineral thorite is exactly opposite depending on the medium it enters. If release is into gas, the REC decreases with increasing prior heat exposure, but if it is into solution, the REC increases with increasing prior heat exposure. This may suggest the importance of heating this mineral to its susceptibility of dissolution. 5. For the mineral zircon, the REC values remain relatively constant for emanation into solution with varying degrees of heating. Unlike the other minerals studied, the REC value for emanation into air is higher than into solution for unheated samples. 78 6. The mineral cerite shows relatively little difference in the REC value into solution at all temperatures. The diffusion coefficient for radon in air is ~0.1cm2 s-1 and in water it is ~10-5 cm2 s-1. Therefore, the diffusion length in a fluid is ~100 times smaller than in air. When comparing the minerals for which the REC was determined into both air and solution, we see the influence of this difference for both monazite and thorite. The REC values are much higher for emanation into solution for both minerals implying that much of the 222Rn that is recoiled from the grains when 226Ra atoms decay is able to embed itself in neighboring grains when emanation is into air, but is stopped short when emanation is into solution. The presence of solution in pore spaces greatly increases the probability that the path of the recoiling 222 Rn atom will be terminated in that pore space, thus increasing the direct-recoil fraction of the REC. The results obtained here suggest that the amount of 222 Rn which gets embedded in neighboring grains in the absence of solution is quite substantial. The mineral zircon, however, yielded a higher value of REC for emanation into gas as opposed to solution, suggesting that the amount of 222 Rn that gets embedded in neighboring grains is small. Of the minerals studied, zircon shows the lowest range of REC values in both gas and liquid, indicating that the radon loss from zircon is small. This observation supports the extensive application of U-Th-Pb chronology on individual zircon crystals, as the differences in ages calculated between 232 238 U-206Pb, 235 U-207Pb and Th-208Pb pairs caused by radon loss are likely to be negligible. For the minerals that had their REC determined for both emanation into liquid and gas (thorite, zircon and monazite), all three exhibited the same temperature trends 79 in REC into gas, but none had the same trends into solution. In fact, none of the four minerals examined for REC into solution showed the same trend as any other. This observation suggests the importance of the individual mineral structures, possibly as a result of radiation damage, in the presence of solution in the pore spaces to the resulting value of REC. The overall differences are attributed to differences in bonding energy between the various atoms in the crystal. For all minerals, however, there is a much larger variation in REC seen with varying degrees of heating for emanation into air as opposed to solution, possibly implying the contribution of dissolution and leaching to the total REC value obtained for the samples in solution. 4.2 Activity Ratios of U-Th series nuclides recoiled and/or leached into solution The concentrations of many of the U-Th series radionuclides that were either recoiled or leached into the 0.1 N HNO3 solution were determined for each mineral at the various temperature subjections and those results were given in Table 11 (Chapter 3). It is useful to interpret these concentrations and their variance with respect to other radionuclides belonging to the same series because we expect all of the nuclides within one decay series to be in secular equilibrium. Any disequilibrium observed between the nuclides implies that a process that exhibits preference (i.e. leaching and/or recoil) is occurring and lends itself to broader interpretation. Ratios of various isotopic pairs were also included in Table 11, and this information was expressed visually as histogram diagrams in Figures 17-19. The isotopic pairs examined were 210 Po/210Pb, and 222 228 Ac/228Ra, 234 Th/238U, 234 U/238U, 212 Pb/224Ra, 224 Ra/228Ra, 224 Ra/228Th Rn/226Ra. The following subsections discuss these results for each mineral. A 80 section comparing the results between all the minerals examined and explaining the possible implications to changes taking place in the minerals after they are exposed to various degrees of heating follows. 4.2.1 Monazite The mineral monazite has a 210 Po/210Pb ratio of approximately zero, like all other minerals, at all temperatures, caused in part by the concentration of 210Pb being below detection level for some samples. The 234U/238U concentration remained at ~1 for all temperatures. The remaining nuclide pairs all have disequilibrium with a widely varying range of values. The value of the 234 Th/238U ratio increases with increasing heat exposure from 0.4 at 25ºC to 1.2 at 200ºC to 3.2 after being annealed at 600ºC. The 224Ra/228Ra ratio behaves similarly to that of cerite with disequilibrium values of 0.6 for the samples at 25º and 200ºC, followed by an essentially equilibrium value for the sample annealed at 600ºC. The value of the 224 Ra/228Th ratio varied from 119 to 960 with the highest value for the sample annealed at 200ºC and the value of the 222 Rn/226Ra ratio varied from 18 to 30 with the highest value for the sample annealed at 600ºC. 4.2.2 Zircon The mineral zircon showed a relatively large number of isotopic ratios that did not vary with the differing exposures to heat. These pairs and the values of their ratios are as follows: is equal to ~ 1.6, 212 210 Po/210Pb is equal to ~ 0, 228Ac/228Ra is equal to ~2, 234U/238U Pb/224Ra and 224 Ra/228Ra are both equal to ~1 and 222 Rn/226Ra is 81 equal to ~2. 224 The pairs for which the disequilibrium varies are Ra/228Th. For 234 234 Th/238U and Th/238U, the largest value is seen for the unheated sample (4.5) but after heating at 200ºC the ratio is reduced to 2.9 and remains that value even after annealing at 600ºC. For the 224 Ra/228Th pair, a near equilibrium value (1.4) is seen for the sample heated to 200ºC, however the unheated sample and the sample heated to 600ºC are grossly out of equilibrium with respective values of 119 and 50. 4.2.3 Cerite The concentrations of 228 Ac, 234 Th and 210 Pb in the leachates of cerite were below the detection limit for at least one of the temperatures, so it is difficult to interpret the ratios of the pairs 228Ac/228Ra, 210Po/210Pb and 234Th/238U. The 234U/238U ratio remained constant at ~1 (equilibrium) for all levels of temperature exposure. The values of 212 Pb/224Ra and 224 Ra/228Ra followed the same trend for this mineral with values of ~0.6 for the unheated sample and the sample heated to 200ºC, followed by approximately equilibrium values for the sample annealed at 600ºC. The values of the 224 Ra/228Th and 222 Rn/226Ra ratios remained relatively constant with varying temperatures with approximate values of ~13 and ~4, respectively. 4.2.4 Thorite The mineral thorite has great variation in the disequilibrium not only between different isotopic pairs but also between the different temperatures in the same isotopic pair. It is difficult to deduce any information about the 234Th/238U pair since the 234Th concentration was below the detection limit for the minerals heated to 200ºC 82 and 600ºC, yet the ratio is ~1000 for the sample at room temperature. The 210Po/210Pb ratio is approximately zero for all temperatures. For the pairs 212 Pb/224Ra and 224 228 Ac/228Ra, 234 U/238U, Ra/228Ra, the greatest deviation from equilibrium is seen for the samples that had been heated to 200ºC. The 224Ra/228Th and 222Rn/226Ra ratios are at a gross disequilibrium with values much, much greater than one for all temperatures with the exception of the 222 Rn/226Ra ratio at 25ºC which is equal to 0.75. It is interesting to note that for this mineral no pairs are at equilibrium for any temperature; there is at least a 0.25 deviation for every detectable pair. 4.3 Mechanims of release of U-Th series radionuclides into solution: The U-Th series radionuclides from the mineral grains can reach the solution phase by the following mechanisms: a) direct and indirect recoil from the grain– this will affect all the daughter products of U-Th series radionuclides, except 232 238 U and Th, the parent nuclides; b) congruent dissolution of mineral grains – in which the nuclide to major ion flux ratio is equal to the concentration ratio in the bulk mineral; c) incongruent dissolution (leaching) where the fractional release rates of nuclides and matrix are not equal; and d) diffusion of nuclides from solid to solution phase. The importance of each of these mechanisms is discussed below with respect to the results reported in the previous chapter. If the mineral grain diameter is less than 20-70 nm (less than the recoil range of nuclides in a mineral grain), then the escape probability of all nuclides undergoing recoil is nearly 1, irrespective of the geochemical property of the nuclide. If a liquid surrounds this mineral grain, we anticipate the activity ratios of daughter/parent in the 83 U-Th series should be close to unity. However, this probability will decrease to 0.54 for grains of 0.9 µm diameter (Tanner, 1978). In our present investigation, we utilized size-range less than 63 µm diameter and the escape probability is expected to be significantly lower than 0.1. The recoiled nuclides in the deeper regions of a mineral grain are unavailable, unless there is a development of large internal surface that may be the result of chemical corrosion, weathering or intensive fracturing on a microscopic scale. Amin and Rama (1986) have shown that openings inside a natural crystalline mineral are narrow with a large internal area and are interconnected to form an extensive network. When mineral grains are soaked in solution, this solution can enter this network and potentially leach the more loosely-bound recoiled daughter nuclides, as opposed to the more tightly-bound parent nuclides that have not undergone any recoil (e.g. 238U, 235U and 232Th). 4.3.1 Congruent Dissolution versus incongruent dissolution: The amounts of 238 U and 232 Th dissolved and/or leached by 0.1 M HNO3 for all 4 minerals (Table 15) indicate that: a) the percentage of dissolution of the mineral grains, based on 238 U and 232 Th values do not have the same value (within 2-σ propagated error) in any of the minerals, irrespective of the temperature to which those minerals were heated. This difference indicates non-congruent dissolution of minerals; b) 238U leaching is higher than 232Th, except for zircon at 200º and 600ºC; c) In the case of monazite, the amount of 238 U leached decreases with increasing temperature however the reverse was found in cerite; in zircon and thorite, higher amounts of 238 U were leached at 200ºC than at room temperatures or 600ºC; d) In 84 cerite, the amount of 232Th leached remained constant at all three temperatures but for the other three minerals, the highest amount of 232Th was leached from 200ºC-heated minerals and generally, lower values were found for unheated minerals. variations in the preferential leaching of 238 U and 232 The Th between minerals are most likely caused by variations in the radiation damage produced by the alpha decay events. In minerals that are resistant to dissolution (e.g. monazite and zircon), the amount of abundant parent-nuclide (232Th in monazite and 238U in zircon) is relatively small compared to less-abundant parent nuclide likely because of the compatibility of these nuclides in the lattice structure. Localized regions with 238 U or 232 Th that are easily leachable can also preferentially contribute U or Th to the leach solution. It could also be caused by a chemical reaction taking place at sites where U/Th are present. The fractional release of U during a 6.8 year leaching experiment with bicarbonate-carbonate solution indicated that the amount of U leached was a factor of ~3 higher than that of 232Th (Olander and Eyal, 1991a). 4.3.2 Leaching of U-Th series radionuclides: The concentrations of U-series radionuclides (234Th, 210 Pb, and 210 Po) and Th-series radionuclides (228Ra, 228 Th, 234 U, 228 230 Ac, Th, 224 226 Ra, 222Rn, Pb, and 212 Pb) from the leach solutions for the four minerals, monazite, zircon, cerite and thorite are given in Table 11. The activity ratios of the daughter to parent nuclides, defined as the relative leaching rate, are given in Tables 12 and 13. In most cases, there is disequilibirum between the daughter and parent nuclides in the leachate. 85 4.3.2a Monazite: In monazite, the amount of 238 U in the leachate decreased with increased annealing temperature but the corresponding values of 234 Th remained constant (within 2-σ), indicating that the amount of leaching decreases and recoil input increases. The amount of recoiled and leached 210 Po, 210 Pb and annealing monazite whereas the concentration decreases for 230 226 Th increases after Ra and 222 Rn after annealing. In the case of Th-series radionuclides, the amount of leached/recoiled 228 Ac and 212Pb is higher at 200ºC annealed samples than that at 600ºC samples. The leaching rate of 228Th in monazite increases dramatically, from 0.88 to 2.47, however the increase in 234U/238U leaching rate is < 10%. The 234U/238U activity ratios are less than unity suggesting that recoil supply of 234 U is also minimal on the time scale of the experiments. For all the particle-reactive nuclides (234Th, as well as 226 Ra and 230 Th, 210 Pb, and 210 Po 222 Rn), the leaching rate increases as the degree of prior heating events increases. The polonium concentration is negligibly small, most likely because a major portion of the Po would have escaped during heating and the small amount of Po measured has been regenerated from the decay of its grandparent, 210 Pb. The activity ratio of 222Rn/238U less than 1 strongly indicates limited leakage of 222Rn from the monazite grains. 4.3.2b Zircon: The amount of 234 Th measured in the solution indicates a large amount of recoil input, as compared to leaching/dissolution input. However, the leaching rates for 222 Rn, 226 Ra, and 210 Pb are higher than unity indicating that the diffusion and 86 leaching of these nuclides is significant, and is higher than is seen for monazite. The leachant could potentially enter the pore space and leach the nuclides. The density of zircon (4.68 g cm-3) is lower than that of monazite (4.6-5.4 g cm-3) suggesting that void space in the mineral structure could potentially be the reason for the differences in the extent of leaching of these nuclides. The leaching rates for 230 Th and 234 U are the highest in annealed minerals, indicating that these nuclides within the crystal structure are efficiently removed, even after annealing. The leaching rates of all the daughter products in the 232 Th series decreased drastically in annealed zircon. Overall, the leaching rate of all nuclides measured (except 230Th and 234U and 210Pb at 600ºC) is lower in the heated samples of zircon than in unheated zircon. This difference is attributed to the annealing of fission- and recoil tracks at these temperatures. Annealing of those tracks does not affect the leaching of long-lived radionuclides. 4.3.2c Cerite: The amount of recoiled and leached 234 Th, 210 Pb and 228 Ac in unannealed cerite is below the detection limit, as compared to 238U and 232Th (Tables 12 and 13). Of all the five minerals, cerite has the lowest alpha decay radiation dose (Table 13) and thus, the radiation damage within the crystal is expected to be the minimum with the least amount of recoil tracks which could potentially lead to loss of radionuclides through these tracks. Annealing cerite at 200ºC results in a higher leaching rate of 228 Ac (Table 13), however annealing at 200ºC and 600ºC results in lower leaching rates of many of the other isotopes compared to the unannealed mineral (except 234U, 87 212 Pb and 228 Ac). The leaching rate for 228 Th is higher than all the other minerals; however, the lower value for the 600ºC-annealed cerite compared to the unannealed sample is likely due to annealing of recoil and fission tracks after heating the mineral. 4.3.2d Thorite: All the daughter products of 232 Th have the highest leaching rates in the unannealed thorite and annealed samples (both at 200 and 600ºC) have leaching rates about 3 orders of magnitude lower. This difference is attributed to the annealing of alpha- and recoil tracks that minimizes the leaching of these nuclides. The leaching rate for 228 Th on the unannealed mineral decreases when it is annealed, whereas the 234 U leaching rate remains constant before and after annealing. The leaching rates of 226 Ra and 210Pb also decrease after annealing the thorite mineral. The leaching rate of 222 Rn increases by more than a factor 5 in annealed sample and it is not clear what is causing this increase in the radon emanation rate. 4.3.3 Activity Ratios of 234U/238U and 228Th/232Th: The disequilibrium between 234 U and 238 U, as well as 228 Th and 232 Th, can provide leaching of these nuclides from fresh and old recoil tracks. Fleischer (1988) reported the annealing time at ambient temperatures is ~400 yrs for a metamict mineral to 2,000-25,000 yr for several crystalline minerals. There is no preferential leaching of 234U from thorite and monazite, and from cerite the 234U/238U activity ratio is within ~10% of the equilibrium value of 1. However, the activity ratio for zircon is higher than unity. The 232Th/228Th activity ratio in unannealed monazite and zircon is 88 close to equilibrium, but in the annealed minerals the activity ratio varies widely. Generally, the lowest 228 Th/232Th activity ratio was found in the 600°C-annealed sample. When alpha tracks are annealed, the accelerated etching by 0.1 N HNO3 of minerals will disappear resulting in a lower amount of leaching of 228 the annealing should also result in less preferential leaching of 234 Th. Similarly, U. A related observation has been reported by Fleischer (1982). The highest activity ratio was found in the cerite samples. In several of the U-Th rich minerals, such as monazite, uraninite, thorianite, betafite, and thorite-melanovite, the increase in the activity ratio in the leachate is ~10% and for 234 U/238U 228 Th/232Th is much larger, i.e., factors of 2X to 7X (Eyal, 1982; Eyal and Kaufman, 1985; Ewing et al., 1986; Fleischer, 1988). When the annealing time scale is comparable to the mean life of 234 U, then only a small portion of the recoiled 234U will be located in fresh damage. Therefore a smaller amount can be leached as compared to 232Th. Hence only this small portion is preferentially leached. 4.3.4 Comparison of Activity Ratios When comparing the different minerals in terms of the behaviors of the various isotopic pairs with differing histories of heat exposure, the following observations can be made: 1. There appears to be a correlation between alpha radiation dose and 232 Th series leaching rates. 2. The 210 Po/210Pb ratio is approximately equal to zero for all minerals at all temperatures with the exception of thorite at 25ºC where the value is 0.25. 89 Due to low melting point of 210Po, it is quite likely that a significant amount of 210 Po has escaped during heating and most of the 210 Po leached is generated from 210Pb from the time the mineral annealed to the time the mineral-solution separation. 3. The amount of recoiled and leached radionuclides mainly depends on whether 238 U and 232 Th are located within the crystal structure or along grain boundaries. From the concentrations and chemical formulae of these nuclides, monazite and thorite incorporate Th into the lattice structure. With the annealing of tracks, the recoil and even leaching of nuclides from these tracks will decrease and that is evident in thorite where the leaching rate decreased 232 over 3 orders of magnitude for all the daughter products of Th. In monazite, the leaching rates for annealed and unannealed minerals vary widely, although a general trend of decreasing was found in the annealed monazite. If U is located along the grain boundary in monazite, then, it is likely that the effect of annealing on radionuclide release is small, if any. On the contrary, annealing could alter the position of some of the impurities leading to increased mobility. Increased leaching rates of monazite and thorite, and increased leaching rates of 222 Rn and 210 Pb and 230 226 Th in Ra in monazite could explain the data presented in Tables 12 and 13. In cerite and zircon, 238 U concentrations are considerably higher than if most of the 238 U is lattice-bound and 232 232 Th (Table 8), and Th is associated as impurities, then annealing will effect U-series radionuclides, rather than the radionuclides. This effect is observed for 230 Th, 226 Ra, and 232 222 Th-series Rn but the 90 leaching rates of 232 Th-series radionuclides remained constant within a factor of 2 between annealed and unannealed cerite. In the case of zircon, the 232 leaching rate of Th-series radionuclides decreased by two orders of magnitude when zircon sample was annealed whereas the daughter products of 238 U-series remained within a factor of 2 (210Pb, There is an increase in 234 U and 230 222 Rn, 226 Ra and 234 Th). Th leaching rates in the annealed zircon that can be attributed to more efficient leaching of these long-lived radionuclides in annealed zircon. 4. The 234 Th/238U ratio does not behave the same for any two of the minerals studied here. Thorite shows a huge disequilibrium (976) for the unheated sample, followed by undetectable amounts of 234Th for both annealed samples. Cerite, however, has undetectable 234 Th for the unheated sample and sample annealed at 200ºC followed by a ratio of 0.2 for the sample annealed at 600ºC. Monazite shows a progression in the ratio value: 0.5 at 25ºC, ~1 at 200ºC and >2 at 600ºC. Zircon has a value of 4.5 for unannealed samples and 2.9 for samples heated at both 200º and 600ºC. 5. The value of the 234 U/238U ratio remains constant and equal to ~1 for monazite, thorite and cerite. The value remains constant and equal to ~1.5 for zircon. 6. The value of the 212 Pb/224Ra ratio varies with temperature from mineral to mineral. For monazite the ratio is ~1 for unheated and samples annealed at 600ºC, but is >2 for samples annealed at 200ºC. For cerite, the ratio is ~0.6 for the unheated sample and sample heated to 200ºC and ~1 for the sample 91 annealed at 600ºC. For thorite the value varies from 1.2 at 25ºC to >2 at 200ºC to 0.2 T 600ºC, and for zircon the value remains constant at ~1. 7. The 224Ra/228Ra ratio behaves similarly for monazite and cerite with values of 0.6 for unheated samples as well as those heated to 200ºC, followed by values of ~1 for the samples annealed at 600ºC. The ratio for zircon remains relatively constant at ~1 with differing degrees of heat, but for thorite varies from 0.75 at 25ºC to >2 at 200ºC to 1.8 at 600ºC. 8. A very large disequilibrium is seen for both the 224 Ra/228Th and 222 Rn/226Ra ratios for all the minerals at all temperatures. With the exception of thorite at 25ºC with a value of 0.7, all other values are much greater than 1. 92 CHAPTER 5 CONCLUSIONS Based on the results and discussion presented in previous chapters, the following conclusions can be drawn: 5.1 Conclusions Regarding the Radon Emanation Coefficient 1. The radon emanation coefficient (REC) for release into gas is highest for all unheated minerals, and lowest for all minerals that have been heated to 600ºC. This difference indicates the importance of recoil tracks as conduit pathways for release of 222Rn. Once these tracks are annealed, the REC is drastically lowered. 2. The REC is distinctly higher for emanation into solution versus gas for monazite and thorite, but relatively constant for zircon, indicating the strong ability of liquid in the pore spaces to prevent the recoiling atom from entering a neighboring grain. This also indicates the importance of the specific characteristics of each mineral in terms of the probability that the recoiling 222Rn atom will embed itself in an adjacent grain. 3. The variation of heating history has a much more profound effect on the REC values for emanation into gas than it does for emanation into liquid for all minerals. The percentage decrease in REC ranges from 55-75% for release into gas, but only 10-40% for release into solution. In addition, the minerals that had the REC determined for release into both gas and solution all have the same trends in REC into gas with varying degrees of heating, however none have the same trends in solution. These two observations suggest that chemical processes, 93 such as dissolution or leaching, taking place in the presence of solution contribute to the overall REC value. 5.2 Conclusions Regarding the Recoil and/or Leaching Rates 1. Of the four minerals studied, thorite has a high alpha radiation dose value and has the highest leached amounts of short-lived radionuclides in the 232 Th and 238 U series for the unannealed minerals. Annealing this mineral reduces the leaching rate by 3 orders of magnitude. This decrease indicates that most of the U and Th series radionuclides are located farther from the surface than the recoil distance and that annealing the recoil tracks eliminates the conduit pathways for diffusion and leaching by the leachant. A similar observation was made for the zircon sample for the 232 Th series nuclides, however, the 238 U series nuclides changed within a factor of 2. This difference between the U and Th series nuclides for zircon is attributed to the distribution (homogeneous vs. along grain boundaries) of U and Th in the mineral grains. 2. Monazite and thorite have the highest leaching rates of all the minerals studied for the 232 Th series radionuclides in unannealed samples, and this is attributed to the large amount of radiation damage caused by the high concentrations of 232Th. 3. In minerals where the leaching rates for 232Th remained constant within a factor of 2 for annealed and unannealed samples, U and Th are likely concentrated along grain boundaries. Annealing of such minerals will not radically affect the leaching/recoil rate. This seems to be the case for monazite and cerite. 94 4. In monazite and thorite, the radon leaching rates increased drastically at 600ºC as compared to the unannealed samples. 230 Th leaching rates also increased in these two cases. These observations are attributed to changes in the mineral structure taking place that lead to enhanced leaching of the nuclides. 5. The location of the parent nuclides 238 U and 232 Th plays a major role in the leaching rates of the daughter products. In those minerals where U and Th parent atoms are homogeneously distributed, the leaching rate decreased after the recoil and alpha tracks had been annealed. In minerals where the U and Th are located along grain boundaries, there was relatively little change in the leaching rate after annealing. 5.3 General Conclusions and Recommendations for Future Work In general there are large variations observed in the results for the experiments conducted in this investigation. Differences are seen from mineral to mineral for both REC values and daughter/parent activity ratios of the U-Th series radionuclides. There are also wide variations in the daughter/parent activity ratios within one decay chain. The data obtained in these experiments suggest that many factors affect both the radon emanation coefficient as well as the leaching and recoil rates in natural minerals. These factors include the heating history of the mineral, the degree of radiation damage within the mineral, the medium into which radon and other nuclides are emanated, the location of parent nuclides within a mineral and the structure and physical properties of each mineral. The potential for much future work exists in this field because of the many variables involved. Suggestions for the direction of future work include: 95 1. Using X-ray Diffraction (XRD) techniques to track the structural changes taking place for minerals subjected to various degrees of heating, in conjunction with leaching experiments to better quantify the leaching observations. 2. Carefully determining the surface area of the grains used for analysis, and comparing these surfaces areas for different minerals to relate the amount of recoil/leaching of radionuclides to the surface area. 3. Mapping the 238 U, 232 Th and radium distributions in grains to determine any heterogeneities and correlating that distribution to the observed leaching/recoil results. 4. Conducting REC and leaching studies on zircons of varying degrees of metamictization (as determined by XRD) to determine the role of the crystallinity of a mineral to its diffusion characteristics and its susceptibility to leaching and/or dissolution. 96 APPENDIX 1: DECAY PLOTS y = -1.2082x + 10.053 R2 = 0.8911 Thorite 1 Ac-228 (25 degrees) 12 Ln Act 10 8 6 4 2 0 0 0.5 1 1.5 2 2.5 Tim e (d) y = -0.5473x + 6.4313 R2 = 0.733 Thorite 4 Ac-228 (200 degrees) 7 6 Ln Act 5 4 3 2 1 0 0 0.5 1 1.5 2 2.5 Tim e (d) Thorite 5 Ac-228 (600 degrees) y = -0.7429x + 5.3789 R2 = 0.8647 Ln Act 6 5 4 3 2 1 0 0 0.5 1 1.5 2 2.5 Time (d) Decay plots used for the determination of 228 Ac for thorite. 97 Thorite 1 Pb-212 (25 degrees) y = -0.9139x + 10.177 R2 = 0.9991 10.2 10 Ln Act 9.8 9.6 9.4 9.2 9 0 0.2 0.4 0.6 0.8 1 1.2 Time (d) y = -0.508x + 7.9497 R2 = 1 Thorite 4 Pb-212 (200 degrees) 8 Ln Act 7.8 7.6 7.4 7.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Time (d) y = -0.617x + 3.885 R2 = 0.9413 Thorite 5 Pb-212 (600 degrees) 5 Ln Act 4 3 2 1 0 0 0.5 1 1.5 Tim e (d) Decay plots used for the determination of 212Pb for thorite. 98 y = -0.0572x + 9.9562 R2 = 0.9617 Thorite 1 Ra-224 (25 degrees) Ln Act 12 10 8 6 4 2 0 0 10 20 30 40 Tim e (d) y = -0.1222x + 7.156 R2 = 0.9786 Thorite 4 Ra-224 (200 degrees) 8 Ln Act 6 4 2 0 0 5 10 15 20 25 Tim e (d) y = -0.0767x + 5.6132 R2 = 0.9889 Thorite 5 Ra-224 (600 degees) Ln Act 6 4 2 0 0 10 20 30 40 50 Time (d) Decay plots used for the determination of 224Ra for thorite. 99 y = -1.1281x + 3.7663 R2 = 0.9969 Cerite 5 Pb-212 (25 degrees) 4 Ln Act 3 2 1 0 0 0.5 1 1.5 2 2.5 Tim e (d) y = -0.7434x + 3.7887 R2 = 0.9992 Cerite 3 Pb-212 (200 degrees) 4 Ln Act 3 2 1 0 0 0.5 1 1.5 2 2.5 Time (d) y = -1.0202x + 3.9911 R2 = 0.996 Cerite 1 Pb-212 (600 degrees) 5 Ln Act 4 3 2 1 0 0 0.5 1 1.5 2 2.5 3 3.5 Tim e (d) Decay plots used for the determination of 212Pb for cerite. 100 y = -0.1761x + 4.21 R2 = 0.987 Cerite 5 Ra-224 (25 degrees) 5 Ln Act 4 3 2 1 0 0 5 10 15 20 25 Tim e (d) y = -0.0459x + 3.9296 R2 = 0.9901 Ln Act Cerite 3 Ra-224 (200 degrees) 3.94 3.92 3.9 3.88 3.86 3.84 3.82 3.8 3.78 3.76 0 0.5 1 1.5 2 2.5 3 3.5 Time (d) y = -0.0895x + 4.0473 R2 = 0.9941 Cerite 1 Ra-224 (600 degrees) 5 Ln Act 4 3 2 1 0 0 10 20 30 40 50 Tim e (d) Decay plots used for the determination of 224Ra for cerite. 101 y = -1.0881x + 7.8559 R2 = 0.986 Monazite 1 Pb-212 (25 degrees) 10 Ln Act 8 6 4 2 0 0 0.5 1 1.5 2 Tim e (d) Monazite 3 Pb-212 (200 degrees) y = -0.8358x + 8.1013 R2 = 0.9796 10 Ln Act 8 6 4 2 0 0 0.5 1 1.5 2 2.5 Tim e (d) Decay plot used for the determination of 212Pb for monazite. 102 y = -0.0928x + 4.3485 R2 = 0.9752 Zircon 1 Ra-224 (25 degrees) 4.4 Ln Act 4.3 4.2 4.1 4 3.9 0 1 2 3 4 5 Tim e (d) Zircon 2 Ra-224 (200 degrees) y = -0.0891x + 4.1977 R2 = 0.9888 Ln Act 4.4 4.2 4 3.8 3.6 0 2 4 6 8 Time (d) Ln Act Zircon 3 Ra-224 (600 degrees) y = -0.0809x + 3.1064 R2 = 0.994 3.5 3 2.5 2 1.5 1 0.5 0 0 2 4 6 8 10 12 Time (d) Decay plots used for the determination of 224Ra for zircon. 103 REFERENCES Amin and Rama, Using Radon as Probe for Investigating Characteristics of Fractures in Crystalline Minerals, Nucl. Instrum. Methods. B17, 527-529, 1986. Baskaran, M. and Naidu, A.S., 210Pb-derived chronology and the fluxes of 210Pb and 137Cs isotopes into continental shelf sediments, East Chukchi Sea, Alaskan Arctic, Geochim. Cosmochim. Acta. 59 (21), 4435-4448, 1995. Ehlert, T.C., Gowda, K.A., Karioris, F.G., and Cartz, L., Differential scanning calorimetry of heavy ion bombarded synthetic monazite, Radiation Effects. 70, 173-181, 1983. Eyal, Y., Isotopic fractionation of thorium and uranium upon leaching of monazite, Scientific Basis for Radioactive Waste Management V. 399-408, 1982. Eyal, Y. and Fleischer, R.L., Timescale of natural annealing in radioiactive minerals affects retardation of radiation-damage-induced leaching, Nature. 314, 518520, 1985a. Eyal, Y. and Fleisher, R.L., Preferential leaching and the age of radiation damage from alpha decay in minerals, Geochim. Cosmochim. Acta. 49, 1155-1164, 1985b. Eyal, Y. and Kaufman, A., Alpha-recoil damage in monazite: Preferential dissolution of the radiogenic actinide isotopes, Nucl. Tech. 58, 77-83, 1982. Eyal, Y., Lumpkin, G.R. and Ewing, R.C., Scientific Basis for Radioactive Waste Management IX. 50, 379, 1986. Eyal, Y., Lumpkin, G.R. and Ewing, R.C., Scientific Basis for Radioactive Waste Management X. 84, 635, 1987. 104 Eyal, Y. and Olander, D.R., Leaching of uranium and thorium from monazite: I. Initial Leaching, Geochim. Cosmochim. Acta. 54, 1867-1877, 1990a. Eyal,Y. and Olander, D.R., Impact of alpha-decay on incongruent actinide isotope leaching from monazite, J. Nucl. Mat. 170, 117-120, 1990d. Fleischer, R.L., Recoiling alpha-emitting nuclei. Mechanisms for uranium-series disequilibrium, Geochim. Cosmochim. Acta. 42, 973-978, 1978. Fleischer, R.L., Alpha-recoil damage and solution effects in minerals: uranium isotopic disequlibrium and radon release, Geochim. Cosmochim. Acta. 46, 2191-2201, 1982. Fleischer, R.L., Theory of alpha-recoil effects on radon release and isotopic disequilibrium, Geochim. Cosmochim. Acta. 47, 779-784, 1983. Fleischer, R.L., Alpha-recoil damage: Relation to isotopic disequilibrium and leaching of radionuclides, Geochim. Cosmochim. Acta. 52, 1459-1466, 1988. Fleischer, R.L. and Mogro-Campero, A., Mapping of integrated radon emantion for detection of long-distance migration of gases within the earth: techniques and principles, J. Geophys. Res. 83, 3539-3549, 1978. Fleischer R.L. and Mogro-Campero, A., Association of subsurface radon changes in Alaska and the northeastern United States with earthquakes, Geochim. Cosmochim. Acta. 49 (4), 1061-1071, 1985. Fleischer, R.L. and Turner, L. G., Correlations of radon and carbon isotopic measurements with petroleum and natural gas at Cement, Oklahoma, Geophysics. 49, 810-817, 1984. 105 Flynn, W.W., The determination of low levels of polonium-210 in environmental materials, Anal. Chim. Acta. 43, 221-227, 1968. Huang, W.H., Maurette, M. and Walker, R.M., Observation of fossil alpha-particle recoil tracks and their implications for dating measurements, Radioactive Dating and Methods of Low-Level Counting: IAEA Publ. 4135. 415-429, 1967. Igarashi, G. and Wakita, H., Groundwater radon anomalies associated with earthquakes, Tectonophysics. 180 (2-4), 237-254, 1990. Igarahi, G., Saeki, S., Takahata, N., Sumikawa, K., Tasaka, S., Sasaki, Y., Takahashi, M., and Sano, Y., Groundwater radon anomaly before the Kobe earthquake in Japan, Science. 269 (5220) 60-61, 1995. Kigoshi, K., Alpha-Recoil Thorium-234: Dissolution into Water and the Uranium234/Uranium-238 Disequilibrium in Nature, Science. 173, 47-48, 1971. Krishnaswami, S. and Seidemann, D.E., Comparative study of 222Rn, 40Ar, 39Ar and 37 Ar leakage from rocks and minerals: Implications for the role of nanopores in gas transport through natural silicates, Geochim. Cosmochim. Acta. 52, 655658, 1988. Lambert, G., Bristeau, P., and Polian, G., Mise en evidence de la faiblesse des migrations du radon a l’interieur des grain de roche [Evidence of little migration of radon within rock grains], C. R. Hebd. Seances Acad. Sci. Ser. D. 274 (25) 3333-3336, 1972. Lambert, G., and Bristeau, P., Migration des atomes implantes dans les cristeaux par energie de recul [Migration of radon atoms implanted in crystals by recoil 106 energy], J. Phys. (Paris), Colloq. C5, Suppl. to No. 11-12, 34 (C5), 137-138, 1973. Lederer, C.M. and Shirley, V.S. (Editors), Table of Isotopes. Wiley-Interscience, New York, N.Y., 1978. Lumpkin, G.R., Foltyn, E.M. and Ewing, R.C., Thermal recrystallization of alpharecoil damaged minerals of the pyrochlore structure types, J. Nucl. Mat. 139, 113-120, 1986. Lumpkin, G.R. and Ewing, R.C., Alpha-decay damage in minerals of the pyrochlore group, Phys. Chem. Minerals. 16, 2-20, 1988. Mathieu, G. G., 222Rn-226Ra technique of analysis. Ann. Tech. Rep. C00-2185-0, Lamont-Doherty Geological Observatory, Palisades, New York, 1977. Mathieu, G.G., Biscaye, P.E., Lupton, R.A., and Hammond, D.E., Systems for measurement of 222Rn at low levels in natural waters, Health Physics. 55, 989992, 1988. Monnin, M.M. and Seidel, J.L., Radon in soil-air and in groundwater related to major geophysical events-a survey, Nucl. Instrum. Methods Physics Research Section A- Accelerators, Spectrometers, Detectors and Associated Equipment. 314 (2), 316-330, 1992. Murakami, T., Chakoumakos, B.C., Ewing, R.C., Lumpkin, G.R. and Weber, W.J., Alpha-decay event damage in zircon, American Mineralogist. 76, 1510-1532, 1991. Olander, D.R. and Eyal, Y., Leaching of uranium and thorium from monazite: II. Elemental leaching, Geochim. Cosmochim. Acta. 54, 1879-1887, 1990b. 107 Olander, D.R. and Eyal, Y., Leaching of uranium and thorium from monazite: III. Leaching of radiogenic daughters, Geochim. Cosmochim. Acta. 54, 18891896, 1990c. Quet, C., Rousseau-Violet, J., and Bussière, P., Recoil emanating power and specific surface area of solids labeled by radium recoil atoms. I. Theory for single solid particles, Radiochim. Radioanal. Letters. 23 (5-6), 359-368, 1975. Raabe, A.G., Kanapilly, G.M. and Boyd, H.A., Studies of in vitro solubility of respirable particles of 238Pu and 239Pu oxides and on accidentally released aerosol containing 239Pu, Inhalation Toxicology Res. Inst. Ann. Report, Lovelace Found. Report LF-46. UC-48, 24-30, 1973. Rama and Moore, W.S., Mechanism of transport of U-Th series radioisotopes from solids into ground water, Geochim. Cosmochim. Acta. 48, 395-399, 1984. Rama and Moore, W.S., Micro-crystallinity in Radioactive Minerals, J. Geophys. Res. 4, 475-478, 1990. Rama and Moore, W.S., Submicronic porosity in common minerals and emanation of radon, Nucl. Geophys. 4, 467-473, 1990. Rosholt, J.N. and Tatsumoto, M., Isotopic composition of uranium and thorium in Apollo 11 samples, Proc. Apollo 11 Lunar Sci. Conf.; Geochim. Cosmochim. Acta. (Suppl. 1) 2, 1499-1502, 1970. Rosholt, J.N. and Tatsumoto, M., Isotopic composition of uranium and thorium in Apollo 12 samples, Proc. Apollo 12 Lunar Sci. Conf.; Geochim. Cosmochim. Acta. (Suppl. 2) 2, 1577-1584, 1971. 108 Semkow, T.M., Recoil-emanation theory applied to radon release from mineral grains, Geochim. Cosmochim. Acta. 54, 425-440, 1990. Tanner, A.B., Radon migration in the ground, The Natural Radiation Environment. Houston, Texas, April 23-28, 1978. Trimble, S.M., The distribution of uranium and thorium series radionuclides in the Canada Basin, Arctic Ocean, M.S. Thesis, submitted to Wayne State University, Department of Geology, April 2003. Vance, E.R. and Metson, J.B., Radiation damage in natural titanites, Phys. Chem. Minerals. 12, 255-260, 1985. Wasserburg, G.J., Diffusion Processes in Lead-Uranium Systems, J. Geophys. Res. 68, 4823-4846, 1963. Weber, W.J., Wald, J.W. and Matzke, Hj., Effects of self-radiation damage in Cmdoped Gd2Ti2O7 and CaZrTi2O7, J. Nucl. Mat. 139, 196-209, 1986. Wetherill, G.W., Discordant Uranium-Lead Ages 2. Discordant Ages Resulting from Diffusion of Lead and Uranium, J. Geophys. Res. 68, 2957-2965, 1963. 109 ABSTRACT MECHANISMS OF RELEASE OF URANIUM AND THORIUM SERIES RADIONUCLIDES FROM A SUITE OF NATURAL MINERALS By ELIZABETH C. GARVER December, 2003 Advisor: Dr. Mark Baskaran Major: Geology Degree: Master of Science The variation in the release of many U- and Th-series isotopes was determined as a function of varying degrees of heating history for a suite of natural minerals consisting of zircon, monazite, thorite, cerite and uraninite. The mineral samples were ground to two size fractions (<63 μm and 1-2 mm) and subjected to heating episodes of 25º, 100º, 200º and 600ºC. The rate of release of 222Rn (Radon Emanation Coefficient or REC) into both gas and solution was measured. The release via recoil and/or leaching of a suite of U-Th-series radionuclides that includes 224 Ra, 212 Pb and 234 U, 234 Th, 232 Th, 230 Th, 228 Ra, 228 Ac, 226 Ra, 210 Po was determined based on their concentrations in a 0.1 N HNO3 solution that the mineral had been placed in for ~25 days. The REC was found to radically decrease for samples that had been heated to 600ºC as compared to unheated samples, suggesting that the alpha-recoil tracks serve as conduit pathways for the release of 222 Rn. Once these tracks are annealed, the REC is drastically lowered. It was also found that the REC is distinctly higher for emanation into 110 solution versus gas for monazite and thorite, but relatively constant for zircon, implying that the denser medium of the solution terminated the path of the recoiling atom before it was able to embed itself in an adjacent grain. A comparison of the variation in the REC value with heating shows a much larger variation for the emanation into gas (55-75%) than it does for solution (10-40%) indicating that leaching and/or dissolution could partially contribute to the overall REC. The results obtained in this investigation suggest that the variation in leaching and/or recoil rates of the various U-Th-series radionuclides with varying degrees of heating history is a function of several variables including the degree of radiation damage in a mineral, the location of the parent nuclides 238 U and 232 Th in the mineral, and the structural and physical properties of each mineral. The differences in the leaching/recoil rates in various minerals may provide information on the internal structure and damage within minerals. 111 AUTOBIOGRAPHICAL STATEMENT I, Elizabeth Garver, am the daughter of Robert and Georgia Williams of Port Sanilac, Michigan. I graduated from Southfield Christian High School, and in December of 2000 graduated with my Bachelor of Science in Geology from Wayne State University. In September of 2001, I began the pursuit of my Master of Science in Geology, also at Wayne State University. I am utterly fascinated and awestruck at the power of the Earth and at the complexity of the processes that occur around us each day. I marvel at the fact that gaining knowledge of these processes also lends insight into the character of our God, who so graciously gave us this creation to study. I aspire to learn more about Him through the magnificence of the world and universe around me, and so give purpose to my academic pursuits.
© Copyright 2024 Paperzz