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.