“Forced-Gamma Emission” Studies Involving Nuclear Isomers Using Fast Neutrons and Bremsstrahlung X Rays N. A. Guardala+, J. L. Price+, J. H. Barkyoumb+, R. J. Abbundi+, G. Merkel* and J.J. Carroll# + NSWC/Carderock Division, 9500 MacArthur Blvd, W. Bethesda, MD 20817-5700 * Army Reserch Lab/Adelphi, 2800 Powder Mill Road, Adelphi MD # Dept. Of Physics and Astronomy, Youngstown State Univ., Youngstow, Ohio Abstract. We propose to perform a series of experiments involving nuclear isomers which will investigate the probabilities and mechanisms for de-exciting the isomeric level down to the ground state upon exposure to external radiation in the form of fast neutrons and bremsstrhalung x rays. The isomers have half-lives on the order of 1 hr to 10 days which is a convenient time scale to measure statistically meaningful changes in the specific activities of the isomeric state. Furthermore, the selected isomers are relatively easy to produce in our laboratory in sufficient quantities so that they can be made in a reasonable time frame and without recourse to any exotic means of production, handling or preparation and without the need for high-purity separated isotopes as the feedstock. We believe that studies undertaken in this fashion will produce fundamentally valuable information on the factors which govern and influence“forcedgamma emission” in nuclear isomers This type of information will potentially be very useful in similar studies involving longer-lived isomers such as:178m2Hf, 242mAm and 108mAg which have the potential to be used in various emerging new technologies in the later part of the 21st Century. and that is the term by which this process has become known widely. While there are many known isomers which have the potential to serve as excellent candidates to observe this phenomena of triggering , in only case, 180mTa (1) has this been observed unequivocally. Another isomer in this mass region, 178m2 Hf (2) has drawn a significant amount of attention in terms of experimental efforts to determine the probability to trigger forced-gamma emission in it. In particular, the efforts have been directed to determine the probability for its triggering using x rays in the 20130 kev region, with reports that the decay of 178m2Hf can be produced by using x rays as low as 20 keV in energy. Studies using synchrotron radiation have attempted to measure the cross section for triggering using radiation in this energy range. INTRODUCTION Recently, there has developed serious interest in the nuclear physics community to study nuclear isomers from not only the point of view of nuclear structure phenomena but also as a means of producing technologically valuable devices mostly for use as compact power sources that require a minimum amount of auxiliary and supporting machinery and equipment. The isomers which would be used in these type of power devices would all have half-lives in the tens to hundreds of years regime and therefore would allow them to be stored “on-the-shelf” for long periods of time. When it was desired to release the stored energy of the isomeric levels in brief time interval it has proposed that exposing the specific isomer to an external radiation of the right energy will result in transitions from the isomeric state to nuclear levels that are no longer forbidden to cascade via γ-ray emission to the ground state. This process of applying suitable external radiation to an isomer and then observing the “normal” γ-ray decay to the ground state has been termed triggering, CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan 2003 American Institute of Physics 0-7354-0149-7/03/$20.00 279 ISOMERS SELECTED FOR “TRIGGERING” STUDIES This method is applicable only in the case of photon triggering and not for triggering brought about through the n,n’γ process. The reason is that the amount of background γ-rays produced from inelastic scattering from other levels which are not related to the isomer is much too large and thereby overwhelms the probabilities of observing γ-ray cascades which would be indicative of triggering events taking place. The photon method is much more “gentle” in terms of the number of states which can be populated both in the isomer and from ground state levels. Therefore coincident measurements of γ-rays will be attempted at NSWC using the suitable isomers which seem to have the possibilities for x-ray triggering based on the knowledge of the appropriate values ∆E, the energy difference between the isomeric level and a level which is strongly connected to a cascade to the ground state and on angular momentum considerations such that the transition form the isomeric level to the intermediate level is favored and then of course that the transition from this intermediate level to the ground state is favored. This set of processes if of course the definition of triggering. The final transition from the intermediate level can either proceed directly to the ground state or to the ground state by a series of γ-ray transitions, i.e. a cascade. We have selected eight isomers to be used in thish study of triggering probabilities and mechanisms. They are in increasing mass order: 87mSr, 103mRh, 111m Cd, 115mIn, 189mOs, 193mIr and 199mHg. The values of t1/2 for these isomers ranges from 42.6 min for 199mHg up to 57.4 d for 125mTe. All of these isomers can be produced via fast neutron inelastic scattering, n,n’γ from the ground state directly to isomeric level or from the ground state to excited states which are strongly coupled to the isomeric level. In certain instances we hope to be able to determine the probabilities for the these two processes as a function of neutron energy to proceed in most of these selected isomers. In all cases, we have prepared samples of these isomers previously in our laboratory by exposing suitable amounts of nuclei to our fast neutron beam and counting the normal decay of the particular isomer. The cross sections for producing these isomers (regardless of which mechanism is responsible) is varies from 0.2 – 2 barns in the energy range of 0.3 – 3 MeV. This is an energy range easily accessible with our acceleratorbased fast neutron source. In addition to fast neutron triggering studies which will be applied to all of the selected isomers, there exists the possibility that some of these isomers may be triggered using bremsstrahlung x rays. These isomers must possess energy levels that are separated by less than 200 keV in energy so that the bremsstrahlung x-ray source available at NSWC/Carderock can be used to study triggering. Properties Of The Selected Isomers The isomers listed above fall into two kinds of groupings based on a generalized assessment of their nuclear properties. These properties are a function of the mass of the nuclide, A and where it appears in the periodic table, as a function of Z. The first group contains nuclei with A < 116 and Z < 50. The second group has A > 125 and Z > 51. In fact, besides 125mTe the majority in this group has 188 > A < 200. It is this particular region of the periodic table where isomeric structures and their relationship to the overall trend in nuclear shape and level structure becomes increasingly complex and hence deserving of study. Experimental MethodsTo Determine Triggering Probabilities and Mechansims The experiments to be performed at NSWC/Carderock to determine if forced-gamma emission i.e.”triggering” is either occurring or has occurred can be divided into two types: a) the method of determining ‘burnup” which is a measurement of the specific activity of the isomer before and after exposure to the triggering radiation source. If triggering has occurred (and minimal or zero production of the isomer has occurred simultaneously) then the specific isomer activity should be less then what would be expected from the normal γ decay . b) In-beam measurements of γ-ray transitions taking place while the external radiation is applied to the isomer. The first group consisting of the lighter–mass isomers are so-called spin isomers (3). These nuclei have single unpaired nucleon spins which are couple to specific values of the orbital angular momentum based on the properties of the orbitals that they occupy. These orbitals are generally “out-of-place” in terms of their relative positions regarding the energy and angular momentum schemes taking place inside their respective nuclei. An inversion can be thought to be taking place where these “intruder” states become lower in energy that the normal situation that would be predicted. This inversion places the isomeric level in a 280 inversion of the normal patterns can occur in terms of angular momentum (now collective or rotational angular momentum values) and energy sequencing. position where it is isolated energetically from states with similar values of angular momentum, i.e. states to which γ-ray transitions would be most favored given the typical selection rules. These selection rules generally place severe restrictions on electromagnetic transitions which require changes in angular momentum with values greater than 3 (4). Collective states nearby to each other may then interact in such a way as to produce mixing, i.e. lose their distinctive angular momentum qualities so that electromagnetic transitions have a greater probability of occurring then what we have been predicted at lower energies. These effects may be further enhanced as the interactions between single particle motion become more strongly coupled to the collective motions at the highest observable energies. The possible γ-ray transitions would become less hindered due to considerations and restrictions based on changes in angular momentum. This mixing would then allow for rapid γ-ray transitions down to the ground state. As an example consider the 43 min 111mCd isomer whch has the isomeric state at 396 keV above the ground state. The isomeric level has a spin of 11/2while the ground state is at spin ½+. A transition between the two states requires the emission of a γ ray with 3 units of angular momentum and this is the primary reason for the 43 min value of t1/2. However a level at only 20.5 keV exists above the isomeric level with spin of 7/2+. This level at 416.7 keV is strongly connected to the first excited state at 245 keV with spin of 5/2+, the downward γ-ray transition between them is 100%. This means that of all the so-called spin isomers selected only this particular 20.5 keV transition has the potential to bring about triggering using the bremsstrahlung x-ray source at our disposal due to the reasonably small change in angular momentum between the isomeric state and the triggering level and the small value of ∆E. These kind of phenomena would be expected in nuclei in which shape coexistence is possible as well as transitions between nuclear shapes. Especially attractive is the situation where quasi-continua are either known or would be expected to exist. It is these kind of states brought about by the nuclear dynamics of shape changes at high spin and high excitation energies that may provide important and stimulating information about the mechanisms and probabilities of triggering. Additionally, this kind of information may be very valuable in designing triggering schemes for the potentially technologically attractive isomers such as: 178m2Hf, 242mAm, and 177mLu. All of these isomers have collective and rotational angular momentum considerations attached them in terms of their isomer levels and for potential triggering modes. The other spin isomers mentioned have energy spacing between the isomeric level and possible triggering levels that is not possible to use the x-ray source at NSWC to bring about triggering. For these other spin-isomers the only alternative is to use the n,n’γ mechanism. Fortunately, fast neutrons with significantly larger energy are available using the NSWC tandem Pelletron and fast neutrons are capable of providing larger inputs of angular momentum via inelastic collisions than photo-absorption can. As much as 8 units of angular momentum can be transferred using neutrons in the 1 MeV energy range while with photons the limit is 3 units. All of the isomers of this second grouping, the collective/rotational isomers have nuclear excited states with sufficiently small values of ∆E that they can be reached using the NSWC bremsstrahlung source so that they can not only be studied via the n,n’γ process (in burnup mode) but they can also be studied with triggering photon excitation in both the burnup and coincident in-beam mode. In the case of the second grouping of isomers the angular momentum considerations based on isomer formation, stability and potential de-excitation are more complicated. This is due to the more complex interaction and arrangement of nuclear levels based on angular momentum coupling which goes beyond the simpler considerations of single particle-orbital angular momentum coupling. Collective angular momentum of the entire nucleus becomes significant and introduces new quantum numbers which in turn place new restrictions on the possible electromagnetic transitions that are possible. This phenomena of collective motion results in typical band structures based on collective excitations (typically rotational excitations) as was the case with the spin isomers Finally these isomers would be ideal to study at high excitation energies, ca. 2-3 MeV, with the most energetic neutrons available from our acceleratorbased source. As previously mentioned in these types of nuclei the possibilities exist that these high-energy states based on band structures have the desired properties of mixing levels and would lead to an enhancement in the forced-gamma emission process. 281 The available fluxes of fast neutrons from the two reactions are ca. 109-1010 n/cm2/s at a position right behind either the Li or Be target (a distance of about 1 cm). For instances where it may be worthwhile to investigate isomer production or burnup with neutron energies lower than 300 keV , suitable moderators such as: high-purity graphite or D2O are available. Experimental Approach The two major experimental facilities available at NSWC for these studies are 1) the 3 MV tandem Pelletron accelerator which produces fast neutrons by either the 7Li(p,n)7Be reaction and the 9Be(p,pn)24He reaction. These reactions have both been used extensively in the past for a variety of neutrons experiments and exposures over a period of nearly ten years of continuous operation. These two fast neutronproducing reactions will be used first to prepare quantities of the particular isomer via the n,n’γ process. In the case of photo-absorption type measurements of triggering the isomers to be studied are:111mCd, 125m Te, 189mOs, 191mIr, 195mPt and 199mHg. The other isomers have values of ∆E too large to be useful with the x-ray source available at NSWC (7). In all of these cases both burnup first and then in-beam coincidence measurements will be performed. After suitable times of irradiation depending on the production cross sections and the value of t1/2 for the particular isomer, the samples will have their specific isomer activity measured at NSWC with a Ge(Li) γray detector for an appropriate time depending on t1/2. Then the sample will be reintroduced into the fast neutron beam produced by either of the protoninduced reactions mentioned previously. The NSWC bremsstrahlung source is capable of delievering 7 x 1010 photons/cm2/sec at a distance of 10 cm from the W target. It is a Phillips MG 225 x-rat source which has and endpoint energy of 225 keV and a maximum intensity of photons at 160 keV. Conclusions Again depending on the amount of isomer material produced and its characteristic t1/2 it will re-irradiated with fast neutrons. This may be done with different samples at a variety of different neutron energies. Then the samples will have their isomer activity remeasured to determine if burnup has taken place and if it has to what extent. In this fashion a traditional excitation function (or for this kind of study a “deexcitation function”) will be generated. We reiterate that although doing in-beam measurements of the γray spectra in particular doing γ-γ coincidence measurements would be the ideal method of providing definitive information concerning triggering mechanisms it is not possible in this instance primarily due to the relative small concentration of isomer (ppmppb’s) in the total mass of sample. An attempt will be made using the facilities of NSWC/Carderock to study systematically some of the fundamental issues and parameters involved in the triggering of nuclear isomers. A variety of isomers each with slightly different nuclear properties will be used. All of these isomers can be produced in sufficient quantity at the NSWC tandem accelerator facility without resort to extraordinary means or expense. After producing them using the n,n’γ method the isomers will be studied systematically by exposing them to a range of fast neutron energies for deexcitation function measurement and a selected number having the right energy spacings will be studied for x-ray triggering probabilities. References This means that only indirect evidence for the states participating in triggering will be obtained. Furthermore, studies of the behaviour of the isomer production cross section using the n,n’γ process will have to be undertaken first as a function of energy to ascertain what the “build-up” factor for each isomer is as a function of neutron energy before determining the burnup factor as a function of neutron energy. All of the selected isomers will be produced using the n,n’γ method and all will be measured for burnip by exposure to neutrons suitable to cause de-excitation. We have discussed previously how fast neutrons are far less selective in causing excitation then photons so that a much larger array of potential triggering states and mechanisms would be available through their use then with x rays. 1. Belic, D., et al, Phys. Rev. Lett. 83, 52425245(2000). 2. Ahmad, I., et al, Phys. Rev. Lett. 87, 07253072255(2001). 3. Evans, R. D., Chapter 6, Effects of Nuclear Moments and Parity on Nuclear Transitions, in The Atomic Nucleus, New York, McGraw-Hill Pub., pp. 229-236(1955). 4. Marmier, P. and Sheldon, E., Chapter 9, Radiative Transitions in Nuclei in Physics of Nuclei and Particles, Volume 1, New York, Academic Press, pp. 414-460,(1969). 282 283
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