Radiation Damage Effects on the Magnetic Properties of Pu (1-x) Am x (x=0.224)
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0986-OO03-03
Radiation Damage Effects on the Magnetic Properties of Pu (1-x) Amx (x=0.224) S. K. McCall1, M. J. Fluss1, B. W. Chung1, M. W. McElfresh1, and R. G. Haire2 1 Lawrence Livermore National Laboratory, Livermore, CA, 94550 2 Oak Ridge National Laboratory, Oak Ridge, TN, 37831
ABSTRACT Pu(Am) is stable in the fcc δ-phase from a few atomic percent to nearly 80 atomic percent Am, expanding the average interatomic separation as the alloy concentration of Am increases. Both Pu and Am spontaneously decay by α-emission creating selfdamage in the lattice in the form of vacancy-interstitial pairs and their aggregates. At sufficiently low temperatures, the damage is frozen in place, but can be removed by thermal annealing at sufficiently high temperatures, effectively resetting the system to an undamaged condition. The magnetic susceptibility and magnetization are observed to increase systematically as a function of accumulated damage in the fcc δ-Pu1-xAmx (x=0.224). Some results of these observations are reported here. INTRODUCTION Plutonium-americium alloys present an interesting material for the study of 5f electron systems, as the two metals form a single stable fcc phase over a wide range of compositions, with the lattice parameter increasing with Am concentration, showing a positive deviation from Vegard’s law. Previous work on radiation damage in PuAm alloys has investigated the consequence of damage accumulation on magnetic susceptibility where the damage was accumulated near room temperature[1, 2]. Recent work[3] on isochronal annealing of PuAm alloys accumulated damage at low temperature with the total number of α-decays per atom approximately 20 times smaller than the room temperature work but the accumulated damage (surviving atomic displacements) was much greater. The isochronal annealing study finds that room temperature is very close to the annealing stage where small vacancy clusters are unstable, removing the last magnetically observable indication of radiation damage. Therefore, “damage” accumulated for extended periods of time at room temperature is not simply the very dilute remnant evolution of the vacancy and interstitial displacements and their diffusion history, but likely reflects also the presence of the helium and daughter products of the nuclear decay itself. The present work reports details of low temperature damage accumulation studies and compares these results to the earlier work. One difference of this experiment compared to the earlier work on PuAm alloys is that the less radioactive Am-243 isotope (t½ = 7,370 years) was used instead of the higher activity Am-241(t½ = 432 years). As a comparison, the Am-243 isotope has slightly more than three times the activity of Pu-239 (t ½ = 24,400 years). Both isotopes of Am decay by spontaneous alpha emission, but the Am-241 decays to the far more stable Np237 (t½ = 2,140,000 years) so while concentrations of this isotope build up in the specimen over time, they are not likely to contribute appreciably to the magnetic
susceptibility*. By co
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