Influence of Radiation Damage and Isochronal Annealing on the Magnetic Susceptibility of Pu 1-x Am x Alloys
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1104-NN01-05
Influence of radiation damage and isochronal annealing on the magnetic susceptibility of Pu1-xAmx alloys Scott K McCall1, Michael J Fluss1, Brandon W. Chung1, and Richard G. Haire2 1 CMELS, LLNL, 7000 East Ave, Livermore, CA, 94550 2 ORNL, Oak Ridge, TN, 37830
ABSTRACT Results of radiation damage in Pu and Pu1-xAmx alloys studied with magnetic susceptibility, χ(T), and resistivity are presented. Damage accumulated at low temperatures increases χ(T) for all measured alloys, with the trend generally enhanced as the lattice expands. There is a trend towards saturation observable in the damage induced magnetic susceptibility data. that is not evident in similar damage induced resistivity data taken on the same specimen. A comparison of isochronal annealing curves measured by both resistivity and magnetic susceptibility on a 4.3at% Ga stabilized δ-Pu specimen show that Stage I annealing, where interstitials begin to move, is largely transparent to the magnetic measurement. This indicates that interstitials have little impact on the damage induced increase in the magnetic susceptibility. The isochronal annealing curves of the Pu1-xAmx alloys do not show distinct annealing stages as expected for alloys. However, samples near 20% Am concentration show an unexpected increase in magnetization beginning when specimens are annealed to 35K. This behavior is also reflected in a time dependent increase in the magnetic susceptibility of damaged specimens indicative of first order kinetics. These results suggest there may be a metastable phase induced by radiation damage and annealing in Pu1-xAmx alloys.
INTRODUCTION Plutonium and its simple binary alloys are known to be extremely sensitive to impurities, defects, and particularly disorder. One interesting method of adding disorder is to alloy plutonium with americium, Pu1-xAmx, forming a stable fcc phase (δ-Pu, β-Am) from 0.06 < x < 0.80 where the lattice parameter increases with x. The electronic structure of americium is 5f6, thus it has a J=0 ground state and its magnetic susceptibility is primarily due to a large Van Vleck contribution[1]. Being a single ion effect this should be independent of the local environment and thus should be insensitive to radiation damage. Thus any changes observed in the magnetic susceptibility of the PuAm alloys that deviate from a simple linear combination of the two end members should reflect a change in the magnetic properties of the plutonium. Another method for adding disorder is through the radioactive decay of Pu and Am which produces a ~5 MeV α-particle and a corresponding recoil (U or Np) that creates a large number of vacancies and interstitials, of which a substantial fraction, after relaxation, remain frozen in place at low temperatures. Disorder from self-damage increases with time and is observable through magnetic susceptibility measurements, where Curie-Weiss behavior evolves and thus demonstrates the creation of local magnetic moments. These emergent moments may be removed by thermal annealing, proving they arise fr
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