INTRODUCTION AND MOTIVATION
A research group used the Cryostation in a unique application of determining the fundamental structure of 𝛿-Pu, working to further understand radiation damage and the effects on the material. In α decay, an α particle and the recoiling nucleus deposit energy as it travels through the crystal structure. This displaces atoms creating vacancies and interstitials, physically changing the material. The changes include conductivity, hardness, electrical resistivity, and susceptibility. By studying these changes the nature of the radiation damage can be determined.
Figure 1: Alpha decay. Source
To study 𝛿-Pu, the researchers cooled the sample down which damaged the sample, and then studied the isochronal annealing, which occurs when the sample goes from cold to warmer temperatures, and repairs the damage induced by the cryogenic temperatures. The group looked at damage repaired at various temperatures, and performed analysis on the data using three methods: amplitude ratio techniques, extended X-ray absorption fine structure (EXAFS) fitting, and a spherical crystallite model.
The 𝛿-Pu sample was prepared and mounted in a Montana Instruments Cryostation; it was annealed first at 375K and then cooled to 5K, the subsequent study lasted 72 days. EXAFS measurements were conducted at the Stanford Synchrotron Radiation Lightsource. The samples were measured in a fluorescence geometry and the isochronal annealing was done by warming to temperatures from 35-300K (in about 10 degree increments), held for 5 minutes and then cooled back to 15K for EXAFS measurements. Data was taken on the Pu L3 edge and the Ga K edge.
The amplitude ratio method of analyzing data considers the Fourier transform of the EXAFS of Ga K and Pu L3 edge after annealing, and the damage fraction of peaks. The EXAFS fitting factors in the loss of amplitude in peaks due to self irradiation damage and determines the damage fraction and change in damage fraction using the amplitude of the path length. The spherical crystallite method compares damaged versus undamaged areas of the atom. It was determined that at the Pu L3 edge, 60% of atoms were lost or disordered in the first shell, in the second shell this went up to 80%. For Ga K the damage was only 40% in the first shell and 60% in the second shell.
This group used various methods of analysis to determine changes at Ga K and Pu L3 edge in 𝛿-Pu from isochronal annealing over 72 days. The changes around Pu and Ga correspond to re-arrangement of interstitials. Ga contributes to restoring lattice order and exhibits less damage than Pu. With analysis, it was possible to determine the density of undamaged regions, disorder, and damage fractions.
Figure 2: EXAFS fitting demonstrating damage fraction, disorder, and bond length vs annealing temperature. Source
Isochronal annealing effects on local structure, crystalline fraction, and undamaged region size of radiation damage in Ga-stabilized 𝛿-Pu arXiv: 1602.03913v1
This work was performed using a Montana Instruments Cryostation. This article should not be considered an endorsement of any product.