Contribution to the study of the physical properties of the Pu delta phase at very low temperatures (4K-300K)

In the framework of a study on the self-diffusion mechanism in the cubic phases of plutonium, the physical properties of the stabilized delta phase of Pu were investigated at low temperatures. The proposed self-diffusion mechanism by activated interstitials must involve a 5f→6d electron transition. Thus the various properties of the delta solution were reviewed and the elastic constants and thermal expansion coefficient of plutonium-scandium alloys in the cryogenic temperature range were studied. The experiments were carried out by means of a specially designed tensile device operating between 4K and 300K, with which it was possible to measure both YOUNG's modulus and the yield strength of the Pu-Sc delta solid solution as a function of the temperature and the scandium concentration. The expansion measurements in the same temperature range were carried out by stress gauge. It is demonstrated that the plutonium atom in delta solid solution contracts when the temperature rises (promotion of f electrons), confirming ZACHARIASEN's hypotheses. This contraction decreases with the addition of solute atoms, but in the case of plutonium allows a relatively easy insertion of the atoms participating directly in self-diffusion