NMR studies of the helium distribution in uranium tritide

Abstract

The distribution of helium (${}_{}{}^3{}_{}{}^{}\mathrm{He}$) created in uranium tritide (U${\mathrm{T}}_3$) by triton decay has been investigated by pulse NMR techniques. The line shapes and relaxation times for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei have been measured during a 3-year period. NMR samples were prepared from five U${\mathrm{T}}_3$ synthesis batches. Because U${\mathrm{T}}_3$ is a highly paramagnetic material which ferromagnetically orders below approximately 180 K, inhomogeneous magnetic field effects contributed to the NMR parameters. Detailed analyses of the frequency, temperature, and age dependences of these NMR parameters indicate most ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms are retained in microscopic gas bubbles with dimensions < 500 Å. No evidence was obtained to suggest significant concentrations of interstitial ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms in the U${\mathrm{T}}_3$ lattice. A simple model based upon the nucleation, growth, and subsequent rupture at critical dimensions of helium bubbles is presented. This model qualitatively describes the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ relaxation times and the observed ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ retention in U${\mathrm{T}}_3$ powders.