Nuclear Gamma-Ray Resonance Study of Hyperfine Interactions inU238

Abstract

Nuclear $\gamma$-ray resonance (the Mössbauer effect) has been observed in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ using the 44.7-keV transition from the first excited state ($2^{+}$) to the ground state ($0^{+}$). The $\gamma$ rays were obtained from the $\alpha$ decay of ${}_{}{}^{242}{}_{}{}^{}\mathrm{Pu}$. The source material (Pu${\mathrm{O}}_2$) gave a single emission line having a width of about 40 mm/sec, which is roughly 1.5 times the natural linewidth. Absorption spectra were taken in the temperature region between 77 and 4.2°K using the following absorbers: U${\mathrm{O}}_2$, UC, U${\mathrm{F}}_4$, (U${\mathrm{O}}_2$) ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$·6${\mathrm{H}}_2$O, U${\mathrm{O}}_3$, U${\mathrm{Fe}}_2$, and $\alpha$-uranium metal. Several compounds showed partially resolved hyperfine spectra. In U${\mathrm{O}}_2$ the resonance line broadens by a factor of 2 when cooling from 77 to 4.2°K. By fitting a pure magnetic hyperfine spectrum to the widened line, a hyperfine field of 2700±200 kOe was deduced, using $g\left(2^{+}\right)=g_R=0.25\mathrm{}$. The resonance in (U${\mathrm{O}}_2$) ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_2$·6${\mathrm{H}}_2$O can be explained by a pure electric quadrupole interaction of $e^{2}qQ=-6100\mathrm{}\pm 225$ MHz. The isomer shift between ${\mathrm{U}}^{4+}$ and ${\mathrm{U}}^{6+}$ compounds is smaller than 2 mm/sec, thus giving a limit for the relative change in nuclear charge radius of $\left|\frac{\delta 〈r^2〉}{〈r^2〉}\right|<~{10}^{-5\mathrm{}}$.