Nuclear Gamma-Ray Resonance Study of Hyperfine Interactions inU238

Abstract

Nuclear $\ensuremath{\gamma}$-ray resonance (the M\"ossbauer 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 $\ensuremath{\gamma}$ rays were obtained from the $\ensuremath{\alpha}$ 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\ifmmode^\circ\else\textdegree\fi{}K using the following absorbers: U${\mathrm{O}}_{2}$, UC, U${\mathrm{F}}_{4}$, (U${\mathrm{O}}_{2}$) ${(\mathrm{N}{\mathrm{O}}_{3})}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}6${\mathrm{H}}_{2}$O, U${\mathrm{O}}_{3}$, U${\mathrm{Fe}}_{2}$, and $\ensuremath{\alpha}$-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\ifmmode^\circ\else\textdegree\fi{}K. By fitting a pure magnetic hyperfine spectrum to the widened line, a hyperfine field of 2700\ifmmode\pm\else\textpm\fi{}200 kOe was deduced, using $g({2}^{+})={g}_{R}=0.25$. The resonance in (U${\mathrm{O}}_{2}$) ${(\mathrm{N}{\mathrm{O}}_{3})}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}6${\mathrm{H}}_{2}$O can be explained by a pure electric quadrupole interaction of ${e}^{2}qQ=\ensuremath{-}6100\ifmmode\pm\else\textpm\fi{}225$ 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 $|\frac{\ensuremath{\delta}〈{r}^{2}〉}{〈{r}^{2}〉}|<~{10}^{\ensuremath{-}5}$.