High-Resolution Study of Isomer Shifts of the 6.2-keV<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>γ</mml:mi></mml:math>Rays of Tantalum-181

Abstract

Isomer shifts of the 6.2-keV $\ensuremath{\gamma}$ rays of $^{181}\mathrm{Ta}$ are reported for dilute impurities of tantalum in transition-metal hosts and for several tantalum compounds. The experimental line-shift-to-linewidth ratios represent an improvement of the resolution by more than an order of magnitude relative to other M\"ossbauer resonances. For $^{181}\mathrm{Ta}$ impurities in transition-metal hosts a systematic variation of the electron density at the nucleus is found. From a comparison of the isomer shifts of $^{181}\mathrm{Ta}$ impurities in transition-metal host with those observed for impurities of $^{57}\mathrm{Fe}$, $^{99}\mathrm{Ru}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$, a value of $\ensuremath{\Delta}〈{r}^{2}〉=\ensuremath{-}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ ${\mathrm{fm}}^{2}$ can be derived for the change of the mean-squared nuclear-charge radius. The negative sign of $\ensuremath{\Delta}〈{r}^{2}〉$ is in agreement with the observed variation of the isomer shifts of LiTa${\mathrm{O}}_{3}$, NaTa${\mathrm{O}}_{3}$, and KTa${\mathrm{O}}_{3}$, as well as with the isomer shift found for TaC.