Isomer Shifts of the First and Second Excited Levels of the Ground-State Rotational Band inYb171

Abstract

The isomer shifts of the 66.7-keV ${\frac{3}{2}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\frac{1}{2}}^{\ensuremath{-}}$ and the 75.9-keV ${\frac{5}{2}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\frac{1}{2}}^{\ensuremath{-}}$ $\ensuremath{\gamma}$ transitions of the \textonehalf{}[521] ground-state rotational band in $^{171}\mathrm{Yb}$ have been investigated, using the M\"ossbauer effect. From the observed shifts, the ratio of change in mean square charge radii for the two excited states was found to be $\frac{\ensuremath{\Delta}〈{r}^{2}〉(76)}{\ensuremath{\Delta}〈{r}^{2}〉(67)}=+0.42\ifmmode\pm\else\textpm\fi{}0.20.$ The decrease of $\ensuremath{\Delta}〈{r}^{2}〉$ for higher rotational excitation is in disagreement with simple collective theory, and is discussed in terms of nonadiabatic effects.From the observed magnetic hyperfine splitting, we also deduced the ratio of nuclear $g$ factors $\frac{g(76)}{g(0)}=+0.411\ifmmode\pm\else\textpm\fi{}0.002.$ This value is in excellent agreement with an earlier measurement and has been used, together with other recent data on all magnetic parameters of the \textonehalf{}[521] band, in discussing the spin polarizations effects. The mixing ratio ${\ensuremath{\delta}}^{2}$ of the $\frac{M1}{E2}$, 66.7-keV $\ensuremath{\gamma}$ transition in $^{171}\mathrm{Yb}$ was redetermined correcting carefully for finite absorber effects. The new result is ${\ensuremath{\delta}}^{2}=0.36\ifmmode\pm\else\textpm\fi{}0.04.$