Experimental evidence of (M1+E2) mixed character of the 9.2 keV transition in ${}^{227}$Th and its consequence for spin-interpretation of low-lying levels

Abstract

The 9.2 keV nuclear transition in ${}^{227}$Th populated in the ${\hat{I}}^2$-decay of ${}^{227}$Ac was studied by means of the internal conversion electron spectroscopy. Its multipolarity was proved to be of mixed character M1+E2 and the spectroscopic admixture parameter $\hat{I}{´}^2$(E2/M1)=0.695 $\pm$ 0.248 (|$\hat{I}´$(E2/M1)|=0.834 $\pm$ 0.210) was determined. Nonzero value of $\hat{I}´$(E2/M1) raises a question about the existing theoretical interpretation of low-lying levels of ${}^{227}$Th.

14 pages, 3 figures; typo in abstract corrected