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

The 9.2 keV nuclear transition in $^{227}$Th populated in the $β$-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 $δ^2$(E2/M1)=0.695 $\pm$ 0.248 (|$δ$(E2/M1)|=0.834 $\pm$ 0.210) was determined. Nonzero value of $δ$(E2/M1) raises a question about the existing theoretical interpretation of low-lying levels of $^{227}$Th.

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