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

TL;DR

This study provides experimental evidence that the 9.2 keV transition in $^{227}$Th has a mixed M1+E2 character, impacting the interpretation of its low-lying nuclear levels.

Contribution

It experimentally determines the multipolarity and admixture parameter of the 9.2 keV transition in $^{227}$Th, challenging existing theoretical models.

Findings

The 9.2 keV transition is of mixed M1+E2 character.

The spectroscopic admixture parameter δ²(E2/M1) is approximately 0.695.

The nonzero E2 component questions current theoretical interpretations.

Abstract

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