First identification of large electric monopole strength in well-deformed rare earth nuclei

TL;DR

This study reports the first measurement of large electric monopole strengths in well-deformed rare earth nuclei, confirming certain excited states as collective beta-vibrational modes through Coulomb excitation experiments.

Contribution

It provides the first experimental evidence of large E0 monopole strengths in specific excited states of $^{154}$Sm and $^{166}$Er, confirming their interpretation as beta-vibrational excitations.

Findings

Large monopole strength in $^{154}$Sm's first excited $0^+$ state.

Large monopole strength in $^{166}$Er's $0_4^+$ state.

Confirmation of beta-vibrational nature of these states.

Abstract

Excited states in the well-deformed rare earth isotopes $^{154}$Sm and $^{166}$Er were populated via ``safe'' Coulomb excitation at the Munich MLL Tandem accelerator. Conversion electrons were registered in a cooled Si(Li) detector in conjunction with a magnetic transport and filter system, the Mini-Orange spectrometer. For the first excited $0^+$ state in $^{154}$Sm at 1099 keV a large value of the monopole strength for the transition to the ground state of $\rho^2(\text{E0}; 0^+_2 \to 0^+_\text{g}) = 96(42)\cdot 10^{-3}$ could be extracted. This confirms the interpretation of the lowest excited $0^+$ state in $^{154}$Sm as the collective $\beta$-vibrational excitation of the ground state. In $^{166}$Er the measured large electric monopole strength of $\rho^2(\text{E0}; 0^+_4 \to 0^+_1) = 127(60)\cdot 10^{-3}$ clearly identifies the $0_4^+$ state at 1934 keV to be the $\beta$-vibrational excitation of the ground state.