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

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 $\ddot{I}{}^2(\text{E0};0_2^{+}\to 0_{\text{g}}^{+})=96(42)\cdot 10^{-3}$ could be extracted. This confirms the interpretation of the lowest excited $0^{+}$ state in ${}^{154}$Sm as the collective ${\hat{I}}^2$-vibrational excitation of the ground state. In ${}^{166}$Er the measured large electric monopole strength of $\ddot{I}{}^2(\text{E0};0_4^{+}\to 0_1^{+})=127(60)\cdot 10^{-3}$ clearly identifies the $0_4^{+}$ state at 1934 keV to be the ${\hat{I}}^2$-vibrational excitation of the ground state.

submitted to Physics Letters B