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

Excited states in the well-deformed rare earth isotopes Sm and 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 Sm at 1099 keV a large value of the monopole strength for the transition to the ground state of ρ(E0; 0+2 → 0g ) = 96(42) · 10 could be extracted. This confirms the interpretation of the lowest excited 0 state in Sm as the collective β-vibrational excitation of the ground state. In Er the measured large electric monopole strength of ρ(E0; 0+4 → 0g ) = 127(60) · 10 clearly identifies the 0+4 state at 1934 keV to be the β-vibrational excitation of the ground state.