Level structure of 123 Cs observed from 123 Ba decay and described using the IBFM and CQPC models

The decay of 123 Ba to 123 Cs has been studied with mass-separated sources. Singles spectra of γ -rays, X-rays and conversion electrons as well as γ - γ - t , γ - e − - t , and X - e − - t coincidences were recorded. A level scheme of 123 Cs has been constructed including 26 new excited states and 82 transitions. The existence of a I π = 11/2 − , T 1/2 = 1.7 s isomer, bandhead of the h 11/2 band, is confirmed. Its excitation energy equals 156.3 keV. A new T 1/2 = 114 ns isomer has been established at 232 keV using the in-beam recoil catcher technique. It is assigned to the I π = 9/2 + bandhead of the gpg 9/2 band. Collective bands based on the 1/2 + ground state and first 3/2 + excited state are proposed. The level structure is described in the frame of the interacting boson-fermion and core-quasiparticle coupling models.