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

The decay of Ba-123 to Cs-123 has been studied with mass-separated sources. Singles spectra of gamma-rays, X-rays and conversion electrons as well as gamma - gamma - t; gamma - e(-) - t, and X - e(-) - t coincidences were rec orded. A level scheme of Cs-123 has been constructed including 26 nerv exci ted states and 82 transitions. The existence of a I-pi = 11/2(-), T-1/2 = 1 .7 s isomer, bandhead of the h(1/2) band, is confirmed. Its excitation ener gy equals 156.3 keV. A new T-1/2 = 114 ns isomer has been established at 23 2 keV using the in-beam recoil catcher technique. It is assigned to the I-p i = 9/2(+) bandhead of the pi g(9/2) band. Collective bands based on the 1/ 2(+) ground state and first 3/2(+) excited state are proposed. The level st ructure is described in the frame of the interacting boson-fermion and core -quasiparticle coupling models.