On the decay of the J(pi)=11/2(-), T-1/2=38.9 h isomer in Ba-133: search for the E5 transition and verification of the EC branch

This paper presents a search for the competing 11/2(-) -> 1/2(+) E5 branch to the main 11/2(-) -> 3/2(+) M4 transition from the T-1/2 = 39 h, J(pi) = 11/2(-) isomeric state in Ba-133. An upper limit of 1.8 x 10(-5) could be established for the E5/M4 intensity ratio. In addition, a longstanding controversy in the EC decay of the same isomer was resolved: only one EC branch to the 11/2(+) state in Cs-133 was observed. The shell-model structures of the involved states in Ba-133 and Cs-133 were studied with the microscopic quasiparticle-phonon-model. Good agreement with the decay rates B(lambda) was obtained. In particular, the B(M4) value is well reproduced and further only one low-lying 11/2 state could be identified in the latter nuclide, supporting the experimental results.