On the decay of the J^π = 11/2⁻, T_1/2 = 38.9 h isomer in ¹³³Ba: 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^π = 11/2⁻ isomeric state in ¹³³Ba. An upper limit of 1.8 × 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 ¹³³Cs was observed. The shell-model structures of the involved states in ¹³³Ba and ¹³³Cs were studied with the microscopic quasiparticle-phonon model. Good agreement with the decay rates B(λ) 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.