Probing Mo-93m Isomer Depletion with an Isomer Beam

The isomer depletion of Mo-93m was recently reported [Chiara et al., Nature (London) 554, 216 (2018)] as the first direct observation of nuclear excitation by electron capture (NEEC). However, the measured excitation probability of 1.0(3)% is far beyond the theoretical expectation. In order to understand the inconsistency between theory and experiment, we produce the 93mMo nuclei using the C-12(Kr-86; 5n) reaction at a beam energy of 559 MeV and transport the reaction residues to a detection station far away from the target area employing a secondary beam line. The isomer depletion is expected to occur during the slowdown process of the ions in the stopping material. In such a low gamma-ray background environment, the signature of isomer depletion is not observed, and an upper limit of 2 x 10(-5) is estimated for the excitation probability. This is consistent with the theoretical expectation. Our findings shed doubt on the previously reported NEEC phenomenon and highlight the necessity and feasibility of further experimental investigations for reexamining the isomer depletion under low gamma-ray background.