Probing ${}^{93m}\mathrm{Mo}$ Isomer Depletion with an Isomer Beam

The isomer depletion of ${}^{93m}\mathrm{Mo}$ 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 ${}^{93m}\mathrm{Mo}$ nuclei using the ${}^{12}\mathrm{C}({}^{86}\mathrm{Kr},5n)$ 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 $2\times {10}^{-5}$ 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.

Figure 1

Experimental setup in the present Letter. The secondary beam line RIBLL is shown with the corresponding distance scale. ${}^{93m}\mathrm{Mo}$ residues were produced at the primary target position and transported to the end of RIBLL to study the isomer depletion. In the lower left area, the isomer depletion of ${}^{93}\mathrm{Mo}$ is sketched together with the spontaneous decay of the long-lived isomer. The setup for implantation and detection is shown in the upper right area.

Figure 2

Spectra acquired by germanium detectors in this measurement. (a) The $\gamma$ spectra of decay events (in purple) and environmental background (in black). (b) The total $\gamma$ spectrum recorded by the germanium detectors during the collection of products. (c) The $\gamma$ spectrum in coincidence with implantation events. The detailed spectrum for the 263- and 268-keV transitions is shown in the inset and fitted by a combination of Gaussian and linear functions. The half-lives of the known isomers are marked for corresponding $\gamma$ peaks, and the lines assigned to an unidentified isomer of ${}^{92}\mathrm{Zr}$ are marked with asterisks. Two $\gamma$ rays with nearly the same energy of 352 keV were identified to originate from ${}^{214}\mathrm{Pb}$ and ${}^{92}\mathrm{Zr}$, respectively. The 352-keV line in (a) and (b) is attributed to ${}^{214}\mathrm{Pb}$, and in (c) this line has two components corresponding to ${}^{214}\mathrm{Pb}$ and ${}^{92}\mathrm{Zr}$.