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Nuclear Excitation by Electron Capture (NEEC) was predicted by Goldanskii and Namiot as the inverse process of internal conversion in 1976[1]. It was expected to play an important role in the isomer depletion, which is a potential path for releasing nuclei energy stored in isomer[2].
The first experimental observation on NEEC was reported in the slowing down process of $^{93}$Mo[3]. The observed isomer depletion probability was too large to be reproduced by Coulomb excitation, and thus attributed to NEEC. However, it also failed to be reproduced by following theoretical works[4,5]. On the experimental side, a comment was addressed on the influence of complex γ background which may cause the overestimation of isomer depletion probability[6]. Later, an independent experiment was performed using a $^{93m}$Mo secondary beam, but no isomer depletion was observed with an accuracy of 2×10$^{-5}$, which was reported as the upper limit of the excitation probability[7]. However, this measurement was performed with lower recoiling energies than the previous experimental work.
A refined experiment has been performed with higher recoiling energy and purity of the $^{93m}$Mo isomer beam. Both lead and carbon foils were used to stop the $^{93m}$Mo ions. Isomer depletion is observed, and the excitation probability is about 210$^{-5}$ for lead, and 310$^{-6}$ for carbon. These results agree well with the calculated probabilities for inelastic reactions, which are suggested to the main mechanisms exciting the $^{93m}$Mo isomer during its stopping process.
[1] V. Goldanskii and V. Namiot, Phys. Lett. B 62 (1976) 393.
[2] P. Walker and G. Dracoulis, Nature 399 (1999) 35.
[3] C. J. Chiara et al., Nature 554 (2018) 216.
[4] Y. Wu et al., Phys. Rev. Lett. 122 (2019) 212501.
[5] J. Rzadkiewicz et al., Phys. Rev. Lett. 127 (2021) 042501.
[6] S. Guo et al, Nature 594 (2021) E1.
[7] S. Guo et al, Phys. Rev. Lett. 128 (2022) 242502.
