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Experimental evidence of the inverse of internal conversion, nuclear excitation by electron capture (NEEC), continues to elude nuclear and plasma physicists alike. Following the reported observation of NEEC in Nature, 2018, using $^{93m}$Mo[1], and its subsequent theoretical disagreement [2], there has been heightened interest in the nuclear excitation mechanism.
Induction and isolation of NEEC versus other competing mechanisms such as CoulEx and NEET remains the most significant challenge when it comes to authenticating NEEC measurements. The $^{93m}$Mo was produced in a fusion-evaporation reaction, then recoiled through a carbon foil, resulting in a considerable spread in the energy and charge state entering the carbon foil where NEEC was suggested to occur. The goal of the present study is to observe NEEC using an accelerated beam of $^{84}$Rb. Just 3 keV above the isomer exists a short-lived 5- state, enabling an excellent signature for this de-excitation route. The accelerated beam allows for a defined charge state and a constrained energy and energy spread. Therefore, the conditions of the present study become favourable for the NEEC process.
[1] Chiara, C., Carroll, J., Carpenter, M. et al. Isomer depletion as experimental evidence of nuclear excitation by electron capture. Nature 554, 216–218 (2018). https://doi.org/10.1038/nature25483
[2] Guo, S., Fang, Y., Zhou, X. et al. Possible overestimation of isomer depletion due to contamination. Nature 594, E1–E2 (2021). https://doi.org/10.1038/s41586-021-03333-5.
