Expanding the Search for Evidence of Nuclear Excitation by Electron Capture to New Nuclear/Atomic Systems

Nuclear excitation by electron capture (NEEC) is a coupled nuclear/atomic process by which the energy released during the capture of an electron into an atomic vacancy excites the nucleus of the atom to a higher-energy state. This phenomenon was proposed theoretically over 40 years ago [1], but the first experimental evidence was not forthcoming until much more recently [2,3], for the nucleus $^{93}$Mo. To expand our knowledge of NEEC beyond this single case, we performed an experiment similar to the successful one for $^{93}$Mo [2] to search for evidence of NEEC in the nucleus $^{127}$Cs. The T$_{1/2} \quad =$ 55-$\mu $s isomer in $^{127}$Cs was populated via the $^{9}$Be($^{123}$Sb, 5n) reaction; this isomer is a candidate for NEEC, with excitation possible to a level lying about 2 keV higher in energy. Considerations that went into the design of the experiment, and its subsequent implementation, will be discussed. \newline \newline [1] V. I. Goldanskii and V. A. Namiot, Phys. Lett. B \textbf{62}, 393 (1976). \newline [2] C. J. Chiara et al., Nature \textbf{554}, 216 (2018). \newline [3] C. J. Chiara et al., Nature \textbf{594}, E3 (2021).