Dominant magnetic-dipole nuclear excitation by electron capture in a beam-based scenario for the Rb84m isomer

Here, we consider nuclear excitation by electron capture (NEEC) in a beam-based experiment for the ${}^{84m}\mathrm{Rb}$ isomer ($I^{\pi }$ = $6^{-}, T_{1/2}=20.26$ min). The NEEC process for the ${}^{84m}\mathrm{Rb}$ isomer allows an excitation by magnetic-dipole ($M1$) and electric-quadrupole ($E2$) transitions into a depletion level ($I^{\pi }$ = $5^{-}$), which subsequently decays, releasing a substantial amount of stored energy. To ensure effective production of the ${}^{84m}\mathrm{Rb}$ isomer, the fusion-evaporation reaction ${}^{82}\mathrm{Se} + {}^7\mathrm{Li}$ is considered. In the multiconfigurational Dirac-Fock calculations of energy released by electron capture into the $M, N, O, P$, and $Q$ shells, we have used the ground-state configurations for ${}^{84m}\mathrm{Rb}$ ions. To focus on the role of $M1$ and $E2$ transition multipolarities, Compton profiles, and excited-state configurations are neglected here. The predictions of the mean equilibrium charge state for the ${}^{84m}\mathrm{Rb}$ recoil ion as a function of its kinetic energy in the C target and the analysis of potentially possible NEEC resonance kinetic energies have been performed. The NEEC resonance strengths and probabilities have been estimated by a theoretical model applied for the $M1$ and $E2$ excitations. It was found that the $M1$ contribution for the NEEC process clearly dominates over the $E2$ one. Moreover, the total NEEC probability ($M1$ plus $E2$) for ${}^{84m}\mathrm{Rb}$ isomer is almost three orders of magnitude higher than that predicted for the ${}^{93m}\mathrm{Mo}$ isomer. This result makes the ${}^{84m}\mathrm{Rb}$ isomer a good candidate for new NEEC beam-based experiment.