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 ⁸⁴ᵐRb isomer (I^π = 6^-, T1/2=20.26 min). The NEEC process for the ⁸⁴ᵐRb isomer allows an excitation by magnetic-dipole ($M1$) and electric-quadrupole ($E2$) transitions into a depletion level (I^π = 5^-), which subsequently decays, releasing a substantial amount of stored energy. To ensure effective production of the ⁸⁴ᵐRb isomer, the fusion-evaporation reaction ⁸²Se + ⁷Li 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 ⁸⁴ᵐRb 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 ⁸⁴ᵐRb 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 ⁸⁴ᵐRb isomer is almost three orders of magnitude higher than that predicted for the ⁹³ᵐMo isomer. This result makes the ⁸⁴ᵐRb isomer a good candidate for new NEEC beam-based experiment.