Monte Carlo simulations for analysis and design of nuclear isomer experiments

The well-established GEANT4 Monte Carlo code was used to analyze the results from a test of bremsstrahlung-induced nuclear isomer switching and to guide development of an experiment to test nuclear excitation by electron capture (NEEC). Bremsstrahlung-induced experiments have historically been analyzed with the assumption that the photon flux of the bremsstrahlung spectrum at a given energy varies linearly with the spectrum's endpoint. The results obtained with GEANT4 suggest that this assumption is not justified; the revised function differs enough to warrant a re-analysis of the experimental data. This re-analysis has been applied to the switching of the unusually long-lived isomer of $^{\mathrm{180}}$Ta (T$_{\mathrm{1/2\thinspace }}$\textgreater 10$^{\mathrm{16}}$ yr.), showing that the energies of its switching states differ by about 30 keV compared to those previously identified. GEANT4 was also employed in the design of a NEEC experiment to test the isomer switching of $^{\mathrm{93}}$Mo via coupled atomic-nuclear processes. Initial work involved modeling a beam of $^{\mathrm{93}}$Mo ions incident on a volume of $^{\mathrm{4}}$He gas and observing the charge exchange process and associated emitted fluorescence. The beam and $^{\mathrm{4}}$He volume, the ionization trails of the electrons liberated from the $^{\mathrm{4}}$He atoms, and the subsequent fluorescence were successfully simulated; however, it was found that GEANT4 does not currently support ion charge exchange. Future work will entail either the development of the requisite code for GEANT4, or the use of a different model that can accurately simulate ion charge exchange.