Investigating high-energy proton-induced reactions on spherical nuclei: Implications for the preequilibrium exciton model

Abstract

Background: A number of accelerator-based isotope production facilities utilize 100- to 200-MeV proton beams due to the high production rates enabled by high-intensity beam capabilities and the greater diversity of isotope production brought on by the long range of high-energy protons. However, nuclear reaction modeling at these energies can be challenging because of the interplay between different reaction modes and a lack of existing guiding cross-section data.Purpose: A Tri-lab collaboration has been formed among the Lawrence Berkeley, Los Alamos, and Brookhaven National Laboratories to address these complexities by characterizing charged-particle nuclear reactions relevant to the production of established and novel radioisotopes.Method: In the inaugural collaboration experiments, stacked-targets of niobium foils were irradiated at the Brookhaven Linac Isotope Producer (${E}_{p}=200\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$) and the Los Alamos Isotope Production Facility (${E}_{p}=100\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$) to measure $^{93}\mathrm{Nb}(p,x)$ cross sections between 50 and 200 MeV. First measurements of the $^{93}\mathrm{Nb}(p,4n)^{90}\mathrm{Mo}$ beam monitor reaction beyond 100 MeV are reported in this work, as part of the broadest energy-spanning dataset for the reaction to date. $^{93}\mathrm{Nb}(p,x)$ production cross sections are additionally reported for 22 other measured residual products. The measured cross-section results were compared with literature data as well as the default calculations of the nuclear model codes TALYS, CoH, EMPIRE, and ALICE.Results: The default code predictions largely failed to reproduce the measurements, with consistent underestimation of the preequilibrium emission. Therefore, we developed a standardized procedure that determines the reaction model parameters that best reproduce the most prominent reaction channels in a physically justifiable manner. The primary focus of the procedure was to determine the best parametrization for the preequilibrium two-component exciton model via a comparison to the energy-dependent $^{93}\mathrm{Nb}(p,x)$ data, as well as previously published $^{139}\mathrm{La}(p,x)$ cross sections.Conclusions: This modeling study revealed a trend toward a relative decrease for internal transition rates at intermediate proton energies (${E}_{p}=20$--60 MeV) in the current exciton model as compared to the default values. The results of this work are instrumental for the planning, execution, and analysis essential to isotope production.