Cosmogenic radionuclide production modeling with Geant4: Experimental benchmarking and application to nuclear spectroscopy of asteroid (16) Psyche

Abstract

Measurements of gamma-ray emissions from the decay of cosmogenic radionuclides provide an opportunity to characterize the elemental composition of a terrestrial planet or asteroid surface. We report on the development of a Geant4 application which models cosmogenic radionuclide production on metal-rich surfaces. The model was benchmarked using measurements of radionuclides produced during 1 GeV proton irradiation of a target made from the Campo del Cielo iron meteorite. The gamma-ray emitting radionuclides ⁵⁸Co, ⁵⁷Co, ⁵⁶Co, ⁵⁴Mn, ⁵²Mn, ⁵¹Cr, ⁴⁸V, ⁴⁶Sc and ²²Na were observed following the irradiation. Our model reproduced the measured radionuclide production rates to within a factor of 2.5 or better. All but two of the elements (⁵⁴Mn, ⁴⁶Sc) have a perfect model-to-data match within their measurement uncertainties. The benchmarked model was used to predict cosmogenic radionuclide production on a large (~100-km radius) metal-rich asteroid. The results are relevant for planned gamma-ray measurements of the metallic asteroid (16) Psyche, which will be visited by the Psyche spacecraft in 2026. We found that galactic-proton-induced radionuclide decay is unlikely to be observed by the Psyche Gamma-Ray Spectrometer (GRS), however an intense solar proton event (>2×10⁶ protons cm^-2 over <5 days) will produce measurable quantities of radioisotopes. Measurements of these radioisotopes could provide an independent method of determining the Ni-to-Fe ratio of materials at Psyche’s surface. Such an analysis will require the use of radionuclide production cross sections to convert GRS-measured cosmogenic radionuclide decay rates to elemental composition information with the ~10% precision required for planetary geochemical studies.