Assessment of secondary neutrons from galactic cosmic rays at mountain altitudes – Geant4 simulations and ground-based measurements of neutron energy spectra

Abstract In this study, the Geant4 Monte Carlo toolkit was used to simulate energy spectra of neutrons from secondary cosmic radiation at mountain altitudes for the Environmental Research Station “Schneefernerhaus” at the Zugspitze mountain, Germany (2660 m a.s.l.) and for Sphinx astronomical observatory at the Jungfraujoch, Switzerland (3585 m a.s.l.). Simulations were performed with different intra-nuclear cascade models available in Geant4, and the results were compared with those of measurements that had been performed at both locations by means of an Extended-Range Bonner Sphere Spectrometer. Measurement conditions were quite different for both locations – at Schneefernerhaus the measurements had been performed on the flank of a hill in March 2018 with much snow, while at Jungfraujoch the measurements had been performed on top of a steep local hill in September 2018 with much less snow. Despite these differences, agreement between measurement and simulation was reasonable at both locations, especially at neutron energies greater than 20 MeV where the (unknown) hydrogen content of the environment did not influence the neutron fluence much (i.e., results from simulations were 6–22% lower than those from the measurements for the Schneefernerhaus, and were 22–29% lower for Jungfraujoch, depending on intra-nuclear model used in the simulations). The agreement was less favorable for lower energies, where environmental hydrogen (e.g., snow cover, soil moisture) is known to influence the shape of the neutron energy spectrum, because the real conditions of the snow accumulation close to the location of the measurements were not known and, therefore, a detailed description of the real hydrogen environment in the simulations was not possible. When the results simulated using different intra-nuclear cascade models were compared with each other, agreement was found within ±5%, ±15%, ±20%, and ±20%, for cascade, evaporation, epithermal and thermal neutrons, respectively. While the latter results are consistent with those of simulations and measurements at the CERN EU High-Energy Reference Field (CERF) facility published recently, a detailed sensitivity analysis of the influence of environmental hydrogen on neutron energy spectra is required before a final quantitative comparison of measurements and simulations can be made. This sensitivity analysis is presently under way. It is concluded that simulation of energy spectra of neutrons from secondary cosmic rays close to the atmosphere-lithosphere interface, validated by the spectrometer measurements, showed differences of less than 30%, for neutron energies greater than 20 MeV, whatever intra-nuclear cascade model was used in the simulations.