Model predicted variations in neutron fluence and dose on the surface of Mars

Abstract

The interactions of primary cosmic radiation with matter create a complex neutron field with energies ranging over many orders of magnitude. The result is a non-negligible contribution of neutrons to the exposure on planetary surfaces or in spacecraft. The measurement of neutrons is complex and an accurate assessment of the neutron dose is a challenge, especially under the existing restrictions in space flight and model calculations can provide valuable information supplementing the measurements. This work presents model-predicted variations caused by solar modulation and atmospheric shielding in the neutron fluence on the Martian surface and corresponding dose rates for different dosimetric quantities. The work is going to support an on-going effort to revise the methodology of neutron measurements of the Radiation Assessment Detector (RAD) of the Mars Science Laboratory. Neutral particle spectra can be derived from RAD measurements using an inversion technique on energy deposition spectra in sensitive detector parts with an additional anti-coincidence condition in the surrounding parts of the detector and resulting neutron and gamma energy spectra on the surface of Mars have been published earlier. The method is currently revised with the goal to publish an update on measured neutron and gamma spectra and to provide a comprehensive picture of the results for the environmental conditions RAD has experienced up to date. These environmental conditions were used to derive boundary conditions for the model calculations. The model calculations were performed using the PLANETOCOSMICS tool for Geant4 using primary galactic cosmic ray spectra from hydrogen to iron. The results can be used to estimate the expected variations in the experimental data and can be benchmarked against RAD results. The model calculated dose rates allow to compare different dose quantities and to estimate the relative contribution of the experimentally accessible part of the neutron spectra to the total.