MSL/RAD Measurements of the Neutron Spectrum in Transit to Mars and on the Martian Surface

Abstract:

The Radiation Assessment Detector (RAD) onboard Mars Science Laboratory's rover Curiosity has measured the energetic charged- and neutral-particle spectra and the radiation dose rate during most of the 253-day 560-million-kilometer cruise to Mars and is now measuring on the Martian surface. An important factor for determining the biological impact of the Martian surface radiation is the specific contribution of neutrons, which possess a high biological effectiveness. In contrast to charged particles, neutrons and gamma rays are generally only measured indirectly, requiring the transfer of kinetic energy from neutral to one or more charged particles. Therefore, the measurement is the result of a complex convolution of the incident particle spectrum with the measurement process. We apply an inversion method to calculate the gamma/neutron spectra from RAD neutral particle measurements. Here we show first measurements of the gamma/neutron spectra during transit and on Mars and compare them to theoretical predictions. Measuring the neutron spectra in transit to Mars and on the Martian surface is an essential step for determining radiation hazard for future human exploration, including shielding designs of potential spacecraft and habitats. The relative contribution of neutrons to the dose equivalent increases considerably with shielding thickness, so our quantitative measurements provide an important baseline to strategies that would mitigate cancer risk.