Measurements of the neutron spectrum on the Martian surface with MSL/RAD

Abstract

Curiosity, measures the energetic charged and neutral particles and the radiation dose rate on the surface of Mars. An important factor for determining the biological impact of the Martian surface radiation is the specific contribution of neutrons, with their deeper penetration depth and ensuing high biological effectiveness. This is very difficult to measure quantitatively, resulting in considerable uncertainties in the total radiation dose. In contrast to charged particles, neutral particles (neutrons and gamma rays) are generally only measured indirectly. Measured spectra are a complex convolution of the incident particle spectrum with the detector response function and must be unfolded. We apply an inversion method (based on a maximum likelihood estimation) to calculate the neutron and gamma spectra from the RAD neutral particle measurements. Here we show the first spectra on the surface of Mars and compare them to theoretical predictions. The measured neutron spectrum (ranging from 8 to 740 MeV) translates into a radiation dose rate of 14 ± 4 μGy/d and a dose equivalent rate of 61 ± 15 μSv/d. This corresponds to 7% of the measured total surface dose rate and 10% of the biologically relevant surface dose equivalent rate on Mars. Measuring the Martian neutron and gamma spectra is an essential step for determining the mutagenic influences to past or present life at or beneath the Martian surface as well as the radiation hazard for future human exploration, including the shielding design of a potential habitat.

The Martian surface radiation environment – a comparison of models and MSL/RAD measurements

“…We compare the presented simulation results with particle spectra measured by RAD during this time period as published in Ehresmann et al (2014) and Köhler et al (2014). Measurements made by the Rover Environmental Monitoring Station (REMS; Gómez-Elvira et al 2014) show that the average atmospheric column depth above the rover in this time frame was~21-22 g/cm 2 (Haberle et al 2014).…”

“…Measurements of particle spectra, dose, and LET Neutral particles are measured in D and E, with C and F acting as anticoincidence against charged particles (see Köhler et al 2014 for details of the neutral particle measurements). Using an inversion technique, we are able to derive separate spectra for c-rays and neutrons, as both D and E have different sensitivities for those particle types.…”

“…The Mars Science Laboratory (Grotzinger et al 2012) -Radiation Assessment Detector (MSL-RAD; Hassler et al 2012) detector on board the Curiosity rover has been the first instrument to measure particle spectra and dose rates both during the cruise to Mars (Zeitlin et al 2013;Guo et al 2015;Köhler et al 2015) and on the Martian surface Hassler et al 2014;Köhler et al 2014;Rafkin et al 2014). Besides the continuous exposure to galactic cosmic radiation (GCR), the MSL-RAD instrument has measured several moderate solar energetic particle events during the cruise phase, from December 2011 to July 2012, and a few small solar particle events on the Martian surface since August 2012.…”

“…The MSL-RAD instrument is, among other things, capable of measuring gamma and neutron spectra (Köhler et al 2014) and several types of charged particles . The published results of the first~200 sols (1 solar day, sol, on Mars is about 24 h and 39 min long) on the Martian surface (between August 2012 and January 2013) are used in this work for a validation of the different numerical radiation transport models.…”

Measurements of Forbush decreases at Mars: both by MSL on ground and by MAVEN in orbit

“…On the surface of Mars, RAD measures a mix of primary GCRs or SEPs and secondary particles generated in the atmosphere including both charged ) and neutral (Köhler et al 2014;Guo et al 2017b) particles. RAD measures dose in two detectors: the silicon detector B and the plastic scintillator E 1 .…”