Fast neutron flux spectrum aboard Mars Odyssey during cruise

Abstract

[1] The energy spectrum of fast neutrons generated by interactions between galactic cosmic rays and the Mars Odyssey spacecraft during quiet interplanetary conditions is determined. It is given byfor energies between 0.7 and 6 MeV. The estimated ratio of this component of spacecraft neutron background to the Martian leakage flux in Mars orbit is expected to amount to between 0.30 and 0.34. This fraction is sufficiently small so that the total background in orbit about Mars is expected to be dominated by Martian leakage neutrons reprocessed by the spacecraft. Nevertheless, the measured asymmetry properties of the Odyssey Neutron Spectrometer response function should allow separation of foreground and background neutron fluxes when in Mars orbit. The fast neutron flux between 0.7 and 6 MeV should only contribute 0.40 Roentgen Equivalent Man (rem) per year to the total radiation dose aboard a Mars Odyssey class spacecraft at solar maximum.

The MESSENGER Gamma-Ray and Neutron Spectrometer

“…Both detectors measure thermal neutrons but are facing in opposite directions. During cruise, the thermal neutron flux is expected to be very low because the few fast neutrons that are created in the spacecraft structure tend to escape without being moderated, and those that are successfully moderated within the fuel tanks are then likely to be absorbed by the large amount of nitrogen (Feldman et al 2002b). If detected, thermal neutrons will appear as a broad Gaussian line centered near channel 24.…”

CO2frost cap thickness on Mars during northern winter and spring

“…The prism that is facing down to Mars (labeled P1) [see Feldman et al , 2002a, Figure 1] is completely covered on the outside with Cd so that it responds only to epithermal and fast neutrons. Discrimination between the epithermal (approximately 0.4 eV to 700 keV) and fast (700 keV to about 2 MeV) energy ranges is made electronically [ Feldman et al , 2002a]. Thermal neutrons are separately measured by making use of the fact that the spacecraft travels faster (3.4 km/s) than a thermal neutron (1.9 km/s, corresponding to an average Martian temperature of about 220 K).…”

Performance of Orbital Neutron Instruments for Spatially Resolved Hydrogen Measurements of Airless Planetary Bodies

“…Feldman et al (2002b) gave an example of a response function calculation for the Mars Odyssey NS; Lawrence et al (2002) gave an example of a sensor response calculation for the LP-NS sensors. In regard to lunar measurements, Elphic et al (2001) described how these counting rates were determined for LP neutron measurements.…”

“…Thermal, epithermal, and fast neutrons from planetary surfaces are separately measured from orbit with standard neutron detection techniques (Feldman et al, 2002b, and these measured neutrons carry information indicative of the underlying near-surface composition. For example, because of their similar masses, epithermal neutrons lose their energy very efficiently in collisions with protons.…”