The radiation environment near the lunar surface: CRaTER observations and Geant4 simulations

Abstract

[1] At the start of the Lunar Reconnaissance Orbiter mission in 2009, its Cosmic Ray Telescope for the Effects of Radiation instrument measured the radiation environment near the Moon during the recent deep solar minimum, when galactic cosmic rays (GCRs) were at the highest level observed during the space age. We present observations that show the combined effects of GCR primaries, secondary particles ("albedo") created by the interaction of GCRs with the lunar surface, and the interactions of these particles in the shielding material overlying the silicon solid-state detectors of the Cosmic Ray Telescope for the Effects of Radiation. We use Geant4 to model the energy and angular distribution of the albedo particles, and to model the response of the sensor to the various particle species reaching the 50 kilometer altitude of the Lunar Reconnaissance Orbiter. Using simulations to gain insight into the observations, we are able to present preliminary energy-deposit spectra for evaluation of the radiation environment's effects on other sensitive materials, whether biological or electronic, that would be exposed to a similar near-lunar environment.

Precise Detections of Solar Particle Events and a New View of the Moon

“…Some fraction of the particles represented in SEPI may be secondary particles produced within the CRaTER instrument by lunar albedo neutrons colliding with atoms in CRaTER. Geant4 simulations found that albedo neutrons contribute ~1% of the particle count rate in D6, comparable to the contrast in the SEPI map; they also found triple‐coincidence neutrons (albedo neutrons that traverse the entire length of CRaTER and trigger all three thick detectors) are negligible compared to other particles (Looper et al, ). The high inverse correlation between the SEPI map and thermal neutron maps suggests that any thermal neutron signal in SEPI is being overpowered by a stronger signal with a positive correlation to SEPI, such as protons or gamma rays.…”

“…Like neutrons, gamma ray photons from the Moon can pass into the CRaTER instrument and interact with matter to produce particles that register in all of CRaTER's detectors, as predicted in simulations (Looper et al, ) and demonstrated in laboratory calibrations of the CRaTER instrument (Spence et al, ). The detector that is least shielded from the nadir direction, D6, should record more counts from albedo gamma rays than the other more shielded detectors.…”

“…Most GCRs have enough energy to pass completely through the CRaTER instrument, and thus register in all three of CRaTER's thick detectors at approximately the same rates, and in all three thin detectors at lower rates. GCRs striking the Moon can initiate several types of nuclear spallation reactions and collisional cascades, resulting in the ejection of many particle types and photons from the lunar surface and back into space where CRaTER can detect them (Looper et al, ; Spence et al, ).…”

“…CRaTER has been measuring GCRs and SEPs from lunar orbit since June 2009. As CRaTER can detect particles arriving from any direction, it also detects various nuclear spallation products propagating up from the Moon (Looper et al, ; Spence et al, ); thanks to LRO's polar orbit (average altitude ~50–90 km), we can make maps of the flux of such “albedo” particles and search for surface features in elemental composition that might affect the albedo proton yield, in a manner similar to the Lunar Prospector gamma ray instrument (Lawrence et al, ; Prettyman et al, ) and neutron detectors on Lunar Prospector and LRO (e.g., Elphic et al, ; Mitrofanov et al, ; Litvak, Mitrofanov, Sanin, Malakhov, et al, ). Wilson et al () attempted the first mapping of lunar albedo protons using the first 18 months of CRaTER data to demonstrate the methodology.…”

Modeling the effectiveness of shielding in the earth-moon-mars radiation environment using PREDICCS: five solar events in 2012

“…Looper et al (2013) used Geant4 to simulate the CRaTER telescope and incoming particles. Part of the study included the electronics box and a simplified mass model of the entire LRO spacecraft.…”