Relative contributions of galactic cosmic rays and lunar proton “albedo” to dose and dose rates near the Moon

Abstract

[1] We use validated radiation transport models of the Cosmic Ray Telescope for the Effects of Radiation instrument and its response to both primary galactic cosmic rays (GCR) and secondary radiation, including lunar protons released through nuclear evaporation, to estimate their relative contributions to total dose rate in silicon (372 μGy/d) and dose equivalent rate at the skin (2.88 mSv/d). Near the Moon, we show that GCR accounts for~91.4% of the total absorbed dose, with GCR protons accounting for~42.8%, GCR alpha particles for~18.5%, and GCR heavy ions for~30.1%. The remaining~8.6% of the dose at Lunar Reconnaissance Orbiter altitudes (~50 km) arises from secondary lunar species, primarily "albedo" protons (3.1%) and electrons (2.2%). Other lunar nuclear evaporation species contributing to the dose rate are positrons (1.5%), gammas (1.1%), and neutrons (0.7%). Relative contributions of these same species to the total dose equivalent rate in skin, a quantity of more direct biological relevance, favor those with comparatively high quality factors. Consequently, the primary GCR heavy ion components dominate the estimated effective skin dose. Finally, we note that when considering the lunar radiation environment, although the Moon blocks approximately half of the sky, thus essentially halving the absorbed dose rate near the Moon relative to deep space, the secondary radiation created by the presence of the Moon adds back a small, but measurable, absorbed dose (~8%) that can and should be now accounted for quantitatively in radiation risk assessments at the Moon and other similar exploration targets.

Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration?

“…The results of this analysis show that declining solar activity and the subsequently weakening heliospheric magnetic field in the protracted solar cycle 23-24 minimum leads to an increasingly hazardous radiation environment due to elevated fluxes of galactic cosmic rays. We project out to the cycle 24-25 solar minimum and find an elevated dose equivalent rate of ∼0.3 cSv/d in interplanetary space behind 16 g/cm 2 nominal spacecraft shielding or equivalently ∼ 0.15 cSv/d on the lunar surface where roughly half of the incident radiation is blocked by the Moon [Spence et al, 2013]. The estimate for interplanetary space is in reasonable agreement with a previous estimate made for similar shielding by Mewaldt et al [2005].…”

“…Applying the modulation model (), we estimate the evolution of dose rates (Figure ). We note that the peak dose rate between cycles 23 and 24 agrees well with the estimate reported recently by Spence et al [], which includes contributions of lunar albedo, of 13.6 cGy/yr. The predictions for both the case of a Gleissberg‐like and Dalton‐like minimum are quite similar.…”

Space Weather Quarterly Volume 12, Issue 1, 2015

“…We project out to the cycle 24-25 solar minimum and find an elevated dose equivalent rate of ∼0.3 cSv/d in interplanetary space behind 16 g/cm 2 nominal spacecraft shielding or equivalently ∼ 0.15 cSv/d on the lunar surface where roughly half of the incident radiation is blocked by the Moon [Spence et al, 2013]. The estimate for interplanetary space is in reasonable agreement with a previous estimate made for similar shielding by Mewaldt et al [2005].…”

Interplanetary space weather effects on Lunar Reconnaissance Orbiter avalanche photodiode performance

“…In situ measurements of the deep space radiation environment near the LOLA SiAPDs are provided by the CRaTER experiment on the LRO, which measures the effects of ionizing energy loss in matter due to solar energetic particles and galactic cosmic rays [Spence et al, 2010]. Furthermore, we note that though LRO measurements are made near the Moon, which is itself a source of secondary ionizing radiation, Spence et al [2013] showed that greater than 90% of the CRaTER dose is from primary cosmic rays, meaning that results obtained in lunar orbit are excellent proxies for deep space radiation conditions.…”