Update on Radiation Dose From Galactic and Solar Protons at the Moon Using the LRO/CRaTER Microdosimeter

Abstract

The NASA Lunar Reconnaissance Orbiter (LRO) has been exploring the lunar surface and radiation environment since June 2009. In Mazur et al. [2011] we discussed the first 6months of mission data from a microdosimeter that is housed within the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument onboard LRO. The CRaTER microdosimeter is an early version of what is now a commercially available hybrid that accurately measures total ionizing radiation dose in a silicon target (http://www.teledynemicro.com/product/radiationdosimeter). This brief report updates the transition from a deep solar minimum radiation environment to the current weak solar maximum as witnessed with the microdosimeter. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) microdosimeter is behind about 4.4 g/cm equivalent aluminum, corresponding to the range of ~55MeV protons. Energetic protons are the dominant source of ionizing radiation at the Moon. The measured dose includes contributions from the low-energy part of the galactic cosmic ray proton spectrum and the high-energy portion of typical solar energetic particle events. Figure 1 is the history to date of the total ionizing dose averaged over 12 h intervals. The figure includes the entire Lunar Reconnaissance Orbiter (LRO) orbital history that ranged from ~50 km circular tomore elliptical orbits with aposelene above ~150 km. We did not correct the observed dose rate for the changing amount of solid angle blocked by the Moon in Figure 1 as one would need to do to derive an interplanetary rate. When averaged over 12 h, the variations among the various orbit modes have less than a 5% and 20% effect on the galactic cosmic ray dose rate and peak solar proton dose rate, respectively.