GCR Dose Rate Observed in Lunar Orbit During the Transition from Solar Cycle 23 to Cycle 24

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft has been measuring the charged particle radiation environment in low-lunar orbit since 2009-06-27. CRaTER’s charged particle measurements are made with six fully-depleted Si solid state detectors organized into three coaxially aligned pairs consisting of a “thin” (~150 μm) and “thick” (~1000 μm) detector. The detector pairs are mounted in a telescope configuration with the adjacent pairs separated by plastic absorbers; the two outermost detectors forming the telescope’s apertures are covered by thin Al end-caps. LRO’s nominal operations are conducted in an ~50 km altitude circular polar orbit with the spacecraft’s attitude oriented to keep the science instruments in a nadir staring direction. This orientation places CRaTER’s longitudinal axis parallel to a radial originating from the center of the Moon, with one pair of detectors facing zenith, another pair facing nadir, with the middle pair shielded on both sides by the plastic absorbers. The daily absorbed dose measured by each detector was computed from the detector’s PHA spectra; for the thin detectors this corresponds to particle interactions with lineal energies between ~1.59 to 1490 keV/ μm and 0.0970 to 63.1 keV/ μm for particle interactions in the thick detectors. During the interval 2009-09-16 to 2010-02-28 the average dose rate (in Si) measured by the three thin detectors were 186 μGy/day, 161 μGy/day, and 171 μGy/day for the zenith-facing, plastic-shielded middle, and nadir-facing detectors, respectively. The corresponding dose rates for the three thick detectors were 255 μGy/day, 248 μGy/day, and 253 μGy/day. These dose rates are in good agreement with the average dose rate of 211 μGy/day reported for the RADOM instrument on the Chandrayyan-1 mission operating in a 100 km circular polar lunar orbit during the period 2008-11-13 to 2008-12-24. In this analysis period the peak dose rates measured by the CRaTER detectors occurred during the period 2009-12-31 to 2010-01-07 followed by the onset of gradual declines. During this period the Thule neutron monitor count rate, a proxy for moderate-to-high-energy GCR proton flux at the top of the Earth’s atmosphere, peaked during the period 2009-10-01 to 2009-12-31 before undergoing a sharp decrease through the remainder of the analysis period. Assuming the peak in the Thule neutron monitor count rate marks the deepest point of solar cycle 23 and the beginning of an unabated increase in solar activity with the onset of cycle 24, these CRaTER measurements may represent the maximum GCR dose rates directly measured since the inception of space-based measurements more than 50 years ago.