The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) sensor has been orbiting the Moon aboard the Lunar Reconnaissance Orbiter (LRO) since 2009. As is implied by its name, it was designed primarily to measure the effects of energetic particles in the near-lunar environment rather than to act as a spectrometer to measure the distribution of those particles. As such, it has now recorded the time-varying spectra of Linear Energy Transfer (LET), or more properly lineal energy, from those particles over nearly a complete 11-year cycle of solar activity. However, we have also been able to use it as a particle spectrometer, in particular to obtain measurements of secondary “albedo” protons ejected from the lunar surface by nuclear interactions of GCR particles with that surface.
Measurements of both albedo protons and GCR LET spectra are subject to backgrounds caused by the abundant flux of GCRs striking the telescope from outside its nominal fields of view. We have devised a variety of empirical corrections for the backgrounds in both sets of measurements to obtain the results published to date, both in journals and, for LET spectra, as a continuously-updated data product available on the World Wide Web. Recently, we have used updated simulations of the response of CRaTER to particles both inside and outside of its nominal fields of view, using the Geant4 Monte Carlo radiation-transport toolkit, to devise improved background corrections based on the detailed physics of the particle interactions with the active and inert components of the sensor. We will describe the details of these corrections, and will present mission-long sets of observations both of the time-varying GCR LET spectrum and of the temporally- and spatially-varying flux of albedo protons coming up from the lunar surface.