Estimating the background count rate in the energy field from 0.55-2.75 MeV for Chang'E-1 gamma-ray spectrometer

With a large geometrical area, the Gamma-ray spectrometer (GRS) onboard Chang'E-1 was designed to detect gamma rays from the moon. The scientific objective is to study the element information including both type and abundance by distinguishing the energy of gamma ray peak relative to elements and calculating the peak area counts. Regretfully, the cislunar spectrum of GRS was not collected. Nevertheless, we give a method to estimate the background count rate in the energy field from 0.55-2.75 MeV. A natural radioactivity count rate map in 2°×2° grids is shown after reducing the background count rate and the uncertainty of the result is discussed.