Background deduction of the Chang’E-1 gamma-ray spectrometer data

Gamma-ray spectrometer (GRS) is used to detect the elemental abundances and distributions on the lunar surface. To derive the elemental abundances, it is vital to acquire background gamma rays except lunar gamma rays. So GRS would observe background spectra in the course of earth-moon transfer on schedule. But in fact, GRS was not switched on in the course of flying toward the moon. After the CE-1 probe finished one-year mission, GRS carried out a test on background data on November 21–22, 2008. The authors did conduct research on the methods of background deduction using 2105 hours of usable gamma-ray spectra acquired at the 200-km orbital height by the GRS and more than 5 hours of gamma-ray spectra acquired in the GRS background test. The final research results showed that the method of deducting the background using the minimum counts in the CE-1 GRS pixels is optimal for the elements, U, K and Th. The method applies to such a case that the elemental abundances in the pixel with the minimum counting rate are 0 μg/g and the continuum background counts are constant over the Moon. Based on the method of background deduction, the full energy peak counts of U, K, and Th are calculated.