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 car-ried 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.
Gamma-ray spectrometer （GRS） is one of the main payloads on the Chang＇E-1 （CE-1） lunar probe, mainly aimed to detect the elemental abundances and distributions on the lunar surface. At 03：58 on 28 November 2007, it performed the first observation of the lunar gamma rays. As of 24 October 2008, 2105 h of effective gamma rays spectra had been acquired by CE-1 GRS, which covers the whole surface of the moon. This paper mainly describes the data processing procedures and methods of deriving the elemental abundances by using the CE-1 GRS time series corrected spectra： first, to bin data into pixels for mapping; then, to perform a background deduction of the cumulative spectra and obtain a peak area of the elements; and finally, to use the elemental abundances inversion model to produce the elemental abundances. Based on these processing methods, the global abundance maps of U, K, and Th at a 5°×5° equal-area pixel are acquired by CE-1 GRS data. The paper gives a preliminary analysis of the uncertainties of the elemental abundances.
The main goal of the gamma-ray spectrometer（GRS） onboard Chang＇E1（CE-1） is to acquire global maps of elemental abundances and their distributions on the moon,since such maps will significantly improve our understanding of lunar formation and evolution.To derive the elemental maps and enable research on lunar formation and evolution,raw data that are received directly from the spacecraft must be converted into time series corrected gamma-ray spectra.The data correction procedures for the CE-1 GRS time series data are thoroughly described.The processing procedures to create the time series gamma-ray spectra described here include channel processing,optimal data selection,energy calibration,gain correction,dead time correction,geometric correction,orbit altitude normalization,eliminating unusable data and galactic cosmic ray correction.Finally,descriptions are also given on data measurement uncertainties,which will help the interested scientists to understand and estimate various uncertainties associated with the above data processing.
Li-Yan ZhangYong-Liao ZouJian-Zhong LiuJian-Jun LiuJi ShenLing-Li MuXin RenWei-Bin WenChun-Lai Li
