| Determining the chemical composition and distrubtuion features of the lunar surface can offer important clues to its origin and evolution. Gamma-rays are emitted by the interactions of cosmic ray with lunar surface materials, and the decay of natural radioactive elements (U, Th, K). Gamma-rays produced in the top few tens of centimeters of the lunar surface can be detected by gamma-ray spectrometer from lunar orbit with little atmosphere. A number of elements in the lunar surface and their abundances can be determined by measuring the energies and the intensities of the characteristic gamma-ray line emissions of specific elements.The Chang'e-1 (CE1) gamma-ray spectrometer (GRS) is one of the Chang'e-1 orbiter's uploads. For acquiring global maps of elemental composition of the moon, the task of CE1-GRS is to carry out lunar orbital gamma-ray measurements from 200 km orbital altitude and a polar orbit during one year of the mission. The CE1-GRS mounted inside the spacecraft, consists of an 118×78mm CsI (Tl) scintillator placed within an anticoincidence shield (ACS) of CsI (Tl) scintillator.Though the data processing methods from foreign experiments of the lunar orbital GRS can provide us some ideas, the differences among the adopted detectors, the structure and performance of various gamma-ray spectrometers, the conditions and parameters of measurement and so on result in the different features of measured spectra data. For the level 3 and level 2C spectra data measured by the CE1-GRS, this paper studies on the methods and data processing procedure of the CE1-GRS spectra, also the inversion and mapping of absolute concentrations of the elements in lunar surface. The preliminary global maps of natural radioactive elements Th, K and U are derived. The paper draws the conclusions as follows:⑴From the features of spectra shape of CE1-GRS level 3 data, it is difficult to determine elemental compositions in the lunar surface. Aimed at this problem, this paper proposes using noise adjusted singular value decomposition (NASVD) method to qualitative analysis of CE1-GRS spectra. The results shows that a number of possible elements such as U, Th, K, Fe, Ti, Si, O, Al, Mg and Ca are qualitatively determined by this method.⑵For each 3s average spectrum from per 5o×5o lunar region, the general features are present that have high gamma-ray background and weak peak signals. Aimed at this problem, this paper proposes using iterative estimation method to estimate the Compton continuum background of each average spectrum. The result shows that in the background subtracted spectra, several full energy peaks can be qualitatively identified.⑶In each 3s average spectrum from per 5o×5o lunar region, statistical noise is still obvious. Aimed at this problem, this paper proposes using the NASVD (noise adjusted singular value decomposition) method to reduce statistical noise in the gamma-ray data set which consists of all the average spectra from each lunar region. The denoising result shows that the quality of processed data is much better than the conventional spectral denoising methods. So this denoising processing can improve the estimation precision of the peak area calculation.⑷A procedure of data processing for level 2C spectra measured from 27 Nov, 2007 to 6 Feb, 2008 by CE1-GRS is proposed by this paper. Using the inversion model of absolute concentrations of natural radioactive elements in lunar surface, the preliminary global distribution maps of elements Th, K and U are derived.The procedure of data processing includes data selection, cosmic ray correction, spectra accumulation, denoising in spectra data, background estimation and subtraction. In the continuum background subtracted spectra, eight elements, namely U, Th, K, Fe, Mg, Si, Al and O are qualitatively identified. An energy band analysis method is applied to calculate the peak area of element Th. And both peak fitting algorithm and energy band analysis method are applied to determine the counts contribution of element K.In lunar spectra, due to the energy resolution of the CE1-GRS, overlap from the 0.609MeV U line, the 0.583MeV Th line and the 0.511MeV line from positron annihilation radiation cannot be separated from each other. The contributions from these gamma-ray lines form a multiplet. In order to calculate the contribution from the 0.609MeV U line it is necessary to estimate the contribution from the 0.511MeV line and the 0.583MeV Th line. This paper estimates the background including the counts from the 0.511MeV line by the relative calibration method. The contribution from the 0.583MeV Th line also can be determined by the decay proportion between the 2.615MeV Th line and the 0.583MeV Th line. Using the inversion model of absolute concentrations of natural radionuclides in lunar surface, the preliminary global distribution maps of Th, K and U are derived. The uncertainty of the inversion results is also analyzed.⑸From the global distribution maps of natural radionuclides in the lunar surface, the distribution features of elements U, Th and K are as follows: There are strong correlations among these three elements. The major distribution features are similar with each other. These elements are primarily concentrated within and around the western-most maria on the nearside and a secondary abundance concentration is located in the South Pole-Aitken basin (SPA). On the farside, these three elements generally show the features of low abundances.
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