| Chang’E-2 spacecraft which is launched in Xichang Satellite Launch Center in October 1,2010 is the forerunner of Chang’E-3.Chang’E-2 flying around the moon height of 100km,compared with Chang’E-1 flight height of 200km is low.Chang’E-2 satellite has polar orbit.Gamma-ray spectrometer is one of the seven payloads in the spacecraft.The science goal is to give the major elements’ distribution,such as O,Si,Fe,Ti,U,Th,K,Mg,A1 and Ca.The environment of lunar is harsh.The impacts of temperature and cosmic ray and so on are seriously on the analysis of gamma-ray spectrum.The procedures of CE2-GRS data process are achieved,which is based on the measurement theory of the gamma ray spectrum and the source and feature of the gamma ray from lunar surface.The data of 2C-level of CE-2 GRS are taken as the research object.The procedures includes data selection,the correction of spectrum drifting,the correction of galactic cosmic ray,accumulation and average of the spectrum,the smoothing of the spectrum,the estimation of the background gamma rays,the qualitative analysis of the spectrum,calculation the peak area,the deconvolution and mapping of the abundances of radioactive elements.After the processing of the gamma ray spectrum,global mearsurement of uranium,thorium and potassium abundance on the lunar surface are presented.The fusion algorithm is proposed and applied for CE-1 GRS and CE-2 GRS.At last,according to the classification of lunar terrain,the distribution of radioactive elements in the lunar surface was analyzed.The main conclusions are as follows:1.Chang,E-2 GRS sensor consists of a 10.8x10.8cm LaBr3 crystal.The gamma-ray energy range extends between 0.3～10MeV.The energy resolution of CE2-GRS is 3.6%for 662 keV.Gamma spectrometer obtain a spectrum for every 3 seconds which contains 512 channel.Combining the processing experience of gamma-ray spectrum of Apollo,Lunar Prospector,SLENE and CE-1,the paper presents the spectrum processing procedures after analyzing the peak drift phenomenon,background composition,statistical fluctuation noise and the elements characteristic peaks of CE-2 gamma-ray spectrum.The data processing flow contains the filter of gamma-ray spectrum,spectrum peak drift correction,the correction of the influence of cosmic ray,the cumulative average of spectrum,the reduction of data noise,the estimation and substraction of background,the qualitative analysis of spectrum,the solving of peak area and mapping.The experiment results show that the abundance maps of radioactive elements can be obtained using the procedures.The data processing method is effective for the analysis of the CE-2 gamma-ray spectrum and the inversion of the radioactive element content of the lunar surface.2.Due to the severe changes in temperature on the surface of the moon and the aging of the electronic components of the gamma spectrometer,the CE-2 gamma-ray spectrum occurs the phenomenon of spectral drifting.The characteristic of spectrum drifting is studied.The line drift is nonlinear,and the drift is mainly linear in low energy.After studing the characteristics of the spectrometer and the spectrum,the methods are proposed for this problem.The method which is based on the theory of correlation is designed for linear drifting of spectrum,and the method which is based on the principle that the energy of element characteristic peak is fixation is for nonlinear drifting.A method for tracking elements characteristic peak is also proposed.The experiments results shows that the the drift channel of characteristic peak of potassium is up to 6 and become 1 after calibration with the method based on the theory of correlation and the peakarea and counts does not change before and afte the calibration.The dirft channel of characteristic peak of potassium become to 4 using the method based on the principle that he energy of element characteristic peak is fixation,while the method will be change the peakarea and peak shape.The tracking for characteristic peak method can avoid correcting drifting for each whole spectrum.The interesting peak window will be selected quickly and conveniently.The experiments show that the success rate of tracking potassium peak is 100%.3.The background of the CE-2 gamma-ray spectrum is mainly composed of the background of the instrument and the scattering background after studing the CE-2 GRS and gamma-ray spectrum.As the main crystal of CE-2 GRS is LaBr3,the elements of La and Ac in the crystal are the dominant factors of the instrument background.The conclusion is validated by experimental data analysis.According to the high scattering background and weak characteristic peak of the CE-2 gamma-ray spectrum,the estimation experiments of scattering background spectrum is complied with SNIP and iterative method.The results shows:the ratio of peakarea and background for potassium characteristic peak increased from 4.52 to 63.06,the peak to background ratio increased by 14 times;the peak to background ratio for iorn elements increased from 0.95 to 16.30,increased by 17 times.4.In order to obtain the abundance of radioactive elements from the corresponding peakarea of CE-2 gamma-ray spectrum,the methods of abundance inversion are studied.A method are carried out which is a model proposed by Lawrence et al.another method are carried out which is based on the least squares method of linear fitting between our peakarea data and the sampled data on the lunar surface by the satellite of Apollo,Luna and so on.The experiments results show that the abundance data obtained by least squares method is closer to the range of the lunar surface and more reasonable.5.The correlation analysis of CE-1 and CE-2 gamma-ray spectrum are carried out base on the data of CE-1 and CE-2 GRS.The results show that the gamma-ray spectrums of the two spectrometers are highly correlated.Based on the weighted average fusion algorithm,the fusion of the two data sets of thorium counting rate of the CE-1 and CE-2 is proposed.After the fusion,the counting rate distribution characteristics of thorium element are closer to the lunar geological features.6.The distribution characteristics of radioactive elements are analyzed based on the results of analysis of the CE-2 gamma-ray spectrum and the classification of the lunar terrain:the KREEP terrane of the Antarctic,the Aiken basin terrane and the highland terrane.A region of high abundance of radioactive elements surrounds near-side Mare Imbrium and Procellarum Ocean,ranging from the southern edge of Imbrium to Fra Mauro crater.The extremely high value occurs on the area of Apenninus Mountains south of Mare Imbrium and the rim of north and northweat of Mare Imbrium.There is a local minimum in the center of Mare Imbrium and Procellarum Ocean.The second high abundance regions of radioactive elements are found around and in South Pole-Aitken basin(SPA),which shows the characteristics of high west to low east.The relatively high thorium abundance region of SPA is Poincare cracter in the weat of SPA and Leibnitz cracter in the northwest of SPA.While it is not obviously that high abundance in the Schirodinger cracter southwest of SPA.The abundance of radioactive elements showed obvious low value in the feldspathic highland terrane(FHT).By analyzing the relationship between the thorium elements counting rate in the region of 5°×5° and the distance from mare Imbrium,it is found that the counting rate decreases with the distance,which comfirms the hypothesis of the formation of mare because of collision. |
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