【作者】 杨佳；

【导师】 葛良全；

【作者基本信息】 成都理工大学 ， 核技术及应用， 2010， 博士

【摘要】 研究月表物质成分及分布特征,能够为月球地质解译提供依据,有助于进一步理解月球的形成及演化过程。月表物质与宇宙射线的相互作用,以及月表天然放射性元素的衰变都会产生伽玛射线。由于月球大气层稀薄,伽玛射线谱仪能够从环月轨道上探测到月表发出的伽玛射线。根据所观测到的伽玛射线的能量和照射量率,能够推定月表元素的种类和丰度。嫦娥一号(CE1)卫星是中国发射的首颗探月卫星。嫦娥一号伽玛射线谱仪(CE1-GRS)是CE1卫星搭载的主要载荷之一,该谱仪的探测任务是从200公里的环月轨道上对月表进行为期一年的有效探测,以此获取月表物质类型和资源分布特征。CE1卫星按极地轨道绕月飞行,CE1-GRS被安放在卫星体内,尺寸为Φ118×78mm的CsI(Tl)主晶体位于仪器中央,周围是实现反符合功能的CsI(Tl)晶体。虽然能够借鉴国外开展环月轨道伽玛射线能谱数据处理的研究方法,但不同的月球轨道伽玛射线测量实验所采用的探测器类型、伽玛谱仪的结构、性能,以及测量条件、参数等方面迥异,从而使获取的月球伽玛能谱特征也存在差异。本文利用CE1-GRS获取的月球伽玛能谱的3级和2C级数据,开展了CE1-GRS数据处理的相关技术方法研究和月表元素含量反演与填图的试验研究;初步完成了月表天然放射性元素U、Th、K的含量填图。得出的主要结论如下:⑴针对CE1-GRS的3级数据难以从谱形特征直接进行定性分析的问题,提出了基于奇异值分解的谱线定性分析方法对CE1-GRS谱线进行定性分析。分析结果表明,该方法能够识别出U、Th、K、Fe、Ti、Si、O、Al、Mg和Ca等10种元素。⑵针对各5o×5o月表区域平均伽玛谱线存在高本底弱特征峰的问题,提出采用基于迭代思想的本底估计方法对散射本底进行估计。结果表明,该方法能够近似估计出谱线中的散射本底数据,从本底扣除后的谱线中能够有效地提取出弱峰信息,有利于对谱线进行定性定量分析。⑶针对各5o×5o月表区域平均伽玛谱线中,统计涨落现象仍然明显的问题,提出了将航空伽玛能谱数据降噪处理的噪声调整的奇异值分解(NASVD)方法用于对月球伽玛能谱数据集进行降噪处理,实验结果表明,其降噪效果明显优于传统的谱线降噪方法,从而提高了求解元素特征峰面积计数的精度。⑷提出了CE1-GRS的2C级数据处理的流程与方法,采用月表天然放射性元素(U、Th、K)含量反演模型,获得了月表天然放射性元素的全月含量分布图。对CE1-GRS第一正飞期2C级谱线数据进行了数据筛选、宇宙射线校正、能谱累计、能谱降噪处理、本底估计及扣除。在散射本底扣除后的谱线中,能够识别出U、Th、K、Fe、Mg、Si、Al和O八种元素。采用能窗峰面积求和法、高斯拟合峰面积求和法,提取出了Th、K元素的特征峰面积计数。由于CE1-GRS的能量分辨率不够高,在伽玛谱线中,U的0.609MeV伽玛射线、Th的0.583MeV伽玛射线以及由电子对效应产生的0.511MeV伽玛射线的计数贡献构成了一个重峰。为了求解U元素的0.609MeV伽玛射线计数值,采用“相对定标本底扣除法”近似估计出包括湮没辐射峰0.511MeV在内的本底计数贡献,通过Th的2.615MeV与0.583MeV的衰变比例确定出0.583MeV伽玛射线的计数贡献,从而计算出了重峰区中U的0.609MeV特征伽玛射线计数值。利用月表天然放射性元素含量反演模型获取U、Th、K元素含量,绘制出这三种元素的全月含量分布图,并对元素含量反演结果的不确定性进行了初步分析。⑸利用所获取的月表天然放射性元素分布图,初步分析U、Th、K元素的分布特征为:月表U、Th、K元素之间具有高度相关性,三种天然放射性元素的主要分布特征基本一致。U、Th、K含量高的区域主要集中在月球正面的西部月海区及其附近;而含量次之的区域位于南极爱特肯盆地内。在月球背面,天然放射性元素含量普遍表现为低值特征。更多还原

【Abstract】 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.更多还原