4~400 MeV宇宙线质子对近月空间轨道γ能谱中湮灭辐射的贡献

Contribution of 4~400 MeV protons galactic cosmic rays to annihilation radiation in orbital gamma spectrum near lunar space

为进一步明确月球表面湮灭辐射的主要来源和影响机理,构建了轨道γ能谱仪中湮灭辐射特征峰定量分析模型,采用GEANT4模拟不同能量质子轰击月岩诱发γ射线微分能谱,并以“嫦娥一号”高能粒子探测器(Chang’e-1 High-energy Particle Detector,CE1-HPD)数据作为输入项,计算了宇宙射线中4~400 MeV质子在月表5种典型岩石中诱发湮灭辐射的特征峰信息。在对“嫦娥一号”γ能谱仪(Chang’e-1 Gamma-ray Spectrometer,CE1-GRS)获取的0.511 MeV特征峰进行本底扣除和天然放射性核素影响剥离后,与4~400 MeV质子诱发湮灭辐射结果进行比较。研究结果表明:级联簇射产生湮灭辐射的概率与入射质子能量呈正相关,在4~400 MeV能量范围内,典型月岩的成分差异对诱发湮灭辐射的影响不明显。虽然该能量段质子的注量率较高,但由于其能量较低,无法有效地通过级联簇射形成正电子,最终导致湮灭辐射贡献率较低,仅为(1.97±0.66)×10^(-4)。

[Background]The annihilation radiation exhibits conspicuous features in the low-energy segment of the orbital gamma spectrum,which contains a substantial amount of geological information about the lunar surface.The fluence rate can reflect the element composition,density,maturity,and other characteristics directly.[Purpose]In order to further clarify the primary source and influencing mechanism of annihilation radiation on the lunar surface.[Methods]A quantitative model for the annihilation radiation characteristic peak of orbital gamma spectrometers was established.The gamma rays induced by protons of varying energies were simulated using GEANT4 to further understand the primary source and mechanism of annihilation radiation on the lunar surface.The data from the"Chang'e-1"high-energy particle detector(CE1-HPD)was used as the input term,and the annihilation radiation characteristic information induced by 4~400 MeV protons galactic cosmic rays interacting with five typical rocks was calculated.After subtracting the 0.511 MeV characteristic peak collected by the"Chang'e-1"gamma spectrometer(CE1-GRS)from natural radioactivity,the results were compared with the annihilation radiation fluence rate induced by 4~400 MeV protons galactic cosmic rays.[Results]The results indicate that the rock's composition have a negligible effect on the annihilation radiation.The probability of cascading shower generating annihilation radiation is directly proportional to the incident proton energy.Additionally,the contribution of 4~400 MeV protons to the annihilation radiation present in the orbital gamma spectrum is relatively low,only(1.97±0.66)×10^(−4).[Conclusions]The established model has proven to be accurate in reflecting the related characteristics of the gamma radiation field on the lunar surface and can be used for quantitative analysis of annihilation radiation.The results indicate that the contribution of 4~400 MeV protons galactic cosmic rays to the annihilation radiation present in the orbital gamma spectrum is minimal.