DAMPE卫星BGO量能器高精度能量测量方法研究

The DArk Matter Particle Explorer (DAMPE) satellite aims at searching for the evidence of the existence of dark matter particles indirectly by measuring the energy spectra of cosmic-rays in the space with high precision. The DAMPE detector has large acceptance, high energy resolution and wide energy coverage, and therefore it is expected to obtain the best measured spectra of cosmic-rays in the space. For this the performance of the BGO calorimeter is crucial, as the key sub-detector of the DAMPE spectrometer. It depends on the precision of the energy measurement essentially, which is based on accurate and reliable energy calibration. This project plans to study the calibration methods of the sensitive unit of the BGO calorimeter, using the minimum ionization energy of heavy ion as standard, measuring the attenuation constant of BGO crystal for energy correction based on the position of incident particle, keeping track of the photomultiplier performance changing with time, taking some special features in the cosmic-ray spectra for reference to cross check, and determining experimently with laser mimicking fluorescence if there is fluorescence saturation effect for electrons above TeV region in the BGO crystal. These studies are important for improving the energy measurement precision of the BGO calorimeter and the particle identification capability, so that accurate cosmic-ray spectra can be measured. It will be the first time to study the response of TeV electron in the BGO crystal, and also a challenge for calibration and verification in a wide energy range from a few tens MeV to a few hundreds GeV.

暗物质粒子探测卫星DAMPE通过精确测量太空中的宇宙线能谱来间接寻找暗物质粒子存在的证据，DAMPE具有接收度大、能量分辨率高、能量覆盖范围宽等优势，期望得到高精度空间宇宙线能谱。由此DAMPE谱仪核心子探测器BGO量能器的性能至关重要，本质上它取决于其能量测量精度，而这依赖于精确的、可靠的能量刻度。本项目拟研究BGO量能器灵敏单元的标定方法，采用轻离子的最小电离作为标定基准，测定BGO晶体衰减系数以根据入射位置进行能量修正，跟踪光电倍增管性能随时间变化情况，并以宇宙线能谱的特殊结构为参照来检验标定效果，通过激光模拟荧光实验判断BGO晶体在TeV以上能区是否出现荧光饱和效应。这些研究对于提高BGO量能器能量测量精度、提高粒子鉴别能力，从而得到准确的宇宙线能谱具有重要意义。实验研究BGO晶体对TeV能量的电子的响应尚属首次，从数十MeV到数百GeV的宽能区实现能量标定与检验也是全新的尝试。

“悟空”暗物质粒子探测卫星(DAMPE)的直接观测对象是高能宇宙线粒子，其核心子探测器BGO量能器的精确标定是测量结果可靠性的基础和保证。本项目基于卫星在轨运行数据，跟踪了BGO量能器各灵敏单元的工作状态，以轻离子的最小电离为标定基准、结合ATMNC3蒙特卡罗模拟对各灵敏单元做了精确刻度，完善了BGO全部灵敏单元的刻度体系，为科学数据重建提供了可靠、可信的基础参数，保证了DAMPE在轨数据的高质量重建。研究了BGO晶体中的荧光衰减表现、双端读出与击中位置之间的关联，并监测衰减长度随时间的变化，开发了逐事例的能量重建、修正以及分析方法，实现了根据每个事例的击中位置进行修正，能量重建的一致性和精度得到了显著改善。借助电子及重核素在地磁场中的截断效应，观测了不同粒子对应的特征结构，验证了10-100GeV之间能量重建的准确性。建立了一套激光模拟荧光测试系统，首次实验研究了BGO晶体对TeV量级电磁簇射能量沉积的响应情况，结果表明无荧光饱和现象。在极高能区采用BGO晶体仅低增益端单端读出，辅以衰减效应修正得到准确能量，使能量测量上限从10TeV提高到了50TeV，有效扩展了能量测量范围。本项目研究提高了BGO量能器能量测量精度，澄清了荧光饱和问题，为DAMPE物理结果提供了有力支撑。基于本项目研究工作已发表文章8篇，被接受待发表3篇，其中合作组文章3篇，包括发表在Sci.Adv.上的40GeV-100TeV质子能谱、Phys.Rev.Lett.上的70GeV-80TeV氦核素能谱。