100PeV 能区宇宙线的射电探测预先研究

High energy cosmic particles are characterized by very low fluxes: 1 particle/m2/year around 1 PeV and decreasing by a factor 1000 for every decade of energy. This has two main consequences: these particles can be detected only indirectly, through the extensive air showers (EAS) they are generated in the atmosphere, and very large detection areas are required. Large detection areas lead to very high costs, while the incomplete knowledge of EAS physics induces systematic biases which can be reduced only by the combination of different detection techniques. Parallel to this challenging experimental situation, detection of EAS with the radio-technique has undergone fast development in the last years: it has been shown (LOPES, CODALEMA ) that EAS energy could be measured from the radio data, while some results (LOPES) show that radio data may also be used to determine the nature of the primary particle. Finally, the Tianshan Radio Experiment for Neutrino Detection (TREND) has shown that EAS could be detected by stand-alone and self-triggered radio arrays. These encouraging developments have triggered strong interest in the community, in particular because radio antennas are cheap detection units, easy to deploy and calibrate, and are therefore very well adapted to giant arrays. Yet, radio-detection of EAS is not mature. It is in particular not yet demonstrated that EAS can be identified with high efficiency over the ultra-dominant background events induced by human activities or natural electric discharges in the air. Polarization of the radio signal could provide valid information for background rejection: EAS are indeed polarized linearly, with a direction perpendicular both to shower direction of origin and Earth magnetic field. The TREND collaboration wishes to test the possibility to use this very peculiar characteristic of EAS to identify them. The basic concept of our proposal is to deploy a hybrid array of 40 antennas and 21 scintillators on the same site with independent triggers. A selection procedure based on the polarization information would be performed on the events triggered by the radio array. A positive result would indicate that the event is indeed an EAS, as there is negligible contamination of the scintillator data by background. This hybrid setup would therefore provide a quantitative evaluation of the EAS identification & background rejection of the radio technique. The design of this hybrid array has been optimized through a detailed MC simulation for the study of EAS with energy around 100 PeV.

由于射电天线造价较低、易于大面积布放和全天候工作，LOPES、CODALEMA和AUGER等实验都在进行宇宙线的射电探测研究。中法合作TREND实验用3台塑料闪烁体探测器做离线符合，在国际上率先用射电自触发方式观测到了高能宇宙线事例。射电自触发模式的成功使用，为射电探测宇宙线技术的大规模应用奠定了基础。射电信号存在人为噪声干扰，有效排除噪声的办法是研究宇宙线射电信号的极化特征。本项目依据100PeV能区宇宙线粒子的模拟计算结果，拟研制出21台塑料闪烁体探测器，与射电天线阵列进行联合实验，以判别射电天线阵列的信号是来自本底还是来自宇宙线事例，从而研究宇宙线射电信号区别于本底的极化特征（其极化方向同时垂直于地磁场方向和初级宇宙线运动方向）。

项目研究背景：宇宙线研究取得了许多重要成果，最引人瞩目的是正电子、 u介子、 π介子和κ介子的发现。但宇宙线研究的进展受到了探测器覆盖面积的严重制约。将射电探测技术用于宇宙线研究中，一方面可使宇宙线探测多一种手段，另一方面可以解决宇宙线研究中探测器造价高、覆盖面积有限这个困扰已久的问题。在此，我们提出联合塑料闪烁体阵列（这个传统的宇宙线探测器）与 21CMA射电天线阵列进行 100PeV能区宇宙线射电探测的预先研究。 .项目主要研究内容是：研制出一个由 21 台（ 3 列 7 行）塑料闪烁体探测器构成的小型 EAS 阵列，联合21CMA射电天线阵列进行宇宙线射电探测；通过交叉检验研究射电信号的极化分量。.本项目取得的主要结果和关键数据有：1.塑料闪烁体原型探测器；2.塑料闪烁体探测器阵列；3.多角度探测器支撑系统；4.优化设计的高压电源系统（去除了电压传输中存在的安全隐患）；5.背景噪音监测；6.射电信号刻度；7.宇宙线事例方位角和天顶角重建等。发表标识文章10篇：1. POS (ICRC2015)1143；2. POS (ICRC2015)632; 3. POS (ICRC2015)1020; 4. POS (ICRC2017)420; 5. JINST 12（2017）P11011; 6. POS（ICRC2017）388; 7. POS (ICRC2017)996; 8. JINST 12（2017）P09023; 9. APJ 819(2016)54; 10. APJ 842 (2017) 31。.本项目的科学意义为：发展宇宙线射电探测技术在国际上的竞争十分激烈，目前的主要问题是本底噪音排除; 主攻方向分布在天线改进，低噪放大器开发，触发和数据获取技术完善方面。谁先掌握这门技术，谁就有可能率先打开极高能宇宙线或中微子天文学的大门。