[论文解读] Commissioning of the COBRA demonstrator and investigation of surface events as its main background

J. Tebrügge|arXiv (Cornell University)|Jan 1, 2016

Neutrino Physics Research参考文献 1被引用 3

一句话总结

本文介绍了COBRA演示器的调试过程，该实验采用平面网格CdZnTe探测器，旨在寻找无中微子双贝塔衰变。表面事件被确定为主要背景，实验室测量与模拟结果揭示了探测器特定表面的灵敏度差异，其中集电电极表面的灵敏度约为非集电电极表面的3倍；改进的保护环装置将残留的α事件减少至千分之一量级，显著提升了表面事件的鉴别能力。

ABSTRACT

The COBRA collaboration investigates 0νββ-decays (neutrinoless double beta-de- cays). Therefore, a demonstrator setup using coplanar-grid CdZnTe detectors is operated at the LNGS underground laboratory. In this work, the demonstrator was commissioned and completed, which is discussed extensively. The demonstrator works reliably and collects low-background physics data. One result of the analysis of the data is that surface events are the dom- inating background component. To better understand and possibly discriminate this background, surface events were studied in detail. This was done mainly us- ing laboratory measurements. For a better interpretation of these measurements, simulations of particle trajectories and ranges were done. The surface sensitivity tests showed large differences between the individual detectors. Often, a dead-layer was determined, especially at the surfaces where the non-collecting anode (NCA) is the outermost anode rail. Due to this, the sensitivity of the surfaces where the collecting anode (CA) is adjacent was typically about a factor of three larger than the NCA sensitivity. A comparison of the pulse shape analysis methods LSE and A/E was done. Laboratory measurements indicate, that the latter performs bet- ter. Alpha scanning measurements were done to spatially investigate the surface sensitivity. Plausible variations were measured. However, no hints were found how to improve the surface event recognition. The instrumentation of the guard ring, which surrounds the anode structure, was tested and improved the surface event discrimination significantly. The fraction of surviving alpha events was at a per-mill level. Furthermore, the electron mobility was determined to (968±28) cm^2/(Vs) is in very good agreement with literature values. A variation at the detector edges was found. Important steps for a future large-scale COBRA experiment are discussed briefly, mainly the use of an integrated read-out system. Overall, the results indicate a large potential in background reduction for the COBRA experiment.

研究动机与目标

• 对COBRA演示器装置进行调试与验证，以实现低本底物理数据的采集。
• 识别并表征CdZnTe探测器中表面事件作为主要本底来源的特性。
• 研究平面网格CdZnTe探测器中集电电极与非集电电极之间特有的表面灵敏度差异。
• 评估脉冲波形分析技术（LSE与A/E）在提升表面事件鉴别能力方面的效果。
• 测试并优化保护环装置，以增强表面事件的抑制能力。

提出的方法

• 在LNGS地下实验室中运行COBRA演示器，采用平面网格CdZnTe探测器。
• 利用α源在实验室中测量表面事件，以绘制探测器表面各区域的灵敏度分布。
• 通过粒子轨迹与射程的模拟，解释实验测得的表面灵敏度数据。
• 应用LSE与A/E两种方法进行脉冲波形分析，以区分表面事件。
• 实施并测试保护环装置，以抑制表面事件本底。
• 通过探测器响应分析测量电子迁移率，获得(968±28) cm²/(Vs)的值。

实验结果

研究问题

• RQ1 COBRA演示器中的主要本底成分是什么，其在探测器表面各区域的分布如何？
• RQ2 在平面网格CdZnTe探测器中，集电电极与非集电电极附近的表面灵敏度有何差异？
• RQ3 在LSE与A/E两种脉冲波形分析方法中，哪一种能更有效地鉴别表面事件？
• RQ4 保护环装置在多大程度上可减少残留表面事件的比例？
• RQ5 电子迁移率在探测器各区域如何变化，是否与文献值一致？

主要发现

• 在COBRA演示器的低本底数据中，表面事件被确定为主要本底成分。
• 靠近集电电极的表面灵敏度约为靠近非集电电极表面的三倍。
• 实验室测量结果表明，A/E脉冲波形分析方法在表面事件鉴别方面优于LSE方法。
• 经过保护环装置的优化，残留α事件的比例降低至千分之一量级。
• 测得电子迁移率为(968±28) cm²/(Vs)，与文献值高度一致，但在探测器边缘区域观察到一定变化。
• 结果表明，通过优化探测器设计与读出系统，未来大规模COBRA实验具备显著降低本底的潜力。