[论文解读] Study of the PICOSEC-Micromegas Detector with Test Beam Data and Phenomenological Modelling of its Response

K. Paraschou|arXiv (Cornell University)|Oct 26, 2020

Particle Detector Development and Performance参考文献 13被引用 4

一句话总结

本论文研究了PICOSEC-Micromegas探测器，这是一种新型气体填充的切伦科夫粒子探测器，通过光电阴极将切伦科夫光子转换为电子，用于超快时间测量。利用测试束数据和现象学模拟，实现了24 ps的时间分辨率，每粒子对应10.4个光电子，将时间偏差效应归因于气体倍增阶段中电离前后电子漂移速度的差异。

ABSTRACT

In this work, a part of the Research and Development effort of the PICOSEC detector is presented. The PICOSEC detector is a novel gas-filled detector, based on the Micromegas detector, which has been developed by the RD-51 PICOSEC collaboration. Instead of relying on traditional direct ionization, the PICOSEC detector takes advantage of the prompt timing characteristics of Cherenkov radiation by converting the Cherenkov photons into electrons through the use of a photocathode. The detector has been put into two type of tests where experimental data are collected. One involves a laser beam and a single photoelectron response at CEA-SACLAY, while the other involves a test beam of 150 GeV muons at the CERN SPS H4 secondary beamline, with multiple photoelectrons. The methods employed in the statistical analysis of the detector's timing properties are summarized and in their application an optimal time resolution of 76 ps is achieved for single photoelectrons. A strange dependence of the mean timing on the size of the electron peak is observed, which mimics the behaviour of the "time walk" effect. A simulation of the detector is developed to study the timing properties, which is also the main scope of the thesis. After a detailed investigation, it is found that the electrons in the gaseous mixture move with a different drift velocity before and after the first multiplication, introducing this "time walk" effect, and a phenomenological explanation of this effect is provided. Finally, a maximum likelihood statistical method for the estimation of the mean number of photoelectron extracted per particle is developed and an optimal time resolution of 24 ps is achieved with a mean number of photoelectrons per particle equal to 10.4. In this work, a deep understanding is acquired on a new detector which has brought upon unprecedented results in the field of gas-filled detectors.

研究动机与目标

• 为了理解并优化PICOSEC-Micromegas探测器的定时响应，该探测器是一种新型基于切伦科夫效应的气体探测器，采用光电阴极触发的电子倍增机制。
• 为了分析来自CEA-Saclay（激光）和CERN SPS（150 GeV μ子）的测试束数据，提取时间分辨率和电子峰特性。
• 为了开发一个现象学模拟模型，重现并解释观测到的定时畸变，特别是“时间偏差”效应。
• 为了通过建模每粒子的平均光电子数并优化统计估计方法，进一步提升时间分辨率。
• 为未来探测器设计和基于高精度时间测量的新型应用提供基础。

提出的方法

• 对CEA-Saclay和CERN SPS的激光和μ子测试束中单光电子和多光电子信号进行统计分析。
• 应用最大似然估计方法，从测量到的信号分布中确定每粒子的平均光电子数。
• 开发现象学模拟模型，重现探测器的定时响应，包括气体混合物中的电子漂移动力学。
• 采用一维具有漂移的布朗运动建模电子输运，其中首次通过时间的概率密度函数（PDF）服从逆高斯分布。
• 将电离雪崩前后的电子漂移速度变化纳入模型，以解释观测到的“时间偏差”效应。
• 利用逆高斯分布推导电子到达时间展宽，其参数依赖于漂移速度和扩散系数。

实验结果

研究问题

• RQ1 在使用单光电子信号时，PICOSEC-Micromegas探测器可实现的最优时间分辨率是多少？
• RQ2 为何电子信号的平均时间延迟表现出对电子峰大小的依赖性，从而呈现出类似‘时间偏差’的效应？
• RQ3 电离雪崩前后电子漂移速度的差异如何导致定时畸变？
• RQ4 现象学模拟模型能否准确重现探测器中观测到的定时行为？
• RQ5 当每粒子的平均光电子数优化为10.4时，可实现的最大时间分辨率是多少？

主要发现

• 在测试束数据中，单光电子信号的最优时间分辨率达到76 ps。
• 观测到平均时间延迟对电子峰大小存在异常依赖关系，类似于‘时间偏差’效应。
• ‘时间偏差’效应被归因于放大区域中首次电离前后电子漂移速度的变化。
• 模拟模型成功重现了定时畸变，证实速度变化源于非弹性电离过程的启动。
• 通过最大似然方法，在每粒子平均光电子数为10.4时，实现了24 ps的最优时间分辨率。
• 基于逆高斯分布模型推导出的理论时间展宽与观测到的时间分辨率一致，验证了该模型的物理解释基础。