Development of Pestov Time-of-Flight Counter for Quark-Gluon-Plasma Experiment

Pestov火花计数器有望在超相对论重离子碰撞等高粒子密度环境中提供良好的定时分辨率（<100ps）和良好的位置分辨率（<2mm）。然而，已知该计数器在时间分布上呈现双重结构，这对使用该计数器进行粒子识别是一个严重的问题。此外，已知老化效应会阻止该计数器的长期稳定性。为了理解这个问题的本质，我们开发了一个用于窄间隙放电的级联模拟代码，其中实现了Townsend/Streamer放电过程和UV发射/吸收过程。结果表明，初始电子的统计涨落主导了一次分量及其宽度，而紫外发射/吸收是第二分量时间分布的主要原因。可以理解，较宽的间隙降低了第二分量的强度，但时序分辨率变差。我们制作了一个计数器的原型，并观察了该计数器的老化效果。产生这种效应的主要原因是由于阴极表面的聚合物层。它是由吸收紫外线光子的猝灭气体化学形成的。我们提出了改善老化效应的方法，这也可能改善时序分布的双重结构。

Pestov Spark Counter is expected to provide good timing resolution (<100ps) and good position resolution (<2mm) in an environment of high particle densities such as in ultra-relativistic heavy ion collisions. However, this counter is known to show double structure in timing distributions, which is a serious problem for the particle identification with this counter. Also an aging effect is known to prevent a long term stability of this counter.To understand the nature of this problem, we have developed a cascade simulation code for narrow gap discharge, in which Townsend/Streamer discharge process and UV emission/absorption processes are implemented. It turns out that statistical fluctuation of initial electrons dominates the primary component and its width, while the UV emission/absorption is the main cause of the second component of the timing distributions. It is understood that wider gap reduces the strength of the second component but the timing resolution becomes worse.We have built a prototype counter and observed the aging effect of this counter. Major cause of the effect is found to be due to a polymer layer built on the surface of the cathode. It is chemically formed from the quencher gas for the absorption of UV photons. We have come up with ideas to improve the aging effect, which might improve the double structure of the timing distribution also.