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

Pestov火花计数器有望在高粒子密度的环境中提供良好的时间分辨率（<100 ps）和位置分辨率（<2 mm），例如在超相对论重离子碰撞中。然而，这种计数器是已知的时间分布，这是一个严重的问题，粒子识别与这种计数器显示双重结构。为了理解这个问题的本质，我们开发了一个窄间隙放电的级联模拟程序，其中实现了汤森/流光放电过程和紫外发射/吸收过程。结果表明，初始电子的统计涨落决定了时间分布的第一分量及其宽度，而紫外发射/吸收是时间分布第二分量的主要原因。可以理解，更宽的间隙降低了第二分量的强度，但定时分辨率变差。我们已经建立了一个原型计数器，并观察了这种计数器的老化效应。该效应的主要原因被发现是由于在阴极表面上构建的聚合物层。它是由猝灭剂气体化学形成的，用于吸收UV光子。我们提出了改善老化效应的想法，这可能也会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.