Introduction

A charged particle traversing the sensitive volume of one GEM-MSGC detector ionizes the gas and the amplified electrons are collected on the MSGC structure. A cluster produced by a minimum ionizing particle has an average width of 3 strips. The center of gravity of such a cluster is called a hit position and used for the track reconstruction. To identify bad areas in a chamber, clusters found by the online hit-preparation program are used to determine the functionality of all strips contributing to the clusters.

A strip is defined as dead if the number of times, when it is used in clusters is 20% below the average number. Another group of problematic strips is defined as hot channels. These channels contribute to clusters more often than the average.

The hit efficiency is defined as the probability to find a hit if a charged particle traverses the sensitive volume of the chamber. For the purpose of off-line reconstruction, the meaningful quantity is the real efficiency of a single chamber, i.e. including dead regions.

For a better understanding of the performance of Micro-Strip Gas Counters (MSGC) with Gas-Electron Multiplier (GEM), it is more interesting to study the pure efficiency, i.e. excluding dead channels from the consideration.