Study of the backside signal of micro-strip gas counters on electronic conducting glass

Microstrip gas counters (MSGC) on electronic conducting glass such as Schott S8900 have a very stable long time behaviour. However, this glass is available in only relatively thick plates. A thick substrate limits the performances of two-dimensional detectors by attenuating the signal of the backside electrode which carries the second position coordinate. In this paper we demonstrate that by using a structure with "open cathodes", where the central area of each cathode is non metallized it is possible to reduce the screening effect by decreasing the backside voltage potential with respect to the cathode voltage potential. This configuration results in a significant increase of the backside signal, whose amplitude becomes even equal to that of the anode signal when the cathodes are not connected to an external potential. As the cathode strips do not contribute to the amplification process in this mode, this leads naturally to a structure where the cathodes are removed entirely. Besides the improvement of the backside signal, another advantage of this new detector design is the very high gas amplification factor that one can achieve. However, as the ions are discharged by a current through the substrate, the voltage drop caused by this current leads to a counting rate dependance of the gas amplification factor.