Effect of the drift field on avalanche gain and charge collection in microgap detectors at high pressure

One of the main drawbacks of the use of microgap structures when used at high pressure (6 bar) at high count rates (∼106 s−1 mm−2) is the limited maximum safe gain at which they can be operated. The use of a preamplification device such as the GEM (gas electron multiplier) can overcome this limitation; however, secondary space charge effects due to charge accumulation at the non-metallic surfaces cannot be neglected and should be minimised. These positive charges are due to the intrinsic multiplication of the GEM device and also to the drift of ions from the anode of the main detector. In this study we present data on the variation of the positive ion collection by the drift electrode versus drift field at several pressures up to 6 bar using a Kr–CO2 mixture. Data were collected with microgaps having several anode and insulator widths. The influence of the drift field in the pulse rise time is also considered.