Gas avalanche pixel detectors

We describe the structure and avalanche gain of microstrip pixel detectors. Each anode is a square 20 μm×20 μm in size connected to an individual pad through a plated center section. The cathodes are plated squares interconnected to a common lead. The anode squares have a pitch of 200 μm in both x-and y-directions. The anodes and cathodes are aluminum layers deposited on amorphous silicon alloyed with carbon (a-Si:C:H) to produce a bulk resistivity of ~10¹³ Ωcm. Measurements on signals from a group of anodes shows an avalanche gas gain close to 10⁴ at a cathode-anode potential of 640 volts using a gas mixture of 50/50 argon-ethane. The avalanche gain is a factor of 3 higher than that of a same pitch microstrip device. For our initial measurements, 16 anodes were connected together to a charge sensitive preamplifier. In a final chamber each anode would be connected to a readout a-Si:H p-i-n diode and signals read out sequentially as in flat screen devices