GAS AVALANCHE PIXEL DETECTORS

We describe the structure and avalanche gain of gaseous pixel detector s. Each anode is a square 20 mu m x 20 mu m in size connected to an in dividual pad through a plated center section. The cathodes are plated squares interconnected to a common lead. The anode squares have a pitc h of 200 mu m in both x- and y- directions. The anodes and cathodes ar e aluminum layers deposited on amorphous silicon alloyed with carbon ( a-Si:C:H) to produce a bulk resistivity of similar to 10(13) Omega cm. Measurements on signals from a group of anodes shows an avalanche gas gain close to 10(4) at a cathode-anode potential of 640 volts using a gas mixture of 50/50 argon-ethane. The avalanche gain is about a fact or of 3 higher than that of microstrip devices we have tested, having the same pitch. For our initial measurements, 16 anodes were connected together to a charge sensitive preamplifier. In a final chamber each anode would be connected to an a-Si:H p-i-n readout diode with signals read out sequentially as in flat screen devices.