Gas avalanche pixel detectors

We describe the structure and avalanche gain of gaseous 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 about a factor 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.