The development of monolithic arrays of multiplexed, high-gain avalanche photodiodes suitable for use in a spectroscopic radiation-imaging device is underway at RMD. We have developed a relatively simple readout scheme using row-column addressing to dramatically reduce the electronics required to support a large array of discrete pixels. By adding a step to the avalanche photodiode creation, it was possible to place two separate diode contacts onto the back of each photodiode in the array. These isolation diodes allow the readout of an entire row or column of photodiodes through a common readout line. A data-decoding matrix uniquely determines the position in the array while simultaneously supporting the goal of reducing the number of signal readout lines and support electronics. This approach reduces the number of pre-amplifiers, pulse-shaping circuits, and sample-and-hold stages from n 2 to 2n (n pixels on a side) per array. Recent research has been carried out with 14 x 14 pixel, planar-processed avalanche photodiode array having pixels 2.00 mm on a side with 2.25-mm pitch. These arrays will be paneled to form the photodetector of a radiation imager of approximately 100-cm 2 sensitive area. To reduce the contribution of noise from each pixel to the common readout lines, research is being carried out to develop a discriminator with an adjustable threshold for each avalanche photodiode readout connection. Initial performance results from multiplexed arrays, a discussion of the active discrimination contacts, and the current status of the imager research project are given.
