VLSI readout for imaging with polycrystalline mercuric iodide detectors

Recently polycrystalline mercuric iodide have become available, for room temperature radiation detectors over large areas at low cost. Though the quality of this material is still under improvement, ceramic detectors have been already been successfully tested with dedicated low-noise, low-power mixed signal VLSI electronics which can be used for compact, imaging solutions. The detectors used are of different kinds: microstrips and pixels; of different sizes, up to about 1 square inch; and of different thickness, up to 600 microns. The properties of this first-generation detectors are quite uniform from one detector to another. Also for each single detector the response is quite uniform and no charge loss in the inter-electrode space have been detected. Because of the low cost and of the polycrystallinity, detectors can be potentially fabricated in any size and shape, using standard ceramic technology equipment, which is an attractive feature where low cost and large area applications are needed. Radiation detectors have been fabricated from very thick films (100 - 600 mu m) of mercuric iodide (HgI2). These devices, which function as nuclear particle counters, have been prepared with single continuos electrical contacts, linear microstrips and square pixel contacts. The word ceramic is used to distinguish the detectors from single crystals which are usually studied for this application. The detectors have been tested with a beta source as well as in a high energy beam of 100 GeV muons at CERN, connected to VLSI electronics, low noise electronics. Charge collection efficiency and uniformity have been studied The charge is efficiently connected even in the space between strips indicating fill factors of 100% could be reached in imaging applications with direct detection of radiation. Single photon counting capability is reached with VLSI electronics. These results show the potential of this material for applications demanding position sensitive, radiation resistant, room-temperature operating radiation detectors, where position resolution is essential, as it can be found in some applications in high energy physics, nuclear medicine and astrophysics.