A CsI-Active Pixel Sensor Based Detector for Gamma Ray Imaging

A Startracker CMOS monolithic active pixel sensor with 525times525 pixels each 25 microns/side has been coupled via a fibre optic stud to a CsI(Tl) crystal 2mm times 20mm times 20mm to form a test detector of gamma rays of energy between 60 keV and 200 keV. The crystal is segmented into 400 micron elements separated by 100 microns of filler. The system records images at 10 f/sec and is interfaced to a PC via an FPGA-based DAQ and optical link. Image acquisition is performed using a MatLab-based GUI. Noise is measured to be ~100 electrons with a temperature dependence of ~0.4%/C indicating that the majority of noise is reset related or fixed pattern in nature rather than statistical thermal noise. Images have been acquired using a Tc99m source placed directly onto the CsI(Tl) crystals as well as through several lead collimators. The structure of the CsI(Tl) scintillator is easy resolved once a noise image is subtracted and an intrinsic spatial resolution of 80-90 microns is measured. The system spatial resolution with a collimator is ~450 microns for 140keV photons. The results indicate that the present APS has too high a noise level for imaging low energy photons but a new device with ~25 electrons noise plus the potential to provide on-chip signal thresholding and cluster analysis could become part of a large-area intelligent gamma ray imager with a sub-mm spatial resolution at 140 keV.