Development of an ASIC for Si/CdTe detectors in a radioactive substance visualizing system

We report on the recent development of a 64-channel analog front-end ASIC for a new gamma-ray imaging system designed to visualize radioactive substances. The imaging system employs a novel Compton camera which consists of silicon (Si) and cadmium telluride (CdTe) detectors. The ASIC is intended for the readout of pixel/pad detectors utilizing Si/CdTe as detector materials, and covers a dynamic range up to 1.4 MeV. The readout chip consists of 64 identical signal channels and was implemented with X-FAB 0.35μm CMOS technology. Each channel contains a charge-sensitive amplifier, a pole-zero cancellation circuit, a low-pass filter, a comparator, and a sample-hold circuit, along with a Wilkinson-type A-to-D converter. We observed an equivalent noise charge of ~500 e− and a noise slope of ~5 e−/pF (r.m.s.) with a power consumption of 2.1 mW per channel. The chip works well when connected to Schottky CdTe diodes, and delivers spectra with good energy resolution, such as ~12 keV (FWHM) at 662 keV and ~24 keV (FWHM) at 1.33 MeV.