A Multi-channel ASIC for the Readout of the HICAM Gamma Camera

The aim of the HICAM project is to build a compact, high-resolution gamma-ray imager for human patients, designed according to the Anger Camera principle. The imager is based on an array of 10 x 10 Silicon Drift Detectors, coupled to a single CsI:Tl scintillator. The array of photo detectors is read out by four 25-channel ASICs, which are the subject of this paper. Each channel of the ASIC features a low-noise preamplifier, compatible with the pulsed reset mode of the SDD, a gain stage, a high order semi-Gaussian shaping amplifier with selectable peaking times, a baseline holder and a peak stretcher. The ASIC also contains a 25:1 analog multiplexer, a digital section and a differential ADC driver. The pulsed reset mode of the preamplifier, which reduces the parallel noise of the system, is particularly well suited for the use with long shaping times, which are required by the CsI:Tl scintillator, due to its long primary decay time constant. The ASIC, however, features peaking times ranging from 1.1 mu s up to 11 mu s, optimized for both X-ray direct conversion measurements and for gamma-ray measurements with LaBr(3):Ce and CsI:Tl. In the paper we describe the design and the first experimental characterization of the ASIC, together with a test SDD. With an ultra-low noise test SDD we have measured an energy resolution of 130 eV FWHM at the Mn K alpha line, showing that the ASIC has a good potential for high resolution gamma-spectroscopy.