Design and performance of the readout electronics for the CsI(Tl) detector

This paper describes a readout electronics system constructed for the CsI(Tl) calorimetry of neutral pions in experiment E246 at KEK. The instrumentation was designed to cover a wide range of energies (from 300 keV to 300 MeV) with high measurement accuracy under a counting rate of a few ten kHz per channel. The readout is provided by one 18 × 18 mm2 (or 28 × 28 mm2) Hamamatsu PIN-photodiode coupled directly to the rear end of each CsI(Tl) crystal. A charge-sensitive preamplifier was developed specifically to have low noise for a high photodiode capacitance. The equivalent noise charge of the photodiode-preamplifier system at a 2 μs shaping time was found to be 640 and 840 electrons for 18 × 18 and 28 × 28 mm2 photodiodes, respectively. A pulse processor consists of a shaping amplifier with two outputs to cover the large dynamic range and a timing filter amplifier. A baseline restorer incorporated into the shaping amplifier retains good energy resolution under rates up to 50 kHz. A multichannel constant-fraction discriminator was designed to accept long risetime signals. A time resolution of 11.4 ns (fwhm) has been achieved in beam runs for the photo energy range from 10 to 200 MeV.