COMPACT CIRCUIT FOR PULSE RISE-TIME DISCRIMINATION

A compact low-power circuit has been developed for rise-time selection in portable systems using thermoelectrically cooled Si-PIN photodiode s and CZT detectors. The principle of operation of the circuit is base d on the comparison of the delayed detector current waveform (current signal) with the shaped preamplifier signal representing the total del ivered charge (charge signal). The delay time of the detector current waveform is chosen to be shorter than the peaking time of the main sha ping amplifier. If the amplitude of the current signal exceeds the amp litude of the charge signal a gate signal is generated allowing pulse acceptance from the multichannel analyzer (MCA); otherwise the pulse i s rejected. The circuit comprises a small number of components mounted on a 50 x 80 mm printed circuit board. The maximum power consumption is 200 mW. Tests performed using Si-PIN photodiodes and CZT detectors indicate a significant reduction of the spectrum background. In the ca se of the CZT detector an improvement of the energy resolution was als o observed.