Optimization of a time-over threshold preamplifier for high-purity germanium detectors

This paper deals with the optimization of a new time-variant preamplifier for High Purity Germanium (HPGe) detectors, featuring an unprecedented 90dB dynamic range in high resolution spectrometry of ionizing particles/radiations. The preamplifier works with a conventional resistor reset for gamma photon energies < 3 MeV while it uses a pulsed fast reset for large pulses of higher energies. The height of the large pulses is measured indirectly from the preamplifier reset time using a Time-over-Threshold (ToT) measurement technique, which is effective even when the signal is much larger than the input voltage swing of the Analog to Digital Converter (ADC). This yields a substantial increase of the overall measurement dynamic range. We studied the time jitter of the ToT parameter by using the concept of “Noise Weighting Function” (NWF). We found that the limiting factor for the signal-to-noise ratio of the ToT measurement comes from the steepness of the NWF’s slope at a certain time position. By smoothing out the NWF’s corner by simply using a passive RC network we were able to improve the time jitter of the ToT measurement of as much as 50%, or from 610 ps to 305 ps (r.m.s.). A noise correlation property is finally observed, which can be used to cancel in part the noise and so improve the spectroscopic resolution in the ToT mode. A detailed study of the ToT technique and its performance is presented in this paper.