An investigation of the rise times of output pulses of semiconductor nuclear radiation detectors

Abstract

This paper is a report of the investigations carried out to determine whether or not the rise time of the output pulses of semiconductor nuclear particle detectors is influenced by the type of ionizing particle striking the detector. Charge collection rise times are calculated in the customary fashion, neglecting interaction between the lines of charge produced by the ionizing particle. A model of the detector is developed on which rise time calculations may be carried out taking into account charge interaction. The calculations are carried out for several different types of ionizing particles. Comparing the calculated rise times shows no difference between the rise time calculated taking into account charge interaction and those neglecting charge interaction: to within the accuracy of the method of calculation. The design of a vide-band amplifier is discussed: and the performance characteristics of the system used to observe detector charge collection rise times are presented. Resistivity and junction capacitances of several detectors were measured. In only one of the available detectors could the charge collection time be observed. Using an eight m.e.v. alpha source, the charge collection time of the detector was measured and found to be several times longer than was predicted. No other types of particle sources were readily available: so it was not possible to determine if this effect varied with the type of ionizing particle striking the detector. This effect (i.e. an apparent lengthening of the charge collection time) has, however, been observed by others: who have determined experimentally that the effect does not depend on the particle type. The conclusion then, of this paper is that there is a definite lengthening of the charge collection time over that calculated by usual methods. This effect does not vary with different types of ionizing particles. No explanation for this effect is offered.