The influence of plasma effects on the timing of fission fragments with semiconductor detectors

Abstract

For fission fragments incident on surface-barrier detectors of low resistivity (ϱ ≈ 400Ω cm) the dependence of the fast current signal on field strength and local resistivity variations of the basic material has been studied. It is well known that with the fast signal as timing reference a time shift is observed when varying the bias voltage. This is due to a change in plasma time. The additional variation of plasma time with mass and/or energy of the heavy ions necessitates a time-walk correction in fission-fragment time-of-flight studies, which has been determined by two independent methods to be about ± 200 ps for the mean light and heavy fragment of binary fission from 252Cf. Local resistivity changes of the silicon wafer the detectors are made from, give rise to local plasma-time shifts, which spoil the time resolution of large-area detectors. For a narrow well-collimated beam of fission fragments an optimum time resolution of 215 ps has been obtained.