Amplitude and risetime compensated time pick-off circuit based on extrapolated leading edge timing technique

Abstract

A time pick-off circuit for use with large high-purity germanium (HPGe) detectors and its performance are described. It uses two discriminators to extrapolate the time origin of the input pulse thereby reducing the walk due to variation in amplitude and risetime. In a commonly used constant fraction discriminator (CFD), the cable decay used for shaping, arming discriminator bias and other circuit parameters are to be critically adjusted to achieve a compensation for the walk. In the circuit described cable delay and arming discriminator are not required and the adjustment of the discriminator biases is simple. The performance of this simple circuit has been found to be comparable to that of a commercially available complex CFD unit (EG&G ORTEC model 473A). A typical delayed coincidence curve pertaining to the 122 keV level of 152Sm is also presented to demonstrate the effectiveness of the circuit in measuring short half-life (1.4 ns) using a 127 cm3 HPGe detector and a BaF2 scintillator combination. This circuit is thus a low-cost and simple alternative to a CFD unit.