A high-resolution BaF2 positron-lifetime spectrometer and experience with its long-term exploitation

A spectrometer for measuring positron lifetimes in solids is described. It consists of a pair of BaF2 scintillators coupled to Philips photomultiplier s XP2020Q and a fast-slow coincidence system composed of standard NIM modul es. To achieve adequate stability and the maximum performance of the spectr ometer, several improvements on commonly used techniques have been introduc ed. Of these, modified dynode timing based on the utilization of the negati ve, "anode-like" fast signal seems to be the most important. At a typical r ate of 35-40 coincidences/s, the best recorded timing resolution power of t he spectrometer for a Na-22 positron source amounted to 139 ps. Although th e photomultipliers used display marked effects of ageing on their dynode sy stems, the timing resolution of the spectrometer after running for more tha n five years has deteriorated to only 146 ps. This long-term behaviour has been achieved as a result of measures for compensating the influence of the gradual loss of photomultiplier gain. Important details relating to the sp ectrometer design, as well as to its optimization and testing, are given. E xamples of measured positron-lifetime spectra are given to demonstrate the performance of the spectrometer. The rich data relating to the long-term be haviour of the spectrometer are broadly discussed. (C) 2000 Elsevier Scienc e B.V. All rights reserved.