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 photomultipliers XP2020Q and a fast-slow coincidence system composed of standard NIM modules. To achieve adequate stability and the maximum performance of the spectrometer, several improvements on commonly used techniques have been introduced. Of these, modified dynode timing based on the utilization of the negative, “anode-like” fast signal seems to be the most important. At a typical rate of 35–40 coincidences/s, the best recorded timing resolution power of the spectrometer for a 22Na positron source amounted to 139 ps. Although the photomultipliers used display marked effects of ageing on their dynode systems, the timing resolution of the spectrometer after running for more than 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 spectrometer design, as well as to its optimization and testing, are given. Examples of measured positron-lifetime spectra are given to demonstrate the performance of the spectrometer. The rich data relating to the long-term behaviour of the spectrometer are broadly discussed.