Performance analysis of a digital positron lifetime spectrometer

A digital positron lifetime spectrometer has been set up and tested comprehensively. The system consists of a fast commercial digitizer connected to a computer, a simple coincidence circuit and software to extract the timing from the collected detector pulses. The digital system has the same time resolution as a conventional analog apparatus using the same detectors. The pulse processing part of the spectrometer is able to analyze and store in real-time several thousands of events per second, which is an order of magnitude more than the count rates in typical positron lifetime experiments. The data acquisition can handle small pulses, down to a few tens of millivolts, and its time-scale linearity and stability are very good. We discuss the advantages of timing with software, e.g. simple setup, use of different timing algorithms and possibility of an offline analysis of lifetime events. The idea is generally applicable to direct measurement of time intervals with picosecond accuracy.