Abstract
The time resolutions of a new positron lifetime spectrometer based on a fast digital oscilloscope are measured using BaF 2 and plastic (BC-422) scintillators. It is found that the plastic scintillator has only a small advantage in the time resolution compared to BaF 2. For the cascade decay of 60Co (1.17 and 1.33 MeV), the time resolution for a pair of BC-422 plastic scintillators having a size of 40 mm in diameter and 20 mm thick is about 120 ps FWHM. This value is very close to that of BaF 2. Although a time resolution of 90 ps FWHM is obtained using a pair of small BC-422 plastic scintillators (30 mm in diameter, 10 mm thick), the counting rate is nearly two orders of magnitude lower than that with BaF 2. It is thus concluded that the BaF 2 is the best scintillator at present for the practical positron lifetime spectroscopy.
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