A new positron lifetime spectrometer using a fast digital oscilloscope and BaF2 scintillators

A new positron lifetime spectrometer has been constructed and tested. It consists of a state-of-the-art digital oscilloscope (LeCroy Wavepro 960) and scintillation detectors based on Hamamatsu H3378 (R2083Q) photomultiplier tubes and BaF2 scintillators. A very good time resolution is obtained with scintillators of commonly used sizes (28 mm in diameter, 10-20 mm thick) and wide energy window settings (40-50%). The time resolution of 144 ps full-width at half-maximum (FWHM) is achieved for the positron lifetime measurement, in which time differences are measured between the arrivals of a 1.27 MeV gamma-ray from Na-22 and one of the two positron annihilation gamma-rays of 0.511 MeV. The time resolution is further improved with a 'double-stop' setup, where timing information carried by the positron annihilation gamma-ray pair is combined to determine the time of the annihilation. The time resolution with this setup is 119 ps FWHM. This improvement is interpreted in terms of the energy deposit in the scintillation detectors by gamma-rays. Applications of the new spectrometer to other scintillation timing measurements are also described. (C) 2001 Elsevier Science B.V. All rights reserved.