Distortion of the Positron Lifetime Spectra in BaF2Spectrometers

Abstract

An essential improvement of nuclear lifetime spectrometry in the subnanosecond range was achieved in 1983, when the new scintillators, BaF2 single crystals, were proposed by Laval and Moszynski [1]. The main advantage of BaF2 comparing to the traditional plastic scintillators, consists in: – very short fluorescence time enabling to reduce the instumental time resolution further, – high efficiency of the gamma ray registration owing to the high Z of barium; the energy spectra contain well-defined full energy peaks, no need to set the windows on the continuous Compton spectum. High efficiency greatly increased the coincidence counting rate and shortened the time of measurement. The high efficiency, being an essential advantage of the new scintillator, can be the source of spectum distortions in one particular case: the positron lifetime measurements. That is due to the summing effects. In the positron lifetime measurements the START pulse is produced by 1274 keV 22Na gamma ray, STOP pulse — by one of two annihilation quanta. Traditional in these measurements is face-toface; two scintillation are usually as close to each other as possible, and the source-sample sandwich is placed between them. When the solid angle subtended by the start scintillator is close to 2π, one of annihilation quanta must enter that scintillator. The majority of positrons in a solid does not survive 1 ns, i.e. they live shorter than the risetime of PM output pulse, thus 1274 keV and annihilation quanta sum perfectly. Total intensity of sum events relative to the normal ones is roughly close to the 511 keV registration efficiency. In the coincidence spectra and linear geometry the summing effect remains unchanged, independent on counter spacing: if the STOP counter is actuated by 511 keV quantum, the other constituent of annihilation pair certainly enters the START counter.