A simple positron lifetime system suitable for low density gases

A conventional positron lifetime system is described which gives data of high statistical accuracy and is especially suitable for gases at low density. Several factors contribute to the significant improvement in performance. The fraction of positrons annihilating in the gas is greatly enhanced by backscattering, and the small dimensions of the pressure vessel coupled with large plastic scintillators yields about 1200 coincidence events per second. Simple electronics are used with no side channel gating, and very clean spectra are obtained with overall resolution better than 1.5 nsec. The true start and stop pulses are counted, which enables the data to be treated by the authors' signal restoration method. The results of measurements in Ar-CO mixtures and data for H₂ and D₂ are presented and discussed.