Geant4 simulation of the effect of backscattered positrons on the lifetime spectra of PLEPS

The Pulsed Low Energy Positron System (PLEPS) allows the measurement of positron lifetime spectra of very high quality with peak-to-background ratios up to 3 · 10⁴. At those peak-to-background ratios small structures appear in the lifetime spectra due to backscattered positrons. Despite their small overall contribution – less than 2 % of the total events in the spectrum even with backscattering coefficient as high as 40 % – those satellite structures can render the data analysis difficult. To understand the origin of those satellite structures and to further improve the performance of the system, comprehensive simulations of the target chamber of PLEPS have been undertaken. The results reproduce fairly well the background of the lifetime spectrum. It is now possible to identify the origin of the background structures and also plan some additional countermeasures.