Monte Carlo simulations of the extraction of slow positrons into gas through thin SiN windows

Monte Carlo simulations have been performed of the extraction of slow positrons into air through thin silicon nitride (SiN) windows. The simulations are based on recent experiments at the positron probe microanalyzer (PPMA) in which slow positron microbeams have been used to analyze samples located outside the vacuum chamber in air. In the present study we calculated the fractional transmission of positrons for SiN windows as a function of positron energy and window thickness, and also calculated their energy and angular spectra after extraction. The effect of a thin air/gas layer between the window and sample was also simulated.