Modeling Positronium induced background in the time-of-flight (TOF) spectra of positron induced secondary electrons using Monte Carlo methods.

The energy spectra of positron induced secondary electrons (PIE) can be determined from the electron time of flight as determined from the difference between the signal produced by the positron annihilation gamma in a scintillator and the signal produced by the secondary electron impacting a channel plate. Measuring the energy of PIE using a TOF spectrometer has the advantage of the parallel collection of electrons with a wide range of energies greatly reducing the data collection time. However delayed annihilations produced by long lived ortho-positronium (o-Ps), an electron-positron bound state, can result in an unwanted background to the PIE signal corresponding to unphysically short electron flight times. Here, we have developed a Monte Carlo based method to model the spectral contribution of o-Ps annihilations to the TOF spectra of PIE. Our modelling extracts the true electron TOF spectra associated with prompt gamma, provides the fraction of positrons which forms o-Ps as a function of positron beam energy and gives an estimate of the energy spectrum of the o-Ps. We report and discuss the results obtained by the application of the method to PIE spectra from pure copper.