Modification of source contribution in PALS by simulation using Geant4 code

Highlights: • Mono-energetic positron distribute in Kapton film and Al. • Positron annihilation ratios in the films are simulated when positron source is ²²Na. • Modification of source contributions for some common materials. • Compared the results of the PALS between the simulation and the experimental. - Abstract: The contribution of positron source for the results of a positron annihilation lifetime spectrum (PALS) is simulated using Geant4 code. The geometrical structure of PALS measurement system is a sandwich structure: the ²²Na radiation source is encapsulated by Kapton films, and the specimens are attached on the outside of the films. The probabilities of a positron being annihilated in the films, annihilated in the targets, and the effect of positrons reflected back from the specimen surface, are simulated. The probability of a positron annihilated in the film is related to the species of targets and the source film thickness. The simulation result is in reasonable agreement with the available experimental data. Thus, modification of the source contribution calculated by Geant4 is viable, and it beneficial for the analysis of the results of PALS.