Monte Carlo simulation to positron emitter standardized by means of 4πβ-γ coincidence system—Application to 22Na

The present work describes the methodology for predicting the behavior of extrapolation curves obtained in radionuclide standardization by 4πβ-γ coincidence measurements, applied to 22Na, developed at the Laboratório de Metrologia Nuclear of IPEN-CNEN/SP (LMN–Nuclear Metrology Laboratory). The LMN system consists of a proportional counter (PC) in 4π geometry coupled to a single or a pair of NaI(Tl) scintillation crystals. Two standardization techniques were used: the Sum-Peak and the Nuclear-Peak methods. The theoretical response functions of each detector have been calculated using the MCNPX Monte Carlo code. The code ESQUEMA, developed at LMN, has been used for calculating the extrapolation curve in the 4πβ-γ coincidence experiment. Modifications were performed in order to include response tables for positrons and coincidences with annihilation photons. From the calibration results it was possible to extract both the activity value and the positron emission probability per decay. The latter was compared with results from the literature.