Monte Carlo simulation of activity measurements by means of 4 pi beta-gamma coincidence system

Abstract

This work describes the methodology developed by the Nuclear Metrology Laboratory (LMN) from IPEN, in Sio Paulo, Brazil, for simulating all detection processes in a 47(,X)-y coincidence system by means of the Monte Carlo technique. In this way, it is possible to predict the behavior of the observed activity as a function of the a 47 detector efficiency. In this approach, the information contained in the decay scheme is used for determining the contribution of all radiation emitted by the selected radionuclide, to the measured spectra of each detector. This simulation yields the shape of the coincidence spectrum, allowing the choice of suitable gamma-ray windows for which the activity can be obtained with maximum accuracy. Therefore, this simulation shall be able to predict a detailed description of the extrapolation curve, mainly in the region where the a 47(Þ,X) efficiency approaches 100%, which is experimentally unreachable due to self absorption of low energy electrons in the radioactive source substrate The theoretical work is being developed with MCNP Monte Carlo code, applied to a gas-flow proportional counter of a 47 geometry, coupled to a pair of Nal(Tl) crystals. The calculated efficiencies are compared to experimental values obtained at the LMN of IPEN. The extrapolation curve will be obtained by means of another Monte Carlo algorithm, being developed in the present work, to take into account fundamental characteristics of a complex decay scheme, including different types of radiation and transitions. The present paper shows preliminary calculated values obtained by the simulation and compared to experimental results.