Application of Monte Carlo simulation to 134Cs standardization by means of 4πβ-γ coincidence system

The methodology recently developed by the Nuclear Metrology Laboratory (LMN) from IPEN, in Sao Paulo, Brazil, for simulating all detection processes in a 4 pi(beta,X) - gamma coincidence system by means of the Monte Carlo technique is described. This procedure makes possible to predict the behavior of the observed activity as a function of the 4 pi beta detector efficiency. The present paper describes its application to the standardization of a typical beta-gamma radionuclide, namely Cs-134. In this approach, information contained in the decay scheme is used for determining the contribution of all radiations 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 energies for which the activity can be obtained with maximum accuracy. The theoretical work applies the MCNP Monte Carlo code to a gas-flow proportional counter with 4 pi detection geometry, coupled to a pair of NaI(TI) crystals. The calculated extrapolation curve showed good agreement when compared to experimental values obtained at the LMN of IPEN.