MCNP simulation and spectrum unfolding for an NaI monitor of radioactivity in aquatic systems

Spectrum unfolding can be implemented when the angular distribution of the radiation fluence is known, e.g. when the source–detector geometry is fixed. In the case of the system developed by Institut National des Radio-Elements (IRE) for monitoring radioactivity in aquatic systems, the source is an infinite homogenous medium surrounding the monitor, with constant density. Its response function for gamma rays is calculated with MCNP4.C2. The simulation model is validated with reference measurements of standard sources diluted in a drum. Results of unfolding with UMG 3.3 are presented for this situation, and for a measurement in sea water.