Monte Carlo simulation of γ-ray spectra from natural radionuclides recorded by a NaI detector in the marine environment

The GEANT4 Monte Carlo code has been used to simulate γ-ray spectra of natural radionuclides collected by a NaI scintillation detector immersed in seawater. The γ-rays emitted from the decay of 40K, and the series of 232Th and 238U, were used to describe the radioactive water source around the NaI crystal. The simulated γ-ray spectra were compared with real data recorded in situ by a newly constructed NaI spectrometer and were found to be in good agreement. The NaI spectrometer was calibrated in the laboratory in a water tank, before its deployment in seawater. Activity concentrations were deduced from the γ-ray spectra and discussed in comparison with results from the literature.