GEANT4 Monte Carlo simulation and validation of an underwater gamma-ray spectrometer based on SrI₂(Eu) scintillator

A GEANT4-based Monte Carlo simulation model of an underwater gamma-ray spectrometer based on europium-doped strontium iodide (SrI₂(Eu)) is developed and validated in this study. The scintillator has superior properties - including excellent energy resolution (∼ 3% at 662keV), high light yield (90 photons/keV), and minimal intrinsic radioactivity - that make it particularly suitable for aquatic radiation monitoring. The GEANT4 model uses the Penelope physics list for accurate low-energy simulation. A direct comparison was made between the baseline Penelope model and the standard electromagnetic physics list (G4EmStandardPhysics). A comprehensive library of detector response functions (DRFs) was generated, covering gamma-ray energies from 50keV to 3MeV to establish the gamma-ray detection efficiency in the full operation range. Experimental validation is performed in an 8m³ water tank using calibrated radioactive solutions of ¹³⁹Ce (gamma-ray energy 165.8keV), ¹³⁷Cs (661.6keV), and ⁸⁸Y (898.0and1836.0keV), which showed excellent agreement ( 1%) with a tank-replication simulation. Another simulation, which used an energy-dependent effective volume (V_eff) to model an infinite water geometry, revealed a significant 12.5% discrepancy at 1836 keV that is attributed to finite tank volume effects.