Comprehensive validation of a GEANT4 model for 2″×1.5″ NaI(Tl) gamma detector: Efficiency calibration and attenuation...

Thallium-doped sodium iodide [NaI(Tl)] remains a widely adopted scintillator material for gamma-ray spectroscopy and radiation monitoring applications. This work presents a comprehensive experimental validation of a GEANT4-based Monte Carlo model for a 2″ × 1.5″ cylindrical NaI(Tl) detector. The simulation framework was benchmarked against measurements obtained with standard gamma-ray sources (⁶⁰Co, ²²Na, ¹³⁷Cs, ²⁴¹Am, and ¹⁵²Eu) spanning the energy range 59.5-1408 keV. Key detector performance parameters, including energy calibration linearity, full-width at half-maximum (FWHM) resolution, absolute and intrinsic detection efficiencies, and mass attenuation coefficients, were systematically evaluated for aluminum, copper, and lead absorbers. Statistical comparison between experimental and simulated results yielded a coefficient of determination R² = 0.952 and a mean absolute error of 2.03% for efficiency curves, demonstrating reliable reproduction of detector response using electromagnetic physics processes alone. These findings establish a reproducible validation protocol that can support efficiency calibration, shielding design, and quality assurance in nuclear instrumentation, radiation protection, and educational laboratory settings where access to extensive calibration sources may be constrained.