Precise Monte Carlo simulation of gamma-ray response functions for an NaI(Tl) detector

Three widely used methods to calculate the response functions for NaI(TI) detectors were investigated. The methods were Berger-Seltzer's method (Nucl. Instrum. Methods 104 (1972) 317), general Monte Carlo (MC) programs, such as EGS4 (The EGS4 code system, Stanford Linear Accelerator Center, 1985) and MCNP4B (MCNP-a general Monte Carlo N-particle transport code, Los Alamos National Laboratory Report, LA-12625-M, 1997), and special MC programs. The pulse height spectra in a 3" x 3" NaI(Tl) detector due to several gamma-ray sources have been measured to verify the calculated results of these methods. The energies of the sources ranged from 0.4118 to 7.11 MeV. The spectra generated by Berger-Seltzer's method and the general MC programs did not agree well with the experimental data. PETRANS 1.0, the special MC program developed in house, was fairly accurate since it also considered the scintillation efficiency and the single escape peak shift.