Abstract

Gamma transmission is a prominent technique for performing nondestructive testing and material analysis, particularly high-precision thickness measurements. However, its accuracy is often compromised by small-angle scattering, in which photons deflected at shallow angles are incorrectly counted by the detector as part of the primary transmitted beam. This is especially pronounced when using common low-resolution scintillation detectors such as NaI(Tl). This study presents a Monte Carlo simulation method, developed with the MCNP-CP software, to correct for the influence of small-angle scattering in gamma transmission measurements. The method is demonstrated using simulations of a standard experimental setup with a 2 × 2 inch NaI(Tl) detector and a collimated gamma-ray beam from three distinct radioisotope sources covering a broad energy spectrum: Co-60 (1173 and 1332 keV), Cs-137 (662 keV), and Am-241 (59.54 keV). By computationally isolating directly transmitted photons from the scattered component, the true linear attenuation coefficients are obtained for various single-element materials, including carbon, copper, and aluminum. The results demonstrate a significant improvement in accuracy; after applying the correction, the calculated attenuation coefficients deviate by less than 3% from the established NIST XCOM reference values, compared with a 6% discrepancy observed without correction. It is also found that the intensity of small-angle scattering has a complex dependence on material thickness, initially increasing to a peak before gradually declining. Analysis of the buildup factor confirms that it has a direct positive correlation with sample thickness, reflecting the increasing contribution of scattered photons in thicker materials. The behavior of the buildup factor is also strongly energy dependent: it is minimal at low energies (Am-241), shows the greatest sensitivity to the collimator’s diameter at intermediate energies (Cs-137), and converges at high energies (Co-60). This work provides a validated method to enhance the precision of gamma-ray measurement systems by compensating for systematic scattering deviations.