The Effects of Radiation and Emitted Light Transport on the Positional Response of 11 cm × 42.5 cm × 5.5 cm NaI(Tl) Detectors

Abstract

Experiments were performed with 30 11 cm × 42.5 cm × 5.5 cm NaI(Tl) detectors to better understand their positional response. Spectra were collected using 0.02 to 0.15 MBq point sources of 241Am, 137Cs, 60Co, and 133Ba positioned on lines parallel and perpendicular to the long axis of the crystal along both the narrow and wide detector faces as well as at different distances from them. A greater density of positions was sampled at the ends of the detector, and repeated measurements were made to examine potential gain drifts during the experiment. Spectroscopic peak counts, spectroscopic pulse heights, and net counts were analyzed. Empirical equations were fit to the aforementioned data for each specific source energy as a function of source position. In addition, a Monte Carlo radiation transport code was used to simulate the expected positionally variable response based solely upon radiation absorption. The simulated radiation transport efficiency functions were compared to the experimental data. The effects of the geometric radiation efficiency, the attenuation and scattering of emitted light within the scintillation crystal, and combined effects such as nonuniformity of the photomultiplier tube, photocathode response, and crystal irregularities were then distinguished. Functions describing each effect were derived. The results suggest potential new corrections to data obtained with large scintillation detectors as well as a novel approach to partial positional gamma-ray detection with minimal collimation, given that the energy resolution is within reason for particular photopeaks.