Application of gamma spectrum analysis techniques for natural radioactivity measurements using NaI(Tl) detector

Abstract

Sodium iodide (NaI(Tl)) scintillation detector is commonly used for gamma-ray spectrometric evaluations in airborne surveys and geophysical well logging. However, the applications related to environment monitoring are often encountered with challenges of low-level counting (LLC), demanding a high-resolution spectrometry system, such as a high purity germanium (HPGe) system. HPGe systems are expensive and cannot be used continuously due to the necessary supply of liquid nitrogen. In this paper, the possibility to use relatively poor resolution NaI(Tl) detector for measuring low-level radioactivity due to principal nuclides by spectrum unfolding has been explored. We quantified the activities of principal radionuclides, ²³²Th, ²³⁸U, and ⁴⁰ K, in soil samples from NaI(Tl) spectral measurements by spectrum decomposition (SD) and matrix deconvolution (MD) techniques. The specific activities of radionuclides were statistically characterized with respect to the measurements made with HPGe detector. The comparison suggests that activities determined with NaI(Tl) detector were underestimated in the majority of cases. However, the activity of ²³⁸U measured with the MD method was overestimated. The results of SD were closer to the HPGe detector measurements as compared to the MD method. Considering the normal distribution of measurements from both detectors, correlation coefficients were computed that led to the development of linear regression models to numerically transform NaI(Tl) measurements to HPGe equivalent activities within statistically acceptable bounds. Thus, a low-resolution but high-efficiency NaI(Tl) detector provides a cost-effective and time-saving alternative to HPGe measurements for the routine environmental radioactivity surveys, since it offers continuous operation and provision for carrying in fields as well, which facilitates sample characterization and thereby quick assessment of radiological hazards.