Practical Considerations for Gamma Ray Spectroscopy with NaI(Tl): A Tutorial

Gamma ray spectroscopy enables the identification of radioactive sources, and, with proper calibration, their activities. In use since the 1950s, NaI(Tl) spectroscopic systems remain popular due to their affordability and high detection efficiency. The crystals and accompanying electronics may vary, and there are practical choices necessary to produce the best possible spectra for a given application or correctly interpret system performance. An overview of the scintillation mechanism as well as the common features of a gamma ray spectrum are presented in this paper. This includes a discussion of the impacts of the size and shape of detector crystals on the spectrum and counting efficiency. A description of supporting electronics is included along with techniques for arranging and optimizing them. Coaxial cables become part of the circuit and can degrade the detector signal if they have mismatched impedance or excessive length. A discussion is included of tradeoffs involved in selecting combinations of individual electronics components for NaI(Tl) spectroscopic applications. Lastly, a comprehensive energy calibration procedure is provided. This paper thus serves as a tutorial on several practical aspects of a NaI(Tl) gamma ray spectroscopy system.