Effects of high count rate and gain shift on isotope identification algorithms

Spectroscopic gamma-ray detectors are used for many research applications as well as Homeland Security screening applications. Sodium Iodide (Nal) scintillator crystals coupled with photomultiplier tubes provide medium-resolution spectral data about the surrounding environment. Nal-based detectors, paired with spectral identification algorithms, are often effective in identifying sources of interest by isotope. However, intrinsic limitations exist for Nal systems because of gain shifts and spectral marring (e.g., loss of resolution and count-rate saturation) at high count rates. These effects are hardware dependent and have strong effects on the radioisotopic identification capability of these systems. In this work, the effects of high count rate on the response of isotope-identification algorithms are explored. It is shown that a small gain shift of a few tens of keV is sufficient to disturb identification. The onset of this and other spectral effects is estimated for Nal crystals, and a mechanism for mitigating these effects by estimating and correcting for them is implemented and evaluated.