Experimental and Simulated Spectral Gamma-Ray Response of a NaI(Tl) Scintillation Detector used in Airborne Gamma-Ray Spectrometry

Abstract

Abstract. The objective of this work is to simulate the spectral gamma-ray response of NaI(Tl) scintillation detectors for airborne gamma-ray spectrometry (AGRS) using the state-of-the-art multi-purpose Monte Carlo code FLUKA. The study is based on a commercial airborne gamma-ray spectrometry detector system with four individual NaI(Tl) scintillation crystals and a total volume of 16.8 L. To validate the developed model, radiation measurements were conducted using 57Co, 60Co, 88Y, 109Cd, 133Ba, 137Cs and 152Eu calibration point sources with known activities and source-detector geometries under laboratory conditions. In addition, empirical calibration and resolution functions were derived from these measurements combined with additional radiation measurements adopting natural uranium, thorium and potassium volume sources. The simulation results show superior accuracy and precision compared to previous AGRS simulation models with a median relative spectral error < 10 % for most of the radiation sources. Moreover, the implementation of a lower level discriminator model and detailed modelling of the laboratory result in a significant improvement in model accuracy for spectral energies < 100 keV compared to previous studies. Yet thorough statistical analysis incorporating statistical and systematic uncertainty estimates revealed statistically significant deviations between the simulated and measured spectra in the spectral region around the Compton edge, which could be attributed to the scintillator non-proportionality. These findings imply that the linear energy deposition model applied in this and previously developed AGRS simulation models should be revised and considered to be replaced by more accurate non-proportional models.

Emulator-based Bayesian inference on non-proportional scintillation models by compton-edge probing

“…To obtain the spectral Compton edge data, we performed gamma-ray spectrometry under controlled laboratory conditions 30 . The adopted spectrometer consisted of four 10.2 cm × 10.2 cm × 40.6 cm prismatic NaI(Tl) scintillation crystals with individual read-out.…”

“…First, it leads to an intrinsic spectral broadening and thereby sets a lower limit on the spectral resolution achievable with the corresponding scintillator 1 , 25 – 28 . Second, various studies stated the conjecture that specific spectral features such as the Compton edges are shifted and distorted as a result of the non-proportional scintillation response 1 , 14 , 15 , 29 , 30 . Furthermore, additional studies revealed a complex dependence of the scintillation non-proportionality on various scintillator properties including the activator concentration, the temperature, and the crystal size, among others 1 , 21 , 22 , 25 , 28 , 31 – 34 .…”

“…NPSMs are therefore particularly relevant for scintillators with large crystal volumes, e.g. in dark matter research, total absorption spectroscopy or remote sensing 1 – 7 , 30 . In addition, especially due to the sensitivity of the activator concentration and impurities 34 , NPSMs need to be calibrated for each individual detector system.…”

“…In this study, we propose Compton edge probing together with Bayesian inversion to infer and calibrate NPSMs. This approach is motivated by the already mentioned conjecture, that the Compton edge shifts as a result of the scintillation non-proportionality 1 , 14 , 15 , 29 , 30 . We obtain the spectral Compton edge data by gamma-ray spectrometry using a NaI(Tl) scintillator and calibrated radionuclide sources for photon irradiations under laboratory conditions.…”

Aeroradiometric measurements in the framework of the Swiss Exercise ARM22

“…A complete day of the ARM22c exercise part was dedicated to support an ongoing PhD thesis at ETHZ and PSI (Breitenmoser et al, 2022). The task of the thesis is the development of a new calibration method for the Swiss airborne gamma-ray spectrometry systems based on the Monte-Carlo radiation transport code FLUKA maintained by CERN (Battistoni et al, 2015).…”