Determination of NaI(Tl) gamma-ray detector efficiency above and below 250 keV

Abstract

ANSTO is implementing a standardised real-time stack monitoring system for monitoring airborne emissions, using identical NaI(Tl) detectors, Multi-channel analyzers (MCA) and locally produced software. This work focuses on gamma energies below 250 keV, as the majority of noble gases discharged during 99Mo production, emit in this region. It was necessary to extend the calculations beyond this region, to ensure they could be used to quantify emissions from the National Medical Cyclotron (NMC) and also used for quantifying 41Ar, produced and discharged by operations at the High Flux Australian Reactor (HIFAR). Determination of NaI(Tl) detector efficiency below 250 keV has always been considered a difficult task, one not made easier by the lack of variety in calibrated low-energy emitting gamma sources or by the lack of specific internal NaI(Tl) detector dimensions required for accurate computer modeling. This project seeks to overcome these obstacles by combining experimental data with computed tomography (CT) imaging, 2D/3D image analysis and reconstruction software,. Monte Carlo code (MCNP-4B) and comparison with high resolution HPGe detectors.