Monte Carlo simulation of an HPGe detector for detection of ²³⁹Pu in waste assay system

This paper discusses the modeling of HPGe detector used in waste assay system using Monte Carlo code FLUKA to determine its detection limits. The modeled detector is validated using experimental point sources efficiencies in the energy range 60 keV-2000 keV. The numerical estimation of efficiency values for ²³⁹Pu present inside a 230 L cotton filled waste drum at various locations along the axis of the detector is discussed. Estimation of the minimum detectable activity (MDA) for ²³⁹Pu at these locations in the presence of 1 MBq of ¹³⁷Cs/⁶⁰Co again along the axis of the detector is examined. The efficiency of ²³⁹Pu decreases from 6.1 × 10^-10 to 7.5 × 10^-11 cps/Bq as the location of ²³⁹Pu moves away from the detector. MDA of ²³⁹Pu increases as the position of ²³⁹Pu and ¹³⁷Cs/⁶⁰Co in the waste drum shifts away from the detector while it decreases as the location of ¹³⁷Cs/⁶⁰CO moves away from ²³⁹Pu along measurement axis. It is observed that for all locations of ¹³⁷Cs/⁶⁰Co with ²³⁹Pu located up to 200 mm from the edge of the drum, the detection limit is within 140.3 mg (allowed limit).