Probabilistic ISOCS Uncertainty Estimator: Application to the Segmented Gamma Scanner

Abstract

Traditionally, high resolution gamma-ray spectroscopy (HRGS) has been used as a very powerful tool to determine the radioactivity of various items, such as samples in the laboratory, waste assay containers, or large items in-situ. However, in order to properly interpret the quality of the result, an uncertainty estimate must be made. This uncertainty estimate should include the uncertainty in the efficiency calibration of the instrument, as well as many other operational and geometrical parameters. Efficiency calibrations have traditionally been made using traceable radioactive sources. More recently, mathematical calibration techniques have become increasingly accurate and more convenient in terms of time and effort, especially for complex or unusual configurations. Whether mathematical or source-based calibrations are used, any deviations between the as-calibrated geometry and the as-measured geometry contribute to the total measurement uncertainty (TMU). Monte Carlo approaches require source, detector, and surrounding geometry inputs. For non-trivial setups, the Monte Carlo approach is time consuming both in terms of geometry input and CPU processing. Canberra Industries has developed a tool known as In-Situ Object Calibration Software (ISOCS) that utilizes templates for most common real life setups. With over 1000 detectors in use with this product, the ISOCS software has been well validated and proven to be much faster and acceptably accurate for many applications. A segmented gamma scanner (SGS) template is available within ISOCS and we use it here to model this assay instrument for the drummed radioactive waste. Recently, a technique has been developed which uses automated ISOCS mathematical calibrations to evaluate variations between reasonably expected calibration conditions and those that might exist during the actual measurement and to propagate them into an overall uncertainty on the final efficiency. This includes variations in container wall thickness and diameter, sample height and density, sample non-uniformity, sample-detector geometry, and many other variables, which can be specified according to certain probability distributions. The software has a sensitivity analysis mode which varies one parameter at a time and allows the user to identify those variables that have the largest contribution to the uncertainty. There is an uncertainty mode which uses probabilistic techniques to combine all the variables and compute the average efficiency and the uncertainty in that efficiency, and then to propagate those values with the gamma spectroscopic analysis into the final result. In the areas of waste handling and environmental protection, nondestructive assay by gamma ray scanning can provide a fast, convenient, and reliable way of measuring many radionuclides in closed items. The SGS is designed to perform accurate quantitative assays on gamma emitting nuclides such as fission products, activation products, and transuranic nuclides. For the SGS, this technique has been applied to understand impacts of the geometry variations during calibration on the efficiency and to estimate the TMU.