The Efficiency Calibration of Non - Destructive Gamma Assay Systems Using Semi - Analytical Mathematical Approaches - 10497

A large variety of non-destructive assay (NDA) systems have been developed for the analysis of waste containers by passive gamma-ray spectroscopy. Examples of such systems include Segmented Gamma Scanners (SGS), Q–style drum counters, Large Box Counters, and far-field measurement systems. The calibration of these systems typically involves the purchase and construction of “calibration” containers that are to varying degree representative of the items to be assayed. Typically these “calibration” containers represent idealizations of the waste stream with homogenized matrix materials and pseudouniform source distributions. But, for complete and correct calibrations, these containers must span the operational range of the system, and consequently include extreme and difficult configurations of matrix, density and source distribution. The NDA systems themselves typically have multiple configurations to deal with different kinds of items, and each configuration must be calibrated separately, often with different calibration sources. If the system is changed or modified, it must subsequently be recalibrated experimentally. The accuracy of such a calibration is typically estimated by performing a limited number of “worst-case” scenarios to establish the bounds of the Total Measurement Uncertainty (TMU). In its entirety, the calibration and uncertainty estimation of an NDA system represents a significant up-front expense. Alternatives exist to the source-based calibrations of NDA systems. Among these alternatives is the mathematical modeling of the efficiency response of a given system to a particular waste configuration. In this situation, the system is calibrated using physically-relevant models of the gamma-ray response, with a relatively small number of validation measurements performed to demonstrate proper operation of the system and establish traceability to national standards. Due to the complexity of NDA systems and waste streams, purely analytically models are typically not feasible, but semi-analytical Monte-Carlo approaches based on the fundamental principles of gamma-ray interactions have proven to be very successful. These semi-analytical approaches are sufficiently versatile to allow the modeling of general waste stream configurations. Systems can be quickly adapted to changing waste streams or detector configurations. Not only can the measurement limits of the TMU be accurately estimated with semianalytical techniques, but its functional form can be properly modeled to allow a more accurate assessment of the entire waste stream. In this paper, we will present a survey of instruments including SGS, Box Counter, and far-field systems that have been calibrated using semi-analytical mathematical models. We will discuss experiences in calibration techniques, modeling of the TMU, and recent advances that further enhance the capabilities of such approaches.