Efficiency Calibration of the H3D H420 Gamma-Ray Imager

The Surplus Plutonium Disposition (SPD) project is down blending tons of Pu for disposal as lowlevel radioactive waste. The bulk of the work will be performed in three glove box lines currently being developed. Material holdup will be an on-going concern for the operation, and we are exploring the use of gamma-ray imaging as a means of reducing the impact of periodic measurements that would normally require stopping work to collect non-destructive analysis data and the large uncertainties associated with the generalized gamma holdup techniques typically employed. Instead, we plan to permanently install modified versions of the H3D, Inc., H420 gamma-ray imagers (4 per line) to conduct the holdup measurements on an ongoing basis. To determine the amount of Pu present in the gamma-ray images requires careful calibration of the imager response function throughout its field-of-view and across its energy spectrum. To perform the calibration, we are employing techniques developed for comparable calibration of a HPGe imager that now returns holdup values for distributed 235U sources that are good to 3%. The calibration includes using GEANT4 Monte Carlo and detector-response models of the imager. The Monte Carlo codes are used to generate events across the imager’s field of view, and the detectorresponse model is used to distribute the events using a realistic detector resolution kernel. The models are validated by comparing results to calibration data collected in the laboratory. The paper will discuss the status of the models, the accuracy achieved, and various issues associated with the mask and detector designs unique to the H420 instrument.