New Algorithms for the Analysis of Waste Characterization

Nondestructive assay of gamma-ray emitting nuclear waste requires modeling because it is not practical to prepare a standard that matches the physical and nuclear properties of every waste item. The use of a high-purity germanium (HPGe) detector simplifies the spectral analysis process. Many models use simplified efficiency determinations and attenuation corrections. These models often work well for medium-sized items measured at detector-to-item distances that are at least half of the largest dimension of the object. Other models use a hybrid Monte Carlo approach, but the detector efficiency parameters must be established at an additional cost. A new algorithm has been developed to reduce these complications and limitations while retaining acceptable accuracy. This algorithm uses a simple mixed-nuclide gamma calibration, the detector crystal diameter and length, crystal type (n-type or p-type), thickness of the germanium dead layer, and distance from the top of the end cap to the germanium active layer to compute the detector intrinsic efficiency. The intrinsic efficiency for the front and side of the detector are correlated with the detector diameter and length. Corrections for the gamma rays from the item being measured are calculated voxel by voxel. These new algorithms have been implemented in the ORTEC ISOTOPIC software version 4. Results show significant improvement for items counted as close as 10 cm, and are useful for the measurement of items positioned in a low-level drum counter. INTRODUCTION When the detector is positioned close to a large item containing radioactivity a significant amount of activity enters the HPGe detector from the side. The classic far-field correction for matrix attenuation does not account for this because it assumes all gamma-rays enter the detector normal to the front surface. An algorithm has been developed that utilizes the crystal diameter and length to automatically estimate the efficiency of the detector from the side surface as well as the front surface. Only one point-source calibration, positioned normal to the front surface of the detector crystal, is needed. This point source, whose activity is traceable to a national standard, must contain gamma rays that span the energy range of the gamma rays emitted by the nuclides present in the measured item.