An MCNPTM Based Method to Determine the Matrix Attenuation Correction Factors for a Gamma Box Counter

A general purpose Gamma Box Counter is a large scale, high-resolution segmented gamma-ray nondestructive assay system for the assay of nuclear waste containers covering a wide range of composition, mass, and geometry. In order to accurately quantify the activity of the radionuclides in a waste container, a correction for the gamma-ray self-attenuation in the waste matrix must be estimated and applied. One of the methods of determining the self-attenuation involves measuring the transmission through the matrix using gamma-rays from an external source. Assuming each segment of the matrix is uniform, the matrix attenuation correction factor can be expressed as a function of the transmission governed by a geometry dependent parameter κ. The parameter κ is a scale parameter applied to the characteristic thickness (or diameter) in the model function, and can vary for different waste container shapes. To determine the parameter κ for the different waste containers in the application of interest, the transmission ratio and attenuation correction factor must be known for each container at a variety of energies. This information can be acquired experimentally but can also be obtained using a transport code to simulate the behavior. In this application simulations also serve to enhance understanding of the problem, provide confidence in the solution by experimental benchmarking, and provide an alternative in the case of difficult-to-measure scenarios.