Determination of 240Pueff within waste drums: improvement of neutron detection efficiency determination by reconstruction of activity spatial distribution

Plutonium determination in waste drums by neutron coincidence methods may be complicated by unknown spatial distribution of the spontaneous fission neutron emission within the waste drum. This work presents a technique capable to reconstruct such activity spatial distribution, thus allowing to calculate more affordable values of the neutron detection efficiency to be utilised in Neutron Coincidence Counting determinations. The technique is based on an application of the Minimum Relative Entropy (MRE) method for the solution of the following one-dimensional linear inverse problem: given the measurements of the well counter detectors, calculate the total value of the contained activity and its distribution for some fictitious partition of the drum volume in voxels. The inverse problems are typically ill conditioned and they have not a unique solution. The MRE method provides a multivariate probability density function and the corresponding expected value is taken as a solution. It has been found that the solution matches well the total activity within the drum both when the activity is uniformly distributed within the drum volume or, on the contrary, when the voxel activities are highly different. In the general case the sampled values are contained within the 90% probability levels of the distribution.