A matrix effect correction method for fissile nuclear material mass measurement by delayed neutrons

Active neutron interrogation (ANI), extensively employed in nuclear safeguards, is sensitive to the presence of special nuclear materials (SNMs). However, the interaction of the matrix material with neutrons weakens the precision of fissile mass measurements by the ANI system. Therefore, it is paramount to ensure that the evaluation of the fissile mass remains unaffected by the matrix and enhances the assay performance of the ANI system. The present work proposes a matrix correction method based on the neutron flux monitor group (NFMG) response to tackle this issue. Based on the varying influence of different matrices on the neutron energies and fluxes, the NFMG response can be used to quantify the matrix effects. This allows the method to enable the identification of matrices already present in the database and has the potential to compensate for unknown matrix effects. To validate the applicability and accuracy of this method, a Shuffler system model and various matrix compositions were developed using the Geant4 toolkit. The results demonstrate that the improved simulated annealing (SA) algorithm exhibits excellent stability when confronted with varying enrichments and distributions of U₃O₈ materials. When the threshold is set at $\alpha$ ≥ 0.90, the improved SA algorithm achieves a successful identification rate of 90.4% for matrices. Simultaneously, using the response equation, the average relative deviation of the corrected ²³⁵U mass is no more than 7%. For the unknown matrix, the correction capability of this method relies primarily on the construction of the reference database and the response equation. In most cases, the average relative deviation of the corrected ²³⁵U mass is less than 28%.