The Importance of Subthreshold Fission Phenomena in Certain Applications of Fission Threshold Detectors: An Example of Neptunium-237 for Nondestructive Assay

A calculational study was made of the response of ²³⁷Np in a “threshold” fission detector to be used in a system for measuring the fissile content of spent fuel subassemblies by nondestructive assay. The results show that the contribution of the subthreshold fission component of this isotope to the total fission activity of the detector precludes the use of ²³⁷Np in this application, thus demonstrating the importance of including these usually ignored subthreshold fission cross sections in the data file.