Optimization of activation foil passive neutron detectors

Abstract

Passive neutron detectors, including activation foils, use neutron absorption reactions with known cross sections to determine a level of neutron radiation received over time. Activation foils are chosen for a specific neutron measurement application based on their reaction probabilities cross section at various neutron energy levels and the properties of the activation products which result from those reactions. Different activation foil nuclides have strongly varying neutron cross sections at different energy levels. By selecting an appropriate set of foils, the energy dependence of the neutron field can be characterized and information about the spectrum and the magnitude of the fluence can be obtained. For the Activation Foil Integrated Detector System (AFIDS), a specific set of activation wires is used to distinguish between different nuclear weapons detonation leakage spectra. The selection of various elements for use in foils and their dimensions for AFIDS was initially based on informed judgement, but no rigorous optimization of the set was done. Since this system could be deployed in cities nationwide, cost is a key consideration alongside the ability to accurately distinguish different spectra. This research will provide a method to optimize both the AFIDS activation detectors and the cost while maintaining feasibility and reliability for this application. A computer code has been developed to perform this optimization and it can be generalized for the use of activation detectors in many different applications and for a range of expected spectra.