Regularized heuristic method for activation monitor neutron spectrum unfolding

This paper describes an activation foil measurement based neutron flux unfolding method utilizing a heuristic optimization approach. A regularization parameter based on the differential neutron flux was introduced into the optimization cost function to remove large nonphysical oscillations in the unfolded neutron energy spectrum. The procedure was completed for two example case studies ranging from thermal energies up to 20 MeV demonstrating recovery of neutron flux energy information on radiation environments where a model was partially missing or there was a discrepancy in a reaction channel. The unfolded results provided a distribution of unfolded spectra that conformed to the activation products but indicated degenerate solutions to the general activation problem. Additionally, increasing fidelity of initial estimate on the neutron spectrum reduced the degeneracies and lowered the uncertainty in the unfolded spectrum, even in a low-fidelity, physics-based estimate of the neutron spectrum. Epithermal neutron energies, where few nuclear reactions are available to inform on the neutron flux, were shown to be the most difficult region. This method can be extended to other heuristic optimization routines and offers an alternative option for traditional activation dosimetry neutron flux unfolding techniques while improving the characterization of the neutron flux in applications with poor initial models of the environment.