PNNL

Abstract

Many evaluations of neutron survey instruments, area monitors and personal dosimeters rely on neutron reference radiations from 252Cf, a source of spontaneous fission neutrons that produces high dose rates from an almost point source geometry. Its characteristics are generally more favorable when compared to (a,n), photoneutron and reactor-based sources of neutrons. The source is typically used in two standardized configurations: unmoderated, to yield a fission energy spectrum, or placed within a heavy-water moderating sphere to produce a softened spectrum that is more appropriate for evaluating devices used in nuclear power plant work environments. The U.S. Department of Energy (DOE) 252Cf Loan/Lease Program was terminated in 2009, and the replacement of high-activity sources is increasingly cost prohibitive for laboratories that formerly benefited from that program, including the Pacific Northwest National Laboratory (PNNL) and National Institute of Standards and Technology (NIST). Neutron generators, based on the D-T and D-D fusion reactions, are becoming economically competitive with 252Cf and are recognized internationally as important calibration and test standards. Researchers from NIST and PNNLare jointly considering the practicality and technical challenges of implementing neutron generators as calibration standards in the U.S. This article reviews characteristics of isotope-based sources, leading to rationale behind increasing favor of electronically-generated neutron options. The evaluation of a D-T system at PNNL has revealed primary challenges in adapting generators to the task of calibration and testing in which neutron characteristics must be accurately determined. Concepts for modifying the generated neutron spectra to achieve particular targeted spectra, simulating 252Cf or workplace environments, are discussed.