The University of New Mexico Mass Spectrometer for High-Resolution Fission Yield Data

Fission cross section and product yields are important for active interrogation, for understanding secondary reactor heating, and for furthering theory on fission preformation conditions. For delayed signals in active interrogation, understanding the fission product yield over a range of neutron energies and actinide targets allows higher fidelity simulations, important for understanding minimum detectable limits. Also, rather than just determining the presence of SNM, better fragment data provides information useful for identifying the interrogated actinide through the delayed gamma-ray and neutron signatures. Very little data exists on fragment distributions on neutron energies outside of the thermal (0.025 eV) and fusion (~14 MeV) neutron energy regions. The first iteration of the UNM Fission Product Spectrometer has been tested at the LANSCE neutron source on the low energy (~0.025 eV – 1 keV) neutron beamline. The spectrometer consists of a heavy ion time-of-flight (TOF) module followed by an ionization chamber (IC), for velocity and KE measurements, respectively, of the ejected fission products. These particle-by-particle measurements, combined, give the masses of the fission products, A. Initial measured product distributions will be presented for nthermal + 235U fission. The IC also provides a range evaluation by the addition of an “active cathode” timing to measure heavy ion penetration depth into the IC gas for determination of the proton number, Z. In the final 2v-2E configuration, we will be able to extract A, Z, and KE of fission products on an event-by-event basis, thus also providing neutron multiplicity and cross sections for each fission product pair. With the range of neutron energies available at the LANSCE facility, the spectrometer will deliver data over a range of incident neutron energies, and will be run for a range of actinide targets relevant to nonproliferation. This work is in parallel with a LANL project to develop a high efficiency spectrometer, SPIDER, to run on the LANSCE high energy neutron beamlines.