The University of New Mexico Fission Spectrometer for High-Resolution Fission Fragment Yield Data, With Initial Results

Fission cross section and fragment yields are important for active interrogation, for understanding secondary reactor heating, and for furthering theory on fission preformation. For delayed signals in active interrogation, understanding the fragment 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 though. We are in a multi-year project to prototype fission fragment spectrometers and perform measurements at the LANSCE neutron source. The first iteration of the UNM Fission Spectrometer is running at LANSCE with low energy neutrons on a U-235 target. 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 fragments. These particle-by-particle measurements, combined, give the masses of the fission fragments, A. Initial measured fragment distributions will be presented for n + U-235 fission. Future iterations of the spectrometer are planned to use active cathode timing to understand fragment penetration into the IC gas for proton number, Z, determination. We will also use spectrometers on both sides around the target to simultaneously measure binary fission fragments. We will extract A, Z, and KE of both fragments on an event-by-event basis, and thus also neutron multiplicity and cross sections for each fragment pair. With the range of neutron energies at LANSCE 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.