Spectrometer Development for High-Resolution Fission Fragment Yield Data

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, and provides information useful for identifying the interrogated actinide. Very little data exists on fragment distributions though. Our group at the University of New Mexico is addressing this need by prototyping a high resolution dual arm spectrometer to study binary fission. The detector arms will be on either side of a fission target with a range of incident neutron energies. The 2E-2v spectrometer is based on time-of-flight fragment velocity measurements and ionization chamber energy measurements. By combining these measurements and examining details of energy loss in the ionization chamber we will extract A, Z, and E of both fragments on an event-by-event basis, providing correlated data, and thus also neutron multiplicity and cross sections for each fragment pair. This detector will run on the low energy neutron beamline at LANSCE, gathering data from millieV through keV neutrons, including thermal neutrons. This work is in parallel with a LANL project to develop a high efficiency spectrometer to run on the high energy LANSCE beamline.