UNM Fission Spectrometer for Improved Nuclear Data

Fission cross section and product yields are important for nonproliferation work such as active interrogation and nuclear forensics while also furthering the understanding of secondary reactor heating, and further developing theory on fission preformation conditions. A better understanding of the fission product yield over a range of incident neutron energies and actinide targets reduces the uncertainty in modeling. Improved fission product data provides information useful for identifying the interrogated actinide through the delayed gamma-ray and neutron signatures. Unfortunately, very little data exists on fission product mass distributions from neutron energies outside of the thermal (0.025 eV), fast (500 eV), and DT (14 MeV) neutron energy regions. The UNM Fission Spectrometer was tested with 252Cf (s.f.) fission source and was used to gather data at the LANSCE neutron source on the low energy neutron beamline with a 235U fission target. Future measurements will include other actinides. The spectrometer consists of a transmission time-of- flight (TOF) module followed by an ionization chamber (IC), for velocity (v) and kinetic energy (E) measurements, respectively, of one of the ejected fission fragments. The IC is used as a time projection chamber, using an active cathode, to provide fission fragment range information within the ionization gas which is used to extract atomic number (Z) information. The LANSCE facility is able to produce a range of incident neutron 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.