Neutron-Induced Fission in the Actinides: Energy-Angle Correlations and Extended-Energy Yields

Neutron-Induced Fission in the Actinides: Neutron Energy-Angle Correlations, and Extended-Energy Yields Michael A. Kovash, Principal Investigator Dept. of Physics and Astronomy, University of Kentucky Abstract The spectrum of evaporation neutrons following neutron-induced fission of actinide targets is not well measured for emitted neutron energies either below 1 or above 6 MeV. The overall goals of this project are to first develop the means to collect reliable spectra in both of these energy ranges, and then to use this equipment to make these measurements with a 235U target. Our initial efforts have been centered on the low energy region, where we used a novel layered scintillation detector to measure neutron spectra from 450 keV to 2 MeV at beam energies from 1 to 20 MeV. The project proposed in this document is focused on the energy range from 3 to 10 MeV, where existing data on a number of targets cannot be described in the standard evaporation model. The discrepancy between the data and models is especially large in the range from 6 to 10 MeV - a region which is characterized by relatively low neutron yield, thus making these experiments both time-consuming, and especially vulnerable to backgrounds. We propose to construct an array of plastic scintillators with high detection efficiency for 3 to 10 MeV neutrons, 32-47%. The array has a 60 degree polar angle coverage, and a very large solid angle, 1 sr. Neutron energies are determined by time-of-flight, and the measured correlation between TOF and energy deposition is used to reduce backgrounds. In addition, the array has position resolution for the neutron interaction points, allowing a measurement of the neutron angular distribution. The array will be located on the 4FP15L channel at LANSCE/WNR, providing a good flux of incident neutrons extending from one to several tens of MeV. The array consists of sixteen scintillator "bars," each of 10 cm x10 cm cross section, and 200 cm length. A light guide and photomultiplier tube is located at each end of the bar. Events are localized along the bar using the measured time difference between the PMT signals, while the measured mean time is used to determine the neutron TOF from the target. The PI has an inventory of 18 of these detectors. The array will be positioned approximately 3 meters above the floor, directly above the fission target. At this location, prompt fission neutrons from 3 to 10 MeV fall in a TOF range which excludes both neutrons which scatter from the walls and floor, as well as gamma rays and neutrons which are generated in the floor and walls by prompt emissions from the target. The non-prompt backgrounds will be measured by recording singles triggers in both the neutron array and fission target chamber. The measured time distributions within the beam macropulse for these trigger types will be used to determine the background accidental coincidence rate. A position-sensitive tagging detector is used to monitor the measured pulse-height and time distributions from the array, simultaneous with the collection of the fission data. Auxiliary mea- surements of the 252Cf spectrum will be made with the array, using a source which is of the same basic design as the fission chamber used to collect the uranium data. Our proposal to run this experiment has been approved by the LANSCE/WNR Program Advisory Committee.