Grants and Contracts Details
Description
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.
Status | Finished |
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Effective start/end date | 9/1/16 → 11/28/18 |
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