High resolution measurement of the 238U neutron capture yield for incident neutron energies from 1 keV to 100 keV

Author

Roger Lawrence Macklin

Date of Award

8-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

R. B. Perez

Committee Members

P. F. Pasqua, D. G. Cacuci

Abstract

The purpose of this work is the precise determination of the ²³⁸U capture yield (i.e., the probability of neutron absorption) as a function of neutron energy with the highest available neutron energy resolution. The motivation for the undertaking of such a complex and lengthy task arises from the central role played by the ²³⁸U neutron capture process in the calculation of the neutron balance of both thermal reactors and fast breeder reactors. Requirements from the reactor design community for improved understanding and representation of the ²³⁸U neutron capture led the Nuclear Energy Agency Nuclear Data Committee (NEANDC) to set up the ²³⁸U Task Force to evaluate the available ²³⁸U neutron capture data. This NEANDC task force recommended that a new high energy resolution measurement of the ²³⁸U capture yield should be performed. With this added incentive, a cooperative program was initiated between the Oak Ridge National Laboratory and the University of Tennessee Nuclear Engineering Department to perform the present measurement using the Oak Ridge Electron Linear Accelerator (ORELA) facility.

The pulsed beam of neutrons from the ORELA facility is collimated on a sample of ²³⁸U. The neutron capture rate in the sample is measured, as a function of neutron time-of-flight (TOP), by detecting the gamma rays from the ²³⁸U(n, γ)²³⁹U reaction with a large gamma-ray detector surrounding the ²³⁸U sample. At each energy, the capture yield is proportional to the observed capture rate divided by the measured intensity of the neutron beam. The constant of proportionality (the normalization constant) is obtained as the ratio of theoretical to experimentally measured areas under small ²³⁸U resonances where the resonance parameters have been determined from high resolution ²³⁸U transmission measurements. The cross section for the reaction ²³⁸U(n, γ)²³⁹U can be derived from the measured capture yield if one applies appropriate corrections for multiple scattering and resonance self-shielding.

Use of fast electronics and the 150-meter flight-path station at ORELA resulted in the world's highest energy resolution measurement of the ²³⁸U neutron capture yield. This feature of the present work will allow the extension of the resolved resonance region in ²³⁸U from its current limit of 4 keV up to 20 keV. In addition, some 200 new ²³⁸U neutron resonances in the energy range from 250 ev to 10 keV have been discovered which had not been detected in previous measurements. These results are truly of historic importance in regard to both reactor design and the wealth of information they provide for the understanding of the structure of the (²³⁸U + n) compound nucleus.

Recommended Citation

Macklin, Roger Lawrence, "High resolution measurement of the 238U neutron capture yield for incident neutron energies from 1 keV to 100 keV. " Master's Thesis, University of Tennessee, 1987.
https://trace.tennessee.edu/utk_gradthes/13535