Uncertainty Budget and Efficiency Analysis for the 239Pu (n,2ny) Partial Reaction Cross-Section Measurements

Abstract

The {sup 239}Pu(n,2n{gamma}){sup 238}Pu partial reaction cross-section, {sigma}{sub (n,2n{gamma})}, has been measured as a function of neutron energy for several transitions in {sup 238}Pu. Partial {gamma}-ray cross sections for yrast, ''collector'' transitions, can provide especially valuable constraints on the magnitude and shape of the total (n,2n) reaction cross-section. In essence, nuclear reaction models will be used to infer the shape and magnitude of the total (n,2n) reaction cross-section from the measured partial {gamma}-ray cross-sections. The reason for undertaking this somewhat indirect approach is that previous measurements of the {sup 239}Pu(n,2n{gamma}) have been hampered by a variety of constraints. Activation measurements have several hurdles: (1) intense flux and long counting times are required to overcome the relatively long half-life of {sup 238}Pu (87 years) and (2) isotopically pure samples of {sup 239}Pu in an environment free of {sup 238}Pu contamination are difficult to come by. Neutron counting experiments are subject to significant uncertainties because (1) large background statistics from fission neutrons and (2) the experimental fission neutron multiplicity spectrum is subject to systematic errors because the flux of low-energy neutrons which induce fissions in thermally-fissile {sup 239}Pu is very difficult to characterize. In this measurement, spallation neutrons are provided by the LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. Neutron flux is monitored in-beam with one {sup 235}U fission chamber and one {sup 238}U fission chamber. The {sup 238}U is not sensitive to background from low-energy neutrons, whereas the {sup 235}U fission chamber has better statistics. Hence, in essence the partial {gamma}-ray cross sections are normalized to the evaluated fission cross sections of {sup 235}U and {sup 238}U. As a check of our normalization to provide additional constraints to the nuclear reaction modeling, benchmark measurements of {sup nat}Fe(n, n{prime}{gamma}) and {sup 235}U(n,2n{gamma}) have also been undertaken. The secondary {gamma}-rays are measured with the GEANIE array. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Any absolute cross section measurement requires a complete understanding of array performance, flux normalization, and target effects. Important items to consider in this experiment include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Radioactive targets give rise to significant counting rates in the GEANIE array resulting a large deadtime. The magnitude, energy dependence, and uncertainties of these effects and other corrections are the subject of this paper.