Evaluation of the 239Pu(n,2n) Integrated Cross Section

Abstract

Recently, new cross section measurements by the GEANIE collaboration have been published for {sup 239}Pu(n, 2n{gamma}) [Ber00] and {sup 235}U(n, 2n{gamma}) [You00] from threshold to 20 MeV. When combined with nuclear reaction calculations [Che00, Cha99, Cha01], these measurements provide the most accurate information available on the shape and magnitude of the {sup 239}Pu(n, 2n) cross section for incident neutron energies, E{sub n} {approx}< 14 MeV. This new data has prompted a re-evaluation of the {sup 239}Pu(n, 2n) {sup 238}Pu reaction cross section considering all available experimental data. The data prior to the measurement of Bernstein et al. [Ber00] is illustrated in Figure 1a. These data sets were considered by previous evaluations [ENDL] of the {sup 239}Pu(n, 2n) cross section. The most precise experiment was an activation measurement done by Lougheed et al. [Lou00] for incident neutron energies, E{sub n}, between 13-15 MeV. In addition, there were two neutron-counting experiments, one by Mather et al. [Mat72] and one by Frehaut et al. [Fre85]. These two measurements cover a wide incident neutron range, with data points from threshold to E{sub n} {approx} 14 MeV. The available data sets are in poor agreement with each other and in some cases do not meetmore » basic expectations. These experiments will be reviewed in detail. The new contributions to this evaluation are (1) the GEANIE data coupled with reaction modeling, illustrated in Figure 1b, and (2) the systematic use of other nuclear data in order to put constraints on the shape and magnitude of the {sup 239}Pu(n, 2n) cross section. The approach of this evaluation has been to use consistency arguments supported by nuclear data to resolve the measurement differences, with the goal of providing: (1) A comprehensive picture of our knowledge on the {sup 239}Pu(n, 2n) cross section; and (2) A new evaluation including the best possible estimate of the cross section and a one-sigma estimate the uncertainties. This paper is organized in the following fashion: Section 2: Evaluation of {sup 239}Pu(n, 2n) measurements--Measurements of the {sup 239}Pu(n, 2n) cross section are evaluated for reliability (systematic uncertainties) and for renormalization possibilities. Section 3: Consistency with other experimental data--Experimental data and nuclear data evaluations for non-elastic, inelastic, fission, (n, 2n), and neutron production cross sections are reviewed in the context of unitarity and other constraints. Conclusions with regard to the shape and magnitude of the {sup 239}Pu(n, 2n) cross section are drawn. Section 4: Recommended {sup 239}Pu(n, 2n) cross section and uncertainties--The results of Section 2 and 3 are combined to provide an overall ''best'' data set with evaluated uncertainties. The recommended {sup 239}Pu(n, 2n) cross sections and relative uncertainties are given in tabular format and compared with other evaluations.« less