Measurement of Several 239Pu(n,xn) Partial (gamma)-Ray Cross Sections for x(<=)3 Using GEANIE at LANSCE/WNR

Previous experimental efforts to measure the {sup 239}Pu(n,2n) reaction cross section have relied on the detection of evaporated neutrons [Mat72,Fre85]. These efforts were hampered by the presence of the large ({sigma} {approx} 2 barns) neutron-induced fission channel which produces on average 3-4 neutrons [How71]. This paper is one of three manuscripts that document an effort to determine the {sup 239}Pu(n,2n) channel cross section using the GErmanium Array for Neutron Induced Excitations (GEANIE) spectrometer [Bec97] at the Los Alamos Neutron Science Center/Weapons Neutron Research (LANSCE/WNR) [Lis90] facility. In this document we report the measurement of several {sup 239}Pu(n,xn{gamma}){sup 240-x}Pu partial cross sections. The other two papers report results for a parallel proof-of-principle experiment using a {sup 235}U target as a surrogate for {sup 239}Pu [Youn00] and a detailed calculation and measurement of the efficiency of the GEANIE spectrometer [McN00]. Results from these two works are frequently referred to in this paper. A later report from Becker et al. will use the partial (n2n{gamma}) cross sections reported here, together with the predictions of the GNASH [Cha99,Cha00] and IDA [Ros99] reaction models, to extract a total cross section for the (n,2n) channel. The results of three experiments are reported here. Two were carriedmore &#187; out in 1998 using a 0.010 inch (referred to as &#39;&#39;thin&#39;&#39;) and 0.020 inch (referred to as &#39;&#39;thick&#39;&#39;) {sup 239}Pu target. The remaining one was run in 1999 using the same thin target. This work is being carried out in parallel with a similar effort to measure (n,xn{gamma}) cross sections on a {sup 235}U target being analyzed by Younes et al., [You00]. This experiment was performed using the same {gamma}-ray spectrometer and a similar analysis approach, thereby allowing a detailed comparison between the two data sets and a check on the techniques used. The report is comprised of three portions. In the first part the experimental techniques used for all three runs will be presented. The next section will describe how the partial (n,xn{gamma}) cross sections were extracted from a combination of all three data sets, including a detailed discussion of the level spectroscopy of levels in {sup 238}Pu populated in this experiment. The last portion of the report will focus on a comparison between the observed partial cross sections and the predictions of the GNASH reaction model [Cha99,Cha00]. Several methods will be suggested in which the model predictions may be coupled with the measured partial cross sections to extract a (n,2n) channel cross section. A separate appendix will present the results from the three separate experiments individually.&#171; less