DOE OSTI2012
Knowledge of the prompt neutron output from neutron-induced fission of 239 Pu is essential to nuclear defense and energy applications. Because the experimental data base is inadequate to constrain models of the spectrum of neutrons from fission, we have embarked on a multi-year program of new measurements of the prompt fission neutron spectrum as a function of the energy of the incident neutron that induces the fission. This is a joint LANL-LLNL project with participation by four university groups in advanced detector development. In the past year much progress has been achieved. The building housing the experiment has been completed and the flight paths including one for this experiment are now constructed. A parallel-plate avalanche counter (PPAC) containing 235 U is installed and working in the beam line. Neutron detectors for both low (<1 MeV) and high (> 0.5 MeV) neutrons have been assembled. The data acquisition system based on waveform digitizers has undergone extensive development and now can handle 10 PPAC channels and 10 neutron detectors simultaneously. In addition, a detector test station centered on a 252 Cf spontaneous fission chamber has been commissioned so that we can characterize our neutron detectors in a well understood experimental environment. Analysis of experiments at this facility has helped us understand the performance of several types of neutron detectors. In addition, the analysis work uncovered that the “standard” 252 Cf fission neutron spectrum in MCNPX disagrees significantly from that presently accepted by the international standards community. One year ago we submitted a report (LA-UR-11-05576) on the accuracies that could be reached under certain assumptions. Several sources of possible systematic uncertainties were identified, and procedures to control these uncertainties were suggested. This report is included as an appendix to this report. One element missing from these estimates was the contribution of backgrounds. It could not be quantified because, at that time, the building and flight path for the experiments had not been constructed. We now have made some measurements of the background and can now add this component to the accuracy projections. This report summarizes the progress in the past year. It also discusses measurements reported in the literature and some factors that should be considered when using these data to optimize parameters in nuclear model codes used in constructing evaluated nuclear data bases.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗