New prompt fission neutron spectra measurements in the²³⁸U(n,f) reaction with a dedicated setup at LANSCE/WNR

Abstract

A new prompt fission neutron spectra (PFNS) measurement in the 238 U(n,f) reaction was performed at LANSCE/WNR facility. Evaluated data show discrepancies on the low (below 1 MeV) and high (above 5 MeV) energy parts in the PFNS for different major and minor actinides. The goal is to improve these measurements in a wide range of incident energy. The energy of the incoming neutron, inducing the fission, and the prompt neutron energies, are measured by time-of-flight method. A dedicated fission chamber was developed, in order to improve alpha-fission discrimination, timing resolution, actinide mass, and to reduce the amount of neutron scattering. To detect prompt neutrons, the 54 Chi-Nu scintillator cells array were surrounding the fission chamber. High statistics were recorded during this experiment, allowing a precise study of PFNS behavior as a function of incident neutron energy, from 1 MeV to 200 MeV. This experiment also showed that all the new tools developed to improve PFNS measurements are performing. Therefore, measurements of PFNS with others actinides such as 239 Pu are planned.

Prompt Fission Neutron Spectra for Neutron-Induced Fission of 239Pu and 235U

“…The outgoing neutrons are then detected in one of two arrays: an array of 22 Li glass scintillation ("'Li-glass") detectors used for measuring the low energy region of the PFNS (10 keV -1.5 MeV) and, in separate experiments, an array of 54 liquid organic scintillators (EJ309) for the high energy region (850 keV and above). Twenty-one of the Li glass detectors are enriched with 6 Li, while one is enriched with 7 Li. Each set of detectors is held in place by a rigid frame, with the front of the Li glass detectors at a distance of 40 cm from the center of the PPAC, and the liquid scintillators at a distance of 1.0 m from the center.…”

“…Early attempts at such measurements were necessarily primitive, but interest in these spectra was such that the first numbered report from the then secret program at Los Alamos was titled A Discussion of the Fission Neutron Spectrum [2]; this document is no longer classified. More recently, there has been a renewed interest in improved PFNS data, with numerous experimental efforts (see, for example, [3][4][5][6][7][8][9][10][11]) and related evaluations [12][13][14] for neutron-induced fission of the major actinides. Improved data on PFNS are needed, particularly for fast neutrons, to improve nuclear data libraries [15,16] used in neutronics calculations for fast systems.…”