The LANL/LLNL Prompt Fission Neutron Spectrum Program at LANSCE and Approach to Uncertainties

Abstract

New data on the prompt fission neutron spectra (PFNS) from neutron-induced fission with higher accuracies are needed to resolve discrepancies in the literature and to address gaps in the experimental data. The Chi-Nu project, conducted jointly by LANL and LLNL, aims to measure the shape of the PFNS for fission of 239 Pu induced by neutrons from 0.5 to 20 MeV with accuracies of 3-5% in the outgoing energy from 0.1 to 9 MeV and 15% from 9 to 12 MeV and to provide detailed experimental uncertainties. Neutrons from the WNR/LANSCE neutron source are being used to induce fission in a Parallel-Plate Avalanche Counter (PPAC). Two arrays of neutron detectors are used to cover the energy range of neutrons emitted promptly in the fission process. Challenges for the present experiment include background reduction, use of 239 Pu in a PPAC, and understanding neutron detector response. Achieving the target accuracies requires the understanding of many systematic uncertainties. The status and plans for the future will be presented.

Evaluation of the 235U prompt fission neutron spectrum including a detailed analysis of experimental data and improved model information

“…The data of Lestone et al [14] and Knitter et al [15] agree similarly well with all three evaluations. However, the preliminary Chi-Nu data [27]-which are not included in any of these evaluations-support a PFNS softer than VII.1. The higher evaluated PFNS at low E is partially caused by extension made to the LAM as shown in Ref.…”

“…The peak from E =6-8 MeV at E inc = 14 MeV is caused by neutrons emitted in a pre-equilibrium process, which are considered in this evaluation and JENDL-4.0 but not in VII.1. The increase of the PFNS around a few [8-11, 14, 15], preliminary Chi-Nu data [27] and evaluations of [2][3][4][5][6] for E inc = thermal, 1.5, 6 and 14 MeV. Experimental data are scaled with respect to this work.…”

The 235U prompt fission neutron spectrum measured by the Chi-Nu project at LANSCE

“…Detection of a fission fragment in a PPAC cell also provides a start signal for the TOF measurement of the outgoing fission neutrons. Those neutrons are then detected in one of two arrays: an array of 21- 6 Li Glass scintillation detectors (LiGl) used for measuring the low energy region of the PFNS (10 keV -2.5 MeV), and an array of 54-liquid organic scintillators for the high energy region (500 keV -15 MeV). Both sets of detectors are held in place by a rigid frame consisting primarily of 1.27 cm thick aluminum plates; with the face of the LiGl detectors being held 40 cm away from the PPAC and the liquid detectors resting 1.0 m away from the PPAC.…”

“…It is a joint effort by Los Alamos National Laboratory (LANL), and Lawerence Livermore National Laboratory (LLNL). Previous descriptions of the Chi-Nu experiment and its progress have been reported in various publications [6][7][8][9][10]. a e-mail: Jaime.Gomez@lanl.gov b e-mail: devlin@lanl.gov…”

Nuclear data activities at the n_TOF facility at CERN

“…Other neutron timeof-flight facilities comprise electron linac-based machines, like GELINA [19,20], IREN [21], KURRI [22], nELBE [23], ORELA (until recently) [24] and PNF [25], RPI [26], and proton-induced spallation targets similar to n TOF, like MLF at J-PARC [27] and LANSCE [28]. All these facilities have their own unique and often complementary characteristics.…”