Measurement of prompt neutron spectra from the ${}^{239}$Pu($n$,$f$) fission reaction for incident neutron energies from 1 to 200 MeV

Prompt fission neutron spectra in the neutron-induced fission of ${}^{239}$Pu have been measured for incident neutron energies from 1 to 200 MeV at the Los Alamos Neutron Science Center. Mean energies deduced from the prompt fission neutron spectra (PFNS) lead to the observation of the opening of the second chance fission at 7 MeV and to indications for the openings of fission channels of third and fourth chances. Moreover, the general trend of the measured PFNS is well reproduced by the different models. The comparison between data and models presents, however, two discrepancies. First, the prompt neutron mean energy seems constant for neutron energy, at least up to 7 MeV, whereas in the theoretical calculations it is continuously increasing. Second, data disagree with models on the shape of the high energy part of the PFNS, where our data suggest a softer spectrum than the predictions.

Figure 1

Schematic view of the WNR facility at the LANSCE. The FIGARO setup is located on the 30${}^{\circ }$ right flight path.

Figure 2

Experimental setup is as follows: FIGARO neutron detectors surrounding the fission chamber are located on seven different detection angles (45${}^{\circ }$, 60${}^{\circ }$, 75${}^{\circ }$, 90${}^{\circ }$, 105${}^{\circ }$, 112${}^{\circ }$, and 135${}^{\circ }$) with respect to the beam direction.

Figure 3

Example of the scattered neutron component subtraction on one neutron time-of-flight spectrum for all incident neutron energies and after neutron–$\gamma$-ray discrimination: (dashed line) raw time-of-flight spectrum, (dotted line) background contribution from scattered neutrons, (solid line) corrected time-of-flight spectrum. The peak on the left side corresponds to undiscriminated prompt fission $\gamma$ rays, whereas the broad peak is from the selected prompt fission neutrons.

Figure 4

Measured relative efficiency for the total FIGARO detection system.

Figure 5

(Solid circles) PFNS obtained in coincidence with incident neutrons from 1 to 5 MeV. (Dashed line) BRC evaluation of PFNS for an incident neutron of 2.5 MeV. The inset shows the lower part of the same spectrum but on a linear scale.

Figure 6

PFNS for several beam energies (indicated inside the square brackets). Experimental data (points) are compared to BRC calculation (solid line), JEFF 3.1 (dashed line), and ENDF/BVII.1 (dotted line) evaluations.

Figure 7

First spectrum (top, left), measured and evaluated PFNS for a neutron beam energy indicated inside square brackets. All other spectra, ratio of the PFNS obtained for several beam energies (indicated inside the square brackets) over the first PFNS (top left) obtained for a beam energy around 1.5 MeV. Experimental data (points) are compared to BRC calculation (solid line), JEFF 3.1 (dashed line), and ENDF/BVII.1 (dotted line) evaluations.

Figure 8

Evolution of the PFN mean energy with the incident neutron energy. (Dotted line) BRC calculation. (Crosses) Experimental results. (Main frame) Experimental and theoretical results are both calculated within a 400 keV to 7 MeV prompt energy cut. (Inset) Total mean energy extracted from a Maxwellian distribution and compared to BRC calculation without energy cut.

Figure 9

Prompt fission neutron mean energy (calculated within a 400 keV to 7 MeV prompt energy cut) as a function of the neutron beam energy, with larger incident neutron energy ranges than in Fig. 8. (Dotted line) BRC calculation. (Crosses) Experimental results.

Figure 10

BRC calculation, without energy cut. (Dotted line) Mean energy of the neutrons evaporated by the com-pound nuclei before fission. (Dashed lines) Mean energy of the neutrons evaporated by the fully accelerated fission fragments in the laboratory frame (short dashed line above 2 MeV) and in the center of mass frame (long dashed line around 1.5 MeV). (Solid line) Total mean energy in the laboratory frame.

Figure 11

Comparison of PFNS (in logarithmic scale and at incoming neutron energy indicated into square brackets) between our data (solid squares), and BRC (solid line), data from Staples (open circles, [21]), and Lestone (NUEX, solid triangles, [22]).