First experimental prompt $\gamma$-ray spectra in fast-neutron-induced fission of ${}^{238}\mathrm{U}$

The knowledge of prompt fission $\gamma$-ray emission has been of major interest in reactor physics for a few years. Only few experimental spectra were published until now for fast-neutron-induced fission, and measurements would be also valuable in order to improve our understanding of the fission process. A simple experimental method was used to measure the first prompt fission $\gamma$-ray spectra up to 10 MeV. In this approach, the $\gamma$ rays are measured with a bismuth germanate (BGO) detector which offers two significant advantages with respect to other $\gamma$-ray detectors: a high peak-to-total ratio and a high efficiency. The prompt fission neutrons are rejected by the time-of-flight technique between the BGO detector and a fission trigger given by a fission chamber. Prompt fission $\gamma$-ray spectra were measured for 1.6 ± 0.1, 5.1 ± 0.2 and 15.0 ± 0.6 MeV neutron-induced fission on ${}^{238}\mathrm{U}$ at the CEA, DAM, DIF Van de Graaff accelerator; average multiplicity and mean photon energy per fission were deduced from the spectra.

Figure 1

Peak to total ratio P/T for different kinds of cylindrical $\gamma$-ray detectors (2 in. × 2 in.) as a function of the incident $\gamma$-ray energy (GEANT 3.21 calculation). The density ρ is 3.67 for NaI, 4.88 for $\mathrm{Ba}{\mathrm{F}}_2$, 5.08 for $\mathrm{LaB}{\mathrm{r}}_3$, 5.23 for Ge, and 7.13 for BGO. The effective atomic number $Z_{\mathrm{eff}}$ is 32 for Ge, 40.5 for $\mathrm{LaB}{\mathrm{r}}_3$, 50 for NaI, 52 for NaI, and 75 for BGO. The P/T for the 3 in. × 6 in. BGO detector used in the experiments is also plotted.

Figure 2

3 in. × 6 in. (diameter × length) BGO detector full-energy peak efficiency at a distance of 90 cm.

Figure 3

Time-of-flight spectrum between the fission chamber and the BGO detector for 1.6 ± 0.1 MeV incident neutrons in the second experiment, for $\gamma$-ray energies greater than 1 MeV. Typical TOF cuts applied to extract the (signal + background) and the background $\gamma$-ray events are shown by vertical lines.

Figure 4

Folded (triangles) and unfolded (squares) spectra for the four measurements: (a) 1.6 ± 0.1 MeV and (d) 15.0 ± 0.6 MeV incident neutron for the first experiment, (b) 1.6 ± 0.1 MeV, and (c) 5.1 ± 0.2 MeV for the second experiment.

Figure 5

Experimental PFGS in the fission of ${}^{238}\mathrm{U}$ induced by 1.6- and 5.1-MeV incident neutrons (dots). Comparison with the one of Verbinski et al. [4] in thermal fission of ${}^{235}\mathrm{U}$ (line).

Figure 6

Experimental PFGS in the fission of ${}^{238}\mathrm{U}$ induced by 15.0-MeV incident neutrons, compared to the one obtained with 1.6-MeV incident neutrons. In addition, PFGS from Ref. [20] for the system $n+{}^{235}\mathrm{U}$ for similar incident neutron energies are shown for comparison.

Figure 7

Comparison of the experimental PFGS (dots) to predictions (lines) from the gef code (release of January 2015, version 1.1) [32] and the fifrelin one [34].