Excitation energy dependence of prompt fission $\gamma$-ray emission from ${}^{241}\mathrm{Pu}{}^{*}$

Prompt fission $\gamma$ rays (PFGs) resulting from the ${}^{240}\mathrm{Pu}(d,p\mathrm{f})$ reaction have been measured as a function of fissioning nucleus excitation energy $E_{\text{x}}$ at the Oslo Cyclotron Laboratory. We study the average total PFG multiplicity per fission, the average total PFG energy released per fission, and the average PFG energy. No significant changes in these characteristics are observed over the range $5.75<E_{\text{x}}<8.25$ MeV. The physical implications of this result are discussed. The experimental results are compared to simulations conducted using the computational fission model FREYA. We find that FREYA reproduces the experimental PFG characteristics within $8\%$ deviation across the $E_{\text{x}}$ range studied. Previous excitation energy-dependent PFG measurements conducted below the second-chance fission threshold have large uncertainties, but are generally in agreement with our results within a $2\sigma$ confidence interval. However, both a published parametrization of the PFG energy dependence and the most recent PFG evaluation included in ENDF/B-VIII.0 were found to poorly describe the PFG excitation-energy dependence observed in this and previous experiments.

Figure 1

Experimental setup for detecting PFGs from the ${}^{240}\mathrm{Pu}(d,p\mathrm{f})$ reaction. Although only three ${\mathrm{LaBr}}_3$ detectors are depicted, 28 were used in the experiment. Two of the four NIFF counters are illustrated. The figure is not to scale.

Figure 2

Fission-gated spectrum showing the time difference $\mathrm{\Delta }{t}_{{\text{LaBr}}_3}$ between a proton in the $\mathrm{\Delta }E$ detector and $\gamma$ rays in OSCAR, plotted against the $\gamma$-ray energy detected by the ${\mathrm{LaBr}}_3$. The time gates used to distinguish PFGs from PFNs via ToF are shown in black.

Figure 3

Unfolded, background-subtracted coincidence matrix, showing the energies $E_{\gamma }$ of $\gamma$ rays from ($d,p\mathrm{f}\gamma$) events as a function of ${}^{241}\mathrm{Pu}^{*}$ excitation energy $E_{\text{x}}$. The values of the inner and outer fission barriers, $6.14\pm 0.5$ MeV and $5.4\pm 0.5$ MeV, respectively [32], are drawn in black.

Figure 4

Extracted prompt fission $\gamma$-ray spectrum for ${}^{252}\mathrm{Cf}$, compared to the previous measurements of Verbinski et al. [33], Billnert et al. [34], and Oberstedt et al. [35] (marked Q489). The latter two measurements were conducted using ${\mathrm{LaBr}}_3$ detectors. (b) shows the same data as (a) magnified to highlight the low-energy region. The uncertainties on the spectrum in this work are statistical. The deviation at low energies can be explained by the difference in relative time gates, see text.

Figure 5

Extracted, uncorrected prompt fission $\gamma$-ray spectra for ${}^{241}\mathrm{Pu}$ for excitation energies in the range 5.5–8.5 MeV, compared to the FREYA prediction at $E_{\text{x}}=6.75$ MeV. Statistical uncertainties are shown for the experimental spectrum in the excitation energy bin $6.5–7.0$ MeV to increase readability. The statistical uncertainties on the FREYA spectrum are smaller than the marker size for lower energies. (b) shows the same data as in (a), magnified to highlight the low-energy region. We observe that the experimental spectra for different $E_{\text{x}}$ are similar.

Figure 6

Evolution of (a) ${\overline{M}}_{\gamma }$, (b) ${\overline{E}}_{\gamma ,\text{tot}}$, and (c) ${\overline{\varepsilon }}_{\gamma }$ with ${}^{241}\mathrm{Pu}^{*}$ excitation energy. The full red, blue, and green lines show the weighted linear interpolation of the uncorrected and corrected experimental data, and the FREYA results, respectively. The statistical uncertainties on the FREYA values are negligible. The uncertainties on the corrected values are the propagated statistical uncertainties. The uncertainty on ${\overline{\varepsilon }}_{\gamma }$ is calculated by assuming the uncertainties on ${\overline{M}}_{\gamma }$ and ${\overline{E}}_{\gamma ,\text{tot}}$ are independent.