Measurement of the ${}^{238}\mathrm{U}(n,f)$ prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons with 1.5–20 MeV energy

With the recent emergence of fast nuclear reactors, there has been a corresponding increasing interest in ${}^{238}\mathrm{U}$-related nuclear data. However, while existing literature data span much of the energy ranges of interest for the prompt fission neutron spectrum (PFNS) for neutron-induced fission of ${}^{238}\mathrm{U}$, most literature data sets are highly correlated, and thus new, independent measurements of this quantity are needed. In this work, we report the results of a new measurement of the ${}^{238}\mathrm{U}$ PFNS at the Los Alamos Neutron Science Center for incident neutron energies from 1.5–20.0 MeV, and outgoing neutron energies of 0.01–10.0 MeV. With some notable exceptions, the present results generally agree with existing literature data, especially with regard to features relating to multichance fission and pre-equilibrium features in the PFNS, thus adding confidence to existing nuclear data evaluations and filling in gaps of knowledge at previously unmeasured incident neutron energies. This result is the third in a series of PFNS measurements by the Chi-Nu collaboration now spanning all three major actinides, ${}^{239}\mathrm{Pu}$, ${}^{235}\mathrm{U}$, and ${}^{238}\mathrm{U}$. Thus, for the first time, we report reliable experimental PFNS ratios and average PFNS energy comparisons for measurements of all three of these isotopes including accurate correlations between the different, but correlated experiments.

Figure 1

An example spectrum of outgoing versus incident neutron energy for the liquid scintillator data obtained for this work summed over all detection angles, and after random-coincidence background subtraction. The neutron energy on the $y$ axis is the measured outgoing neutron energy from time of flight, before conversion to a PFNS result, and counts along the $z$ axis refer to counts in each logarithmically spaced bin before corrections for varying bin size. See the text for a discussion of features in this spectrum.

Figure 2

The relative uncertainty from all non-negligible sources of uncertainty in the present data set for the final PFNS results from (a) $E_n^{\mathrm{inc}}=2.0$–3.0 MeV, (b) 8.0–9.0, and (c) 12.0–13.0 MeV. These $E_n^{\mathrm{inc}}$ ranges correspond to one range present in each of Secs. 4a, 4b, and 4c.

Figure 3

The present results for Li-glass (black diamonds) and liquid scintillator (red circles) data are shown for (a) $E_n^{\mathrm{inc}}=1.5$–2.0, (b) 2.0–3.0, (c) 3.0–4.0, and (d) 4.0–5.0 MeV, corresponding to average incident neutron energy, $\langle E_n^{\mathrm{inc}}\rangle$, values of 1.77, 2.51, 3.51, and 4.50 MeV, respectively. ENDF/B-VIII.0 [3] evaluations are shown as solid and dashed blue lines, JEFF-3.3 [45] as the dash-dotted red lines, and JENDL-5.0 [46] as dotted green lines. The shaded blue region corresponds to the uncertainty of the lower $E_n^{\mathrm{inc}}$ ENDF/BB-VIII.0 evaluation on each plot. All other evaluation uncertainties can be assumed to be similar. Literature data are plotted as compiled in EXFOR [47]. All data are shown as a ratio to a 1.32 MeV Maxwellian distribution.

Figure 4

The present results for Li-glass (black diamonds) and liquid scintillator (red circles) data are shown for (a) $E_n^{\mathrm{inc}}=5.0$–5.5, (b) 5.5–6.0, (c) 6.0–7.0, (d) 7.0–8.0 MeV, (e) 8.0–9.0, and (f) 9.0–10.0 MeV, corresponding to average incident neutron energy, $\langle E_n^{\mathrm{inc}}\rangle$, values of 5.25, 5.76, 6.54, 7.50, 8.50, and 9.49 MeV, respectively. ENDF/B-VIII.0 [3] evaluations are shown as solid and dashed blue lines, JEFF-3.3 [45] as the dash-dotted red lines, and JENDL-5.0 [46] as dotted green lines. The shaded blue region corresponds to the uncertainty of the lower $E_n^{\mathrm{inc}}$ ENDF/BB-VIII.0 evaluation on each plot. All other evaluation uncertainties can be assumed to be similar. Literature data are plotted as compiled in EXFOR [47]. All data are shown as a ratio to a 1.32 MeV Maxwellian distribution.

Figure 5

The present results for Li-glass (black diamonds) and liquid scintillator (red circles) data are shown for (a) $E_n^{\mathrm{inc}}=10.0$–11.0, (b) 11.0–12.0, (c) 12.0–13.0, (d) 13.0–14.0, and (e) 14.0–15.0 MeV, corresponding to average incident neutron energy, $\langle E_n^{\mathrm{inc}}\rangle$, values of 10.50, 11.49, 12.51, 13.51, and 14.51 MeV, respectively. ENDF/B-VIII.0 [3] evaluations are shown as solid and dashed blue lines, JEFF-3.3 [45] as the dash-dotted red lines, and JENDL-5.0 [46] as dotted green lines. The shaded blue region corresponds to the uncertainty of the lower $E_n^{\mathrm{inc}}$ ENDF/BB-VIII.0 evaluation on each plot. All other evaluation uncertainties can be assumed to be similar. Literature data are plotted as compiled in EXFOR [47]. All data are shown as a ratio to a 1.32 MeV Maxwellian distribution.

Figure 6

The present results for Li-glass (black diamonds) and liquid scintillator (red circles) data are shown for (a) $E_n^{\mathrm{inc}}=15.0$–16.0, (b) 16.0–17.0, (c) 17.0–18.0, (d) 18.0–19.0, and (e) 19.0–20.0 MeV, corresponding to average incident neutron energy, $\langle E_n^{\mathrm{inc}}\rangle$, values of 15.51, 16.49, 17.52, 18.49, and 19.51 MeV, respectively. ENDF/B-VIII.0 [3] evaluations are shown as solid and dashed blue lines, JEFF-3.3 [45] as the dash-dotted red lines, and JENDL-5.0 [46] as dotted green lines. The shaded blue region corresponds to the uncertainty of the lower $E_n^{\mathrm{inc}}$ ENDF/BB-VIII.0 evaluation on each plot. All other evaluation uncertainties can be assumed to be similar. Literature data are plotted as compiled in EXFOR [47]. All data are shown as a ratio to a 1.32 MeV Maxwellian distribution.

Figure 7

Mean PFNS energies are a function of $E_n^{\mathrm{inc}}$ are shown in the top panel with the total (black) and statistical (red) uncertainties of the data reported here. ENDF/B-VIII.0, JEFF-3.3, and JENDL-5.0 mean energies are shown as the solid green, dashed red, and dotted blue lines, respectively. The trends of total (solid black) and statistical (dashed red) uncertainties of the present results are shown in the bottom panel.

Figure 8

Comparison of mean PFNS energies from the present results (black diamonds) and those of Ethvignot et al. [16] (open, red circles) integrated over the range $E_n^{\mathrm{out}}=0.65$–7.5 MeV. Statistical uncertainties are shown for each experiment. See the text for a discussion.

Figure 9

${}^{235}\mathrm{U}$/${}^{238}\mathrm{U}$ PFNS ratios for $E_n^{\mathrm{inc}}$ = (a) 2.0–3.0, (b) 7.0–8.0, and (c) 14.0–15.0 MeV. The ${}^{239}\mathrm{Pu}$/${}^{238}\mathrm{U}$ ratios for the same $E_n^{\mathrm{inc}}$ values are shown (d), (e), and (f), and those for the ${}^{239}\mathrm{Pu}$/${}^{235}\mathrm{U}$ ratio are shown in (g), (h), and (i), respectively. ENDF/B-VIII.0 evaluation comparisons are shown as the solid red lines, with uncertainties shown as the shaded regions.

Figure 10

Mean PFNS energy results from Chi-Nu measurements of the PFNS of ${}^{239}\mathrm{Pu}$ (black diamonds), ${}^{235}\mathrm{U}$ (red circles), and ${}^{238}\mathrm{U}$ (blue squares) are shown here with ENDF/B-VIII.0 results for each of these nuclei shown as the black dotted, red dashed, and blue dash-dotted lines, respectively. Uncertainties shown here are the total uncertainty of each measurement. The ${}^{238}\mathrm{U}$ data have been shifted up by 0.2 MeV in incident energy to allow the differences between these data and the ${}^{235}\mathrm{U}$ results to be discerned more clearly.

Figure 11

The mean energy of Chi-Nu results of the PFNS of ${}^{238}\mathrm{U}$ (left side of figure), ${}^{235}\mathrm{U}$ (middle), and ${}^{239}\mathrm{Pu}$ (right) versus the nuclear $Z^2/A$ parameter for incident energy ranges of $E_n^{\mathrm{inc}}=2.0$–3.0 (black diamonds), 7.0–8.0 (green squares), 13.0–14.0 (open, red circles), and 19.0–20.0 MeV (blue stars). Data points from the same $E_n^{\mathrm{inc}}$ range are connected with dashed lines of the same color to guide the eye. The $E_n^{\mathrm{inc}}=13.0$–14.0 MeV data points for ${}^{238}\mathrm{U}$ and ${}^{235}\mathrm{U}$ have been shifted by 0.03 along the $x$ axis to avoid overlap with other data. Results from the CGMF [40] code are also shown as the shaded regions of the same color as the Chi-Nu data. The range of the CGMF calculations correspond to the $E_n^{\mathrm{inc}}$ ranges shown in the legend.