Monoenergetic photon-induced fission cross-section ratio measurements for ${}^{235}\mathrm{U}$, ${}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ from 9.0 to 17.0 MeV

Background: High-accuracy data are needed to advance microscopic descriptions of the nuclear fission process, to improve the predictive precision of phenomenological models, and for applications in nuclear energy and homeland security.

Purpose: The main goal of this work is to provide high-accuracy cross-section data for photofission of ${}^{235}\mathrm{U}$, ${}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ in the energy range from the fission threshold to the high-energy tail of the giant dipole resonance. These new data should contribute significantly to the reduction of the systematic uncertainty in evaluated photofission cross-section databases.

Method: Cross-section ratios for photofission of the “big three” actinide nuclei were measured using a quasimonoenergetic photon beam and dual-fission chambers. The measurements were performed at the $\mathrm{HI}\gamma \mathrm{S}$ facility using a photon beam produced from Compton backscattering of free-electron laser light. The dual-fission chamber enabled simultaneous counting of fission events from two targets. This method allows for cross-section ratio measurements with very small systematic errors. The largest source of systematic error in the determination of the cross-section ratios is the uncertainty in the ratio of the target thicknesses.

Results: We report photon-induced fission cross-section ratios for $\sigma [{}^{235}\mathrm{U}(\gamma ,f)/{}^{238}\mathrm{U}(\gamma ,f)]$, $\sigma [{}^{235}\mathrm{U}(\gamma ,f)/{}^{239}\mathrm{Pu}(\gamma ,f)]$, and $\sigma [{}^{238}\mathrm{U}(\gamma ,f)/{}^{239}\mathrm{Pu}(\gamma ,f)]$ at photon energies from 9.0–17.0 MeV. More than 20 data points were measured for each ratio, and the systematic uncertainties are less than $2\%$.

Conclusion: The present photofission cross-section ratio data sets are compared to ratios computed from previous measurements, to the corresponding neutron-induced fission cross-section ratios, and to ratios computed from evaluated databases. The data obtained in this work for $\sigma [{}^{235}\mathrm{U}(\gamma ,f)/{}^{238}\mathrm{U}(\gamma ,f)]$ are consistent with the existing data, while for $\sigma [{}^{235}\mathrm{U}(\gamma ,f)/{}^{239}\mathrm{Pu}(\gamma ,f)]$ and $\sigma [{}^{238}\mathrm{U}(\gamma ,f)/{}^{239}\mathrm{Pu}(\gamma ,f)]$ the present data are systematically lower.

Figure 1

Schematic diagram (not to scale) of the experimental setup using a dual-fission chamber and a $123\%$ efficient (relative) HPGe detector. In the present photofission measurement, the thick activation target was not used (shown by the dashed-line at the center of the fission chamber).

Figure 2

$\gamma$-ray spectrum (in red) measured with a $123\%$ HPGe detector positioned at $0^{\circ }$ relative to the incident photon beam of 12.25 MeV. The full-energy peak (FEP), single escape (SE), and double escape (DE) are labeled. In the low-energy part of the spectrum, room background lines (indicated by the asterisk) are clearly seen. Taking the HPGe detector response into account, the unfolded spectrum of the FEP (shown in blue) is obtained.

Figure 3

Fission chamber pulse-height spectra for the downstream (FC1) and upstream (FC2) chambers are shown for 11.2 MeV incident photon beam. These chambers were loaded with ${}^{238}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ reference foils, respectively.

Figure 4

Experimental fission cross-section ratios for ${}^{235}\mathrm{U}/{}^{238}\mathrm{U}$ are compared with previous measurements [14, 15, 18, 22, 24, 26, 28, 39] and the ENDF/B-VII.1 [30] and JENDL/PD-2004 [31] evaluations, and the TENDL-2009 [30, 31] calculation. The error bars on our data points represent the total experimental uncertainties.

Figure 5

Experimental fission cross-section ratios for ${}^{235}\mathrm{U}/{}^{239}\mathrm{Pu}$ are compared with previous measurements [15, 16, 18, 22, 24, 26, 28, 39] and the ENDF/B-VII.1 [30] evaluation, and the TENDL-2009 [30, 31] calculation. The error bars on our data points represent the total experimental uncertainties.

Figure 6

Experimental fission cross-section ratios for ${}^{238}\mathrm{U}/{}^{239}\mathrm{Pu}$ are compared with previous measurements [14, 15, 18, 22, 24, 26, 28, 39] and the ENDF/B-VII.1 [30] evaluation and the TENDL-2009 [30, 31] calculation. The error bars on our data points represent the total experimental uncertainties.

Figure 7

Experimental fission cross-section ratios for ${}^{235}\mathrm{U}/{}^{238}\mathrm{U}$ are compared with the previous measurements [15, 39] and the evaluations ENDF/B-VII.1 [30], JENDL/PD-2004 [31], and the TENDL-2009 [30, 31] calculation. The error bars on our data points represent the total experimental uncertainties. Also shown (in the bottom panel) are the differences (in $\%$) between the ENDF/B-VII.1 evaluation and our present data. The relative difference in our data and the ENDF/B-VII.1 evaluation range from $0\%$ to about $16\%$ (the average deviation is $\sim 6.8\%$) with our data constantly higher than the evaluation values.

Figure 8

Experimental fission cross-section ratios for ${}^{235}\mathrm{U}/{}^{239}\mathrm{Pu}$ are compared with the previous measurements [15, 16] and the evaluation ENDF/B-VII.1 [30] and the TENDL-2009 [30, 31] calculation. The error bars on our data points represent the total experimental uncertainties. Also shown (in the bottom panel) are the differences (in $\%$) between the ENDF/B-VII.1 evaluation and our present data. The relative difference in our data and the ENDF/B-VII.1 evaluation range up to $32\%$ (the average deviation is $\sim 16\%$) with our data constantly lower than the evaluation values.

Figure 9

Experimental fission cross-section ratios for ${}^{238}\mathrm{U}/{}^{239}\mathrm{Pu}$ are compared with previous measurements [15, 16] and the evaluation ENDF/B-VII.1 [30] and the TENDL-2009 [30, 31] calculation. The error bars on our data points represent the total experimental uncertainties. Also shown (in the bottom panel) are the differences (in $\%$) between the ENDF/B-VII.1 evaluation and our present data. The relative difference in our data and the ENDF/B-VII.1 evaluation range from 0 to $70\%$ (the average deviation is $\sim 25\%$) with our data constantly much lower than the evaluation values, except for the energy regime around 12 MeV.

Figure 10

Present photon-induced cross-section ratio data are compared to their neutron-induced counterparts [42, 43, 44].