Analysis of the Daya Bay Reactor Antineutrino Flux Changes with Fuel Burnup

We investigate the recent Daya Bay results on the changes in the antineutrino flux and spectrum with the burnup of the reactor fuel. We find that the discrepancy between current model predictions and the Daya Bay results can be traced to the original measured ${}^{235}\mathrm{U}/{}^{239}\mathrm{Pu}$ ratio of the fission $\beta$ spectra that were used as a base for the expected antineutrino fluxes. An analysis of the antineutrino spectra that is based on a summation over all fission fragment $\beta$ decays, using nuclear database input, explains all of the features seen in the Daya Bay evolution data. However, this summation method still allows for an anomaly. We conclude that there is currently not enough information to use the antineutrino flux changes to rule out the possible existence of sterile neutrinos.

Figure 1

(top) The ratio of the ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ $\beta$ spectra. The data are from [6], and the curves are the ratios obtained by fitting the individual ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ data, using different assumptions. The different assumptions are explained in the text. Excellent fits were obtained in all cases. (bottom) The ratio of the antineutrino spectra resulting from the fits. We note that the jagged structures largely reflect the fact that the fits only require about 25 endpoints; these effects are normally smoothed in published expected spectra.

Figure 2

The ratio of the ${}^{235}\mathrm{U}$ to ${}^{239}\mathrm{Pu}$ aggregate $\beta$ spectra as a function of the kinetic energy of the electron, for the Schreckenbach et al. [6] measurement (squares), and the summation method (curve).

Figure 3

The IBD yield per fission as a function of the fraction of fissions from ${}^{239}\mathrm{Pu}$. The data are from Daya Bay [1], while the straight (dashed) curves are the absolute (renormalized) predictions from the summation calculations. The slope of the summation predictions for the change in the IBD yield with ${\mathrm{F}}_{239}$ is in agreement with experiment, but the absolute value of the predicted IBD yield is 3.5% high.

Figure 4

The variation of the IBD yield in four prompt energy ranges. The data are from [1], while the straight lines are the predictions of the summation method. The summation predictions for ${\overline{S}}^{-1}d{S}_j/d{F}_{239}$ are ($-0.143$, $-0.273$, $-0.521$, $-0.678$) for $j=1–4$, to be compared with the experimental values of ($-0.16\pm 0.07$, $-0.23\pm 0.04$, $-0.49\pm 0.05$, $-0.69\pm 0.12$).

Figure 5

The variation of the IBD yield for different prompt energy ranges. The data are from [1], the solid line is the prediction of the summation method, while the dashed line is obtained from converting the ILL data to antineutrino spectra and using the database for ${}^{238}\mathrm{U}$.