Dissecting Reactor Antineutrino Flux Calculations

Current predictions for the antineutrino yield and spectra from a nuclear reactor rely on the experimental electron spectra from ${}^{235}\mathrm{U}$, ${}^{239}\mathrm{Pu}$, ${}^{241}\mathrm{Pu}$ and a numerical method to convert these aggregate electron spectra into their corresponding antineutrino ones. In the present work we investigate quantitatively some of the basic assumptions and approximations used in the conversion method, studying first the compatibility between two recent approaches for calculating electron and antineutrino spectra. We then explore different possibilities for the disagreement between the measured Daya Bay and the Huber-Mueller antineutrino spectra, including the ${}^{238}\mathrm{U}$ contribution as well as the effective charge and the allowed shape assumption used in the conversion method. We observe that including a shape correction of about $+6\%{\mathrm{MeV}}^{-1}$ in conversion calculations can better describe the Daya Bay spectrum. Because of a lack of experimental data, this correction cannot be ruled out, concluding that in order to confirm the existence of the reactor neutrino anomaly, or even quantify it, precisely measured electron spectra for about 50 relevant fission products are needed. With the advent of new rare ion facilities, the measurement of shape factors for these nuclides, for many of which precise beta intensity data from TAGS experiments already exist, would be highly desirable.

Figure 1

Daya Bay measured IBD cross section folded antineutrino spectrum divided by the calculated ones using the Huber and Mueller antineutrino spectra (black squares), as calculated in this work using the Huber prescription (blue circles), and as calculated using the Mougeot prescription (red triangles). To facilitate the viewing, uncertainties were not plotted and the Huber (Mougeot) prescription points are shifted to higher (lower) energies by 0.05 MeV.

Figure 2

Antineutrino spectra calculated using the summation method multiplied by the IBD cross section for ${}^{235,238}\mathrm{U}$ and ${}^{239,241}\mathrm{Pu}$ (full lines). The dashed line corresponds to the ${}^{238}\mathrm{U}$ antineutrino spectrum adjusted to match the measured Daya Bay antineutrino spectrum.

Figure 3

Effective $Z$ values as a function of the end-point energy used in the conversion method for ${}^{235}\mathrm{U}$.

Figure 4

Number of $\beta$-minus decaying levels as a function of the antineutrino energy for the Daya Bay reactors, together with the number of levels needed to account for 25%, 50%, 90%, and 99% of the antineutrino spectrum.

Figure 5

(a) Daya Bay spectrum and predictions using the Huber-Mueller spectra and our Mougeot-prescription spectra with and without a linear term ($a_1$) equal to 0.03 for transitions with end-point energy in the 3–6 MeV region. The antineutrino spectra for these $a_1$ values for $Z=45$ and $Q=5\mathrm{MeV}$ are shown in the inset. (b) Ratio of Daya Bay spectrum to Mougeot prescription calculations with $a_1=0.03$. For clarity, calculation uncertainties are not plotted.