Extraction of the ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ Antineutrino Spectra at Daya Bay

This Letter reports the first extraction of individual antineutrino spectra from ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ fission and an improved measurement of the prompt energy spectrum of reactor antineutrinos at Daya Bay. The analysis uses $3.5\times {10}^6$ inverse beta-decay candidates in four near antineutrino detectors in 1958 days. The individual antineutrino spectra of the two dominant isotopes, ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$, are extracted using the evolution of the prompt spectrum as a function of the isotope fission fractions. In the energy window of 4–6 MeV, a 7% (9%) excess of events is observed for the ${}^{235}\mathrm{U}$ (${}^{239}\mathrm{Pu}$) spectrum compared with the normalized Huber-Mueller model prediction. The significance of discrepancy is $4.0\sigma$ for ${}^{235}\mathrm{U}$ spectral shape compared with the Huber-Mueller model prediction. The shape of the measured inverse beta-decay prompt energy spectrum disagrees with the prediction of the Huber-Mueller model at $5.3\sigma$. In the energy range of 4–6 MeV, a maximal local discrepancy of $6.3\sigma$ is observed.

Figure 1

The weekly fission fractions for the four major isotopes in the six reactors in 1958 days including four to six refueling cycles for each. The solid line represents an approximately linear relation between fission fractions of ${}^{239}\mathrm{Pu}$ and ${}^{241}\mathrm{Pu}$.

Figure 2

(Top) Comparison of the extracted ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ spectra and the corresponding Huber-Mueller model predictions with the normalization factors 0.92 and 0.99, respectively. The error bars in the data points are the square root of the diagonal terms of the covariance matrix of the extracted spectra. The error bands are the uncertainties from the Huber-Mueller model. (Middle) Ratio of the extracted spectra to the predicted spectra. The ${}^{239}\mathrm{Pu}$ data points are displaced for visual clarity of error bars. (Bottom) Local significance of the shape deviations for the extracted ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$ spectra compared to the model predictions.

Figure 3

(Top) Comparison of the extracted ${}^{235}\mathrm{U}$ spectrum and $s_{\text{combo}}$ as a combination of ${}^{239}\mathrm{Pu}$ and ${}^{241}\mathrm{Pu}$ with the corresponding Huber-Mueller predicted spectra with the normalization factors 0.92 and 0.99. (Bottom) The fractional size of the diagonal elements of the covariance matrix for extracted spectra with and without the combination of ${}^{239}\mathrm{Pu}$ and ${}^{241}\mathrm{Pu}$.

Figure 4

(Top) Predicted and measured prompt energy spectra. The prediction is based on the Huber-Mueller model and is normalized to the number of measured events. The blue and red filled bands represent the square root of diagonal elements of the covariance matrix for the flux prediction and the full systematic uncertainties, respectively. (Middle) Ratio of the measured prompt energy spectrum and the normalized predicted spectrum. The error bars on the data points represent the statistical uncertainty. (Bottom) The local significance of the shape deviation in a sliding 2-MeV window showing a maximum $6.3\sigma$ discrepancy in 4–6 MeV.