Final Measurement of the ${}^{235}\mathrm{U}$ Antineutrino Energy Spectrum with the PROSPECT-I Detector at HFIR

This Letter reports one of the most precise measurements to date of the antineutrino spectrum from a purely ${}^{235}\mathrm{U}$-fueled reactor, made with the final dataset from the PROSPECT-I detector at the High Flux Isotope Reactor. By extracting information from previously unused detector segments, this analysis effectively doubles the statistics of the previous PROSPECT measurement. The reconstructed energy spectrum is unfolded into antineutrino energy and compared with both the Huber-Mueller model and a spectrum from a commercial reactor burning multiple fuel isotopes. A local excess over the model is observed in the 5–7 MeV energy region. Comparison of the PROSPECT results with those from commercial reactors provides new constraints on the origin of this excess, disfavoring at 2.0 and 3.7 standard deviations the hypotheses that antineutrinos from ${}^{235}\mathrm{U}$ are solely responsible and noncontributors to the excess observed at commercial reactors, respectively.

Figure 1

Time evolution of DE and SE segment numbers with reactor-on (RxOn) and reactor-off (RxOff) running, and definition of the five DS periods. The number of DE segments can be seen to decrease over time, while the sum of DE and SE segments decreases slightly. The DE and SE segment numbers at the end of each DS period define the detector configuration used.

Figure 2

First panel: Solid blue and red lines represent total reactor-on (RxOn) and -off (RxOff) prompt spectra, respectively, for all periods, while the point distribution is the result of their subtraction. Colored areas represent measured IBD prompt spectra for each period. Second to sixth panels: Point distributions represent the ratio between periods 1–5 and the total spectrum normalized to the integrated counts of each period, while the dotted line displays the expected behavior according to Monte Carlo simulations of the different detector configurations.

Figure 3

Unfolded antineutrino spectrum from HEU reactor at HFIR. First panel: Points represent PROSPECT’s unfolded antineutrino spectrum; the magenta line depicts HM ${}^{235}\mathrm{U}$ prediction; and the gray shaded histogram corresponds to the reactor nonfuel and nonequilibrium contributions, which have been already subtracted from the data. The transformed covariance matrix is also displayed. Second panel: data to model ratio together with the Gaussian function $G_{\mathrm{PRO}}(A·r,\mu ,\sigma )$ obtained from the simultaneous fit of PROSPECT’s and DYB’s unfolded spectra. Third panel: the local $p$ value obtained from a 1.25-MeV-wide sliding windows analysis.

Figure 4

$\mathrm{\Delta }{\chi }^2$ distributions for a Gaussian-like excess simultaneous fit of PROSPECT’s and DYB’s unfolded spectra. Profile from the common fit (solid black line) displays an asymmetric behavior due to the negligible impact of DYB’s spectrum in rejecting the no-${}^{235}\mathrm{U}$ hypothesis. The best-fit value of $r=0.79_{-0.25}^{+0.35}$ is found with a $\mathrm{\Delta }{\chi }^2=13.6(3.9)$ with respect to $r=0(1.77)$, amplitude ratio required to satisfy no-${}^{235}\mathrm{U}$ (all-${}^{235}\mathrm{U}$) hypothesis and represented by the magenta (red) line. The green dotted line represents the equal-isotope hypothesis, i.e., $r=1$, which crosses the profile at $\mathrm{\Delta }{\chi }^2=0.4$. When DYB’s uncertainties are largely downscaled (dotted black line), $\mathrm{\Delta }{\chi }^2$ becomes symmetric.