Evolution of the Reactor Antineutrino Flux and Spectrum at Daya Bay

Abstract

The Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 GW$_{\textrm{th}}$ reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective $^{239}$Pu fission fractions, $F_{239}$, from 0.25 to 0.35, Daya Bay measures an average IBD yield, $\bar{\sigma}_f$, of $(5.90 \pm 0.13) \times 10^{-43}$ cm$^2$/fission and a fuel-dependent variation in the IBD yield, $d\sigma_f/dF_{239}$, of $(-1.86 \pm 0.18) \times 10^{-43}$ cm$^2$/fission. This observation rejects the hypothesis of a constant antineutrino flux as a function of the $^{239}$Pu fission fraction at 10 standard deviations. The variation in IBD yield was found to be energy-dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at 3.1$\sigma$. This discrepancy indicates that an overall deficit in measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes $^{235}$U, $^{239}$Pu, $^{238}$U, and $^{241}$Pu. Based on measured IBD yield variations, yields of $(6.17 \pm 0.17)$ and $(4.27 \pm 0.26) \times 10^{-43}$ cm$^2$/fission have been determined for the two dominant fission parent isotopes $^{235}$U and $^{239}$Pu. A 7.8% discrepancy between the observed and predicted $^{235}$U yield suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.Comment: 7 pages, 5 figure

Reactor fuel fraction information on the antineutrino anomaly

“…[11], in section 4 we perform the combined analysis, and in section 5 we draw our conclusions. Table 1.…”

“…However, this is not possible in practice because the data do not constrain them in a sufficient way. Indeed, also the Daya Bay collaboration [11] constrained σ f,238 and σ f,241 around the theoretical values, albeit with an arbitrary 10% uncertainty. We adopt the theoretical uncertainties because they are well defined and motivated, and it is possible that the corresponding predictions are correct, whereas those of σ f,235 and σ f,239 are not.…”

“…In this paper we present the results of analyses of the Daya Bay evolution data [11] with least-squares functions that take into account explicitly the uncertainties of the theoretical calculation of the four cross sections per fission. Moreover, we consider additional models with independent variations of the 235 U and 239 Pu fluxes with and without active-sterile neutrino oscillations, and we extend the analysis taking into account also the information on the cross sections per fission of all the other reactor antineutrino experiments which have different fuel fractions.…”

“…The Daya Bay collaboration presented in figure 2 of ref. [11] the values of σ f for eight values of the effective 239 Pu fission fraction F 239 . They fitted these data allowing variations of the two main cross sections per fission σ f,235 and σ f,239 , with the assumption that σ f,238 and σ f,241 have the Saclay+Huber theoretical values [4,5,9] with enlarged 10% uncertainties.…”

“…The Daya Bay collaboration presented recently [11] the results of the measurement of the correlation between the reactor fuel evolution and the changes in the antineutrino detection rate in the Daya Bay experiment, which detectsν e 's produced by two complexes of power reactors at distances of about 360 m and 500 m. Theν e detection rate is quantified by the cross section per fission σ f , given by…”