Spectral Structure of Electron Antineutrinos from Nuclear Reactors

Recent measurements of the positron energy spectrum obtained from inverse beta decay interactions of reactor electron antineutrinos show an excess in the 4 to 6 MeV region relative to current predictions. First-principles calculations of fission and beta decay processes within a typical pressurized water reactor core identify prominent fission daughter isotopes as a possible origin for this excess. These calculations also predict percent-level substructures in the antineutrino spectrum due to Coulomb effects in beta decay. Precise measurement of these substructures can elucidate the nuclear processes occurring within reactors. These substructures can be a systematic issue for measurements utilizing the detailed spectral shape.

Figure 1

(a) Ab initio nuclear calculation of the cumulative ${\beta }^{-}$ energy spectrum per fission of ${}^{235}\mathrm{U}$ exposed to thermal neutrons (solid red), including $1\sigma$ uncertainties due to fission yields and branching fractions. The measured ${\beta }^{-}$ spectrum from Ref. [6] is included for reference (dashed blue). (b) Ratios of each spectrum relative to the BILL measurement. (c) The corresponding ${\overline{\nu }}_e$ spectrum per fission in a nominal reactor weighted by the inverse beta decay cross section (solid red), compared with that obtained by the ${\beta }^{-}$ conversion method (dashed blue [12], dotted green [11]), and an alternate ab initio calculation (dash-dotted blue-green [17]). See text for discussion of uncertainties. (d) Ratios of each relative to the Huber calculation. Measurements of the positron spectra (green [23], brown [24], black [25]) are similar to our ab initio calculation, assuming the approximate relation $E_{\overline{\nu }}\simeq E_{e^{+}}+0.8\mathrm{MeV}$. To compare with the calculated spectral shape, measurement normalizations were adjusted approximately $-5\%$.

Figure 2

(a) Calculated reactor ${\overline{\nu }}_e$ spectrum per fission in a nominal nuclear power reactor multiplied by the cross section for inverse beta decay (solid red), in the 5–7 MeV region. The eight most prominent decay branches in this region provide 42% of the total counts (long dashed magenta), and combine to produce a local excess relative to the ${\beta }^{-}$ conversion method (dashed blue [12], dotted green [11]). The remaining $>1100$ decay branches each provide less than 2% of the total rate in this region, and combined provide a smooth shape (dash-dotted black). (b) Individual spectra from the eight most prominent branches. Uncertainties are the same as for Fig 1.

Figure 3

(a) Calculated ${\overline{\nu }}_e$ energy spectrum from a nominal nuclear reactor (solid black) divided by a smooth approximation [Eq. (3)], including the $1\sigma$ uncertainties due to the fission yields and branching fractions (gray band). Significant discontinuities are caused by the Coulomb correction to the spectra of prominent beta decays. Random variation of fission yields and branching fractions can alter the particular pattern (dashed colored lines). (b)–(d) Same spectra after accounting for detector energy resolution. The current generation of experiments with $\sim 8\%$ energy resolution is sensitive to the larger variations. Future high-resolution experiments would detect some of the substructures.