Nuclear structure insights into reactor antineutrino spectra

Antineutrino spectra following the neutron induced fission of ${}^{235}\mathrm{U},{}^{238}\mathrm{U},{}^{239}\mathrm{Pu}$, and ${}^{241}\mathrm{Pu}$ are calculated using the summation approach. While each system involves the decay of more than 800 fission products, the energy region of the spectra most relevant to neutrino oscillations and the reactor antineutrino anomaly is dominated by fewer than 20 nuclei, for which we provide a priority list to drive new measurements. The very-high-energy portion of the spectrum is mainly due to the decay of just two nuclides, ${}^{92}\mathrm{Rb}$ and ${}^{96}\mathrm{Y}$. The integral of the signal measured by antineutrino experiments is found to have a dependence on the mass and proton numbers of the fissioning system. In addition, we observe that $\sim 70\%$ of the signal originates from the light fission fragment group and about $50\%$ from the decay of odd-$Z,$ odd-$N$ nuclides.

Figure 1

Calculated electron spectra (solid blue [gray] line) following the thermal fission of (a) ${}^{235}\mathrm{U}$, (b) ${}^{239}\mathrm{Pu}$, and (c) ${}^{241}\mathrm{Pu}$ and (d) the fast fission of ${}^{238}\mathrm{U}$ compared with the high-resolution data from ILL [25] as well as the recently published data for ${}^{238}\mathrm{U}$ [26] (black squares). The thin gray lines indicate the individual $\beta$ spectrum from each fission fragment, $I_i\left(E_{\overline{\nu }}\right)$, and thick lines highlight the 20 most important individual contributors at 5.5 MeV.

Figure 2

Comparison of the measured $\beta$ spectrum [23] with present calculations for (a) ${}^{92}\mathrm{Rb}$ and (b) ${}^{96}\mathrm{Yb}$ using a $95\%$ ground state $\beta$-feeding intensity and assuming an allowed shape.

Figure 3

Antineutrino spectra multiplied by the $\overline{\nu }+p\to n+e^{+}$ cross section for the thermal fission of ${}^{235}\mathrm{U}$ and ${}^{239,241}\mathrm{Pu}$ and the fast fission of ${}^{238}\mathrm{U}$.

Figure 4

$\langle \sigma I_{\overline{\nu }}\rangle$ as a function of $(3Z-A)$ for fissioning systems ranging from ${}^{232}\mathrm{Th}$ to ${}^{245}\mathrm{Cm}$. (a) The total as well as the contributions from the light fission fragments (LFF) and heavy fission fragments (HFF) are shown. (b) Same as panel (a), except the individual contributions from nuclides with even-even (EE), even-odd (EO), odd-even (OE), and odd-odd (OO) combinations of protons and neutrons are indicated.