Improved Predictions of Reactor Antineutrino Spectra

Abstract: We report new calculations of reactor antineutrino spectra including the latest information from nuclear databases and a detailed error budget. The first part of this work is the so-called ab initio approach where the total antineutrino spectrum is built from the sum of all beta-branches of all fission products predicted by an evolution code. Systematic effects and missing information in nuclear databases lead to final relative uncertainties in the 10 to 20% range. A prediction of the antineutrino spectrum associated with the fission of 238U is given based on this ab initio method. For the dominant isotopes 235U and 239Pu, we developed a more accurate approach combining information from nuclear databases and reference electron spectra associated with the fission of 235U, 239Pu and 241Pu, measured at ILL in the 80's. We show how the anchor point of the measured total beta-spectra can be used to suppress the uncertainty in nuclear databases while taking advantage of all the information they contain. We provide new reference antineutrino spectra for 235U, 239Pu and 241Pu isotopes in the 2-8 MeV range. While the shapes of the spectra and their uncertainties are comparable to that of the previous analysis of the ILL data, the normalization is shifted by about +3% on average. In the perspective of the re-analysis of past experiments and direct use of these results by upcoming oscillation experiments, we discuss the various sources of errors and their correlations as well as the corrections induced by off equilibrium effects.

• The paper improves reactor antineutrino spectra predictions by combining an ab initio approach with experimental electron data.
• It reduces systematic uncertainties from 10–20% by merging updated nuclear databases with ILL reference measurements.
• The refined predictions enhance accuracy for reactor neutrino experiments and support effective nuclear reactor monitoring.

Improved Predictions of Reactor Antineutrino Spectra

The paper "Improved Predictions of Reactor Antineutrino Spectra" primarily addresses the need for accurate predictions of the antineutrino spectra emitted by nuclear reactors. These predictions are crucial for applications in reactor neutrino oscillation experiments and for monitoring nuclear reactors for non-proliferation purposes. The study introduces enhanced computational frameworks to refine these predictions by employing updated nuclear databases and innovative calculation methodologies.

The authors initiate the discussion with the {\it ab initio} approach. This method calculates the total antineutrino spectrum by aggregating all beta-branches of fission products computed through an evolution code. They emphasize that while the {\it ab initio} approach provides a theoretical foundation, it is limited by systematic uncertainties arising from incomplete and imprecise nuclear databases. The resulting relative uncertainties of such direct calculations lie in the range of 10 to 20%. The paper presents a specific prediction for the antineutrino spectrum associated with the fission of $^{238}$U using this approach.

Moreover, the dominant isotopes $^{235}$U and $^{239}$Pu are examined through an alternative method that merges nuclear database information with empirical data from reference electron spectra. These spectra were initially measured in the 1980s at the Institut Laue-Langevin (ILL) reactor. By applying this mixed-method approach, the paper achieves a reduction in uncertainties by leveraging the experimental data to constrain the theoretical models, achieving a notable shift in the normalization of the predicted antineutrino spectra by about 3%.

The numerical analysis provided in this study is comprehensive, presenting electron and antineutrino spectra with explicit statistical and systematic uncertainties, while also delineating the sources of these errors. The prediction shows an improvement in accuracy over previous models and exhibits decreased systematic deviation by effectively addressing the missing information through the integration of physical constraints derived from empirical data.

The paper further explores implications for experimental physics, particularly for analyzing past reactor neutrino experiments and guiding future ones. Given the refined predictions, there is a clear avenue for reinterpreting historical data from experiments such as Double Chooz, as well as improving the accuracy of upcoming oscillation measurements. The research establishes a foundation for recalibrating analyses that depend on antineutrino detection, potentially influencing the evaluation of the mixing angle $\theta_{13}$ in neutrino physics.

In conclusion, this work advances the fundamental understanding of reactor antineutrino spectra by optimizing theoretical predictions through improved databases and methodological innovation. The implications extend beyond theoretical predictions, influencing both experimental pursuits in neutrino physics and practical applications in nuclear reactor monitoring. Future developments should aim to further reduce uncertainties and expand the precision and applicability of these predictions across diverse reactor conditions.