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Direct determination of the $^{235}$U to $^{239}$Pu inverse beta decay yield ratio in the power reactor neutrino experiments

I. Alekseev, V. Belov, A. Bystryakov, M. Danilov, D. Filosofov, M. Fomina, P. Gorovtsov, Ye. Iusko, S. Kazartsev, V. Khvatov, S. Kiselev, A. Kobyakin, A. Krapiva, A. Kuznetsov, I. Machikhiliyan, N. Mashin, D. Medvedev, V. Nesterov, D. Ponomarev, I. Rozova, N. Rumyantseva, V. Rusinov, E. Samigullin, Ye. Shevchik, M. Shirchenko, Yu. Shitov, N. Skrobova, D. Svirida, E. Tarkovsky, E. Yakushev, I. Zhitnikov, A. Yakovleva, D. Zinatulina

TL;DR

The paper introduces a direct method to determine the IBD yield ratio $\sigma_5/\sigma_9$ by analyzing how the reactor antineutrino detection rate evolves with fuel burnup, thereby canceling detection-efficiency uncertainties. Using about $8$ million antineutrinos collected by the DANSS detector over $7.5$ years, they extract a normalized slope $S_n = -0.380 \pm 0.032$ and deduce $\sigma_5/\sigma_9 = 1.529 \pm 0.057$, the most precise reactor-based result to date. The result is consistent with the Daya Bay measurements and the Huber–Mueller model, while showing a modest tension with the Kopeikin (KI) claim; it also highlights the role of minor isotopes and fission-fraction derivatives in the uncertainty budget. The method, which relies on fuel-evolution data and cross-campaign averaging, provides a robust, efficiency-independent cross-check of reactor-model predictions and can be extended to other power reactors to improve global constraints on antineutrino production models.

Abstract

The yields of the inverse beta decay events produced by antineutrinos from a certain nuclear reactor fuel component are used by many experiments to check various model predictions. Yet measurements of the absolute yields feature significant uncertainties coming, mainly, from the understanding of the antineutrino detection efficiency. This work presents a simple novel approach to directly determine the $^{235}$U to $^{239}$Pu inverse beta decay yield ratio using the fuel evolution analysis. This ratio can be used for a sensitive test of reactor models, while the proposed method, results in smaller systematic uncertainties. The DANSS result on this ratio is one of the most precise among reactor neutrino experiments, yet does not significantly contradict to any previous measurement.

Direct determination of the $^{235}$U to $^{239}$Pu inverse beta decay yield ratio in the power reactor neutrino experiments

TL;DR

The paper introduces a direct method to determine the IBD yield ratio by analyzing how the reactor antineutrino detection rate evolves with fuel burnup, thereby canceling detection-efficiency uncertainties. Using about million antineutrinos collected by the DANSS detector over years, they extract a normalized slope and deduce , the most precise reactor-based result to date. The result is consistent with the Daya Bay measurements and the Huber–Mueller model, while showing a modest tension with the Kopeikin (KI) claim; it also highlights the role of minor isotopes and fission-fraction derivatives in the uncertainty budget. The method, which relies on fuel-evolution data and cross-campaign averaging, provides a robust, efficiency-independent cross-check of reactor-model predictions and can be extended to other power reactors to improve global constraints on antineutrino production models.

Abstract

The yields of the inverse beta decay events produced by antineutrinos from a certain nuclear reactor fuel component are used by many experiments to check various model predictions. Yet measurements of the absolute yields feature significant uncertainties coming, mainly, from the understanding of the antineutrino detection efficiency. This work presents a simple novel approach to directly determine the U to Pu inverse beta decay yield ratio using the fuel evolution analysis. This ratio can be used for a sensitive test of reactor models, while the proposed method, results in smaller systematic uncertainties. The DANSS result on this ratio is one of the most precise among reactor neutrino experiments, yet does not significantly contradict to any previous measurement.

Paper Structure

This paper contains 9 sections, 4 equations, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Method
  3. Numeric estimates
  4. Minor IBD yield ratios
  5. Fission fractions and derivatives
  6. Normalized slope
  7. $\sigma_5/\sigma_9$ yield ratio
  8. Conclusions
  9. Additional figures

Figures (6)

  • Figure 1: Fission fractions of the four major isotopes, superimposed for all campaigns; the spread is within the line thickness. Vertical lines separate the $f_9$ intervals for the data averaging.
  • Figure 2: Count rate of the DANSS detector as a function of the $^{239}$Pu fission fraction (full circles) and it’s linear fit (solid line); similar measurements from the DB experiment DayaBay:2017jkb (empty circles and dashed line). All data are normalized by corresponding linear fit values at $f_9$=0.3.
  • Figure 3: Daya Bay results on the $^{235}$U and $^{239}$Pu IBD yields from the original DayaBay:2017jkb (green ellipses) and final DayaBay:2025fuel (pink triangle with cross) analyses; black dot and ellipse give the H-M model predictions HuberMuellerDayaBay:2017jkbHayes:2017res; lines with $1\sigma$ error corridors represent the $\sigma_5/\sigma_9$ ratio obtained by KI Kopeikin:2021ugh (dotted cyan) and the result of this work (solid blue)
  • Figure 4: Relative IBD count rate of the DANSS detector, normalized by the reactor power, as a function of the calendar time, all backgrounds subtracted, all corrections applied, statistical errors only; orange, blue and green points correspond to the top (closest to the reactor), middle and bottom positions of the detector; only measurements at full reactor power presented. The position matching uses a toy geometrical MC of the reactor core and the detector. A one month period from November 18, 2016 (shown with red dotted rectangle) is used to establish the correspondence between the IBD rate and the reactor power and this common normalization is used for the whole analysis period. Four full fuel campaigns are clearly seen, separated by the reactor-off periods for the fuel reload; other blank gaps correspond to either pauses for the DANSS maintenance or the intervals of the reactor operation at low power.
  • Figure 5: Relative IBD count rates of the DANSS detector as functions of the $^{239}$Pu fission fraction (full circles) with their linear fits (solid lines) for the four full fuel campaigns (a-d), statistical error only; empty circles and dashed lines repeat the DB result DayaBay:2017jkb for comparison. All data are normalized by corresponding linear fit values at $f_9$=0.3.
  • ...and 1 more figures