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Worldwide Reactor Neutrino Propagation to Underground Labs: Matter Effects and Flux Predictions

Keyu Han, Juncheng Qian, Shaomin Chen

Abstract

As a unique probe for geophysical research, geoneutrinos can reveal the distribution of internal heat sources in the Earth by detecting electron antineutrinos produced by the radioactive decay of $^{238}$U, $^{232}$Th, and $^{40}$K. However, commercial nuclear power plants continuously produce the same type of electron antineutrinos, which constitute a primary background difficult to eliminate in geoneutrino experiments. As geoneutrino measurements and reactor background modeling approach sub-percent precision, even small matter-induced corrections to reactor antineutrino propagation require quantitative assessment. In this paper, we develop a high-precision prediction framework for reactor neutrino fluxes at underground labs, using global reactor operating data, reactor-to-detector distances, and matter effects (MSW) on neutrino propagation through the Earth. To solve the three-flavor MSW evolution efficiently, we implement a second-order Strang-splitting solver in the vacuum mass basis. Within this framework, we have calculated the reactor neutrino oscillation probabilities, including the MSW effect under one-dimensional (spherically symmetric) and three-dimensional (including lateral inhomogeneities) Earth models, and compared them with the vacuum oscillation scenario, to assess the impact of Earth's structural features on the accuracy of reactor neutrino flux predictions.

Worldwide Reactor Neutrino Propagation to Underground Labs: Matter Effects and Flux Predictions

Abstract

As a unique probe for geophysical research, geoneutrinos can reveal the distribution of internal heat sources in the Earth by detecting electron antineutrinos produced by the radioactive decay of U, Th, and K. However, commercial nuclear power plants continuously produce the same type of electron antineutrinos, which constitute a primary background difficult to eliminate in geoneutrino experiments. As geoneutrino measurements and reactor background modeling approach sub-percent precision, even small matter-induced corrections to reactor antineutrino propagation require quantitative assessment. In this paper, we develop a high-precision prediction framework for reactor neutrino fluxes at underground labs, using global reactor operating data, reactor-to-detector distances, and matter effects (MSW) on neutrino propagation through the Earth. To solve the three-flavor MSW evolution efficiently, we implement a second-order Strang-splitting solver in the vacuum mass basis. Within this framework, we have calculated the reactor neutrino oscillation probabilities, including the MSW effect under one-dimensional (spherically symmetric) and three-dimensional (including lateral inhomogeneities) Earth models, and compared them with the vacuum oscillation scenario, to assess the impact of Earth's structural features on the accuracy of reactor neutrino flux predictions.

Paper Structure

This paper contains 22 sections, 45 equations, 8 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Worldwide nuclear reactor survey
  3. Reactor anti-electron neutrino spectral model
  4. Pathlength and oscillation in vacuum
  5. Neutrino propagation: vacuum vs. matter effect inside the Earth
  6. Evolution equation
  7. A numerical approach to the matter effect: the Strang-splitting method
  8. First-order analytical approximation to the matter effect
  9. Density models for the Earth
  10. The 1D radial reference model
  11. The 3D Earth density model construction
  12. Flux and spectrum predictions: 1D vs. 3D models
  13. Default parameters setup and uncertainty propagation
  14. The flux and IBD event rate predictions
  15. Predictions for CJPL
  16. ...and 7 more sections

Figures (8)

  • Figure 2.1: Worldwide distribution of operating reactor sites in 2024. The color bar indicates the annual-average effective thermal power of each reactor site, and the locations of underground laboratories are also shown. (The map was produced using Cartopy cartopy, with base features from Natural Earth.)
  • Figure 3.1: Detectable reactor $\bar{\nu}_e$ spectra of the four main fissile isotopes after weighting by the IBD cross section, computed with the SM2023 spectral model SM2023. The contributions from the four isotopes are shown separately.
  • Figure 4.1: Reactor $\bar{\nu}_e$ flux varies with distance for CJPL.
  • Figure 7.1: Predicted reactor $\bar{\nu}_e$ flux spectrum at CJPL from the adopted global reactor sample. The left panel uses the vacuum survival probability in Eq. \ref{['eq:vac_osc']}. In contrast, the right panel includes the three-flavor MSW matter effect in the Earth evaluated with the Strang-splitting solver Eq. \ref{['eq:strang_P']}.
  • Figure 7.2: Predicted IBD event rate spectrum at CJPL from the adopted global reactor sample, expressed in TNU. The left panel uses the vacuum survival probability Eq. \ref{['eq:vac_osc']}, while the right panel includes the three-flavor MSW matter effect in the Earth evaluated with the Strang-splitting solver Eq. \ref{['eq:strang_P']}.
  • ...and 3 more figures