Sources of Neutrinos — Part 1: Solar neutrinos and the Sun as a nuclear factory

This is the first part of the Sources of Neutrinos series. Each part walks through a major source of neutrinos in nature or in the laboratory: how it produces them, what the energy spectrum looks like, and which experiments have measured it. We begin with the largest neutrino source we can directly study: the Sun.

The Sun as a fusion reactor

The Sun’s energy comes from nuclear fusion in its core. The dominant process is the proton-proton (pp) chain: four protons eventually combining into a helium-4 nucleus, releasing 26.7 MeV of energy in the process. A small fraction (~1% in the Sun) comes from the CNO cycle, in which carbon, nitrogen, and oxygen nuclei catalyze the same net process.

Each fusion reaction step emits at least one electron neutrino. The neutrinos escape the Sun’s interior essentially instantly — they pass through the entire Sun without interacting. The photons that the same fusion produces take hundreds of thousands of years to random-walk their way out. So the Sun we see in light is the Sun of a hundred millennia ago. The Sun we see in neutrinos is the Sun of eight minutes ago.

The energy spectrum

Solar neutrinos arrive at Earth across a wide energy range. Different fusion steps produce neutrinos of characteristic energies:

• pp neutrinos (~0.4 MeV max, continuous spectrum): The proton-proton fusion that starts the chain. By far the most numerous — about 86% of all solar neutrinos.
• pep neutrinos (1.44 MeV monoenergetic): Subdominant pp-chain branch with two protons and an electron.
• ⁷Be neutrinos (0.86 MeV monoenergetic): Beryllium-7 electron capture. About 8% of the total.
• ⁸B neutrinos (continuous up to ~15 MeV): Boron-8 beta decay. Rare (~0.01%) but the highest-energy and easiest to detect.
• hep neutrinos (up to ~18 MeV): Helium-3 + proton fusion. Extremely rare.
• CNO neutrinos (continuous up to ~1.7 MeV): From the CNO catalytic cycle. About 1% of the total.

The total flux at Earth is about 6 × 10¹⁰ per cm² per second — every square centimeter of Earth, every second.

How we measure them

Solar neutrino detectors fall into roughly three categories, distinguished by energy threshold and detection mechanism.

Radiochemical detectors count individual electron-neutrino captures on a target nucleus. Davis’s classic experiment at Homestake used 600 tons of perchloroethylene cleaning fluid — chlorine-37 captures an electron neutrino to become argon-37, which is chemically extracted and counted. Davis’s threshold was 0.814 MeV (above pep, ⁷Be, and ⁸B). GALLEX and SAGE used gallium-71, with a threshold of 0.233 MeV — low enough to capture pp neutrinos for the first time.

Water Cherenkov detectors like Kamiokande, Super-Kamiokande, and SNO detect the recoil electron produced when a solar neutrino scatters off an atomic electron, or in SNO’s case, the breakup of deuterium nuclei. Their energy thresholds are ~5 MeV — only ⁸B and the hep tail.

Liquid scintillator detectors like Borexino can reach much lower thresholds — about 0.2 MeV with sufficient radio-purity. Borexino is the only detector to have measured pp, ⁷Be, pep, ⁸B, and CNO neutrinos individually with the same instrument.

The solar neutrino problem and its resolution

For 33 years, every solar neutrino experiment found a deficit. Davis measured about a third of Bahcall’s predicted ⁸B flux. GALLEX, SAGE, and Kamiokande all confirmed the deficit at different energy thresholds. The discrepancy became known as the solar neutrino problem.

SNO settled it in June 2001. By exploiting the unique reactions available in heavy water, SNO measured separately the electron-neutrino flux and the total all-flavor flux from the Sun. The total matched Bahcall’s prediction. The electron-neutrino fraction was about a third — confirming that the missing two-thirds had oscillated to muon and tau flavors during the eight-minute trip from the Sun.

The solar neutrino problem dissolved in one experimental campaign. The Standard Solar Model was vindicated. Neutrino oscillation became established physics. Kajita (atmospheric oscillation, Super-K) and McDonald (solar oscillation, SNO) shared the 2015 Nobel Prize.

What we’ve learned

Solar neutrinos remain a precision probe of two things:

The Sun’s interior. Every fusion reaction’s neutrino flux has been measured. The CNO measurement by Borexino in 2020 closed the loop on the Standard Solar Model. Future precision could constrain the Sun’s metallicity (its content of elements heavier than helium) — a current minor puzzle in solar physics.

Neutrino oscillation. The MSW effect — neutrino oscillation in the presence of solar matter — was first established in solar neutrino data, and remains the cleanest astrophysical measurement of θ₁₂ and Δm²₂₁.

What’s still open

A few subtle questions remain:

• The day-night asymmetry caused by neutrino regeneration during their nighttime path through the Earth. Detected by SNO and Super-K at marginal significance.
• The transition between vacuum and matter-dominated oscillation at intermediate solar-neutrino energies. The MSW effect has a sharp turnover at a few MeV; precision measurements of the transition test the standard three-flavor framework.
• The solar metallicity question — small inconsistencies between helioseismology measurements and standard models could be probed by precise CNO measurements.

What’s next

Solar neutrinos will continue to be measured by Borexino’s successors. SNO+ (the SNO detector reconfigured with liquid scintillator) is now running and will produce competitive measurements. JUNO at 20 kilotons will produce the most statistics ever on solar ⁷Be and ⁸B neutrinos.

Some next-generation 0νββ detectors will also serve as solar-neutrino spectrometers: large liquid xenon and germanium detectors are intrinsically sensitive in the low-MeV regime where solar neutrinos live.

The next part of this series turns to the second-largest natural neutrino source on Earth: cosmic rays raining down through the atmosphere.

Neutrino Times Editorial Team. (2026, February 20). Sources of Neutrinos — Part 1: Solar neutrinos and the Sun as a nuclear factory. Neutrino Times. https://neutrino-times.com/articles/sources-of-neutrinos-part-1-solar/

Neutrino Times Editorial Team. "Sources of Neutrinos — Part 1: Solar neutrinos and the Sun as a nuclear factory." Neutrino Times, February 20, 2026. https://neutrino-times.com/articles/sources-of-neutrinos-part-1-solar/.

Neutrino Times Editorial Team. "Sources of Neutrinos — Part 1: Solar neutrinos and the Sun as a nuclear factory." Neutrino Times, 20 Feb. 2026, https://neutrino-times.com/articles/sources-of-neutrinos-part-1-solar/.

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  note         = {Accessed: 2026-02-20}
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