Which reactions in the Sun make neutrinos?
The Sun does not send us a single neutrino spectrum from a single reaction. Its fusion pathways leave different energy signatures—and each detector sees only part of that mixture.
Making helium requires more than joining protons
The Sun’s main fusion network gradually converts hydrogen into helium. A helium-4 nucleus contains two protons and two neutrons, so starting from hydrogen requires changes in particle identity as well as bringing nuclei together.
Weak-interaction steps supply those changes and produce electron neutrinos. The main network is called the proton–proton chain, often shortened to the pp chain. A second network uses carbon, nitrogen and oxygen nuclei as catalysts and is called the CNO cycle.
Both networks release energy, some of which leaves with neutrinos. Most of the energy remains in the star. Neutrinos thus offer a way to test the reactions powering the Sun, rather than only the light that finally escapes its surface.
The common first step makes low-energy neutrinos
In the ordinary pp reaction, two protons produce deuterium, a positron and an electron neutrino. Deuterium is a nucleus containing one proton and one neutron. This begins a sequence of further reactions leading towards helium.
The available energy can be divided among the products in different ways. The pp neutrinos therefore form a continuous spectrum rather than one fixed-energy line. They are the most numerous solar neutrinos, but their energies are low: the spectrum ends at about 420 keV.
A related route, called pep, involves two protons and an electron. It also produces deuterium and an electron neutrino, but gives a much narrower neutrino-energy feature. Similar contributions to fusion can leave very different clues in an energy spectrum.
- 01pp and pep
Different energy features from the early chain
- 02Beryllium and boron
Line-like and higher-energy continuous components
- 03A detector’s window
Thresholds and interactions weight the mixture
Beryllium and boron mark different branches
Later in the pp chain, the paths branch. Beryllium-7 can capture an electron and become lithium-7, emitting a neutrino. Because the nuclear final states are specified, this produces line-like features, broadened somewhat by conditions in the Sun.
Alternatively, beryllium-7 can capture a proton and form boron-8. The subsequent beta decay produces a continuous neutrino spectrum extending to much higher energies than the pp component. This route is rare compared with the reactions producing pp neutrinos.
CNO nuclei also produce neutrinos through particular decay steps. Their contribution carries information about a different fusion network and the material in the solar core. The Sun’s neutrino spectrum is consequently a mixture of reaction signatures, with predictions that carry nuclear and solar-model uncertainties.
The Sun’s most common neutrinos may be below the threshold
A detector does not count every neutrino passing through it. Its target must permit an interaction, enough energy must be available, and the resulting signal must survive background cuts and reconstruction. A threshold is therefore a physical and experimental selection.
The chlorine capture reaction used at Homestake has a threshold of about 814 keV. Ordinary pp neutrinos cannot reach it. The gallium reaction opens at about 233 keV and can capture part of that low-energy population. These two radiochemical measurements do not sample identical parts of the Sun’s spectrum.
Water Cherenkov experiments and SNO’s principal solar analyses studied higher-energy neutrinos, especially boron-8. Their prominence in those detectors does not make boron-8 the dominant solar source. Interaction probabilities and detection requirements can make a rare component the easiest one to observe.
Separate what the Sun makes from what reaches the instrument
A source model predicts neutrinos produced in the solar core. Flavor evolution on the way to Earth changes what an electron-flavor-sensitive detector receives. The detector’s response then determines which of those arrivals become recorded events. These are three different parts of the calculation.
Experiments with different thresholds and channels can test the parts together. Radiochemical measurements integrate capture over an exposure; spectroscopic measurements use the energy distribution of visible recoil particles to disentangle components, with overlapping backgrounds included in the fit.
Borexino’s studies of pp-chain components and its 2020 CNO result extended that approach to low energies. Reading the solar-neutrino history becomes easier once the source mixture is clear: each experiment opened a particular window, and their windows had to be compared rather than treated as interchangeable counts.
Primary sources & revision
- Adelberger et al. · Solar fusion cross sections II: the pp chain and CNO cycles (2011)
- John Bahcall · How the Sun shines
- Borexino Collaboration · Comprehensive measurement of pp-chain solar neutrinos (2018)
- John Bahcall · Gallium Solar Neutrino Experiments (1997)
- John Bahcall · The beryllium-7 solar neutrino line (1994)
- Borexino Collaboration · Evidence of neutrinos from the CNO fusion cycle (2020)
- Brookhaven National Laboratory · Raymond Davis Jr. and solar neutrinos
- SNO Collaboration · Direct Evidence for Neutrino Flavor Transformation (2002)
- Particle Data Group · Neutrino Masses, Mixing, and Oscillations (2024)
First published and source-checked on 9 October 2026. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.