Which reactions in the Sun make neutrinos?

Making helium requires more than joining protons

The Sun’s main fusion network gradually turns hydrogen into helium. A helium-4 nucleus has two protons and two neutrons, so building it from hydrogen means turning some protons into neutrons, not just bringing nuclei together.

Weak-interaction steps make those changes and emit electron neutrinos. The main network is the proton–proton chain, or pp chain. A second network, the CNO cycle, uses carbon, nitrogen and oxygen nuclei as catalysts.

Both networks release energy. Neutrinos carry some of it away; most stays in the star. Neutrinos therefore let us test the reactions that power the Sun, instead of relying only on the light that finally escapes its surface.

Sources: [1] · [2] · [3]

The common first step makes low-energy neutrinos

In the ordinary pp reaction, two protons make a deuterium nucleus, a positron and an electron neutrino. Deuterium has one proton and one neutron. Further reactions then carry on towards helium.

The products can share the released energy in many ways, so pp neutrinos have a continuous spectrum, not a single fixed energy. They are the most numerous solar neutrinos, but their energies are low: the spectrum ends at about 420 keV.

A related reaction, called pep, combines two protons and an electron. It also makes deuterium and an electron neutrino, but its neutrinos have a much narrower energy spread. Two steps that play similar parts in fusion can leave very different marks in the spectrum.

Sources: [1] · [4] · [3]

Beryllium and boron mark different branches

Later in the pp chain, the path splits. Beryllium-7 can capture an electron, becoming lithium-7 and emitting a neutrino. The final nuclear states are fixed, so the neutrinos appear as near-sharp lines, slightly broadened by conditions in the Sun.

Beryllium-7 can instead capture a proton and form boron-8. Boron-8 then beta-decays, and its neutrinos have a continuous spectrum that reaches much higher energies than pp neutrinos. This route is rare compared with the reactions that make pp neutrinos.

Particular decays in the CNO cycle also emit neutrinos. These carry information about the second fusion network and about the material in the solar core. The Sun’s neutrino spectrum is therefore a mix of reaction signatures, and its predictions carry uncertainties from nuclear physics and from solar models.

Sources: [1] · [5] · [3] · [6]

The Sun’s most common neutrinos may be below the threshold

A detector counts only some of the neutrinos passing through it. The target must allow an interaction, the neutrino must bring enough energy, and the signal must survive background cuts and reconstruction. A threshold therefore comes from both the physics and the experiment.

The chlorine capture reaction at Homestake has a threshold of about 814 keV, which ordinary pp neutrinos cannot reach. The gallium reaction starts at about 233 keV and can capture part of that low-energy population. The two radiochemical measurements do not sample the same part of the Sun’s spectrum.

Water Cherenkov experiments and SNO’s main solar analyses studied higher-energy neutrinos, especially those from boron-8. Boron-8 stands out in those detectors, yet it is not the Sun’s main source. Interaction probability and detection requirements can make a rare component the easiest one to see.

Sources: [4] · [7] · [8] · [9]

Separate what the Sun makes from what reaches the instrument

Getting from the solar core to a detector’s events takes three separate steps. A solar model predicts which neutrinos the core produces. Flavor change on the way to Earth alters what a detector sensitive to electron flavor receives. The detector’s response then decides which arriving neutrinos become events.

Experiments with different thresholds and interaction channels can test the three steps together. Radiochemical experiments count the total captures over an exposure. Spectroscopic experiments separate components using the energy spectrum of the visible recoiling particles, and fit the overlapping backgrounds at the same time.

Borexino’s studies of pp-chain components, and its 2020 CNO result, carried that approach to low energies. Once the mix of sources is clear, the history of solar neutrinos is easier to follow. Each experiment looked through its own window, so their counts had to be compared window by window, not treated as interchangeable.

Sources: [3] · [6] · [9] · [4]

Primary sources & revision

1. Adelberger et al. · Solar fusion cross sections II: the pp chain and CNO cycles (2011)
2. John Bahcall · How the Sun shines
3. Borexino Collaboration · Comprehensive measurement of pp-chain solar neutrinos (2018)
4. John Bahcall · Gallium Solar Neutrino Experiments (1997)
5. John Bahcall · The beryllium-7 solar neutrino line (1994)
6. Borexino Collaboration · Evidence of neutrinos from the CNO fusion cycle (2020)
7. Brookhaven National Laboratory · Raymond Davis Jr. and solar neutrinos
8. SNO Collaboration · Direct Evidence for Neutrino Flavor Transformation (2002)
9. Particle Data Group · Neutrino Masses, Mixing, and Oscillations (2024)

First published 2026-10-09; last revised 2026-10-09. 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.