The Sun’s missing neutrinos: a tank, a calculation and a long disagreement

How do you find a handful of atoms in a huge tank?

In a mine in South Dakota, Raymond Davis Jr. and his collaborators filled a large tank with roughly 615 tonnes of a chlorine-bearing cleaning fluid. Very occasionally, a solar electron neutrino could turn a chlorine-37 nucleus into argon-37.

Counting those argon atoms gave a chemical measure of rare interactions. After each exposure, the team extracted argon from the liquid and placed it in small counters, where the radioactive decays of argon-37 revealed the atoms created during the run.

The task was extraordinary: recover a tiny number of atoms from an enormous amount of liquid, then tell their decays apart from background. Extraction efficiency, counting efficiency and argon made by other processes all mattered. A big tank alone could not make the answer reliable.

Sources: [1] · [2]

Turning a solar model into a prediction for the tank

John Bahcall and his collaborators calculated the neutrinos expected from nuclear reactions inside the Sun. Predicting what Homestake would count took another step: combining those neutrino energies with the probability that chlorine responds at each energy.

The chlorine reaction has an energy threshold of about 0.814 MeV, and the abundant low-energy neutrinos from the main proton–proton reaction lie below it. Homestake therefore did not count all solar neutrinos with equal sensitivity; the smaller high-energy component was especially important to its prediction.

So when Homestake is said to have seen only a fraction of the expected signal, it was a fraction of the predicted chlorine capture rate. It does not show that every nuclear reaction chain in the Sun produced that same reduced fraction of neutrinos.

Sources: [2] · [3] · [4]

The low count kept coming back

From the late 1960s, results showed fewer captures than expected. The gap persisted as the experiment ran on and the solar calculations improved, and it became known as the solar neutrino problem.

Several explanations deserved serious attention. The solar model might have the wrong rates for important reactions. An extraction or counting problem might have lowered the measured yield. Or the neutrinos might not reach Earth in the form the calculation assumed.

Both sides had work to do. The experimenters tested whether argon could be recovered and recognized; solar researchers re-examined nuclear inputs, composition and the structure of the Sun. A disagreement becomes more interesting once both prediction and measurement have survived specific searches for an ordinary mistake.

Sources: [1] · [3] · [4]

The tank could answer one question very well

Homestake’s method also limited what it could tell us. Extracting the accumulated argon gave a capture rate averaged over an exposure, with no direction, arrival time or energy for individual neutrinos.

At solar energies, the chlorine reaction was sensitive to electron neutrinos. Any that arrived as muon or tau flavor would not add to that capture signal. A low rate could therefore mean a change during the journey, not just a weak source.

Homestake alone could not separate these possibilities, and that was a reason to build other kinds of detector. One that measured direction, reached lower energies or responded differently to each flavor could ask a question the chlorine tank could not.

Sources: [2] · [3]

A new kind of comparison changed the argument

Later solar experiments used other materials and methods. SNO’s heavy water allowed an especially revealing comparison: it measured the arriving electron-neutrino component and also the total flux of the three active flavors.

SNO found the electron component low while the total active flux agreed with solar expectations. KamLAND then tested the relevant oscillation picture with antineutrinos from reactors on Earth. A question rooted in solar modelling could now be checked with a very different source.

Homestake’s first low count did not prove the final explanation. Its importance lay in keeping a difficult discrepancy alive long enough for sharper measurements to become possible. The answer grew out of a long exchange between calculation, chemical technique and new detector designs.

Sources: [5] · [6] · [3]

Primary sources & revision

1. Brookhaven National Laboratory · Raymond Davis Jr. and solar neutrinos
2. Particle Data Group · Neutrino Masses, Mixing, and Oscillations (2024)
3. John Bahcall · Solving the mystery of the missing neutrinos
4. John Bahcall · How the Sun shines
5. SNO Collaboration · Direct Evidence for Neutrino Flavor Transformation (2002)
6. KamLAND Collaboration · First Results: Evidence for Reactor Anti-Neutrino Disappearance (2002/2003)

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.