What Is a Neutrino? A Particle That Barely Notices Matter
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Around sixty billion neutrinos from the sun pass through every square centimetre of your body every second, and essentially none of them interact with anything on the way. That combination, enormous abundance and almost total indifference to matter, makes neutrinos both extraordinarily difficult to detect and uniquely useful, since they arrive carrying information from places nothing else escapes.
Why they were proposed
The neutrino was invented on paper to save a conservation law. In beta decay, a nucleus emits an electron and becomes a different element, and measurements in the 1920s showed the emitted electrons carried a range of energies rather than the single fixed value that energy and momentum conservation demanded. The apparent options were that energy is not conserved in nuclear processes, a position Niels Bohr was prepared to entertain, or that something undetected was carrying away the balance. Wolfgang Pauli proposed the second in 1930 in a letter addressed to a physics meeting he was skipping, describing it as a desperate remedy and apologising for postulating a particle that could not be detected. Enrico Fermi built it into a theory of beta decay and gave it the name, meaning little neutral one. It was finally detected in 1956 by Clyde Cowan and Frederick Reines, using a nuclear reactor as a source and a large tank of liquid as a target, twenty-six years after Pauli's apology.
What makes them hard to catch
Neutrinos are electrically neutral, have almost no mass and interact only through the weak nuclear force and gravity, which means they ignore the electromagnetic interactions that stop everything else. The practical consequence is a mean free path through solid matter measured in light years. Detecting them therefore requires an enormous target and extreme patience, and the standard techniques reflect that:
- •Very large volumes of material, from thousands of tonnes of purified water to a cubic kilometre of Antarctic ice
- •Deep underground or under ice placement, since the overburden blocks cosmic rays that would otherwise swamp the signal
- •Detection of the secondary particle produced when a neutrino does interact, rather than the neutrino itself
- •Cherenkov radiation, the faint blue light emitted when a charged particle moves through a medium faster than light does in that medium, detected by thousands of photomultiplier tubes lining the tank
- •Radiochemical methods, counting the few atoms transmuted by neutrino capture in a tank of cleaning fluid, which was the first solar neutrino experiment
- •Extreme purity requirements, since natural radioactivity in the detector material produces far more events than neutrinos do
The solar neutrino problem
The first sustained measurement of solar neutrinos, run by Raymond Davis from the 1960s in a tank of perchloroethylene deep in a South Dakota mine, found roughly a third of the number that solar models predicted. The discrepancy lasted three decades and could have meant that the models of how the sun burns were wrong, that the experiment was wrong, or that something happened to the neutrinos in transit. The answer turned out to be the third. Neutrinos come in three types, associated with the electron, the muon and the tau, and they oscillate, changing from one type into another as they travel. The sun produces electron neutrinos, Davis's detector was sensitive only to electron neutrinos, and by the time they arrived a substantial fraction had become something else. The Sudbury observatory in Canada confirmed this in 2001 by measuring all three types and finding the total matched predictions exactly. The result carried a further consequence: oscillation is only possible if neutrinos have mass, which contradicted the standard model as then written and remains the clearest experimental evidence for physics beyond it.
What they are used for
Because neutrinos escape from places photons cannot, they carry information available no other way. They stream out of the sun's core directly, so they report on fusion happening now, while the light from the same reactions takes many thousands of years to random-walk out to the surface. Neutrinos from supernova 1987A arrived hours before the light, since they leave the collapsing core immediately while the shock wave takes time to reach the surface, and about two dozen detected events confirmed the theory of core collapse in a single stroke. High-energy neutrinos detected in Antarctic ice have been traced back to distant active galaxies, opening a channel of astronomy that is not blocked by dust, distance or magnetic fields. Closer to home, neutrinos from reactors are used to study oscillation precisely and have been proposed as a means of verifying that a reactor is operating as declared, since the flux cannot be shielded or faked, which is a non-proliferation application currently under development.
The takeaway
Pauli proposed the neutrino in 1930 to account for missing energy in beta decay and apologised for suggesting an undetectable particle, and it was detected in 1956. Interacting only weakly, it passes through matter almost freely, so detectors use enormous shielded volumes and catch the rare secondary particle. The solar neutrino deficit, a third of the predicted count, was explained by oscillation between three types, confirmed in 2001, which also proved neutrinos have mass. They carry information out of stellar cores that light cannot.