It’s the early 1950s, and people pretty much agree that a neutrino exists. No one has found it, and because it’s nearly massless and completley chargeless no one can figure out how to find it. Learn how two scientist finally managed it.
The Paper Particle
It was 1949 and the neutrino definitely existed on paper. According to the people who studied how subatomic particles behaved, there was a small particle, emitted every day, that no one had found yet. When it came to equations and paper, there was no getting away from this little particle. People had known it existed since 1931, when Wolfgang Pauli was studying beta decay. During beta decay, an atom emits an electron (or a positron), but Pauli noticed that it emitted this electron in a wide and continuous spectrum of different energy levels. It was almost as if the electron were sharing the total energy with another particle, each carrying a proportion. Three years later, Enrico Fermi confirmed the existence of a particle in his comprehensive theory of radioactive decay. According to Enrico Fermi, this particle could do impressive things. It could, for example, turn a proton into a neutron. When a proton captures a neutrino, it turns into a neutron, and in the process gives off a positron.
What it couldn’t do is show up on any instrument at all that measured charge, because it had no charge. (This is what led Fermi to give it the name “neutrino.”) It couldn’t show up on an instrument that measured mass, because it was almost completely massless. Whenever people sat down and worked out how radioactive decay worked, the neutrino was unavoidable. When they studied actual decay, going on in front of them in a lab, the neutrinos they knew were there disappeared.
The Key to the Discovery
So looking for a neutrino was, at the time, impossible. But Fermi had given the world clues as to how to find the neutrino when he described its power—including its power to turn a proton into a neutron. This was the key that allowed scientists to prove the neutrino existed. The two people who picked up on it were the neutrino researchers Clyde Cowan and Frederick Reines. It wasn’t a smooth journey from intention to discovery for them. Their first idea for an experiment involved detonating a nuclear bomb. But they realized that a neutrino turning a proton into a neutron meant the neutrino was interacting with the world instead of just passing through. That particular interaction resulted simultaneously in a neutron and a positron. The simultaneity of the generation meant that they had a way to prove the neutrino’s existence in the physical world.
Unlike a neutrino, a neutron and a positron were both nearly guaranteed to interact with the world sooner or later. The positron would interact sooner. It would hit an electron and annihilate, giving off a gamma ray. The neutron would interact later, when it hit the right nucleus at the right time. It would also give off a gamma ray. The two particles could interact, and give off a gamma ray, at any time, but on average the time between the positron’s flash and the neutron’s flash would be five microseconds. So if they built a detector, put it next to a nuclear reactor which would be giving off neutrons, and got flashes five microseconds apart, they were seeing the evidence of neutrinos.
Cowan and Reines built a tank, filled it full of liquid that would send off flashes of light when disturbed, and surrounded the tank with light detectors. The flashes came as predicted, five microseconds apart. The rest is Nobel history.
[Source: Neutrino Hunters]
Top Image: The Daya Bay Antineutrino Detector. Second Image: MiniBooNE Neutrino Detector, Fred Ullrich