By Don Lincoln, Fermilab
How do you make neutrinos? In 1955, a team of researchers led by American physicists Frederick Reines and Clyde Cowan decided to use a nuclear reactor. Their detector was pretty complicated, consisting of huge tanks of water, sandwiched between layers of what is called scintillator. The neutrinos would interact with protons in the water and kick out a neutron and a positron, which is an antimatter electron.
The Benefit of Using a Scintillator
The antimatter electron would annihilate with an electron in the detector and make a couple of high-energy photons called gamma rays. The gamma rays would make blinks of light in the scintillator, which they could detect. There are more technical details, but that’s the big idea. And, of course, given that nobody had ever seen a neutrino, their expectation might have been wrong. But this was consistent with the theory of weak nuclear force interactions that was held at the time.
So, Reines and Cowan put their detector near a powerful nuclear reactor at Savannah River, South Carolina. The reactor provided a huge flux of neutrinos, about 50 trillion neutrinos per every square centimeter every second. A square centimeter is a little smaller than a postage stamp and the detector was pretty big, about the size of a 50-gallon drum or thereabouts.
So, every second, something like 500 quadrillion neutrinos passed through the detector, which is about two sextillion neutrinos per hour. Now, odds are that you’ve never even heard about a sextillion, but a sextillion is, very roughly speaking, about 100 times more than every grain of sand on every beach on the entire Earth.
The Achievement of Reines and Cowan
So, Reines and Cowan shot a ton of neutrinos through their detector every hour, in the ballpark of 100 times as many as there are grains of sand on Earth. And how many neutrinos do you think they observed in their detector? About three per hour. That’s just crazy.
Now, you might be asking how they were sure that these interactions they saw in their detector were from neutrinos. Well, it’s simple actually. They turned the nuclear reactor on and off and saw a difference.
On, and they saw three interactions per hour. Off, and they didn’t.
And, on June 14, 1956, Reines and Cowan sent a telegram to Wolfgang Pauli announcing their discovery. Reines and Cowan called their apparatus Project Poltergeist, which is a completely apt name for an experiment that saw the first evidence for ghosts of the subatomic world.
Reines and Cowan had found neutrinos, but the ghost of the particle world had surprises in store for physicists. And the next surprise would be observed in 1962. Until the 1960s, physicists hadn’t studied high energy neutrinos, but rather only lower energy ones.
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How the Particle Accelerator Helped
Now it may seem kind of weird to say that neutrinos from nuclear reactions are low energy, but particle accelerators of the 1960s could easily achieve energies 1,000 times higher than those in nuclear reactors. Nowadays, it’s more like a million, but let’s not get ahead of ourselves.
In any event, in 1962, a team of researchers led by a trio of American scientists, by the name of Melvin Schwartz, Jack Steinberger, and Leon Lederman, was making a beam of pions that decayed into muons and neutrinos.
The beam used a particle accelerator located at Brookhaven National Laboratory on Long Island in New York. They used magnets to deflect muons away, leaving a beam of high energy neutrinos.
They basically wanted to see what happened when high-energy neutrinos slammed into a target. They expected to see that the high energy neutrinos would occasionally hit a proton and out would pop either an electron or a muon. Easy enough, right?
This experiment was big, not so different than the size of a room. It consisted of slabs of metal, interspersed with detectors that would trace out the path of particles crossing the detector. The signature of a muon being created was a long path of a particle crossing the entire detector, while an electron would be created and stopped, with a super short path.
The Different Types of Neutrinos
So what did they see? Every single interaction they saw made a muon, never an electron. What could it mean? They thought about things for a bit. Either these high energy neutrinos only made muons, or the neutrinos remember that they were made in collisions where muons were made. (Remember that the neutrino beam was made by pions decaying into muons and neutrinos.)
After a lot of additional work, it became clear that there wasn’t a neutrino, but rather two kinds of neutrinos, one associated with muons and one with electrons. They were called, rather unimaginatively, electron neutrinos and muon neutrinos.
And, several decades later, a third kind of neutrino was observed, this time called the tau neutrino. The tau is a heavier version of the electron and muon and it was discovered in 1975. The tau neutrino actually waited until 2000.
But there you have it. There are three kinds of neutrinos. And in 1962, when the muon neutrino was discovered, it seemed very much like they were distinct and separate and different particles. After all, remember that the muon discovery experiment proved that neutrinos made with muons remembered their heritage and only made muons in subsequent interactions.
Common Questions about Experiments for Observing Different Types of Neutrinos
Frederick Reines and Clyde Cowan used a complicated detector, consisting of huge tanks of water, sandwiched between layers of a scintillator. The neutrinos would interact with protons in the water, ejecting a neutron as well as a positron, thus shooting a ton of neutrinos every hour.
This trio of American scientists led a team of researchers. The team used a particle accelerator to make a beam of pions that decayed into muons and neutrinos, and also managed to use magnets to deflect muons away from high-energy neutrinos.
These include electron neutrinos, muon neutrinos, and tau neutrinos, the latter of which is a heavier version of the electron and muon.