The Discovery of Positrons, Antiprotons, and Antineutrons


By Don LincolnFermilab

You’ve heard about Einstein’s equation, E = mc2, and this means that energy can be converted to matter and vice versa, but only if an equal amount of antimatter is made at the same time. Make an electron, and you have to make a positron. Thinking bigger, if you make a proton or a neutron, you have to make an antimatter proton and neutron, too.

E=mc² Written on a blackboard
According to the relativity equation, energy can be converted to matter when the same amount of antimatter is made at the same time. (Image: Santeri Viinamäki/Public domain)

Cosmic Rays and the Discovery of Positively Charged Electrons

Carl Anderson was one of the first people to use the cloud chamber to study cosmic rays. He surrounded his cloud chamber with a big electromagnet; so powerful that the lights dimmed in the laboratory when he turned it on. The magnet would make the tracks curve if the particle was charged and, indeed, all tracks curved, which isn’t so shocking since it’s the electric charge that makes the little jet contrail.

Anderson set up his cloud chamber and magnet and used a camera to take pictures. Most photos were empty, but some recorded tracks, and he could use those photos to characterize what was going on in cosmic rays. He certainly saw what looked like electrons, meaning particles curving in a way that clearly indicated that they were light, fast, and negatively charged. 

But he also found ones that moved upward and looked like they were light, fast, and positively charged. In fact, to all intents and purposes, they looked just like positively charged electrons.

What Anderson Did to Be Proven Right 

Portrait of Carl David Anderson
Carl Anderson used a lead sheet to prove that he had discovered positively charged electrons and not the downward-moving negative particles. (Image: Smithsonian Institution/Public Domain)

But, of course, nobody had ever seen such a thing, and his thesis advisor and other senior physicists were super skeptical. Heck, I would have been skeptical. They suggested that perhaps they were downward moving negative particles, which would curve the same way as upward moving positive particles. That would have explained everything. 

But Anderson put a lead sheet in his cloud chamber and proved that he was right. What he was seeing was an upward-moving, positively charged particle that was light and fast. He was seeing what seemed to be impossible. He was seeing a positive electron.

But first, why was the lead important? A particle like an electron will lose energy when it passes through lead. Lower energy particles will curve more in a magnetic field than high energy ones. So, by having the particle pass through lead and seeing whether the path was curved more on the top or bottom, he could unambiguously determine the particle’s direction. 

If it curved more above the lead, so the particle was moving upward. And if you know the direction a particle is traveling and whether it curves to the right or left, you can know if it is positively or negatively charged.

This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We KnowWatch it now, on Wondrium.

Positrons Versus Antiprotons 

So, where did the positron come from? It’s not like you can find it in nature, just laying around somewhere. And, furthermore, when the positron slowed down, it would annihilate with a stray electron that was kicking around. That was a question that needed to be answered. Luckily, the answer was there, staring out of Dirac’s equations.

So, what was happening was that the energy of those high-energy cosmic rays was being converted into pairs of electrons and positrons. Antimatter had been discovered. Antiproton, the antimatter equivalent of the proton, which would be like the proton, but with a negative charge. Did Anderson discover that too? Actually, no, he just didn’t have enough energy.

The Michigan Mark I FFAG accelerator in 1956
Ironically, antiprotons were discovered by a particle accelerator and not by studying cosmic rays. (Image: Lawrence W. Jones/Public domain)

The proton weighs just shy of 2,000 times more than the electron, and the same thing is true for the antiproton and positron. You need a lot more energy to make antiprotons than you do positrons. And it is indeed possible to make antiprotons in cosmic ray interactions, but it’s just super, incredibly, rare.

Anderson just didn’t get lucky. In fact, while he discovered positrons in 1931, it took until 1955 for the antiproton to be discovered, and it wasn’t done using cosmic rays. It actually took a super powerful particle accelerator, built especially for that purpose, to do it.

The Finding of Antineutrons 

The Bevatron was a particle accelerator built near the University of California in Berkeley. The name comes from the prefix Bev—or B – E – V—which is short for billion electron volts, and the atron is the end name of lots of particle accelerators. Bev is kind of confusing, since we don’t use that term anymore, we now use the letter G for billion. 

This is the standard for the metric system, with G being short for giga, which is the prefix for a billion. The proper pronunciation for that G is a hard G, and scientists say G – E – V, or GeV, but we also say gigaelectron volts. To paraphrase Ralph Waldo Emerson, consistency is for little minds. What are you going to do?

In any event, the Bevatron ran at an energy of eight GeV, which is just enough to make antiprotons and it allowed Owen Chamberlain and Emilio Segrè to discover them. The antineutron was discovered at the Bevatron a year later.

Common Questions about the Discovery of Positrons, Antiprotons, and Antineutrons

Q: What is the comparison between positrons and antiprotons?

Just like protons and electrons, antiprotons and positrons are different in many aspects. For example, antiprotons need a lot more energy to be made than positrons. On the other hand, while positrons can be made in cosmic ray interactions, antiprotons need super-powerful particle accelerators to be made.

Q: How did Carl Anderson discover positron?

He did this using a cloud chamber surrounded by a powerful electromagnet. Anderson took photos from the cloud chamber to characterize the events in the cosmic rays in which he noticed some positively charged particles that certainly weren’t electrons. They were proved to be positive electrons or positrons.

Q: What did Carl Anderson do to prove that he was discovering a positively charged electron?

He used a lead sheet in his cloud chamber to determine the particle’s direction. By determining the direction of the particle and whether it curves to the left or the right, he could guess if it was negatively or positively charged; and what he had discovered were positrons.

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