APS News

August/September 2004 (Volume 13, Number 8)

This Month in Physics History

August 1932: Discovery of the Positron

Anderson's cloud chamber picture of cosmic radiation
Photo Credit: Carl D. Anderson, Physical Review Vol.43, p491 (1933)
Anderson's cloud chamber picture of cosmic radiation from 1932 showing for the first time the existence of the anti-electron. The particle enters from the bottom, strikes the lead plate in the middle and loses energy as can be seen from the greater curvature of the upper part of the track.

Star Trek creator Gene Roddenberry incorporated a lot of actual science into what has become one of the most successful series franchises of all time. One of those is the matter/antimatter engines that power the Enterprise, enabling it to supposedly travel at speeds faster than the speed of light.

In 1928, British physicist Paul Dirac showed that Einstein's relativity implied that every particle in the universe has a corresponding antiparticle, each with the same mass as its twin, but with the opposite electrical charge.

The hunt was on to find experimental verification of this hypothesis; a Caltech postdoc named Carl D. Anderson would win the race.

Anderson was born in 1905 to Swiss parents in New York City. When he was 7, the family relocated to Los Angeles, and his parents divorced shortly thereafter. Anderson helped support the family at a very young age, but still managed to get a college education at Caltech. He originally intended to study electrical engineering but switched to physics after taking a particularly inspiring class in the subject. He ultimately went on to earn a PhD in physics engineering (now known as applied physics) from Caltech.

Anderson spent most of his career at Caltech. His early research was on X-rays, but then Victor Hess discovered cosmic rays in 1930. At the advice of his mentor, Robert A. Millikan, Anderson turned his attention to studying those high energy particles. Most scientists were doing this by using cloud chambers: a short cylinder with glass end plates containing a gas saturated with water vapor. If an ionizing particle passes through the chamber, it leaves a trail of water droplets, which can be photographed. By measuring the density of the droplets, scientists can deduce how much ionization is produced—indicating the kind of particle that passed through.

Anderson built his own, improved version of a cloud chamber, incorporating a piston so that he could get the pressure to drop very rapidly. He also used a mixture of water and alcohol in the chamber. And he obtained much better photographs than his colleagues. He surrounded his chamber with a large electromagnet, which caused the paths of ionizing particles to bend into circular paths. By measuring the curvature of those tracks, he could calculate the particles' momentum and determine the sign of the charge.

The resulting photographs surprised Anderson by revealing that cosmic rays produced showers of both positively and negatively charged particles, and the positive charges could not be protons, as one might expect, because the track radius would specify a proton stopping distance much shorter than the length of the track.

Anderson and Millikan speculated that perhaps the positively charged particles were electrons traveling in the opposite direction.

To test the hypothesis, Anderson placed a lead plate in the chamber. When particles passed through the plate, they would emerge from the other side at a lower energy than when they started, so the direction of travel could be deduced.

In August 1932, Anderson recorded the historic photograph of a positively charged electron (now known as a positron) passing through the lead plate in the cloud chamber. It was definitely a positively charged particle, and it was traveling upwards.

Despite initial skepticism from the scientific community, Anderson's result was confirmed the following year, and scientists concluded that the positron was one of a pair of positive and negative electrons produced when a gamma ray converted into matter.

His discovery snagged Anderson a Nobel Prize in Physics in 1936, at the age of 31—the youngest person to be so honored. Antiprotons—protons with a negative instead of the usual positive charge—were discovered by researchers at the University of California, Berkeley in 1955, and the antineutron was discovered the following year. It would take another 30 years before scientists created the first anti-atoms.

In 1995, CERN researchers used the Low Energy Antiproton Ring (LEAR) to slow down rather than accelerate antiprotons. By so doing, they managed to pair positrons and antiprotons together, producing nine hydrogen anti-atoms, each lasting a mere 40 nanoseconds.

Within three years, the CERN group was producing as many as 2000 anti-hydrogen atoms per hour.

That's still not enough to achieve practical antimatter propulsion. It would take tons of antiprotons to travel to interstellar destinations, yet the CERN facility only produces enough antiprotons in one year to light a 100 watt bulb for three seconds. And that's not considering the huge amounts of energy required to power the intense beams that produce the antiprotons.

Nonetheless, in 2000 NASA scientists announced early designs for an antimatter engine that might be capable of fueling a spaceship for a trip to Mars using only a millionth of a gram of antimatter.

The positron has found one useful application: positron emission tomography (PET). This medical imaging technique uses low energy annihilations of electrons and positrons to image the inner workings of the brain, injecting radioactive nuclei into a patient and observing the resulting pairs of gamma rays. The energy produced is insufficient to form even the lightest particle and antimatter and emerges instead as two gamma rays.

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Editor: Alan Chodos
Associate Editor: Jennifer Ouellette