The positron or antielectron is the antiparticle or the antimatter counterpart of the electron.
The positron has an electric charge of +1e, a spin of ½, and has the same mass as an
electron. When a low-energy positron collides with a low-energy electron, annihilation occurs,
resulting in the production of two or more gamma ray photons.
Positrons may be generated by positron emission radioactive decay, or by pair production from
a sufficiently energetic photon.
History Theory
In 1928, Paul Dirac published a paper proposing that electrons can have both a positive charge
and negative energy. This paper introduced the Dirac equation, a unification of quantum
mechanics, special relativity, and the then-new concept of electron spin to explain the Zeeman
effect. The paper did not explicitly predict a new particle, but did allow for electrons
having either positive or negative energy as solutions. The positive-energy solution
explained experimental results, but Dirac was puzzled by the equally valid negative-energy
solution that the mathematical model allowed. Quantum mechanics did not allow the negative
energy solution to simply be ignored, as classical mechanics often did in such equations; the
dual solution implied the possibility of an electron spontaneously jumping between positive
and negative energy states. However, no such transition had yet been observed experimentally.
He referred to the issues raised by this conflict between theory and observation as "difficulties"
that were "unresolved". Dirac wrote a follow-up paper in December
1929 that attempted to explain the unavoidable negative-energy solution for the relativistic
electron. He argued that "... an electron with negative energy moves in an external
[electromagnetic] field as though it carries a positive charge." He further asserted that
all of space could be regarded as a "sea" of negative energy states that were filled,
so as to prevent electrons jumping between positive energy states and negative energy
states. The paper also explored the possibility of the proton being an island in this sea,
and that it might actually be a negative-energy electron. Dirac acknowledged that the proton
having a much greater mass than the electron was a problem, but expressed "hope" that a
future theory would resolve the issue. Robert Oppenheimer argued strongly against
the proton being the negative-energy electron solution to Dirac's equation. He asserted
that if it were, the hydrogen atom would rapidly self-destruct. Persuaded by Oppenheimer's
argument, Dirac published a paper in 1931 that predicted the existence of an as-yet
unobserved particle that he called an "anti-electron" that would have the same mass as an electron
and that would mutually annihilate upon contact with an electron.
Feynman, and earlier Stueckelberg, proposed an interpretation of the positron as an electron
moving backward in time, reinterpreting the negative-energy solutions of the Dirac equation.
Electrons moving backward in time would have a positive electric charge. Wheeler invoked
this concept to explain the identical properties shared by all electrons, suggesting that "they
are all the same electron" with a complex, self-intersecting worldline. Yoichiro Nambu
later applied it to all production and annihilation of particle-antiparticle pairs, stating that
"the eventual creation and annihilation of pairs that may occur now and then is no creation
or annihilation, but only a change of direction of moving particles, from past to future,
or from future to past." The backwards in time point of view is nowadays accepted as
completely equivalent to other pictures, but it does not have anything to do with the macroscopic
terms "cause" and "effect", which do not appear in a microscopic physical description.
Experimental clues and discovery Dmitri Skobeltsyn first observed the positron
in 1929. While using a Wilson cloud chamber to try to detect gamma radiation in cosmic
rays, Skobeltsyn detected particles that acted like electrons but curved in the opposite
direction in an applied magnetic field. Likewise, in 1929 Chung-Yao Chao, a graduate
student at Caltech, noticed some anomalous results that indicated particles behaving
like electrons, but with a positive charge, though the results were inconclusive and the
phenomenon was not pursued. Carl D. Anderson discovered the positron on
August 2, 1932, for which he won the Nobel Prize for Physics in 1936. Anderson did not
coin the term positron, but allowed it at the suggestion of the Physical Review journal
editor to which he submitted his discovery paper in late 1932. The positron was the first
evidence of antimatter and was discovered when Anderson allowed cosmic rays to pass
through a cloud chamber and a lead plate. A magnet surrounded this apparatus, causing
particles to bend in different directions based on their electric charge. The ion trail
left by each positron appeared on the photographic plate with a curvature matching the mass-to-charge
ratio of an electron, but in a direction that showed its charge was positive.
Anderson wrote in retrospect that the positron could have been discovered earlier based on
Chung-Yao Chao's work, if only it had been followed up. Frédéric and Irène Joliot-Curie
in Paris had evidence of positrons in old photographs when Anderson's results came out,
but they had dismissed them as protons. Production
New research has dramatically increased the quantity of positrons that experimentalists
can produce. Physicists at the Lawrence Livermore National Laboratory in California have used
a short, ultra-intense laser to irradiate a millimetre-thick gold target and produce
more than 100 billion positrons. Applications
Certain kinds of particle accelerator experiments involve colliding positrons and electrons
at relativistic speeds. The high impact energy and the mutual annihilation of these matter/antimatter
opposites create a fountain of diverse subatomic particles. Physicists study the results of
these collisions to test theoretical predictions and to search for new kinds of particles.
Gamma rays, emitted indirectly by a positron-emitting radionuclide, are detected in positron emission
tomography scanners used in hospitals. PET scanners create detailed three-dimensional
images of metabolic activity within the human body.
An experimental tool called positron annihilation spectroscopy is used in materials research
to detect variations in density, defects, displacements, or even voids, within a solid
material. See also
Beta particle Radioactive decay
List of particles Positron emission tomography
Positronium Proton
Positronic brain References
Notes
Citations
External links What is a Positron?
Website about positrons and antimatter Positron information search at SLAC
Positron Annihilation as a method of experimental physics used in materials research.
New production method to produce large quantities of positrons
Website about antimatter. Positron Laboratory, Como, Italy
Website of the AEgIS: Antimatter Experiment: Gravity, Interferometry, Spectroscopy, CERN
Synopsis: Tabletop Particle Accelerator ... new tabletop method for generating electron-positron
streams.