Positron

I

Introduction

Positron, elementary particle identical to the electron except for its electric charge and its magnetic moment (a property that determines how it behaves in a magnetic field). Positrons are elementary particles, which are fundamental constituents of matter—that is, they cannot be divided into smaller units. Positrons have uses in medicine and in industry, particularly in a form of imaging known as positron emission tomography (PET).

II

Characteristics and Behavior

All elementary particles have basic characteristics called mass, charge, and spin (a property analogous to angular momentum). The positron has the same mass—amount of matter—as the electron, and the same spin. The two particles also have the same amount of electric charge, but the positron’s charge is positive and the electron’s is negative. For this reason, the positron is sometimes called a positive electron. Although positrons and electrons have a measurable mass, charge, and spin, they have no measurable size, shape, or structure. Scientists therefore consider them pointlike. Other pointlike elementary particles include neutrinos and quarks.

Every elementary particle has an equal and opposite antiparticle. The positron is the antiparticle of the electron. Just as particles combine to form ordinary matter, antiparticles combine to create antimatter. When a particle and its antiparticle collide, they destroy each other, releasing energy. This feature makes positrons useful in creating PET scans, images of the brain and other soft tissues inside the body. To create a PET scan, positron-emitting substances are injected into the body. Computers track the energy released inside the body by positron-electron collisions and use this information to form images. PET scans are especially helpful in identifying and locating brain tumors and in studying other disorders in the brain. Positrons are also used in industry to reveal defects on metal surfaces and in semiconductors.

Positrons are emitted by certain radioactive substances that scientists create in the laboratory. They are also produced within stars and by collisions of cosmic rays (high energy particles that originate in space). But positrons are short-lived because they soon collide with electrons. In the laboratory, scientists create positrons by a method known as pair production. In this method a gamma ray (particle of electromagnetic energy) interacts with the nucleus of a very heavy atom, producing a positron and an electron.

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III

History

British physicist Paul Dirac predicted the existence of the positron in 1928. Dirac arrived at his prediction by applying the theory of relativity of German-born American physicist Albert Einstein to observations of the motion of particles with electric charge. To reconcile Einstein’s theory with his own mathematical analyses of the motion of electrons, Dirac had to assume the existence of a new kind of particle that was identical to the electron in all ways except the sign of its electric charge.

Dirac’s theoretical particle received the name positron, although the scientific community was reluctant to accept his findings. This reluctance vanished in 1932, when American physicist Carl David Anderson discovered the positron while tracking the paths of subatomic particles using an instrument called a cloud chamber. Some of the paths Anderson analyzed had the same curve as electron paths but with positive charges. Thus, they indicated the existence of particles that possessed the characteristics Dirac had predicted mathematically.

In 1951 Austrian physicist Martin Deutsch discovered positronium—an unstable structure in which a positron is very briefly bound together with an electron. After less than one-billionth of a second, the electron and positron destroy one other, producing energy in the form of two photons.

In 1995 scientists using positrons produced the first artificially created atom of antimatter: antihydrogen. Hydrogen is the simplest atom, combining an electron and a subatomic particle called a proton. To produce antihydrogen, a team of scientists at the European Organization for Nuclear Research (CERN) in Switzerland combined a positron and an antiproton (the antiparticle of the proton).

In 2007 a team of physicists at the University of California at Riverside reported creating molecules of positronium in a laboratory for the first time. Called di-positronium, the molecules each consisted of a pair of electrons and a pair of positrons bound together like two atoms. Thousands of positronium molecules were created when intense bursts of positrons were compressed by a magnetic field and fired into silica in a vacuum chamber. A porous silica film slowed down the positrons, allowing them to be captured by electrons to form briefly stable molecules. The molecules only existed for 0.25 nanoseconds (a nanosecond is one-billionth of a second). Such positronium molecules could provide a new way to study antimatter. When positrons and electrons annihilate each other, they release high-energy gamma-ray radiation. Creating molecules of positronium could be a step toward technologies such as extremely powerful gamma-ray lasers.