Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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).


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.