A positron is a positively charged, with a resting energy of at least 511 keV, subatomic particle. A positron is the antiparticle of an electron, identical in mass and spin.
Positrons can be generated by positron decay or pair production.
Positron emission tomography detects positrons from the decay of radioactive tracers.

See also Beta Decay.

An atom is the basic particle of matter, the smallest (~10^-8cm) particle of a chemical element. Atoms consist of a nucleus with neutrons, positive charged protons and orbiting negative charged electrons. The chemical properties of elements are defined largely by the number of protons in the nucleus. The number of electrons is similar to the number of protons. An atom is ionized when the number of electrons is not equal to the number of protons; the resulting electrical charge depends on the difference between the number of protons and the number of electrons.

See Rutherford-Bohr Atom Model, Ionization.

An electron is a negatively charged subatomic particle that orbits the positively charged nucleus of an atom and determines chemical properties. The mass of an electron is around 1/1837 that of the proton.

See also Rutherford-Bohr Atom Model, Beta Particle.

An ion is an atomic particle that is electrically charged, either negatively or positively by loss or addition of one or more electrons. The simplest ions are for example hydrogen ions (a proton, H+), or an alpha particle (helium ion, He2+).
Positively-charged ions have fewer electrons than protons. They are cations due to the attraction to cathodes.
Negatively charged ions have more electrons in the electron shells than they have protons in the core. Due to their attraction to anodes they are named anions.

Henri Becquerel demonstrated beta particles in 1900. Identical with electrons is there negative charge at -1. Their mass is 549 millionths of one AMU, 1/2000 of the mass of a proton or neutron. Beta particles consist of high energetic electrons emitted by radioactive nuclei or neutrons. By the process of beta decay, one of the neutrons in the nucleus is transformed into a proton and a new atom is formed which has one less neutron but one more proton in the core. Beta decay is accompanied by the emission of a positron (the antiparticle of the electron), a positive charged antineutrino. Beta particles have a greater range of penetration than alpha particles but less than gamma rays or x-rays. The name beta was coined by Rutherford in 1897. The traveling speed of beta particles depends on their energy. Because of their small mass and charge beta particles travel deep into tissues and cause cellular damage and possible cancer.

See also Radiation Shielding.