Atom: the smallest particle of an element that can enter into a chemical reaction.
Decay, Radioactive: disintegration of the nucleus of an unstable atom by the release of radiation.
Half-Life: the time any substance takes to decay by half of its original amount. See also biological half-life, decay constant, effective half-life, radioactive half-life.
mrem: One thousandth of a rem (0.001 rem). Definition provided by the United States Nuclear Regulatory Commission (NRC).
Neutron: a small atomic particle possessing no electrical charge typically found within an atom's nucleus. Neutrons are, as the name implies, neutral in their charge. That is, they have neither a positive nor a negative charge. A neutron has about the same mass as a proton. See also alpha particle, beta particle, gamma ray, nucleon, x-ray.
Nucleus: the central part of an atom that contains protons and neutrons. The nucleus is the heaviest part of the atom.
Positrons: particle equal in mass but opposite in charge to the electron. A positive electron. Definition provided by NRC.
Protons: a small atomic particle, typically found within an atom's nucleus, that possesses a positive electrical charge. Even though protons and neutrons are about 2,000 times heavier than electrons, they are tiny. The number of protons is unique for each chemical element. See also nucleon.
Radionuclides: an unstable and therefore radioactive form of a nuclide.