The decay constant is inversely proportional to the radioactive half-life time. It is the fraction of a number of atoms of a radionuclide that disintegrates in a unit of time.

See also Half-Life, Decay.

See Daughter Nuclide.

[Beta Plus Decay] If an atom is unstable because there are too many protons in the nucleus, a proton is converted into a neutron and a positron is emitted. The atomic mass of the atom stays unchanged, but the number of protons increases by one, the number of neutrons decreases by one, which transforms the atom to a different element.

See also Beta Decay.

Radioactive decay is the change of instable atoms to a more stable state. This change to a different nuclide by the spontaneous emission of radiation such as alpha or beta particles, gamma rays, or by electron capture follows an element-specific decay chain. Each step in the decay chain has a definite half-life.
Sometimes also the reduction of excitation energy of the nucleus by e.g. internal conversion is mentioned as radioactive decay.

See also Decay Chain, Radioisotope.

Law which describes the reduction in radioactivity of a material in dependence of time.

See also Decay and Half-Life.