Certain atoms contain an unstable complement of neutrons and protons in their nuclei, and undergo spontaneous disintegration to form other atoms plus radiation. The rate of radioactive decay is measured in half-lives, the time required for ½ of the atoms of a sample to decay, which vary from a fraction of a second to hundreds of thousands of years. The three principle forms of radiation emitted by decay are alpha particles, beta particles, and gamma radiation. Alpha (α) radiation consists of two protons bound to two neutrons and is the least penetrating form of radiation, but it is the most damaging form, with direct exposure, because of its relatively great mass. Beta (ß) radiation is a similar to an electron and is more penetrating than alpha radiation. Gamma (γ) radiation has no mass and can pass through buildings. It interacts with biological tissue causing mutations, sickness, and death, depending on the dose level.