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The three basic modes of radioactive decay are the emission of alpha, beta and gamma radiation:

Alpha–Unstable nuclei frequently emit alpha particles, actually helium nuclei consisting of two protons and two neutrons. By far the most massive of the decay particles, it is also the slowest, rarely exceeding one-tenth the velocity of light. As a result, its penetrating power is weak, and it can usually be stopped by a piece of paper. But if alpha emitters like plutonium are incorporated in the body, they pose a serious cancer threat.

Beta–Another form of radioactive decay is the emission of a beta particle, or electron. The beta particle has only about one seven-thousandth the mass of the alpha particle, but its velocity is very much greater, as much as eight-tenths the velocity of light. As a result, beta particles can penetrate far more deeply into bodily tissue and external doses of beta radiation represent a significantly greater threat than the slower, heavier alpha particles. Beta-emitting isotopes are as harmful as alpha emitters if taken up by the body.

Gamma–In some decay processes, the emission is a photon having no mass at all and traveling at the speed of light. Radio waves, visible light, radiant heat, and X-rays are all photons, differing only in the energy level each carries. The gamma ray is similar to the X-ray photon, but far more penetrating (it can traverse several inches of concrete). It is capable of doing great damage in the body.

Common to all three types of nuclear decay radiation is their ability to ionize (i.e., unbalance electrically) the neutral atoms through which they pass, that is, give them a net electrical charge. The alpha particle,
carrying a positive electrical charge, pulls electrons from the atoms through which it passes, while negatively charged beta particles can push electrons out of neutral atoms. If energetic betas pass sufficiently close to atomic nuclei, they can produce X-rays which themselves can ionize additional neutral atoms. Massless but energetic gamma rays can knock electrons out of neutral atoms in the same fashion as X-rays, leaving them ionized. A single particle of radiation can ionize hundreds of neutral atoms in the tissue in multiple collisions before all its energy is absorbed. This disrupts the chemical bonds for critically important cell structures like the cytoplasm, which carries the cell’s genetic blueprints, and also produces chemical constituents which can cause as much damage as the original ionizing radiation.
U.S. Arms Control and Disarmament Agency (1975), WORLDWIDE EFFECTS OF NUCLEAR WAR – SOME PERSPECTIVES, Note 3: Radioactivity