There are three types radiation: alpha radiation, beta radiation, and gamma radiation.

Alpha Radiation

Alpha radiation is a heavy, short-range particle, which contains the constituent parts of a helium nucleus. Although alpha radiation is not able to penetrate human skin, it can be quite harmful to humans if these materials are ingested. It has been consistently demonstrated that alpha emitting material can cause cancer if inhaled, swallowed, or absorbed through open wounds. An alpha particle can be easily blocked by a thin layer of water, paper, or any other thin layer of material. Some examples of alpha emitters are radium, radon, uranium, and thorium.

Beta Radiation

Beta radiation, like alpha particles, is a light, relatively short-range particle. It is actually an electron ejected from a decaying atom. Beta radiation may travel several feet and is moderately penetrating. For example, beta radiation will penetrate human skin to the the lowest layer where formation of replacement skin cells occurs. Extended exposure of high levels of beta can result in dermal insult and injury. While most beta sources can be easily detected with a Geiger counter, some produce at very low-energy levels where detection is more problematic. Examples of some pure beta emitters are strontium-90, carbon-14 and tritium.

Gamma Radiation

Gamma radiation or x-rays is long range, penetrating electromagnetic radiation. Gamma radiation is able to penetrate deep into human tissue and through most materials. Both gamma and x rays are electromagnetic radiation like visible light, radiowaves, and ultraviolet light. To effectively block such a high velocity particle requires thick concrete or lead shielding. Gamma radiation is extremely hazardous to humans, with high levels or prolonged exposure resulting in tissue damage and serious disruptions to cellular level functions. This is a high energy radiation and is commonly detected by standard survey meters. Examples of some gamma emitters are iodine-131, cesium-137, cobalt-60 and radium-226