In air, beta particles may be stopped by plastic or wood. Gamma and X-rays are forms of electromagnetic radiations because they have both electric and magnetic properties. Gamma rays, or photons, come from the nucleus when materials decay. X-rays are a result of electron removal or rearrangement in atoms. Gamma and x-ray radiations are used frequently in medicine because they can easily penetrate the human body.
Neutrons are heavy, uncharged particles that cause the atoms that they strike to become ionized.
External Exposure comes from a source outside the body, such as a medical x-ray. To do harm, the radiation must have enough energy to penetrate the body. The longer a person is exposed to a source of radiation, the higher the radiation dose. The relationship between distance and exposure is not as simple because the intensity of radiation falls off very quickly. At three feet, the rate will be one-ninth of 1rem per hour or 0.11 rem per hour.