Gamma and X-Ray Radiation

Gamma rays are electromagnetic radiation (photons) that originate in the nucleus of an atom. X-rays are electromagnetic radiation that is identical to gamma radiation except that they originate in the electron cloud surrounding the nucleus. Gamma radiation is emitted in discrete (monoenergetic), small packets of energy known as photons. X-rays can have either discrete energies or a whole spectrum of energies (polyenergetic) if produced by the process of bremsstrahlung. The specific properties of gamma and x-ray photons are determined by their energy which is proportional to the frequency (E=hv).

Photons interact with matter primarily by three processes: photoelectric effect, Compton scattering, and pair production. A lengthy explanation of these processes will not be presented here, but note that all three processes transfer energy to electrons and can produce electrons with sufficient energy to ionize or excite other atoms in the absorber. These interactions permit the detection of gammas and x-rays.

Attenuation refers to the reduction in the intensity of radiation (e.g., gammas and x-rays) as they pass through an absorbing material. In many cases, it may be necessary to use shielding materials to reduce exposure. The higher the photon energy, the more shielding (absorbing) material that will be needed to reduce the exposure rate. The amount of absorbing material needed to attenuate the intensity of a particular photon to one-half of its original value is called the half-value layer. The amount of absorbing material needed to attenuate the intensity of a particular photon to one-tenth of its original value is called the tenth-value layer. Photons with an energy of less than about 10 keV are attenuated mostly in the skin of the body, thus these photons present an external radiation hazard only to the skin. Higher energy photons can penetrate considerable distances into and through the body. Therefore, photons of high energy are considered an external radiation hazard to the whole body. The external radiation hazard of gamma and x-ray emitters can be eliminated with lead foil for low energy x-rays and gamma radiation or lead bricks for high-energy x-rays and gamma radiation. Gamma and x-ray emitters can cause an internal hazard and precautions must be taken to prevent the internal uptake of these emitters.

Gamma and/or x-ray emissions seldom occur alone, and are usually accompanied by other types of radiation. Common gamma emitters on campus include ¹²⁵I, ¹³¹I, ²²Na, ⁸⁶Rb, and ⁵¹Cr. X-rays are produced by some radioisotopes, but are usually machine-produced. Information in Appendix I can be used as a basis for designing shielding for some of the more common gamma emitters.

 Previous | TOC | Next