Gamma and X-rays are the most commonly used physical mutagens [22,23]. Gamma rays are emitted in the process of the decay of the radioisotopes cobalt-60 (60Co), cesium-137 (137Cs) and to a less extent, plutonium-239 (239Pu). Gamma sources containing one of these radioisotopes are typically installed as gammacell irradiators. A gammacell is used mostly for acute irradiation (i.e. for short periods). For chronic irradiations, i.e. the exposure of plants to irradiation for extended periods of times, irradiators are installed in specially designed gamma rooms (or chambers), greenhouses or fields. In general, the irradiators are sealed sources which are normally encapsulated within a stainless steel casing as a safety measure against unintended irradiation. In all cases, only adequately trained and, depending on national statutory requirements, certified personnel operate the sources. Also, special precautions, including strictly controlled access to the facility, are imposed in order to prevent accidents.