An irradiator is a large piece of equipment commonly used in research and medical laboratories. This machine subjects a sample such as human or animal cells, bacteria, yeast, or even live animals such as mice to a precise amount of gamma radiation from a radioactive source. This source is normally a thin rod of Cesium-137 or Cobalt-60 that contain hundreds to millions of Curies of radiation. Samples are exposed to the source for an adjustable time of seconds to minutes to obtain a certain amount of radiation exposure, usually measured in Gray or Rads. The cells, tissues and/or whole animals are then further analyzed to better understand the resulting damage to DNA, proteins and other macromolecules. Irradiators are also used to better understand and fine-tune radiotherapy used to treat various cancers.
Direct exposure to the radioactive source in irradiators would result in a lethal dose for humans in mere minutes. In order to avoid this irradiators consist of two separate chambers, an upper, cubic chamber with a door and a lower, cylindrical chamber. Both chambers are shielded with several inches of lead to prevent the radioactivity from leaking out of the machine. The radioactive source is kept in the lower chamber that is sealed away from the upper chamber when not in use. Scientists place their samples in the upper chamber and then seal it by closing the door. The source is then activated and it rises from the lower chamber to the upper, exposing the samples to radioactivity for a timed period. The samples are generally placed on a slowly rotating turntable so they are uniformly exposed to the source. When the time is up the source automatically lowers itself and the scientist can open the upper chamber and retrieve the samples.
Besides being used in research facilities, irradiators are also used to expose certain foods, especially raw meat, to radioactivity. The exposure kills any harmful bacteria in the food that can cause food poisoning. More foods today are being irradiated before being sold, but the public has been rather cautious about accepting the method. The major concern is that irradiating the food somehow causes it to become radioactive, which is completely untrue. (Check out gwenllian's writeup here or Whipster's writeup here for more info.)
Irradiators are always locked away from the general population due to their radioactivity hazard and only those who have been properly trained are allowed access. However, after September 11th many institutions are taking even more steps to protect their irradiators because of concerns that the gamma radiation sources could be stolen by terrorists and used to make dirty bombs. These precautions are certainly a good idea, however it would be rather difficult to steal either an irradiator or its source. Whole irradiators weigh a huge amount, requiring a fork lift or other device to move. Additionally, improperly removing the source from the irradiator risks lethal exposure to radioactivity. One can only hope these factors are enough of a deterrent.