Cobalt60 irradiator. The Gammacell 220 ® (MDS Nordion, Ottawa, Canada), an example of a conventional self-shielded irradiator. In the irradiate position the sample chamber is surrounded by several rods or "pencils" of the isotope. The dose rate of the cell is determined by the activity of the source, and the absorbed dose delivered to the insects controlled by adjusting the exposure time. The sample chamber is a vertical cylinder, approximately 150 mm diameter by 200 mm tall (3.7 L) and has a typical dose uniformity ratio of about 1.7. Such self-shielded isotopic irradiators (60Co or less commonly 137Cs) are the main means of insect sterilization for SIT programmes worldwide.