While gamma rays have been a reliable source, the isotopes, primarily cobalt-60 and cesium-137, used to generate these gamma rays have come under heavy scrutiny worldwide in recent years. To quote the abstract of a recent publication, “Recent fears of terrorism have provoked an increase in delays and denials of transboundary shipments of radioisotopes. This represents a serious constraint to sterile insect technique (SIT) programs around the world as they rely on the use of ionizing radiation from radioisotopes for insect sterilization (A New Generation of X Ray Irradiators for Insect Sterilization, T. Mastrangelo, Oct 21, 2009).”
In addition to the terrorism fears, disposal options have become a real concern. In the United States as many as 36 states do not have disposal options for isotopes that have reached the end of their useful service lives (Sealed Source Disposal and National Security: Problem Statement and Solution Set, Deliverable […] December 2009 Table 2, page 12).