The Irradiation industry: how is irradiation carried out?

As radiation passes through dense materials such as lead, concrete or even water, it loses energy and ceases to be dangerous to microbes or humans. This is known as ‘attenuation’. The key factor in the design and operation of an irradiation plant is safety. The process is carried out within an enclosure (cell) whose walls and roof are 2 metre thick. The source of gamma radiation, the element cobalt 60, is stored in a deep water pond or a shielded trench. Government controlled safety precautions are taken to protect people working with radiation. Exposure to high doses of radiation could be fatal. It is important to remember, however, that when a product leaves the irradiation process it is not radioactive because the energy of the radiation used in processing is relatively low.

PRODUCING GAMMA RADIATION

All atoms of the naturally occurring element cobalt 59 have 27 protons and 32 neutrons present in the nucleus and 27 electrons moving outside the nucleus. Isotopes are atoms of the same element but with different numbers of neutrons. By bombarding cobalt 59 with neutrons an additional neutron can be captured by the nucleus converting it into cobalt 60. Cobalt 60 is specifically made for use in gamma irradiation plants – it is not a waste product of nuclear power plants.

Cobalt 60 is radioactive and is called a radioisotope. It is unstable and in trying to become stable it emits a beta-particle and two photons of gamma radiation. An isotope of nickel remains. The half-life of cobalt 60 is 5.27 years. A pellet of cobalt 60 has a useful life of 20 years by which time its activity is reduced to about 1/14th of the original. Long term storage will allow the activity of the pellet to decay to safe levels. The gamma processing plant is designed for continuous operation. Material to be irradiated is moved around the source by a conveyor. Before starting the process, checks are made to ensure that no personnel are inside the cell and that the door is locked. When irradiation is completed, the source is lowered. The cell is now safe for personnel because no radiation remains. Safety precautions ensure that the entrance door to the plant can only be opened when the source is in a safe storage position. (When cobalt 60 rods need replacing they are transferred from a shielded container to the storage position).