Industrial irradiation is a very effective way of sterilisation of goods for primarily medical or laboratory use. An irradiation facility is mainly composed of:

• a chamber made of thick (2 m) concrete walls that prevent radiation to escape to the environment.
• a source of ionizing radiation inside the chamber.
• a warehouse where irradiated products are physically separated from non-irradiated products.
• a conveyor belt that takes the products from the untreated area of the warehouse to the irradiation chamber, and then from the irradiation chamber to the treated area of the warehouse.
• computerized systems to control the time of exposure of the products to the source of radiation.
• safety systems that ensure that nobody can enter the irradiation chamber when it is in operation and that nobody can start operation when there is someone inside the chamber.
• various auxiliary systems for ventilation, supply of compressed air and preservation of water quality where there is a pool.

The source of radiation can be,

• Cobalt-60, a man-made radioisotope-emitting gamma rays.
• an electron beam machine producing electrons of high energy (beta rays).

The emission of gamma rays by Cobalt-60 cannot be stopped. In gamma irradiators, the operation is stopped by lowering the source into a 6-7 meter deep pool where there is a 2 meter deep layer of water on top of the source. The water cover absorbs the radiation making entry into the chamber perfectly safe. Gamma allows treating dense and thick products. The exposure time is generally counted in hours and minutes. The half life of cobalt is 5.25 years. This means that the activity in 150 years will reach 2.7 mCi., which is the limit for classification as a non-radioactive material. The residual will be a stable Nickel isotope.

Radioactivity means that instable atoms fall apart and thereby emit ionising radiation. Co60 is one example of a radioactive isotope that emits so called gamma rays (= high energy photons). The activity is measured in amount of broken atoms per second. This unit is called Bequerel (Bq). Note that it is a very small unit. A normal load of a reasonably big gamma plant, measured in the older unit, could be 500 kCi ( kilo Curie). It will correspond to 1.85*1016 Bq. This big amount of zeroes makes Bequerel not a very practical unit. Curie is therefore the prevailing unit in the business.

Electron beams (beta rays) are produced by an electrically powered accelerator and can be interrupted at will. They are less penetrating than gamma radiation and do not allow the uniform processing of dense or thick packages. The exposure time is counted in seconds.

GAMMA AND BETA DIFFERENCES

Beta is given at a high dose rate. Each box can therefore be treated individually and a sterilisation dose can be given within a few seconds. Therefore goods can be treated batch-wise, one box after another. In gamma deep penetration is traded for a low dose rate. Depending on the amount of cobalt loaded it may take several hours or even days to accumulate a high dose. A box-by-box treatment is therefore not feasible. All goods are loaded simultaneously on a hanging or standing conveyor that transports the goods towards the radioactive source. Beta treatment can be carried out while the customer is waiting. This is not the case for gamma. The production schedule and the Quality Assurance manuals are adjusted to these different circumstances.

GAMMA AND BETA SIMILARITIES

Gamma is an electromagnetic wave emanating from radioactive material. In industrial practice Cobalt-60 is the only practical alternative. The electromagnetic wave from Cobolt-60 has a very good penetration (meters). It interacts with matter in a complicated way and after a series of chemical reactions the end result is slow electrons.

In beta, or E-beam, the electrons emitted in the electron gun at the back of the accelerator will be accelerated in order to get fast electrons which are the only electrons capable of penetrating matter deep enough. The acceleration is initiated and maintained by microwaves (like in radar) and the electrons are riding on these microwaves. At the end of the acceleration the fast electrons continue by themselves through a window and create a curtain of electrons through which the goods are transported. The fast electrons interact with the matter and the end results, as in gamma, are slow electrons.

The slow electrons create free radicals. So, in terms of function, gamma and beta are the same. Therefore a sterilization dose of 25 kGy is the same independent of which source of radiation you have - beta or gamma. These concepts are the common wisdom today and are the basis for regulations and standards.