Monte Carlo calculations for a commercial cobalt-60 gamma-ray irradiator : Ionising Radiation Modelling : Topics : National Physical Laboratory

Monte Carlo calculations for a commercial cobalt-60 gamma-ray irradiator

NPL scientists carried out Monte Carlo calculations of the dose rates and dose distribution around a commercial cobalt-60 gamma irradiator, and compared them with the results from the NPL real time dosemeter.

About half of all sterile single-use medical devices are sterilised by irradiation, to an average dose of several tens of kilograys. Much of this irradiation is carried out in facilities with typically a few petabecquerel (PBq), or a few times 10¹⁵ becquerel (Bq), of cobalt-60, as several hundred individual source pencils loaded into modules with a few dozen pencils each. Since cobalt-60 decays with a half-life of 5.7 years, source racks are replenished every 6-12 months, with some source pencils replaced, and others repositioned within the source rack, aiming to maintain the form of the dose distribution in products with perhaps an adjustment in the time that product totes remain close to the source rack.

A model was constructed, using the Monte Carlo electron-photon transport code EGSnrc, of a Nordion JS9600 commercial irradiator, as used by Isotron in Bradford, Yorkshire. The product is run in along the lower deck of a conveyor, shuffles from position to position each side of the source rack, with a 'dwell time' dependent on the dose range required by the product and on the activity present in the source rack. It is then moved to the upper level and, after shuffling back past the source rack, is run out back to the warehouse.

The inside of the modelled irradiator vault (left) and a photo of Cherenkov light from the cobalt-60 source rack in the water storage pond (right – courtesy of Mark Bailey).

Dose rates around the plant will vary from location to location, with the maximum dose rates in locations very close to the source rack of the order 100 kGy/hour. The product will typically spend about two hours in the vault being irradiated.

The models were compared with measurements made using the NPL’s real time dosemeter, which records signals from an ionisation chamber onto VHS tape in a radiation-resistant package.

A graph showing the dose rates calculated from Monte Carlo models (blue) and measured using the real-time dosemeter (yellow), at the centre of a product irradiation tote.