For many years, the determination of gamma-ray dose-rate in and around irradiation facilities has been performed using simple methods. This is because apart from radiation streaming along mazes, the geometry of the plant and irradiated product lends itself to hand calculations using point kernel methods with build-up factors accounting for scattered radiation.

Recently, however, the irradiation of denser products than before (possibly including food) means that more accurate calculations are required in order to optimise the dose uptake throughout the product.

RANKERN has been used to determine the dose uptake through irradiated material. A large rectangular source of Co60 irradiates the totes which contain the product - in this case represented by polythene of density 1.0g/cc.

The geometry can be modelled to any degree of detail, even down to individual sources. RANKERN has the capability to specify one source, then replicate it in arrays so that the whole source structure is easily modelled in as much detail as required. Similarly, one tote can be modelled accurately and repeated along the stack. Because RANKERN can perform a series of calculations with various parameters, e.g. location of a tote, changing with each calculation, the movement of the totes through the plant can also be simulated.

The variation of dose-rate at mid-height through one of the totes (along and either side of the green line) shows how RANKERN can rapidly provide information which can be used to optimise the irradiation programme.