BEAM SCANNING FOR DOSE UNIFORMITY

A low max/min ratio of absorbed dose is a prime objective in any indus trial irradiation process. The IMPELA(R) electron beam irradiator has been further developed to deliver a beam distribution that can be tail ored to match the required dose uniformity for specific products and a pplications. Thus, for the first time, the beam source can be readily adjusted to create an acceptable max/min ratio. Prior knowledge of pro duct geometry and density anisotropy, together with off-line dosimetry measurements, are used to specify the beam distribution necessary to provide a uniform absorbed dose. The characteristics of two scanning m ethods are examined: deflection of the beam across the product width w ithin each beam pulse, and a slow scan covering the width with many be am pulses. Product-specific distributions are achieved by reducing the duration of individual pulses and tailoring the scan waveform during a slow scan. Examples will be given of irradiations carried out on the 10 MeV, 50 kW accelerator at Chalk River Laboratories.