Norbert H. Drewell

The measurement, control, and validation of critical parameters in an electron beam sterilization facility

Abstract The delivery and validation of a specified dose to a medical device are key concerns of operators of electron beam irradiation facilities. In an IMPELA-based irradiator 1 , four of the parameters that directly influence the absorbed dose distribution in the product are controllable in real time — the electron energy, average beam current, scanned area, and the product exposure time. The 10 MeV accelerator operates at 50 kW with a stream of 200 μs wide, 100 mA pulses at a repetition rate of 250 Hz. The combination of short-term intra-pulse regulation with long-term pulse-to-pulse stability makes the IMPELA output attractive for the sterilization of medical products. The measurement and closed-loop control techniques used in the IMPELA design with be described with reference to facilitating compliance with medical sterilization standards.

Beam scanning for dose uniformity

A low max/min ratio of absorbed dose is a prime objective in any industrial irradiation process. The IMPELA® electron beam irradiator has been further developed to deliver a beam distribution that can be tailored to match the required dose uniformity for specific products and applications. Thus, for the first time, the beam source can be readily adjusted to create an acceptable max/min ratio. Prior knowledge of product 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 methods are examined: deflection of the beam across the product width within each beam pulse, and a slow scan covering the width with many beam 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.

Photon energy limits for food irradiation: a feasibility study

The penetrating nature of the photons produced by the X-ray (bremsstrahlung) process makes them attractive for the treatment of dense materials in industrial radiation processing. The inefficiency of the conversion process can be balanced by the exposure of high Z materials to electrons with high energy. However, activation of the product being irradiated and the equipment imposes energy limits. Results of a theoretical and experimental study to investigate the key parameters have resulted in recommendations for the design of X-ray converters in the electron energy range 7 to 11 MeV. Results of the Monte Carlo calculations and the methods used in conversion experiments with the 50 kW IMPELA accelerator are reported.

AECL IMPELA electron beam industrial irradiators

Abstract A family of industrial irradiators is being developed by AECL to cover an electron-beam energy range from 5 to 18 MeV at beam powers between 20 and 250 kW. The IMPELA family of irradiators is designed for push button, reliable operation. The major irradiator components are modular, allowing for later upgrades to meet increased demands in either electron or X-ray mode. Interface between the control system, irradiator availability and dose quality assurance is in conformance with the most demanding specifications. The IMPELA irradiators use a klystron-driven, standing-wave, L-band accelerator structure with direct injection from a rugged, triode electron gun. Direct control of the accelerating field during the beam pulse ensures constant output beam energy, independent of beam power. The first member of the family, the IMPELA 10 50 (10 MeV, 50 kW), is in the final stages of assembly at Chalk River Nuclear Laboratories. The IMPELA 10 50 is constructed around a 3.25 m long, high-power-capacity accelerator structure operated at a duty factor of 5%. Beam loading exceeds 60%. The rf power is provided by a 2 MW/150 kW modulated-anode klystron protected from load mismatches by a circulator. This prototype will be used to demonstrate the reliability and dose uniformity targets of the IMPELA family. Full beam operation of the IMPELA 10 50 is scheduled for early 1989.

The impela accelerator, field performance and new developments

Experience with three industrial irradiators with over 20,000 hours of beam operation is reported. Research has now shifted from accelerator development to new shielding designs, split beams, photon conversion, dose monitoring, beam scanning and QA controls. Activities in these areas are described.

Custom-tailored dose with the IMPELA electron irradiator

Abstract For maximum utilization of the electron beam, the exposure dose should be tailored to match the geometric and density variations in the product. Such tailoring has already been demonstrated [1] with the spot scanner, which lays down a series of overlapping “spots” of controlled intensity across the product. The new IMPELA macroscanner sweeps every beam pulse across the entire width of the product, and can also be adjusted to provide a non-uniform surface dose. Experience with the spot scanner is summarized, and new experimental results with the macroscanner are presented. The experiments illustrate the flexibility in exposure-dose distribution that can be achieved by varying the beam pulse duration and timing relative to the scan waveform.