Joseph McKeown

Radiation Processing Using Electron Linacs

The performance capabilities of a modern linac make it a commercially viable tool for enterprising applied radiation chemists and biologists. Good economics requires efficient transformation of mains power to beam power and although pulsed travelling wave linacs have been used in industrial applications for several years, their high cost per watt is a disadvantage. The cw linac with its variants is a developed technology which could be used profitably in selected applications. More integrated designs like the self-excited linac and the induction linac also offer promise for the near future. Linac design optimization in relation to the physical and chemical processes desired in the irradiated product is discussed.

Beam Position Monitor Using a Single Cavity

Beam tests have been carried out with a cylindrical cavity tuned to 2.415 GHz which is the third harmonic of the accelerator frequency. The beam excites a TM110-like mode when the centroid is displaced horizontally and an orthogonal mode when the beam is displaced vertically. The phase reversal detected when the beam crosses the plane of symmetry identifies the cartesian quadrant occupied by the beam. The signals are proportional to beam current and displacement with average slope 0.35 mW1/2.mm-1. mA-1.

Heat Transfer, Thermal Stress Analysis and the Dynamic Behaviour of High Power RF Structures

A general purpose finite element computer code called MARC is used to calculate the temperature distribution and dimensional changes in linear accelerator rf structures. Both steady state and transient behaviour are examined with the computer model. Combining results from MARC with the cavity evaluation computer code SUPERFISH, the static and dynamic behaviour of a structure under power is investigated. Structure cooling is studied to minimize loss in shunt impedance and frequency shifts during high power operation. Results are compared with an experimental test carried out on a cw 805 MHz on-axis coupled structure at an energy gradient of 1.8 MeV/m. The model has also been used to compare the performance of on-axis and coaxial structures and has guided the mechanical design of structures suitable for average gradients in excess of 2.0 MeV/m at 2.45 GHz.

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.

Accelerating Structure with on-Axis Couplers for Electron Storage Rings

A biperiodic structure with on-axis couplers is described that uses electric coupling between cavities via a relatively large beam aperture. A system with nine accelerating cavities has been designed for operation in the ¿/2 standing-wave mode at 804 MHz with 2.1% intercavity electric coupling. Results of calculations, laboratory modeling measurements and a comparison with other structures are given.

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.

Real-time confirmation of electron-beam dose

Abstract A new development in electron beam irradiation offers the opportunity to make real time measurements during irradiation to enhance quality assurance measures. With the new CDose TM scheme, the product identity and an image of the two dimensional product dose are presented to the operator as a moving electrographic image and stored to provide a permanent record of the irradiation. The electrographic images show each item on the tray and provides a non-intrusive method of assuring the orientation and packing of each item. The prescribed integrated-dose is also assured by the image. The image is produced by collecting the electron current from the accelerators beam stop and plotting the calculated product dose as a function of the instantaneous beam and tray position. The techniques described in this paper are most useful when irradiating high value items such as advanced composite aircraft parts of medical devices where evidence is required to show that each unit was processed correctly.

The Chalk River Electron Test Accelerator; A Prototype for Industrial Irradiator Designs

A cw electron linear accelerator is suggested as an efficient source of high power radiation for large scale material processing. Beams with a radiation equivalent greater than 5 MCi of 60Co have been obtained with the Chalk River Electron Test Accelerator operating at 4 MeV and from simple scaling of the design concepts of the accelerator, radiation yields at least one order of magnitude higher can be confidently predicted. Research in window development is in progress. In a waste water treatment experiment, a 0.76 mm stainless steel window was used at a power dissipation of 1 W/mm2 during a continuous 16 hour run without deterioration. In a separate facility, a 0.25 mm water cooled aluminum window designed to withstand 2 W/mm2 is used to bring the electron beam into air. Results of electron beam distribution measurements in air are discussed. Recent technological developments both at Chalk River and at electron storage facilities using similar technology indicate that the cw electron linear accelerator will predominate in applications where beam powers greater than 200 kW are required.

Electron sterilization of sewage sludge: A real case comparison with other processes

Abstract The seventies and early eighties provided high expectations for the adoption of electron treatment of waste. Despite unassailable evidence of assured disinfection, more than a decade has passed and work continues to be concentrated in the laboratory. Reasons for the lack of commitment to build municipal systems are complex. An indication of underlying hurdles may be found form AECLs experience with a recent proposal on a Sludge Management Plan submitted to the City of Edmonton, Canada. A study of this case provides economic assessments of competing technologies and establishes electron sterilization as a competitive process.

Operational Experience with Coupled-Cavity Structures in a High Duty Factor Accelerator

The Chalk River Electron Test Accelerator is a facility to study beam behaviour in a multi-tank accelerator and to develop control systems for high power operation. Two standing-wave structures have been operated at energy gradients of 0.75 MeV/m and their accelerating fields held constant under 50% beam loading. During start-up, the control systems must accommodate resonant frequency shifts exceeding ten bandwidths in both of the dissimilar structures and their resonant frequencies must differ by less than a tenth of a bandwidth before locking to the accelerator frequency. The central mini-computer controls the run-up and among other things controls the structure temperature.