Electron Accelerators

All electron accelerators include a source (of electrons), an evacuated accelerating chamber, and a system for extraction from the vacuum and distribution over the product surface. Most obtain their electrons from a heated filament source (similar to that of a TV tube) called the electron gun. The energy of these electrons is then increased in one or more stages as they pass through a vacuum with an applied electric field. There are numerous ways to generate this electric field. DC accelerators generate and maintain the full accelerating voltage between just two electrodes. As the voltage is raised to millions of volts, electrical insulation becomes a major engineering problem. Even at low powers, these accelerators can have dimensions exceeding 15m (45 feet). For systems with both the energy and the power needed for industrial irradiation, the most common commercially available models are the Dynamitron®, and Insulated Core Transformer (ICT). The maximum voltage available is usually less than 5 MeV.

A second family of particle accelerators is based on radio frequency (rf) power technology. These high frequency waves generate very intense electrical and magnetic fields in suitably shaped conducting cavities. By matching the field oscillations with the injection of charged particles, the rf fields can drive the particles to high energies without having to create the full final potential at any one instant. They do not therefore require the large scale insulation of DC units and are more compact. They can easily reach energies of 10 MeV, and currently are available in models reaching up to 200 kW of electron beam power.

The rf accelerators that employ a linear series of rf cavities are called linacs. The assembly of copper cavities is referred to variously as a structure, a guide or a waveguide. They operate either at an rf frequency of 3 GHz (known as S-band; more compact but generally limited to beam powers below 20 kW) or 1 GHz (L-band; physically larger but capable of beam powers considerably above 20 kW). The accelerating structure of a typical 10 MeV industrial linac is 2-4m long. The main power tube for linacs is the klystron. These tubes are expensive and have limited operating life. Their replacement expense can become a significant fraction of the operating cost.

A more compact rf accelerator (the Rhodotron®) has been developed. This device generates radial accelerating fields in a single cavity and uses an array of magnets to pass the beam through this cavity zone on repeated orbits. The accelerating structure for the higher power models is a tank about 1.5 m diameter systems).

Differences in physical size between dc and rf accelerators are significant because dc accelerators require costly larger buildings and shields. Differences between the rf models are less important once the total cost of the shielded facility has been included..

Production staff must be protected from the radiation source. This protection or shielding is usually provided by placing the source inside a concrete vault and passing the product into the radiation zone through a maze. Commonly used material for radiation shields are concrete, steel and lead. The thickness of a lead shield will be about 1/5th that of concrete. The higher cost of lead and steel however generally make them uneconomic for all but in-line sterilizers.

A reduction of the level of radiation to less than 1/1,000,000 of the source level is necessary and this requires about 2.5 m of concrete for a 10 MeV beam or 2.0 m for 5 MeV accelerators.

Radiation itself causes degradation in the product conveyor components, particularly where any plastics or lubricants are used. In addition, radiation beams form high concentrations of ozone in air and this is both corrosive and hazardous. To combat the corrosion, extensive use is made of stainless steel and a regular inspection and replacement schedule for sensitive parts has to be established. All radiation plants need powerful ventilation systems to remove the ozone.

Equipment: Recent Developments.

The industrial uses of accelerated electrons, and gamma rays from cobalt 60, had both been identified well before 1980. Industries based on low energy electrons (<750 keV) and medium energy electrons (< 5 MeV) had been established for coatings, tires, cable and heat-shrink tubes. The DC technology is very mature and reliable. It dominates in the coatings, wire, cable and tire industries. The use of cobalt-60 for sterilization is routine and several hundreds of plants have been installed.

Prior to 1990, a number of S-band 10 MeV rf linear accelerators were installed in Europe for medical device sterilization. These 10 MeV machines performed satisfactorily up to about 10 kW.

DC accelerators with energies near 5 MeV, fitted with X-ray converters were commissioned in Japan to compete with cobalt in that market. The conversion technology was demonstrated to be satisfactory.

In the 1990’s the power available from accelerators jumped significantly with AECL’s 10 MeV, 50 kW linac followed quickly by IBA’s Rhodotrons at 100 kW.

The linear rf technologies are fully reliable at 10 MeV and up to 75kW at L-band frequencies, but despite the efforts of four manufacturers it seems that compact S-band units cannot reliably provide more than 30 kW.

When Russian technology became accessible to western buyers several different types of accelerators were offered for sale. Of these, perhaps the most interesting is the single resonant cavity ILU from the Budker Institute in Novosibirsk. The ILU’s provide energies to 5 MeV at modest powers compared with the Dynamitron and Insulated Core Transformers but are much small in physical size.

Suppliers Of Electron Beam Accelerators

Accelerator Type Supplier Country

DC (Dynamitron) RDI or Radiation Dynamics (a division of IBA) USA

DC Machine of the ICT Type Nissin High Voltage Japan

S Band Linac L3-PSD (formerly Titan) USA

S Band Linac Linac Systems France

S Band Linac Mevex Canada

S Band Linac Mitsubishi Heavy Industries Japan

L Band Linac Iotron Industries Canada

Rhodotron IBA (Belgium) Belgium

ILU Budker Laboratories Russia

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