ELECTRON BEAM RADIATION PROCESSING FACILITIES

EB accelerators have two important functional parameters: beam energy and beam current. The beam energy determines the penetration depth the beam can achieve, while the beam current controls the throughput that can be obtained.

EB accelerators used in radiation processing possess beam energies in the range 0.1 MeV to 10 MeV. An upper energy limit of 10 MeV for EB applications has been set to avoid, with a very high level of confidence, any induction of radioactivity in irradiated products through photonuclear reactions. High beam current is the main distinguishing feature differentiating industrial EB accelerators from equipment that is used for research purposes. While the industrial accelerators have beam currents in the tens of milliampere range (over 10 mA), the research equipment, such as Van de Graaff accelerators, Pelletrons, and many linacs, operates in the microampere range, which is orders of magnitude lower in beam

current than industrial equipment. High beam currents are desired in industry because product throughput rates are proportional to beam current.

An EB accelerator typically consists of the following subsystems:

— Source of electrons: heated cathode which emits electrons;

— Focusing device: electrons are focused into a beam with an extraction electrode;

— Acceleration unit: electrons are accelerated within an evacuated space with a strong electric field;

— Extraction window: electrons pass into the air through a thin titanium foil window.

The electrons are produced through a thermal electron emission effect by an electric device called an ‘electron gun’. The emitted electrons are focused and accelerated in a vacuum by different mechanisms to attain the final electron energy. These accelerated high energy electrons then cross a mechanically resistant thin window and are allowed to strike the objects to be irradiated.

Accelerators are capable of producing beams that are either pulsed or continuous.

Electrons emitted by accelerators have fairly narrow spectral energy limits (usually less than ±10% of the nominal energy). The energy of the electrons reaching the product is further controlled by the bending magnets of the beam handling system, if applicable.

Based on electron energy, EB accelerators used for radiation processing are classified as low, medium or high energy accelerators [6.6, 6.7].

Low energy accelerators: Accelerators in the energy range of 100 keV to 700 keV are in this category. This type of equipment is available with beam widths from approximately 0.5 m up to approximately 1.8 m. Low energy accelerators are generally self-shielded. Their applications are found in areas including surface curing of thin films and laminations, production of antistatic and antifogging films, and wood surface coatings. The maximum range of penetration could be up to 60 mg/cm².

Medium energy accelerators: Scanned beam systems with energies between 1 MeV and 5 MeV fall in this category. This type of equipment is available with beam widths from 0.5 m to 1.8 m. These units are characterized by beam powers from 25 kW to 700 kW. Because of their useful penetration ranges, these accelerators are the workhorses of the radiation processing industry with a range of applications: cross-linking of materials with thicker cross-sections, polymer rheology modification, colour enhancement of gemstones, sterilization of medical products and food irradiation (to a limited extent). Typical penetration depths in unit density material are in the range of 5 mm to 25 mm.

High energy accelerators: Accelerators with an energy range from 5 MeV to 10 MeV provide the highest penetration depth and are best suited to bulk product irradiation. Scanned beams with power levels from 25 kW to 350 kW are available with beam widths up to 1.8 m. With the penetration depth for 10 MeV electrons typically being 50 cm (when irradiated from both sides) for 0.15 g/cm³ product density, this category of accelerator is commonly used for applications such as medical product sterilization, cross-linking of thick section products, disinfestation, wastewater treatment, polymer rheology modification, colour enhancement of gemstones and shelf life extension for food and fruits.

Medium and high energy EB accelerator facilities, like a gamma radiation processing facilities, consist of the following:

— Appropriate radiation shielding surrounding the irradiation room;

— Control room housing;

— Product transport system to move in and take out the products;

— Product containers to store the products for transport during irradiation;

— Control and interlocks for safe operation of the facility;

— Loading/unloading areas for storage of products.

Figure 6.2 shows the layout of a typical EB processing facility designed for processing a high volume of products. The products enter on a conveyer through a labyrinth that permits access but stops radiation from escaping.

FIG. 6.2. Layout of a typical EB irradiation facility (courtesy of IBA, Belgium).

The treatment room houses the accelerator itself and is constructed of thick concrete to protect workers from radiation. In the treatment room, the materials pass under the accelerator for processing. After being irradiated with accelerated electrons, the materials continue on the belt until they exit the irradiation room.

The equipment area contains the electrical, electronic and cooling equipment required to run the accelerator. EB processing can provide an extremely fast treatment process with high dose rate that results in faster turnaround times and may be more compatible with a wider range of materials [6.8, 6.9].

For the disinfection of cultural heritage artefacts, high energy EBs are typically required to achieve penetration of the product and packaging. When evaluating EB irradiation for the purpose of sterilization, product density, size, orientation, and packaging must be considered. In general, EB irradiation is most suitable for irradiating low density and uniformly packaged products. It is worth emphasizing that this treatment lasts only several seconds. EBs may very often be sufficient to disinfect or sterilize small cultural heritage objects. In particular, it is useful to use EB irradiation to treat books and documents.

A very conservative market survey indicates that presently there are over 1400 high energy EB units in commercial use. The IAEA published a Directory of Electron Beam Irradiation Facilities in Member States in 2008 [6.10].