3. Electron accelerators for radiation sterilization
3.1. INTRODUCTION
Industrial radiation processes using high power electron accelerators are attractive because the throughput rates are very high and the treatment costs per unit of product are often competitive with more conventional chemical processes. The utilization of energy in e-beam processing is more efficient than typical thermal processing. The use of volatiles or toxic chemicals can be avoided. Strict temperature or moisture controls may not be needed. Irradiated materials are usable immediately after processing. These capabilities are unique in that beneficial changes can be induced rapidly in solid materials and preformed products [3.1, 3.2].
In recent years, e-beam accelerators have emerged as the preferred alternative for industrial processing as they offer advantages over isotope radiation sources, such as (a) increased public acceptance since the storage, transport and disposal of radioactive material is not an issue; (b) the ability to hook up with the manufacturing process for in-line processing; (c) higher dose rates resulting in high throughputs. During the 1980s and 1990s, accelerator manufacturers dramatically increased the beam power available for high energy equipment. This effort was directed primarily at meeting the demands of the sterilization industry. During this era, the perception that bigger (higher power, higher energy) was always better prevailed, since the operating and capital costs of accelerators did not increase with power and energy as fast as the throughput. High power was needed to maintain low unit costs for the treatment. During the late 1980s and early 1990s, advances in e-beam technology produced new high energy, high power e-beam accelerators suitable
for use in sterilization on an industrial scale [3.3]. These newer designs achieved high levels of reliability and proved to be competitive with gamma sterilization by 60Co and fumigation with EtO. In parallel, technological advances towards ‘miniaturization’ of accelerators also made it possible to integrate self-shielded systems in-line.
3.2. ACCELERATOR CLASSIFICATION
The first charged particle accelerator was constructed nearly 80 years ago. The fast growth of accelerator development was connected to the rapid growth of nuclear experimental studies at that time. The cascade generator, electro- static accelerator, linear accelerator and cyclotron were constructed in a short period of time at the beginning of the 1930s. The main differences between those accelerators were based on the method of electric field generation, related to accelerating section construction and accelerated particles trajectory shape. The primary accelerator application was strictly related to the field of nuclear physics. The fast development of accelerator technology created the opportunity to increase the field of application towards chemistry, medicine and industry. New ideas for accelerator construction and progress in the technical development of electrical components were the most important factors in the process of accelerator technology perfection.
The progress in accelerator technology development means not only a growing number of units but also lower cost, compact size suitable for the production line, beam shape specific to the process, reliability and other parameters, which are important in the radiation processing application.
Advances in high power switches technology, core amorphous ferro- magnetic materials, modulator macropulses technology, and the continuous wave operation of microwave generators are being transferred continuously to the development of industrial accelerators. The computers for automatic control and parts, such as power switches, thyristors, thyratrons and the new generation of microwave sources, are the best examples of the technology transfer that allowed perfecting accelerator construction. Industrial accelerator development is still in progress, not only because of new kinds of applications, but also because of demands of lower cost, more compact size suitable for the production line, beam shape specific to the process and other parameters which are important for radiation processing implementation [3.4, 3.5]. Electron accelerators used for radiation processing are classified
— Low energy: The accelerators in the energy range of 400 keV to 700 keV are in this category. Even lower energies in the range of 150 keV to 350 keV, single gap, unscanned beams with extended electron source and beam currents from a few mA to more than 1000 mA are available for surface curing applications. In the 400 keV to 700 keV range, the beam currents are available from 25 mA to approximately 250 mA. This type of equipment is available in beam width from approximately 0.5 m up to approximately 1.8 m. All the low energy accelerators are generally the self-shielded type. The applications are found in the areas of surface curing of thin films, laminations, the production of antistatic, antifogging films, wood surface coatings, etc. The maximum range of penetration could be up to 60 mg/cm2. More recently, a 200 keV, 1–10 mA accelerator with a scanned beam was developed by Linac Technologies for a new application, namely, surface sterilization of a pharmaceutical component [3.7].
— Medium energy: Scanned beam systems with energy between 1 MeV and 5 MeV fall in this category. This type of equipment is available in beam width from 0.5 m to 1.8 m. These units are characterized by beam powers from 25 kW to 300 kW. These units are used for 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 use) because of higher penetration ranges. Typical penetration depths in unit density material will be in the range of 5 mm to 25 mm.
—High energy: The accelerators having an energy range from 5 MeV to 10 MeV provide the highest penetration thickness and are best suited to bulk product irradiation. Scanned beams with power levels from 25 kW to 350 kW are available with beam width up to 1.8 m. With a penetration depth for 10 MeV electrons typically being 50 cm (when irradiated from both sides) for 0.15 g/cc product density, this category of accelerators is commonly used for medical product sterilization, cross-linking of thick section products, food disinfestation, wastewater treatment, polymer rheology modification, colour enhancement of gemstones, and shelf-life extension for food and fruits, etc.
Table 3.1 lists various applications suitable for different e-beam energies, along with the maximum thickness of the product that can be irradiated with acceptable dose variation.
Accelerators used for radiation processing extract the beam in a larger area defined by beam width or scanning width that is typically between 0.5 m and 2 m. The product is conveyed to and from under this zone to get the
required dose using suitable product conveyors. The electron penetration is proportional to the energy and inversely proportional to the product density. Following is the basic formula that describes penetration:
Penetration (cm) = (0.524E – 0.1337)/
where E is the beam energy in MeV and is the density in g/cm3. This equation applies to electron energies greater than 1 MeV.
The main parameters of interest in electron accelerators are the beam energy and current. The energy decides the thickness of the product over which it can be irradiated with acceptable dose variation, and the dose rate at which the product can be irradiated is decided by the current. The process thickness, which is defined as the depth at which the dose equals the entrance (surface) dose, is a crucial parameter to be evaluated for the material of interest for the selection of appropriate beam energy. This can be seen in Fig. 3.1, where depth– dose distributions for different electron energies are shown; dose distribution for 60Co gamma rays is also shown for comparison. To increase the process thickness, the product is irradiated from two opposite sides. The following expressions give the relation between the process thickness, d (in cm), and the energy [3.8]:
3.3. SERVICE CENTRE ACCELERATORS
Accelerators are made in three types: DC type, where a constant beam is extracted; microwave pulsed type (GHz), where the output beam is repeated at a low frequency (repetition rate); and pulse or continuous wave type, where lower radiofrequency (100–200 MHz) accelerates electrons with each amplitude. All of them — DC, RF and microwave accelerators — have become the workhorse of radiation processing and are extensively employed. DC accel- erators give high average beam power whereas the microwave accelerators, operated in the pulsed mode, give low average power. On the other hand, microwave accelerators have high energy gain per unit length, thus are more compact in construction compared to the DC accelerators.
Continuous wave RF type accelerators provide a DC-like beam current at higher energies. Due to the penetration range and the fact that products of different density are delivered to the service centres, almost exclusively accel- erators of electron energy from 5 MeV to 10 MeV are used in this case [3.9]. Low energy electrons cannot penetrate a product deeply and lower electric power has a smaller throughput.
DC voltage is used to accelerate electrons in the direct acceleration method (Fig. 3.2(a)). The necessary DC voltage power supplies are usually based on high power, oil or gas filled HV transformers with a suitable rectifier circuit. They are simple and reliable accelerator components. An HV cable is usually used to connect an HV power supply to the accelerating head, for a voltage level not higher than 0.8 MV. The MV level in a conventional
transformer is impractical because of technical problems with the insulation and dimensions of such a device. Different types of inductance or capacitance coupling make it possible to increase relatively low primary voltage up to 5 MV by multistage cascade systems.
An RF accelerator is based on a large, single cavity operating at a frequency between 100 MHz and 200 MHz. The high power vacuum tubes are applied to provide necessary electromagnetic energy that is used to accelerate electrons in a single pass (Fig. 3.2(b)) or multipass system. These inexpensive and reliable components require relatively simple and compact DC or pulse modulators to generate UHF oscillations. Medium and high electron energy levels with high beam power can be obtained.
The main feature of a linear accelerator is related to the use of microwave energy in the electron accelerating process. Power supplies consist of pulsed microwave generators. A large number of small resonant cavities are used (Fig. 3.2(c)). The accelerating structure can provide an electric field over 10 MV/m as compared to 2 MV/m for DC accelerators, due to magnetic isolation that is present in such systems. That makes linear accelerator construction very compact. However, the overall electrical efficiency of a microwave linac is 10–20% because of the power loss in the microwave generator and accelerating tube.