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Source: Institute of High Energy Physics, Chinese Academy of Sciences
Electron irradiation accelerators come in many types with different performance characteristics and applications. Based on their energy ranges and main applications, they can be divided into three categories:
(1) Low-energy (0.1 MeV - 0.3 MeV) accelerators: The representative type is the electron curtain accelerator, a high-voltage accelerator that has no accelerating tube or scanning device. It is small in size, regular in appearance, self-shielding, and relatively simple in structure, with a beam power typically of 150 - 200 kW. It is mainly used for radiation sterilization of agricultural products such as grain, radiation curing of surface coatings—especially for curing coatings on engineering plastics, panels, glass, and other building decoration materials—preparation of functional membranes such as seawater desalination membranes and microporous composite separators for lithium batteries, waste gas treatment, rubber vulcanization, and more. A new application of electron curtain accelerators is the disinfection and sterilization of mail or package surfaces. The U.S. Postal Service once purchased 8 electron beam accelerators to sterilize documents destined for key government agencies to ensure security.
(2) Medium-energy (0.3 MeV - 5 MeV) accelerators: The representative type is the high-frequency high-voltage accelerator, also known as the Dynamitron accelerator. It is currently the most widely used type of irradiation accelerator in the world, with relatively stable performance. Its beam energy is typically 2 - 5 MeV, with power ranging from tens to over a hundred kW. It is mainly used for radiation crosslinking of polyethylene insulation materials for wires and cables and polyethylene foamed plastics, rubber vulcanization, and radiation production of high-strength temperature-resistant polyethylene heat-shrinkable tubes.
(3) High-energy (5 MeV - 10 MeV) accelerators: The representative type is the linear accelerator, which uses microwave electromagnetic fields to accelerate electrons. Based on the microwave type, it can be classified into traveling-wave and standing-wave types. The electron energy is generally relatively high (>5 MeV), with output power ranging from a few kW to tens of kW. The maximum electron energy of electron linear accelerators has exceeded hundreds of GeV, but due to industrial radiation safety restrictions, the maximum energy of general industrial irradiation electron linear accelerators is limited to 10 MeV. Within the scope of radiation processing safety permits, high-energy (>5 MeV) electrons have deeper effective penetration into irradiated objects, and the conversion efficiency to X-rays is also higher, allowing conversion to X-rays for irradiation, and thus they are widely used. They are mainly used for radiation disinfection of medical products, food preservation, modification of electronic components, tire vulcanization, and other applications.
Electron linear accelerator (developed by the Institute of High Energy Physics, Chinese Academy of Sciences)
Electron Linear Accelerator
A linear accelerator (linac) is an accelerator device in which charged particles are accelerated by high-frequency electric fields and travel along a straight path. In a linear accelerator, the axial high-frequency electric field is generated by microwave power fed from a microwave power source (typically at a frequency of 0.5 - 30 GHz). The beam is synchronously accelerated with the alternating electric field, forming multiple beam bunches. The energy gained by the beam depends on the average electric field strength and the length of the acceleration path. Therefore, linear accelerators are not constrained by the difficulty of further increasing the high voltage of high-voltage power supplies in high-voltage accelerators, and beam energies can reach the GeV range. Compared with other high-energy accelerators, linear accelerators have the advantages of simple injection and extraction, high beam utilization efficiency, and high beam intensity. Since linear accelerators can easily achieve higher beam energies, the high-energy accelerators in the 5 – 10 MeV range used in radiation processing are mainly electron linear accelerators.
The basic components of an electron linear accelerator mainly include the electron gun, accelerating structure, focusing system, modulator, power source, microwave transmission system, vacuum system, control system, constant-temperature system, and beam transport system. The figure below is a structural block diagram of an electron linear irradiation accelerator system. The main functions of each component are as follows: the modulator generates two high-voltage pulses; one excites the power source, and the high-power RF pulse generated by the power source is fed through the microwave transmission system into the accelerating tube, establishing an accelerating electric field in the tube. The other high-voltage pulse is applied to the electron gun to extract the electron beam, which is injected into the accelerating tube and accelerated by the accelerating electric field. The focusing system ensures that the beam always travels along the central orbit during acceleration, maintaining a small beam cross-section. The irradiation accelerator also includes a scanning magnet and scanning horn system. The high-energy electron beam emerging from the accelerating tube is transported through the beam transport system to the scanning magnet, which deflects the beam back and forth within a fixed angle. After passing through a relatively long scanning horn, the scanning width of the emerging electron beam is increased, i.e., the irradiation range is expanded, allowing irradiation of larger-sized items.
Irradiation accelerator layout
Product Conveyor System
Irradiated products are placed on a chain conveyor belt, which can handle various packaging or different types of products. The conveyor belt passing through the electron beam is made of flexible thin metal sheets. Product loading and unloading operations can be performed manually or automatically. Products are arranged in a planar layout on the conveyor belt, controlled to achieve maximum beam utilization and reproducible dose distribution. The conveyor speed is controlled to match the electron beam current and beam width so that the irradiated products receive the predetermined dose. When boxes need to be turned over for double-sided irradiation, the simplest method is to have the boxes transported out of the irradiation room and turned over manually. A safer and more reliable method is automatic turning. Generally, the container conveying speed is 0.5 - 15 m/min, with a control accuracy of 0.8%.
Personnel Radiation Protection System
Shielding walls are required around the irradiation area to attenuate X-rays, which are generated when high-energy electrons strike objects such as products, conveyor belts, walls, and floors. The thickness of the shielding walls increases with electron energy and decreases with increasing atomic number of the shielding material. For a 10 MeV electron accelerator, the side concrete shielding thickness is approximately 2 m, and the direct-beam concrete shielding thickness is approximately 2.5 m. To reduce scattered radiation, the conveyor belt and personnel entrances and exits should be designed as maze passages.
To prevent personnel radiation accidents, a safety interlock system must be installed in areas where irradiation equipment is installed to ensure that personnel are not accidentally exposed. In addition, personal film dosimeters are used to record the cumulative dose of personnel.
Dosimetry Experiment System
Absorbed dose value—the quantitative indicator of radiation processing.
Accurate and reliable dose measurement is of great significance for studying and reproducing radiation effects, calibrating and monitoring processing operating parameters, improving production efficiency, and ensuring the quality of irradiated products. Only sufficiently reliable dosimetry systems recognized by national or international dose standards can issue formal documentary evidence demonstrating that irradiation processing has achieved the required results. In addition, in industrial irradiation processing, food irradiation preservation and sterilization and disinfection of medical supplies directly affect people's health and safety, so health and environmental protection departments will impose strict requirements and formulate legal provisions. China has formulated and published the national standard GB/T 16841-1997 "Guide for Dosimetry in an Electron Beam Radiation Processing Facility for Energies Between 300 keV and 25 MeV," which has clear provisions on the characteristics of radiation sources, composition of irradiation facilities, processing parameters, dosimetry systems, qualification of irradiation facilities, process qualification, routine production processing, certificates, documents and archives, and uncertainty. In addition, there is also the national standard GB/T 16640-2008 "Guide for Selection and Calibration of Dosimetry Systems for Radiation Processing" regarding the selection and calibration of dosimetry systems for radiation processing, which must be complied with.
Product Handling System (Product Receiving, Pre-Irradiation, Post-Irradiation, and Storage Areas)
As a whole, the irradiation facility requires a set of reasonable operating parameters for different products and processing requirements, as well as a scientific management and operation system. These play a crucial role in ensuring product quality.
Processing parameters include:
Characteristics of the irradiation unit (dimensions, bulk density, and non-uniformity);
Irradiation conditions (processing geometry, single-sided irradiation, double-sided irradiation)
Operating parameters
Electron beam characteristics (electron energy E, average electron beam current I)
Product conveyor speed
Electron beam expansion parameters (scan width)
Operating parameters need to be measured online, and their relationship with absorbed dose values must be established for automatic control.
Applications of Irradiation Accelerators
Using high-energy electron linear accelerator technology for irradiation processing projects can be applied not only to chemical products but also to agricultural product processing and medical services. It plays a significant role in upgrading the industrial and agricultural levels of a region, increasing the export capacity of local products, and promoting the development of an export-oriented economy. The main application areas are as follows:
1) Using electron beam irradiation on fast switching thyristors and chips can modify and optimize product performance.
2) Degrading residual chloramphenicol in aquatic products and bee products to meet the pesticide residue hygiene standards of exporting countries.
3) Preservation of agricultural and sideline products, food, seafood, and aquatic products to achieve sterilization, insect disinfestation, sprout inhibition, and shelf-life extension.
4) Sterilization and disinfection of disposable medical and health supplies (medical dressings, gauze, gloves, surgical medical devices, etc.)
5) Treatment of industrial wastewater, domestic wastewater, and aquaculture wastewater. After treatment, the biochemical oxygen demand, chemical oxygen demand, and total organic carbon can be significantly reduced; bacteria, viruses, and other harmful microorganisms in wastewater can be thoroughly killed.
6) Sterilization of traditional Chinese patent medicines and health products. Using electron beams, various harmful bacteria in traditional Chinese patent medicines and health products can be effectively killed, meeting practical hygiene requirements.
7) Color enhancement of topaz, pearls, and crystals to improve product quality and increase product added value.
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