Basic structure of accelerator

Industrial radiation sterilization began in the 1950s to carry out sterilization test on suture products on electron accelerator. In the 1960s, CO sources were widely used for sterilization of medical products because they could provide higher productivity than the electron accelerators produced in the 1970s, so powerful electron accelerators were developed. This has led to a change in trend. The number and total activity of "common sources" in the United States and Europe are gradually decreasing, and the total power of electron beam equipment is increasing. In 1999, the number of commercial electron beam devices used worldwide was eight to nine times that of commercial gamma ray irradiators.

 

The industrial sterilization process is as follows: irradiation sterilization (accelerator and isotope source), 46%; gas (ethylene oxide) sterilization, 52.2%; thermal sterilization, 1.8%.

 

In recent ten years, while many countries began to restrict the practice of gas sterilization, the share of radiation sterilization was also increasing.

 

The key parameter in the process of radiation sterilization is the penetration depth of ionizing radiation. The electric energy of most direct current electron accelerators is less than 3 MeV, which limits its application in sterilization. RF type accelerators with energy of 5MeV or higher are usually used. The maximum energy of electron beam should not exceed 10MeV to avoid radioactive problems in the energy range of 2.5-10mev (for EB sterilization). The dependence of the import and export dose on the electron energy is similar It is linear. The penetration depth limits the surface thickness of the treated product. Double side irradiation can increase the allowable thickness of the treated product by 2.4 times. Another method is to increase the penetration depth and use heavy metal target to generate bremsstrahlung (X-ray). The X-ray produced by 5-7.5 MeV electron beam has the same penetrating ability as y-ray in the range of 30-40 GCM.

 

In Europe and the United States, many service centers provide low or medium energy cobalt sources and electron beam irradiators. Most of the products are treated with electron beam, while the products that cannot be treated with electron beam are irradiated with "co - γ rays".

 

The RF power supply system is based on the use of reliable transistor (pulse power is not less than 2 MW) of self starting motor. The RF system does not need precise frequency adjustment of the structure and generator, which makes the design of accelerator control system and generator relatively simple.

 

To a certain extent, the self-excited RF generator simplifies the design of the accelerator, reduces the cost, improves the reliability and reduces the maintenance cost. The ilu-8 machine with an energy range of 0.8-1 MeV is mainly used for the crosslinking of wires, tubes and films. Since 1983, accelerators with energy up to 2.5 MeV have been used for sterilization of pharmaceutical products. The accelerator can produce beams up to 5 mV. In order to meet the increasing demand of high energy radiation processing industry, a new type of accelerator was developed. Its energy range is 7.5-10 MeV. The accelerator can be used to process products with energy up to 10 MeV in electron beam mode and in X-ray production mode with energy of 7.5 MeV (maximum electron energy produced by X-ray).

 

At 5 MeV, the penetration depth of X-ray generated by accelerator is 30 g · cm-2, which is equivalent to the penetration depth of co - γ ray. The irradiation equipment based on accelerator and mobile X-ray converter can process all products processed by 6co source. Some accelerator machines have been operating in China for many years, including the accelerator of Shanghai Irradiation Center established by Shanghai Institute of Applied Physics.

 

Some accelerators operate at a voltage of 4-5 MeV, which is sufficient for disinfection. The basic structure of the accelerator is an annular copper hole with a working frequency of 116mhz. It has an axial protrusion to form an acceleration gap of 270 mm. The protruding shape is selected to form and focus the electron beam so as to complete the incident, acceleration and further passing through the beam scanning and extraction system with minimum loss. Put the copper RF cavity 2 into the stainless steel vacuum box 1. The triode electron injector 5 is formed by a cathode unit and a grid installed in the upper protrusion. A triode system is composed of a low capacitance projection and an injector

 

Under the lower electrode of the cavity, there is a magnetic lens to form an electron beam in the capacitor channel. The linear beam scanning and extraction device 6 is directly installed in two RF self excitation generators and nine power triodes on the vacuum box. The operating frequency of generator 9 is about 116 MHz, close to the specific frequency of the cavity. The anode circuit is coupled with the hole through an inductance loop. The coupling rate is determined by the square of the loop and can be adjusted by the anode circuit. The feedback in the generator using the common gate circuit is provided by the additional capacitance of the anode and cathode of the connecting tube, which is about 20pF. The fine tuning of feedback value and phase is realized by the cathode short-circuit tail driven by notor with movable quick contact. When the accelerator is preset, the coupling rate between the cavity and the generator is adjusted by changing the capacity of the vacuum capacitor 8 and the square of the coupling circuit. The chamber is sealed with metal vacuum (copund indium) and the working vacuum is 1.33x10pa.