(This chapter is more specialized, and readers who are not interested can skip it directly and move on to the next chapter without affecting the coherence of the plot.) ）

Whether it is a low-energy medical electron linear accelerator or a medium and high-energy medical electron linear accelerator, although it has its own advantages in structure, the basic structure is the same, mainly including acceleration tube, pulse modulator, electron gun, microwave system, vacuum system, frequency stabilization, temperature control and inflation system, beam extraction system, treatment head, treatment bed.

1. Electron gun

The electron gun is the core device of the electron emission system, which is a device that generates, accelerates and gathers a high energy density electron beam, which emits an electron beam with a certain energy, a certain beam current, speed and angle (also known as electron note). The quality of the electronic injection parameters directly affects the quality of the accelerator tube.

In addition to the electron injection emitted by the accelerator tube to the electron gun, it is also required to have a certain injection current intensity, injection voltage, sufficient range, and a certain injection angle and injection waist radius.

2. Microwave system

The microwave system is composed of a microwave power source and a microwave transmission system, the microwave source provides the RF power required for the accelerator tube to establish the acceleration field, and the magnetron and klystron are used as the microwave source, and the accelerator of our factory uses the magnetron as the microwave power source.

3. Beam transmission system

The beam transmission system is an automatic control system set up for beam focusing, beam guidance and beam deflection division of electrons in the acceleration process. It can be divided into three parts: focusing system, guiding system and deflection system.

4. Frequency stabilization, temperature control and inflation system

The auxiliary systems of the microwave system include automatic frequency stabilization system, automatic temperature control system and waveguide inflation system.

The microwave automatic frequency stabilization system is an important link to coordinate the electromagnetic oscillation frequency consistency between the microwave source and the accelerator tube.

The automatic temperature control system is also an important part of the medical electronic linear accelerator, because in the medical electronic linear accelerator, there are many components that have to emit different heat when working, and these components can only ensure stable operation under constant temperature conditions.

The waveguide gas filling system refers to a set of devices that fill the microwave transmission system with a specific gas of a certain pressure, the purpose of which is to increase the density of gas molecules in the waveguide and shorten the average free path of the gas molecules, so as to improve the breakdown strength threshold of the waveguide and prevent the ignition phenomenon in the waveguide that may occur during microwave power transmission.

5. Vacuum system

The vacuum system can maintain a high vacuum state in the electron movement area and the accelerator tube, which can prevent the cathodic poisoning of the electron gun and the oxidation of the thermoon or filament of the tungsten filament material on the one hand, and on the other hand, it can avoid the discharge breakdown in the accelerator tube, and can also reduce the collision loss between the electrons and the residual gas.

6. High voltage pulse adjustment system

In accelerators that use microwave electric fields to accelerate electrons, in order to obtain the highest possible accelerating electric field, the instantaneous microwave power is very high, so the microwave source is pulsed.

The pulse modulator is a power supply that provides pulse power to this microwave source, and its working principle is to use the principle of energy storage and discharge to form a high-voltage pulse, and the voltage is further amplified by the pulse modulator for the use of the microwave power source.

7. Radiation system

The function of the radiation system is to convert and average the output of the electron beam according to the need, or directly equalize the output of electron rays, and monitor and limit the beam irradiation of the output X-ray or electron ray in real time.

The basic structure of the radiating head: the accelerator tube is installed in the upper part of the radiant head, and the target is close to the window of the accelerator tube, followed by the primary collimator, the beam homogenization filter or scattering foil, the ionization chamber, the radiation field optical simulation system, a pair of upper collimators, a pair of lower collimators, and an accessory disk.

8. Dose monitoring system

The dose monitoring system consists of a dose monitoring ionization chamber and a dose monitoring circuit.

The ionization chamber is located within the radiation system and is installed between a homogenization filter or a secondary collimator of the scattering foil and photonic lines, and consists of a number of pole pieces, two pairs of which are used to monitor the uniformity of the two directions perpendicular to each other in the radiation field, one to monitor the energy change of the radiation, and two to detect the absorbed dose of the radiation.

Most use flat plate ionization chambers, the size of which should cover the entire therapeutic field, and a few use finger ionization chambers, whose function is to monitor the dose rate of X-rays, electron beams, integral doses, and symmetry and flatness of the shooting field.

9. Mechanical system

The mechanical system is composed of a base, a rotating frame, a radiant head, a treatment bed and other structures, and the modern medical linear accelerator adopts an isocentric motion system, that is, the rotation axis of the frame, the radiant head and the treatment bed intersect at a point, which is called the isocentric center.

10. Control system

The control system consists of the following parts:

(1) Various power supplies.

(2) Interlocking protection: including water flow, water temperature, water pressure, high-voltage overload, microwave power source ignition and other protection.

(3) Automatic control: including automatic frequency control, automatic dose rate control, automatic uniformity control, automatic wedge filter control, arc rotation control, etc.

(4) Program control of normal treatment: including program control of several states such as standby, preset, preparation, beam discharge, and morning inspection.

(5) Safety interlocking: safety interlocking to ensure the safety of equipment, safety interlocking of control rays, etc.

(6) Safety grounding and interference shielding: electrical shielding, magnetic field shielding, electromagnetic shielding, etc.

During the use of medical electron linear accelerators, some common fault problems will be encountered.

(1) Automatic frequency stabilization fault: With the use of medical electronic linear accelerator, automatic frequency stabilization often has problems, because the detection diode is a more vulnerable component, most of the automatic frequency stabilization faults are caused by the damage of the detection diode, and the loosening of the AFC motor will also cause automatic frequency stabilization failure.

(2) Dose failure: Dose abnormality may occur when the device is damaged on the dose integration board, the dose and dose rate decrease with the passage of time of the accelerator tube, and the fluctuation of voltage will also cause dose abnormality.

(3) Ionization chamber failure: The ionization chamber is a relatively easy to damage component, and the output signal line of the ionization chamber is also easily damaged, resulting in an open circuit.

(4) Microwave system failure: damage to the cathode filament of the microwave source, damage to the circulator, etc.

(5) Fault of distance measuring lamp and light field lamp: distance measuring lamp and light field lamp are located on the radiation head, due to the large power of the ranging lamp and light field lamp, it will reach a very high temperature, and it is easy to burn out if there is a problem with heat dissipation.

(6) Failure of the treatment bed: damage to the potentiometer limit switch, damage to the lead screw bearing, failure of the panel button, and damage to the motor.

(This chapter borrows from the Baidu article, although the author has professional experience, but to systematically describe the composition and principle of medical accelerators, it is still not as coherent as the Baidu article.) ）