[Related professional knowledge] X-ray tube

The role of the X-ray tube: produce X-rays, which are used to penetrate the human body and observe human images after being received by a special receiving position.

According to the anode form classification is divided into fixed anode X-ray tube and rotating anode X-ray tube, according to the shell material is divided into glass shell, metal shell.

Fixed anode X-ray tube: consists of anode, cathode and tube shell.

1. Anode: receiving electron impact to produce X-ray,

It is composed of anode head, anode cap and anode handle. The part where the electron beam hits to produce X-rays is called the target surface. When the electron beam hits the target surface, most of the energy (99%) is converted into heat.

The target surface material should have the following conditions: high atomic number, high X-ray generation efficiency; high melting point; low metal evaporation rate; high conductivity; high thermal conductivity.

The fixed anode X-ray tube generally uses tungsten as the target surface material to make a certain thickness of the plate, which is closely embedded in the copper anode body, which uses the high melting point of tungsten (3370 ℃) and the high thermal conductivity of copper.

The anode handle extends out of the tube and is provided with a heat dissipation block, which is immersed in high-voltage insulating oil. The heat of the anode head is dissipated by heat conduction.

X-ray generation efficiency = X-ray energy/cathode ray energy = KVZ

The efficiency of X-ray generation is related to the tube voltage and the atomic number of the target substance.

The angle between the impact surface (target surface) of high-speed electrons and the vertical plane of the long axis of the X-ray tube is called the anode inclination angle. Under the premise of a certain filament size, the anode inclination angle is large, the actual focus area is large, the effective focus area is large, and the load power is large. The inclination angle of the fixed anode is generally 17 ° ~ 19 °.

An anode cap is provided around the anode to prevent longitudinal stresses caused by secondary electrons accumulating on the tube wall. Fixed anode X-ray tubes are mostly used in low-power X-ray machines, such as special surgical machines and dental machines.

2. Cathode: The role in the X-ray tube is to emit electrons and focus them properly.

The cathode is composed of a filament and a collector. The filament is made of tungsten wire, usually wound into a spiral tube, and its width and length determine the shape of the focus.

The use of tungsten wire is due to the high melting point of tungsten, low evaporation rate, easy processing, etc. The number of emitted electrons depends on the temperature of the filament.

The shape of the collector and the position of the filament in the collector determine the degree to which the electron beam is focused.

3. Tube shell: it is a sealed shell to maintain the high vacuum in the tube and support the fixed cathode and anode in a determined position.

The degree of vacuum in the tube was maintained at 133.3 × 10 − 6Pa(1 × 10 − 6mmHg) or less.

The shell material should have the following conditions: good insulation performance; high mechanical strength to withstand high vacuum pressure; small thermal expansion coefficient, no deformation after heating; easy processing, can be welded with metal; less absorption of X-rays. The original and now commonly used X-ray tube is the use of glass tube shell.

Rotating anode X-ray tube: consists of rotating anode, cathode and tube shell.

1. Anode structure: the anode is made of tungsten disc, forming a circular ring belt.

Compared with the fixed anode tube, its focus is small and its power is large.

The rotor of the rotating anode is inside the X-ray tube shell, and the stator is outside the shell. The rotor drives the target disk to rotate through the rotating magnetic field. The bearing of the rotating anode is in the tube, and the lubricant of the bearing is usually a solid metal lubricating material, such as silver, lead, molybdenum dioxide, etc. The service life of rotating anode X-ray tube is mostly ended due to bearing damage. The heat of the target disk is mainly dissipated by the thermal radiation of the target disk and absorbed by the insulating oil outside the tube.

There are three types of target disk materials: pure tungsten, rhenium-tungsten alloys, and molybdenum-based or graphite-based rhenium-tungsten alloys.

Rhenium tungsten alloy target (rhenium accounted for 10%, tungsten accounted for 90%), can reduce the occurrence of cracking;

The molybdenum-based or graphite-based rhenium-tungsten alloy target has the highest heat capacity.

The weight of molybdenum is half that of tungsten, and the specific heat capacity is twice that of tungsten.

The specific gravity of graphite is 1/10 of tungsten and the specific heat capacity is 10 times that of tungsten.

Rhenium tungsten alloy target plate with molybdenum-based and graphite-based composite target rotating anode X-ray tube increases the anode heat capacity.