x ray circuit and x ray tube Flashcards
an x ray tube provides
- Electrons which are free to move
- A means of getting these electrons to travel at high speed
- A force causing these fast moving electrons suddenly to change direction
Supply of electrons by thermionic emission
Outer electrons of the filament atoms of an X-ray tube are more loosely bound because they are further from the nucleus. When applying heat to the filament it increases the kinetic energy of its atoms. The outer electrons may be dislodged from an atom. Higher temperature increases the kinetic energy of electrons.
Space charge effect
If a body is electrically neutral before the application of heat (e.g. Tungsten filament), electrons leaving the body result in a net +ve charge.
After a short time a state of equilibrium exists where electrons are attracted back at the same rate as they are emitted.
When the exposure button is fully pressed:
- High electrical potential difference is applied across the X-ray tube (kilovoltage)
- The anode positive and the cathode negative
- Electrons gain kinetic energy when they are accelerated from cathode to anode
- Electrons travel freely as they are in vacuum
Electrons sudden change in direction
- Narrow beam of electrons flow to X-ray tube target
- The target is made of tungsten (Z = 74)
- Target’s nuclei exert strong +ve attractive forces on the electrons (high number of protons)
- These are the forces which cause the sudden change in direction
Electrons lose energy when they collide with target atoms
- Interactions between electrons and outer electrons surrounding target atoms (Heat 99%)
- Interactions between electrons and the nuclei of target atoms (Bremsstrahlung)
- Interactions between electrons and individual inner electrons of the target atoms (Characteristic)
X ray tube
Inside the x ray tube there is a filament and target. The filament has a step down transformer to reduce the voltage and to give a heating current. When we press down the button we heat up the filament to the required temperature to give us the x number of electrons to filament to target when we apply the kv across the x ray tube
During the prep stage the electron boil up through thermionic emission. The electrons boil off but do not go anywhere unless the button is fully pressed. The space charge effect means the electrons do not go anywhere as they are attracted back at the same rate. Once the voltage is applied the electrons move and are attracted to the anode target. The target is also made of tungsten and when the electrons get to the target they interact with the tungsten target atoms. There are 3 types of interactions.:
3 types of interactions
Bremsstrahlung-incident electrons interact with incoming electrons and slow down change direction lose energy and the energy is converted to x rays
Characteristic- incident electrons gets close to the nuclease but not quite and they interact with the inner shell electrons and overcome the binding energy of that shell electron and this leaves a vacancy which is filled by the other electrons from the other shells and this gives off energy in the from of a characteristic x ray.
Heat
x ray tube
Outside of this there I a tube insert there is a vacuum which prevents the electrons from interacting with anything else as this could slow them down. Beyond the tube there is a tube housing made of led lining which absorbs the x rays that leave. There are also high tension sockets that come in through the tube housing which provides the electric supply for the anode and one for the cathode. Between the tube housing and the glass there is oil which absorbs some heat via convection and is a good insulator to prevent electrocution.
glass envelope
Made from boroscikate glass
String enough to support vacuum
Holds electrodes in their precise positions
Joined at cathode and anode by ree-entrant seals
Allows thermal expansion to prevent cracking
Must be a good electrical insulator to prevent current flow
Radiolucent to transmit the x ray beam with minimum attenuation
cathode
Electrode operates at high neagtive potential
Consist of the filamnet focusisng cup, conecting wires and cathode support
The focusing cup is made from nickel or steel
Has a high melting point and is a poor thermionic emmitter to prevent it from
The area of the focal spot is reduced by the negaitve bias on the focussing cup
filmament
Made of tungsten
Low work function of 4.5
Emmits electrosn easily to form a space charge cloud
High melting point 3370
Tungsten has low vapour pressure and doesn’t easily evaporate
Tunngsten is strong and can be drawn into a thin spiral which incrreases surfaces area so more electrons
May be two filamnets which is known as a dual focus
anode
Copper ccylinder supports rectangualr target
Acts as an elecrical conductor positive
Conducts heat from the target
Copper has a low atomic number making it a poor choice as a source of x rays.
The remote end of the cylinder lies outide tube insert sorrounded by oil to help cool it down
Oil acts as an electrical insulator
Oil carries heat away to tube housing convection
Target tugnsten
Tungsten is use as a target material
High melting point 3370
Low vapur pressure 5000 at high tempearture
High atomic number of 74
Tungsten is an efficent converter of the bombarding electrons kinetic energy into x rays bremsstrahlung and characteristic x rays
Can be made into a smooth block with an expansion rate similar to copper keeping it in place when its temperature rises
Good thermal conductivity
tube housing
Electrical and radiation safety for staff and patients
Earthed aluminum or steel
Radiation should not escape apart from useful beam
Provides support to x ray tube and cables
Insulation between the insert and shield to avoid electrical breakdown
Expansion of the cooling oil must be provided
Adequate filtration at tube port to remove low energy x ray photons