x ray circuit and x ray tube

Question 1

an x ray tube provides

Answer 1

1. Electrons which are free to move
2. A means of getting these electrons to travel at high speed
3. A force causing these fast moving electrons suddenly to change direction

Question 2

Supply of electrons by thermionic emission

Answer 2

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.

Question 3

Space charge effect

Answer 3

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.

Question 4

When the exposure button is fully pressed:

Answer 4

• 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

Question 5

Electrons sudden change in direction

Answer 5

• 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

Question 6

Electrons lose energy when they collide with target atoms

Answer 6

• 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)

Question 7

X ray tube

Answer 7

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.:

Question 8

3 types of interactions

Answer 8

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

Question 9

x ray tube

Answer 9

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.

Question 10

glass envelope

Answer 10

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

Question 11

cathode

Answer 11

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

Question 12

filmament

Answer 12

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

Question 13

anode

Answer 13

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

Question 14

Target tugnsten

Answer 14

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

Question 15

tube housing

Answer 15

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

Question 16

electrical safety

Answer 16

1. Insulation of live components – This exists between the live components and the housing in the form of oil in its housing. The resistance of an insulator diminishes as the its temperature rises and so the role of the oil in heat dissipation is also important from the point of view of electrical safety
2. Earthing of component housings – The tube housing is connected to earth via the outer braiding of high tension cables and so the casing will always remain at earth potential. If a live wire within the casing becomes disconnected and touches the shield, then the current will flow to earth and the casing will provide minimal electrical hazard to someone touching it at the time. Layers of rubber and other insulators are used to provide resistance between the innermost conducting core and the outside to prevent current flow across the cable. High tension cables operate up to potentials of at least 150 kV.
3. Restricted access to live components – The live components are secured inside the tube shield and the ends of the high tension cable connectors are securely fixed. This means there is no access to the live components. Circuits may be isolated from the mains by switches, fuses and breakers.

Question 17

reaosns for focusing cup

Answer 17

•The real (true) focus can be longer than the effective focus. The filament may be relatively long without giving rise to excessive geometric unsharpness. This means that thermionic emission from the filament is proportional to its surface area and so a long filament will provide the high values of mA required by many exposures.

2 The area over which the heat is deposited is the larger area of the real focus. The temperature rise experienced by a large area for a given amount of heat is less than that experienced by a small area since more atoms are able to take part in the heat dissipation processes. The line focus principle means that the area of effective focus is three times less than the real (true) focus. This minimises temperature rise at the target but allows a small focal spot to reduce geometric unsharpness.

Question 18

heat loss

Answer 18

• Some heat is lost from the focal spot by radiation through the vacuum to be absorbed by glass envelope and oil.
• Most heat is lost by conduction through the tungsten target into copper block and along anode stem.
• The end of the stem is in contact with oil and this sets up convection currents which warm the oil. The oil expands and eventually expansion bellows trip a microswitch to prevent further exposures.
• Heat then passes from the oil to the metal housing by conduction

Question 19

radiation protection

Answer 19

Lead lining of the tube housing
X rays emitted in all directions from the focus
Useful beam leaves through the port
Any break in lead lining allows radiation leakage
Inherent filtration glass and oil
Added filtration aluminium
Total filtration(inherent filtration plus added filtration) must be a minimum of 2.5 mm

Question 20

Operation of x ray tube

Answer 20

X rays are produced when electrons are accelerated from cathode to anode
The number of electrons is controlled by the ma seeclro
When selecting ma the radiographer is selecting the required filament current to produce the required ma
Kinetic energy of the electrons is controlled by the kv selector
Kvp controls the peak potential difference across the tube- if increased the higher energy x rays are produced

Question 21

reason for a rotating anode

Answer 21

Area bombarded by electrons is a broad ring shaped focal track
Rotation carries the recently heated area out of the way of the incoming electrons
This can achieve a shorter exposure time and increased tube rating

Question 22

x-ray tube faults

Answer 22

Glass envelope and enclosed vacuum
• Deposition of tungsten resulting in reduced insulating properties of the glass, making puncture more likely. This results in loss of vacuum with drawing in of oil and immediate uncontrollable tube current through ionisation.
• Glass may be broken by careless handling
• Stress factures due to strain of anode assembly weight and rapid rotation of anode

Anode, target and rotor
Heat can lead to crazing of the target surface, causing unsharpness and reduction in X-ray output.

Filament
Eventually breaks due to vaporisation

Oil seal and shield
Radiation leakage

Stator windings
No rotation of anode