Production of x-rays Flashcards
Describe how x-rays are produced
1.Electrons are produced in a filament containing Tungsten within the Cathode (-). When a current is passed through, the filament heats up causing the electrons to be released by thermionic emission.
2.The thermionic electrons are then fired and attracted to the target by high positive voltage within the anode (+).
3.The kinetic energy of the electrons when brought to rest within the anode is converted into heat (99%) and X-rays (1%).
why is kinetic energy converted to heat and x-rays when producing x-rays
under the principle of conservation of energy - energy cannot be created or destroyed.
Name the four interactions that occur at the target (Tungsten within the anode)
- Excitation of the target atoms (NOT RELEVANT FOR TR)
- Ionisation of outer shells of atoms
- Ionisation of inner shells of atoms
- Interaction with the nucleus of atoms (MOST IMPORTANT).
Describe excitation
The electron passes close to an outer electron, the electron is raised to a higher energy level and then returns to its former position. The excess energy is given of as heat.
Describe outer shell ionisation
The incident electron collides with an outer shell electron and removes it from the shell. For this to happen the incident electron must have energy greater than the binding energy of the orbiting electron. The electron that has been liberated moves through the target material giving up its energy as heat.
Describe inner shell ionisation
The incident electron penetrates the electron cloud and removes an inner shell electron i.e. an electron from ‘K’. This leaves a hole in a shell; the atom rearranges the electrons to fill the hole. In this process, an electron from an outer shell will fill the gap, excess energy will be released as an x-ray photon.
*The shells have different levels of energy; the lowest energy is closest to the nucleus therefore for an electron to move into a lower shell it must lose energy (X-ray Photon)
*This type of radiation is known as characteristic radiation as it is characteristic of the target producing it i.e. Tungsten will always produce the same energy photon when an L electron fills a K ‘hole.
* This produces a line spectrum (the graph only has one value)
What is characteristic radiation
Characteristic radiation that is released as the atom rearranges is dependent on:
1. What orbit was ionised.
2. What orbit is donating an electron to the ionised orbit to fill the hole.
If the ‘hole’ in the L shell is filled by an M shell electron, what type of photon is produced is.
Lα (alpha radiation).
If the ‘hole’ in the L shell is filled by an N shell electron, what type of photon is produced is.
Lβ (Beta radiation)
Describe how electrons interact with the nucleus is
The incident electron released from the cathode filament passes through the electron cloud to interact with the nucleus.
* The electrons are slowed down at different levels; the kinetic energy is converted into X-ray photons.
The combination of high speed and the positive attraction of the nucleus causes the electrons to be slowed down and their direction changes.
The closer the electron passes by the nucleus, the greater the braking effect (secondary radiation that is produced whilst slowing down electrons and is continuous Bremsstrahlung radiation) and the greater the release of energy.
When are stationary anodes used?
dental radiography and mammography
What are the two main parts of an x-ray tube
- The tube housing (the protective outer casing)
- The tube insert – inner ceramic or glass insert.
Why is the Tungsten at an Angle
soft X-rays can be attenuated back in the anode assembly
What is the function of thermal L-cut out switch
When the machine heats up, oil is heated up and expands. If the oil expands and surpasses a critical point the machine will cut out.
what is the function of the step-up transformer
used to increase normal UK voltage to voltage of the X-ray Tube.
