Lecture 3: Xray production and interactions Flashcards
What 2 types of energy are connected to electrons within an atom?
- binding energy
- electron energy
What is binding energy?
- It is the force holding the electron in place.
- It is the amount of energy that would have to be transferred to an electron to completely remove it from the atom.
For electrons, where is binding energy highest?
Where is it lowest?
It is highest among inner shell electrons
It is lowest among outer shell electrons
What is electron energy?
-The energy actually possessed by the electron
Where is electron energy highest?
Where is it lowest?
It is highest among outer shell electrons
it is lowest among inner shell electrons
What is the conduction band?
It only exists in some elements (conductors)
This exists beyond the valence band, electrons can jump up into the conduction band from the valence band when excited.
When the electrons are in these orbitals, they have enough energy to move freely in the material
What is excitation?
It occurs when orbiting electrons are given sufficient energy to raise them from their current shell but not enough to remove them from the atom
What is the process of excitation and how the electron is affected?
1- External stimulus gives energy to the electron
2-Electron is raised in proportion to the energy transferred
3-When the external stimulus is removed the electron returns to the original position giving up the additional energy as it falls
What is ionisation?
It occurs when an external stimulus transfers enough energy to the electron to overcome the binding energy and expel it from the atom.
What is the process of ionisation and how is the electron affected?
1- High energy external stimulus
2- The electron has been expelled from the atom and the inner shell has an electron gap
3- An electron from the shell further out falls to fill the gap
4-This process continues until the gap eventually appears on the valence band
5- As the electrons fall they have to give up energy as they can only occupy a shell if they have the correct energy
6-At the end of this process, the chemical properties of the atom would have changed
In the xray machine, what is the filament made from?
Tungsten that has been treated with Thalium so electrons leave it when heat its added
In the xray machine, what is the glass cylinder made from?
boro-silicate glass
What % of kinetic energy is produced into xrays?
1-5%
Why is the Tungsten target sloping?
For a more precise concentration of electrons
What 2 types of radiation are produced from Xrays?
1- Bremsstrahlung Radiation (continuous spectra)
2- Line or Characteristic Radiation (line spectra)
What does intensity mean?
How many photons are present in a particular area
What is Bremsstrahlung radiation?
also known as ‘braking radiation’
-It describes the radiation emitted when electrons are decelerated when fired at a metal target.
-It is characterised by continuous distribution of radiation. It becomes more intense and shifts towards higher frequencies when energy of bombarding electrons is increased.
What is line or characteristic radiation?
- It describes the radiation produced when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is “characteristic” to each element.
- It is characterised by multiple spikes on the graph, each set of spikes is for an energy band
How is characteristic radiation produced?
1- High energy electrons from the filament hit a bound electron in an atom
2- The target electron is ejected, leaving the atom with a vacant energy level (in the K shell)
3- The outer shell electrons fall into inner shells to fill the gap and re-adjust
4-As they do this, they emit quantised photons with an energy equivalent to the energy difference between higher and lower states
Do Xrays travel as a single wave?
No, they travel as multiple individual photons/quanta
What is attenuation?
- It is the fractional reduction of the intensity of an x-ray beam as it traverses matter.
- It is the proportion of the beam that is not measured on exit (the beams that don’t hit the target). This is expressed as a %
What 4 factors affect attenuation?
1- atomic number of the material (high atomic number materials will have more electrons and a bigger nucleus, this means more xray photons will interact with more electrons)
2- density
3- thickness of material
4- Kv of x-ray beam
In terms of attenuation, what happens to low energy photons?
- They will deflect further from their original course than high energy photons.
- They have a wide angle scatter
- They will travel up and down through the patient’s body, impacting on critical tissues
In terms of attenuation, what happens to high energy photons?
- They will deflect very little from their original path.
- They have a narrow angle scatter
- They are more likely to exit the patient’s body in a similar direction to the original direction of the x-ray beam
What does total attenuation always consist of?
It consists of the elements:
- absorption (photo-electric absorption)
- scatter (Compton Scattering)
What happens to an x-ray photon that is absorbed?
It ceases to exist, all its’ energy has been given up in the interaction
What happens to an x-ray photon that is scattered?
It will continue to exist after the interaction but it will travel in a new direction with a lower energy value
How would a low radiation dose affect the radiograph image produced?
It will mean less contrast and less detail
When is absorption more likely to occur?
- when a low energy photon interacts with an electron
- when a photon interacts with an inner shell electron
When is scattering more likely to occur?
- when a high energy photon interacts with an electron
- when a photon interacts with an outer shell electron
What is Compton scattering?
It is the scattering of a photon by a charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), called the Compton effect.
