Summary
1. X-Rays are high-energy EM waves, with λ considerably shorter than light.
2. To produce X-Rays:
a. A filament is heated by a low voltage transformer, causing electrons to ‘boil’ off the surface of the filament.
b. A high voltage transformer provides a voltage between the filament and the anode.
c. The electrons boiling off accelerate towards the anode.
d. As the electrons are accelerated enormously, they produce X-Rays when they hit the dense anode.
e. The typical efficiency of X-Ray production is about 1%.
f. The other 99% is heat, and so the anode is made of tungsten, which has a high melting point to absorb most of the heat, and it is rotating to spread out the heat around its surface.
g. The X-Rays are produced in all directions, so lead shielding is used to cut out the X-Rays not wanted.
3. When a high energy electrons are fired at a heavy atom (high Z number):
a. Electrons approaching the nucleus are slowed down, and their loss of energy results in the release of X-Rays (braking radiation).
b. Some electrons knock orbital electrons out of the inner shells, and in the process of being replaced, X-Rays will be produced (ionisation).
4. For a spectrum of X-Ray energies and intensities:
a. The continuous spectrum illustrates braking radiation – every wavelength is possible.
b. The peaks at specific wavelengths illustrate ionisation (the line spectrum)
c. The maximum energy of X-Rays is given by E max and λ min .
5. One electron-volt (1eV) is the energy gained when one electron is accelerated through a potential difference of 1V. Energy in joules = energy in eV × charge on one electron.
6. As the tube voltage is increased:
a. E max increases and λ min decreases.
b. The peak intensity of the continuous spectrum moves to a higher energy.
c. The total intensity (area under the curve) increases – proportional to V02.
d. More ionisation peaks may appear on the spectrum.
7. An AC power supply will give a greater proportion of low energy radiation than a DC supply.
8. As the tube current is increased:
a. E max remains constant (V 0 is constant).
b. The shape of the spectrum is unchanged.
c. The total intensity increases – proportional to the tube current.
9. As the Z number of the target material increases:
a. E max remains constant (V 0 is constant).
b. The total intensity increases, as there is a greater probability of a collision.
c. The ionisation peaks are shifter to higher energies.
10. A filter (e.g. thin aluminium) is used to reduce the lower energy X-Rays so that less are absorbed by the skin and soft tissues.
11. The following terms are used:
a. Quality – the penetration of the X-Rays. This increases as V0 increases.
b. Hardness – hard X-Rays have had the lower energy components removed (by filtering).
12. The quality of an X-Ray beam can be described by the half-value thickness (HVT). This is the thickness of material that is needed to reduce the intensity to half of its original value.
13. For diagnostic X-Rays, the tube voltage will be in the region of 30 kV (X-Ray energy of 30 keV), so that the absorption is proportional to the Z3 of the body material.
14. Photographic emulsion is not very sensitive to X-Rays:
a. The film can be double coated with emulsion.
b. Intensifying screens can be used to convert X-Rays into visible light, which interacts better with the emulsion (but this decreases the resolution).
15. Artificial contrast mediums are used to study areas such as the digestive tract – i.e. radio-opaque barium is used.
16. The scattering of X-Rays in the body reduces both contrast and resolution of the image, so a lead grid is used to only allow normal X-Rays through.
