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.

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