X-rays are high energy photons with short wavelengths, range 10-8 to 10-13m. They are produced in X-ray tubes:
- Evacuated tube containing two electrodes (evacuated so electrons pass through without interacting with gas atoms)
- An external power supply is used to create a large p.d. between these two electrodes
- Cathode produces electrode by thermionic emission. These are accelerated towards the positive anode, made out of a target metal with a high melting point (e.g. tungsten)
- Fast-moving electrons collide with anode
- Decelerate, with KE being transferred to X-rays
- K-lines due to electron deexcitation (excited by incident photon then drop down energy levels)
Other structural features:
- Less than 1% of KE is converted to electromagnetic energy of X-rays; remainder is transformed to thermal energy of anode. Anode is rotated and oil is used to cool.
- Anode is shaped so X-rays are emitted in the desired direction, through a window. X-ray tube is lead lined to shield radiographer.
To find the shortest wavelength of X-rays produced, consider the case when all the energy of the electron is converted to electromagnetic radiation.
When X-ray photons interact with the matter they pass through, they are scattered and absorbed. This results in a decrease in intensity, and is called attenuation. 4 attenuation mechanisms exist.
- Simple scatter: photon interacts with an electron in the atom, bouncing off it without a change in energy.
- Photoelectric effect: electron absorbs photon, using the energy to escape the atom
- Compton scattering: photon interacts with electron, ejecting it from the atom. But photon emerges with reduced energy.
- Pair production: photon interacts with the nucleus of the atom. It disappears and its energy is used to produce an electron and a positron.
For a given substance, the transmitted intensity of X-rays depends on the thickness of substance. This is described by:
Soft tissues have low attenuation coefficients, so a contrast medium is used to improve the visibility of internal structures.
- Large atomic number so large attenuation coefficient
- Can image outline of soft tissues with good contrast
Iodine is often used to view blood flow. Barium is often used in “barium meals” to image the digestive system.
Computerised axial tomography (CAT) scanners are used to produce 3D images of patients’ internal structures.
- X-ray tube rotates around patient, with the beam angle changing
- The X-ray beam is thin and fan-shaped
- Ring of detectors around patient allows images of slices through the patient to be taken
- The X-ray tube moves along the patient, describing a spiral path
- Computer uses electrical signals to produce a 3D image
3D image allows doctors to assess the shape, size and A single traditional X-ray scan is quicker and cheaper position of disorders such as tumours Prolonged exposure to X-rays can be harmful
Provides good contrast between different soft tissues with Patents must remain still for prolonged lengths of time similar attenuation coefficients