X-rays can be polarised.
X-rays are electromagnetic waves.
To produce x-rays a high voltage and a good vacuum in a tube with an anode and a cathode is required.
Electrons are accelerated from the cathode to the positive anode.
- If they have enough energy some x-rays will be emitted.
- The cathode is often heated to increase the output of electrons.
- Only 1% of energy is the electron beam.
X-rays as particles.
X-rays use the wave-particle duality.
x-rays and the photoelectric effect:
- x-rays can cause emission of photoelectrons.
- The work function of x-rays are so small it's ignored.
- The emitted photoelectrons have a maximum kinetic energy equal to the photon energy of x-rays.
- These electrons cause ionisation.
- Measuring the energy of the electrons is a method of detecting and measuring the energy of x-rays.
- Beam of high-frequency x-rays produce a positron and an electron.
- 1.02MeV would be needed for an x-ray photon.
- The effect is only seen in high-voltage systems.
The Compton effect.
- Compton filtered out all wavelengths except those with the strongest sharp peaks.
- He found some deflected x-rays had a longer wavelength than the initial x-rays.
- This was explained using the quantum theory.
- Any photon deflected through a large angle will have lost more energy so its wavelength will be longer.
The intensity of a beam of x-rays is defined as the power per unit cross-sectional area.
X-ray beams may be collimated in which the intensity hardly changes with distance.
- The amount of absorption varies with the frequency of the x-rays.
- At higher frequencies the compton effect is the main method.
- At very high frequencies pair production is most important.
- The distance for halving the intensity is called the half-value thickness.
X-ray film is the traditional way of making an x-ray image and requires considerable exposure.
Other methods are used:
- Use photographic film that is more sensitive to x-rays or put a fluorescent plate behind the film. Both result in less exposure.
- Use an x-ray absorbing substance as a contrast medium. Patients are given a barium meal before having an x-ray taken of the digestive tract.
- Use an image intensifier, many radiography departments now use digital methods.
3D X-ray images.
- Traditional x-ray show a shadow of the part of the body which x-rays have passed.
- If a bone is broken the break shows up as a shadow
- An x-ray of the heart is taken and digitised
- A contrast medium is then injected into the blood stream, and a second x-ray is then taken and digitised.
- The computer images are then subtracted from one another to only leave changes on the final photo.
Computerised axial tomography (CAT) scan
- Use x-rays to create a 3D image.
- The source is shielded so the rays emerge from a point and spread out through the patient.
- The x-ray beam is fan shaped and has little thickness.
- Once passed through the patient x-rays are detected by a ring of detectors.
- The x-ray source…