Diagnostic X-Rays

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Diagnostic X-Rays

Each type of atom (or element) has its own characteristic electromagnetic spectrum and its own X-ray output. We can examine these to identify a material.

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The electromagnetic spectrum

  • X rays lie at the high-frequency end of an atom’s spectrum and are characteristic of each atom.
  • We are able to look at these characteristics and use them in medical imaging.
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X-ray production

  • Electrons are emitted from a hot filament and then accelerated with a high voltage, this gives them a large kinetic energy.
  • The electrons are then fired at the anode.
  • There are two processes by which x rays are produced.
    • The deceleration of electrons produces x rays, and these x rays are called bremsstrahlung, or braking radiation.
    • The atomic structure produces characteristic x rays from electrons moving shells.
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X-ray curve

  • An X-ray spectrum is obtained when energetic electrons strike a material.
    • The smooth part of the spectrum is called bremsstrahlung radiation.
    • The peaks are characteristic of the anode material.
      • A different anode material would have characteristic X-ray peaks at different frequencies.

Rotating Anode X-Ray Tube

X-rays to be used on patients can be created using a rotating anode X-ray tube.

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Rotating anode

  • The anode is a tungsten disc, rotated at high speed (3000 rpm).
  • The rotation ensures that the target area is spread, permitting larger tube currents and voltages to be used, and hence more powerful X-ray beams to be produced, without overheating the target.
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Beam intensity

  • The beam intensity increases with an increase in the applied voltage across the tube and with an increase in the tube current.
  • The X-ray photon energy is a continuous spectrum with a maximum that depends upon the voltage between anode and cathode.
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Characteristic X-rays

  • The 'characteristic' X-ray photon energy depends upon the changes in electron energy level that produce the photon.
  • These depend upon the structure of the target material but are fixed for that material.
  • Common anode materials are tungsten, molybdenum and rhodium, which are used for specific purposes because of the different characteristic photon energies.
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Image produced

  • The contrast between parts of the X-ray scan depends upon the proton numbers of the materials being scanned.
  • Image sharpness depends upon the size of the anode and the physical dimensions of the X-ray set-up, as well as blurring caused by movement of the subject.
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Patient safety

  • The patient dose must be kept to a minimum, which is aided by using image intensifiers and improved detection devices.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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