CIE A Level Physics (9702) 2019-2021

Revision Notes

27.4.2 Production & Use of X-rays

Production of X-rays

  • X-rays are short wavelength, high-frequency part of the electromagnetic spectrum
    • They have wavelengths in the range 10−8 to 10−13 m
  • X-rays are produced when fast-moving electrons rapidly decelerate and transfer their kinetic energy into photons of EM radiation

Producing X-rays

  • At the cathode (negative terminal), the electrons are released by thermionic emission
  • The electrons are accelerated towards the anode (positive terminal) at high speed
  • When the electrons bombard the metal target, they lose some of their kinetic energy by transferring it to photons
  • The electrons in the outer shells of the atoms (in the metal target) move into the spaces in the lower energy levels
  • As they move to lower energy levels, the electrons release energy in the form of X-ray photons
  • When an electron is accelerated, it gains energy equal to the electronvolt; this energy can be calculated using:

Emax = eV

  • This is the maximum energy that an X-ray photon can have
  • Therefore, the maximum X-ray frequency fmax, or the minimum wavelength λmin, that can be produced is calculated using the equation:

Production of X-rays equation 1

Production of X-rays equation 2

Production of X-rays equation 3

  • Where:
    • e = charge of an electron (C)
    • V = voltage across the anode (V)
    • h = Planck’s constant (J s)
    • c = speed of light (m s-1)

Worked example: Production of X-rays

Production_of_X-rays_Worked_Example_-_Production_of_X-rays_Question, downloadable AS & A Level Physics revision notes

Part (a)

    • Photons are produced whenever a charged particle is accelerated towards a metal target
    • The wavelength of the photons depends on the magnitude of the acceleration
    • The electrons which hit the target have a distribution of accelerations, therefore, a continuous spectrum of wavelengths is observed

Part (b)

    • The minimum wavelength is equal to

Production of X-rays Worked Example equation 1

    • This equation shows the maximum energy of the electron corresponds to the minimum wavelength
      • Therefore, the higher the acceleration, the shorter the wavelength
    • At short wavelengths, the sharp cut-off occurs as each electron produces a single photon, so, all the electron energy is given up in one collision

Using X-rays in Medical Imaging

  • X-rays have been highly developed to provide detailed images of soft tissue and even blood vessels
  • When treating patients, the aims are to:
    • Reduce the exposure to radiation as much as possible
    • Improve the contrast of the image

Reducing Exposure

  • X-rays are ionising, meaning they can cause damage to living tissue and can potentially lead to cancerous mutations
  • Therefore, healthcare professionals must ensure patients receive the minimum dosage possible
  • In order to do this, aluminium filters are used
    • This is because many wavelengths of X-ray are emitted
    • Longer wavelengths of X-ray are more penetrating, therefore, they are more likely to be absorbed by the body
    • This means they do not contribute to the image and pose more of a health hazard
    • The aluminium sheet absorbs these long wavelength X-rays making them safer

Contrast & Sharpness

  • Contrast is defined as:

The difference in degree of blackening between structures

  • Contrast allows a clear difference between tissues to be seen
  • Image contrast can be improved by:
    • Using the correct level of X-ray hardness: hard X-rays for bones, soft X-rays for tissue
    • Using a contrast media
  • Sharpness is defined as:

How well defined the edges of structures are

  • Image sharpness can be improved by:
    • Using a narrower X-ray beam
    • Reducing X-ray scattering by using a collimator or lead grid
    • Smaller pixel size

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