A LevelPhysicsCambridge (CIE)Revision Notes24. Medical PhysicsProduction & Use of X-raysProduction & Use of X-rays

Production & Use of X-rays (Cambridge (CIE) A Level Physics) : Revision Note

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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:

E subscript m a x end subscript space equals space e V

  • 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:

E subscript m a x end subscript space equals space e V space equals space h f subscript m a x end subscript space equals space fraction numerator space h c over denominator lambda subscript m i n end subscript end fraction

Maximum frequency: f subscript m a x end subscript space equals space fraction numerator e V space over denominator h end fraction

Minimum wavelength: lambda space equals space fraction numerator space h c over denominator e V end fraction

  • 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

A typical spectrum of the X-ray radiation produced by electron bombardment of a metal target is shown below.

Explain why:

(a) a continuous spectrum of wavelengths is produced.

(b) the spectrum has a sharp cut-off at short wavelengths. 

Answer:

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

lambda subscript m i n end subscript space equals fraction numerator space h c over denominator E subscript m a x end subscript end fraction

  • 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 less penetrating which means 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 and sharpness

  • Contrast is defined as:

    The difference in the 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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Ashika

Author: Ashika

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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