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

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:

• This is the maximum energy that an X-ray photon can have

• Therefore, the maximum X-ray frequency f_max, or the minimum wavelength λ_min, that can be produced is calculated using the equation:

Maximum frequency:

Minimum wavelength:

• 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)

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