24.2 Production and Use of X-rays

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Explain how X-rays are produced by electron bombardment of a metal target, and calculate the minimum wavelength of X-rays from the accelerating potential difference (voltage).
  2. Understand how X-rays are used to image the inside of the body, including what the term contrast means in X-ray imaging.
  3. Recall and use the attenuation equation I = I₀e^(–μx) to calculate how X-ray intensity changes as X-rays pass through matter.
  4. Understand how CT (computed tomography) scanning builds a 3D image of the body by combining multiple 2D X-ray images taken from different angles across different sections.

1. Production of X-rays

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What are X-rays?

X-rays are a type of electromagnetic radiation — the same family as visible light and radio waves, but with a much shorter wavelength (roughly 10⁻⁸ m down to 10⁻¹³ m). Because their wavelength is so short, they carry a lot of energy and can pass through soft materials like skin and muscle, but are stopped by denser materials like bone.

The X-ray Tube

X-rays are produced inside a device called an X-ray tube. Here is what happens step by step:

Step 1 — Thermionic emission A metal wire called the filament (cathode) is heated by passing an electric current through it. When it gets hot enough, electrons are shaken loose from its surface. This process of releasing electrons by heating is called thermionic emission.

Step 2 — Acceleration of electrons A high voltage — typically between 20 kV and 100 kV — is applied between the cathode (negative terminal) and a metal target called the anode (positive terminal). Because electrons are negatively charged, they are strongly attracted toward the positive anode and accelerate rapidly across the gap, gaining kinetic energy.

Step 3 — Electron bombardment of the metal target The fast-moving electrons slam into the metal target (often made of tungsten). When they hit:

  • They decelerate very suddenly.
  • Their kinetic energy is converted into X-ray photons (packets of electromagnetic energy).
  • A large amount of energy is also lost as heat, which is why the anode needs to be cooled.

Step 4 — X-rays are emitted The X-ray photons are released from the target and directed out of the tube to be used for imaging.

Key idea: X-ray photons are produced when fast electrons rapidly decelerate upon hitting a metal target.

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