X-rays in OCR A Level Physics

Contents

• X-ray Tube
• X-ray Attenuation Mechanisms
• Calculating X-ray Attenuation
• X-ray Imaging
• CAT Scans

X-ray Tube

Structure

• Converts electrical input into X-rays.
• Main components:
  • Heated cathode
  • Anode
  • Metal target
  • High voltage power supply
• Uses: medical imaging, security, industrial imaging.

Role of Components

• Heated Cathode:
  • Negative terminal, heated to liberate electrons via thermionic emission.
• Anode:
  • Positive terminal with high voltage to accelerate electrons, mostly converting kinetic energy to heat.
• Metal Target:
  • Electrons strike target, emitting X-ray photons.
• High Voltage Power Supply:
  • Provides potential difference to accelerate electrons.
• Other Components:
  • Lead shielding for safety.
  • Vacuum chamber prevents electron collisions before target.

Production of X-ray Photons

• Bremsstrahlung:
  • Deceleration of electrons produces a spectrum of X-ray energies.
• Characteristic Radiation:
  • Electrons knocked from inner shells produce discrete energy X-rays.

X-ray Attenuation Mechanisms

• Simple Scattering:
  • Low-energy X-rays deflected without energy change.
• Photoelectric Effect:
  • X-ray absorbed, ejecting inner electron (photoelectron).
• Compton Scattering:
  • X-ray deflected, losing energy to ejected electron.
• Pair Production:
  • High-energy X-ray produces electron-positron pair near nucleus.

Calculating X-ray Attenuation

• Formula: ( I = I_0 e^{-\mu x} )
  • ( I_0 ): Incident beam intensity.
  • ( x ): Distance through material.
  • ( \mu ): Linear absorption coefficient.
• Intensity decays exponentially with thickness.

X-ray Imaging

Reducing Exposure

• Ionising nature requires minimal dosage.
• Use of aluminium filters to remove long-wavelength X-rays.

Contrast & Sharpness

• Contrast:
  • Difference in blackening between structures.
  • Improved by correct X-ray hardness and contrast media.
• Sharpness:
  • Defined edges, improved by narrower beam and collimators.
• Contrast Media:
  • Substances like barium, iodine increase contrast by absorbing X-rays.

CAT Scans

• Computed Tomography Scanning:
  • X-ray tube rotates around patient, creating 3D images from 2D slices.
  • Offers detailed images, higher resolution, and eliminates overlapping.
• Comparison to X-rays:
  • CAT scans provide more detail but involve higher exposure.
  • Differences in exposure due to multiple angle imaging in CAT scans versus single exposure in X-rays.