Fonctionnement du Tube à Rayons X

X-ray Tube Functioning

• An X-ray tube is a vacuum tube that produces X-rays when a high voltage is applied across two electrodes: the cathode and the anode.
• The cathode is the negative electrode and emits electrons.
• The anode is the positive electrode and absorbs the electrons and converts their kinetic energy into X-rays.
• The process begins with the heating of a filament within the cathode, causing thermionic emission of electrons.
• A high voltage applied across the tube accelerates these electrons towards the anode.
• Upon impact with the anode target, electrons lose kinetic energy, which is converted into X-rays.
• A significant portion of the energy is lost as heat, necessitating cooling mechanisms for the anode.

X-ray Tube Components

• Cathode:
  • Filament: A heated tungsten wire that emits electrons via thermionic emission.
  • Focusing cup: A conductive cup surrounding the filament that focuses the emitted electrons toward the anode.
• Anode:
  • Target: A dense metal (e.g., tungsten) with high atomic number, placed to absorb electron impact and convert their kinetic energy into X-rays.
  • Anode rotation: Some X-ray tubes use rotating targets for efficient heat dissipation. This rotation spread the heat across the surface.
  • Cooling systems: crucial to prevent overheating. Often include forced air or oil cooling mechanisms.
• Vacuum Envelope:
  • Encloses the cathode and anode, holding the entire system in a vacuum. This vacuum is necessary to prevent electron collisions with air molecules, which would reduce the number of X-rays produced and can damage the tube over time.
• High Voltage Generator:
  • Provides the high voltage (~ 60,000 volts to millions of volts) needed to accelerate the electrons towards the anode.

X-ray Tube Types

• Fixed Anode Tubes: Simpler design, but generate less X-rays than rotating anode systems and are limited in the high intensities obtainable.
• Rotating Anode Tubes:
  • Target rotates, spreading heat over a larger surface area.
  • Capable of producing higher X-ray intensities for more efficient imaging.
  • Faster imaging rates due to high intensity.
  • Preferred in medical and industrial applications.

X-ray Production Mechanisms

• Bremsstrahlung Radiation: The most common type of X-ray produced. It is generated when electrons are decelerated by the strong electric field of the anode target nuclei.
• Characteristic Radiation: Generated when the electron impact excites inner-shell electrons in the target material, causing them to fall back to a lower energy level and release energy in the form of X-rays. The energy of these X-rays is characteristic of the target material.
• Heat production: A large percentage of initial electron energy is converted into heat in the target.