X-ray Production and Interactions

Interactions at the Target

• Electrons and the Target:

  • Electrons are emitted from a filament and interact with tungsten atoms at the anode (the target).

  • These interactions are crucial as they lead to X-ray production.

• Two Key Concepts:

  • Interactions at the Target: Refers to the initial interactions that produce X-ray photons.

  • Interactions with Matter: How generated X-ray photons interact with biological tissue.

X-Ray Production in Detail

• Inciting Motion of Electrons:

  • High-speed electrons travel from the cathode to strike the anode. These are known as incident electrons.

  • They possess kinetic energy, which is crucial for the interactions that lead to X-ray creation.

• Kinetic Energy Characteristics:

  • Incident electrons travel at half the speed of light, transferring their energy to target atoms in the anode, producing X-ray photons during interactions.

Types of Interactions at the Target

Bremsstrahlung Interaction (Brems)

• Definition and Nature:

  • Named from the German word for "breaking" or "slowing".

  • Accounts for approximately 85% of the photon beam created in X-ray production.

• Mechanism:

  • Occurs when incident electrons are slowed down by the electric field of the atomic nucleus of the target atoms.

  • As they decelerate, they release energy in the form of X-ray photons.

  • Energy of the emitted photon is the difference between the electron's incoming and outgoing energy.

• Efficiency:

  • It's important to note that less than 1% of the incident electrons actually produce X-ray photons; 99.8% are converted to heat.

Characteristic Interaction

• Definition:

  • Occurs when an incident electron interacts with an inner shell electron of a tungsten atom.

  • Results in ionization, as the inner shell electron is ejected from its orbit.

• Energy Requirement:

  • The incident electron must possess energy greater than the binding energy (69.5 KeV) of an inner shell electron to eject it.

• Cascading Effect:

  • When an inner shell electron is ejected, electrons from outer shells drop to fill vacancy, generating more photons in a cascade effect. These photons have energies characteristic of the differences in binding energies between shells.

Concepts of Energy and Photons

• Polyenergetic Nature of X-rays:

  • Both Bremsstrahlung and characteristic photons are considered polyenergetic as they possess various energy levels.

  • The X-ray emission spectrum can predict their energy distributions accurately.

• Photon Energy Ranges:

  • X-ray energies can range from zero to the peak kVp set on the control panel, indicating a wide variety of energies from the electrons.

• Ionization Potential:

  • Characteristic interactions are ionizing, and can therefore affect biological matter significantly, while Bremsstrahlung are non-ionizing in their direct interaction.

Recap of Learning Outcomes

• Understanding that both different interactions have distinct processes and outcomes is crucial in medical imaging and radiography.

• Recognizing the efficiency issues in X-ray production highlights the importance of managing heat within X-ray tubes for effective and safer operations in imaging practices.