26. Ionizing radiations.

Question 1

Define ionization

Answer 1

Ionization:

process by which an atom or a molecule acquires a negative or positive charge by gaining or losing e^- to form ions

We distinguish between:

• direct ionization: caused by particles with charge (α, β^-, β⁺)
• indirect ionization: caused by particles without charge (γ-, X-rays)

Question 2

Define effective range of radiation

Answer 2

Effective range:

distance (not path!) in which the particle can produce ion pairs (= cause ionization). At the end of this distance, the particle loses the excess E, and the particle has only E_thermal

→ measured in [mm], [cm]

Depends strongly on the type of radiation (mass)

Question 3

Describe the **interaction **of α-radiation with matter

Answer 3

Interaction of α-radiation with matter:

• α-particle = ⁴₂He²⁺
• direct ionization
• travels in a straight path (m_α >>m_e^- when α-particle collides with e^-, α-particle doesn’t change its direction)
• highest specific ionization (2+ charges)
• effective range: shortest (10-100μm in soft tissue)
• highest linear **energy transfer **

Question 4

Describe the **interactions **of β-radiation and name an application

Answer 4

Interaction of β-radiation:

• β-particles (positron, electron) **collide **(= annihilation)
• 2 γ-rays emitted, half E each (511 keV)

Interaction of β-radiation with matter:

• direct ionization
• travel in a zigzag path (m_β = m_e^-)
• medium specific **ionization **
• effective range: medium (few cm in soft tissue)
• medium linear energy transfer

Application:

Positron Emission Tomography (PET)

Question 5

Describe the interaction of γ-radiation/X-rays with matter

Answer 5

Interaction of γ-radiation/X-rays with matter:

• effects:
  
  • ​photoeffect
  • pair production
  • Compton scattering
  • elastic scattering
• indirect ionization (= pair production → β-particle ionize further atoms)
• Compton effect: γ-photon changes the direction (no remaining γ-photons after pair production / photoeffect)
• lowest specific ionization
• effective range: highest (few dm in soft tissue)
• lowest linear energy transfer

Question 6

Define annihilation

Answer 6

Annihilation denotes the process that occurs when a **subatomic particle collides **with its respective antiparticle.