Pair Production in Physics

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Questions and Answers

Which factor(s) do the photoelectric atomic attenuation coefficients depend on?

Atomic number

Wavelength

Both atomic number and energy (correct)

Energy

For which type of photons is the photoelectric effect the major contributor to mass attenuation at relatively low energies?

Photons with energies below 0.5 MeV

Photons with energy close to or less than the binding energy (correct)

Photons with very high energy

Photons with energy much greater than the binding energy

What happens at absorption edges in terms of photon energy and binding energy of a shell?

Photon energy is lower than binding energy

Photon energy equals binding energy (correct)

Photon energy has no relation to binding energy

Photon energy is higher than binding energy

In the Compton effect, why is the orbital electron considered 'free'?

Because its energy is much greater than the binding energy Signup and view all the answers

What does the Compton electronic attenuation coefficient depend on?

Electron density Signup and view all the answers

What is the half value layer (HVL) of concrete for a 10 MeV photon beam?

21.32 cm Signup and view all the answers

Which property does the energy loss of a charged particle in matter depend on?

All of the above Signup and view all the answers

What is the total stopping power defined as?

The sum of collision stopping power and radiation stopping power Signup and view all the answers

What is characteristic of atomic excitation during electron interactions with matter?

Transfer of an atomic orbital electron to a higher energy level orbit Signup and view all the answers

What type of interaction between incident electrons and absorber nuclei results in no energy loss?

Elastic collision Signup and view all the answers

Which factor characterizes the loss of kinetic energy through bremsstrahlung during electron-nucleus interactions?

Radiation stopping power Signup and view all the answers

What is the energy threshold for nuclear pair production to occur?

1.022 MeV Signup and view all the answers

What type of pair production occurs in the Coulomb field of an absorber orbital electron?

Electronic pair production Signup and view all the answers

Which interaction generally predominates at low photon energies?

Photoelectric effect Signup and view all the answers

What does the linear attenuation coefficient m depend on?

Energy of the photon and absorber's atomic number (Z) Signup and view all the answers

What does the half-value layer (HVL) represent?

Absorber thickness attenuating beam intensity to 50% Signup and view all the answers

Which absorber thickness reduces beam intensity to 10% of the original intensity?

Tenth-value layer (TVL) Signup and view all the answers

How is the linear attenuation coefficient determined experimentally?

By using a narrow collimated mono-energetic photon beam Signup and view all the answers

Study Notes

Pair Production

Production of e- - e+ pair occurs when an incident photon is completely absorbed by an absorber atom
Requires energy (Eg) greater than 2mec2 = 1.022 MeV, where mec2 is the rest energy of e- and e+
Two types of pair production:
- Nuclear pair production: occurs when the collision partner is the absorber atomic nucleus, with Eg > 2mec2
- Electronic pair production or triplet production: occurs when the collision partner is the absorber orbital electron, with Eg > 4mec2 = 2.044 MeV
Pair production probability is zero for Eg < 2mec2 and increases rapidly with Eg > threshold
Nuclear pair production contributes more than 90% to the pair production attenuation coefficient

Attenuation Coefficients

Linear attenuation coefficient (m) is the probability per unit path length that a photon will have an interaction with the absorber
Depends on photon energy (hn) and absorber atomic number (Z)
Exponential absorption of photon beam in absorber occurs, with intensity decreasing as absorber thickness increases
Attenuation coefficient (m) is determined experimentally by measuring beam intensity with varying absorber thicknesses

Characteristic Absorber Thicknesses

Three special thicknesses used to characterize photon beams:
- Half-value layer (HVL or x1/2): absorber thickness that attenuates original intensity by 50%
- Mean free path (MFP or λ): absorber thickness that attenuates beam intensity by e = 36.8%/1
- Tenth-value layer (TVL or x1/10): absorber thickness that attenuates beam intensity to 10% of the original intensity

Photoelectric Effect

Occurs when an orbital electron is ejected from the atom as a photoelectron, with kinetic energy EK = hν - EB
Threshold energy: Eg = hν > EB, where EB is the binding energy of the photoelectron
Photoelectric atomic attenuation coefficients depend on atomic number and energy
Photoelectric effect is the major contributor to attenuation coefficient at relatively low Eg

Compton Effect

Occurs when an incident photon interacts with a loosely bound ("free") electron, transferring part of its energy to the electron
Photon is scattered through a scattering angle, with energy E'g = hn' lower than incident energy Eg
Basic Compton equation: λ' - λ = λC = 0.024Å, where λC is the Compton wavelength of the electron
Relationship between scattered and incident energies: Eg' = (Eg / (1 + (Eg / mec2)(1 - cos(θ)))

Electron Interactions with Matter

Energetic charged particles undergo Coulomb interactions with absorber atoms, resulting in:
- Ionization loss
- Radiation loss
Interactions characterized by specific cross-section (probability) and stopping power
Energy loss depends on particle properties and absorber properties
Total stopping power: stot = scol + srad, where scol is collision stopping power and srad is radiation stopping power

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Description

Learn about pair production in physics, which involves the creation of an electron-positron pair with the complete absorption of an incident photon by an absorber atom. Explore the conditions and characteristics of nuclear pair production and electronic pair production (or triplet production).