5. Photon Electron Interactions Flashcards
What is radiation length (Xo)?
Characteristic distance denoting the effectiveness of a medium in attenuating photons as they propagate in it
Radiation length is the distance in a medium at which the photon intensity reduces to 1/e (~37%) of its initial value
I(Xo) = Io/e
I(x) = Io x e^(-μx)
μ = 1/Xo (attenuation coefficient)
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What is the mass attenuation coefficient?
μp = μ/ρ
Units if gcm^-2
What is Rayleigh scattering?
Elastic scattering of photons off atoms or molecules in the medium
Coherent scattering where the atom recoils
Doesn’t excite or ionize the atom
Results in very small loss of energy
What is the photoelectric effect?
A photon is absorbed and an electron is set free from an atom or molecule
The atom is ionized
Results in free moving electron and no photon
Probability for this process increases with the atomic number of interacting medium
What is Compton scattering?
Elastic scattering of a photon off a free electron
It is incoherent scattering, where an electron leaves the atom and the atom is ionized
There is still a photon after the interaction at reduced energy
What is pair production?
Photon is converted into an electron and a positron pair
All of the photons energy is shared by the particle-antiparticle pair and the recoil electron or the nucleus
Photon must have a minimum energy of 1.022 MeV (twice the mass of electron) to produce particle-antiparticle pairs
What is photo nuclear reactions?
Interaction of photons with atomic nuclei
Very small chance for energy loss
Not significant enough to be considered for the calculation of the energy loss of photons
What are photon interaction cross sections?
When we send a beam of photons into a medium, the effect is loss of intensity
Attenuation coefficient can be related to the physical processes through atomic cross sections as:
μρ = σ(tot)/uA
u is constant and A is mass number
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What is Bremsstrahlung radiation?
This radiation is produced when a charged particle traverses a medium
The charged particles experience an accelerating/decelerating force and the result is radiation
What is Cherenkov radiation?
This is emitted when a charged particle traverses a medium with a velocity greater than that of the speed of light in that medium
Vlight = c/n (refractive index)
Cherenkov radiation is emitted if v>vlight=c/n or βn>1
βmin = 1/n
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What is transition radiation?
Emitted when a particle traverses the boundary of two media with different dielectric constants
Dielectric medium is a poor conductor of electricity
What is synchrotron radiation?
Radiation emitted by electrons as they pass through a magnetic field
Particle accelerators where electrons reach higher and higher energies and are bent to retrace their path for further acceleration
What is photon flux?
Flux = 2.46x10^13 x E x I x G1(x)
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