Photon Interaction Mechanisms 2 Flashcards
What is Compton scattering?
Incident photon inelastically scatters an outer free electron (recoil electron) and photon of lower energy is scattered from atom
What did Compton find?
Incident photons scattered off graphite target leads to an increased wavelength at increased angles
Tested at low intensity to dismiss chance of being a Doppler shift
What happens in elastic photon scattering?
Photon changes direction on interaction without losing energy
(creates noise and spread of dose)
What is Thompson scattering?
γ + e- -> γ
incident γ interacts with an oscillating atomic electron and emission of photon with same wavelength (change direction but no net effect)
What is Rayleigh scattering?
Incident γ interacts with whole atom causing bound electrons to vibrate and emission of photon with same wavelength (occurs in forward direction)
What is the equation of the attenuation coefficient for elastic scatter?
Φ ∝ Z^2 / E
When does elastic scatter occur?
More at lower energies and high Z materials
(no contribution to absorption as no energy lost)
What is the energy of the recoil electron?
It shares its energy with photon (they travel in opposite directions)
Why is Compton scattering different to other types of scattering?
it is inelastic / incoherent
it is ionising
What is the atomic Compton scatter attenuation coefficient σ?
A measure of the chance of a Compton scatter interaction
σ ∝ Z/E
What does σ depend on?
High enough electron density (enough outer shell electrons)
Being more dominant than PE effect (as they happen around same energy but PE occurs first)
Z
E
What is Compton shift?
Change in wavelength (or energy) of incident photon after a Compton scatter
(energy and momentum must be conserved)
What is the de Broglie wavelength equation?
p = h / λ = hv / c
What is the recoil electron energy?
T = E_i - E_f
How does the scatter angle change the energy of scatted photon?
Greater the scatter angle (up to 180 degrees), the greater the loss of energy