x rays Flashcards
2 types of attenuation?
Absorption and scattering
4 Types of possible interactions?
Compton, coherent, photoelectric, pair production
What is compton scatter
Photons are scattered out of the beam with a reduced energy and recoil electrons (which are considered free in calculations) are emitted in a forward direction with kinetic energies up to Tmax. These ionise the material until energy is lost.
What are the two types of compton scattering?
inelastic and incoherent
Maximum energy transfer to recoil electron
Maximum energy transfer to recoil electron occurs when photon is backscattered and Tmax = (hv)/(1+(1/2a))
What is coherent scatter?
The photon hits one of the loose outershell electrons “electron cloud” and the photon is elastically scattered.
(No electron is dislodged.)
Conditions:
- Incident wavelength of photon λ > electron spacing.
- Confined to small angles.
What does the probability of compton or coherent scatter depend on?
Whether the electrons are seen as almost free or tightly bound.
Describe the photoelectric effect
Photons incident (E=hv) on strongly bound electrons can be totally absorbed as atom as a whole takes up recoil momentum. The excited electron is then emitted with energy T=hv-Be where Be is the binding energy of the electron
What happens when the ionised atom de-excites?
Characteristic x-ray (Electron drops from a higher shell, energy of x-ray depends on relative shell energies) or auger electron (energy given to outer shell electron - emitted , more likely for low atomic number materials see graph)
energy dependence of photoelecrtric effect
as photo energy increases, photoelectric absorption decreases. ( ~1/E^3) hence it constantly decreases but there are exceptions at attenution edges.
as long as incident x-ray energy is less than binding energy of k-shell electron, x-ray can only kick out L and M shell electrons.
Summary of photoelectric absorption?
1) electron ejected from atom followed by emission of either a characteristic x-ray or an auger electron
2) Total absorption of photon
(Dominates at low x-ray energy, normally decreases rapidly with energy (~E^-3) but increases sharply immediately above an absorption edge)
What is Pair production ?
At photon energies > 1.02MeV an interaction with the electrostatic field of the nucleus produces a positron and electron pair.
What does the electron and positron from pair production do?
which moves through material causing ionisation. The positron annihilates giving two 0.511MeV gamma rays.
Photoelectric absorption at low energies depends on z and E how?
Z^3 and E^-3
Compton scatter depends on what?
Density of material alone
