Radiation Interactions - Photons

What is Ionization?

Removing Bound electrons (irradiation)

Excitation?

Exciting bound electrons (heat)

Nuclear interaction?

interactions involving nuclear charge

What is Rayleigh Scattering (Coherent)?

Attenuation without absorption.. Photon’s E-field causes e^- to oscillate (absorbs energy), photon is scattered (irradiated) with same energy as incident… ZERO ENERGY CHANGE

Probability of Coherent scattering?

Z/E²

What is Photoelectric Effect?

Photon COMPLETELY absorbed by inner-shell electron, ejected electrons leaves vacancy that is filled → characteristic x-ray

What is Characteristic x-ray?

photon emitted from outer shell electron dropping into empty lower shell space

What is energy of ejected electron in photoelectric effect?

E = hν - Φ… Φ is binding energy difference between shells

What are the dominant and significant energy ranges for photoelectric effect?

Dominant: 10-30 keV

Significant; 10-70 keV

What is Auger Electron? What interaction type is it most associated with?

Photoelectric effect… characteristic x-ray kicks another bound electron → Auger Electron

Probability of photoelectric effect?

Z³/E³

Reason for characteristic peaks in absorption coefficient curves?

Resonance phenomenon in photoelectric effect (inverse probability to E)… Near binding energy of shell, probability sharply increases

Compton Scattering description?

Incident electron kicks bound outer-shell electron, scattering at an angle

What shell does Compton interact with?

Outer Shell

Dominant energies for Compton Scattering?

30 keV - 25 MeV

Probability of Compton Scattering?

1/E… technically Z/A, but this ratio is constant for everything beyond the hydrogen atom

Energy of scattered electron from Compton?

E’ = E_o / (1 + (E_o/0.511)(1 - cosΦ))

Max energy of backscattered compton photon? (phi = 180)

0.255 MeV

Max energy of side scattered compton photon? (phi = 90)

0.511 MeV

Pair Production description?

Photon passes near nucleus → electron-positron pair created

Energy requirement for Pair Production?

1.022 MeV

What happens to excess energy in pari production?

converted to Kinetic, split between positron and electron… e.g. anything over the 1.022 MeV is split. So if phton has 2.022 MeV, the pair is created, each with 0.5 MeV each

Dominant energies of pair production?

Over 25 MeV

Probability of Pair Production?

Zln(E)

Condition for Triplet production?

if the production occurs near a bound electron (2 electrons, 1 positron). It is rare

Minimum energy of Triplet production?

2.04 MeV

What is Photonuclear interaction?

high energy interaction with the nucleus causes photodisintegration

What is Photodisintegration?

neutron ejection as a result of high energy photon interaction with nucleus… THIS is why neutron shielding is important for high energy Linacs

Energy for Photonuclear interactions?

Over 8 MeV. Common at ~10 MeV

2 types of Coherent Scattering?

Rayleigh - elastic scattering of light by ALL e^- in atom

Thompson - elastic scattering of light by A SINGLE e^- in atom