Photon Interactions (Detection Physics)

Question 1

Photoelectric Absorption

Answer 1

Cross-section Absorption:
sigma ~ Z^nE^-3

Shape is sharp absorption edges with step-like features at atomic shell energies, dominate at low E. Steeply decreases with energy.
Z= atomic #
n ~ 4-5 (strong Z dependence)

Photon is completely absorbed. Electrons ejected from atom. All photon energy deposited locally which produces sharp spectral features.

Question 2

Dominant interaction at 5keV?

Answer 2

Photoelectric absorption.

Low energy —> strong E^-3 dependence so dominated.

Question 3

Compton Scattering

Answer 3

Photon scatters off a quasi-free electron so energy is partially transferred.

Question 4

Compton Scattering: important regimes?

Answer 4

1. Thomson (low energy) - hv «_space;mec^2 with IC power; small photon energy change
2. Klein Nishina (high energy) - hv > mec^2; large photon energy change

Thomson —> easy scattering, photons barely changes.
Klein-Nishina —> hard scattering, quantum effects reduce cross-section

Question 5

Physical behavior of Compton Scattering?

Answer 5

Photon survives scattering, energy redistributed, produces Compton continuum in detectors.

Question 6

Dominant at 100 keV?

Answer 6

Compton scattering.

Intermediate energy regime.

Question 7

Pair Production

Answer 7

Photon converts to: gamma —> e- + e+ which must occur near nucleus to conserve momentum.

Photon disappears and creates electron-positron pair which dominates at high gamma ray energies.

Question 8

Threshold energy for Pair-Production:

Answer 8

E > 2mec^2 = 1.02 MeV

Question 9

Dominant at 5MeV?

Answer 9

Pair production.