physics 4

Question 1

look at graph in slide 1: what’s happening in photon A?

Answer 1

• collided with the atom and has been deflected
• known as scattering
• could result in loss of photon energy

Question 2

look at graph in slide 1: what’s happening in photon B?

Answer 2

• has interacted with atom and lost all of the energy to atom
• ABSORPTION

Question 3

look at graph in slide 1: what’s happening in photon C?

Answer 3

• passes through material without interacting with any of the atoms.

Question 4

define ATTENUATION

Answer 4

the loss of intensity of the whole radiation beam, not just single photons.

Question 5

what happens when a beam of radiation passes through a medium?

Answer 5

it is attenuated by the process of absorption and scattering

Question 6

define Elastic (Coherent) scattering

Answer 6

• occurs when photon energy is less than binding energy of e- in atom
• photon deflected with no loss of energy
• attenuation v small, present as only scattered through small angle

Question 7

define photoelectric absorption

Answer 7

• Photon involved in an inelastic collision with orbiting e- of atom
• Photon deposits total energy (and disappears) to the electron
• e- ejected from the atom
• An outer shell e- jumps to vacancy in the inner shell.
• Characteristic radiation produced

note: Will only occur if the incident photon energy is equal to or greater than binding energy of e-

Question 8

define Compton scatter

Answer 8

• Incident photon of higher energy collides with ‘free’ valence e-
• e- ejected from atom and partial absorption occurs
• The photon is scattered with it’s remaining energy
• Ejected e- is always scattered in forward direction relative to photon
• Interaction is common in medical radiography

Question 9

define pair production

Answer 9

(conversion of energy to mass)

• Incident photon of min 1.02MeV energy passes close to atomic nucleus of medium and interacts with electrical field
• Incident photon disappears and is replaced by electron and positron of equal energies 0.51 MeV
• Energy has been converted to mass!!!
• Positron eventually collides with another e- within the medium
• Both positron and e- will be replaced by 2 photons of equal energy – known as annihilation radiation

Question 10

what is a positron?

Answer 10

Positrons are antimatter, that is, positively charged beta-rays. more mass than proton