photon interaction processes

Question 1

what are the 4 mechanisms of photon interactions

Answer 1

PE, CS, RS, PP

Question 2

what happens during photoelectric effect?

Answer 2

1. x-ray photon ejects K-shell electron, transferring all of its energy and leaving a vacancy in its place
2. an I shell electron with higher energy fills the K shell vacancy, emitting the excess energy as a characteristic photon

Question 3

photoelectric effect: the photon energy = _____ energy + _____ energy of emitted _____

Answer 3

binding, kinetic, photoelectron

Question 4

a _____ photon is emitted in the photoelectric effect

Answer 4

characteristic

Question 5

characteristic photon energy = difference in __ and __ shell energies

Answer 5

K, L

Question 6

____ ____ is the dominant interaction at low energies

Answer 6

photoelectric effect

Question 7

soft tissue has low/high Z and k-shell binding energies are high/low. metals has low/high Z and k-shell binding energies are high/low

Answer 7

low, low, high, high

Question 8

in soft tissue, characteristic radiation is ____ ____ while in metals, characteristic radiation ____ ____

Answer 8

absorbed locally, can escape

Question 9

two things which can be emitted when electron vacancy filled

Answer 9

characteristic x-rays or auger electrons

Question 10

fluorescent yield formula (omega_k)

Answer 10

omega_k=no. of k x-ray photons/no. of k shell vacancies

Question 11

if omega_k = 1, then __ Auger electrons, if omega_k = 0, then __ Auger electrons

Answer 11

no, all

Question 12

what must be satisfied for fluorescence to occur

Answer 12

the energy required to remove a photoelectron from the K shell of an atom must be satisfied, (energy of the absorbed quantum must be greater than the ionization energy)

Question 13

what is mass attenuation coefficient (mu/rho)

Answer 13

probability of photon being removed from the beam by any process

Question 14

what is photoelectric mass attenuation coefficient (tau/rho)

Answer 14

probability of photon being removed from the beam by photoelectric effect

Question 15

relationship between PE mass coefficient (tau/rho), atomic number Z and photon energy (E)

Answer 15

tau_rho proportional to Z^3/E^3

Question 16

what enables clear imaging of bone

Answer 16

relatively high Z and density

Question 17

mammo: use low/high energy to maximise ____ effect because of no difference in ____ between normal and pathological tissue making differences in ____ subtle

Answer 17

photoelectric, density, attenuation

Question 18

rough mean Z of 1. bone, 2. soft tissue/muscle, 3. fat

Answer 18

1. 14
2. 7.5
3. 6

Question 19

compton scattering is an interaction between an ____ and ____ ____ ____

Answer 19

electron, high energy photon

Question 20

compton scatter: A photon interacts with a ___ ___ ___ in the ___ shell of an atom.
The photon undergoes ____ scatter and ___ the ___ electron from its shell.
The photon is ____ and loses ___.
During this process, the photon imparts ___ ___ to the electron.

Answer 20

loosely bound electron, outer, removes, inelastic, recoil, energy, kinetic energy

Question 21

in compton scatter: the change in photon energy depends on ___ and ___

Answer 21

initial photon energy, angle of scatter

Question 22

change in photon wavelength equation for compton scatter

Answer 22

delta lambda = h/m_e*c (1-cos(theta)), where theta = angle through which photon scattered, h = Planck’s constant, m_e = rest mass of electron, c = speed of light

Question 23

compton scattering: higher photon energy results in a ___ % of energy retained by photon and ___ % of energy taken by electron

Answer 23

lower, greater

Question 24

compton scattering: as photon energy increased, a greater proportion of its energy is retained by the photon or transferred to the electron?

Answer 24

transferred to electron

Question 25

how does the probability of compton interaction vary with energy

Answer 25

decreases with increasing energy

Question 26

what happens in compton interaction at low energies

Answer 26

most of the energy is retained by the photon

Question 27

compton scattering: at higher energies, even though scatter is less likely, why can it have effect on image quality

Answer 27

more penetrating photons resulting from scatter which may escape more easily and reach detector

Question 28

compton mass attenuation coefficient (sigma/rho): approx ____ in diagnostic range proportional to ___ if E > 100 keV and is independent of __

Answer 28

constant 1/E Z

Question 29

there are N_0 ( ___ ___ ) atoms in A ( ___ ___ ) grams, so the number of atoms per gram = __ / __. there are __ electrons per atom therefore, the equation for electron density, defined as the number of electrons per gram = _______

Answer 29

avagadros no, atomic mass, N_0/A Z (N_0 Z)/A

Question 30

disadvantages of compton scatter (2)

Answer 30

1. staff dose 2. reduces image contrast

Question 31

formula for backscatter factor

Answer 31

backscatter factor = dose at P with scatter/ dose at P without scatter

Question 32

typical backscatter factors

Answer 32

1.2-1.3

Question 33

is undercouch or overcouch tube preffered for reducing backscatter

Answer 33

undercouch

Question 34

what happens during elastic scattering

Answer 34

1. the whole atom absorbs the recoil 2. "bound" electrons resonate and photon frequency 3. electrons re-radiate energy at same frequency and energy of photon 4. scattering in forward direction

Question 35

what is the k-absorption edge effect

Answer 35

where the photon energies exceed the k shell binding energy so there is an abrupt increase in the photoelectric effect

Question 36

how can k edge be used in imaging

Answer 36

maximise absorption in detector by using image detector with high absorption matches energy of the k-edge approx. to peak of x-ray spectrum

Question 37

what benefit from using k edge in imaging

Answer 37

fewer photons for given optical density meaning lower dose to patient

Question 38

contrast agents are __ and __

Answer 38

iodine, barium

Question 39

iodine Z=__, k-edge=__

Answer 39

53, 33 kev

Question 40

why is iodine used as contrast agent

Answer 40

high atomic number

Question 41

example application of iodine as contrast

Answer 41

angio

Question 42

how does iodine work as contrast agent in angiography

Answer 42

greater absorption means greater difference between transmitted intensity through the iodine in the blood vessel and soft tissue

Question 43

pair production: ___ __ ___ hits ___ producing __ __ pair

Answer 43

high energy photon nucleus e- e+

Question 44

energy equivalent of e- and e+ rest mass

Answer 44

m(e) c^2 = 511 keV = 0.511 MeV

Question 45

pair production: how much energy needed to produce electron positron pair

Answer 45

2 x rest mass energy of electron = 1.02 MeV

Question 46

pair production: why must both positron and electron be produced

Answer 46

to conserve charge because the incident photon is neutral

Question 47

pair production: how do both electrons and positrons lose energy

Answer 47

by interactions with other electrons

Question 48

pair production: __ positron eventually __ with a __ __

Answer 48

slow annihilates free electron

Question 49

product of annihilation: two ___, each with energy of __ __ emitted in __ __

Answer 49

photons 0.511 MeV opposite directions

Question 50

is pair production important in dr

Answer 50

no because not possible <1.02 Mev and dr is up to 150 kev

Question 51

what applications is it used

Answer 51

nm: positron emitting radionuclides for pet scanning rt: important interaction

Question 52

what is the energy and z dependence of elastic interactions (eta)

Answer 52

1/E^2 Z^2

Question 53

what is the energy and z dependence of photoelectric effect (tau)

Answer 53

1/E^3 Z^3

Question 54

what is the energy and z dependence of compton scatter (sigma)

Answer 54

for 10-100keV, constant dependence of energy, for >100 keV, varies with 1/E independent of Z

Question 55

what is the energy and z dependence of pair production (pi)

Answer 55

E-1.02 Z

Question 56

how to approximate mass energy coefficient (mu/rho) for mixtures and compounds

Answer 56

weight by the fraction by weight

Question 57

how are neutrons classified

Answer 57

according to energy fast, thermal and intermediate

Question 58

how do fast neutrons lose energy

Answer 58

via elastic and inelastic scattering

Question 59

neutron interactions: fast neutrons losing energy - more energy is lost in __ __ __ materials

Answer 59

low atomic number

Question 60

how may thermal neutrons be captured

Answer 60

by other nuclei

Question 61

__ __ results in production of radioactive material

Answer 61

neutron capture

Question 62

what does neutron capture result in the production of

Answer 62

radioactive material

Question 63

where can neutron activation occur

Answer 63

in heads of high energy linacs

Question 64

what material for neutron shielding and why

Answer 64

boron has low atomic number