photon interaction processes Flashcards
what are the 4 mechanisms of photon interactions
PE, CS, RS, PP
what happens during photoelectric effect?
- x-ray photon ejects K-shell electron, transferring all of its energy and leaving a vacancy in its place
- an I shell electron with higher energy fills the K shell vacancy, emitting the excess energy as a characteristic photon
photoelectric effect: the photon energy = _____ energy + _____ energy of emitted _____
binding, kinetic, photoelectron
a _____ photon is emitted in the photoelectric effect
characteristic
characteristic photon energy = difference in __ and __ shell energies
K, L
____ ____ is the dominant interaction at low energies
photoelectric effect
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
low, low, high, high
in soft tissue, characteristic radiation is ____ ____ while in metals, characteristic radiation ____ ____
absorbed locally, can escape
two things which can be emitted when electron vacancy filled
characteristic x-rays or auger electrons
fluorescent yield formula (omega_k)
omega_k=no. of k x-ray photons/no. of k shell vacancies
if omega_k = 1, then __ Auger electrons, if omega_k = 0, then __ Auger electrons
no, all
what must be satisfied for fluorescence to occur
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)
what is mass attenuation coefficient (mu/rho)
probability of photon being removed from the beam by any process
what is photoelectric mass attenuation coefficient (tau/rho)
probability of photon being removed from the beam by photoelectric effect
relationship between PE mass coefficient (tau/rho), atomic number Z and photon energy (E)
tau_rho proportional to Z^3/E^3
what enables clear imaging of bone
relatively high Z and density
mammo: use low/high energy to maximise ____ effect because of no difference in ____ between normal and pathological tissue making differences in ____ subtle
photoelectric, density, attenuation
rough mean Z of 1. bone, 2. soft tissue/muscle, 3. fat
- 14
- 7.5
- 6
compton scattering is an interaction between an ____ and ____ ____ ____
electron, high energy photon
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.
loosely bound electron, outer, removes, inelastic, recoil, energy, kinetic energy
in compton scatter: the change in photon energy depends on ___ and ___
initial photon energy, angle of scatter
change in photon wavelength equation for compton scatter
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
compton scattering: higher photon energy results in a ___ % of energy retained by photon and ___ % of energy taken by electron
lower, greater
compton scattering: as photon energy increased, a greater proportion of its energy is retained by the photon or transferred to the electron?
transferred to electron
how does the probability of compton interaction vary with energy
decreases with increasing energy
what happens in compton interaction at low energies
most of the energy is retained by the photon
compton scattering: at higher energies, even though scatter is less likely, why can it have effect on image quality
more penetrating photons resulting from scatter which may escape more easily and reach detector
compton mass attenuation coefficient (sigma/rho):
approx ____ in diagnostic range
proportional to ___ if E > 100 keV
and is independent of __
constant
1/E
Z
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 = _______
avagadros no, atomic mass, N_0/A
Z
(N_0 Z)/A
disadvantages of compton scatter (2)
- staff dose
- reduces image contrast
formula for backscatter factor
backscatter factor = dose at P with scatter/ dose at P without scatter
typical backscatter factors
1.2-1.3
is undercouch or overcouch tube preffered for reducing backscatter
undercouch
what happens during elastic scattering
- the whole atom absorbs the recoil
- “bound” electrons resonate and photon frequency
- electrons re-radiate energy at same frequency and energy of photon
- scattering in forward direction
what is the k-absorption edge effect
where the photon energies exceed the k shell binding energy so there is an abrupt increase in the photoelectric effect
how can k edge be used in imaging
maximise absorption in detector
by using image detector with high absorption
matches energy of the k-edge approx. to peak of x-ray spectrum
what benefit from using k edge in imaging
fewer photons for given optical density meaning lower dose to patient
contrast agents are __ and __
iodine, barium
iodine Z=__, k-edge=__
53, 33 kev
why is iodine used as contrast agent
high atomic number
example application of iodine as contrast
angio
how does iodine work as contrast agent in angiography
greater absorption
means greater difference between transmitted intensity through the iodine in the blood vessel and soft tissue
pair production: ___ __ ___ hits ___ producing __ __ pair
high energy photon
nucleus
e- e+
energy equivalent of e- and e+ rest mass
m(e) c^2 = 511 keV = 0.511 MeV
pair production: how much energy needed to produce electron positron pair
2 x rest mass energy of electron = 1.02 MeV
pair production: why must both positron and electron be produced
to conserve charge because the incident photon is neutral
pair production: how do both electrons and positrons lose energy
by interactions with other electrons
pair production: __ positron eventually __ with a __ __
slow
annihilates
free electron
product of annihilation: two ___, each with energy of __ __ emitted in __ __
photons
0.511 MeV
opposite directions
is pair production important in dr
no because not possible <1.02 Mev and dr is up to 150 kev
what applications is it used
nm: positron emitting radionuclides for pet scanning
rt: important interaction
what is the energy and z dependence of elastic interactions (eta)
1/E^2
Z^2
what is the energy and z dependence of photoelectric effect (tau)
1/E^3
Z^3
what is the energy and z dependence of compton scatter (sigma)
for 10-100keV, constant dependence of energy, for >100 keV, varies with 1/E
independent of Z
what is the energy and z dependence of pair production (pi)
E-1.02
Z
how to approximate mass energy coefficient (mu/rho) for mixtures and compounds
weight by the fraction by weight
how are neutrons classified
according to energy
fast, thermal and intermediate
how do fast neutrons lose energy
via elastic and inelastic scattering
neutron interactions: fast neutrons losing energy - more energy is lost in __ __ __ materials
low atomic number
how may thermal neutrons be captured
by other nuclei
__ __ results in production of radioactive material
neutron capture
what does neutron capture result in the production of
radioactive material
where can neutron activation occur
in heads of high energy linacs
what material for neutron shielding and why
boron
has low atomic number
