radiation physics topic

1. INTERACTION OF RADIATIONINTERACTION OF RADIATION
WITHWITH
MATTER(PHOTOELECTRICMATTER(PHOTOELECTRIC
AND PAIR PRODUCTION)AND PAIR PRODUCTION)
DR SAQIB AHMAD SHAHDR SAQIB AHMAD SHAH
Dept. of RadIATION ONCOLOGYDept. of RadIATION ONCOLOGY
SKIMSSKIMS
MODERATOR:-DR TARIQMODERATOR:-DR TARIQ
RASOOLRASOOL

2. Radiation
 The termThe term radiationradiation applies to the emission andapplies to the emission and
propagation of energy through space or apropagation of energy through space or a
material medium .material medium .
 Radiation may beRadiation may be
Electromagnetic RadiationElectromagnetic Radiation
Particle RadiationParticle Radiation
 When radiation passes through matter it mayWhen radiation passes through matter it may
interact with the material , transferring some orinteract with the material , transferring some or
all of its energy to the atoms of that material.all of its energy to the atoms of that material.

3. ElectromagneticElectromagnetic RadiationRadiation
Constitutes theConstitutes the
mode of energymode of energy
propagation forpropagation for
such phenomenasuch phenomena
as light waves,as light waves,
heat waves, radioheat waves, radio
waves, u v rays, xwaves, u v rays, x
rays and γ rays .rays and γ rays .
Spectrum ofSpectrum of
electromagneticelectromagnetic
radiation rangesradiation ranges
fromfrom 101077
m (radiom (radio
waves) to 10waves) to 10-13-13
mm
(ultra high energy(ultra high energy
X rays) .X rays) .

4. Particulate RadiationParticulate Radiation
 Refers to the energy propagated by travelingRefers to the energy propagated by traveling
corpuscles – which have definite rest mass ,corpuscles – which have definite rest mass ,
definite momentum and a defined position at anydefinite momentum and a defined position at any
instant .instant .
 Atomic particles are electrons (charge – 1) ,Atomic particles are electrons (charge – 1) ,
protons (charge + 1) and neutrons (zero charge).protons (charge + 1) and neutrons (zero charge).
 Some common subatomic particles are positronsSome common subatomic particles are positrons
(charge + 1) , neutrinos (zero charge) and(charge + 1) , neutrinos (zero charge) and
mesons .mesons .

5. Interaction of Photons with MatterInteraction of Photons with Matter
 When an X ray or γ ray beam passesWhen an X ray or γ ray beam passes
through a medium , interactionthrough a medium , interaction
occurs between the photon and theoccurs between the photon and the
matter and energy is transferred tomatter and energy is transferred to
the medium .the medium .

6. Process Definition
Attenuation Removal of radiation from the beam by the matter.
Attenuation may occur due to scattering and
absorption
Absorption The taking up of the energy from the beam by the
irradiated material. It is absorbed energy, which is
important in producing the radiobiological effects in
material or soft tissues.
Scattering Refers to a change in the direction of the photons and its
contributes to both attenuation and absorption
Transmission Any photon, which does not suffer the above processes is
transmitted.
Photon-beam InteractionsPhoton-beam Interactions

7. Attenuation Coefficient (1)Attenuation Coefficient (1)
 Fraction of photons removed from a monoFraction of photons removed from a mono
energetic beam of x-ray or gamma ray per unitenergetic beam of x-ray or gamma ray per unit
thickness of material is calledthickness of material is called linear attenuationlinear attenuation
coefficientcoefficient ((µµ), typically expressed in), typically expressed in cmcm-1-1
..
 Number of photons removed from the beamNumber of photons removed from the beam
traversing a very small thicknesstraversing a very small thickness ∆∆x:x:
wherewhere nn = number removed from beam,= number removed from beam,
NN = number of photons incident on the material,= number of photons incident on the material,
and minus sign is placed before μ to indicate thatand minus sign is placed before μ to indicate that
no. of photons decreases as the absorberno. of photons decreases as the absorber
thickness increases.thickness increases.
xNn ∆=µ

8. Attenuation Coefficient (2)Attenuation Coefficient (2)
 For a mono energetic beam of photons incidentFor a mono energetic beam of photons incident
on either thick or thin slabs of material, anon either thick or thin slabs of material, an
exponential relationship exists between numberexponential relationship exists between number
of incident photons (of incident photons (NN00 ) and those transmitted) and those transmitted
(N) through thickness (x) without interaction:(N) through thickness (x) without interaction:
 The number of photons indicate the Intensity ofThe number of photons indicate the Intensity of
the beam and can also be written as ( I ).the beam and can also be written as ( I ).
x
eNN µ−
= 0

9. Attenuation Coefficient (3)Attenuation Coefficient (3)
 TotalTotal Linear attenuation coefficientLinear attenuation coefficient is the sum ofis the sum of
individual linear attenuation coefficients for eachindividual linear attenuation coefficients for each
type of interaction:type of interaction:
 For a given thickness of material , probability ofFor a given thickness of material , probability of
interaction depends on number of atoms the xinteraction depends on number of atoms the x
ray or gamma ray encounter per unit distance.ray or gamma ray encounter per unit distance.
The density (ρ ) of material affects this number.The density (ρ ) of material affects this number.
 Linear attenuation coefficient is proportional toLinear attenuation coefficient is proportional to
the density of the material.the density of the material.
pairComptonphotoRayleigh µµµµµ +++=

10. Mass Attenuation CoefficientMass Attenuation Coefficient
 For a given thickness probability of interaction isFor a given thickness probability of interaction is
dependent on the number of atoms per volume.dependent on the number of atoms per volume.
 This dependency can be overcome byThis dependency can be overcome by
normalizing linear attenuation coefficient fornormalizing linear attenuation coefficient for
density of material –density of material –
 Mass Attenuation CoefficientMass Attenuation Coefficient (μ / ρ ) =(μ / ρ ) =
Linear attenuation coefficientLinear attenuation coefficient
Density of the materialDensity of the material
 Mass attenuation coefficient is independent ofMass attenuation coefficient is independent of
density of the material.density of the material.

11. List of InteractionsList of Interactions

12. High Speed
Electrons
Photon

13. Photoelectric Effect(1)Photoelectric Effect(1)
 History:1887History:1887
Henrich HertzHenrich Hertz
discovereddiscovered
electrodeselectrodes
illuminated by uvilluminated by uv
light createslight creates
electric sparkselectric sparks

14.  1905:-Albert1905:-Albert
Einstein publishedEinstein published
paper explainingpaper explaining
phenomenon ofphenomenon of
photoelectric effectphotoelectric effect
(noble prize in(noble prize in
1921)1921)

15. Photoelectric Effect (2)Photoelectric Effect (2)
 All of the incident photon energy is transferred toAll of the incident photon energy is transferred to
an electron, which is ejected from the atom.an electron, which is ejected from the atom.
 Kinetic energy of ejected electron called theKinetic energy of ejected electron called the
photoelectronphotoelectron ((EECC ) is equal to incident photon) is equal to incident photon
energy (energy (EEOO ) minus the binding energy of the) minus the binding energy of the
orbital electron (orbital electron (EEBB ))
EECC ==EEOO -- EEBB

17. Photoelectric Effect (3)Photoelectric Effect (3)
 Incident photon energy must be greater than orIncident photon energy must be greater than or
equal to the binding energy of the ejectedequal to the binding energy of the ejected
photon.photon.
 The ionized atom regains electrical neutrality byThe ionized atom regains electrical neutrality by
rearrangement of the other orbital electrons. Therearrangement of the other orbital electrons. The
electrons that undergo these rearrangementselectrons that undergo these rearrangements
surrender some of the energy in form of a photonsurrender some of the energy in form of a photon
known as theknown as the characteristic radiation of the atom.characteristic radiation of the atom.
 Absorption of these characteristic radiationAbsorption of these characteristic radiation
internally in the atom may result in emission ofinternally in the atom may result in emission of
Auger electronsAuger electrons . These electrons are mono. These electrons are mono
energetic in nature.energetic in nature.

18. Photoelectric Effect (4)Photoelectric Effect (4)
 Probability of photoelectric absorption per unitProbability of photoelectric absorption per unit
mass is approximately proportional tomass is approximately proportional to
 Energy dependence explains, in part, why imageEnergy dependence explains, in part, why image
contrast decreases with higher x-ray energies.contrast decreases with higher x-ray energies.
 Process can be used to amplify differences inProcess can be used to amplify differences in
attenuation between tissues with slightly differentattenuation between tissues with slightly different
atomic numbers, improving image contrast.atomic numbers, improving image contrast.
33
/ EZ

19. Photoelectric Effect (5)Photoelectric Effect (5)
 Graph of probability of photoelectric effect, as aGraph of probability of photoelectric effect, as a
function of photon energy, exhibits sharpfunction of photon energy, exhibits sharp
discontinuities calleddiscontinuities called absorption edgesabsorption edges ..
 Photon energy corresponding to an absorptionPhoton energy corresponding to an absorption
edge is the binding energy of electrons in aedge is the binding energy of electrons in a
particular shell or sub shell .particular shell or sub shell .
 ImportanceImportance
• 1) Low-energy photons are less attenuated and1) Low-energy photons are less attenuated and
therefore more penetrating than high energy photons.therefore more penetrating than high energy photons.
• 2) A substance is relatively transparent to its own2) A substance is relatively transparent to its own
characteristic radiation. This effect is important whencharacteristic radiation. This effect is important when
filters are considered as the filters will be “transparent”filters are considered as the filters will be “transparent”
to their own characteristic radiation.to their own characteristic radiation.

22. Pair Production (1)Pair Production (1)
History:-Patrick BlackettHistory:-Patrick Blackett
discovered pair production whilediscovered pair production while
his invention of cloud chamberhis invention of cloud chamber
got noble prize in 1948got noble prize in 1948

23. Pair Production (2)Pair Production (2)
Definition:-refers to the creation ofDefinition:-refers to the creation of
an elementary particle and itsan elementary particle and its
antiparticle when usually photon(orantiparticle when usually photon(or
another neutral boson) interacts withanother neutral boson) interacts with
nucleus or another boson..This isnucleus or another boson..This is
allowed,if enough energy is providedallowed,if enough energy is provided
to create the pair( equal to restto create the pair( equal to rest
mass of pair) with conservation ofmass of pair) with conservation of
energy and momentum.energy and momentum.

24. Pair Production (2)Pair Production (2)
ExampleExample:-When the photon with energy in excess:-When the photon with energy in excess
of 1.02 M e V passes close to the nucleus ofof 1.02 M e V passes close to the nucleus of
an atom, the photon disappears, and a positronan atom, the photon disappears, and a positron
and an electron appear. This effect is known asand an electron appear. This effect is known as
pair production.pair production.
 Pair production results in attenuation of the beamPair production results in attenuation of the beam
with absorption.with absorption.
 The positron created as a result loses its energyThe positron created as a result loses its energy
by interaction with an electron to give rise to twoby interaction with an electron to give rise to two
annihilation photons, each having 0.511 M e Vannihilation photons, each having 0.511 M e V
energy. Again because momentum is conservedenergy. Again because momentum is conserved
in the process two photons are rejected inin the process two photons are rejected in
opposite directions. This reaction is known as anopposite directions. This reaction is known as an
annihilation reaction.annihilation reaction.

27. Pair Production (3)Pair Production (3)
 Other examples:-tau and anti tau,,moun and antiOther examples:-tau and anti tau,,moun and anti
mounmoun
 Pair production results from an interaction withPair production results from an interaction with
the electromagnetic field of the nucleus and asthe electromagnetic field of the nucleus and as
such the probability of this process increasessuch the probability of this process increases
rapidly with the atomic number (rapidly with the atomic number (ZZ22
).).
 In addition, the likelihood of this interactionIn addition, the likelihood of this interaction
increases as the photon energy increases, inincreases as the photon energy increases, in
contrast to the Compton effects and thecontrast to the Compton effects and the
photoelectric effect.photoelectric effect.

28. Relative Importance of theRelative Importance of the
Various ProcessesVarious Processes
The relative importance of the 3 principal modes ofThe relative importance of the 3 principal modes of
interaction pertinent to radiation therapy- theinteraction pertinent to radiation therapy- the
Photoelectric , Compton and Pair production processes -Photoelectric , Compton and Pair production processes -
as a function of Incident beam energy and Atomicas a function of Incident beam energy and Atomic
number of absorber matter shows -number of absorber matter shows -
For an absorber with Z approximately equal to that ofFor an absorber with Z approximately equal to that of
soft tissue - 7 , and for mono energetic photons ,soft tissue - 7 , and for mono energetic photons ,
Photoelectric effect is the dominant interaction belowPhotoelectric effect is the dominant interaction below
about 50 k e v.about 50 k e v.
Above 50 k e v Compton effect remains dominant andAbove 50 k e v Compton effect remains dominant and
remains so,remains so,
Until about 24 M e v , after which Pair Production effectUntil about 24 M e v , after which Pair Production effect
becomes dominant .becomes dominant .

29. Relative ImportanceRelative Importance

30.  The μ/ρ is large for lowThe μ/ρ is large for low
energies and high Z mediaenergies and high Z media
(eg.Lead ) because of the(eg.Lead ) because of the
predominance ofpredominance of
Photoelectric interactionsPhotoelectric interactions
under these conditions.under these conditions.
The μ /ρ decreases rapidlyThe μ /ρ decreases rapidly
with energy until thewith energy until the
photon energies far exceedphoton energies far exceed
the electron bindingthe electron binding
energies and Comptonenergies and Compton
effect becomes theeffect becomes the
predominant mode ofpredominant mode of
interaction.interaction.

31. Photon Energy
(MeV)
Relative Number of Interactions (%)
P.E. (τ/ρ) Compton (σ/ρ) Pair Prod. (π/ρ)
0.01 95 5 0
0.026 50 50 0
0.060 7 93 0
0.150 0 100 0
4.00 0 94 6
10.00 0 77 23
24.00 0 50 50
100.00 0 16 84
Data from Johns HE, Cunningham JR. The physics of radiology. 3rd ed. Springfield,
IL: Charles C Thomas, 1969.
Relative Importance OF P.E.Relative Importance OF P.E. ((ττ)), Compton (, Compton (σσ) And Pair) And Pair
production (production (ππ ) processes in Water) processes in Water

32. Figure: Plot of total mass attenuation coefficient (μ/ρ) as a
function of photon energy for lead and water. (from Johns HE,
Cunningham JR. The physics of radiology, 3rd ed.)
Energy
Range
Dominant Effects
Up to 50KeV PE (Photo Electric)
effect is important
60 KeV -
90 KeV
Both PE & Compton
effect
200 KeV - 4
MeV
Compton effect
Beyond 20
MeV
Pair Production

33. Practical ImplicationsPractical Implications
 The photoelectric effect has severalThe photoelectric effect has several
important implications in practicalimportant implications in practical
radiology:radiology:
 In diagnostic radiology , the primary modeIn diagnostic radiology , the primary mode
of interaction is photoelectric. It is alsoof interaction is photoelectric. It is also
responsible for the contrast effect.responsible for the contrast effect.
 In therapeutic radiology , low-energyIn therapeutic radiology , low-energy
beams in orthovoltage irradiation causesbeams in orthovoltage irradiation causes
excessive absorption of energy in bone.excessive absorption of energy in bone.

34. Due to the kilovoltage energies used, plain x-raysDue to the kilovoltage energies used, plain x-rays
are attenuated predominately by the photoelectricare attenuated predominately by the photoelectric
effect. Attenuation is therefore related to the cube ofeffect. Attenuation is therefore related to the cube of
the atomic number (Z3). In human tissues, this leadsthe atomic number (Z3). In human tissues, this leads
to a marked difference in attenuation between softto a marked difference in attenuation between soft
tissues and fat (Z ~ 8) and bone (Z ~ 13). This istissues and fat (Z ~ 8) and bone (Z ~ 13). This is
seen on the image, where bone causes significantseen on the image, where bone causes significant
attenuation, soft tissues cause some attenuation andattenuation, soft tissues cause some attenuation and
air/lung cause minimal attenuationair/lung cause minimal attenuation....
ThetwoThetwo X-rayX-ray contrastmediacontrastmedia iodineiodine andand bariumbarium havehave
ideal K shell binding energies for absorption of X-rays,ideal K shell binding energies for absorption of X-rays,
33.2 keV and 37.4 keV, respectively, which is close to33.2 keV and 37.4 keV, respectively, which is close to
the mean energy of most diagnostic X-ray beams.the mean energy of most diagnostic X-ray beams.

35.  PPositron emission tomographyositron emission tomography (PET) works on(PET) works on
principle of positron annihalation.Fludeoxyglucose (FDG)principle of positron annihalation.Fludeoxyglucose (FDG)
is taken up by metabolically active cells undergoesis taken up by metabolically active cells undergoes
positron decay travels in the tissue about shortpositron decay travels in the tissue about short
distance(1mm) looses kinetic energy where it interactsdistance(1mm) looses kinetic energy where it interacts
with electron leading to annihalation .The photons arewith electron leading to annihalation .The photons are
released in opposite direction fall onreleased in opposite direction fall on
scintillator(lumninesence property),creating a burst ofscintillator(lumninesence property),creating a burst of
light detected by silicon diodes..the technique dependslight detected by silicon diodes..the technique depends
on simultaneous detection of the pair of photonson simultaneous detection of the pair of photons
travelling in opposite direction .these images aretravelling in opposite direction .these images are
reconstructed and results are obtained..reconstructed and results are obtained..