INTERACTION OF RADIATION INTERACTION OF RADIATION WITH WITH

MATTER(PHOTOELECTRIC MATTER(PHOTOELECTRIC AND PAIR PRODUCTION)AND PAIR PRODUCTION)

DR SAQIB AHMAD SHAHDR SAQIB AHMAD SHAHDept. of RadIATION ONCOLOGY Dept. of RadIATION ONCOLOGY

SKIMSSKIMS

MODERATOR:-DR TARIQ MODERATOR:-DR TARIQ RASOOLRASOOL

Radiation The term The term radiation radiation applies to the emission and applies to the emission and

propagation of energy through space or a propagation of energy through space or a material medium . material medium .

Radiation may be Radiation may be Electromagnetic RadiationElectromagnetic Radiation Particle RadiationParticle Radiation

When radiation passes through matter it may When radiation passes through matter it may interact with the material , transferring some or interact 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.

ElectromagneticElectromagnetic Radiation RadiationConstitutes the Constitutes the mode of energy mode of energy propagation for propagation for such phenomena such phenomena as light waves, as light waves, heat waves, radio heat waves, radio waves, u v rays, x waves, u v rays, x rays and γ rays .rays and γ rays .

Spectrum of Spectrum of electromagnetic electromagnetic radiation ranges radiation ranges from from 101077 m (radio m (radio waves) to 10waves) to 10-13-13 m m (ultra high energy (ultra high energy X rays) .X rays) .

Particulate RadiationParticulate Radiation

Refers to the energy propagated by traveling Refers 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 any definite 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 positrons Some common subatomic particles are positrons (charge + 1) , neutrinos (zero charge) and (charge + 1) , neutrinos (zero charge) and mesons .mesons .

Interaction of Photons with MatterInteraction of Photons with Matter

When an X ray or γ ray beam passes When an X ray or γ ray beam passes through a medium , interaction through a medium , interaction occurs between the photon and the occurs between the photon and the matter and energy is transferred to matter and energy is transferred to the medium . the medium .

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

Attenuation Coefficient (1)Attenuation Coefficient (1)

Fraction of photons removed from a mono Fraction of photons removed from a mono energetic beam of x-ray or gamma ray per unit energetic beam of x-ray or gamma ray per unit thickness of material is called thickness of material is called linear attenuation linear attenuation coefficientcoefficient ( (µµ), typically expressed in ), typically expressed in cmcm-1-1 ..

Number of photons removed from the beam Number of photons removed from the beam traversing a very small thickness traversing a very small thickness ∆∆x:x:

where where 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 that and minus sign is placed before μ to indicate that

no. of photons decreases as the absorber no. of photons decreases as the absorber thickness increases.thickness increases.

xNn ∆=µ

Attenuation Coefficient (2)Attenuation Coefficient (2)

For a mono energetic beam of photons incident For a mono energetic beam of photons incident on either thick or thin slabs of material, an on either thick or thin slabs of material, an exponential relationship exists between number exponential 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 of The 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 ).

xeNN µ−= 0

Attenuation Coefficient (3)Attenuation Coefficient (3)

TotalTotal Linear attenuation coefficientLinear attenuation coefficient is the sum of is the sum of individual linear attenuation coefficients for each individual linear attenuation coefficients for each type of interaction:type of interaction:

For a given thickness of material , probability of For a given thickness of material , probability of interaction depends on number of atoms the x interaction 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 to Linear attenuation coefficient is proportional to the density of the material.the density of the material.

pairComptonphotoRayleigh µµµµµ +++=

Mass Attenuation CoefficientMass Attenuation Coefficient For a given thickness probability of interaction is For 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 by This dependency can be overcome by

normalizing linear attenuation coefficient for normalizing linear attenuation coefficient for density of material – density of material –

Mass Attenuation CoefficientMass Attenuation Coefficient (μ / ρ ) = (μ / ρ ) = Linear attenuation coefficient Linear attenuation coefficient Density of the materialDensity of the material Mass attenuation coefficient is independent of Mass attenuation coefficient is independent of

density of the material.density of the material.

List of InteractionsList of Interactions

High Speed Electrons

Photon

Photoelectric Effect(1)Photoelectric Effect(1) History:1887 History:1887

Henrich Hertz Henrich Hertz discovered discovered electrodes electrodes illuminated by uv illuminated by uv light creates light creates electric sparkselectric sparks

1905:-Albert 1905:-Albert Einstein published Einstein published paper explaining paper explaining phenomenon of phenomenon of photoelectric effect photoelectric effect (noble prize in (noble prize in 1921)1921)

Photoelectric Effect (2)Photoelectric Effect (2)

All of the incident photon energy is transferred to All 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 the Kinetic 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

Photoelectric Effect (3)Photoelectric Effect (3)

Incident photon energy must be greater than or Incident photon energy must be greater than or equal to the binding energy of the ejected equal to the binding energy of the ejected photon.photon.

The ionized atom regains electrical neutrality by The ionized atom regains electrical neutrality by rearrangement of the other orbital electrons. The rearrangement of the other orbital electrons. The electrons that undergo these rearrangements electrons that undergo these rearrangements surrender some of the energy in form of a photon surrender some of the energy in form of a photon known as the known as the characteristic radiation of the atom.characteristic radiation of the atom.

Absorption of these characteristic radiation Absorption of these characteristic radiation

internally in the atom may result in emission of internally 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.

Probability of characteristic x-ray emission Probability of characteristic x-ray emission decreases as Z decreasesdecreases as Z decreasesDoes not occur frequently for diagnostic energy Does not occur frequently for diagnostic energy

photon interactions in soft tissuephoton interactions in soft tissue

Photoelectric Effect (4)Photoelectric Effect (4)

Probability of photoelectric absorption per unit Probability of photoelectric absorption per unit mass is approximately proportional tomass is approximately proportional to

Energy dependence explains, in part, why image Energy 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 in Process can be used to amplify differences in attenuation between tissues with slightly different attenuation between tissues with slightly different atomic numbers, improving image contrast.atomic numbers, improving image contrast.

33 / EZ

Photoelectric Effect (5)Photoelectric Effect (5) Graph of probability of photoelectric effect, as a Graph of probability of photoelectric effect, as a

function of photon energy, exhibits sharp function of photon energy, exhibits sharp discontinuities called discontinuities called absorption edgesabsorption edges . .

Photon energy corresponding to an absorption Photon energy corresponding to an absorption edge is the binding energy of electrons in a edge 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 and 1) 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 own 2) A substance is relatively transparent to its own

characteristic radiation. This effect is important when characteristic 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.

Pair Production (1)Pair Production (1)

History:-Patrick Blackett History:-Patrick Blackett discovered pair production while discovered pair production while his invention of cloud chamber his invention of cloud chamber

got noble prize in 1948got noble prize in 1948

Pair Production (2)Pair Production (2)

Definition:-refers to the creation of Definition:-refers to the creation of an elementary particle and its an elementary particle and its antiparticle when usually photon(or antiparticle when usually photon(or another neutral boson) interacts with another neutral boson) interacts with nucleus or another boson..This is nucleus or another boson..This is allowed,if enough energy is provided allowed,if enough energy is provided to create the pair( equal to rest to create the pair( equal to rest mass of pair) with conservation of mass of pair) with conservation of energy and momentum. energy and momentum.

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 of of 1.02 M e V passes close to the nucleus of an atom, the photon disappears, and a positron an atom, the photon disappears, and a positron and an electron appear. This effect is known as and an electron appear. This effect is known as pair production. pair production.

Pair production results in attenuation of the beam Pair production results in attenuation of the beam with absorption. with absorption.

The positron created as a result loses its energy The positron created as a result loses its energy by interaction with an electron to give rise to two by interaction with an electron to give rise to two annihilation photons, each having 0.511 M e V annihilation photons, each having 0.511 M e V energy. Again because momentum is conserved energy. Again because momentum is conserved in the process two photons are rejected in in the process two photons are rejected in opposite directions. This reaction is known as an opposite directions. This reaction is known as an annihilation reaction. annihilation reaction.

Pair Production (3)Pair Production (3)

Other examples:-tau and anti tau,,moun and anti Other examples:-tau and anti tau,,moun and anti mounmoun

Pair production results from an interaction with Pair production results from an interaction with the electromagnetic field of the nucleus and as the electromagnetic field of the nucleus and as such the probability of this process increases such the probability of this process increases rapidly with the atomic number (rapidly with the atomic number (ZZ22 ).).

In addition, the likelihood of this interaction In addition, the likelihood of this interaction increases as the photon energy increases, in increases as the photon energy increases, in contrast to the Compton effects and the contrast to the Compton effects and the photoelectric effect. photoelectric effect.

Relative Importance of the Relative Importance of the Various ProcessesVarious Processes

The relative importance of the 3 principal modes of The relative importance of the 3 principal modes of interaction pertinent to radiation therapy- the interaction 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 Atomic as 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 of For 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 below Photoelectric 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 and Above 50 k e v Compton effect remains dominant and remains so, remains so,

Until about 24 M e v , after which Pair Production effect Until about 24 M e v , after which Pair Production effect becomes dominant .becomes dominant .

Relative ImportanceRelative Importance

The μ/ρ is large for low The μ/ρ is large for low energies and high Z media energies and high Z media (eg.Lead ) because of the (eg.Lead ) because of the predominance of predominance of Photoelectric interactions Photoelectric interactions under these conditions. under these conditions. The μ /ρ decreases rapidly The μ /ρ decreases rapidly with energy until the with energy until the photon energies far exceed photon energies far exceed the electron binding the electron binding energies and Compton energies and Compton effect becomes the effect becomes the predominant mode of predominant mode of interaction.interaction.

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 (π π ) processes in Water) processes in Water

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

Practical Implications Practical Implications

The photoelectric effect has several The photoelectric effect has several important implications in practical important implications in practical radiology: radiology:

In diagnostic radiology , the primary mode In diagnostic radiology , the primary mode of interaction is photoelectric. It is also of interaction is photoelectric. It is also responsible for the contrast effect. responsible for the contrast effect.

In therapeutic radiology , low-energy In therapeutic radiology , low-energy beams in orthovoltage irradiation causes beams in orthovoltage irradiation causes excessive absorption of energy in bone.excessive absorption of energy in bone.

Due to the kilovoltage energies used, plain x-rays Due to the kilovoltage energies used, plain x-rays are attenuated predominately by the photoelectric are attenuated predominately by the photoelectric effect. Attenuation is therefore related to the cube of effect. Attenuation is therefore related to the cube of the atomic number (Z3). In human tissues, this leads the atomic number (Z3). In human tissues, this leads to a marked difference in attenuation between soft to a marked difference in attenuation between soft tissues and fat (Z ~ 8) and bone (Z ~ 13). This is tissues and fat (Z ~ 8) and bone (Z ~ 13). This is seen on the image, where bone causes significant seen on the image, where bone causes significant attenuation, soft tissues cause some attenuation and attenuation, soft tissues cause some attenuation and air/lung cause minimal attenuationair/lung cause minimal attenuation....

Thetwo Thetwo X-rayX-ray contrastmediacontrastmedia iodineiodine and and bariumbarium have have 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 to 33.2 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.

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 undergoes is taken up by metabolically active cells undergoes positron decay travels in the tissue about short positron decay travels in the tissue about short distance(1mm) looses kinetic energy where it interacts distance(1mm) looses kinetic energy where it interacts with electron leading to annihalation .The photons are with electron leading to annihalation .The photons are released in opposite direction fall on released in opposite direction fall on scintillator(lumninesence property),creating a burst of scintillator(lumninesence property),creating a burst of light detected by silicon diodes..the technique depends light detected by silicon diodes..the technique depends on simultaneous detection of the pair of photons on simultaneous detection of the pair of photons travelling in opposite direction .these images are travelling in opposite direction .these images are reconstructed and results are obtained..reconstructed and results are obtained..