Interaction of Radiation With Matter- Anupam

7/31/2019 Interaction of Radiation With Matter- Anupam

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: The smallest indivisible part of anelement having all the characteristics of that

element is known as Atom.

Atoms are specified as ZXA

Where, Z = atomic number, and A = mass number.

According to Niels Bohr, electrons revolve inspecific orbits around the nucleus, named as K,L,Metc; K being innermost orbit.

These orbits are synonymous with energy levels.

Here energy refers to the potential energyof theelectron. This energy depends upon the coulombforces of attraction b/w the nucleus and the orbitalelectrons. Higher the atomic number greater isthis binding energy.

Energy level diagram (H2nucleus


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The term radiation applies to the emission and propagation of energy through

space or a material medium.RADIATION

ELECTROMAGNET

IC

PARTICULATE

:

Refers to the energy propagated by traveling

corpuscles, which have definite rest mass, definitemomentum and a defined position at any instant.

Elementary atomic particles: electron, proton, neutron.

Positron, neutrino and mesons are subatomic particles.


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Mode of energy propagation forphenomena such as light waves,X

Rays, Rays etc. Was defined by Maxwellin terms of

oscillating electrical and magneticfields being at right angles to eachother.

Travel with velocity of 310m/s in free space

Transfer energy from place to place in quanta (E= h)

In passing through matter the intensity is reduced because ofabsorption and scatter

Obeys inverse square law in free space, i.e intensity at any

place varies inversely as the square of distance (I 1/d2 )

EM radiation ranges from the

wavelength of 107 m(radio waves) to 10-13 m (Ultra high-energy x-rays).


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TWO MAJOR FORMS OF ELECTROMAGNETIC RADIATION USED INMODERN DAY RADIOTHERAPY:

Discovered by Wilhelm Rontgen in 1895.

Two different mechanism of X Ray production:

Characteristic X rays:Emitted when incidentelectrons with a kinetic energy > BE of the orbitalelectrons, ejects the orbital electron and leaves the

atom ionized. The resulting vacancy when filled byouter electrons result in emission of characteristicradiation

Bremsstrahlung: Radiative collision(interaction) b/w a high speed incident electron andnucleus. The electron passing near a nucleus, suffers a

sudden deflection and deceleration due to Coulombforces of attraction lose a part of its energy asbremsstrahlung radiation

The nuclei with unstable n/p combinations undergo intranuclear

disintegration to achieve more stable state with release of charged particle/ EM radiationsknown as GAMMA RAYS.

Co Ni+ e- + (2 photons of energies 1.17 and 1.33 MeV)


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DIRECTLY IONISING INDIRECTLYIONISINGCharged particles:

electrons

protonsalpha particle

Uncharged ParticlesNeutrons

Protons

photoelectric effect compton effect pairproduction

High speed electrons

ionisation and exitation

Biological Effects


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The photon-beam may undergo the following four processes:

Attenuation: removal of radiation from the beam by the matter.

Attenuation may occur due to scattering and absorption.

Absorption: taking up of the energy from the beam by the irradiated

material. It is absorbed energy, which is important in producing the

radiobiological effects

Scattering: change in the direction of the photons and it contributes to

both attenuation and absorption. Its a cause ofproblem in both

diagnostic and therapeutic radiology.

Transmission: Any photon, which does not suffer the above processes


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Attenuation occurs exponentially, that

is a given fraction of the photos is

removed for a given thickness of the

attenuating material. Thus its

impossible to reduce this beam to

nothing.

Attenuation (dN i.e. reduction in number of photon) is proportional to numberof incident photons (N) and thickness of absorber (dx)

dN Ndx

Also Higher Z more attenuation; Higher photon energy smaller

A reduction in intensity of beam of radiation when it passes through anymaterial


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The fractionalreduction

produced in any monoenergeticphoton beam; is constant for anygiven material per unit thickness (cm-1).

dI/I = - dx = 0.693/ HVL

Depends on:

1. energy of the photons2. nature of material i.e. the no. of electrons present in that

thickness and densityof the material.

dividing linear attenuationcoefficient by density; is a more fundamental coefficient. Does notinvolve density but rather the atomic composition.

Other coefficients which are more fundamental include:Atomic attenuation coefficient (atoms/cm)Electronic attenuation coefficient (electrons/ cm)

= 0.1/cm


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Thickness of a given material requiredto attenuate the intensity of a beam to

half of its original value. = 0.693/HVL

HVL is an expression of thequality/penetrating power of the

beam.

For a heterogeneous beam,1st andsubsequent HVLs are not constant, but1st HVL is less than subsequent HVLs.

An important clinical implication of hvlis in SHIELDING.

For practical purposes, the shielding

material which reduces beamtransmission to 5% of its original is

considered acceptable.

Semilog plot showing exponential

attenuation of a monoenergetic beam


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After passing through 5HVL, intensityof a monoenergetic beam reduces toless than 5% of original

The recommended minimumthickness of lead for shieldingforvarious megavoltage beam qualitiesis

Co60 - 5.5cm

4MV - 6.0cm

6MV - 6.5cm

10MV - 7.0cm

25MV - 7.0cmHVL MEASUREMENT: Material for HVL measurement vary with energy ofradiation.

Radiation upto 30kV-CellophaneRadiation at 30-150kV-AluminiumRadiation at 120-600kV-Copper

Radiation at 500kV-2MV-Lead


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Coherent scattering

Photoelectric effect

Compton effect

Pair production.

Photodisintegration


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(ELASTIC SCATTERING/UNMODIFIED SCATTERING/CLASSICAL SCATTERING/RAYLEIGH SCATTERING)

Process described by considering

radiation as waves rather than asparticles.

Involves bound electrons.

Occurs with higher atomic number

materials and low energy radiations(


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In this phenomena, the photon

disappears altogether afterinteracting with the bound electron.

Entire energy of the photon istransferred to an orbital electron.While part of the incident photonenergy is used to overcome thebinding energy, the rest of it is usedas the K.E. of the photo electron.

h= BE + mv

The process results in ionization ofthe atom and the resulting vacancythus created is filled by an outer shell

electron with emission ofcharacteristic X rays (fluorescentradiation) of the atom.

internal atomicabsorption

Auger electrons

(internal PE effect)

The angular distribution of emittedphoto electrons depend on thephoton energy:

For low energy: 90 degreesFor higher energies:

progressively

forward direction.


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Mass photo electric coefficient (/) :

/ = k Z/E

ABSORPTION EDGES :

/ versus photon energy plot shows

discontinuities at levels of energies

corresponding with the binding energies of

the various electronic shells.

Two important consequences

Near the absorption edges, the lower

energy photons are less attenuated andmore penetrating than higher energy

photons.

Any substance is transparent to its own

characteristic radiation, energies of which

are always a little less than corresponding

binding energies.

Mass photoelectric attnuation coeffecient plottedagainst photon energy


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PE effect forms the basis of many applications in diagnosticradiology.The difference in atomic number of various tissuessuch as bones, nucleus and fat amplifies the differencein X ray absorption to Z times.

This Z dependence is also exploited when usingcontrast mediums with high atomic numbers (BaSO4and hypaque).

The Z dependence also leads to higher absorption ofenergy in bones while using orthovoltage machines, wherePE effect is dominant.


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(MODIFIED/INCOHERENT/INELASTIC SCATTERING)

Arthur Holly Comptonin 1923

Predominates between 200KeV and

4MeV.Photons interact with freeelectrons

The photon collides with electron andhands over part of its energy to it and

itself continues in a new direction (i.e.its scattered) but with reduced energyand hence increased wavelength.

The wavelength change depends neither

on the material being irradiated nor onthe radiation energy, but only upon theangle through which the radiation isscattered.

The Compton effect results in both

attenuation and absorption.

If the angle by which the electron isscattered is and the angle bywhich the photon is scattered is, then the following formuladescribes the change in thewavelength ()of the photon:

1 2 = = 0.024 ( 1- cos )


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Energy of scattered electron is :

E = h ( 1- Cos)1 + (1- Cos)

Energy of scattered photon is :

h = h .1 + ( 1- Cos)

Where, = h /mc { also, mc = 0.511MeV}h = energy of incident photon

Direct hit (= 180 degree, = 0degrees): Will result in electronreceiving maximum possibleenergy while the scatteredphoton will be left with minimum

possible energy.

Grazing hit(= 0 degree, =90 degrees): Will result inscattered electron with noenergy and scattered photon

with maximum energy.

Independent of Z,

depends only on thenumber of electrons pergram.

The coefficient is

practically same for allelements excepthydrogen, since electronper gram for all elements

except hydrogen is


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The Compton attenuation coefficient does

not depend on the atomic number, so, for

shielding in megavoltage radiation any

material can be used with similar efficacy.

There is no excess radiation absorption by

bones as compared to soft tissues in the

megavoltage range where Compton effect is

dominant.

No bone shielding effect, as seen in

orthovoltage radiation with dominant

photoelectric effect.


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The fraction of the energy imparted to the recoil electron

increases as the beam energy increases. So, higher beam

energies allow greater absorption of radiation dose in the

body with less scattering of energy.

As the photon energy increases there is a corresponding

increase in the forward scatter of the beam, resulting in

better dose distribution.

The radiation scattered is independent of incident energy

and has a maximum energy of 0.511 MeV at 90 degrees and

0.255MeV for the radiation scattered backwards; the photons

scattered at angles < 90 degrees will have energies more

than 0.511MeV gradually approaching the incident photon

energy


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Photon of energy > 1.02 MeV interacts strongly withelectromagnetic field of nucleus, the photon disappears, and apositron (e+) and an electron (e-) appear. This effect is known aspair production

Particles travel in a forward direction relative to the incidentphoton. Any energy distribution is possible, but

usually divided equally i.e. halfof the

available energy (hv - 1.02) MeV

ANNIHILATION RADIATION

e+ + e-

Ray

0.511Mev

Ray

0.511Mev


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Pair production results froman interaction with theelectromagnetic field of

the nucleus and as suchthe probability of thisprocess increases rapidlywith the atomic number( Z2)

In addition, the likelihood ofthis interaction increasesas the logarithm of theincident photon energy

above the treshold ( lo E

CLINICAL

IMPLICATIONS: Beyond 4 MeV pair

production results inincreasing mass

attenuation coefficientsspecially for higheratomic numberelements.

High energy radiations

(>20 MeV) are less

penetrating than somelower energy radiations

and are not used inradiothera .


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Interaction of a high energy photon with an atomic nucleus leading to

emission of one or more nucleons, mostly neutrons.

Occurs when the photon has energy greater than the binding energyof the nucleus itself.

Example : Cu + Cu + n Threshold energy: the difference between the rest energy of the

target nucleus and that of the residual nucleus plus the emittednucleons. For majority of atoms the threshold energy for this effect isabout 10 MeV.

Chances of occurrence increases rapidly with increasing energy untilmaximum is reached at 5MeV above threshold, after this the chancefalls off with equal rapidity.

Nowadays, the main use of this reaction is for energy calibration of

machines producing high energy photons.


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Total mass attenuationof beam is sum of four individual coefficients

(/) = ( /) + (coh/) + (/) + (/)

total P.E coherent compton pair

So, the energies important for radiotherapy ranges from orthovoltage(50 - 500kev) to megavoltage( 4-20 MeV).

At low energies PE is

dominant decreases rapidly

with increase in energy to

reach a minimum at 0.1

MeV, in low Z materials.

medium energy range i.e

from 0.2 to 4 MeV, Compton

effect is the predominant

mode of attenuation in low

atomic number materials

pair production starts at

1.02MeV and increases with

photon energy, making very

high energy(>20 MeV) less

penetrating


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Taking up of energy from thebeam by the irradiated material

Most of electrons set in motionby the above interactions loseenergy by inelastic collisionswith the atomic electrons of thematerial.

A few electrons will lose energyby the Bremsstrahlunginteractions with the nuclei. Thisenergy is irradiated out of thelocal volume as X-rays and istherefore not included in thecalculation of locally absorbedenergy.

In most interactions involvingthe soft tissues, thebremsstrahlung component is

negligible

ENERGY ABSORPTION

COEFFICIENT:The product of the

energy transfercoefficient(tr) and (1-g)where g is the fraction of

energy of secondarycharged particles lost toBremsstrahlung in thematerial.

en =tr(1-g)

Mass energy absorptioncoefficient: en/


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91% 15% 46% 71% 96%

% of attenuated energy which is absorbed


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FEATURE ORTHOVOLTAGE MEGAVOLTAGE

ENERGY 50-150-500 KeV 4-20Mev

SOURCE Electricity Electricity/Radioisotopes

SKIN SPARING Absent Present

PENETRATION Less More

BONE SPARING Absent Present

BONE SHEILDING Present Absent

DOSE RATE Less High

PDD Lower Higher

SSD 40 cm 80-100 cm

BEAM MODIFICATION Difficult Easy

RBE More Less

LET More Less

ORTHOVOLTAGE VERSUS MEGAVOLTAGE


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PARTICULATE RADIATION

Ionizing/ Charged particles Uncharged particles

Electrons (e+) Neutrons

Protons (p+)

Alfa particles (4He2)

Pi-mesons (+/-/0)

The charged particle interactions are mainly mediated byCoulomb forcesbetween the electric fields of the travellingcharged particle and the electric fields of orbital electron andnuclei of atoms


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modes of interactions of charged particles

STOPPING POWER

Rate of kinetic energy loss per unit path length of the particleS=dE/dx

Rate of energy loss or stopping power is proportional to the square ofparticle charge and inversely to the square of its velocity.

S= k charge/velocity

Thus as the particle slows down, its rate of energy loss to the

medium increases

COLLISIONAL LOSSCollision between the particleand the electron cloudresulting in ionization orexcitation ( more importantin low Z elements)

RADIATIVE LOSSCollision between nucleus andcharged particle, resulting in

Bremsstrahlung radiation


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:

As the particle slows down itsrate of energy loss increasesand so does the ionization orabsorbed dose to themedium. This peaking ofdose at the end of theparticle range is called theBraggs peak.

Seen with protons andheavier charged particles,not with electrons.

The protons and heavier charged particles provide a muchsought after advantage in radiotherapy; their ability toconcentrate dose inside a target volume and minimize dose to

the surrounding normal tissues

CLINICALIMPLICATION :


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Electrons are light particles with negligible mass and single negative charge. As

a result they penetrate deeperthan other charged particles but at the same time

undergo greater scattering

Electrons lose energy predominantly by ionization and excitation.

May interact with the electric field of the nucleus and result in Bremsstrahlung

radiation. The probability of Bremsstrahlung increases with electron energy and

atomic number of medium.

Suffer multiple scattering and direction change, because of smaller mass. This

leads to a smudging of the Bragg's peak which is hence not seen in electrons.

DELTA RAY: An electron ejected as a result of ionization, having sufficient

energy to produce an ionization track of its own is called secondary electron or

delta ray.


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The ionization pattern produced by a beam of electrons ischaracterized by a constant value from the surface to adepth equal to about half the range, followed by a rapid

dose falloff to almost zero at a depth equal to the range.

ThisRapid dose fall off is speciallyis specially seen in

electrons in the energy range of6 -15 MeV making these useful treatment modality forsuperficial lesions.


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Neutrons are indirectly ionizing.

Interact basically by two processes:

1) Nuclear disintegrations, with emission of heavy charged particles,

gamma rays and neutrons; contributing around 30% of the total dose in

tissues.

2) Recoiling protons (predominant process) from hydrogen and recoiling

heavy nuclei from other elements, leading to redistribution of energy

between the colliding particles.

The most efficient recoil is seen if the colliding particle have same

mass(e.g. hydrogen nucleus) and this leads to the maximumabsorption.

This phenomenon has some practical implications: Hydrogenous materials like fats absorb neutrons more than heavier

materials and thus there is a 20% greater absorption in fat relative to

muscle.


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The recoil protons, set in motion after interaction with neutrons,

further cause ionization. The dense ionization produced by these

particles in the vicinity, results in high LET values more likely toinduce lethal damage in cells due to the dense ionization they

produce.

Biological effects of neutrons do not depend much on the

presence or absence of oxygen i.e. low Oxygen Enhancement

Ratio (OER).

Neutrons, being uncharged particles also penetrate deeply into

matter.

Despite these attractive radiobiological and physical properties,

neutrons are not commonly used in practical radiotherapy,

because of technical difficulties in production of these beams as

well as their complicated dosimetry.


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Have a very high linear energy transfer (LET) that is they havea very high ionization density

Exhibit the phenomena ofBraggs peak

However there are several practical and theoretical difficultieswith the use of these charged particles. Some of them include:

Width of the Braggs peak is very small (0.5cm) leading toinhomogeneous tumor dose in larger tumors

Generation of these charged particles requires expensive

and large machines. These large machines necessary forproduction of these beams often make it necessary to movethe patient instead of the gantry!


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The reactions that most commonly lead to cell damage usually occur at thelevel of the DNA although they may occur at the level of cell membranes,proteins etc.

Lethal double stranded DNA breaks are often irreparableand persist inthe form of micronuclei formation, chromosomal aberrations and loss ofreproductive integrity.

Some degree of protectionfrom chemical effects can be provided byaddition of anti oxidantswhich compete and take up some of the free

radicals

Radiation interacts with the atoms ofthe DNA molecule, or some othercellular component critical to the

survival of the cell

When radiation interacts with water toproduce free radicals cell

destructionIonizing radiation + HO HO + e

HO + HOHO +OH cell

damage


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Cells can die in several ways: Apoptosis Mitotic catastrophe Senescence Necrosis Autophagy

: delayed, over aperiod of days.

Most important for the effect ofradiotherapy of solid tumors is mitoticcatastrophe, which is caused by lethalchromosome damage.After irridiation, cells can passthrough one or few mitotic cyclesbefore mis-segregation of

chromosomes leads to loss ofreplicative potential (or clonogenicity) ofcells multimicronucleated cells canbe detected.

: A characteristicof radiosensitive cells eg. neoplastichematopoitic or lymphatic cells, is

death from radiation induced apoptosisvia the intrinsic, caspase 9 dependentpathway.Early/premitoticP53 dependent;occurs within 2 - 6 hours afterirradiation before cells enter mitosis

(primarily a consequence of DNAdamage)Late/postmitosis occurs aftermitosis; due to radiation induced lethalchromosome abberation.

Radiation induced senescence(p53 mediated cell cycle arrest) plays an

important role for development of normal tissue damage, for example fibrosis.


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The three major forms of interaction of radiation with matter,which are of clinical importance in radiotherapy are:

1. Compton effect.

2. Photoelectric effect.

3.

Pair production. Compton effect is the most important in modern-day megavoltage

radiation therapy.

Photoelectric effect is primarily important in diagnostic radiology.

There are several unresolved issues pertaining to the use ofparticulate radiations.

Radiation ultimately affects cells by two mechanisms, direct (DNAdependent) and Indirect (free radical dependent).


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COMPTON EFFECT


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BREMMSTRALUNG RADIATION CHARACTERISTIC X-RAY


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Pair productionPHOTOELECTRIC EFFECT


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PHOTO-DISINTEGRATIONCHARACTERISTIC X

RAY


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Interaction of Radiation With Matter- Anupam

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