INTERACTION OF IONIZINGRADIATION

DR/ OMAR HASHIM

Interaction of radiation (x or gamma ray ) with the matter lead to ejectionElectron , this high speed electron transfer their energy by producingIonization &excitationIonization ;- the process by which a neutral atom acquires +ve or -ve charge .Removal of an orbital electron (- ve ) leaving the atom + ve charge in ionPair .Charge particles such as electron , proton , and alpha particle are known asDirectly ionizing radiation ---- collision .When the energy produced lost by incident particle is not sufficient to ejectElectron but is used to raise the electron to higher energy level , the pro-Cess known as excitation . Un charged particle such as neutron & photons Are indirectly ionizing radiation .Ionizing photons interact with atoms by 3 major process ;--I. Photoelectric effect .II. Compton effect .III. Pair production

Photon beam description ;- 1 the fluence ( Q ) OF photon is quotient Dn by da where DN is the numberOf photon that enter an imaginary sphere of cross sectional areaQ = Dn/da .2 Fluency rate or density is the fluency per unit time Q = Do /dt .3 Energy fluency is the quotient of de where de is the sum of the energyof all the photon that enter sphere of cross sectional area==de/da ,4 energy fluence rate , energy flux , or intensity .Photon beam attenuation ;- experimental arrangement ;- a narrow beam of monoenergetic photon is On an absorber of variable thickness . Detector is placed at fixed distanceFrom the source .then photon pass through the absorber is measured ,Under this condition the reduction in the number of the photon is Proportional to the number of the incidence photon (N)& to the thickness ofThe absorber(DX) dn = -att N dx .The number of photon decrease as the absorber thickness increase

If thickness is expressed as length then daltta called attenuation coefficient . If thick-Ness is measured in centimeters , the factors 1/cm

H VL (half value layer ) ;-Denied as the thickness of an absorber required to attenuate the intensityOf attenuate the intensity of the beam to half its original value . -

Interaction of photon with matter ;- A ) Photoelectric effect ;- is phenomena in which a photon interacts with anAtom and ejects one of the orbital electrons from the atom .The energy first absorbed to the atom then transferred to atomic electron . the kinetic energy of the ejected electron (photoelectric ) =hv-Eb .Eb binding energy of the electron . This can take place in ( K—L -- M ---OR N ) .After the electron has been ejected vacancy is created in the shill . LeavingThe atom in excited state . The vacancy will be filled by outer orbital electronWith the emission of characteristic x ray . These is the possibility of emissionOf auger electron . Which is monoenergetic electrons produced by absorptionOf characteristic x ray by the atom .Because the binding energy of the soft tissues are very low ( 0.5 kev ) theEnergy of the characteristic photons produced in biologic absorbers isVery low .for higher energy photon and higher atomic number material . TheCharacteristic photon are of higher energy may deposit energy at large Distance . In such cases , the local energy absorption is reduced by the energyEmitted as characteristic radiation .

The probability of photoelectric absorption depend on the photon energy . WhereThe mass photoelectric attenuation coefficient ( t/p ) is plotted as a function ofPhoton energy . 3Water ( allow atomic number ) & lead ( high atomic number ) .t/p proportional 1/E .IF photon energy increase . The probability of photoelectric attenuation decrease .The data for various materials indicated that photoelectric attenuation dependsStrongly on the atomic number of the absorbing material . This relationship fromThe basic many application in the diagnosis radiology .the differences in the Z OFThe various tissue such as bone , muscles , & fat amplified diff3erence in x ray Absorption . The 1ry mode interaction is photoelectric . This Z dependence is alsoExploited when using contrast media such as BaSO4 .In the therapeutic radiology , the low energy beams produced by superficial andOrthovoltage machines cause un necessary high absorption of x ray energy in bonesDue to high Z

3 3

t/p pro-z / E

COMPTON EFFECT ;- IN the compton process the photon interacts with an atomic electron as throughIt were free electron .The eletron receive some energy from the photon and is emitted an angle .Special case of compton effect A ) direct hit ;- if photon make direct hit with the electron . The electronThe scattered photon will travel backward . The electron receive willMaximum energy .& the scattered photon will be left with minimum energy B) Grazing Hit ;-If a photon make grazing hit with the electron will be emitted right angles= 90 degree & the scattered photon will in the forward direction .Degree photon scatter ;-If a photon is scattered at right angles to its original direction = 90 oneCalculate E & hv .

a) Interaction of allow energy photon ;- if the incident photon energy is Much less than the rest energy of the electron , only small part of the Energy is imparted to the electron , result in scattering photon with theSame energy as incident photon

b) Interaction of high energy photon ;- if the incident photon has very high energy( much greater than the ret energy of the electron ) . The photon loses mostOf its energy to the Compton electron , the scattered photon has much less energy

Energy and atomic affect in Compton ;-As mentioned before the Compton effect is interaction between photon and freeEletron . Practically this means that the energy of incident photon must be Large compared with the eletron binding energy . Althougth the photon energy increaseLead to decrease of the compton effect .Because the compton interaction involve free electron in the absorbing material .Its independent of atomic number z .From the previous discussion , it follow that if the energy of the beam is in theRegion where the compton effect is only possible mode of interaction . ApproximatelyThe same attenuation of the beam occur in any material of equal density thick –Ness ( attenuation per g/cm for bone = is the same as for soft tissues

Pair production ;-If the energy of the photon is greater than 1.02 mev , the photon may interact withThe matter throught mechanism of pair production . In these process the electronInteracts strongly with the electromagnetic field of atomic nucleus and give of allOf its energy in the process of creating apiary consisting of -ve eletron and + ve Eletron .Because the rest mass energy of the electron = 0.51 mev . Minimum energy of 1.o2Mev . The photon energy in excess of this threshold is shared between the particlesAs kinetic energy .The most probable distribution of energy for each particle to acquire half the availableKinetic energy. Although any energy distribution is possible .The pair production is event in which energy is converted into mass . As predicted byEinstein s equation .—The reverse process , the conversion of mass in toEnergy take 2 place when positron combines with an electron E = M CTo produce two photon called the annihilation radiation .Positron loss its energy as its traverses the matter by the same type of interactionAs electron does ( ionization ,excitation & bermsstrahlunge ) . At the end of the slowly Moving positron combines with one electron in its vicinity to give rise to twoAnnihilation photons each one has o.51 mev , the two photon ejected in oppositedirection

Because the pair production result from interaction with the electromagnetic Field of the nucleus , the probability of this process increase rapidly with atomicNumber . Relative important of various types of interacton ;-The total mass attenuation coefficient m/pTotal m/p = photoelectrical/p + coherent + Compton + pair . Cohered scattering is Only impotent for very low photon ( less than 10 kev ) & high Z material .The attenuation coefficient decrees rapidly with energy until the photon energyFar exceeds the electron and the Compton effect becomes the predominantMode of interaction . In the Compton range of energy ate- co- of the lead & waterDo not differ greatly , since this type of interaction is independent of atomic Number . Co - however decrease with energy until pair production begins to Become important at energy more than 1.02 mev .Interaction of charged particles ;-( electrons , protons , alpha particles and nuclei ) interact principally by ionizationAnd excitation .radiative collision in which the charged particles interact by Coulomb force between the electric field of traveling particle & electeric fieldOf orbital electron and nuclei of atoms . Collision between the particle and theAtomic electron result in ionization & excitation