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INTERACTION OF IONIZINGRADIATION<br />DR/ OMAR HASHIM<br />
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Interaction of radiation (x or gamma ray ) with the matter lead to ejection<br />Electron , this high speed electron transfer their energy by producing<br />Ionization &excitation<br />Ionization ;- the process by which a neutral atom acquires +ve or -ve charge .<br />Removal of an orbital electron (- ve ) leaving the atom + ve charge in ion<br />Pair .<br />Charge particles such as electron , proton , and alpha particle are known as<br />Directly ionizing radiation ---- collision .<br />When the energy produced lost by incident particle is not sufficient to eject<br />Electron but is used to raise the electron to higher energy level , the pro-<br />Cess known as excitation . Un charged particle such as neutron & photons <br />Are indirectly ionizing radiation .<br />Ionizing photons interact with atoms by 3 major process ;--<br />Photoelectric effect .<br />Compton effect .<br />Pair production<br />
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Photon beam description ;- <br /> 1 the fluence ( Q ) OF photon is quotient Dn by da where DN is the number<br />Of photon that enter an imaginary sphere of cross sectional area<br />Q = Dn/da .<br />Fluency rate or density is the fluency per unit time <br />Q = Do /dt .<br />Energy fluency is the quotient of de where de is the sum of the energy<br />of all the photon that enter sphere of cross sectional area<br />==de/da ,<br />4 energy fluence rate , energy flux , or intensity .<br />Photon beam attenuation ;-<br />experimental arrangement ;- a narrow beam of monoenergetic photon is <br />On an absorber of variable thickness . Detector is placed at fixed distance<br />From the source .then photon pass through the absorber is measured ,<br />Under this condition the reduction in the number of the photon is <br />Proportional to the number of the incidence photon (N)& to the thickness of<br />The absorber(DX) dn = -att N dx .<br />The number of photon decrease as the absorber thickness increase<br />
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If thickness is expressed as length then daltta called attenuation coefficient . If thick-<br />Ness is measured in centimeters , the factors 1/cm <br />
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H VL (half value layer ) ;-<br />Denied as the thickness of an absorber required to attenuate the intensity<br />Of attenuate the intensity of the beam to half its original value .<br />-<br />
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Interaction of photon with matter ;- <br />A ) Photoelectric effect ;- is phenomena in which a photon interacts with an<br />Atom and ejects one of the orbital electrons from the atom .<br />The energy first absorbed to the atom then transferred to atomic electron .<br /> the kinetic energy of the ejected electron (photoelectric ) =hv-Eb .<br />Eb binding energy of the electron . This can take place in ( K—L -- M ---OR N ) .<br />After the electron has been ejected vacancy is created in the shill . Leaving<br />The atom in excited state . The vacancy will be filled by outer orbital electron<br />With the emission of characteristic x ray . These is the possibility of emission<br />Of auger electron . Which is monoenergetic electrons produced by absorption<br />Of characteristic x ray by the atom .<br />Because the binding energy of the soft tissues are very low ( 0.5 kev ) the<br />Energy of the characteristic photons produced in biologic absorbers is<br />Very low .for higher energy photon and higher atomic number material . The<br />Characteristic photon are of higher energy may deposit energy at large <br />Distance . In such cases , the local energy absorption is reduced by the energy<br />Emitted as characteristic radiation .<br />
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The probability of photoelectric absorption depend on the photon energy . Where<br />The mass photoelectric attenuation coefficient ( t/p ) is plotted as a function of<br />Photon energy . 3Water ( allow atomic number ) & lead ( high atomic number ) .<br />t/p proportional 1/E .<br />IF photon energy increase . The probability of photoelectric attenuation decrease .<br />The data for various materials indicated that photoelectric attenuation depends<br />Strongly on the atomic number of the absorbing material . This relationship from<br />The basic many application in the diagnosis radiology .the differences in the Z OF<br />The various tissue such as bone , muscles , & fat amplified diff3erence in x ray <br />Absorption . The 1ry mode interaction is photoelectric . This Z dependence is also<br />Exploited when using contrast media such as BaSO4 .<br />In the therapeutic radiology , the low energy beams produced by superficial and<br />Orthovoltage machines cause un necessary high absorption of x ray energy in bones<br />Due to high Z<br />3 3 <br /> t/p pro-z / E<br />
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COMPTON EFFECT ;- <br />IN the compton process the photon interacts with an atomic electron as through<br />It were free electron .<br />The eletron receive some energy from the photon and is emitted an angle .<br />Special case of compton effect <br />A ) direct hit ;- if photon make direct hit with the electron . The electron<br />The scattered photon will travel backward . The electron receive will<br />Maximum energy .& the scattered photon will be left with minimum energy<br /> B) Grazing Hit ;-<br />If a photon make grazing hit with the electron will be emitted right angles<br />= 90 degree & the scattered photon will in the forward direction .<br />Degree photon scatter ;-<br />If a photon is scattered at right angles to its original direction = 90 one<br />Calculate E & hv .<br />Interaction of allow energy photon ;- if the incident photon energy is <br />Much less than the rest energy of the electron , only small part of the <br />Energy is imparted to the electron , result in scattering photon with the<br />Same energy as incident photon<br />
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Interaction of high energy photon ;- if the incident photon has very high energy<br />( much greater than the ret energy of the electron ) . The photon loses most<br />Of its energy to the Compton electron , the scattered photon has much less <br />energy<br />
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Energy and atomic affect in Compton ;-<br />As mentioned before the Compton effect is interaction between photon and free<br />Eletron . Practically this means that the energy of incident photon must be <br />Large compared with the eletron binding energy . Althougth the photon energy increase<br />Lead to decrease of the compton effect .<br />Because the compton interaction involve free electron in the absorbing material .<br />Its independent of atomic number z .<br />From the previous discussion , it follow that if the energy of the beam is in the<br />Region where the compton effect is only possible mode of interaction . Approximately<br />The same attenuation of the beam occur in any material of equal density thick –<br />Ness ( attenuation per g/cm for bone = is the same as for soft tissues<br />
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Pair production ;-<br />If the energy of the photon is greater than 1.02 mev , the photon may interact with<br />The matter throught mechanism of pair production . In these process the electron<br />Interacts strongly with the electromagnetic field of atomic nucleus and give of all<br />Of its energy in the process of creating apiary consisting of -veeletron and + ve<br />Eletron .<br />Because the rest mass energy of the electron = 0.51 mev . Minimum energy of 1.o2<br />Mev . The photon energy in excess of this threshold is shared between the particles<br />As kinetic energy .<br />The most probable distribution of energy for each particle to acquire half the available<br />Kinetic energy. Although any energy distribution is possible .<br />The pair production is event in which energy is converted into mass . As predicted by<br />Einstein s equation .—The reverse process , the conversion of mass in to<br />Energy take 2 place when positron combines with an electron <br /> E = M C<br />To produce two photon called the annihilation radiation .<br />Positron loss its energy as its traverses the matter by the same type of interaction<br />As electron does ( ionization ,excitation & bermsstrahlunge ) . At the end of the slowly <br />Moving positron combines with one electron in its vicinity to give rise to two<br />Annihilation photons each one has o.51 mev , the two photon ejected in opposite<br />direction <br />
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Because the pair production result from interaction with the electromagnetic <br />Field of the nucleus , the probability of this process increase rapidly with atomic<br />Number . <br />Relative important of various types of interacton ;-<br />The total mass attenuation coefficient m/p<br />Total m/p = photoelectrical/p + coherent + Compton + pair . Cohered scattering is <br />Only impotent for very low photon ( less than 10 kev ) & high Z material .<br />The attenuation coefficient decrees rapidly with energy until the photon energy<br />Far exceeds the electron and the Compton effect becomes the predominant<br />Mode of interaction . In the Compton range of energy ate- co- of the lead & water<br />Do not differ greatly , since this type of interaction is independent of atomic <br />Number . Co - however decrease with energy until pair production begins to <br />Become important at energy more than 1.02 mev .<br />Interaction of charged particles ;-<br />( electrons , protons , alpha particles and nuclei ) interact principally by ionization<br />And excitation .radiative collision in which the charged particles interact by <br />Coulomb force between the electric field of traveling particle & electeric field<br />Of orbital electron and nuclei of atoms . Collision between the particle and the<br />Atomic electron result in ionization & excitation <br />
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