Interaction  of  ionizing
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  • 1. INTERACTION OF IONIZINGRADIATION
    DR/ OMAR HASHIM
  • 2. Interaction of radiation (x or gamma ray ) with the matter lead to ejection
    Electron , this high speed electron transfer their energy by producing
    Ionization &excitation
    Ionization ;- the process by which a neutral atom acquires +ve or -ve charge .
    Removal of an orbital electron (- ve ) leaving the atom + ve charge in ion
    Pair .
    Charge particles such as electron , proton , and alpha particle are known as
    Directly ionizing radiation ---- collision .
    When the energy produced lost by incident particle is not sufficient to eject
    Electron 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 ;--
    Photoelectric effect .
    Compton effect .
    Pair production
  • 3. Photon beam description ;-
    1 the fluence ( Q ) OF photon is quotient Dn by da where DN is the number
    Of photon that enter an imaginary sphere of cross sectional area
    Q = Dn/da .
    Fluency rate or density is the fluency per unit time
    Q = Do /dt .
    Energy fluency is the quotient of de where de is the sum of the energy
    of 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 distance
    From 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 of
    The absorber(DX) dn = -att N dx .
    The number of photon decrease as the absorber thickness increase
  • 4. If thickness is expressed as length then daltta called attenuation coefficient . If thick-
    Ness is measured in centimeters , the factors 1/cm
  • 5. H VL (half value layer ) ;-
    Denied as the thickness of an absorber required to attenuate the intensity
    Of attenuate the intensity of the beam to half its original value .
    -
  • 6. Interaction of photon with matter ;-
    A ) Photoelectric effect ;- is phenomena in which a photon interacts with an
    Atom 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 . Leaving
    The atom in excited state . The vacancy will be filled by outer orbital electron
    With the emission of characteristic x ray . These is the possibility of emission
    Of auger electron . Which is monoenergetic electrons produced by absorption
    Of characteristic x ray by the atom .
    Because the binding energy of the soft tissues are very low ( 0.5 kev ) the
    Energy of the characteristic photons produced in biologic absorbers is
    Very low .for higher energy photon and higher atomic number material . The
    Characteristic photon are of higher energy may deposit energy at large
    Distance . In such cases , the local energy absorption is reduced by the energy
    Emitted as characteristic radiation .
  • 7. The probability of photoelectric absorption depend on the photon energy . Where
    The mass photoelectric attenuation coefficient ( t/p ) is plotted as a function of
    Photon 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 depends
    Strongly on the atomic number of the absorbing material . This relationship from
    The basic many application in the diagnosis radiology .the differences in the Z OF
    The various tissue such as bone , muscles , & fat amplified diff3erence in x ray
    Absorption . The 1ry mode interaction is photoelectric . This Z dependence is also
    Exploited when using contrast media such as BaSO4 .
    In the therapeutic radiology , the low energy beams produced by superficial and
    Orthovoltage machines cause un necessary high absorption of x ray energy in bones
    Due to high Z
    3 3
    t/p pro-z / E
  • 8. COMPTON EFFECT ;-
    IN the compton process the photon interacts with an atomic electron as through
    It 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 electron
    The scattered photon will travel backward . The electron receive will
    Maximum 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 one
    Calculate E & hv .
    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 the
    Same energy as incident photon
  • 9. 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 most
    Of its energy to the Compton electron , the scattered photon has much less
    energy
  • 10. Energy and atomic affect in Compton ;-
    As mentioned before the Compton effect is interaction between photon and free
    Eletron . Practically this means that the energy of incident photon must be
    Large compared with the eletron binding energy . Althougth the photon energy increase
    Lead 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 the
    Region where the compton effect is only possible mode of interaction . Approximately
    The 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
  • 11. Pair production ;-
    If the energy of the photon is greater than 1.02 mev , the photon may interact with
    The matter throught mechanism of pair production . In these process the electron
    Interacts strongly with the electromagnetic field of atomic nucleus and give of all
    Of its energy in the process of creating apiary consisting of -veeletron and + ve
    Eletron .
    Because the rest mass energy of the electron = 0.51 mev . Minimum energy of 1.o2
    Mev . The photon energy in excess of this threshold is shared between the particles
    As kinetic energy .
    The most probable distribution of energy for each particle to acquire half the available
    Kinetic energy. Although any energy distribution is possible .
    The pair production is event in which energy is converted into mass . As predicted by
    Einstein s equation .—The reverse process , the conversion of mass in to
    Energy take 2 place when positron combines with an electron
    E = M C
    To produce two photon called the annihilation radiation .
    Positron loss its energy as its traverses the matter by the same type of interaction
    As electron does ( ionization ,excitation & bermsstrahlunge ) . At the end of the slowly
    Moving positron combines with one electron in its vicinity to give rise to two
    Annihilation photons each one has o.51 mev , the two photon ejected in opposite
    direction
  • 12. Because the pair production result from interaction with the electromagnetic
    Field of the nucleus , the probability of this process increase rapidly with atomic
    Number .
    Relative important of various types of interacton ;-
    The total mass attenuation coefficient m/p
    Total 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 energy
    Far exceeds the electron and the Compton effect becomes the predominant
    Mode of interaction . In the Compton range of energy ate- co- of the lead & water
    Do 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 ionization
    And excitation .radiative collision in which the charged particles interact by
    Coulomb force between the electric field of traveling particle & electeric field
    Of orbital electron and nuclei of atoms . Collision between the particle and the
    Atomic electron result in ionization & excitation