Lesson3_1 - Lesson 03 Quantities for Describing the...

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Lesson 03 Quantities for Describing the Interaction of Radiation with Matter MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1
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Quantities for describing the interaction of radia- tion with matter Three nonstochastic quantities that are useful to describe the interaction of radiation with matter are: Kerma ( K ) - First step in energy deposition by indirectly ionizing radiation Absorbed Dose ( D ) - Energy imparted to matter by ionizing radiation Exposure ( X ) - Describes X -rays or γ rays ability to ionize air Stochastic Quantities Radiant energy ( R ) ( ICRU 1980) Energy of the particle (excluding rest energy) emitted, received or transferred Radiative losses : conversion of charged particle kinetic energy to photon energy through either bremsstrahlung or in-flight annihilation of positrons In the latter case, the kinetic energy possessed by the positron at the instant of annihilation is classified as radiative energy loss. Energy transferred ( tr ) in a volume V tr = ( R in ) u - ( R out ) nonr u + Q ( R in ) u = Radiant energy of uncharged particle entering V ( R out ) nonr u = Radiant energy of uncharged particle leaving V , except that which originated from radiative losses of kinetic energy by charged particles while in V Q = Net energy derived from rest mass in V ( m E positive E m negative ) 2
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Kerma ( K ) Kerma stands for the k inetic e nergy r eleased per unit ma ss in the matter. This is a nonstochastic quantity only relevant to the fields of indirectly ionizing radiation. K = d tr dm tr = expectation value of the energy transferred in the finite volume V for a time t . d tr = sum of initial kinetic energies of secondary charge particles in a mass dm of the infinitesimal volume of dv . Kerma is the first step of dissipating energy by indirectly ionizing radiation. definition Kerma is the expectation value of the energy transferred to charged particles per unit mass at a point of interest, including radiative loss energy but excluding energy passed from one charged particle to another. Units of Kerma is J.kg - 1 or Gy . Exponential Attenuation Figure 1: Exponential Attenuation 3
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n = number of interactions in distance dx n dx and n N n = μNdx μ = linear attenuation coefficient μ = ( n N ) dx Units of μ are m - 1 or cm - 1 Definition of μ Probability of a photon interaction with matter by one process or another per unit distance traversed. μ depends on the density of material and the photon energy. Change in number of photons, dN = ( N - n ) - N = - n = - μNdx N N 0 dN N = L 0 - μ.dx N = N 0 e - μL N = number of photons passed through material without interaction. Mass Attenuation Coefficient μ divided by the density of the material is defined as the mass attenuation coefficient. Mass attenuation coefficient = ( μ ρ ) Units: cm 2 .g - 1 4
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Definition of ( μ ρ ) ( μ ρ ) = ( n N ) ρdx The probability of an interaction per unit thickness in g.cm - 2 of material tra- versed.
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