This preview shows pages 1–5. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentThis preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
Unformatted text preview: Lesson 03 Quantities for Describing the Interaction of Radiation with Matter MP200 Radiation Physics  2010 Duke Medical Physics Graduate Program 1 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 Xrays 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 inflight 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 + X 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 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 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 Z N N dN N = Z L μ.dx N = N 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....
View
Full
Document
This note was uploaded on 10/30/2010 for the course MP 200 taught by Professor Guna during the Fall '10 term at Duke.
 Fall '10
 Guna

Click to edit the document details