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Lesson5_1 - Lesson 05 Charged Particle and Radiation...

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Unformatted text preview: Lesson 05 Charged Particle and Radiation Equilibria MP200 Radiation Physics - 2010 Duke Medical Physics Graduate Program 1 Introduction The concepts of radiation equilibrium (RE) and charged particle equilibrium (CPE) are useful as a means of relating certain basic quantities. • CPE- relates the absorbed dose to the collision kerma K c • RE- makes dose D equal to the net rest mass converted to energy per unit mass. Conditions to obtain Radiation Equilibrium (RE) Consider an extended volume V containing a distributed radioactive source . Con- sider a very small volume v in its center. Figure 1: Radiation equilibrium. Volume V, contains a homogeneous medium and homogeneous isotropic source distribution If d is the maximum range of any particle, the RE exits for v if, • The atomic composition of the medium is homogeneous • The density of the medium is homogeneous • The radioactive source is uniformly distributed • There are no electric or magnetic fields present to perturb the charged-particle paths, except the fields associated with the randomly oriented individual atoms. If we consider a plane T , that is a tangent to volume v at any point P and consider rays crossing the plane per unit area. The rays of each type and the energy crossing both ways are the same, because S contains a spherically symmetrical radioactivity distribution. 2 Definition of RE RE is the condition where for each type and energy of ray entering v , another iden- tical ray leaves. Absorbed Dose in a media at RE When the medium is in radiation equilibrium, ( ¯ R in ) u = ( ¯ R out ) u ( ¯ R in ) c = ( ¯ R out ) c The energy imparted, ¯ = ( ¯ R in ) u- ¯ R out ) u + ( ¯ R in ) c- ( ¯ R out ) c + X Q ¯ = X Q nonstochastic case, v-→ dv about point P, Absorbed dose, D = d dm where = ∑ Q If radiation equilibrium exists at a point in a medium, the absorbed dose is equal to the expectation value of the energy released by the radioactive material per unit mass at that point, ignoring neutrinos. Practical Importance of RE The concept of RE has practical importance in medicine and radio-biology, where distributed radioactive sources may be introduced in the human body or other bio- logical systems for diagnostic. The resulting absorbed dose at any given point in such circumstances depends on the size of the object relative to the radiation range and on the location of the point within the object. Isotropicity Required for Radiation Equilibrium? • Isotropicity is not a requirement for RE in volume v . The only requirement is that the flow of identical particles entering v is the same of particles leaving v . 3 • Any source anisotropy, or distortion of charged particle tracks, that is homoge- neously present everywhere in V will have no effect on RE in v Figure 2: Radiation equilibrium in homogeneous, but anisotropic radiation field Assume that radiation moves from left to right. Homogeneity and symmetry require particles A from dv to dv are identical to the particles...
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Lesson5_1 - Lesson 05 Charged Particle and Radiation...

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