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Unformatted text preview: Lesson 08 Absorbed Dose in Radioactive Media MP200 Radiation Physics  2010 Duke Medical Physics Graduate Program 1 Introduction The computation of absorbed dose is straightforward for CPE and RE conditions. But it is more difficult for intermediate situations. If radiation emitted consists of CP and long range γ rays, One can determine if CPE or RE present depending on the size of the radioactive object. • Case 1 In a small radioactive object V ( mean radius not much greater than the maximum chargedparticle range d ), CPE is well approximated at any internal point P that is at least a distance d from the boundary of V . If d << 1 μ all photons escape from the object and D is equal to the energy per unit mass of the medium that is given to the charged particles • Case 2 In a large radioactive object ( mean radius r >> 1 μ ), RE is approx imated at any internal P , and the dose is equal to the sum of the energy per unit mass of the medium that is given to CP and γ rays. To determine the size of the object Deciding a maximum γ ray range for case 2, requires a quantitative criterion. mean free path = 1 μ Less than 1% of gamma ray penetrate through a layer five mean free paths in thick ness, 2 Less than 0 . 1% through 7 mean free paths. From the figure, ( 1 MeV,γ rays, broad place beam in water) at a depth of 7 mean free paths the true attenuation is closer to 10 2 than 10 3 . Roughly 10 mean free paths are necessary to reduce beam penetration to < . 1%. If the relevant data for buildup factor or effective attenuation coefficient ¯ μ is not available, the use of ¯ μ = μ en will overestimate the size of the object and the use of ¯ μ = μ will underestimate the size of the object, by ignoring scattered rays. Intermediate Size Object To estimate γray dose at any internal point in an intermediate sized object, we define a quantity called ”Absorbed Fraction(AF)” AF = γ ray radiant energy absorbed in target volume γ ray radiant energy emitted by source V is a radioactive object filled by homogeneous and uniformly distributed γ ray source. V is surrounded by: • Infinite homogeneous medium identical to that in V , but nonradioactive ( organ in the body) or, • An infinite vacuum ( an object surrounded by air) 3 Case 1: V is surrounded by another volume with homogeneous medium identical to that of V , but nonradioactive...
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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

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