Lecture 9 - Health Physics Principles EEE 498 EEE 591...

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1/23/2012 Lecture 1 1 Health Physics Principles EEE 498, EEE 591 Lecture 9 Interaction of Radiation with Matter: Gamma Rays Spring 2012, ASU R. Metzger, [email protected], BYEN335 No Defined Range Gamma rays do not have any definite range in matter. Good geometry: A term used to describe a well-collimated, narrow beam of radiation. Lecture 9 2
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1/23/2012 Lecture 1 2 Exponential Absorption The attenuation of gamma rays follow an exponential curve: where I is the gamma ray intensity, t is the absorber thickness, and μ is the linear attenuation coefficient (slope of the absorption curve). Lecture 9 3 t e I I t I I 0 0 ln ln Attenuation Coefficients μ l is the linear attenuation coefficient and its unit is “cm -1 .” μ m (or sometimes μ/p ) is the mass attenuation coefficient and its unit is “cm 2 /g.” μ m is almost independent of the atomic number of the absorber in the Compton region. μ a is the atomic attenuation coefficient and its unit is “cm 2 .” It is the probability that an absorber atom will interact with one of the photons in the beam. Lecture 9 4 3 1 - 2 3 -1 2 atoms/cm cm cm g/cm cm /g cm N l a l m
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1/23/2012 Lecture 1 3 Cross Section The atomic attenuation coefficient is also called the microscopic cross section, σ . The units of σ are cm 2 and barn (b): 1 b = 10 -24 cm 2 . Macroscopic cross section, Σ : Σ is usually used for neutrons and we have: Lecture 9 5 3 2 1 - cm atoms atom cm cm N Example: Calculate the mass attenuation coefficient of UO 2 for 1 MeV γ -rays: The molecular weight of UO 2 is 238+(2x16)=270. 238/270=88.1% is U and the remaining is O. μ m for U is 0.0757 cm 2 /g and for O is 0.0636 cm 2 /g. Thus: μ m = 0.881x0.0757+0.119x0.0636=0.0743 cm 2 /g. The mean free pass = λ =1/ μ m =1/0.743=1.35 cm. For a mixture of materials, we have: where N n is the number of atoms per cm 3 of the n th element. Lecture 9 6 t Nt t m l e I e I e I I 0 0 0 n n n an a a l N N N 1 2 2 1 1 ...
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1/23/2012 Lecture 1 4 Mass Attenuation Coefficient Lecture 9 7 Mass Attenuation Coefficient We can find the mass attenuation coefficient for different elements here: http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html Lecture 9 8
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