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Unformatted text preview: 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 wellcollimated, narrow beam of radiation. Lecture 9 2 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 ln ln Attenuation Coefficients • μ l is the linear attenuation coefficient and its unit is “cm1 .” • 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 31 2 atoms/cm cm cm g/cm cm /g cm N l a l m 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 = 1024 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 γrays: 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 n n n an a a l N N N 1 2 2 1 1 ......
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This note was uploaded on 02/11/2012 for the course EEE 498 taught by Professor Barnaby during the Spring '08 term at ASU.
 Spring '08
 Barnaby

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