This preview shows pages 1–4. 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: 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, rlmetzger@asu.edu, 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 is the linear attenuation coefficient (slope of the absorption curve). Lecture 9 3 t e I I t I I ln ln Attenuation Coefficients l is the linear attenuation coefficient and its unit is cm1 . m (or sometimes /p ) is the mass attenuation coefficient and its unit is cm 2 /g. m is almost independent of the atomic number of the absorber in the Compton region. 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, . The units of are cm 2 and barn (b): 1 b = 1024 cm 2 . Macroscopic cross section, : 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 rays: The molecular weight of UO 2 is 238+(2x16)=270. 238/270=88.1% is U and the remaining is O. m for U is 0.0757 cm 2 /g and for O is 0.0636 cm 2 /g. Thus: m = 0.881x0.0757+0.119x0.0636=0.0743 cm 2 /g. The mean free pass = =1/ 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 ......
View Full
Document
 Spring '08
 Barnaby

Click to edit the document details