PHYS 3330 Module 6.1

Photon Attenuation

Learning Objectives

By the end of this section, you will be able to:

• Describe the exponential behavior of photon attenuation and connect it to the principles learned in 4: Exponential Behaviors.
• Calculate and interpret the linear and mass attenuation coefficients for different materials.
• Understand and calculate the mean free path and half-value layer for X-rays and gamma rays in various substances.

Exponential Attenuation of Photons

Half-Value Layer: HVL

NIST Standard Reference Database 126: X-Ray Mass Attenuation Coefficients

Tables and graphs of the photon mass attenuation coefficient μ/ρ and the mass energy-absorption coefficient μ_en/ρ are presented for all of the elements Z = 1 to 92, and for 48 compounds and mixtures of radiological interest. The tables cover energies of the photon (x-ray, gamma ray, bremsstrahlung) from 1 keV to 20 MeV. The μ/ρ values are taken from the current photon interaction database at the National Institute of Standards and Technology, and the μ_en/ρ values are based on the new calculations by Seltzer described in Radiation Research 136, 147 (1993). These tables of μ/ρ and μ_en/ρ replace and extend the tables given by Hubbell in the International Journal of Applied Radiation and Isotopes 33, 1269 (1982).

Effective Attenuation Coefficient