Answer to Question #1632 Submitted to "Ask the Experts"

Q

What is the physical significance of mass attenuation coefficient, molar extinction coefficient, and effective atomic number? Will a good shielding material have high values of these parameters or low?

A

The mass attenuation coefficient is used to describe the total reduction of x and gamma radiation (photons) at a detector due to both energy absorption and scattering. The intensity of photons reaching a detector decreases as the mass attenuation coefficient increases for the same absorber thickness and photon energy. The mass attenuation coefficient tends to increase with increasing atomic number at the same photon energy, so materials with high atomic numbers (and, hence, high mass attenuation coefficients) are normally chosen to shield x and gamma radiation. Values of the mass attenuation coefficient as a function of photon energy are provided for a large number of elements and common shielding materials by the National Institute of Science and Technology (NIST) Web site. The molar extinction coefficient is analogous to the mass attenuation coefficient, but it is used to describe the absorption of light in a translucent medium. Light intensity reaching a detector decreases as the molar extinction coefficient increases for the same thickness of absorber and at a particular wavelength of light, so materials with high molar extinction coefficients are most effective at reducing light intensity. The molar extinction coefficient is not related to the effective atomic number, but to the orbital electronic structure of the atoms or molecules used as an absorber. Total light attenuation for a given absorber thickness at a particular wavelength depends on the molar concentration of absorber (the number of molecules per unit volume)—hence the name, molar extinction coefficient. The molar extinction coefficient is an important intrinsic property and is extensively used in quantitative chemical analysis. More information about the molar extinction coefficient (also called molar absorptivity) and its use can be found in most basic chemistry textbooks and by searching the Internet for Beer's Law or the Beer-Lambert Law. One such Web site is provided by Sheffield Hallam University. James S. Bogard, PhD, CHP Oak Ridge National Laboratory