Radiation Protection and Shielding
Lesson 7: Interaction coefficients
In this lesson, we take concepts that you have already familiar from your previous study of neutron cross sections and relate them to the slightly different notation that is predominant in describing photon interactions. For this course, we need to be familiar with both nomenclatures. In addition, this lesson describes the scattering interaction coefficients in terms of distributions in energy and direction for the particle after the scattering event.
Interaction coefficient = Macroscopic cross-section
In previous courses, you have learned the concept of the macroscopic cross section, , for a material as the probability of interaction per unit path, with units of . For photons, the traditional symbol for this is ; same idea, same unit.
Other variations that your are used to carry over to the new notation:
Use of mass interaction and attenuation coefficients
One other convention that we will have to get used to is that the photon interaction coefficients themselves are not usually tabulated (i.e., presented in data tables or problem descriptions) as the values we have discussed, but instead as this value divided by the material density, , which has units of and is referred to as the mass interaction (or attenuation) coefficients. (i.e.,The word "linear" is replaced with the word "mass".)
This has been found to be useful for a number of reasons:
where I = number of isotopes
= number density of isotope i (nuclei/barn/cm)
= microscopic total cross section of isotope i (barns)
No such juggling of units is needed if we stay on a per mass basis, since:
where =mass fraction of isotope i in the material
Return to Course Outline İRonald E. Pevey. All rights reserved.