Radiation Protection and Shielding
Lesson 18 - Photon response functions
When we turn our attention from neutron response functions to photon response functions, are job gets more complicated because of two facts:
First, let's deal with the notational difference. So far, our equation for the local response functions (by which we mean the response function for material that affects the flux) has been the one developed in Lesson 16:
where is the energy transferred to the material medium by secondary charged particles due to neutron reaction type j of isotope i.
We have already seen that photon reaction coefficients are historically expressed in terms of the linear attenuation coefficient:
Well, since the are dependent on the i and j summations, it is difficult to provide a direct analog of the first equation using the . How the traditional photon notation has treated this difficulty has been to define a family "linear deposition coefficients" with the general form of:
which results in the response function being defined as:
Several important points need to be made about the :
Matching of flux with deposition coefficient
If you look at the tables in Appendix C, you will not find a listing for . We have been using this symbol as a "stand-in" for linear coefficients that will allow us to approximate the dose rate -- when multiplied by the energy E of the incident photon and by the photon flux.
This last phrase "... and by the photon flux" is where the problem comes in.
The photon reaction mechanisms that we studied earlier -- photoelectric effect, Compton scattering, and pair production -- result in the release of a range of secondary photons. Some of them have relatively high energies -- especially the pair production photons at 0.511 MeV and the Compton photons, which usually have energies comparable to the incident photon. Others, however, have very low energies -- especially the photons from Bremsstrahlung and the fluorescent photons from the photoelectric reactions. The question is: Are each of these categories of photons included in the flux calculation or not?
You should become familiar with Figure 5.4 in the book, so that you can properly match up the energy deposition coefficient to use with the details of the flux calculation. In summary, you should use:
You should closely study the examples in section 5.7.4 of the text.
Return to Course Outline © 1998 by Ronald E. Pevey. All rights reserved.