Answer to Question #10010 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Is the concept of stopping power defined exclusively for charged particles that are directly ionizing while the analogous concept for indirectly ionizing photons is attenuation coefficient?
However, I always come across other people saying "stopping power for photon beam . . ."—for example, in P. Andreo and A. Brahme's "Stopping Power Data for High-Energy Photon Beams" (Physics in Medicine and Biology, v 31, 8, p. 839; 1986).
You are correct in your inference that the quantity stopping power is limited to charged particles. The commonly used mass stopping power represents the energy lost by the charged particle per unit mass density thickness travelled and has common units of MeV per g cm-2.
Since stopping power refers to energy lost per unit thickness travelled, and photon attenuation coefficient refers to fractional number of photons lost per unit thickness travelled, the quantities are not directly analogous, although they may show generally similar trends with energy. The physical quantity for photons that would be more comparable, in what it represents, to the stopping power for electrons would be the product of the photon energy and the photon energy transfer coefficient since, under charged particle equilibrium conditions, this represents the energy transferred from photons to charged particles per unit thickness travelled.
When the term "stopping power" is used in reference to photons, as seems to be the case for the example you give, it is not really being used for the photons themselves, but for the electrons set free by the photon interactions. Thus, in the paper by Andreo and Brahme, what they are actually reporting are results of Monte Carlo simulations used to determine the energy distributions of electrons (which may also include positrons and secondary electrons down to an acceptably low cutoff energy) generated by photon interactions. The stopping powers evaluated are for these distributions of electrons.
The use of the "stopping power" term in reference to photons is technically incorrect and seems to be grounded in historical usage, especially in the medical therapy community, although I have not thoroughly researched this association.
I hope this clarifies the issue for you.
George Chabot, PhD