What is the reason foils are not used for photon beams in the same manner as for electron beams?

The use of foils with photon beams differs significantly from their use with electron beams primarily due to the fundamental differences in how photons interact with matter compared to electrons.

Photons, which are high-energy particles, interact with matter primarily through processes such as Compton scattering and pair production, depending on the energy of the photons. Unlike electrons, which are charged particles that can easily lose energy through ionization and collisions as they traverse materials, photons maintain their energy when passing through materials to a greater extent, especially when considering materials of low atomic number (such as those used in foils).

In contrast, electrons lose energy rapidly as they interact with atoms in the material, resulting in a significant reduction in their energy over shorter distances. This rapid energy loss enables the use of foils as a tool to customize the energy of electron beams. On the other hand, since photons do not lose energy in the same direct way as electrons, foils would not serve the same purpose in controlling the energy of photon beams as they do for electrons.

Therefore, the characteristics of photon interaction with materials explain why foils are not employed for photon beams in the same way as for electron beams.