Consider Fig. 5.7. Explain why the three response functions diverge at lower energies. Explain why, for 20-keV photons, the response function for tissue dose in air exceeds those for the effective dose and the deep dose index. Solution The low energy photons in a medium is suppressed because of interactions between the produced photon and the electrons in the medium. This suppression occurs because the emission takes place over on a long distance scale, allowing for destructive interference between different instantaneous photon emission amplitudes. 20 Kev photons, the response function for tissue dose in air exceeds those for the effective dose and deep dose index is because of longitudinal density effect..