Photoelectric versus Compton
The curve represents, in the case of copper, the probability of gamma interaction (or cross section) as a function of their energy. The stopping power of gamma is measured by this quantity. The figure also shows the contribution of various mechanisms of interaction, the main ones in the energy range of radioactivity gamma are, the photoelectric effect and Compton effect. For an absorber such as copper, the photoelectric effect is much weaker than for lead (the threshold of the K-shell is less than 10 keV and nuclear charge of 28 instead of 82). The Compton effect prevails from about 120 keV.