Photoelectric effect: the example of lead
In the case of lead atoms, the probability (cross section) of the photoelectric effect decreases by several orders of magnitude with the energy of gamma rays. The photoelectric stopping power of lead decreases by more than 2000 times in the energy range shown. This impressive decrease is however hampered by the crossing of interaction thresholds of the photon with electrons of the atom’s innermost layers L and K. For a lighter absorber such as copper, we would not see the K-shell threshold below 10 keV. The curve would be similar but shifted left and downwards.
IN2P3 (Source Encyclopaedia Britannica)