Why Probability of Photoelectric and Campton decrease as photon energy increases??
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The rate of interaction of a photon with normal matter depends on a matrix element, which derives from the integral of the product of the electron and photon wavefunctions. The orbitals of electrons in atoms are on the scale of the Bohr radius, 529 nm. For any wavelength of light shorter than that, one can expect that the oscillatory light wavefunction will alternate from positive to negative more rapidly than the electron wavefunction does, and that means the matrix element sums to... zero.
Thus, a downward trend is expected with increasing energy; photon energy is inversely proportional to photon wavelength.
The outer electrons (largest volume) are thus out of the picture at modest photon energies (a few dozen eV), and even inner electrons have little effect on gamma rays (tens of thousands of eV).
Other interactions, as with nuclei, dominate at the highest photon energies.