Of course this is not really a fundamental answer, since I don't have any physical reason to offer as to why $A$ is only logarithmic in $x$ at high energies. So it seems that the high-energy behavior of the cross-section is mainly just kinematic, except for the rather gentle logarithmic dependence on energy that is present as well. The high-energy limit of the integrated Klein-Nishina formula is $\sigma\approx \pi r_0^2\ln x/x$, where $r_0$ is the classical electron radius, so $A\approx \pi/\ln x$. The only dimensionless Lorentz scalars we can form are $x=p_\gamma\cdot p_e/p_e\cdot p_e=E_\gamma/m$ and functions of $x$. In terms of relativistic transformation properties, we expect $\sigma=A/F$ to be invariant under longitudinal boosts, where $A$ is a Lorentz scalar and $F=mE_\gamma$ is the Moller flux factor. The probabilities are affected by competition from pair production. Note that the title of the question refers to probabilities, but the graph is of cross-sections. Fundamentally, in the Compton effect, a photon is elastically scattered by a charge which recoils due to conservation of energy and momentum whereas in the photoelectric effect, a photon is completely absorbed by a solid and an electron is ejected in the process.This is not a complete answer, but I think I partially understand why the cross-section falls off roughly like $1/E_\gamma$ at high energies. The photoelectric effect demonstrated the result when a photon gives up all its energy after interaction with an electron while the Compton effect showed that the consequence after a photon gives up some of its energy after interaction with an electron. It provided verification to quantum mechanics hypothesis that electromagnetic radiation came in discrete photons with energy proportional to frequency. The shift in wavelength could not be explained by classical theory, which stated that under the influence of the periodic electric field of an electromagnetic wave, an electron should oscillate with a frequency equal to that of the wave and consequently should radiate secondary (scattered) waves of the same frequency. He observed that X-rays scattered in paraffin have a longer wavelength than that of incident rays. In contrast, the Compton effect, was discovered by American physicist Arthur Holly Compton, in 1923. The discovery, upon further investigation, caused a major upset on the prevailing hypothesis that light is a wave and gave rise to the theory that light exists as a particle and wave. This verified that electromagnetic waves of wavelength shorter than those of visible light was irradiating the first gap and causing it to break down. He noticed that the gap between a pair of oppositely charged electrodes broke down, or sparked, quite readily when a second spark gap was fired in the immediate neighborhood. It was by accident that he discover it the phenomenon by accident while conducting some of his monumental studies on the electromagnetic wave. Another photon that is of lesser energy, then moves off at an angle to the direction of motion of the primary photon.ĭiscovery of the photoelectric effect phenomenon was by German physicist Heinrich Hertz in 1887. When a primary photon collides with an electron, some of the energy of the photon is transferred to the electron, which is then ejected from the atom. The Compton effect, on the other hand, is the scattering of photons from gamma or X rays by the electrons orbiting around the atoms. In an electric circuit consisting of a photocell, when electromagnetic radiation, such as ultraviolet rays is incident on a photocell, an electric current flows and the current is called photocurrent. The photoelectric effect refers to the emission of electrons when electromagnetic radiation is incident on a metal surface.
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