1.5 COMPTON EFFECT

 

1.5.1 Background and actual interpretation

 

            According to theory of electromagnetic interaction with charged particle, developed by Thompson, an incident radiation of frequency f0 should accelerate an electron in the direction of propagation of the incident radiation, and the electron should undergo forced oscillations and re-radiation at frequency f, where f < f0. The frequency of the scattered radiation should depend upon the length of time of electron exposure to the incident radiation as well as the intensity of the incident radiation.

            Compton experiment demonstrates contrary, more precisely, wavelength shift of x-rays scattered at a given angle is independent of both the intensity of the incident radiation and the length of exposure to the incident radiation, and depends only upon the scattering angle.

In original experiment, Compton bounced x-rays into a graphite target using three different scattering angles; 45º, 90ºand 135º. The interpretation of Compton experiment is based in actual quantum mechanic on the corpuscular aspect of electromagnetic radiation.

According to this photons are massles particle with following energy and momentum:

;  (1.22)

            If we allow a beam of x-rays to strike a target, some of the photons in the beam will interact, according to quantum mechanics, with free electrons in the material. When the incoming photon gives part of its energy to the electron, then the scattered photon is measured to have a lower energy than the original photon. Those photons which, after scattering, come out at an angle θ relative to the incident beam direction “add up” to form the scattered beam of x-rays observed at that angle.

 Without presenting the entire mathematical demonstration, available in any book about quantum mechanic, the wavelength of scattered beam is given by :

 (1.23)

            1.5.2  Proposed  interpretation

 

            In book about Corpuscular nature of light we will show that photons are mass particle, which obey to classical mechanic low, and consequently we have the classical energy and momentum formula:

 (1.24)

(1.25)

We describe the Compton effect in terms of collisions between individual electrons and individual photons. General rules of kinematics apply to such collisions, and can be used to determine the properties of the scattered photons. The Compton effect shows that, in these collisions, photons act precisely like particles.

We simply consider a photon as a particle with mass  τ momentum p, and energy ε, and proceed to work out the kinematics of the collision between such a particle and an electron.

The incoming photon is incident on the electron considered at rest and after collision the outgoing photon is scattered under the angle q, relative to the initial direction of the incident photon, and the electron is scattered under the angle j. The electron is considered initially at rest, so its initial momentum and energy are zero.

Initial energy of photons is:  (1.26) respectively after scattering:   (1.27) where τ is the mass of photon. The moment of electron after scattering is mv.

Figure 1.9 Photon scattering in Compton effect

The equation for conservation of momentum in the collision:

 (1.28)

on axes:

 (1.29)

 (1.30)

Making some tricks:

 (1.31)

 (1.32)

            With following simplifications:

 (1.33)

 (1.34)

 (1.35)

Writing the equation for conservation of energy in the collision:

 (1.36)

 (1.37)

 (1.38) where

 (1.39)

 (1.40)

 (1.41)

 (1.42)

 (1.43)

            The analysis of equation 1.43 and 1.23 indicate the same dependency  of photon energy related to the angle and mass of electron.

They differ regarding to a constant: mass of photon in proposed explanation and Planck constant in case of quantum mechanics.

            According to quantum theory, the effect should be the same in case of photon from IR, VIS, UV, X-ray.

            The proposed relation is photon mass dependent, and this fact is observed in experiments. In the same conditions the difference of energy is smaller for a photon from visible domain comparatively with an X-ray or g-ray photon. With these formula is it possible, like in actual interpretation to determine the mass of electron, but also it is possible to estimate the mass of photon.

 

1.5.3 Inverse Compton effect

 

            In astrophysics inverse Compton scattering is actually more important than Compton scattering. Inverse Compton scattering, takes place when the electron is moving, and has sufficient kinetic energy (E1) compared to the photon (e1). In this case net energy may be transferred from the electron to the photon (E1>E2, e1<e2).

Figure 1.10 Inverse Compton scattering

 

            The study of inverse Compton scattering follow the same classical rules up presented with a specification: there is no possible to have scattering with electromagnetic wave. (radio, TV and microwave domain of energy).