# Rayleigh Scattering

Rayleigh scattering, also known as coherent scattering, occurs when a photon interacts with an atom or molecule and is scattered slightly, with negligible change in energy. This interaction is not an ionizing interaction, but can compete with the ionizing interactions described below. The total Rayleigh scattering attenuation coefficient \sigma_R can be approximated using the atomic form factor F(q,Z) by integrating over its angular scattering attenuation coefficient:

\frac{\sigma_R}{d\Omega} = \frac{r_0^2}{2}(1+cos^2\theta)[F(q,Z)]^2

where r_0 is the classical electron radius, \theta is the scattering angle, q = 2ksin(\theta / 2), k = E / m_e c^2, E is the photon energy, and m_e is the electron rest mass (0.511 MeV/c2). As an approximation, the probability of Rayleigh scattering increases with atomic number and decreases with increasing photon energy:

\sigma_R \propto \frac{Z^2}{E^2}

Since the photon energy is unchanged, Rayleigh scattering does not deliver radiation dose to matter. Rayleigh scattering is noticeable for lower-energy photon beams, but is overshadowed by the ionizing effects at higher energies.