Interaction of Photons with Matter

The energies of photons in the deep-red region and in the blue region of the visible light are 1.5 and 3.0 eV respectively. The UV radiation covers a wide energy range from a few hundreds eV. Energy of X- and gamma-ray photons range from keV to MeV. The energy E of a photon with frequency n and wavelength l is E = h n
    = h c / l
where c is the speed of light.

Interaction of X- and Gamma-ray Photons with Matter

The X-ray and gamma-ray photons lose their energy in a stopping medium mainly by these major processes: photoelectric effect, Compton effect, and pair production. Photoelectric and Compton effects produce ion pairs, and pair production produces a pair of electron and positron from a photon.

When all the energy of a photon is used to release liberate the electron from a molecule, the process is called photoelectric effect. The kinetic energy of the electron equals to the energy of the photon minus the binding energy of the electron to the atom. These photoelectrons can be very energetic, and act like beta particles. Photoelectric effect is the major means of interaction for low-energy photons, including those in the UV region.

The Compton effect is equivalent to inelastic collision of photon with electrons. Part of the photon energy is lost to the electron, and a less-energetic photon bounce off.

At the vicinity of an atom, a photon with energy greater than 1.02 MeV creates a positron-electron pair, and such a process is called pair production. Pair production also occurs in the field of an atomic electron, especially for photons with energy of more than 2.04 MeV. Pair production is not exactly the reverse of annihilation, because the former involves only one photon, and two photons are emitted in annihilation.
The major mechanisms of interaction are depicted in the diagram here.

Attenuation of Gamma Rays

Gamma-ray intensity I decreases exponentialy from its initial intensity, Io I = Io e- m x where m is the absorption eoefficient of the medium. Its values are high if the medium consists of heavy elements. The x is the thickness.
The Gamma-ray Spectrometry Center has a catalogue of many gamma ray spectra available in pdf format.

©cchieh@uwaterloo.ca