| MadSci Network: Physics |
The effectiveness of a material, such as lead, as a gamma ray shield is dependent on the electron density of that material. Gamma rays will interact by one of three general mechanisms: photoelectric effect, compton effect, or pair production. In the photoelectric effect the gamma ray photon transfers all of its energy to a bound electron, which is ejected from the atom. In the compton effect, which is predominant at moderate energies, the photon transfers a portion of its energy to an electron. Finally, in pair production, which is only dominant at very high energies, the photon is interacting with the Coulomb field of the nucleus.
Just to be complete gamma ray photons may also interact through photonuclear reactions, or Rayleigh scattering. However, from a shielding perspective these are minor players.
Lead has a high atomic number (Z = 82) and therefore also has a high electron density as compared to many other materials. The impact of this electron density on the probabilty of the three interactions briefly described above is the reason for its effectiveness as a shielding material.
If you are interested in learning the gory details of these interactions and thumbrules for estimating their probability, I highly recommend the text "Introduction to Radiological Physics and Radiation Dosimetry" by Attix.
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