In 1914 Rutherford and Andrade managed to diffract
gamma-rays by a crystal, showing them to be electromagnetic in nature with a wavelength of
about 10-13 m. Gamma-rays are the result of an initial emission of a alpha-particle, a beta-particle or a neutron from a nucleus, leaving the nucleus in an excited state. The nucleus
then loses energy, just as an excited atom does, but the nucleus does not radiate visible
light but gamma radiation.
The absorption of gamma radiation in a material is given by the equation:
ln I = Io - mt
where Io is the incident intensity, I is the intensity at a distance t in the material and mlinear absorption coefficient. For a gamma ray of energy 1 MeV the linear absorption coefficient is about 0.8 cm-1. (This rises to almost 60 for 100 keV gamma radiation). One cm of lead will reduce the intensity of gamma rays by 50%.
Sources of gamma radiation show a line spectrum
similar to that obtained for X-rays; for example, cobalt-60 gives two gamma rays of energies
1.17 and 1.33 MeV, while caesium 137 shows one line at 660 keV.
