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.
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.
