John Sealy Townsend became a research student at Cambridge
at the same time as Ernest Rutherford, and Hans Geiger studied
under Ernest Rutherford at the University of Manchester. Ernest
Rutherford distinguished alpha and beta particles. Alpha decay is
the emission of a particle containing two protons and two
neutrons; beta decay is the emisson of an electron or positron.
Gamma radiation is the emision of high-energy photons.
See also in The book of science:
Readings on wikipedia: