Scientists eventually identified several distinct types of radiation, the particles resulting from radioactive decays. The three types of radiation were named after the first three letters of the Greek alphabet: (alpha), (beta), and (gamma).

Alpha particles are helium nuclei (2 p, 2 n):

Beta particles are speedy electrons:

Gamma radiation is a high-energy photon:

These three forms of radiation can be distinguished by a magnetic field since

• the positively-charged alpha particles curve in one direction,
• the negatively-charged beta particles curve in the opposite direction,
• and the electrically-neutral gamma radiation doesn't curve at all.

Alpha particles can be stopped by a sheet of paper, beta particles by aluminum, and gamma radiation by a block of lead. Gamma radiation can penetrate very far into a material, and so it is gamma radiation that poses the most danger when working with radioactive materials, although all types of radiation are very dangerous. Sadly, it took scientists many years to realize the perils of radioactivity...