The classifications were originally made according to the penetrating power of the radiation. Alpha rays were found to be the nuclei of helium atoms, two protons and two neutrons bound together. Alpha rays have a net positive charge. Alpha particles have weak penetrating ability, a couple of inches of air or a few sheets of paper can effectively block them. Beta rays were found to be electrons, identical to the electrons found in atoms.
Beta rays have a net negative charge.
Beta rays have a greater penetrating power than Apha rays and can penetrate 3mm of aluminum. Gamma rays are high energy photons. This has the greatest penetrating power being able to pass through several centimeters of lead and still be detected on the other side. This Geiger Counter is sensitive to all three types of radioactivity.