Alpha Particles
Alpha particles are produced by radioactive decay. Basically any form of radioactive decay occurs when an element or compound emitting a particle as it changes into a lighter element. When an alpha particle is emitted by an element it is emitting a hydrogen nucleus, or two neutrons and two protons. This kind of decay is called " decay and it is what we are measuring in this lab.
Actually it is more complicated than that. Every atom emits alpha particles at different levels of energy. This experiment measures the energies of the alpha particles that are detected by the equipment and from that measurement the experimenter is expected to determine the atom that is emitting the particles.
The picture on the lower axis below, contributed by Jim Wren, shows the number of alpha particles detected on the horizontal axis vs. the energy the alpha particles on the vertical axis. This sample is Thorium 228.