Particulate radiation comes from subatomic particles carousing about. An atom with too many or too few neutrons makes for an unstable nucleus. The nucleus becomes radioactive. A disintegrating atom produces particulate radiation, including alpha and beta particles.
When radium, uranium, and polonium decay, they effervesce alpha particles: 2 protons and 2 neutrons. Alpha particles are relatively chunky and slow. Protons and neutrons that can travel only short distances.
Alpha particles can be stopped by a piece of paper, or skin. Inhaling or ingesting alpha particles is dangerous, in exposing sensitive internal tissues to radiation.
Beta particles are hightailing electrons or positrons. Beta decay may be β−: where a neutron converts into a proton, an electron, and an electron-type antineutrino, or β+: where a proton transforms into a neutron, a positron, and an electron-type neutrino.
Lower-energy beta particles can be slowed, or even stopped, by clothing or aluminum foil. Higher-energy beta decay can reach speeds that are ultrarelativistic: approaching the speed of light.
When passing through matter, particulate radiation ionizes whatever it encounters, incrementally losing energy in the process.
The distance to where a charged particle is spent is called the range of the particle. Range depends on particle type, initial energy, and what matter is in the way.
Similarly, energy loss per path length, stopping power, depends on the particle’s type and energy, and the material bombarded. Stopping power, which equates to density of ionization, increases until it reaches an apex, the Bragg peak, just before the energy drops to zero. English physicist William Henry Bragg discovered the elemental dynamics of ionizing radiation in 1903.