Alpha particles or α particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as He²⁺ or ⁴₂He²⁺. They are a highly ionizing form of particle radiation, and have low penetration. Alpha radiation consists of helium-4 nucleus and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminum plate. Gamma radiation is eventually absorbed as it penetrates a dense material. Lead is good at absorbing gamma radiation, due to its density.

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