Emitted by many high-atomic-number natural and man-made radioactive elements such as thorium, uranium and plutonium, alpha decay does not lead directly to stable nu clei. Intermediate isotopes are first produced, then these undergo further decay. The relatively high mass of the alpha particle means that for a given energy, the velocity is relatively low. The heavy, slow-moving, highly charged particles are completely absorbed by a few centimeters of air. After absorption, they are released to the atmosphere as harmless helium gas.
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