Editing Radiation (section)

=== Alpha radiation ===
{{Main|Alpha decay}}
[[File:Alpha particle detected in an isopropanol cloud chamber.jpg|thumb|[[Alpha particle]] detected in an [[isopropanol]] [[cloud chamber]]]]
Alpha particles are [[helium-4]] [[Atomic nucleus|nuclei]] (two protons and two neutrons). They interact with matter strongly due to their charges and combined mass, and at their usual velocities only penetrate a few centimetres of air, or a few millimetres of low density material (such as the thin mica material which is specially placed in some Geiger counter tubes to allow alpha particles in). This means that alpha particles from ordinary [[alpha decay]] do not penetrate the outer layers of dead skin cells and cause no damage to the live tissues below. Some very high energy alpha particles compose about 10% of [[cosmic ray]]s, and these are capable of penetrating the body and even thin metal plates. However, they are of danger only to astronauts, since they are deflected by the Earth's magnetic field and then stopped by its atmosphere.

Alpha radiation is dangerous when alpha-emitting [[radionuclide|radioisotopes]] are inhaled or ingested (breathed or swallowed). This brings the radioisotope close enough to sensitive live tissue for the alpha radiation to damage cells. Per unit of energy, alpha particles are at least 20 times more effective at cell-damage than gamma rays and X-rays. See [[relative biological effectiveness]] for a discussion of this. Examples of highly poisonous alpha-emitters are all isotopes of [[radium]], [[radon]], and [[polonium]], due to the amount of decay that occur in these short half-life materials.