| Abstract from DBPedia | Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+ or 42He2+ indicating a helium ion with a +2 charge (missing its two electrons). Once the ion gains electrons from its environment, the alpha particle becomes a normal (electrically neutral) helium atom 42He. Alpha particles have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV, and a velocity in the vicinity of 4% of the speed of light. (See discussion below for the limits of these figures in alpha decay.) They are a highly ionizing form of particle radiation, and (when resulting from radioactive alpha decay) usually have low penetration depth (stopped by a few centimeters of air, or by the skin). However, so-called long range alpha particles from ternary fission are three times as energetic, and penetrate three times as far. The helium nuclei that form 10–12% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes, and thus may be highly penetrating and able to traverse the human body and also many meters of dense solid shielding, depending on their energy. To a lesser extent, this is also true of very high-energy helium nuclei produced by particle accelerators.アルファ粒子（アルファりゅうし、α粒子、英: alpha particle）は、高い運動エネルギーを持つヘリウム4の原子核である。陽子2個と中性子2個からなる。放射線の一種のアルファ線（アルファせん、α線、英: alpha ray）は、アルファ粒子の流れである（アルファ線、およびベータ線はラザフォードが発見）。 固有の粒子記号は持たず、ヘリウム4の2価陽イオンとして He2+（より厳密には 4He2+）と表される。 (Source: http://dbpedia.org/resource/Alpha_particle) |
