α衰变Alpha decay

重核中库仑斥力很大，放出结合紧密的 ⁴He 核能释放能量（衰变能 Q 通常为 4–9 MeV）。按经典力学，α 粒子能量低于核表面的库仑势垒，不应逃出；1928 年伽莫夫以及格尼与康登用量子隧穿效应解释了 α 衰变，这是量子力学在核物理中的首次成功应用。盖革-努塔尔定律指出，衰变能越大，半衰期越短，二者呈对数关系。

In heavy nuclei Coulomb repulsion is large, and emitting the tightly bound ⁴He nucleus releases energy (decay energies Q are typically 4–9 MeV). Classically the alpha particle lacks the energy to cross the Coulomb barrier at the nuclear surface; in 1928 Gamow, and independently Gurney and Condon, explained alpha decay by quantum tunnelling, the first success of quantum mechanics in nuclear physics. The Geiger–Nuttall law relates higher decay energy to exponentially shorter half-life.

1899 年卢瑟福发现铀的射线中有一种穿透力很弱、能被几张纸挡住的成分，命名为 α 射线。1909 年卢瑟福和罗伊兹把 α 粒子收集在真空玻璃管中，通过光谱证明其就是氦。同年盖革与马斯登的 α 粒子散射实验导致了 1911 年原子核模型的提出。

In 1899 Rutherford found that uranium's radiation included a weakly penetrating component, stopped by a few sheets of paper, which he named alpha rays. In 1909 Rutherford and Royds collected alpha particles in an evacuated glass tube and showed spectroscopically that they were helium. Geiger and Marsden's alpha-scattering experiment that year led to the nuclear model of 1911.

α 粒子电离能力强、射程短（空气中几厘米，不能穿透皮肤），外照射危害小，但吸入或摄入后内照射危害极大，如 ²¹⁰Po 和氡的子体。应用包括烟雾探测器（²⁴¹Am）、航天器放射性同位素热电源（²³⁸Pu）和靶向 α 放射治疗（²²³Ra、²²⁵Ac）。地壳中的氦主要来自铀、钍的 α 衰变。

Alpha particles ionise strongly but travel only a few centimetres in air and cannot penetrate skin, so external exposure is of little concern, but inhaled or ingested emitters such as ²¹⁰Po and radon progeny are very hazardous. Uses include smoke detectors (²⁴¹Am), radioisotope thermoelectric generators for spacecraft (²³⁸Pu) and targeted alpha therapy (²²³Ra, ²²⁵Ac). Most of the helium in the Earth's crust comes from alpha decay of uranium and thorium.