放射性Radioactive decay

单个原子核何时衰变是随机的，但大量核的衰变服从一级动力学：N = N₀e^(−λt)，λ 为衰变常数。半衰期 t₁/₂ = ln2/λ，从 ²¹²Po 的 0.3 μs 到 ²³⁸U 的 44.7 亿年不等。放射性活度 A = λN，SI 单位为贝可勒尔（Bq），1 Bq 表示每秒衰变 1 次。

When an individual nucleus decays is random, but large numbers follow first-order kinetics: N = N₀e^(−λt), with decay constant λ. The half-life t₁/₂ = ln2/λ ranges from 0.3 μs for ²¹²Po to 4.47 billion years for ²³⁸U. Activity A = λN is measured in becquerels (Bq), one decay per second.

1896 年贝可勒尔发现铀盐能使包在黑纸中的照相底片感光。1898 年居里夫妇发现钋和镭，并创造 radioactivity（放射性）一词。1899 年起卢瑟福区分出 α 射线和 β 射线，1900 年维拉德发现 γ 射线。1902 年卢瑟福和索迪提出放射性是元素的嬗变。贝可勒尔与居里夫妇共获 1903 年诺贝尔物理学奖。

In 1896 Becquerel found that uranium salts fogged photographic plates wrapped in black paper. In 1898 the Curies discovered polonium and radium and coined the word radioactivity. From 1899 Rutherford distinguished alpha and beta rays, and Villard discovered gamma rays in 1900. In 1902 Rutherford and Soddy explained radioactivity as the transmutation of elements. Becquerel and the Curies shared the 1903 Nobel Prize in Physics.

主要衰变方式有 α 衰变（重核放出 ⁴He）、β⁻ 衰变（中子过多的核）、β⁺ 衰变和电子俘获（质子过多的核）、γ 跃迁（激发态退激）以及自发裂变。核素的稳定性取决于中子数与质子数之比，稳定核素分布在“稳定岛”附近；Z > 83 的元素都没有稳定同位素。

The main decay modes are alpha decay (heavy nuclei emitting ⁴He), beta-minus decay (neutron-rich nuclei), beta-plus decay and electron capture (proton-rich nuclei), gamma transitions (de-excitation) and spontaneous fission. Stability depends on the neutron-to-proton ratio, and stable nuclides lie along the valley of stability; no element beyond Z = 83 has a stable isotope.