β衰变Beta decay

在夸克层次上，β⁻ 衰变是中子中的一个下夸克通过发射 W⁻ 玻色子变为上夸克，W⁻ 再衰变为电子和电子反中微子。中子过多的核（位于稳定线左上方）倾向 β⁻ 衰变，质子过多的核倾向 β⁺ 衰变或电子俘获。释放的能量由电子和中微子随机分配，因此 β 粒子能谱是连续的。

At the quark level, beta-minus decay turns a down quark in a neutron into an up quark by emitting a W⁻ boson, which decays into an electron and an electron antineutrino. Neutron-rich nuclei tend to undergo β⁻ decay, and proton-rich ones β⁺ decay or electron capture. The energy released is shared randomly between electron and neutrino, so the beta spectrum is continuous.

1899 年卢瑟福命名 β 射线，后来证明它是高速电子。1914 年查德威克发现 β 粒子能谱是连续的，似乎违反能量守恒。1930 年泡利提出存在一种看不见的中性粒子带走了能量，1933 年费米据此建立了 β 衰变理论并命名为“中微子”。1956 年柯温和莱因斯在核反应堆旁首次探测到中微子。1956–1957 年李政道、杨振宁提出弱相互作用中宇称可能不守恒，吴健雄用钴-60 的 β 衰变实验证实。

Rutherford named beta rays in 1899, and they were later shown to be fast electrons. In 1914 Chadwick found the beta spectrum to be continuous, apparently violating energy conservation. In 1930 Pauli proposed an unseen neutral particle that carries off energy, and in 1933 Fermi built his theory of beta decay on it and named it the neutrino. Cowan and Reines detected neutrinos beside a reactor in 1956. In 1956–57 Lee and Yang proposed that parity might not be conserved in weak interactions, and Wu confirmed it with the beta decay of cobalt-60.

β 粒子穿透力比 α 强，可被几毫米铝板挡住。¹⁴C 的 β 衰变用于考古测年，³H、³²P、¹³¹I 用作示踪剂和放射治疗，⁹⁰Sr 用于同位素电池。核反应堆中裂变产物的 β 衰变产生的“衰变热”在停堆后仍需持续冷却，福岛事故正是因此发生。

Beta particles penetrate further than alpha particles and are stopped by a few millimetres of aluminium. Beta decay of ¹⁴C underpins radiocarbon dating; ³H, ³²P and ¹³¹I serve as tracers and in radiotherapy, and ⁹⁰Sr powers isotope batteries. Decay heat from beta-decaying fission products must be removed even after a reactor shuts down, which is what failed at Fukushima.