电子俘获Electron capture

电子俘获与 β⁺ 衰变的结果相同，但不需要满足 1.022 MeV 的能量阈值，因此在衰变能较小的质子富余核中是唯一可能的方式。K 层电子最靠近原子核，被俘获的概率最大。内层出现空位后，外层电子跃迁填补，放出子原子的特征 X 射线或俄歇电子。由于涉及核外电子，电子俘获的速率会因化学环境略有变化（如 ⁷Be 在不同化合物中差约 0.1%），这是“衰变不受外界影响”的少数例外。

Electron capture gives the same result as β⁺ decay but needs no 1.022 MeV threshold, so for proton-rich nuclei with small decay energies it is the only option. K electrons, closest to the nucleus, are most likely to be captured. Outer electrons then fill the vacancy, emitting the daughter's characteristic X-rays or Auger electrons. Because it involves atomic electrons, the rate changes slightly with chemical environment (about 0.1% for ⁷Be in different compounds), a rare exception to the rule that decay is unaffected by external conditions.

1935 年汤川秀树和坂田昌一从理论上预言了电子俘获。1937 年阿尔瓦雷斯在钒-48 衰变中观测到子体钛的特征 X 射线，首次实验证实了这一过程。

Hideki Yukawa and Shoichi Sakata predicted electron capture theoretically in 1935. In 1937 Luis Alvarez observed characteristic titanium X-rays from the decay of vanadium-48, the first experimental confirmation.

⁴⁰K 约 11% 通过电子俘获衰变为 ⁴⁰Ar，这是钾-氩测年的基础。¹²⁵I（电子俘获）用于放射免疫分析和近距离放疗；⁵⁵Fe 是常用的 X 射线校准源；¹¹¹In、²⁰¹Tl、⁶⁷Ga 用于单光子核医学显像。俄歇电子射程极短，正在研究用于细胞核靶向治疗。

About 11% of ⁴⁰K decays by electron capture to ⁴⁰Ar, the basis of potassium–argon dating. ¹²⁵I (an electron-capture nuclide) is used in radioimmunoassay and brachytherapy, ⁵⁵Fe is a standard X-ray calibration source, and ¹¹¹In, ²⁰¹Tl and ⁶⁷Ga are used for single-photon nuclear imaging. The very short range of Auger electrons is being explored for therapy targeted at cell nuclei.