正电子发射Positron emission

自由质子比中子轻，不能自发转变；但在原子核内，若子核的结合能足够大，使母原子与子原子的质量差大于两个电子质量（1.022 MeV），β⁺ 衰变就能发生；否则只能进行电子俘获。放出的正电子在物质中减速后与电子湮灭，产生两个方向相反、能量各为 511 keV 的 γ 光子。

A free proton is lighter than a neutron and cannot convert spontaneously, but inside a nucleus β⁺ decay can occur when the daughter is bound strongly enough that the parent atom outweighs the daughter atom by more than two electron masses (1.022 MeV); otherwise only electron capture is possible. The emitted positron slows in matter and annihilates with an electron, producing two 511 keV gamma photons travelling in opposite directions.

1928 年狄拉克的相对论电子方程预言了反电子。1932 年安德森在宇宙射线云室照片中发现正电子，获 1936 年诺贝尔物理学奖。1934 年约里奥-居里夫妇用 α 粒子轰击铝得到放射性磷-30，它通过放出正电子衰变，这是人工放射性的首次发现，二人获 1935 年诺贝尔化学奖。

Dirac's relativistic electron equation predicted the anti-electron in 1928. In 1932 Anderson found the positron in cloud-chamber photographs of cosmic rays and received the 1936 Nobel Prize in Physics. In 1934 the Joliot-Curies bombarded aluminium with alpha particles to make radioactive phosphorus-30, which decays by positron emission; this first discovery of artificial radioactivity earned them the 1935 Nobel Prize in Chemistry.

正电子发射断层成像（PET）利用湮灭产生的一对 511 keV 光子做符合探测，重建放射性药物在体内的三维分布。最常用的示踪剂是 ¹⁸F 标记的氟代脱氧葡萄糖（¹⁸F-FDG，半衰期 110 min），肿瘤细胞摄取葡萄糖多，因而显像明亮。其他正电子核素有 ¹¹C、¹³N、¹⁵O 和 ⁶⁸Ga，多由回旋加速器生产。

Positron emission tomography (PET) detects the paired 511 keV annihilation photons in coincidence and reconstructs the three-dimensional distribution of a radiopharmaceutical. The commonest tracer is ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG, half-life 110 min); tumours take up more glucose and show brightly. Other positron emitters include ¹¹C, ¹³N, ¹⁵O and ⁶⁸Ga, mostly made in cyclotrons.