γ射线Gamma ray

原子核像原子一样有分立的能级，α 或 β 衰变后子核常处于激发态，退激时放出能量确定的 γ 光子，因此 γ 能谱是线状的，可用来鉴别核素（如 ¹³⁷Cs 的 662 keV、⁶⁰Co 的 1173 和 1332 keV）。γ 射线与物质主要通过光电效应、康普顿散射和电子对效应（能量 > 1.022 MeV）相互作用，强度随厚度呈指数衰减。

Nuclei have discrete energy levels like atoms; after alpha or beta decay the daughter is often excited and emits gamma photons of definite energy as it relaxes, so gamma spectra are line spectra that identify nuclides (662 keV for ¹³⁷Cs; 1173 and 1332 keV for ⁶⁰Co). Gamma rays interact with matter mainly through the photoelectric effect, Compton scattering and pair production (above 1.022 MeV), and their intensity falls exponentially with thickness.

1900 年法国化学家维拉德在研究镭的辐射时，发现一种不被磁场偏转、穿透力远强于 α、β 射线的成分。1903 年卢瑟福将其命名为 γ 射线。1914 年卢瑟福和安德拉德通过晶体衍射测定其波长，证明 γ 射线是电磁波。

In 1900 the French chemist Paul Villard, studying radium's radiation, found a component undeflected by a magnetic field and far more penetrating than alpha or beta rays. Rutherford named it gamma rays in 1903. In 1914 Rutherford and Andrade measured its wavelength by crystal diffraction, proving gamma rays are electromagnetic waves.

屏蔽 γ 射线需要铅、混凝土等高密度厚材料。应用包括 ⁶⁰Co 放射治疗和“伽马刀”、医疗器械和食品辐照灭菌、工业探伤、核医学单光子成像（⁹⁹ᵐTc）、γ 能谱分析以及穆斯堡尔谱。天文学中观测到的伽马射线暴是宇宙中最剧烈的爆发现象。

Shielding gamma rays needs thick, dense materials such as lead and concrete. Uses include ⁶⁰Co radiotherapy and the Gamma Knife, sterilisation of medical devices and food irradiation, industrial radiography, single-photon nuclear imaging with ⁹⁹ᵐTc, gamma spectrometry and Mössbauer spectroscopy. Gamma-ray bursts seen by astronomers are the most violent explosions in the universe.