中子为什么相对质量都大于1（核言碎语279）

194 外源中子和外部中子源

Source Neutrons and External Source of Neutrons

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外源中子和外部中子源

②(α,n) reactions. In certain light isotopes, the ‘last’ neutron in the nucleus is weakly bound and is released when the compound nucleus formed following α-particle bombardment decays. For example, the bombardment of beryllium by α-particles leads to the production of neutrons by the following exothermic reaction: 4He 9Be→12C n 5.7 MeV. This reaction yields a weak source of neutrons with an energy spectrum resembling that from a fission source. Another very important source is (α,n) reaction with oxygen-18. This element is present in the uranium oxide fuel. The neutron-producing reaction is: 18O(α,n)21Ne. Alpha particles are commonly emitted by all of the heavy radioactive nuclei occurring in fuel (uranium), as well as the transuranic elements (neptunium, plutonium, or americium).

②（α，n）反应。在某些轻同位素中，原子核中的“最后”中子是弱束缚的，当α粒子轰击衰变厚形成复合核时释放出来。例如，α粒子轰击铍通过以下放热反应产生中子：⁴He ⁹Be→¹²C n 5.7 MeV。这个反应产生一个弱中子源，其能谱与裂变源的能谱相似。另一个非常重要的来源是（α，n）与氧-18的反应。这种元素存在于铀氧化物燃料中。中子产生反应为：18O（α，n）21Ne。α粒子通常由燃料（铀）中的所有重放射性核以及超铀元素（镎、钚或镅）发射。

③(γ,n) reactions – photoneutrons. Neutrons can also be produced in (γ, n) reactions, and therefore they are usually referred to as photoneutrons. A high-energy photon (gamma-ray) can, under certain conditions, eject a neutron from a nucleus. It occurs when its energy exceeds the binding energy of the neutron in the nucleus. The lowest threshold has 9Be with 1.666 MeV and 2D with 2.226 MeV. Therefore photoneutrons are very important, especially in nuclear reactors with D2O moderator (CANDU reactors) or Be reflectors (some experimental reactors).

③（γ，n）反应–光中子。中子也可以在（γ，n）反应中产生，因此它们通常被称为光中子。在某些条件下，高能光子（伽马射线）可以从原子核中弹射出中子。当它的能量超过原子核内中子的结合能时就会发生。最低阈值为9Be（1.666 MeV）和2D（2.226 MeV）。因此，光中子非常重要，尤其是在带有D2O慢化剂（CANDU反应堆）或Be反射体（一些实验反应堆）的核反应堆中。

（待续）