中子资料

▪快中子能量高于1电子伏特、0.1兆电子伏特或者接近1兆电子伏特，有不同的定义。

▪慢中子能量小于等于0.4电子伏特。

▪超热中子能量在1电子伏特至10电子伏特之间。

▪高热中子能量约0.2电子伏特。

▪热中子能量约0.025电子伏特。

▪冷中子能量约5x10−5电子伏特至0.025电子伏特。

▪甚冷中子能量约3x10−7电子伏特至5x10−5电子伏特。

▪极冷中子能量小于3x10−7电子伏特。

▪连续区间中子能量从0.01兆电子伏特至25兆电子伏特。

▪共振区间中子能量从1电子伏特至0.01兆电子伏特。

▪低能区间中子能量低于1电子伏特。

[编辑]快中子

此处介绍的快中子的动能接近1兆电子伏特（100TJ/kg），速度接近14000千米/秒。将它们命名为快中子可以将其区别于于低能的热中子、以及通常在宇宙射线或者加速器中产生的高能中子。快中子通常有由核反应例如核裂变产生。

核聚变反应中产生的中子通常的能量都远大于1兆电子伏特，例如，氘氚核聚变的中子能量达到14.1兆电子伏特（1400 TJ/kg，速度约52000千米/秒，达到了光速的17.3%）。这样高能量的中子可以很容易使得铀-238与其他超铀元素发生裂变。

快中子可以通过中子慢化过程转变为热中子。中子慢化主要依靠减速剂。在核反应堆中，通常使用重水、轻水、或石墨来使中子减速。

热中子是动能约为0.025电子伏特（大约4.0×10−21焦，2.4MJ/kg，速度约2.2千米/秒）的自由中子。这个速度也是对应于290K（摄氏17度）时麦克斯韦-玻尔兹曼分布下的最可能速度。

最可能能量和最可能速度对应的能量、平均能量是不同的。最可能能量是最可能速度对应的能量的一般，而平均能量比最可能速度对应的能量大50%。

在中子与常温下减速介质的原子核发生若干次碰撞后，如果中子还没有被俘获，它们就会达到这个能量。热中子通常有比快中子大得多的有效中子俘获截面，也因此会更容易被原子核吸收，形成更重的、通常也不稳定的同位素。这个现象也被称为中子活化。

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Moderated and other, non-thermal neutron energy distributions or ranges are listed in the table below:

▪Fast neutrons have an energy greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.

▪Slow neutrons have an energy less than or equal 0.4 eV.

▪Epithermal neutrons have an energy from 1 eV to 10 keV.

▪Hot neutrons have an energy of about 0.2 eV.

▪Thermal neutrons have an energy of about 0.025 eV.[1]

▪Cold neutrons have an energy from 5x10−5 eV to 0.025 eV.

▪Very cold neutrons have an energy from 3x10−7 eV to 5x10−5 eV.

▪Ultra cold neutrons have an energy less than 3x10−7 eV.

▪Continuum region neutrons have an energy from 0.01 MeV to 25 MeV.

▪Resonance region neutrons have an energy from 1 eV to 0.01 MeV.

▪Low energy region neutrons have an energy less than 1 eV.

[edit]Fast neutrons

A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (100 TJ/kg), hence a speed of 14,000 km/s. They are named fast neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as nuclear fission.

Neutrons from fusion reactions are usually considerably more energetic than 1 MeV; the extreme case is deuterium-tritium fusion which produces 14.1 MeV neutrons (1400 TJ/kg, moving at 52,000 km/s, 17.3% of the speed of light) that can easily fission uranium-238 and other non-fissile actinides.

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a neutron moderator. In reactors, typically heavy water, light water, or graphite are used to moderate neutrons.

A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (approx.

4.0×10−21J; 2.4 MJ/kg, hence a speed of 2.2 km/s) which is the energy corresponding to the most probable velocity at a temperature of 290 K (17°C or 62°F), the mode of

the Maxwell–Boltzmann distribution for this temperature.

After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, neutrons arrive at about this energy level, provided that they are not absorbed.

Thermal neutrons have a different and often much larger effective neutron

absorption cross-section for a given nuclide than fast neutrons, and can therefore often be