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          Common classification of neutron rays

          2015/2/3 16:19:07 (Browse times: 2015/2/3 16:19:07)

          1. Thermal neutron
          Thermal neutrons are free neutrons that conform to the Maxwell-Boltzmann distribution and have the most probable kinetic energy of approximately kT = 0.0253 electron volts (4.0 × 10−21 Joules). The rate corresponding to this kinetic energy is approximately 2.2 km / second. This speed also corresponds to the most probable rate under Maxwell-Boltzmann distribution at 290K (17 degrees Celsius). After several collisions between the neutron and the nucleus of the medium at room temperature, this rate will be reached if it is not captured. Thermal neutrons usually have much larger effective neutron capture cross-sections than fast neutrons, and are therefore more easily absorbed by atomic nuclei, forming heavier and often unstable isotope. This phenomenon is also called neutron activation. Some fission reactors use a moderator to slow down fast neutrons, also known as "thermal neutronization." In a fast neutron breeder reactor, fast neutrons are used directly without a deceleration step.

          Cold neutrons
          Cooling hot neutrons to extremely low temperatures yields cold neutrons, such as liquid hydrogen or liquid deuterium. Such cold neutron sources are typically placed in moderators for research reactors or spalled neutron sources. Cold neutron sources are very important for neutron scattering experiments. The energy of a cold neutron is between 5x10−5 electron volts and 0.025 electron volts.
          The fusion reaction rate rises sharply with temperature, reaches a peak, and then gradually decreases. Compared with other nuclear fusion reactions that are promising for power generation, the deuterium-tritium (DT) reaction rate peaks at a lower temperature (70 kiloelectron volts, about 800 million K) and is higher than other reactions.

          3.Ultra-cold neutrons
          Cold neutrons can obtain ultra-cold neutrons after inelastic scattering with substances that have a temperature of only a few K (such as solid deuterium or superfluid liquid helium). Its energy is less than 3x10−7 electron volts.

          4.Fast neutrons
          Fast neutrons are free neutrons generated in nuclear fission reactions, and their kinetic energy can reach 1 megaelectron volts (1.6 × 10−13 Joules, corresponding to a speed of about 14000 km / s, equivalent to 5% of the speed of light. They are They are called fast neutrons to distinguish them from thermal neutrons and high-energy neutrons produced by cosmic rays or accelerators. Neutrons produced in nuclear reactions conform to the Maxwell-Boltzmann distribution, and their energy ranges from 0 to 14 megaelectron volts. The average energy of neutrons produced by uranium −235 is 2 megaelectron volts, and more than half of the neutrons are not fast neutrons. Therefore, neutrons generated by fission of uranium −235 alone cannot trigger breeding materials (such as uranium −238 And 钍 −232).
          Fast neutrons can become thermal neutrons by slowing down. In nuclear reactors, light water, heavy water, or graphite are commonly used to slow down neutrons.
          5.Fusion neutrons
          The deuterium-tritium (DT) fusion reaction produces a higher energy neutron with a kinetic energy of 14.1 megaelectron volts, corresponding to a speed equivalent to 17% of the speed of light. These neutrons are nearly 10 times faster than fast neutrons. The deuterium-tritium reaction is also one of the easiest reactions to ignite. When the kinetic energy of deuteron and tritium reaches one thousandth of 14.1 megaelectron volts, the reaction almost reaches its peak reaction rate.
          Fusion neutrons can effectively trigger the fission of non-fissionable heavy elements (such as actinides) and release more neutrons. Therefore, it has been proposed to use future tokamak deuterium-tritium fusion reactors to transmutate transuranic elements in nuclear waste. Spallation neutron sources also use 14.1 megaelectron volts to generate neutrons.
          Because fusion neutrons either cause fission or spallation, it is difficult to be absorbed by other nuclei. The hydrogen bomb nuclear weapon takes advantage of this feature. First, fusion reactions produce high-energy neutrons. Next, fissionable materials such as uranium-238 are fissioned by these neutrons. This obviously brings some nuclear security and proliferation issues: if someone has mastered the fusion reaction, they may be able to make thermonuclear weapons with nuclear materials that cannot make atomic bombs, such as depleted uranium and reactor-grade plutonium.
          Other fusion reactions produce lower neutron energy. For example, deuterium-deuterium (DD) fusion has a 50% chance to generate a 2.45 megaelectron volt neutron and a helium-3 nucleus; a 50% chance to generate a tritium nucleus and a proton. Deuterium-helium-3 (D-3He) fusion does not generate neutrons.

          6.Neutron
          Neutrons with energy between fast and thermal neutrons are called neutron neutrons. The energy of this neutron is between 1 electron volt and 10 electron volt. Neutron capture cross sections for neutron capture and nuclear fission have multiple resonance peaks in this energy range. Neutron neutrons are not important in fast neutron reactors and thermal neutron reactors. However, in thermal neutron reactors with poor deceleration, neutron neutrons may cause changes in the reactivity of the chain reaction, making the control of the reaction more difficult.
          Some nuclear fuels do not necessarily fission after absorbing neutrons, such as plutonium-239. This property can be described by the capture / fission ratio. Because the capture event not only wastes a neutron, but also usually generates a thermal or neutron neutron that cannot be split. Uranium-233 is an exception. For any energy neutron, the uranium-233 capture / fission ratio is good.

          7.High-energy neutrons
          High-energy neutrons are secondary particles produced by an accelerator bombarding a target or high-energy cosmic rays bombarding the atmosphere. Its energy is much higher than fast neutrons. Some high-energy neutrons can have tens of joules of kinetic energy. They have extremely strong ionizing properties and can cause cell damage and death more than X-rays and protons.

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