Thermal neutron,It usually refers to kinetic energy of about 0.025 eV（speedAbout 2.2 km/s).This speed also corresponds to 290K(17 ℃)Maxwell Boltzmann distributionThe most probable speed under.
The speed of thermal neutrons is about several kilometers per second, and the kinetic energy is between a few millielectron volts and a hundred electron volts, which is about the same as the average kinetic energy of the thermal motion of ideal gas molecules at the same temperature.The high-energy neutrons produced in the nuclear reaction collide with light nuclei in the retarder composed of light elements, transferring part of the energy to the struck nucleus, and the neutrons themselves bounce back, reducing the energy.The lighter the collided nucleus, the greater the neutron energy loss.After several collisions, the kinetic energy of neutrons will be reduced to the same energy as the thermal motion energy, thus becoming thermal neutrons.This deceleration process of neutrons is called "normalization".[1]
effect
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Thermal neutrons are easy to generate nuclear fission for absorption of heavy nuclei, making heavy nuclei split into medium mass nuclei, and releasing huge energy at the same time.Since the 1930s, research on nuclear fission has led to the emergence of atomic bombs and nuclear power plants.
Another important application of thermal neutron is itsInelastic scattering。Since the kinetic energy of thermal neutrons is similar to the vibration, rotation, diffusion and other motion energies of molecules and atoms in most materials at room temperature, the microscopic motion information inside materials can be detected through the scattering effect of thermal neutrons on materials.According to the characteristic that neutrons have magnetic moments and are sensitive to isotopes, thermal neutron scattering can be used to measure some data that cannot be obtained by such technical means as X-ray, infrared, ultrasonic, etc.
Since the 1960s, with the development of nuclear reactorsParticle acceleratorWith the appearance of isointense neutron sources, thermal neutron scattering has developed into a relatively mature frontier discipline, which has been increasingly widely used in many fields such as solid state physics, chemistry, magnetism, metallurgy, etc.
The first large-scale atomic reactor designed by China and built in early 1981 has a maximum thermal neutron flux of 62 billion/s · cm.This high-throughput atomic reactor can simultaneously produce multipleradio isotopeIt can also carry out activation analysis of trace elements and technical research and production of neutron transmutation of monocrystalline silicon, providing an important means for building nuclear power plants and further developing atomic science.[1]
Thermal reactor
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Thermal reactorIt is a device that uses moderators to reduce the speed of fast neutrons to make them become thermal neutrons (or slow neutrons), and then uses thermal neutrons to carry out chain reaction.Since thermal neutrons are more likely to cause U-235 and other fission reactions, chain fission reactions can be obtained with a small amount of fissile material.Moderators are substances that contain light elements but absorb less neutrons, such as heavy water, beryllium, graphite, water, etc.Thermal reactorGenerally, fuel elements are regularly arranged in moderator to form reactor core.The chain reaction is carried out in the core.
Neutron distribution interval
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Fast neutronThere are different definitions for energies higher than 1 MEV, 0.1 MEV or close to 1 MEV.
Slow neutronEnergy is less than or equal to one thousand electron volts.
Epithermal neutronThe energy is between 1 electron volt and 10 electron volts.
High thermal neutronThe energy is about 0.2 eV.
thermal neutronThe energy is about 0.025 eV.
Cold neutronEnergy is about 5x10-3Electron volts to 0.025 electron volts.
Very cold neutronEnergy is about 3x10-7Electronic volts to 5x10-3Electron volts.
Extremely cold neutronEnergy less than 3x10-7Electron volts.
Continuous interval neutronThe energy ranges from 0.01 MEV to 25 MEV.
Resonance interval neutronThe energy ranges from 1 eV to 0.01 MEV.
Low energy interval neutronThe energy is less than 1 electron volt.
