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The magical transition from fast neutrons to thermal neutrons: How do neutrons reveal the microscopic world of matter?
Neutron scattering is an important physical phenomenon involving the irregular scattering of free neutrons and matter. Not only does this process occur naturally, it is also used in a variety of experimental techniques to explore the fundamental characteristics of matter. In nuclear engineering and science, the significance of natural phenomena is self-evident; as an application of experimental technology, understanding and manipulating neutron scattering is crucial to fields such as crystallography, physics, physical chemistry, biophysics, and materials research. important.
Neutron scattering has a wide range of applications, especially when exploring the structure and properties of materials. Through neutron scattering, scientists can deeply analyze the microstructure of materials.
When conducting neutron scattering experiments, researchers use research reactors and fission neutron sources, which provide varying intensities of neutron radiation. Depending on their needs, researchers can choose elastic scattering (neutron diffraction) to analyze material structure, or use inelastic neutron scattering to explore atomic vibrations and other exciting phenomena.
Scattering of fast neutrons
Fast neutrons, defined as neutrons with kinetic energy exceeding 1 MeV, can scatter with molten materials. In such collisions, fast neutrons transfer most of their kinetic energy to the scattering nuclei (i.e., the atomic nuclei of matter). With each collision, the fast neutrons slow down, eventually reaching a state of thermal equilibrium with the material from which they were scattered. To produce thermal neutrons, neutron reducers are often used, which produce thermal neutrons with a kinetic energy of less than 1 eV.
By regulating the scattering of fast neutrons, we can reveal the structure and behavior of many substances at the microscopic level.
Neutron-matter interaction
Because neutrons are electrically neutral, they can penetrate matter more deeply, giving them greater advantages as detectors than charged particles of the same kinetic energy. Neutrons mainly interact with the magnetic field generated by atomic nuclei and unpaired electrons, which allows them to cause significant interference and energy transfer effects in neutron scattering experiments.
There are significant differences in neutron scattering and absorption cross-sections between different isotopes, with hydrogen having the highest scattering cross-section, which makes neutrons useful in analyzing low atomic number materials, especially proteins and surfactants.
Inelastic neutron scattering
Inelastic neutron scattering is an important experimental technique that is widely used in condensed matter research to explore the motion of atoms and molecules and the excitation of magnetism and crystal fields. This technique makes distinctions based on the change in kinetic energy after the neutrons collide with the sample, and the results are usually expressed in terms of dynamic structure factors.
Inelastic neutron scattering allows us to observe the dynamic behavior of matter in a new light, untying the limitations of traditional spectroscopy.
History and facilities of neutron scattering
The first neutron diffraction experiments date back to the 1930s, but with the advent of nuclear reactors in 1945, the accessibility of high neutron fluxes made in-depth structural studies possible. With the development of science and technology, researchers installed the first neutron scattering instruments in the beam tubes of multi-purpose research reactors. By the 1960s, the construction of high-flux reactors allowed practitioners of beam experiments to obtain higher neutron scattering instruments. sub-flux.
Currently, most neutron scattering experiments are conducted by research scientists who apply for beam time from neutron sources through a formal proposal process. Because neutron scattering experiments involve low count rates, relatively long beam times (on the order of days) are typically required to obtain usable data sets.
Evolution of technology
With the deepening of research, many neutron scattering technologies have continued to evolve, including neutron diffraction, small-angle neutron scattering, spin echo and non-hero reflection. These technologies help scientists obtain more accurate data to analyze the microscopic properties of matter. structure.
With the development of neutron scattering technology, we can unveil the mystery of the microscopic world of matter. However, the principles behind the technology still make people think deeply: What new technologies may appear in the future that we can use to deeply explore the mysteries of this microscopic world?
