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The Amazing Dance of Neutrons and Atoms: Why Can Neutrons Pass Through Matter?
Neutron scattering is a fascinating physical phenomenon that involves the anomalous scattering of free neutrons with matter. This is not only a physical process that occurs in nature, but also an experimental technique that scientists use to study materials. The charge-free nature of neutrons allows them to penetrate deep into matter, probing its internal structure more efficiently. Natural neutron scattering processes are of great interest to nuclear engineering and science, and play a key role in a variety of materials research.
Neutron scattering technology has become an indispensable tool in fields such as crystallography, physics and biophysics.
The basis of the experiments lies in the understanding and manipulation of neutron scattering, which requires the use of different neutron sources, such as research reactors and detonation neutron sources. These sources are capable of providing neutron radiation of varying intensities for research purposes. Neutron diffraction (elastic scattering) allows scientists to analyze the structure of materials, while inelastic neutron scattering is used to study the vibrations and other excited states of atoms.
Fast neutron scattering
So-called "fast neutrons" have kinetic energies above 1 MeV. These neutrons can be scattered by condensed matter, and this is a valid experimental approximation, which can be regarded as an elastic collision. In each collision, the fast neutron transfers a significant portion of its kinetic energy to the scattered nucleus, with the energy transfer in this process depending on the type of nucleus. As multiple collisions proceed, the fast neutrons gradually slow down and eventually reach thermal equilibrium with the matter. This phenomenon requires the assistance of a neutron moderator to produce thermal neutrons below 1 eV.
Interaction of neutrons with matter
Because neutrons have no charge, they can penetrate deeper into matter than charged particles of the same kinetic energy, making them a powerful tool for probing bulk properties. Neutrons interact primarily with atomic nuclei, unlike X-rays, which interact primarily with the surrounding electron cloud. For example, hydrogen has the highest scattering cross section of all isotopes, making neutrons particularly effective in analyzing low-atomic-number materials such as proteins and surfactants.
Neutron scattering and absorption cross sections vary from isotope to isotope, and scattering can be either incoherent or coherent, depending on the isotope used.
Inelastic neutron scattering
Inelastic neutron scattering is a commonly used experimental technique in condensed matter research to study atomic and molecular motions as well as magnetic and crystal field excitations. This technique accounts for the changes in kinetic energy that occur during collisions between neutrons and the sample, and the results are usually reported in terms of dynamic structure factors. In such experiments, most measurements will focus on either type of elastic or inelastic scattering, depending on the needs of the research question.
History and Current Technology
The first neutron diffraction experiments were performed in the 1930s, and with the advent of nuclear reactors in 1945, the technology for high neutron fluxes became available. This field developed rapidly with the construction of multi-purpose research reactors in the 1960s. By the 1980s, research based on high-flux sources made neutron scattering technology more mature and widely used in various material research.
Current neutron scattering experimental facilities and technologies
Most current neutron scattering experiments are conducted by scientists applying for experimental time at neutron sources through a formal proposal process. The experimental recovery rate is relatively low, so it usually takes several days of experimental time to obtain a usable data set. Many different neutron scattering techniques, such as small-angle neutron scattering, reflectivity, inelastic neutron scattering, etc., are being used in research around the world.
How can we give full play to the advantages of neutrons in the development of future technologies to promote related research in materials science?
These studies not only provide us with a deeper understanding of the nature of matter, but also inspire endless possibilities for material performance. What impact will exploring the interaction between neutrons and matter have on the development of science and technology?
