Tungsten Has Become the Key Material of New Generation Neutron Scattering Technology in China

Recently, China's CSNS facility in Dongguan has been put into use, which is not only a major step forward by Chinese scientists, but also a major event in the international scientific community. Among them, the important component of the facility "neutron target station" uses tungsten materials.

CSNS uses neutron scattering technology: when a neutron is incident on the sample, the incident neutron is interacted with the nucleus of the sample and is scattered. Measuring the energy and momentum changes of scattered neutrons, we can study the microstructure and motion laws of various substances on the atomic and molecular scales, and tell people the position of atoms and molecules and their state of motion.

The spallation neutron source technology plays an irreplaceable role in the fields of biology, life, medicine, new energy and Aerospace Science and technology, and is complementary to the synchrotron radiation source. It has become one of the most important platforms for basic scientific research and new materials research and development. For example, proton and neutron from spallation neutron sources can be used for radiotherapeutic research of tumors, and have been applied in many developed countries. Furthermore, in the development and utilization of new clean energy ice, the neutron diffraction technique under the high pressure of the spallation neutron source can be used to study the formation mechanism and stability conditions of methane hydrate in combustible gas, and the research results will provide a scientific basis for the safe and efficient exploitation and utilization of combustible ice.

CSNS mainly consists of three parts: proton accelerator, neutron target station and neutron scattering spectrometer, in which the target station is a device for converting the spallation neutrons produced by the high-energy proton beam of the accelerator system to the neutron beam that can be carried out in the scattering experiment. The target station consists of target, reflector, moderator, cooling device and shield. The target material of the neutron target station is tungsten, which is the core component of the neutron target station and the whole CSNS facility. The spallation neutron source produces neutrons through the proton impact on the tungsten target.

It is understood that the material has the characteristics of small neutron absorption cross section, high atomic density, high neutron yield, good thermal conductivity, strong corrosion resistance and radiation damage resistance when selecting target materials for spallation neutron source. In the spallation reaction, the amount of the neutron yield is proportional to the atomic mass of the target material, so the heavy metal with high atomic mass is generally chosen as the target material. The design power of the CSNS phase is 100k W, and the two phase is 500kW. Under these two power, the performance of the solid target is much higher than that of the liquid target.

Solid target materials include tantalum, lead, uranium, tungsten, gold and so on. Among them, the atomic density and neutron yield of lead are too low. Although the neutron yield of uranium is very high, it will also have a fission reaction in the spallation environment. The fission reaction will increase the number of retarded neutrons, thus increase the background of neutrons, which is not conducive to the control of the neutron beam, and the radiation damage of uranium is very obvious, causing uranium targets. The service life is very low; tantalum has poor thermal conductivity and high activity after irradiation. CSNS selects tungsten as the target material for producing neutrons. Compared with other materials, tungsten has the advantages of high melting point, good thermal conductivity, high neutron yield and low activity. The Monte Carlo simulation shows that the leakage neutron of the flat shape target is higher than that of the cylindrical target and the square target. At the same time, the distance between the moderator and the neutron producing region is shortened, the coupling efficiency of the target moderator is improved and the neutron intensity of the slower output is improved, so the CSNS is determined to use the flat shape tungsten target.

CSNS uses tungsten as a metal target. Compared with the method of producing neutron beam using nuclear reactor, this method is more economical and safe. Because neutrons have no charge, they can penetrate more easily, and neutrons are more sensitive to hydrogen and other elements, so they can also be used to detect the manufacturing materials of fuel cells. Similar facilities are available only in the United Kingdom, the United States, Japan and Switzerland, while the relevant facilities in Sweden are under construction.

The CSNS facility will invest about 2.2 billion RMB. It will not only encourage Chinese researchers to stay in domestic development, but it will also attract more and more international top physicists and material scientists to participate in China’s research. Undoubtedly, China’s spallation neutron source The instrument will be an important asset for the Earth, environment and energy sciences as well as physics, chemistry and materials science. (Chinatungsten Online: Weiping)

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