A few days ago, China National Scattered Neutron Source (CSNS), a major national science and engineering project in Dongguan, Guangdong Province, successfully hit the target and obtained neutron beams. This marked the successful completion of CSNS main project and entered the trial operation stage. What is the source of spallation neutrons in China? How long is it? What is the use? Security is not safe? The reporter went to the scene and announced for you one by one.
One question: What is it?
Like a "super microscope" that can study the microstructure of matter
"When a neutron is incident on a research sample, it interacts with its nuclei or magnetic moments to generate scattering. By measuring the changes in the energy and momentum of the scattered neutrons, we can study the various atomic and molecular scales. The microscopic structure of matter and the laws of motion tell people the position and movement of atoms and molecules. This research method is called neutron scattering technology.†said Chen Hesheng, an academician and project manager of China Spallation Neutron Source Engineering and an academician of the Chinese Academy of Sciences.
In 1932, the British physicist Chadwick discovered neutrons. People began to realize that the nucleus consists of positively charged protons and uncharged neutrons. The discovery and application of neutrons is one of the most important technological achievements of the 20th century. Because neutrons are uncharged, have a magnetic moment, and are highly penetrable, they can resolve light elements, isotopes, and neighboring elements and are non-destructive. These properties make neutron scattering an ideal probe for studying the structural and dynamic properties of materials. First, like X-rays, it is a powerful tool for detecting the microstructure and atomic movement of matter in multidisciplinary studies.
Chen Hesheng explained further: “For example, suppose there is an invisible net in front of us. We constantly throw a lot of glass marbles. Some of the marbles pass through the net, while others play online and flick to different angles. If you record the trajectory of these marbles, you can roughly infer the shape of the net; if the marbles are made enough, dense enough, and strong enough, you can accurately describe this net and even infer its material. ."
However, neutron scattering research requires the use of neutrons as probes. There must be a proper source of neutrons. Advanced neutron sources are the basis of neutron science research. How to obtain the neutron source that produces high-flux neutrons has always been a goal pursued by scientists.
High flux neutron sources include reactor neutron sources and spallation neutron sources. Nuclear reactors are a stable and continuous source of neutrons and have played a significant role in neutron science research. However, reactor neutron flux reached saturation in the 1960s and 1970s due to reactor cooling technology limitations. With the advancement of science and technology, corresponding research systems such as thin films, nanoclusters, biological macromolecules, and proteins have larger scale distributions and it is more difficult to obtain samples in the order of grams. Therefore, fast, high-resolution neutron scattering measurements for small samples urgently require a new generation of higher-throughput, wider-band neutron sources. Spallation neutron sources have emerged as a matter of course.
"It is the use of high-energy protons from large accelerators to bombard heavy metal targets, causing spallation reactions of metal atoms, releasing large amounts of neutrons. These neutrons form very strong neutron beams, neutrons slow down (neutrons and media The phenomenon of neutrons colliding and causing neutron energy to decrease and decelerate) scatters with the sample, and is finally received by a neutron scattering spectrometer. Our researchers then analyzed the microscopic characteristics of the observed object based on these neutron scattering data. The spallation neutron source is like a 'super microscope' that can study the microstructure of DNA, crystalline materials, polymers, etc." said Chen Hesheng.
Second question: How are you?
Very large, built underground from 13 to 18 meters deep
Due to these unique properties of spallation neutron sources and the increasingly stringent requirements of the international community for safety assessment, neutron scattering studies have been widely used in developed countries for spallation neutron sources. At present, the countries in the world that are operating pulsed spallation neutron sources are mainly the United Kingdom, the United States, and Japan. After the completion of China's spallation neutron source, it will become the first spallation neutron source in developing countries and rank among the world's four major sources of pulse spallation neutrons, which will greatly increase the level of basic research and high technology in China.
Although the neutrons are so small, the source of spalled neutrons that produce strong neutron beams is an extremely large device, and it is a whole assembly of various high, precise, and pointed devices.
The reporter saw on the scene that the CSNS tunnel installation was built underground in the depth of 13 to 18 meters. The main construction includes a negative hydrogen ion linear accelerator with a beam energy of 80 megaelectron volts, a fast cycle proton synchrotron with a beam energy of 1.6 billion electron volts, two beam transport lines, and a target station. And 3 spectrometers, as well as supporting facilities and civil engineering.
Chen Hesheng said: “The number of neutron spectrometers in the CSNS Phase I project will be three, and we can build up to 20 in the future. At present, the beam power is 100 kilowatts. We have also set aside a further increase in beam power to 500 kilowatts and we will Second target station upgrade space."
The first three spectrometers were general powder diffractometers, multi-function reflectors, and small-angle scatterometers.
“Universal powder diffractometer is mainly used to study the crystal structure and magnetic structure of the material; multifunction reflectometer analyzes the reflected neutrons from the sample to study the surface and interface structure of the material; small angle scatterometer is used to detect the material system in 1-100. Nano-scale microscopic and mesoscopic (a system between the macroscopic and microscopic structures). Chen Hesheng said.
Three questions: What's the point?
Makes high-speed rail and aircraft safer and more comfortable, and can be used for radiotherapy research of tumors
The spallation neutron source can generate strong pulsed neutrons, and by measuring the scattering reaction process of the neutron beam in the sample, detecting the location and movement of the sample nuclei, therefore, in materials science and technology, life science, physics, chemical industry, Many areas such as resource and environment, new energy, etc. have extensive application prospects.
Chen Hesheng gave an example. “In June 1998, an intercity express train in Germany accidentally derailed, and the final culprit of the accident was turned out to be the wheel of aging. The wheel was detected on the source of spallation neutrons in the UK and it was found that there were internal cracks.â€
In fact, both the high-speed rails and the turbines and wings of the aircraft have stress, which determines the service life and safety of the high-speed rail and the aircraft. However, this stress cannot be seen or touched, and its research has become the key to avoiding disasters. Now scientists have been able to measure the residual stress on the spalled neutron source to study the wheel and rail and wing, optimize the machining process, make the high-speed rail and the aircraft more secure and comfortable.
Chen Hesheng said: "The spallation neutron source technology also plays an important role in the field of biology, life, medicine and Other research. It is complementary to the synchrotron radiation source and becomes the most important platform for basic scientific research and new material research and development." For example, protons and neutrons of spallation neutron source can be used for radiotherapy studies of tumors, and have been applied in many developed countries.Also, in the development and utilization of new clean energy combustible ice, spallation neutron source The neutron diffraction technique under high pressure can be used to study the formation mechanism and stable conditions of combustible gas methane hydrate. The research results will provide a scientific basis for the safe and efficient exploitation and utilization of flammable ice.â€
The construction of the CSNS device will also greatly strengthen the international exchange and cooperation of the domestic neutron scattering science and application sector, and provide a good opportunity for China's neutron scattering technology and applications to occupy a place in the international frontier.
“In the construction of CSNS, the construction of proton accelerators, target stations, and neutron spectrometers all involve a large number of high-tech strategic significance. These technical difficulties require China’s highest level of scientific research to work together to overcome. Therefore, the construction of the project will attract A large number of world-class scientists have come to carry out scientific research activities and will attract more than 600 experts and scholars at home and abroad to gather in Dongguan for research after the completion of the research. Sun Mu, party secretary of the Institute of Physics of the Chinese Academy of Sciences, said.
Four questions: Is it safe?
Lived for a year in the vicinity and received radiation equivalent to only one long-haul aircraft
What is the security of such a huge scientific device? Will it cause pollution to the surrounding environment?
Chen Hesheng said: "CSNS is based on a new generation of accelerators, does not require nuclear materials, its power comes from electric energy, and radiation is strictly controlled within the scope of environmental protection. We have done calculations and lived in the vicinity of spallation neutron source for one year, and the radiation received by residents is only It's equivalent to a long-haul plane."
According to reports, a monitoring station is built near CSNS to monitor radiation at any time. The "Scattered Neutron Source Project Environmental Impact Report" also put forward an accident emergency plan. Since the spallation neutron source is a ray device, the main radiation generated by the neutron source disappears as soon as the power is shut down.
In addition, the spallation neutron source is completely different from the nuclear device of the reactor type. It does not require nuclear materials, there is no chain nuclear reaction, and a small amount of particles are lost during the accelerator operation to generate secondary particles, such as gamma rays, gamma rays, and The sons and daughters, all of which are radioactive, are called instantaneous radiation.
"These radioactive materials are 5 to 6 orders of magnitude lower than the neutron source of the reactor and have a much smaller impact on the environment, and these radioactive materials will be stored in the national designated waste repository." Manager Chen Yuanbai said.
“As long as the accelerator stops, the main radiation source that causes environmental impact disappears, and it can be said that it is safe and reliable like a faucet. The accelerator is controlled automatically by electronic means. Once an error occurs, the accelerator can be turned off within a few milliseconds. If an earthquake occurs In other natural disasters, the spalled neutron source machine will cease operation and the radiation field will immediately disappear. After repeated argumentation and research, spallation neutron source is a relatively clean ray device,†said Chen Hesheng. (Wu Yuehui)
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