In a lush lychee forest in Dongguan, Guangdong, there is a "neutron factory".
This is the largest scientific project with the largest single investment in our country so far - the China Spallation Neutron Source (CSNS). It was supported by the National Development and Reform Commission. Its completion has made our country the largest scientific project after the United Kingdom, the United States, and Japan. The fourth country in the world to have a spallation neutron source.
to the microstructure of the material. The spallation neutron source is a tool that uses neutrons to explore the microscopic world .
Why is neutron scattering an "ideal probe" for exploring the microstructure of matter? How does the "neutron factory" continuously "produce" neutron ? What are the application scenarios for these neutrons? With these questions in mind, Beijing News reporters recently visited the China Spallation Neutron Source, which is undergoing summer shutdown for maintenance.
The "ideal probe" for studying the microscopic world
In 1998, a high-speed train accident occurred in Germany. The train derailed while passing through the town of Eschede, resulting in the death of 101 people. Since then, scientists have been embroiled in controversy over the analysis of the cause of the accident. They finally discovered through the neutron device that the aging wheels caused the tragic accident.
This means that if neutrons are used to detect residual stress and metal fatigue in rails, turbines, wings and other components in advance, similar disasters may be avoided from happening again.
neutrons were discovered by physicist Chadwick in 1932. Together with protons , they form the nucleus . Neutrons have become one of the "ideal probes" for studying the structure of matter due to their uncharged, strong penetrating and non-destructive properties.
"Neutrons are everywhere, and almost half of our bodies are composed of neutrons." Chen Yanwei, deputy director of Institute of High Energy Physics, Chinese Academy of Sciences and director of the Dongguan Research Department, said that because neutrons are uncharged, they are not susceptible to charged protons and The obstruction of electrons allows them to penetrate matter more easily.
When a neutron beam hits the object being studied, most will pass through the object, but some will interact with their nuclei and change their direction of motion. This is like throwing glass marbles at a net. Some will pass through the net, and some will hit the net and bounce in all directions. Neutrons are like glass marbles, and the object being studied is like this net. By studying the trajectory of the glass marbles and the changes in their energy and momentum, we can infer the structure of the net.
Like X-rays, neutrons are penetrating, but X-rays have a limited penetration depth and cannot penetrate most metals. Neutrons do not have this limitation. They can even penetrate thick sealed lead pipes and "see" what's inside.
Although free neutrons exist in the air, their intensity is very weak. "Studying the microstructure of matter requires a large amount of strong neutrons." Chen Yanwei said that previously, scientists used the reactor to generate neutrons, but the intensity of neutrons continuously produced by this method was close to the limit due to heat removal limitations. To this end, scientists have designed a new device to generate neutrons - a spallation neutron source.
Its working principle is that the accelerator first accelerates protons to close to the speed of light, so that the proton beam bombards the heavy metal target like a bullet. The atomic nuclei of the metal target are impacted to emit protons and neutrons. Scientists "collect" neutrons through special devices to carry out various projects. kind of experiment.
Pulsed spallation neutron source is technically complex, costly, and experimentally difficult. Previously, only three countries in the world had such devices, namely the Rutherford Laboratory in the United Kingdom, the Oak Ridge National Laboratory in the United States, and the Japan Atomic Energy Agency's laboratory in Tokai, Ibaraki Prefecture.
"National Treasure" grows in the lychee forest
Academician of the Chinese Academy of Sciences and commander-in-chief of the China Spallation Neutron Source Project Chen Hesheng described such an experience: He once visited the Japanese Spallation Neutron Source to explore the performance of lithium batteries. Experimental station, but the other party refused to visit "because you are competitors."He secretly made up his mind at that time, "We will do it ourselves when we return to China!"
In the late 1990s, veteran scientists from the Institute of High Energy Physics of the Chinese Academy of Sciences and the China Institute of Atomic Energy pointed out the necessity of building a spallation neutron source for the development of national science and technology. sex. Since then, the spallation neutron source has been included in the national "Eleventh Five-Year Plan" for the construction of large scientific facilities. In 2006, the spallation neutron source was located in Guangdong. This was also the first time that a major scientific facility in my country was located in the Pearl River Delta region.
Chen Yanwei, deputy director of the Institute of High Energy Physics of the Chinese Academy of Sciences and director of the Dongguan Research Department, explained in front of the sand table of the China Spallation Neutron Source. Photographed by Beijing News reporter Zhang Lu
In the six years from 2012 to 2017, in the mountains of Songshan Lake Science City in Dongguan , the China Spallation Neutron Source, "the most important weapon of the country", "grew" from a lychee forest. come out. There is an important reason for choosing this site. The geological conditions here are very good and can withstand the target station and spectrometer weighing nearly 10,000 tons. The annual settlement is less than 0.2 mm.
In the autumn of 2017, the China Spallation Neutron Source successfully obtained a neutron beam for the first time in its target shooting. One year later, the China Spallation Neutron Source passed national acceptance and was officially put into operation and open to domestic and foreign scientists, engineers and technicians, and industrial enterprises, with usage time allocated based on the scientific value of their experiments and projects.
"In the past, when China did not have a spallation neutron source, Chinese scientists had to queue up abroad to apply for experiments, but the time they gave us may not meet the demand. The most important point is that many of my country's core and key equipment cannot go abroad for experiments. For example, parts of important equipment, axles of high-speed railways, etc.," Chen Yanwei said.
As soon as the China Spallation Neutron Source was opened, it experienced strong user demand. In the four years since its opening, there have been more than 3,800 registered users around the world, and more than 800 scientific topics have been completed, including residual stress and service performance testing of large engineering components such as deep-sea submersible welding simulation parts, etc., providing high-performance structural materials that are urgently needed in our country. It provides a key technology platform for solving key problems. In the past operating year, the number of user applications has doubled, and currently, the device can only meet 29% of the needs.
Recently, while the China Spallation Neutron Source was undergoing summer "maintenance", reporters saw this "popular" large scientific device hidden 17 meters underground and learned how the "super microscope" works.
Advances in accelerator technology have led to the creation of a powerful weapon for cancer treatment
The China Spallation Neutron Source consists of a negative hydrogen ion linear accelerator, a fast cycle synchrotron, a target station, multiple neutron spectrometers and corresponding supporting facilities.
At the starting point of the installation, the 200-meter-long tunnel of the linear accelerator can be seen endlessly. Complex equipment of various colors and connected to various pipelines are arranged here. The researchers first used hydrogen gas to generate negative hydrogen ions, that is, hydrogen atoms with one more electron, and accelerated them in a linear accelerator.
Wang Sheng, deputy director of the Dongguan Research Department of the Institute of High Energy Physics, Chinese Academy of Sciences, introduced relevant equipment in the linear accelerator tunnel. Photographed by Beijing News reporter Zhang Lu
This process sounds abstract, but in the process of research and development of key technologies, very specific and practical applications have been derived.
Boron neutron capture cancer treatment research has a history of 70 years. In the past, neutrons generated by the nuclear reactor were used for treatment, but the nuclear reactor was difficult to enter the hospital. In the past 10 years, with the development of accelerator technology, researchers have been able to make accelerators that are compact, easy to maintain and operate and placed in hospitals.
"The boron neutron capture therapy BNCT device is the first industrialization project spawned by using the technology related to China's spallation neutron source. Its experimental device was built in August 2020, and the first clinical device is being installed in the hospital and will be able to treat cancer." According to Wang Sheng, a member of the Institute of High Energy Physics of the Chinese Academy of Sciences and deputy director of the Dongguan Research Department, during treatment, the patient is first injected with a boron-containing drug. This drug has a strong affinity with cancer cells and will quickly accumulate in the cancer cells. Within the cell, it is equivalent to "marking" cancer cells.Then the high-flow protons generated by the accelerator are used to target the target area to produce high-flux neutrons to irradiate the lesion. When the irradiated neutrons are captured by the boron in the cancer cells, highly lethal α particles and lithium ions are produced, which can accurately "kill" "Death" cancer cells within 8 cm of the skin.
α particles and lithium ions have a very short range, just about the length of a cell, and can "kill" cancer cells without damaging the surrounding cell tissue . The
BNCT experimental device has been used for cell experiments and animal experiments. Currently, the research team is working with Dongguan People's Hospital to promote clinical trial related work.
BNCT device accelerator. Photo courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences
The "neutron factory" that works for more than 5200 hours a year
When the energy of negative hydrogen ions in the linear accelerator reaches 80 million electron volts , they will enter the next stage, "sprint" into the ring-shaped fast cycle synchrotron, pass through the carbon film, peel off the two electrons and convert them into protons, and travel on the 228-meter-long "runway" in 0.02 seconds. It ran 20,000 times inside, increasing the energy by 19 times to 1.6 billion electron volts, and reaching a speed of 0.92 times the speed of light.
Schematic diagram of the accelerator and target station experiment hall. Photo courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences
Within one second, 25 waves of negative hydrogen ions entered the fast cycle accelerator for acceleration.
According to Wang Sheng, the spallation neutron source device is extremely large, has many equipment components, and the process is extremely complex. The manufacturing and installation process has overcome many difficulties. Among them, the development of fast cycle synchrotron has encountered challenges. For example, the 25 Hz AC magnet and excitation resonant power supply were developed for the first time in my country, and the difficulty was beyond imagination. The role of the magnet is to control and confine the proton beam. The purpose of a 25 Hz AC magnet is to make the magnetic field go from the highest to the lowest to the highest again, 25 times per second. This rapid change in current will cause vibration cracking, eddy current heating, etc. of the core and coil . This is everyone's experience. new issues outside. "The key technologies are in the hands of one or two large foreign companies. It is impossible for us to obtain technical information through scientific and technological exchanges." The scientific researchers and the factory engineering and technical personnel of the Institute of High Energy Physics of the Chinese Academy of Sciences jointly tackled key problems. After 6 years, they overcame the technical difficulties one by one. Finally, we developed qualified magnets on our own, and the key technical indicators exceeded similar foreign equipment.
Part of the China Spallation Neutron Source Fast Cycle Synchrotron. Photographed by Beijing News reporter Zhang Lu
Proton beams approaching the speed of light rush towards heavy metal targets like bullets, just like machine guns , which fire 2 million "bullets" every day. Metal nuclei undergo spallation reactions under the bombardment of high-energy protons, releasing protons and neutrons. Scientists use special devices to "collect" neutrons for experiments.
In order to meet as many user needs as possible, the device operates 24 hours a day to generate neutron beams. In addition to one day of maintenance per week and more than two months of maintenance in the summer, the device can only take a "7-day holiday" during the Spring Festival. "Last year, the device's operating time exceeded 5,200 hours, which is the best in the world. Other spallation neutron source devices generally only have about 4,000 hours." Wang Sheng said that the actual operating efficiency of the device reached 97%, which is the best in the world. The ratio of the actual operating time of the fission neutron source to the planned operating time. The higher the number, the lower the device failure rate and the better the stability of .
"Super Microscope" reveals the microstructure
Walking out of the 17-meter underground accelerator tunnel, the reporter came to the spectrometer hall of the target station. Here, after the proton beam bombards the heavy metal target, the neutron beam pulse generated instantly is short and strong. After the neutrons are slowed down, they are introduced into a spectrometer.
Blue, orange, green, purple... Multiple spectrometers are centered on the target station, extending outward like the colorful petals of a sunflower . After neutrons are introduced into the spectrometer, they interact with the nuclei of the sample material and cause scattering. By measuring scattered neutrons, the microstructure and dynamics of a sample can be studied.
In the spectrometer hall of the target station, each spectrometer with different functions has a different color.Photo courtesy of the Institute of High Energy Physics, Chinese Academy of Sciences
China Spallation Neutron Source has built three spectrometers in the first phase, and many more spectrometers will be built and put into use in the future. What are the typical applications of spectrometers? Liang Tianjiao, deputy director of the Dongguan Research Department of the Institute of High Energy Physics, Chinese Academy of Sciences, came together one by one.
is a general-purpose powder diffractometer with a green appearance. It is mainly used to study the crystal structure and magnetic structure of objects. It is also currently carrying out residual stress testing of small parts. Due to the strong penetrating power of neutrons, the engineering materials spectrometer that is about to be commissioned can also study the residual stress and metal fatigue of large components such as high-speed rail wheels and aerospace engine blades without damaging the samples, meeting major national needs. .
Residual stress refers to the stress retained in materials and components during processing, service, etc., which may cause deformation or even failure of engineering components. "For example, the shell of the 10,000-meter submersible is welded by titanium alloy. Relevant units took the welding simulation parts made by different welding processes to the spectrometer for testing, so as to understand the residual stress parameters of different welding processes and provide guidance for the shell. Provide key data support for body life prediction and welding process selection. "
universal powder diffractometer. During the experiment, the screen door will be closed and the experimenter will read the data in the control room. Photographed by Beijing News reporter Zhang Lu
The spallation neutron source can not only meet the demand for cutting-edge equipment in major scientific research projects such as aerospace and deep-sea exploration, but can also serve people's daily lives and shine in fields such as medical and health.
small-angle neutron scattering instrument and micro-angle neutron scattering instrument detect the nanometer-scale microstructure inside the material. "It can also study how to adjust the ingredient formula of ice cream to make it more delicious." Liang Tianjiao used an example to bring the two esoteric equipment closer to everyone. They can be used in life sciences, food science and other fields. At present, small-angle neutron scattering instruments have been used to conduct research on polymer materials, shale, new cancer treatment drugs, etc., providing micro-mechanical support for drug research and development.
Neutrons are sensitive to carbon, hydrogen, oxygen, nitrogen and other light elements that are extremely important in the fields of energy and life sciences. For example, "flammable ice" distributed in the deep sea or terrestrial permafrost is a crystalline substance formed by methane and water under high-pressure and low-temperature conditions. Scientists must place them in thick metal containers to simulate thousands of ice. meter water depth to study the changes in its microstructure under pressure and low temperature conditions. With the super penetration ability of neutrons, the properties of flammable ice can be studied through thick containers.
"A key issue in combustible ice mining is that when its pressure and temperature conditions change, changes in microstructure may lead to changes in macroscopic physical properties, which may bring safety risks. Through high-pressure powder diffractometers, we can study the microstructure of combustible ice in different states. Changes and impacts. "
spectrometer can also assist in the study of cultural relics. Liang Tianjiao said that after the energy-resolving neutron imaging spectrometer is completed, it can perform lossless neutron "CT" on many precious cultural relics and understand their three-dimensional structure.
The "upgrade" plan is on the agenda
At present, the China Spallation Neutron Source has "exceeded" its design targets. In February 2020, the target beam power reached the design indicator of 100 kilowatts, one and a half years ahead of schedule. The current beam power has reached 125 kilowatts, and an "upgrade" plan for higher beam power has been put on the agenda.
Wang Sheng said that the design of the first phase of the project has reserved space for upgrading and transformation. In the next step, the target beam power of the China Spallation Neutron Source will be increased from the current 125 kilowatts to 500 kilowatts. This means that the device will produce more neutrons in the same time, which will not only effectively shorten the experimental time and improve the experimental resolution The efficiency is higher and neutron scattering experiments can also be carried out on smaller samples.
"After the new spectrometer and experimental terminal are completed, it will meet more users' measurement requirements for smaller samples and higher precision." Liang Tianjiao said that by then, the device will carry out structural characterization, dynamic measurement, and in-situ research and high spatial resolution detection and other multidisciplinary interdisciplinary research to provide more powerful support.
" The most important weapon of the country " will be "new"
as the Beijing Electron Positron Collider , China Spallation Neutron Source, High Altitude Cosmic Ray Observatory, Jiangmen Neutrino Experiment , High Energy As a "big user" of many large scientific devices such as synchrotron radiation light source , in the future, what plans does the Institute of High Energy Physics, Chinese Academy of Sciences have in its "national importance" plan?
"The first is accelerator-based research on high energy physics." Wang Yifang, academician of the Chinese Academy of Sciences and director of the Institute of High Energy Physics of the Chinese Academy of Sciences, said that the Beijing Electron Positron Collider will complete its scientific mission in a few years. The Circular Electron Positron Collider (CEPC) is a next-generation high-energy particle accelerator project under planning. Scientists hope to use it to study the Higgs particle and related scientific issues, and to find new physics beyond the "Standard Model" .
Second, in terms of space, the Institute of High Energy Physics of the Chinese Academy of Sciences has planned two projects. One is the future Based on China's first space X-ray satellite "Wisdom Eyes", it will not only measure X-ray energy, but also measure the polarization of X-rays. It will lead international research on extreme astrophysics such as black holes and neutron stars, and become the world's leading X-ray Astronomical device.
The other one is the Chinese Space Station High Energy Cosmic Radiation Detection Facility (HERD), which is building a cosmic ray detector and a gamma ray detector on the Chinese Space Station to continue the ongoing AMS experiment on the International Space Station to improve the understanding of the universe. The acceptance and energy range of line measurements are used to search for dark matter and measure the energy spectrum and composition of cosmic rays. "It is also the calibration of the cosmic ray energy measured by the ground high-altitude cosmic ray observatory (LHAASO). The two will be combined to study the origin and acceleration mechanism of cosmic rays."
Wang Yifang said, in neutrinos On the other hand, the Jiangmen Neutrino Experiment will be completed next year and will be upgraded in the future. "We are also discussing the establishment of a neutrino underwater or under-ice device to thoroughly understand the origin and acceleration mechanism of cosmic rays through the combination of gamma rays , charged particles and neutrinos."
new Beijing News reporter Zhang Lu
editor Fan Yijing proofread Liu Jun
