What is a Cloud Chamber?

A cloud chamber is a device that displays the track of particles that can cause ionization. Early nuclear radiation detectors were also the earliest charged particle detectors. It was proposed by CTR Wilson in 1896, hence the name Wilson Cloud Room. ^[1]

The cloud room is a device for carrying out different cloud physics experiments under simulated cloud and fog conditions in a certain space. The volume is large or small. Large-volume cloud chambers with a volume of more than cubic meters are fixed and can be used for a variety of cloud physics experiments; small hybrid cloud chambers with a volume of several to tens of liters are mainly used for observation of natural ice cores in the field, and can also be used for cloud seeding. Testing of catalyst ice-forming performance. ^[2]

In the fall of 1895, the German physicist Roentgen discovered that X-rays reached the United Kingdom. In early 1896, J. Thomson, director of Cavendish Laboratory, began to study the electrical characteristics of air after X-ray exposure, and proposed a gas Ionization theory. Wilson had the opportunity to touch the X-ray tube in its original form. He irradiated the cloud chamber with X-rays and found that clouds formed when the expansion ratio reached a certain limit. This experiment shows that X-rays produce a large number of condensed nodules, which are in the same category as the very small number of nuclei produced in the air. In the next two years, Wilson used his invented dilatometer to study X-rays, newly discovered uranium rays, ultraviolet rays, tip discharges, and other condensed nodules in the air. The experimental results show that the minimum supersaturation required for the condensation of water vapor by the nucleus generated by pure ionization is all the same. The nature of the condensed nodules generated by ionization in the electric field indicates that they are indeed charged ions. This supports J. Thomson's theory of gas ionization. These research reports were submitted to the Royal Society in the fall of 1898.

The charged particles are invisible, but as charged nodules, water vapor can be condensed into fog beads around it, which can be seen. This property can be used to show the traces of charged particles. Unfortunately, as Wilson was busy with other research work, he did not continue his research in this area.

Until 1910, Wilson resumed the research on the expansion of the cloud chamber. The problem he is thinking about now is to make known charge ions become visible, countable and photographic traces by condensing. At that time, the concept of particulateness of and rays had been confirmed. He thought that when charged particles passed through the air, they would ionize gas molecules due to collisions with gas molecules in the air, thereby generating a large number of incident particles on the path of motion. The supersaturated water vapor will use these positive and negative ions as the core to condense into fog beads, and the fog beads are visible and photographable. Armed with this idea, Wilson began experimenting with the most appropriate shape of the expansion instrument, looking for an effective method for taking instant photos of cloud particles. In the spring of 1911, the test was not completed. One day, he tested whether he could see certain traces with a rough instrument that had been made. The experiment used X-rays. Although he did not have much hope of success, the result made him happy: clouds and fog formed in the cloud chamber. Thin lines, these lines are the tracks of electrons generated by the action of rays. Later, he put the metal sheet containing radium on the scintillation mirror into the cloud chamber. For the first time, he saw a very beautiful cloud and fog image condensed along the alpha particle track. When he brought an appropriate radioactive source close to the cloud chamber, he also saw There is a long linear track of fast beta particles. In the summer of 1911, the photo-expandable cloud chamber was finally designed. He used the photographs of the track of the alpha particles to confirm WH Prague's recent analysis of X-ray particle properties. Wilson pointed out that to obtain a good cloud chamber track photo, two conditions need to be met: first, expansion cannot stir the gas. In order to ensure this, a flat and wide cloud chamber can be used, whose bottom can be suddenly lowered, and the volume can be increased according to requirements. Second, there must be no "dust" particles or ions in the cloud room, except for the ionized ions to be observed. To do this, an electric field is applied between the top and bottom of the cloud chamber. ^[3]

Cloud room is showing can cause

The structure is mainly composed of the following parts: a refrigerant interlayer composed of two copper concentric cylinders, a cloud chamber tube concentric with the refrigerant interlayer, a compressor refrigeration system, an electric heater, a thermal insulation layer, an ice crystal access device, and a multi-point measurement Thermometer, ultrasonic atomizer, microscope and cold stage. ^[2]

In addition to the construction of fixed cloud chambers with a volume of 96m ³ and 2m ³ and multiple portable hybrid cloud chambers with a small volume in China, there are also static diffusion cloud chambers and uniform water droplet freezing experimental devices that study specific ice core activation mechanisms. Using these equipments, a lot of work has been done in the research of ice core activation, the development of cloud seeding catalysts and the observation of natural ice cores. At present, the structure and basic performance of cloud chambers widely used in cloud and precipitation physics are as follows.

3L Portable Hybrid Cloud Room

The main body of the cloud chamber is a double-layer jacket structure, and the sandwich layer is filled with coolant. Starting from the lowest temperature, the activation temperature spectrum of the ice nucleus is detected during the temperature rise. They have the advantages of simple operation and portability for on-site detection, but due to their small size and large boundary effects, the detection results are more discrete.

2m³ isothermal cloud room

The main body of the cloud chamber is a hollow cylinder with jackets on the top and bottom, with an inner diameter of 1.2m and a maximum height of 2.08m. The refrigerant is circulated in the jacket, and a 10cm thick polyester foam layer is used for heat preservation outside the jacket. Use ultrasonic atomizer to make fog. The cloud chamber is equipped with a diluted wind tunnel, and its detection results can be compared with those of the CUS isothermal cloud chamber.

96m³ Medium Cloud Room

The main body of the cloud chamber is 14.8m high and the inner diameter is 3m. The temperature can be adjusted from 30 to -45 ° C. The fogging system can be divided into steam fog, water spray fogging, and ultrasonic fogging. Various simulation testing tests can be performed.

1m³ isothermal cloud room

The cloud room is 1.76 m high and 0.88 m inner diameter. The temperature of the cloud chamber can be set in advance and automatically adjusted, and can be maintained for a long time after reaching the preset temperature, and the temperature fluctuation is less than 0.1 ° C. The fog in the cloud chamber is generated by an ultrasonic atomizer, and the humidity and drop spectrum concentration can be manually adjusted.

The small hybrid cloud room has two irreplaceable features:

The first is to sample and observe the atmospheric ice nucleus to understand the background concentration and time and space changes of the atmospheric ice nucleus;

The second is the development and testing of cloud-seeking catalysts, especially when testing the nucleation rate of bomb-laden catalysts, which can be set near the explosion site for testing.

However, the activation of ice nuclei is extremely sensitive to temperature and humidity conditions. It is difficult to strictly control the temperature of small hybrid cloud chambers due to volume restrictions. The subcooled mist in the cloud chamber has a short maintenance time. The fogging can easily cause the gas sample volume, temperature, and humidity to be disturbed Such shortages will inevitably cause great uncertainty. ^[4]

Observation and Research of Atmospheric Ice Nuclei Concentration

Aerosol particles in the atmosphere affect the microphysical structure of the cloud and the precipitation process through its phase change promotion effect on water, thereby affecting weather and climate processes. These have attracted the attention of the international atmospheric science community. In view of the fact that the ice crystal process plays an important role in the precipitation process in a large area of northern China, many observations of atmospheric ice cores were conducted in China in the 1960s and 1970s. The results show that: Sol particles are the main source of ice nuclei. In the 1980s, the study of stratiform cloud artificial precipitation in northern China observed and analyzed the regional distribution characteristics of cloud condensing nodules in a large area of northern China. After the 1990s, Youlaiguang equal organized two observations of the atmospheric ice core concentration in Beijing in 1995 and 1996, and found that there is a positive correlation between the ice crystal concentration and the aerosol particle concentration in the clouds.

Study on High-efficiency Silver Iodine Flame Reagent and Its Ice-forming Performance

-AgI is a hexagonal crystal system. The lattice parameters of the crystal are very close to that of ice. Its particles can act as ice nuclei at temperatures below -4 ° C. Ice crystals are generated in supercooled clouds through heteronucleation. If the difference between the parameters of AgI and ice lattice can be further reduced, the ice formation efficiency may be further improved. In this regard, some experimental work has been carried out at home and abroad, with the aim of generating composite aerosols of AgI and other substances to change the lattice parameters of the AgI crystals so as to improve the ice-forming performance of the flame retardant. The pros and cons of the performance of flame formulations for ice formation are mainly understood through cloud chamber testing. The "Research on New Catalytic Technology System for Artificial Rain Enhancement", which was concluded in 1999, carried out unified detection and analysis of the nucleation rate of various AgI flame retardants in China in a 2m ³ isothermal cloud chamber.

Experimental Study on the Anti-fog and Ice-forming Performance of Liquid Nitrogen (LN)

Liquid nitrogen (LN) is seeded into cold clouds and mixed with saturated humid air and cloud droplets. Like other catalysts, the Bergeron process appears, which produces a large number of ice crystal embryos and ice crystals. Zhang Ye et al. Studied the anti-fog and ice-forming performance of liquid nitrogen. The experimental results show that the correlation between the nucleation rate of liquid nitrogen and the cloud temperature is not obvious, but it is related to the spread of liquid nitrogen. Cao Xuecheng, Ren Jie, Wang Weimin, Han Guang, etc. studied the ice nucleation and characteristics of liquid nitrogen (LN), and the results showed that the high temperature section of 0 -5 , the ice nucleation rate of LN was> 1010.g ^-1 ;- The ice nucleation rate at 5 to -10 ° C is 10 ¹⁰ to 10 ¹² .g ^-1 ; -10 ° C the ice nucleation rate is 10 ¹² to 10 ¹³ .g ^-1 . Because LN has a high ice nucleation rate in the high temperature section, it has a wider temperature range in terms of artificial rain enhancement for cold cloud catalytic operation and artificial supercooling fog elimination for catalytic operation.

Experimental Study on the Ice-forming Properties of Rare Earth Compounds

In the work of weather modification, silver iodide is widely used at home and abroad. In order to overcome some of the shortcomings of silver iodide aerosols in practical seeding (such as the attenuation of activity caused by sunlight), there are also seeding in the form of hydrosols. However, both are consumed A lot of silver. In order to find new economical and effective ice-forming catalysts, many researchers in various countries have conducted experimental studies on the ice-forming properties of many inorganic and organic compounds over the years.

In the early 1970s, Matsubara reported the ice-forming properties of seven rare earth element oxides and believed that their ice-forming ability was medium, but there was no comparable standard. Based on this, Mo Tianlin et al. Prepared several mixed rare earth compound hydrosols by chemical methods, and preliminary screened their formation conditions and freezing performance, and obtained rare earth fluoride (RF3), rare earth iodate [R ( IO3) 3] The results are similar to the average freezing temperature of hydrosol and silver iodide hydrosol. Based on China's abundant rare earth resources, studying the ice-forming properties of rare earth compounds, and looking for non-silver catalyst alternatives to the widely used silver iodide catalysts, not only can it expand new applications of rare earths, gain independent intellectual property rights, but also be sustainable for artificially affected weather. Development and protection of the ecological environment are of practical significance.

Experimental and Observational Analysis of Ice and Snow Crystals Colliding and Increasing

During the life of the cloud, condensation alone cannot grow to raindrops of the order of millimeters. There must be a collision process, but droplets with a diameter of less than 50 m are very inefficient. It seems that it takes a long time for cloud droplets to condense and reach effective rainfall. However, it is often observed that clouds form precipitation within 1 h, and hail formation sometimes occurs within 1 to 2 h, thanks to the appearance of ice crystals and phase transitions. Experiments and field observations have found that the growth of ice crystals also has an acceleration process similar to that of droplets. The ice crystals collide with the droplets (undercooling) to form a radon, and then the supercooled water droplets grow into hail; the ice crystals collide with each other and are connected , Climbing, rapid growth into snowflakes or snow clusters, such as melting into millimeters of raindrops, will accelerate the growth of water quality points and accelerate the precipitation process.

People have paid attention to the observation and research of the phenomenon of collision and coherence between ice and snow crystals, and believe that it is mainly related to temperature. L -5 is a frequent occurrence region, and its mechanism is mainly adhesion, followed by -12 -17 , mainly collusion and attachment. ; The most crystalline forms are flakes and spokes, and needles form bunches. Huang Geng et al. Believed that the growth process of ice crystals occurred only under the conditions of saturated and supersaturated water. Among them, the highest efficiency is from 13 ° to 17 ° C. The branches and stellate crystals in this layer are the main areas for hooking and climbing, and also the main area for the growth of ice crystals. The growth rate is the fastest, and it is the artificial precipitation that spreads artificial ice nuclei. Temperature range with high catalytic efficiency. In short, the study of ice-snow crystals colliding and coherent is of great significance to natural precipitation (snow), especially in artificial seed-catalyzed precipitation enhancement. ^[4]

Improvement of cloud chamber and contribution to particle physics:

The Wilson cloud chamber can show the movement trajectory of particles that are too small to be directly observed, and can even shoot the situation where the high-speed particles collide with each other to change the direction of movement. Therefore, it was immediately popularized by people once it was invented Attention and application have made an indelible contribution to testing the theory and exploring new particles.

In 1923, Arthur Holly Compton (1892-1962) discovered the phenomenon that the wavelength becomes longer after X-ray scattering, namely the Compton effect, and he used the law of conservation of momentum and energy when photons collided with electrons to explain. When doubted about this, Wilson used the track of the recoil electrons taken by the cloud chamber to convincingly confirm the Compton scattering theory and provide an experimental basis for Einstein's photon theory. Because of this work and his invention In the cloud room, he and Compton won the 1927 Nobel Prize in Physics.

Later, many new types of particles were discovered using the Wilson Cloud Chamber. The first was the discovery of positrons. In 1932, Carl David Anderson (Carl David Anderson, 1883-1964) used the Wilson Cloud Chamber to study cosmic rays. In the photo of the cosmic ray cloud chamber, a positron trail was found. This was the first antiparticle found in the cloud chamber, a positron, confirming PAM Dirac's prediction of the existence of a positron. Anderson Therefore, he won the Nobel Prize in Physics in 1936. In 1937, Anderson used it to discover the theoretical meson that Yugawa Hideki predicted in 1935. In 1955, Chinese scientist Wang Changchang and his collaborators used a large cloud chamber to discover Anti-sigma negative superson.

In the Cavendish laboratory in 1925, the young Brackett (MS Blackett, 1897-1974), under the guidance of Rutherford and Wilson, committed to using a cloud chamber to study the problem of a particle hitting a nitrogen nucleus. He started from Eight of the more than 20,000 photographs of the cloud room were taken, confirming Rutherford's earliest artificial nuclear reaction experiment in the world in 1919.

In 1932, PMS Blacken and UPS Occhialini collaborated to study the cosmic rays using the Wilson cloud chamber. Because the cosmic rays are scarce, if the cloud chamber is randomly expanded and photographed, about every 100 photos There are only 2 to 5 pictures with traces of cosmic rays, which made them think of the automation problem of cloud room photography. The solution was to place a Geiger counting tube on the upper and lower sides of the cloud room vertically, so that the cloud passes The cosmic rays in the chamber must pass through the two counting tubes. Brackett designed a circuit that can only trigger the expansion of the cloud chamber to produce recorded photos when the signals from the two counting tubes are combined. Kate uses this automated technology to control cloud room photography. About 80% of the photos have ray trails. Through analysis of about 7,000 photos, they confirmed the positrons discovered by Anderson a few months ago. The creation and annihilation of positron-positron pairs. In 1933, Brackett transferred to the University of London's Birkbeck College as a professor. There, he continued to study the cosmic rays using the cloud chamber method, and he developed the Big and well-balanced Magnetic field device, and used this device to take a lot of pictures of cosmic ray trails. Bracket won the 1948 Nobel Prize in Physics for his improvements in cloud chamber technology and a series of new discoveries on nuclear physics and cosmic rays. .

In 1952, DA Ulaser of the University of California in the United States invented the bubble chamber directly with a liquid instead of a gas-vapor mixture in the cloud chamber. The emergence of the bubble chamber provides an effective method for detecting high-energy charged particles. Grasse won the 1960 Nobel Prize in Physics. ^[3]

(1) The small portable hybrid cloud chamber is used for observation and sampling of atmospheric ice cores; the development and testing of cloud seeding catalysts, especially when testing the nucleation rate of bomb-borne catalysts, have its unique convenience. It can be set up near the scene for testing. However, due to the volume limitation of small hybrid cloud chambers, the temperature is difficult to strictly control. The supercooled fog in the cloud chambers has a short maintenance time. The through-fog can easily cause the disturbance of gas sample volume, temperature and humidity in the cloud chambers. Coupled with the lack of strict and uniform testing procedures, it will inevitably cause a lot of uncertainty and make the test results differ in magnitude. In the future, in order to make the cloud chamber have better stability and repeatability, improvements should be made in the cooling system of the cloud chamber, the design of the subcooled mist, and the method of receiving ice crystals.

(2) As an excellent artificial precipitation catalyst, it should be cost-effective and easy to use. Generally, the ice-forming threshold and the nucleation rate are used to measure the ice-forming performance of a substance. When selecting a catalyst, it should be noted that the substance used as the catalyst should be rich in resources, cheap, non-toxic and non-corrosive. It should also be noted that the conditions for transportation and storage are not too harsh. The method of generating particles during spreading should be simple and easy, and the incidence of kernels per unit time should be high. ^[4]

A more modern design is the diffusion cloud chamber. In this device, a large temperature difference is maintained between the top and the bottom of the cloud chamber. Dry ice is usually used to cool the bottom of the cloud chamber. The room temperature at the top means that the alcohol in the top felt will It rolls down along the wall of the chamber and mixes with the heavy cold air near the bottom of the chamber, and then suspends there. The cloud chamber is filled with air and alcohol vapor. When the temperature spreads at the bottom, the vapor becomes supersaturated. The low temperature at the bottom means that once the steam drops, it will be excessively cooled, that is, it will be in a steam state at a temperature where it is impossible to generate steam, so the steam is easy to condense into a liquid, and a little cosmic rays will ionize the steam, that is Cosmic rays deprive many gas molecules of electrons and charge the atoms, so the ionized particles attract each other to trigger the condensation process, forming a path of cosmic particles.