Extremely-low-background radioactivity constant-temperature container for rare case detection experiment and assembling and using method of extremely-low-background radioactivity constant-temperature container

Abstract

An ultra-low background radioactivity constant temperature container for a rare example detection experiment comprises an innermost layer container which is made of an organic glass material with ultra-low background radioactivity and is used for containing a detection target substance; the middle-layer container is arranged outside the innermost-layer container in a sleeving manner, is made of a low-background metal material and is used for containing a liquid or gas medium with low-background radioactivity, so that the pressure generated by the detection target substance is transmitted to the middle-layer container, and meanwhile, the middle-layer container serves as a shielding layer to block radioactivity of the outside and the middle-layer container; and the outer-layer container is sleeved outside the middle-layer container, is a vacuum container, and provides a vacuum heat insulation layer so as to keep the low-temperature operation of the middle-layer container and the innermost-layer container. According to the invention, a multi-layer container design (the innermost layer, the middle layer and the outer layer) is adopted to realize extremely low background radioactivity and constant temperature control, and the internal pressure and temperature are accurately controlled to adapt to the detection requirements of rare cases.

Description

Technical Field

[0001] The invention relates to a rare event detector in the field of high energy physics experiments, in particular to a sealed container with extremely low background radioactivity. Background Art

[0002] Rare event particle detection experiments such as dark matter detection and neutrino detection are important research directions in the field of high energy physics. The expected signals are very weak and rare, and even on a 10-year time scale, there is only a signal equivalent to the decay intensity of a single atom. The radioactivity level of the detector itself has a decisive influence on the detection sensitivity and needs to be strictly controlled. And with the deepening of research, the requirements for the background of detector materials are becoming increasingly stringent. Reducing the background radioactivity in detector materials is an eternal pursuit in this field. In extremely deep underground laboratories, the background brought by cosmic rays after being blocked and attenuated by thousands of meters of rocks has been greatly reduced or even ignored. As a result, the background radioactivity brought by the structural materials, functional materials, components, etc. of the particle detector itself accounts for an increasing proportion, which has an important impact on the sensitivity of the experimental device. According to statistical theory, in high energy physics experiments, the sensitivity of particle detectors is proportional to the quality of the detection medium and the square root of the operating time, and inversely proportional to the square root of the background event rate. Reducing the background of rare event particle detectors can reduce the demand for effective operating time of the detector, which is of great significance. Therefore, the measurement, screening, and production of low background materials have always been an important task in rare event experiments.

[0003] Since the detected signal is very weak and rare, the radioactivity level of the detector itself has a decisive influence on the sensitivity of the experiment. With the deepening of research, the requirements for the background radioactivity of the detector material are becoming increasingly stringent. Reducing the background radioactivity in the detector material is the key to improving the sensitivity of the experiment and is also the goal that this field continues to pursue.

[0004] Low-background radioactive (pressure) vessels are essential components of rare event particle detectors. For example, the target material xenon of PandaX, LZ, XENON and other series of dark matter detectors works in gas-liquid two-phase state, with a rated design pressure of about 0.5MPa and a rated operating temperature of about -100℃. Low-background radioactive pressure vessels are needed as structural parts to install detection components such as photomultiplier tubes and time drift chambers and hold target materials. At the same time, they should try to avoid bringing too much radioactive background and prevent the invasion of radioactive gases Kr, Rn and electronegative impurities such as nitrogen, oxygen, carbon and hydrogen in the air. The NEXT experiment plans to use a stainless steel pressure vessel with a design pressure of 1.5MPa to seal the xenon gas, and then line it with 120mm thick oxygen-free copper to shield the radioactivity of the stainless steel. "The EXO-200 detector, part I: detector design and construction PUBLISHED BY IOPPUBLISHING FOR SISSA MEDIALAB" The EXO-200 experiment has an oxygen-free copper double-layer container in the shielding body, which is filled with low-background liquid medium as a shielding material and transmits the pressure load of the inner container to the outer container. Even for pure solid detection target materials, such as crystal detectors such as GERDA and CUORE, vacuum-sealed containers are required to avoid the influence of Kr and Rn in the air. These containers are very heavy and close to the sensitive area of ​​the detector. Their radioactive background accounts for a high proportion in the detector, which is one of the keys to background radioactivity control.

[0005] Patent document CN105047241B discloses a container for transporting radioactive materials, which is a sealed can with a hollow structure. The thickness of the sealed can can be reduced to about 30% of that of a lead can while ensuring the same radiation shielding ability as lead, thereby improving the heat dissipation performance and reducing the weight.

[0006] In the past, stainless steel, pure titanium, oxygen-free copper and other metal materials were generally used to manufacture low-background pressure vessels. In the past, oxygen-free copper had lower background radioactivity than stainless steel and pure titanium. These metal materials can be mass-produced, and the background radioactivity has a certain stability. The technology of manufacturing pressure vessels with stainless steel and pure titanium is very mature. Although there are many technical challenges in using oxygen-free copper as a pressure vessel material, with the development of industrial technology, especially the development and popularization of electron beam welding technology, the design and manufacturing technology of oxygen-free copper pressure vessels has also tended to mature. Non-pressure vessels can be made of organic glass. For example, the JUNO Jiangmen neutrino experiment plans to use an organic glass ball with a diameter of 35m to hold 20,000 tons of liquid scintillator, and the DEAP3600 experiment uses an organic glass spherical container with a diameter of 1700mm to hold 3500kg of liquid argon. However, the background radioactivity of these metal materials such as stainless steel, pure titanium, and oxygen-free copper is a thousand times or more than that of organic glass. There is no feasible technical solution to further reduce the background radioactivity of the above metal materials to the level of organic glass. However, organic glass is a brittle material, and there are great technical and safety risks in using it to manufacture pressure vessels. The requirements for the background radioactivity of rare event detector containers are endless. It is necessary to comprehensively consider the mechanical properties, manufacturing process, etc. of the above-mentioned low background radioactivity materials, and make the best use of their strengths and avoid their weaknesses to provide an environment with lower background radioactivity for rare event experiments. Summary of the invention

[0007] In view of the increasingly stringent background radioactivity requirements of rare event detection experiments, the present invention proposes a three-layer constant temperature container that can maintain low background radioactivity while meeting pressure and safety requirements.

[0008] The technical solution of the present invention is as follows:

[0009] First, the present invention provides an extremely low background radioactive constant temperature container for rare event detection experiments, which is characterized by comprising:

[0010] The innermost container is made of organic glass material with extremely low background radioactivity and is used to hold the detection target material;

[0011] The middle container is mounted outside the innermost container and is made of a low-background metal material. It is used to contain a liquid or gas medium with low background radioactivity, so as to transfer the pressure generated by the detection target material to the middle container. At the same time, it serves as a shielding layer to block the radioactivity of the outside world and the middle container itself.

[0012] Outer container: It is a vacuum container installed outside the middle container and provides a vacuum insulation layer to keep the middle container and the innermost container running at low temperature.

[0013] Preferably, the innermost container is composed of an upper half and a lower half, wherein the upper half includes an organic glass upper cover, an organic glass pipe and an organic glass pipe flange connected in sequence from bottom to top; and the lower half includes an organic glass lower flange, an organic glass cylinder and an organic glass bottom plate connected in sequence from top to bottom; wherein the organic glass upper cover is connected to the organic glass lower flange.

[0014] Preferably, the sealing between the upper part of the organic glass container and the lower part of the organic glass container is achieved by bulk polymerization or epoxy resin bonding to achieve a helium mass spectrometer leak detection level, and the leakage rate is less than 1×

[0015] 10 -12 Pa·m 3 / s.

[0016] Preferably, when the innermost container is in operation, the internal pressure is 0 to 0.1 MPa lower than that of the middle container.

[0017] Preferably, the inner diameter of the organic glass pipe is 50 to 300 mm, and the length is between 500 mm and 1000 mm, and is used to provide thermal resistance to separate the normal temperature area and the low temperature area.

[0018] Preferably, the plexiglass bottom plate is placed on an plexiglass support frame; the plexiglass support frame has 4 cantilevers, each cantilever has a through hole at the end, and is suspended on a hook inside the upper flange of the pressure vessel by a low-background suspension rod with a diameter of about 10 mm; the upper head of the pressure vessel has a connecting flange, which corresponds to the number and position of the connecting flange of the plexiglass container; the connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, and the hook together constitute the upper cover of the middle container.

[0019] Preferably, the time drift chamber of the detector, the photomultiplier tube and the temperature control ring are installed inside the organic glass cylinder. The temperature control ring is a V-shaped integral structure, made of low-background oxygen-free copper, and two are arranged up and down in the organic glass container assembly. The outer diameter is slightly smaller than the inner diameter of the organic glass container assembly. It is a V-shaped integral structure. The liquid xenon pipeline with an inner diameter of about 10mm directly passes through the temperature control ring from the outside, and the distance from the V-shaped bottom is 10mm to 25mm to ensure that the low-temperature liquid xenon drips into the temperature control ring. Otherwise, the low-temperature liquid xenon directly drips on the room-temperature organic glass, which will cause excessive thermal stress to damage the organic glass structure. The temperature sensor is arranged 10mm from the edge of the temperature control ring, about 4 to 6 are evenly distributed, and one temperature sensor is as close to the liquid xenon pipeline as possible. After the liquid xenon drips into the temperature control ring, if the temperature of the temperature control ring is high, the liquid xenon will directly vaporize and cool the temperature control ring at the same time. The vaporized xenon has a low temperature and will cool the nearby structures. At the same time, the temperature control ring itself is a good thermal conductive structure, its own temperature tends to be uniform, and it also has a large contact area with the gas xenon. The relatively high temperature structure is lower than the temperature control ring, which will cause convection heat transfer, which can effectively and gently cool the organic glass container assembly and other internal structures. A thermal insulation pad made of tough organic material with a thickness of 10mm to 20mm is set under the temperature control ring. Optional materials include polyethylene and polytetrafluoroethylene with low radioactive background, etc., to avoid direct contact between the temperature control ring and the organic glass structure to avoid excessive thermal stress.

[0020] Preferably, the design pressure range of the middle container is -0.1MPa to 1MPa, corresponding to the vacuum working condition before the detection medium is injected and the rated working condition of the detector working normally, and the temperature range is -196°C to 60°C.

[0021] Preferably, the pressure of the outer container is -0.1 MPa to -0.2 MPa.

[0022] Second, the present invention also provides a method for assembling an extremely low background radioactive constant temperature container for rare event detection experiments, which is characterized by comprising:

[0023] Step 1. Sealing connection between the first bellows 206 and the organic glass flange:

[0024] Step 2. After the time drift chamber, photomultiplier tube and other detector components are loaded, the organic glass upper cover 102 and the organic glass lower flange are sealed and connected by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper half and the lower half;

[0025] Step 3. Place the plexiglass support frame on the middle container and cover it to provide support for the innermost container;

[0026] Step 4. The connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, and the hook together form the upper cover of the middle container;

[0027] Step 5: Lower the height of the upper cover of the middle container so that the first bellows extends more than 10 cm outside the connecting flange. The knife-edge flange of the first bellows is connected to the adapter flange, and the adapter flange is connected to the connecting flange, using an indium wire sealing design.

[0028] Step 6. Lift the upper cover of the layer container and the innermost layer container, and slowly place them into the lower cylinder of the middle layer container;

[0029] Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes.

[0030] Thirdly, the present invention also provides a method for using an extremely low background radioactive constant temperature container for rare event detection experiments, which is characterized in that it includes:

[0031] Step 1. Connect the first bellows to the sealing of the plexiglass flange:

[0032] Step 2. After the time drift chamber, photomultiplier tube and other detection components are loaded, the organic glass upper cover and the organic glass lower flange are sealed and connected by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper half and the lower half;

[0033] Step 3. Place the plexiglass support frame on the middle container and cover it to provide support for the innermost container;

[0034] Step 4. The connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, and the hook together form the upper cover of the middle container;

[0035] Step 5: Lower the height of the upper cover of the middle container so that the first bellows extends more than 10 cm outside the connecting flange. The knife-edge flange of the first bellows is connected to the adapter flange, and the adapter flange is connected to the connecting flange, using an indium wire sealing design.

[0036] Step 6. Lift the upper cover of the layer container and the innermost layer container, and slowly place them into the lower cylinder of the middle layer container;

[0037] Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes;

[0038] Step 8, vacuuming;

[0039] Step 9, precooling and injecting a medium such as liquid scintillator, gaseous xenon or liquid xenon;

[0040] Step 10, the detector is running;

[0041] Step 11, recycle the media, and complete the run.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1) The radioactivity of the detector container itself is greatly reduced, which significantly improves the sensitivity of the detector.

[0044] 2) The present invention adopts a multi-layer container design (innermost layer, middle layer, outer layer) to achieve extremely low background radioactivity and constant temperature control, and precisely controls the internal pressure and temperature to meet the needs of rare event detection.

[0045] 3) The present invention adopts a specific sealing method and support structure to achieve a helium mass spectrometer leak detection level seal BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 Very low background radioactivity sealed container

[0047] Figure 2 Organic glass container assembly 100

[0048] Figure 3 Cooling ring

[0049] Figure 4 Schematic diagram of cooling ring position

[0050] Figure 5 Assembly process 2

[0051] Figure 6 Organic glass container assembly 100 flange seal diagram

[0052] Figure 7 Assembly process diagram Figure 1

[0053] Figure 8 Assembly process 2

[0054] Fig. 9 Assembly process three

[0055] Fig.10 Assembly process four

[0056] In the figure: an organic glass container assembly 100, an organic glass container upper part 101, an organic glass container upper cover 102, an organic glass pipe 103, an organic glass pipe flange 104, an organic glass container lower part 110, an organic glass lower flange 111, an organic glass cylinder 112, an organic glass bottom plate 113, an organic glass support frame 114,

[0057] Pressure vessel 200,

[0058] Vacuum container 300. DETAILED DESCRIPTION

[0059] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. The technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the invention and the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0060] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. The experimental methods without specific conditions in the following examples are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. Unless otherwise specified, all percentages, ratios, proportions or parts are by weight.

[0061] An extremely low background radioactive container, particularly suitable for rare event detectors. The container includes, from inside to outside:

[0062] The innermost container is made of organic glass with extremely low background radioactivity to ensure the sealing of the target material; the middle container is made of low background metals such as stainless steel, pure titanium, and oxygen-free copper. It is a vacuum container designed and manufactured according to the standards of pressure vessels and needs to comply with the requirements of the national standard pressure vessel GB150 and the safety technical supervision regulations for fixed pressure vessels TSG21. For use outside China, it can also be designed and manufactured according to the US ASME standard, the German TUV standard or other pressure vessel standards to ensure reliability and impact resistance. The outer container is made of low background metals such as stainless steel, pure titanium, and oxygen-free copper. It is a vacuum container that provides a vacuum layer for insulation for the low-temperature detector.

[0063] There is a low-background radioactive liquid or gas medium between the middle container and the inner container, which is used to transmit pressure and reduce the actual working pressure of the inner organic glass container.

[0064] The key of the present invention is the following 4 points

[0065] a) Low temperature sealing of organic glass containers;

[0066] b) The pressure difference of the fluid medium inside and outside the organic glass container must be controlled within a limited range;

[0067] c) The thermal stress of the organic glass container should be controlled within a limited range during the cooling and heating process;

[0068] d) High recovery rate of expensive detection media.

[0069] The difficulty of design and manufacturing can be reduced by taking the middle metal pressure vessel and the inner organic glass container into consideration and jointly bearing the working pressure of the detector target material. The middle metal container can contain liquid scintillator or other fluid medium with low background radioactivity or part of solid medium as shielding material, which can block the radioactivity of the shielding container itself and the outside world. The middle metal pressure vessel can share the working pressure of the target material in the organic glass container transmitted through the fluid medium. The control system can control the fluid medium inside and outside the organic glass container to operate within the design pressure range, which can reduce the design requirements of the organic glass container and improve the safety and reliability of the organic glass container.

[0070] The middle metal pressure vessel only bears internal pressure, and the operating pressure P s The inner organic glass container may be subjected to both internal pressure and external pressure during operation, and its design pressure is -P D1 ~P D2 (P D1 ≥0), the operating pressure of the target material in the organic glass container P W , the error range of the pressure control systems of the two cavities is ±δP.

[0071] Theoretically, the following relationship can be obtained:

[0072] P D1 ≥P s -P W +2δP

[0073] P D2 ≥P W -P s +2δP

[0074] Simplifying, we get:

[0075] P D1 +P D2 ≥4δP

[0076] In order to reduce the manufacturing difficulty of organic glass containers and reduce weight, the operating pressure P of the middle metal pressure vessel can be increased. s method to achieve this, but P W -P s When it approaches 0, considering the fluid pressure control error, P D1 , P D2 The sum cannot be too small, and then the rated working pressure is determined based on the accuracy of the control system and the properties of the plexiglass container.

[0077] Since the background radioactivity of plexiglass is extremely low, the weight of the plexiglass container itself is not a factor that needs to be strictly controlled. Considering the manufacturing feasibility, it is more reasonable to have a wall thickness of 50mm for cylinders and container structures with a diameter of more than 2m. This is different from metal containers. Metal materials generally have a larger yield strength and allowable stress. The 6mm wall thickness of the pressure vessels of previous PandaX detectors was determined by the external pressure stability, that is, the external air pressure of 0.1MPa during the vacuum process. The strength of metal materials is far more than that. The compressive yield strength of plexiglass is greater than the tensile yield strength, so plexiglass containers are more suitable for working under external pressure. After the structural form and wall thickness of the plexiglass container are determined, the maximum allowable external and internal pressures can be calculated.

[0078] The working pressure range of organic glass container is P D1 +P D2 In order to ensure safety, it is necessary to set a safety factor, which is proportional to the accuracy δP of the control system:

[0079] P D1 +P D2 =n×δP, n>4

[0080] Where n is the safety factor, and its size is closely related to the reliability of the control system. A reliable control system can select a smaller safety factor, and a precise control system can ensure a smaller pressure fluctuation range δP.

[0081] According to the above analysis, the core problem of the rare event detector extremely low background container is the structural design of the organic glass container, which exerts the ultimate mechanical properties of the material to control the weight. The key parameters include the working pressure range of the organic glass container is P D1 +P D2 , control system error ±δP. These parameters are strongly correlated. The present invention can transfer the difficulty of designing and manufacturing the extremely low radioactive background organic glass container to the control system. The existing control technology is relatively mature, and it does not need to control the radioactivity when placed outside the detector shielding body, so it is easy to realize the design function.

[0082] Another key point of the present invention is the sealing of the organic glass container. The organic glass container has multiple flanges, one type is a larger diameter equipment flange, and the other type is a smaller diameter pipe flange. In order to improve the detection efficiency of rare event detectors, PandaX, LZ, XENON, etc. all have larger time drift chambers, whose diameters are slightly smaller than the required container diameter, so the detector container must have a large diameter equipment flange. The inner diameters of the PandaX detector flanges of successive generations are 750mm, 800mm, and 1335mm respectively. The long-term planned detector diameters are 2000mm, 2500mm, etc., and this equipment flange needs to be opened several times during the service of the rare event detector, about 10 times. The operating temperature of these detectors is about -100°C, which exceeds the operating temperature range of existing rubber seals. Considering the high purity requirements of the target material, metal seals are generally used, such as indium wire, or outer silver spring energy storage seals. However, metal seals require a large preload, that is, the flange is subjected to a large local stress, which is a harsh working condition for organic glass flanges and difficult to achieve. The present invention proposes a method combining gluing and mechanical cutting to achieve reliable sealing of the flange of the organic glass device, and removes the bonding structure by mechanical cutting to facilitate maintenance of the time drift chamber of the rare event detector, etc. These processes will not contaminate the internal time drift chamber.

[0083] The sealing surface of the equipment flange has a 5mm chamfer. After the time drift chamber is installed inside the organic glass container, a space with an upward opening is formed outside the equipment flange with a film material, a foil material or a mold. Epoxy resin is injected into this space. After the epoxy resin is cured, the two equipment flanges are bonded to complete the sealing. When it is necessary to open the organic glass container to maintain the time drift chamber, the epoxy resin is cut off and cleaned. After cutting, the organic glass container can be opened.

[0084] The diameter of the take-over flange is relatively small, and it is mainly used for the in and out of the target material fluid and the passage of optical fiber and cable. The external equipment corresponding to the design function of these take-over flanges are all metal flanges or metal joints, which need to complete the transition from organic glass to metal parts, and ensure the sealing reliability. The solution of the present invention is to extend the length of the organic glass take-over, use radiation heating or conduction heating to ensure that the organic glass take-over flange is at room temperature, the organic glass flange is connected to the metal transition flange of the same caliber stainless steel, titanium, nickel, etc., and low melting point metal gallium or low melting point alloy is used as a sealing material, and the sealing of the organic glass flange and the metal transition flange can also be achieved by epoxy resin bonding to achieve high vacuum sealing. The connection between the organic glass flange and the metal transition flange is permanent. Once connected and the sealing is confirmed by a helium mass spectrometer leak detector until the equipment is retired, it will not be disassembled. A detachable metal flange is welded on the other side of the metal transition flange, which is convenient for the detector to be disassembled and assembled multiple times during use. The contact force between the small diameter fluid pipeline for fluid in and out and the organic glass take-over is very small, and the thermal resistance is very large, which will not bring a lot of cooling load to the organic glass take-over flange. In order to further reduce the heat transfer between the fluid pipeline and the organic glass pipe flange, a polytetrafluoroethylene (PTFE) support ring can be added outside the fluid pipeline.

[0085] The thickness of the organic glass container is much greater than that of metal materials such as stainless steel, pure titanium, and oxygen-free copper, and the thermal conductivity and thermal diffusion coefficient are much lower than those of metal. Therefore, during the startup phase of the use process of the present invention, it is necessary to carefully consider and formulate a cooling plan to control the maximum temperature difference and the cooling rate to avoid large thermal stress in the organic glass material.

[0086] The present invention takes the next generation 20-ton PandaX liquid xenon detector as an example to illustrate the design, manufacturing and use methods of the extremely low background radioactive sealed container for rare event detection experiments. The innermost detector time drift chamber has a diameter of about 2m and a height of 2m.

[0087] The extremely low background radioactive sealed container has a three-layer structure, which includes, from the inside to the outside, a plexiglass container assembly 100, a pressure container 200, and a vacuum container 300. Figure 1As shown. The time drift chamber and photomultiplier tube of the detector are installed in the organic glass container assembly 100, and the high-purity target material is contained; the space between the pressure vessel 200 and the organic glass container assembly 100 contains liquid scintillator or other solid low-background shielding materials. The pressure difference between the inside and outside of the organic glass container assembly 100 is extremely small, with only liquid static pressure, and all of these work in a low-temperature state. The design pressure of the pressure vessel 200 is -0.1MPa to 1MPa, which corresponds to the vacuum working condition before the detection medium is injected and the rated working condition of the detector working normally, and the design temperature is -196℃ to 60℃. The vacuum container 300 provides a vacuum environment for the pressure vessel 200 to reduce the thermal load of the system. The external medium can be air or high-purity water as a shielding material. The design pressure is -0.1MPa. If used in a pure water shielding body, the design pressure is -0.2MPa according to the static pressure exerted by the water.

[0088] First is the innermost organic glass container assembly 100, which is divided into an upper part 101 of the organic glass container assembly and a lower part 110 of the organic glass container assembly. The upper part 101 of the organic glass container includes an organic glass upper cover 102, an organic glass pipe 103, and an organic glass pipe flange 104; the lower part 110 of the organic glass container assembly includes an organic glass lower flange 111, an organic glass cylinder 112, and an organic glass bottom plate 113. The organic glass pipe 103 has an inner diameter of about 150 mm and a length of 500 mm to 1000 mm, providing a large thermal resistance to ensure that the organic glass upper cover 102 and the lower part 110 of the organic glass container assembly operate at low temperatures while the organic glass pipe flange 104 operates at room temperature without causing large heat leakage. The organic glass container assembly 100 is internally installed with a time drift chamber for detecting rare events, a photomultiplier tube, a target material injected during operation, and a temperature control ring 131. The structure of the temperature control ring is shown in the figure. Two temperature control rings are arranged in the upper and lower parts of the organic glass container assembly 100. The outer diameter is slightly smaller than the inner diameter of the organic glass container assembly 100. It is a V-shaped overall structure, made of low-background oxygen-free copper, with a thickness of about 3mm to 5mm, and each side is about 50mm long. There is a liquid xenon pipe 130 with an inner diameter of about 10mm directly connected to the temperature control ring 131 from the outside, and the distance from the V-shaped bottom is 10mm to 25mm to ensure that the low-temperature liquid xenon drips into the temperature control ring 131. Otherwise, the low-temperature liquid xenon directly drips on the room-temperature organic glass, which will cause excessive thermal stress and damage the organic glass structure. The temperature sensor 131 is arranged 10mm from the edge of the temperature control ring 131, about 4 to 6 are evenly distributed, and one temperature sensor is as close to the liquid xenon pipe 130 as possible. After the liquid xenon drips into the temperature control ring 131, if the temperature of the temperature control ring is high, the liquid xenon will directly vaporize and cool the temperature control ring 131 at the same time. The vaporized xenon has a low temperature and will cool the nearby structures. At the same time, the temperature control ring 131 itself is a good heat-conducting structure, and its own temperature tends to be uniform. It also has a large contact area with the gas xenon. The structure with a relatively high temperature is lower than the temperature control ring 131, which will cause convection heat transfer, and can effectively and gently cool the organic glass container assembly 100 and other internal structures. A heat-insulating pad 132 made of tough organic material with a thickness of 10mm to 20mm is set under the temperature control ring 131. The optional materials include polyethylene and polytetrafluoroethylene with low radioactive background, etc., to avoid direct contact between the temperature control ring 131 and the organic glass structure to avoid causing excessive thermal stress.

[0089] The target material used in the PandaX experiment is liquid xenon, which has a density about 3 to 4 times that of the external liquid scintillator. Calculated based on the 3m depth of the internal liquid xenon, the pressure difference caused by the difference in medium density on both sides of the bottom of the lower half 110 of the plexiglass container assembly under normal working conditions is about 4 to 10×104Pa, so a larger thickness is required. Fortunately, the background radioactivity of plexiglass is extremely low and its density is slightly greater than that of liquid scintillator, and its shielding effect is better than that of liquid scintillator, so it is not necessary to strictly limit the thickness of the plexiglass structure. However, it should be noted that during the cooling and heating process, the liquid scintillator will have convection and the temperature gradient is small, while plexiglass is solid and heat transfer depends only on heat conduction, but its thermal conductivity is limited, and there is a risk of temperature gradient and excessive thermal stress. This issue will be discussed in detail in the thermal management solution section.

[0090] The assembly of the organic glass container assembly 100 requires first heating the first bellows 206 and the organic glass flange 104 to 40-50°C, aligning them, injecting gallium or other low melting point alloys at the same temperature into the gap to seal, and then tightening them with screws, waiting for natural cooling to achieve sealing. In order to improve reliability, the opening of the organic glass pipe 103 at the organic glass upper cover 102 can be temporarily sealed with a blind plate, vacuum seal, silicone pad, etc., and the connection between the first bellows 206 and the organic glass flange 104 is subjected to helium mass spectrometry vacuum leak detection to confirm the sealing performance. If a leak is found, slowly heat it again to 40-50°C, open it, and then reconnect it. After confirming the sealing, the first bellows 206 and the organic glass flange will not be opened during the entire service period.

[0091] The connection between the first bellows 206 and the organic glass flange 104 can also be sealed by epoxy resin bonding, and after the helium mass spectrometer vacuum leak detection is carried out to confirm the sealing performance, it will not be opened again during the entire service life.

[0092] After the components inside the organic glass container assembly 100 are installed in place, it is necessary to seal the organic glass upper cover 102 with the organic glass lower flange 111. The bolt connection force required by the previous indium wire sealing, polytetrafluoroethylene gasket sealing, etc. is very large, that is, there is a large stress in the flange, and the allowable stress of organic glass is relatively small, so we adopt the method of bulk polymerization or epoxy resin bonding to achieve sealing. Because when the organic glass container assembly 100 is running, the internal and external pressure difference of this sealing place is very small, and the external pressure is slightly greater than the internal pressure, so the sealing material only needs to ensure the sealing function, and does not need to bear a large structural stress, which reduces the technical difficulty of structural design and manufacturing. When it is necessary to open the organic glass container assembly 100 to repair and maintain the internal components, the sealing material can be machined off with a tool such as a grinding wheel or a milling cutter. The annular tooling is designed to make the tool perform annular motion to ensure a high processing accuracy. When removing the sealing material, the removal of the organic glass background material is less, which can be controlled below 0.5mm. When the organic glass container assembly 100 is designed and manufactured, this margin is taken into account and can be used multiple times.

[0093] like Figure 4 As shown, the plexiglass upper cover 102 and the plexiglass lower flange 111 are respectively processed with 5mm chamfers in the initial state. After the two are aligned and clamped with a C-ring, a flexible plastic template 121 is used to surround the joint between the two and inject the plexiglass body polymer material or epoxy resin into the chamfered space to achieve sealing, and then the excess sealing material is ground off and polished. It should be noted that this sealing structure cannot withstand external forces. When the plexiglass container assembly 100 needs to be moved or lifted, it is necessary to pass through the plexiglass support frame 114. The plexiglass support frame 114 is placed on the head of the pressure vessel 200 to provide support for the plexiglass container assembly 100, especially when there is a large pressure difference between the inside and outside of the plexiglass bottom plate 113 in the working state, the plexiglass support frame 114 can improve the stress state of the plexiglass bottom plate 113.

[0094] The organic glass support frame 114 has 4 cantilevers, each with a through hole of 12 mm in diameter at the end of the cantilever, and is suspended on the hook 204 inside the upper flange 203 of the pressure vessel by a low-background suspension rod 205 with a diameter of about 10 mm. The suspension rod can be made of stainless steel or pure titanium. The upper head 202 of the pressure vessel has a connecting flange 201, which can also be designed to be multiple as needed, corresponding to the number and position of the connecting flange 104 of the organic glass container. The connecting flange 201, the upper head 202 of the pressure vessel, the upper flange 203 of the pressure vessel, and the hook 204 together constitute the upper cover 210 of the pressure vessel. Figure 6 shown.

[0095] Then lower the height of the upper cover 210 of the pressure vessel, the first bellows 206 extends out of the pipe flange 201 by more than 10 cm, the knife-edge flange of the first bellows 206 is connected to the adapter flange 207 of the pressure vessel 200, the lower side of the adapter flange 207 has a corresponding knife-edge, which is sealed with a copper gasket, and finally the adapter flange 207 is connected to the pipe flange 201, and an indium wire sealing design is adopted. The structure composed of the upper cover 210 of the pressure vessel and the organic glass container assembly 100 is lifted, and slowly placed into the lower cylinder of the pressure vessel 200, and the flange is locked after laying the indium wire for sealing. Among them, the interface 208 has a small diameter, within the range of 10mm to 50mm, and the number is 4 to 6. The corresponding pipeline is made of stainless steel, which brings a small radioactive background, has greater flexibility, and is easy to assemble. Its main function is to transport liquid scintillator and other working gases, including vacuuming, which is used for the cooling process of the rare event detector startup stage, liquid scintillator filling and circulation, or pressure balance between the xenon gas space and the liquid scintillator space.

[0096] After the above assembly is completed, the pressure vessel upper head 202, organic glass container 100, organic glass support frame 114, low background suspension rod 205 and other components are placed in the lower part 206 of the pressure vessel. After being placed in place, ensure that the suspension rod 205 is in a slightly compressed state, tighten the bolts to achieve sealing, and then assemble the second bellows 207 and the connecting flange 201, and seal with a copper gasket. The completed assembly is as follows: Figure 7 shown.

[0097] Finally, the assembled pressure vessel 200 and the second bellows 207 are placed in the vacuum vessel 300 as a whole, and the second bellows 207 is connected to the vacuum flange 301. A small diameter pipe is used to connect the connecting pipe 308 and the interface 208 on the vacuum flange 301. The small diameter pipe needs to be designed with a bending structure for thermal compensation or other buffering. Figure 8 shown.

[0098] How to use

[0099] Since the organic glass container 100 cannot withstand a large internal and external pressure difference, the startup, operation, and shutdown of the system need to carefully consider the safety of various working conditions such as pressure and temperature changes during the transition process. The use process is divided into the following steps: vacuuming, medium injection and thermal management, operation, shutdown, etc.

[0100] Vacuum

[0101] The target material of the detector is high-purity xenon, and air pollution needs to be avoided. Before injecting high-purity xenon into the detector, the inside of the organic glass container 100 needs to be evacuated to 1×10 -4Pa, in order to ensure the pressure balance inside and outside the organic glass container 100, it is necessary to evacuate the inside of the pressure container 200 synchronously. In order to ensure reliability, it is necessary to connect the two chambers and use the same vacuum pump group to evacuate, and at the same time control the evacuation rate of the vacuum pump group when the air pressure is higher than 1000Pa. Since the space between the organic glass container 100 and the pressure container 200 is filled with liquid scintillator, there is no high vacuum requirement. Therefore, when the vacuum degree is below 10Pa and the molecular pump start condition is reached, the valves of the pressure container 200 and the vacuum pump group are closed and the molecular pump is started. The molecular pump only evacuates the inside of the organic glass container 100 until 1×10 -4 Pa, and maintain for a certain period of time.

[0102] Initial injection of medium

[0103] Since the space inside and outside the organic glass container assembly 100 requires different media, this step is more difficult than vacuuming. The injection process requires more patience and safety margin, and the pressure difference between the inside and outside is strictly controlled. Buffer containers are set in the injection pipeline, with a volume of 10L~50L, and valves are set at the front and back. For the internal space, first ensure that the rear valve is closed, inject xenon into the buffer container to 0.1~0.2MPa, then close the front valve, open the rear valve, and release xenon into the organic glass container 100. At the same time, for the external space, a similar method is used to first fill the buffer container with high-purity nitrogen or argon, and then inject it into the space inside the pressure container 200 outside the organic glass container assembly 100. Repeat this operation many times, always pay attention to the pressure difference on both sides should not exceed 10kPa, when there is a significant difference in the pressure difference on both sides, fill the low-pressure side several times until the pressure difference is far away from 10kPa. After the absolute pressure of the space on both sides reaches 0.2MPa, suspend the medium injection and enter the cooling stage.

[0104] Precooling

[0105] The plexiglass container assembly 100 has a diameter of 2000 mm, a height of 3000 mm, a wall thickness of 50 mm, and a surface area of ​​approximately 25 m 2 The thermal conductivity of organic glass is low, only at the order of 0.1W / (m·K), and the allowable stress is about 3MPa. In order to avoid excessive thermal stress caused by excessive temperature gradient during the cooling process, the cooling rate needs to be strictly controlled. The organic glass container assembly 100 adopts a double-sided cooling strategy. According to calculations, the cooling rate is controlled at 10K / day, i.e., 1×10 -4K / day can control the maximum temperature difference of each part of the organic glass container to be less than 1K, and the required cooling power is about 100W. Calculated based on the room temperature of 298K, it takes 12 days to cool down to the rated operating temperature of 178K of the PandaX detector. During this time, the liquefaction of 20 tons of gaseous xenon can be completed simultaneously, and the filling of liquid xenon can be completed within 1 day after the cooling is completed. Compared with the pre-cooling and filling time of about 1 week for previous generations of PandaX detectors, two weeks is acceptable.

[0106] Inside the organic glass container assembly 100, liquid xenon is injected into the cooling ring 131 through the liquid xenon pipe 130. The vaporization heat of xenon is about 100 J / g, and the injection flow rate of liquid xenon is about 1 g / s in the initial stage. First, the liquid xenon cools the liquid xenon pipe 130 and vaporizes itself. Low-temperature xenon gas is input into the organic glass container assembly 100. As the cooling proceeds, the temperature of the gaseous xenon output from the liquid xenon pipe 130 gradually decreases and finally liquid xenon is output, dripping on the cooling ring 131. Because the temperature of the cooling ring 131 is relatively high, the liquid xenon immediately vaporizes into xenon gas with a temperature lower than the surrounding environment and cools the cooling ring 131 at the same time. After heat exchange with the detector assembly, a cooling effect is achieved. Since the cooling ring 131 is made of oxygen-free copper with high strength and good toughness, the thermal stress caused by the dripping of liquid xenon will not exceed the tensile strength of oxygen-free copper, and it can operate safely for a long time. The temperature of the cooling ring 131 is lower than that of the surrounding environment. The oxygen-free copper used has a high thermal conductivity of 400W / (m·K). The thermal conductivity coefficient will quickly tend to be uniform. The heat exchange area on the lower side is about 0.6m 2 The natural convection heat transfer coefficient of the gas surface is about 100W / (m 2 ·K), which can effectively exchange heat with the surrounding xenon gas and cause xenon convection, thereby achieving uniform and gentle cooling of the organic glass and other components of the detector. A part of the low-temperature xenon gas vaporized on the upper side of the cooling ring 131 directly overflows the cooling ring 131, and a part flows inside the cooling ring 131 to other parts, thereby accelerating the temperature uniformization of the cooling ring 131 itself. In short, the cooling ring 131 made of oxygen-free copper material can directly use liquid xenon to cool the organic glass assembly 100 and other detector components, and can ensure that no large thermal stress occurs during the cooling process. During the cooling process, the internal pressure will decrease as the temperature decreases. When the pressure is lower than the rated value, xenon gas needs to be supplemented to maintain pressure stability and balance with the pressure in the liquid scintillator space.

[0107] For the pressure vessel 200, the welding of the main flange, the head and the cylinder is double welded, and a fluid interlayer is designed between the double welds, with a cross-sectional area of ​​not less than 3cm 2, ethanol coolant can be circulated to cool the pressure vessel 200. The low-temperature refrigerator has a refrigeration power of 2kw, and the lowest temperature of the output ethanol can reach -110℃, so the pressure vessel 200 can be cooled to -95℃. The thickness of the main flange is about 150mm, the width is about 80mm, and the maximum distance from the fluid interlayer is no more than 200mm. According to heat transfer theory, it can be calculated that the temperature of each part of the main flange tends to be consistent with the temperature of the ethanol coolant on a time scale of 1h. After the main flange is cooled, the internal nitrogen and argon will be cooled, and the surface heat transfer coefficient of natural convection between gas and solid is about 10W / (m 2 ·K), the nitrogen (or argon) flows downward after cooling, gradually cooling the structure at a geometric position lower than the main flange. Since the main flange of the pressure vessel 200 is more than 500mm away from the organic glass component 100, and the surface heat transfer coefficient of the gas-solid interface is low, the process of gas cooling is relatively gentle. In order to more effectively control the cooling process, temperature sensors are set at the upper, middle and lower parts of the organic glass component 100, 1cm away from the organic glass, and the temperature of each point is cooled at a rate of 10K / day

[0108] Medium injection

[0109] After the cooling process is completed, the medium injection can be started. The liquid scintillator needs to be precooled to about 178K. During the injection process, it is necessary to consider that the buoyancy of the liquid scintillator on the organic glass component 100 should be less than the weight of the liquid xenon to avoid excessive buoyancy causing damage to the thermostat. At the same time, it is necessary to pay attention to the pressure difference on both sides not exceeding the design value. After the medium injection is completed, the detector can enter the normal operation stage.

[0110] Media Recovery

[0111] After the detector is finished running, the medium is recovered. It is also necessary to pay attention to the buoyancy of the liquid scintillator, and the recovery rate needs to be greater than the recovery rate of liquid xenon. The recovery process also needs to pay attention to the pressure difference on both sides within the design range. In the process of liquid scintillator recovery, a part of nitrogen or argon needs to be filled for replacement. After all the liquids are recovered, xenon is expensive and needs to be recovered in full. In contrast to the process of initial medium injection, the internal space of the organic glass component 100 is connected to the low-temperature recovery device through a gas pipeline, and the external space is connected to the vacuum pump through a pipeline. After opening the valve, it is necessary to pay attention to the pressure difference on both sides not exceeding the set range until the internal air pressure of the organic glass component 100 drops below 1Pa, that is, the internal xenon can be ignored. The recovery is completed. Then the organic glass container assembly 100, the pressure vessel 200, and the vacuum container 300 are injected with nitrogen or argon again to 0.1MPa, and then the system is naturally restored to room temperature. Each container can be opened to complete the decommissioning work or maintenance work. After the maintenance work is completed, the aforementioned assembly, vacuuming, initial medium injection, precooling, medium injection, detector operation and other work need to be repeated.

Claims

1. A very low background radioactive constant temperature container for rare event detection experiments, characterized in that: include: The innermost container is made of organic glass material with extremely low background radioactivity and is used to hold the detection target material; The middle container is mounted outside the innermost container and is made of a low-background metal material. It is used to contain a liquid or gas medium with low background radioactivity, so as to transfer the pressure generated by the detection target material to the middle container. At the same time, it serves as a shielding layer to block the radioactivity of the outside world and the middle container itself. Outer container: It is a vacuum container installed outside the middle container and provides a vacuum insulation layer to keep the middle container and the innermost container running at low temperature.

2. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, characterized in that: The innermost container is composed of an upper half and a lower half. The upper half includes an organic glass upper cover, an organic glass pipe and an organic glass pipe flange connected in sequence from bottom to top; the lower half includes an organic glass lower flange, an organic glass cylinder and an organic glass bottom plate connected in sequence from top to bottom; wherein the organic glass upper cover is connected to the organic glass lower flange.

3. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 2, characterized in that: The sealing between the upper part of the organic glass container and the lower part of the organic glass container is achieved by bulk polymerization or epoxy resin bonding to achieve a helium mass spectrometer leak detection level, and the leakage rate is less than 1×10 -12 Pa·m 3 / s.

4. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 2, characterized in that: When the innermost container is in operation, the internal pressure is 0 to 0.1 MPa lower than that of the middle container.

5. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 2, characterized in that: The inner diameter of the organic glass pipe is 50 to 300 mm, and the length is between 500 mm and 1000 mm, and is used to provide thermal resistance to separate the normal temperature area and the low temperature area.

6. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 2, characterized in that: The plexiglass bottom plate is placed on the plexiglass support frame; the plexiglass support frame has 4 cantilevers, each cantilever has a through hole at the end, and is suspended on a hook inside the upper flange of the pressure vessel by a low-background suspension rod with a diameter of about 10 mm; the upper head of the pressure vessel has a connecting flange, which corresponds to the number and position of the connecting flange of the plexiglass container; the connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, and the hook together constitute the upper cover of the middle container.

7. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 2, characterized in that: The time drift chamber of the detector, the photomultiplier tube and the temperature control ring are installed inside the organic glass cylinder. The temperature control ring is a V-shaped integral structure, made of low-background oxygen-free copper. Two are arranged up and down in the organic glass container assembly, and the outer diameter is slightly smaller than the inner diameter of the organic glass container assembly. It is a V-shaped integral structure. The liquid xenon pipeline with an inner diameter of about 10mm directly passes through the temperature control ring from the outside, and the distance from the V-shaped bottom is 10mm to 25mm to ensure that the low-temperature liquid xenon drips into the temperature control ring. Otherwise, the low-temperature liquid xenon directly drips on the room-temperature organic glass, which will cause excessive thermal stress and damage the organic glass structure. The temperature sensor is arranged 10mm from the edge of the temperature control ring, about 4 to 6 are evenly distributed, and one of the temperature sensors is as close to the liquid xenon pipeline as possible. After the liquid xenon drips into the temperature control ring, if the temperature of the temperature control ring is high, the liquid xenon will directly vaporize and cool the temperature control ring at the same time. The vaporized xenon has a low temperature and will cool the nearby structures. At the same time, the temperature control ring itself is a good thermal conductive structure, its own temperature tends to be uniform, and it also has a large contact area with the gas xenon. The relatively high temperature structure is lower than the temperature control ring, which will cause convection heat transfer, which can effectively and gently cool the organic glass container assembly and other internal structures. A thermal insulation pad made of tough organic material with a thickness of 10mm to 20mm is set under the temperature control ring. Optional materials include polyethylene and polytetrafluoroethylene with low radioactive background, etc., to avoid direct contact between the temperature control ring and the organic glass structure to avoid excessive thermal stress.

8. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, characterized in that: The design pressure range of the middle container is -0.1MPa to 1MPa, corresponding to the vacuum working condition before the detection medium is injected and the rated working condition of the detector working normally, and the temperature range is -196℃ to 60℃.

9. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, characterized in that: The pressure of the outer container is -0.1 MPa to -0.2 MPa.

10. A method for assembling an extremely low background radioactive isothermal container for rare event detection experiments, characterized in that: include: Step 1. Sealingly connect the first bellows 206 to the organic glass flange; Step 2. After the time drift chamber, photomultiplier tube and other detector components are loaded, the organic glass upper cover 102 and the organic glass lower flange are sealed and connected by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper half and the lower half; Step 3. Place the plexiglass support frame on the middle container and cover it to provide support for the innermost container; Step 4. The connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, and the hook together form the upper cover of the middle container; Step 5: Lower the height of the upper cover of the middle container so that the first bellows extends more than 10 cm outside the connecting flange. The knife-edge flange of the first bellows is connected to the adapter flange, and the adapter flange is connected to the connecting flange, using an indium wire sealing design. Step 6. Lift the upper cover of the layer container and the innermost layer container, and slowly place them into the lower cylinder of the middle layer container; Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes.

11. A method for using an extremely low background radioactive constant temperature container for rare event detection experiments, characterized in that: include: Step 1. Connect the first bellows to the sealing of the plexiglass flange; Step 2. After the time drift chamber, photomultiplier tube and other detection components are loaded, the organic glass upper cover and the organic glass lower flange are sealed and connected by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper half and the lower half; Step 3. Place the plexiglass support frame on the middle container and cover it to provide support for the innermost container; Step 4. The connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, and the hook together form the upper cover of the middle container; Step 5: Lower the height of the upper cover of the middle container so that the first bellows extends more than 10 cm outside the connecting flange. The knife-edge flange of the first bellows is connected to the adapter flange, and the adapter flange is connected to the connecting flange, using an indium wire sealing design. Step 6. Lift the upper cover of the layer container and the innermost layer container, and slowly place them into the lower cylinder of the middle layer container; Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes; Step 8, vacuuming; Step 9, precooling and injecting a medium such as liquid scintillator, gaseous xenon or liquid xenon; Step 10, the detector is running; Step 11, recycle the media, and complete the run.

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