An extremely low background radioactive constant temperature container for rare event detection experiments and its assembly and use method

Abstract

An extremely low-background radioactive constant-temperature container for rare event detection experiments comprises: an innermost container, made of an organic glass material with extremely low background radioactivity, for containing a detection target material; a middle container, mounted outside the innermost container, made of a low-background metal material, for containing a liquid or gas medium with low background radioactivity, thereby transmitting the pressure generated by the detection target material to the middle container and, at the same time, acting as a shielding layer to block the radioactivity of the outside world and the middle container itself; and an outer container, mounted outside the middle container, which is a vacuum container and provides a vacuum insulation layer to maintain low-temperature operation of the middle and innermost containers. The present invention adopts a multi-layer container design (innermost, middle, and outer layers) to achieve extremely low background radioactivity and constant temperature control, and precisely controls internal pressure and temperature to meet the needs of rare event detection.

Description

Technical Field

[0001] The present 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 areas in high-energy physics. The desired signals are extremely weak and rare, sometimes as weak as the decay of a single atom on a 10-year timescale. The detector's inherent radioactivity levels have a crucial influence on detection sensitivity and require strict control. Furthermore, with advancing research, requirements for the background radioactivity of detector materials are becoming increasingly stringent. Reducing this background radioactivity in detector materials is a constant pursuit in this field. In extremely deep underground laboratories, the background radioactivity from cosmic rays attenuated by thousands of meters of rock has been significantly reduced to negligible levels. Consequently, the background radioactivity from the particle detector's structural, functional, and component materials contributes an increasing proportion, significantly impacting the sensitivity of the experimental setup. In high-energy physics experiments, according to statistical theory, particle detector sensitivity is directly 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 radioactivity in rare-event particle detectors can reduce the detector's effective operating time, a crucial task. Therefore, the measurement, screening, and production of low-background materials have always been crucial tasks in rare-event experiments.

[0003] Because the signals detected are extremely weak and rare, the radioactivity level of the detector itself has a decisive influence on the sensitivity of the experiment. As research deepens, the requirements for the background radioactivity of detector materials are becoming increasingly stringent. Reducing background radioactivity in detector materials is key to improving experimental sensitivity and a persistent goal in this field.

[0004] Low-background radioactivity (pressure) vessels are essential components of rare-event particle detectors. For example, the target xenon in dark matter detectors like the PandaX, LZ, and XENON series operates in a gas-liquid two-phase state, with a rated design pressure of approximately 0.5 MPa and a rated operating temperature of approximately -100°C. These vessels serve as structural components for mounting detector components such as photomultiplier tubes and time drift chambers, as well as for containing the target material. These vessels also minimize excessive radioactive background and prevent the intrusion of radioactive gases such as Kr and Rn from the air, as well as electronegative impurities such as nitrogen, oxygen, carbon, and hydrogen. The NEXT experiment plans to use a stainless steel pressure vessel with a design pressure of 1.5 MPa to enclose the xenon gas, lined with a 120 mm thick oxygen-free copper shield against the stainless steel's radioactivity. The EXO-200 detector, part I: detector design and construction, published by IOP, is a double-layered container within the shield. The space between the containers is filled with a low-background liquid medium, which acts as a shielding material and transfers the pressure load of the inner container to the outer container. Even for purely solid-state target materials, such as crystal detectors like GERDA and CUORE, vacuum-sealed containers are required to avoid the effects of Kr and Rn in the air. These containers are very heavy and close to the detector's sensitive area. Their radioactive background contributes significantly to the detector, making them a key factor in background radioactivity control.

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

[0006] In the past, metal materials such as stainless steel, pure titanium, and oxygen-free copper were typically used to manufacture pressure vessels with low background radiation. Oxygen-free copper has historically exhibited lower background radiation than stainless steel and pure titanium. These metal materials can be mass-produced, and their background radiation is relatively stable. The technology for manufacturing pressure vessels using stainless steel and pure titanium is well established. While oxygen-free copper as a pressure vessel material presents significant technical challenges, the development of industrial technology, particularly the advancement and widespread use of electron beam welding, has also matured the design and manufacturing of oxygen-free copper pressure vessels. Non-pressure vessels can be manufactured using organic glass. For example, the JUNO Jiangmen neutrino experiment plans to use a 35-meter-diameter organic glass sphere to hold 20,000 tons of liquid scintillator, and the DEAP3600 experiment will use a 1700-mm-diameter organic glass sphere to hold 3500 kg of liquid argon. However, the background radiation of these metal materials, such as stainless steel, pure titanium, and oxygen-free copper, is thousands or more times that of organic glass. Further reducing the background radiation of these metal materials to the level of organic glass remains a feasible technical solution. However, organic glass is a brittle material, and its use in pressure vessel manufacturing carries significant technical and safety risks. 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, make the best use of their strengths and avoid their weaknesses, and 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 solutions of the present invention are 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 including:

[0010] The innermost container is made of organic glass 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, thereby transmitting the pressure generated by the detection target material to the middle container. At the same time, it acts 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, 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.

[0014] Preferably, the seal 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, with a leakage rate of less than 1×10 -12 Pa·m 3 / s.

[0015] Preferably, when the innermost container is in operation, the internal pressure is 0 to 0.1 MPa lower than that of the middle container.

[0016] 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.

[0017] Preferably, the plexiglass base 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.

[0018] Preferably, the interior of the plexiglass cylinder houses the detector's time drift chamber, photomultiplier tube, and temperature control ring. The temperature control ring is a V-shaped, monolithic structure made of low-background, oxygen-free copper. Two rings are positioned one above the other within the plexiglass container assembly, with an outer diameter slightly smaller than the inner diameter of the plexiglass container assembly. A liquid xenon pipe with an inner diameter of approximately 10 mm extends directly through the temperature control ring, maintaining a distance of 10 to 25 mm from the bottom of the V-shape to ensure that low-temperature liquid xenon drips into the ring. Otherwise, direct dripping of low-temperature liquid xenon onto the room-temperature plexiglass would cause excessive thermal stress and damage the plexiglass structure. Approximately four to six temperature sensors are evenly spaced 10 mm from the edge of the temperature control ring, with one sensor positioned as close to the liquid xenon pipe as possible. When liquid xenon is dripped into the temperature control ring, if the ring is too hot, it vaporizes directly, cooling the ring. The vaporized xenon is cooler, cooling nearby structures. The ring itself is a good thermal conductor, tending to maintain a uniform temperature and providing a large contact area with the xenon gas. Furthermore, the relatively high-temperature structures are lower than the ring, resulting in convective heat transfer, effectively and gently cooling the plexiglass container assembly and other internal structures. A 10mm-20mm thick thermal insulation pad made of a tough organic material, such as polyethylene or polytetrafluoroethylene (PTFE), is placed beneath the ring to prevent direct contact between the ring and the plexiglass structure, which could cause excessive thermal stress.

[0019] Preferably, the design pressure range of the middle container is -0.1MPa to 1MPa, corresponding to the vacuum working condition before the injection of the detection medium and the rated working condition of the detector under normal operation, and the temperature range is -196°C to 60°C.

[0020] Preferably, the pressure of the outer container is -0.1 MPa to -0.2 MPa.

[0021] 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:

[0022] Step 1. Seal the first bellows 206 to the organic glass flange:

[0023] Step 2. After installing the time drift chamber, photomultiplier tube and other detector components, the organic glass upper cover 102 is sealed to the organic glass lower flange by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper and lower halves;

[0024] Step 3. Place the organic glass support frame on the middle container and seal it to provide support for the innermost container;

[0025] 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;

[0026] 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. Connect the knife-edge flange of the first bellows to the adapter flange, which is then connected to the connecting flange, using an indium wire sealing design.

[0027] Step 6. Lift the top cover of the upper container and the innermost container and slowly place them into the lower cylinder of the middle container;

[0028] Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes.

[0029] Third, 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 by comprising:

[0030] Step 1. Connect the first bellows to the sealing of the plexiglass flange:

[0031] Step 2. After installing the time drift chamber, photomultiplier tube and other detection components, the organic glass upper cover and the organic glass lower flange are sealed together by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper and lower halves;

[0032] Step 3. Place the organic glass support frame on the middle container and seal it to provide support for the innermost container;

[0033] 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;

[0034] 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. Connect the knife-edge flange of the first bellows to the adapter flange, which is then connected to the connecting flange, using an indium wire sealing design.

[0035] Step 6. Lift the top cover of the upper container and the innermost container and slowly place them into the lower cylinder of the middle container;

[0036] Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes;

[0037] Step 8, vacuuming;

[0038] Step 9, pre-cooling and injecting a medium such as liquid scintillator, gaseous xenon or liquid xenon;

[0039] Step 10, detector operation;

[0040] Step 11: Recycle the media and complete the run.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] 1) The radioactivity of the detector container itself is greatly reduced, which significantly improves the sensitivity of the detector.

[0043] 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.

[0044] 3) The present invention adopts a specific sealing method and support structure to achieve the sealing level of helium mass spectrometry leak detection BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Very low background radioactivity sealed container

[0046] Figure 2 Organic glass container assembly 100

[0047] Figure 3 Temperature control ring

[0048] Figure 4 Schematic diagram of the temperature control ring position

[0049] Figure 5 Assembly process 2

[0050] Figure 6 Organic glass container assembly 100 flange seal diagram

[0051] Figure 7 Assembly process diagram Figure 1

[0052] Figure 8 Assembly process 2

[0053] Figure 9 Assembly process three

[0054] Figure 10 Assembly process four

[0055] In the figure: organic glass container assembly 100, organic glass container upper part 101, organic glass container upper cover 102, organic glass pipe 103, organic glass pipe flange 104, organic glass container lower part 110, organic glass lower flange 111, organic glass cylinder 112, organic glass bottom plate 113, organic glass support frame 114,

[0056] Pressure vessel 200,

[0057] Vacuum container 300. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, 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 solutions 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 embodiments described are only some embodiments of the present invention, not all embodiments.

[0059] All other embodiments obtained by persons of ordinary skill in the art based on the embodiments herein without inventive effort are intended to fall within the scope of protection of the present invention. Experimental methods in the following examples, where specific conditions are not specified, were generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise indicated, all percentages, ratios, proportions, and parts are by weight.

[0060] An extremely low background radioactivity container, particularly suitable for rare event detectors. The container comprises, from the inside out:

[0061] The innermost container is made of organic glass with extremely low background radioactivity to ensure a tight seal around 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 pressure vessel standards and must comply with the national standard for pressure vessels (GB150) and the Technical Supervision Regulation for Safety of Stationary 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 and provides a thermally insulating vacuum layer for the cryogenic detector.

[0062] 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.

[0063] The key of the present invention is the following 4 points

[0064] a) Low-temperature sealing of organic glass containers;

[0065] b) The pressure difference of the fluid medium inside and outside the organic glass container must be controlled within a limited range;

[0066] c) Control the thermal stress of the organic glass container within a limited range during the cooling and heating process;

[0067] d) High recovery rate of expensive detection media.

[0068] Taking the middle metal pressure vessel and the inner organic glass container into consideration, so that they can jointly bear the working pressure of the detector target material, can reduce the difficulty of design and manufacturing. The middle metal container can hold liquid scintillator or other low-background radioactive fluid medium or some solid medium as a 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. Through the control system, the fluid medium inside and outside the organic glass container is controlled 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.

[0069] 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 two cavity pressure control systems is ±δP.

[0070] Theoretically, the following relationship can be obtained:

[0071] P D1 ≥P s -P W +2δP

[0072] P D2 ≥P W -P s +2δP

[0073] Simplifying, we can get:

[0074] P D1 +P D2 ≥4δP

[0075] In order to reduce the manufacturing difficulty of organic glass containers and reduce weight, the operating pressure of the middle metal pressure vessel P 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 according to the accuracy of the control system and the properties of the organic glass container.

[0076] Because organic glass has extremely low background radioactivity, the weight of organic glass containers themselves is not a factor that requires strict control. Considering manufacturing feasibility, a wall thickness of 50mm is more reasonable for cylinders and container structures with a diameter of more than 2m. This is different from metal containers, which generally have a larger yield strength and allowable stress. The 6mm wall thickness of previous PandaX detector pressure vessels 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 organic glass is greater than its tensile yield strength, so organic glass containers are more suitable for working under external pressure. After the structural form and wall thickness of the organic glass container are determined, the maximum allowable external and internal pressures can be calculated.

[0077] 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:

[0078] P D1 +P D2 =n×δP, n>4

[0079] 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.

[0080] According to the above analysis, the core problem of the rare event detector's extremely low background container is the structural design of the organic glass container, which takes advantage of the material's extreme mechanical properties to control the weight. The key parameters include the operating pressure range of the organic glass container, P D1 +P D2 , control system error ±δP. These parameters are strongly correlated. The present invention can shift the difficulty of designing and manufacturing the ultra-low radioactive background organic glass container to the control system. Existing control technology is relatively mature, and placement outside the detector shield does not require radioactivity control, making it easy to implement the design function.

[0081] Another key aspect 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 is a smaller diameter pipe flange. To improve detection efficiency, rare event detectors such as PandaX, LZ, and XENON all have larger time drift chambers, whose diameters are slightly smaller than the required container diameter. Therefore, the detector container requires an equipment flange with a large diameter. The inner diameters of the flanges of previous PandaX detectors have been 750mm, 800mm, and 1335mm, respectively, with future planned detector diameters of 2000mm, 2500mm, and so on. Furthermore, this equipment flange needs to be opened several times during the service life of the rare event detector, approximately ten times. These detectors operate at temperatures of approximately -100°C, which exceeds the operating temperature range of existing rubber seals. Given the high purity requirements of the target material, metal seals, such as indium wire or spring-energized seals with a silver outer layer, are generally used. However, metal seals require a significant preload, which means the flange is subject to significant localized stress, making them difficult to achieve for organic glass flanges. The present invention proposes a method that combines gluing and mechanical cutting to achieve reliable sealing of the flange of the organic glass equipment, and removes the bonding structure by mechanical cutting to facilitate maintenance of the rare event detector time drift chamber, etc. These processes will not contaminate the internal time drift chamber.

[0082] The sealing surface of the device flange has a 5mm chamfer. After the time drift chamber is installed inside the plexiglass container, a film, foil, or mold is used to create an upward-opening space outside the device flange. Epoxy resin is injected into this space. Once the epoxy resin cures, the two device flanges are bonded and sealed. If the plexiglass container needs to be opened to maintain the time drift chamber, the epoxy resin is cut away and cleaned. The plexiglass container can be opened after the cuts are removed.

[0083] The pipe flange has a relatively small diameter and is primarily used for the inlet and outlet of target fluids and the passage of optical fibers and cables. The external devices corresponding to these pipe flange design functions are all metal flanges or metal joints, requiring a transition from the plexiglass to the metal component and ensuring a reliable seal. The solution of the present invention is to extend the length of the plexiglass pipe flange and maintain the plexiglass pipe flange at room temperature using radiation heating or conduction heating. The plexiglass flange is then connected to a metal transition flange of the same caliber, such as stainless steel, titanium, or nickel, and sealed with low-melting-point gallium or a low-melting-point alloy. The seal between the plexiglass flange and the metal transition flange can also be achieved by epoxy bonding to achieve a high-vacuum seal. The connection between the plexiglass flange and the metal transition flange is permanent. Once connected and the seal is confirmed with a helium mass spectrometer leak detector, it is not removed until the equipment is decommissioned. A detachable metal flange is welded to the other side of the metal transition flange, facilitating multiple disassembly and assembly of the detector during use. The contact force between the small-diameter fluid pipe for fluid inlet and outlet and the plexiglass pipe flange is very low, resulting in high thermal resistance and no significant cooling load on the plexiglass pipe 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.

[0084] 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 its thermal conductivity and thermal diffusivity 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 cooling rate to avoid large thermal stress in the organic glass material.

[0085] The present invention uses the next generation 20-ton PandaX liquid xenon detector as an example to illustrate the design, manufacturing and use of an extremely low background radioactive sealed container for rare event detection experiments. The innermost detector time drift chamber has a diameter of approximately 2m and a height of 2m.

[0086] The extremely low background radioactive sealed container has a three-layer structure, which is composed of an organic glass container assembly 100, a pressure container 200, and a vacuum container 300 from the inside to the outside. Figure 1As shown. The organic glass container assembly 100 is equipped with the detector's time drift chamber, photomultiplier tube, and high-purity target material. 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 the static pressure of the liquid. All of these operate at low temperatures. The design pressure of the pressure vessel 200 is -0.1MPa to 1MPa, corresponding to the vacuum working condition before the injection of the detection medium and the rated working condition of the normal operation of the detector, respectively. The design temperature is -196℃ to 60℃. The vacuum container 300 provides a vacuum environment for the pressure vessel 200 to reduce the system's thermal load. 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 shield, the design pressure is -0.2MPa based on the static pressure exerted by the water.

[0087] First, the innermost organic glass container assembly 100 consists of an upper organic glass container assembly 101 and a lower organic glass container assembly 110. The upper organic glass container assembly 101 includes an organic glass upper cover 102, an organic glass pipe 103, and an organic glass pipe flange 104. The lower organic glass container assembly 110 comprises an organic glass lower flange 111, an organic glass cylinder 112, and an organic glass base 113. The organic glass pipe 103 has an inner diameter of approximately 150 mm and a length of 500 mm to 1000 mm, providing high thermal resistance to ensure that the organic glass upper cover 102 and the lower organic glass container assembly 110 operate at low temperatures while the organic glass pipe flange 104 operates at room temperature without significant heat leakage. The organic glass container assembly 100 houses the time drift chamber for detecting rare events, a photomultiplier tube, target materials injected during operation, and a temperature control ring 131. As shown, the temperature control rings are arranged one above the other within the plexiglass container assembly 100. Their outer diameter is slightly smaller than the inner diameter of the plexiglass container assembly 100, forming a V-shaped, monolithic structure. Made of low-background, oxygen-free copper, they are approximately 3mm to 5mm thick and each side is approximately 50mm long. A liquid xenon pipe 130 with an inner diameter of approximately 10mm extends directly through the temperature control ring 131, positioned 10mm to 25mm from the bottom of the V-shape to ensure that low-temperature liquid xenon drips directly into the ring 131. Otherwise, dripping directly onto the room-temperature plexiglass would cause excessive thermal stress and damage the plexiglass structure. Four to six temperature sensors 131 are evenly spaced 10mm from the edge of the ring 131, with one sensor positioned as close as possible to the liquid xenon pipe 130. After liquid xenon drips into temperature control ring 131, if the ring is at a high temperature, it will directly vaporize, cooling ring 131. The vaporized xenon is lower in temperature and cools nearby structures. Temperature control ring 131 itself is a good thermal conductor, tending to maintain a uniform temperature. It also has a large contact area with the xenon gas. Furthermore, relatively high-temperature structures are lower than ring 131, resulting in convective heat transfer, effectively and gently cooling the organic glass container assembly 100 and other internal structures. A 10-20 mm thick thermal insulation pad 132 made of a tough organic material is placed beneath ring 131. Optional materials include low-radioactive polyethylene and polytetrafluoroethylene. This prevents direct contact between ring 131 and the organic glass structure, which could cause excessive thermal stress.

[0088] The target material used in the PandaX experiment is liquid xenon, which has a density approximately 3–4 times that of the external liquid scintillator. Based on a 3-meter depth of liquid xenon, the pressure difference between the two sides of the bottom of the lower half of the plexiglass container assembly (110 mm) due to the density difference between the two media is approximately 4–10×10⁴ Pa during normal operation, necessitating a relatively large thickness. Fortunately, the background radioactivity of plexiglass is extremely low and its density is slightly greater than that of liquid scintillator, providing superior shielding effectiveness. Therefore, strict restrictions on the thickness of the plexiglass structure are not necessary. However, it is important to note that during the cooling and heating process, the liquid scintillator undergoes convection, resulting in a smaller temperature gradient. However, plexiglass is a solid, and heat transfer relies solely on conduction. However, its thermal conductivity is limited, which poses the risk of temperature gradients and excessive thermal stress. This issue will be discussed in detail in the thermal management solution section.

[0089] To assemble the plexiglass container assembly 100, first heat the first bellows 206 and the plexiglass flange 104 to 40-50°C. After alignment, inject gallium or another low-melting-point alloy at the same temperature into the gap to seal it. Then, tighten with screws and wait for it to cool naturally to achieve a seal. To improve reliability, the opening of the plexiglass connecting pipe 103 at the plexiglass upper cover 102 can be temporarily sealed with a blind plate, vacuum sealant, silicone gasket, or other method. A helium mass spectrometer vacuum leak test is performed on the connection between the first bellows 206 and the plexiglass flange 104 to confirm the sealing performance. If a leak is detected, slowly heat it again to 40-50°C, open it, and reconnect it. Once the sealing is confirmed, the first bellows 206 and the plexiglass flange should not be opened during the entire service life.

[0090] The connection between the first bellows 206 and the organic glass flange 104 can also be sealed by epoxy resin bonding. After the helium mass spectrometry vacuum leak test is carried out to confirm the sealing performance, it will not be opened again during the entire service life.

[0091] After the components within the organic glass container assembly 100 are installed, the organic glass upper cover 102 needs to be sealed to the organic glass lower flange 111. Conventional methods such as indium wire sealing and polytetrafluoroethylene gasket sealing require significant bolting force, resulting in significant stress within the flange, while the permissible stress of organic glass is relatively low. Therefore, bulk polymerization or epoxy resin bonding is used to achieve the seal. Because the internal and external pressure differential at this seal is minimal during operation of the organic glass container assembly 100, with the external pressure slightly greater than the internal pressure, the sealing material only needs to ensure a seal without having to withstand significant structural stress, which reduces the technical difficulty of structural design and manufacturing. When the organic glass container assembly 100 needs to be opened to repair and maintain the components within, the sealing material can be removed using a tool such as a grinding wheel or milling cutter. Designing a circular tooling tool that allows the tool to perform a circular motion ensures high machining accuracy. While removing the sealing material, minimal organic glass base material is removed, typically less than 0.5 mm. By taking this margin into account during the design and manufacture of the organic glass container assembly 100, the assembly can be reused multiple times.

[0092] like Figure 4 As shown, the organic glass upper cover 102 and the organic glass 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 organic glass 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 organic glass container assembly 100 needs to be moved or lifted, it is necessary to pass through the organic glass support frame 114. The organic glass support frame 114 is placed on the head of the pressure vessel 200 to provide support for the organic glass container assembly 100, especially when there is a large pressure difference between the inside and outside of the organic glass bottom plate 113 in the working state. The organic glass support frame 114 can improve the stress state of the organic glass bottom plate 113.

[0093] The organic glass support frame 114 has four cantilevers, each with a through hole of 12 mm in diameter at the end. It 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.

[0094] The pressure vessel upper cover 210 is then lowered, and the first bellows 206 extends more than 10 cm beyond the connecting flange 201. The knife-edge flange of the first bellows 206 is connected to the adapter flange of the pressure vessel 200. The adapter flange has a corresponding knife-edge on its underside, which is sealed with a copper gasket. Finally, the adapter flange is connected to the connecting flange 201, using an indium wire seal. The structure consisting of the pressure vessel upper cover 210 and the organic glass container assembly 100 is lifted and slowly placed into the lower body of the pressure vessel 200. The sealing indium wire is laid and the flange is tightened. The ports 208 have a small diameter, ranging from 10 mm to 50 mm, and are four to six in number. The corresponding pipes are made of stainless steel, resulting in a low radioactive background and high flexibility for easy assembly. Their primary function is to transport liquid scintillator and other working gases, including vacuuming, for the rare event detector startup cooling process, liquid scintillator filling and circulation, and pressure balancing between the xenon gas space and the liquid scintillator space.

[0095] 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 complete the assembly of the second bellows 207 and the connecting flange 201, and seal it with a copper gasket. The completed assembly is as follows: Figure 7 shown.

[0096] Finally, place the assembled pressure vessel 200 and the second bellows 207 into the vacuum vessel 300, connect the second bellows 207 to the vacuum flange 301, and use a small diameter pipe to connect the connecting pipe 308 on the vacuum flange 301 and the interface 208. The small diameter pipe needs to be designed with a curved structure for thermal compensation or other buffering. Figure 8 shown.

[0097] How to use

[0098] Because the organic glass container 100 cannot withstand large internal and external pressure differences, the system startup, operation, and shutdown processes require careful consideration of various operating conditions such as pressure and temperature changes during transient processes. The operation process is divided into the following steps: vacuuming, medium injection and thermal management, operation, and shutdown.

[0099] Vacuum

[0100] 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 interior of the organic glass container 100 needs to be vacuumed to 1×10 -4Pa, in order to ensure the pressure balance inside and outside the organic glass container 100, it is necessary to simultaneously evacuate the interior of the pressure container 200. To ensure reliability, it is necessary to connect the two chambers and use the same vacuum pump group to evacuate the air, while controlling the vacuum pump group's pumping rate 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 drops below 10Pa and meets the molecular pump starting condition, close the valves between the pressure container 200 and the vacuum pump group and start the molecular pump. The molecular pump only evacuates the interior of the organic glass container 100 until 1×10 -4 Pa, and maintain it for a certain period of time.

[0101] Initial injection of medium

[0102] Because the spaces inside and outside the plexiglass container assembly 100 require different media, this step is more challenging than vacuuming. The injection process requires greater patience and a safety margin, and the pressure differential between the inside and outside must be strictly controlled. Buffer containers with capacities ranging from 10L to 50L are installed in the injection pipeline, each with a valve installed at the front and rear. For the internal space, first ensure the rear valve is closed and inject xenon gas into the buffer container to a pressure of 0.1-0.2 MPa. Then, close the front valve and open the rear valve to release the xenon gas into the plexiglass container 100. 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 vessel 200 outside the plexiglass container assembly 100. Repeat this process multiple times, ensuring that the pressure differential between the two sides does not exceed 10 kPa. If there is a significant pressure difference between the two sides, fill the lower-pressure side several more times until the pressure difference is below 10 kPa. Once the absolute pressure between the two spaces reaches 0.2 MPa, pause the injection of the medium and enter the cooling phase.

[0103] Precooling

[0104] The organic glass container component 100 has a diameter of 2000mm, a height of 3000mm, a wall thickness of 50mm, and a surface area of ​​approximately 25m 2 The thermal conductivity of organic glass is low, only about 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, that is, 1×10 -4K / day can keep the maximum temperature difference across the plexiglass container below 1K, requiring approximately 100W of cooling power. Based on a room temperature of 298K, cooling to the PandaX detector's rated operating temperature of 178K takes 12 days. Within this timeframe, 20 tons of gaseous xenon can be liquefied simultaneously, and refilling with liquid xenon can be completed within one day of cooling. This two-week timeframe is more than acceptable compared to the approximately one-week pre-cooling and refilling time required by previous PandaX detector generations.

[0105] Inside the organic glass container assembly 100, liquid xenon is injected into the temperature control ring 131 through a liquid xenon pipe 130. The heat of vaporization of xenon is approximately 100 J / g, and the initial injection rate is approximately 1 g / s. The liquid xenon first cools the liquid xenon pipe 130 and vaporizes itself, injecting low-temperature xenon gas into the organic glass container assembly 100. As the cooling progresses, the temperature of the gaseous xenon output from the liquid xenon pipe 130 gradually decreases, eventually releasing liquid xenon, which drips onto the temperature control ring 131. Because the temperature control ring 131 is at a higher temperature, the liquid xenon immediately vaporizes into xenon gas at a lower temperature than the surrounding environment, simultaneously cooling the temperature control ring 131 and exchanging heat with the detector assembly to achieve a cooling effect. Because the temperature control ring 131 is made of high-strength and high-toughness oxygen-free copper, the thermal stress caused by the dripping liquid xenon does not exceed the tensile strength of the oxygen-free copper, ensuring safe and long-term operation. The temperature of the temperature control 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 induce xenon convection, achieving uniform and gentle cooling of the organic glass and other components of the detector. A portion of the low-temperature xenon gas vaporized on the upper side of the temperature control ring 131 directly overflows the temperature control ring 131, while a portion flows within the temperature control ring 131 to other parts, accelerating the temperature uniformity of the temperature control ring 131 itself. In short, the temperature control ring 131, made of oxygen-free copper material, can directly use liquid xenon to cool the organic glass assembly 100 and other detector components, while ensuring that the cooling process does not cause significant thermal stress. During the cooling process, the internal pressure will decrease as the temperature drops. When the pressure falls below the rated value, xenon gas needs to be added to maintain pressure stability and balance with the pressure in the liquid scintillator space.

[0106] For pressure vessel 200, the welding between the main flange and 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, which can provide the circulation of ethanol coolant to realize the cooling of pressure vessel 200. The cooling power of the low-temperature refrigerator is 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 the 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), nitrogen (or argon) cools and flows downward, gradually cooling the structure geometrically below the main flange. Because 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 gas cooling process is relatively gentle. To more effectively control the cooling process, temperature sensors are installed at the top, middle, and bottom of the organic glass component 100, 1cm away from the organic glass. The temperature at each point is cooled at a rate of 10K / day.

[0107] Medium injection

[0108] After the cooling process is complete, dielectric injection can begin. The liquid scintillator needs to be pre-cooled to approximately 178K. During the injection process, it is important to ensure that the buoyancy of the liquid scintillator on the organic glass assembly 100 is less than the weight of the liquid xenon to prevent damage to the thermostat caused by excessive buoyancy. Furthermore, the pressure differential between the two sides must not exceed the designed value. After dielectric injection is complete, the detector can enter normal operation.

[0109] Media Recovery

[0110] After the detector operation is completed, 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 ensure that the pressure difference on both sides is within the design range. During the liquid scintillator recovery process, a portion of nitrogen or argon needs to be filled for replacement. After all the liquid is recovered, the xenon gas 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 recovery is completed when the internal xenon gas can be ignored. Then, the organic glass container component 100, pressure vessel 200, and vacuum container 300 are re-injected with nitrogen or argon to 0.1MPa, and then the system is allowed to naturally return to room temperature. Each container can be opened to complete the decommissioning work or maintenance work. After completing the maintenance work, the aforementioned assembly, vacuuming, initial medium injection, pre-cooling, medium injection, detector operation, etc. 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 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, thereby transmitting the pressure generated by the detection target material to the middle container. At the same time, it acts as a shielding layer to block the radioactivity of the outside world and the middle container itself. Outer container: It is a vacuum container that is placed outside the middle container and provides a vacuum insulation layer to keep the middle container and the innermost container running at low temperatures; The innermost container consists 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; the detector's time drift chamber, photomultiplier tube and 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.

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

3. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, 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.

4. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, 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.

5. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, characterized in that: The organic glass bottom plate is placed on the organic glass support frame; the organic glass support frame has 4 cantilevers, each cantilever has a through hole at the end, and is suspended on the hook inside the upper flange of the pressure vessel with a low-background suspension rod with a diameter of 10mm; the upper head of the pressure vessel has a connecting flange, which corresponds to the number and position of the connecting flange of the organic glass container; the connecting flange, the upper head of the pressure vessel, the upper flange of the pressure vessel, The hooks together form the upper cover of the middle container.

6. The extremely low background radioactive constant temperature container for rare event detection experiments according to claim 1, characterized in that: Two temperature control rings are arranged above and below the plexiglass cylinder assembly. Their outer diameter is slightly smaller than the inner diameter of the plexiglass container assembly, forming a V-shaped integral structure. A liquid xenon pipe with an inner diameter of 10 mm extends directly through the temperature control ring from the outside, 10 mm to 25 mm from the bottom of the V-shape to ensure that low-temperature liquid xenon drips into the temperature control ring. Otherwise, if the low-temperature liquid xenon drips directly onto the room-temperature plexiglass, it will cause excessive thermal stress and damage the plexiglass structure. Four to six temperature sensors are evenly distributed 10 mm from the edge of the temperature control ring, with one temperature sensor as close as possible to the liquid xenon pipe. A 10 mm to 20 mm thick thermal insulation pad made of a tough organic material is placed under the temperature control ring. The insulation pad is made of one of polyethylene and polytetrafluoroethylene with low radioactive background to prevent direct contact between the temperature control ring and the plexiglass structure, thereby preventing excessive thermal stress.

7. 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 under normal operation, and the temperature range is -196℃ to 60℃.

8. 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.

9. A method for assembling an extremely low background radioactive constant temperature container for rare event detection experiments, based on the extremely low background radioactive constant temperature container for rare event detection experiments according to any one of claims 1 to 8, characterized in that: include: Step 1. Sealingly connect the first bellows (206) to the organic glass flange; Step 2. After the time drift chamber and the photomultiplier tube detector are installed, 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 and lower parts; Step 3. Place the organic glass support frame on the middle container and seal 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. Connect the knife-edge flange of the first bellows to the adapter flange, and the adapter flange to the connecting flange, using an indium wire sealing design. Step 6. Lift the top cover of the upper container and the innermost container and slowly place them into the lower cylinder of the middle container; Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes.

10. A method for using an extremely low background radioactive constant temperature container for rare event detection experiments, based on the extremely low background radioactive constant temperature container for rare event detection experiments according to any one of claims 1 to 8, characterized in that: include: Step 1. Connect the first bellows to the organic glass flange; Step 2. After the time drift chamber and photomultiplier tube detector are installed, the organic glass upper cover and the organic glass lower flange are sealed together by bulk polymerization or epoxy resin bonding, thereby achieving a sealed connection between the upper and lower halves; Step 3. Place the organic glass support frame on the middle container and seal 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. Connect the knife-edge flange of the first bellows to the adapter flange, and the adapter flange to the connecting flange, using an indium wire sealing design. Step 6. Lift the top cover of the upper container and the innermost container and slowly place them into the lower cylinder of the middle container; Step 7: Place the assembled middle container into the outer container and connect the relevant flanges and pipes; Step 8, vacuuming; Step 9, pre-cooling and injecting liquid scintillator, gaseous xenon or liquid xenon medium; Step 10, detector operation; Step 11: Recycle the media and complete the run.

Images