A vacuum system for precise control of gas density in a sub-dose probe

Abstract

A kind of vacuum system for nanodose detector gas density precision control belongs to ionizing radiation nanodose measurement field.The present application includes vacuum pump group unit, vacuum degree detection unit, logic control unit.The present application is aimed at the requirement of nanodose detector to vacuum degree and gas concentration, and realizes that working air pressure is in 1Torr by the pump group in vacuum system and the cooperation of PLC logic control and data monitoring, and gas pressure control precision reaches 0.01Torr.The system can provide ionizing radiation gas and low vacuum environment for nanodose detector, ensure that nanodose detector measurement signal is accurate and stable.The ionization degree of nanodose detector depends on the vacuum degree in cavity, and the vacuum degree of cavity depends on the concentration of gas in sensitive volume.The present application has the advantages of high precision, high accuracy, high reliability and the like.Through the present application, the vacuum environment of nanodose detector can be constant and sensitive gas is single, which can support the accurate measurement of nanodose detector.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of vacuum systems for the precision control of nanodose detector gas density, belong to ionizing radiation nanodose measurement field. BACKGROUND

[0002] Deeply understand the energy deposition in the process of ionizing radiation and human tissue and its damage mechanism is the key to realize accurate radiotherapy.Nanodose detector is a kind of detection device for measuring nanometer scale dose characteristics in the process of radiotherapy, which needs to maintain a single type and constant concentration of sensitive gas in the cavity of the gas-tight chamber to realize ion counting measurement of the detector, so as to understand the damage mechanism in the process of radiation beam action.In practical application, the vacuum degree in the cavity of the device needs to be constantly kept at 1Torr (gas density is about 2.3×10-6g / cm3), however, the traditional gas concentration control method often uses a single vacuum pump to adjust the vacuum pressure, it is difficult to realize quantitative and precise control of gas concentration, and it is difficult to meet the current measurement conditions of nanodose detector.The concentration of sensitive gas in the cavity of nanodose detector is too high or too low, which will cause the detection signal to deviate seriously, how to control the vacuum degree and gas concentration in the cavity within a reasonable range has become one of the research focuses of nanodose detector.In order to solve this problem, it is urgent to develop a kind of vacuum system device for precise control of gas density of nanodose detector by multiple vacuum pumps. SUMMARY

[0003] In order to overcome the above-mentioned measurement difficulties of nanodose detector, the purpose of the present application is to provide a kind of vacuum system for precise control of gas density of nanodose detector, according to the requirements of nanodose detector for vacuum degree and gas concentration, the working pressure is realized within 1Torr by pump group and PLC logic control in the vacuum system and data monitoring, and the gas pressure control precision reaches 0.01Torr.The system can provide ionizing radiation gas and low vacuum environment for nanodose detector, to ensure that the measurement signal of nanodose detector is accurate and stable.The ionization degree of nanodose detector depends on the vacuum degree in the cavity, and the vacuum degree in the cavity depends on the concentration of gas in the sensitive volume.The present application has the advantages of high precision, high accuracy, high reliability, etc.Through the present application, the vacuum environment of nanodose detector can be kept constant and the sensitive gas can be single, which can support the accurate measurement of nanodose detector and help to establish the internal relationship between nanodosiology and macroscopic water absorption dose.

[0004] The purpose of the present application is realized by the following technical scheme.

[0005] The present application discloses a kind of vacuum systems for the precision control of nanodose detector gas density, including vacuum pump group unit, vacuum degree detection unit, logic control unit.

[0006] The vacuum pump group unit is composed of a molecular pump, a scroll pump a, a scroll pump b, a first high-pressure electromagnetic valve, and a second high-pressure electromagnetic valve. The scroll pump a gas inlet is connected with the high-pressure electromagnetic valve through a vacuum pipeline, the scroll pump b gas inlet is connected with the molecular pump gas outlet through a vacuum pipeline, and the third high-pressure electromagnetic valve is connected with the molecular pump gas inlet through a vacuum pipeline. The high-pressure electromagnetic valve and the other side of the high-pressure electromagnetic valve are connected with the gas outlet below the nanodose detector through a KF interface. The scroll pump a is used for pre-extraction to ensure that the internal vacuum degree of the cavity is reduced to below 20 Pa. Since the vacuum degree when the molecular pump starts needs to be below 10 Pa, the scroll pump b is installed as a molecular pump trailer pump, thereby ensuring that the molecular pump and the gas inlet and gas outlet on both sides are less than 10 Pa to complete the opening of the molecules. The cooperation of the pump group ensures the stable operation of the molecular pump. If the molecular pump is directly opened at atmospheric pressure, the blades in the pump will be damaged, thereby damaging the molecular pump.

[0007] The vacuum degree detection unit includes a low vacuum pressure sensor, a high vacuum pressure sensor, and a static vacuum pressure sensor. The vacuum degree detection unit is used for detecting the vacuum degree of the pipeline and the cavity. The low vacuum pressure sensor is installed on the sixth vacuum pipeline between the bottom gas outlet and the third high-pressure electromagnetic valve, and is used for detecting the vacuum degree of the sixth vacuum pipeline in real time. The high vacuum pressure sensor is installed on the seventh vacuum pipeline between the molecular pump and the third high-pressure electromagnetic valve, and is used for detecting the vacuum degree on the seventh pipeline to detect whether the pipeline vacuum degree reaches the molecular pump starting condition. The static vacuum pressure is connected with the middle gas outlet of the nanodose detector, and is used for detecting the static vacuum degree in the nanodose detector cavity. The vacuum degree detection unit ensures the detection of the pipeline and cavity vacuum degree, efficiently cooperates with the PLC logic controller in the logic control unit to judge the current vacuum degree state, and controls the high-pressure electromagnetic valve to act, thereby cooperating with the MFC flow controller to transport effective gas to reach the preset vacuum degree condition and gas concentration. The pipeline refers to the sixth vacuum pipeline and the seventh vacuum pipeline.

[0008] The logic control unit comprises a PLC controller, an MFC flow control meter, a gas cylinder A, a gas cylinder B, a fourth high-pressure electromagnetic valve, and a first high-pressure electromagnetic valve. The fourth high-pressure electromagnetic valve is connected with the gas cylinder A, and the fourth high-pressure electromagnetic valve is connected with the first high-pressure electromagnetic valve. One end of the MFC flow control meter is connected with the gas cylinder B, and the other end is connected with the first high-pressure electromagnetic valve. The other side of the first high-pressure electromagnetic valve is connected with a gas inlet at the top of the cavity. The PLC controller is connected with the first high-pressure electromagnetic valve, the second high-pressure electromagnetic valve, the third high-pressure electromagnetic valve, the fourth high-pressure electromagnetic valve, the MFC, and a static vacuum pressure sensor communication interface. Through serial communication multi-device collaborative control, the PLC logic controller judges the current vacuum degree state and controls the high-pressure electromagnetic valve to act, and then cooperates with the MFC flow controller to transport effective gas, so as to ensure that the concentration of sensitive gas in the cavity of the nanodose probe is constant and the negative pressure environment realizes the measurement environment of the nanodose probe, and the preset vacuum degree condition and gas concentration requirement are achieved.

[0009] In order to ensure the remote control precision and sensitivity, preferably, the first high-pressure electromagnetic valve, the second high-pressure electromagnetic valve, the third high-pressure electromagnetic valve, and the fourth high-pressure electromagnetic valve are all selected from high-pressure electromagnetic valves.

[0010] The working method of the vacuum system for nanodose probe gas density precision control disclosed in the application is as follows:

[0011] Step 1: The vacuum pump set unit is started. In order to ensure the stable start of the molecular pump and the vortex pump, the complete start process of the pump set and the system self-checking need to be completed. Before starting, it is necessary to check whether the first high-pressure electromagnetic valve, the second high-pressure electromagnetic valve, the third high-pressure electromagnetic valve, the fourth high-pressure electromagnetic valve, and the MFC flow control meter are closed, so as to ensure that the pipeline connected with the molecular pump and the vortex pump is in a sealed cover state, and prevent the molecular pump from being damaged. When the high-vacuum pressure sensor on the pipeline shows a value below 20 Pa, it means that the vacuum degree of the pipeline meets the start condition of the molecular pump. When the start condition is met, the molecular pump is started, and when the rotating speed of the molecular pump is stable, the vacuum pump set unit is started.

[0012] Step two: through PLC cooperative control of the second high pressure solenoid valve open, complete the nanometer dose detector cavity and the pipeline of the vortex pump, this process is the nanometer dose detector cavity rough pumping process, quickly the nanometer dose detector cavity internal vacuum degree reaches 20 Pa or less. When the low vacuum pressure sensor value ≤ 10 Pa, then indicates that the pipeline vacuum degree has reached the limit of the vortex pump a, realize the nanometer dose detector cavity pre-evacuation. The nanometer dose detector cavity air is extracted, then need to use the molecular pump with stronger pumping speed, at this time the second high pressure solenoid valve is closed, the third high pressure solenoid valve is opened, complete the nanometer dose detector cavity and the pipeline of the molecular pump, realize the nanometer dose detector cavity precision pumping, more fine gas molecules are extracted, the molecular pump has stronger pumping speed, so that the nanometer dose detector cavity is kept at a lower vacuum degree.

[0013] Step three: due to the actual situation of the nanometer dose detector cavity and the pipeline exists leakage rate and the nanometer dose detector cavity internal dead volume problem, so that the nanometer dose detector cavity cannot be extracted to absolute vacuum, at this time need through gas replacement to make the nanometer dose detector cavity internal gas impurity to a minimum. When the low vacuum pressure sensor value reaches 1-3 Pa, the second high pressure solenoid valve is closed, the first high pressure solenoid valve of the gas inlet valve, the second high pressure solenoid valve is opened, the nitrogen in the gas cylinder A is transported to the nanometer dose detector cavity, so that the nitrogen molecules in the nanometer dose detector cavity occupy the position, and then the vacuum degree in the cavity is rapidly increased to atmospheric pressure. When the low vacuum pressure sensor value ≥ 10 -5 Pa, the first high pressure solenoid valve and the second high pressure solenoid valve are closed to complete the nitrogen gas inlet process.

[0014] Step four: cycle step two and step three, through the cycle of nitrogen gas washing can control the nanometer dose detector cavity internal other gas content decline to 0.1% or less, cycle step two and step three 15-20 times, finally keep the nanometer dose detector cavity in vacuum state and the first high pressure solenoid valve, the second high pressure solenoid valve, the fourth high pressure solenoid valve are in the closed state. The vacuum state refers to the vacuum degree ≤ 1.2 Pa.

[0015] Step five: establish the relationship between the vacuum pressure and the propane gas concentration by introducing the effective gas in cylinder B through the MFC flow meter. Set the flow rate of the MFC flow meter to 0.01-0.1 mL / min, and the PLC detects the value of the static vacuum gauge in real time. When the value is equal to the set value, close the MFC flow meter and the first high-pressure electromagnetic valve. After the pressure in the nanodosimeter cavity is balanced, the actual vacuum degree in the nanodosimeter cavity is ensured to be within 5% of the set value, thereby ensuring the accuracy and stability of the measurement signal. The 5% deviation is caused by the dead volume inside the nanodosimeter cavity and the adsorption of gas molecules by the deposited oil. The actual vacuum degree and the set value deviation are determined by fitting, and the vacuum degree is accurately controlled according to the fitting relationship between the actual vacuum degree and the set value deviation, that is, the gas density in the nanodosimeter cavity is precisely controlled.

[0016] Preferably, according to steps one to five, the vacuum degree of the nanodosimeter cavity is controlled within 1 Torr-30 Torr, ensuring the accuracy and stability of the measurement signal of the nanodosimeter. The nanodosimeter reads the signal count of the ion avalanche event, and establishes the internal relationship between the nanodosimetric quantity and the macroscopic water absorption dose according to the readout signal count result.

[0017] Further preferably, according to steps one to five, the vacuum degree of the nanodosimeter cavity is controlled within 2 Torr-8 Torr, further improving the accuracy and stability of the measurement signal of the nanodosimeter.

[0018] Advantages:

[0019] 1. The vacuum system for precise control of gas density of a nanodosimeter disclosed in the present application is based on the gas precision extraction method of a combination of a front-stage scroll pump and a molecular pump. The PLC logic controller collects the vacuum degree value of the pipeline through the serial port protocol to start the pump group composed of the molecular pump and the scroll pump. Compared with a single scroll pump, the present application can quickly respond and make the nanodosimeter cavity reach a lower vacuum degree. By gas washing, the air content is reduced to a minimum, thereby reducing the gas content in the nanodosimeter cavity.

[0020] 2. The vacuum system for precise control of gas density of a nanodosimeter disclosed in the present application guarantees the detection of the vacuum degree of the pipeline and the nanodosimeter cavity through the vacuum degree detection unit. The high-efficiency logic control unit cooperates with the high-pressure electromagnetic valve control to realize precise control of the pressure inside the nanodosimeter cavity, meeting the high standard requirements of ion avalanche events on vacuum degree and gas content, i.e. the vacuum degree is within 1 Torr-30 Torr.

[0021] 3. This invention discloses a vacuum system for precise control of gas density in a nanodose detector. By installing a vortex pump b as a trailer pump for the molecular pump, the pressure on both sides of the molecular pump and the inlet and outlet is ensured to be less than 10 Pa to complete the molecular activation. The coordinated use of the pump set ensures the stable operation of the molecular pump. If the molecular pump is activated directly at atmospheric pressure, it will cause damage to the pump blades and thus damage the molecular pump.

[0022] 4. The present invention discloses a vacuum system for precise control of gas density in a nanodose detector. By setting the flow rate of the MFC flow meter, the PLC detects the value of the static vacuum gauge in real time. When the value is equal to the set value, the MFC flow meter and the first high-pressure solenoid valve are closed. After the pressure equalization of the nanodose detector cavity is completed, the actual vacuum degree in the nanodose detector cavity is guaranteed to deviate from the set value by less than 5%, thereby achieving the ionization reaction conditions of the nanodose detector.

[0023] 5. This invention discloses a vacuum system for precise control of gas density in a nanodose detector. Based on the beneficial effects described in points 1, 2, 3, and 4, and through extensive experimental and theoretical analysis, it has been determined that controlling the vacuum level of the nanodose detector cavity within the range of 1 Torr to 30 Torr ensures the occurrence of ionization avalanche events in the nanodose detector, thus ensuring the accuracy and stability of the nanodose detector's measurement signal. By counting the readout signals of the ionization avalanche events using the nanodose detector, an internal relationship between nanodose biometrics and macroscopic water absorption dose is established based on the count results. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a vacuum system for precise control of gas density in a nanodose detector according to the present invention;

[0025] Figure 2 This is a schematic diagram of the logic control of the present invention.

[0026] Among them, 1-molecular pump, 2-vortex pump a, 3-vortex pump b, 4-gas cylinder A, 5-gas cylinder B, 6-MFC flow controller, 7-static vacuum pressure sensor, 8-low vacuum pressure sensor, 9-high vacuum pressure sensor, 10-first high-pressure solenoid valve, 11-second high-pressure solenoid valve, 12-third high-pressure solenoid valve, 13-fourth high-pressure solenoid valve, 14-PLC controller. Detailed Implementation

[0027] like Figure 1 As shown in the figure, the vacuum system for precise control of gas density in a nanodose detector disclosed in this embodiment consists of a vacuum pump unit, a vacuum degree detection unit, and a logic control unit.

[0028] The vacuum pump unit consists of a molecular pump 1, a vortex pump a2, a vortex pump b3, a first high-pressure solenoid valve 10, and a second high-pressure solenoid valve 11. The inlet of vortex pump a2 is connected to the high-pressure solenoid valve via a vacuum pipe; the inlet of vortex pump b3 is connected to the outlet of molecular pump 1 via a vacuum pipe; and the third high-pressure solenoid valve 12 is connected to the inlet of molecular pump 1 via a vacuum pipe. The other side of each high-pressure solenoid valve is connected to the outlet below the nanodose detector via a KF interface. Vortex pump a2 is used for pre-evacuation to ensure the vacuum level inside the chamber drops below 20 Pa. Since the vacuum level for molecular pump 1 to start requires below 10 Pa, vortex pump b3 is installed as a secondary pump for molecular pump 1, thereby ensuring that the pressure on both sides of molecular pump 1 and its inlet and outlet is less than 10 Pa to complete the molecular pumping process. The coordinated use of the pump unit ensures the stable operation of molecular pump 1. If molecular pump 1 is started directly at atmospheric pressure, it will damage the pump blades and ultimately destroy molecular pump 1.

[0029] The vacuum detection unit includes a low vacuum pressure sensor 8, a high vacuum pressure sensor 9, and a static vacuum pressure sensor 7. This unit is used to detect the vacuum level inside the pipelines and cavities. The low vacuum pressure sensor 8 is installed on the sixth vacuum pipeline between the bottom outlet and the third high-pressure solenoid valve 12, and is used to detect the vacuum level of the sixth vacuum pipeline in real time. The high vacuum pressure sensor 9 is installed on the seventh vacuum pipeline between the molecular pump 1 and the high-pressure third high-pressure solenoid valve 12, and is used to detect the vacuum level in the seventh pipeline to determine whether the pipeline vacuum level has reached the starting condition of the molecular pump 1. The static vacuum pressure sensor is connected to the middle outlet of the nanodose detector and is used to detect the static vacuum level inside the nanodose detector cavity. The vacuum detection unit ensures the detection of the vacuum level in the pipelines and cavities, efficiently cooperating with the PLC logic controller in the logic control unit to determine the current vacuum level status and control the high-pressure solenoid valve, thereby coordinating with the MFC flow controller to deliver effective gas and achieve the preset vacuum level conditions and gas concentration. The pipelines refer to the sixth and seventh vacuum pipelines.

[0030] The logic control unit includes a PLC controller 14, an MFC flow meter 6, gas cylinder A4, gas cylinder B5, a fourth high-pressure solenoid valve 13, and a first high-pressure solenoid valve 10. The fourth high-pressure solenoid valve 13 is connected to gas cylinder A4 and to the first high-pressure solenoid valve 10. One end of the MFC flow meter 6 is connected to gas cylinder B5, and the other end is connected to the first high-pressure solenoid valve 10. The other side of the first high-pressure solenoid valve 10 is connected to the air inlet at the top of the cavity. Additionally, the PLC controller 14 is connected to the communication interfaces of the first high-pressure solenoid valve 10, the second high-pressure solenoid valve 11, the third high-pressure solenoid valve 12, the fourth high-pressure solenoid valve 13, the MFC, and the static vacuum pressure sensor 7. The PLC logic control collects branch vacuum values ​​and the status and flow information of the MFC flow meter. Through serial communication and multi-device collaborative control, the PLC logic controller determines the current vacuum level and controls the high-pressure solenoid valve to operate. This, in turn, coordinates with the MFC flow controller to deliver effective gas, ensuring a constant concentration of sensitive gas inside the nanodose detector cavity and a negative pressure environment to achieve the measurement environment of the nanodose detector, thus reaching the preset vacuum level and gas concentration.

[0031] The working method of a vacuum system for precise gas density control of a nanodose detector disclosed in this embodiment is as follows:

[0032] Step 1: Vacuum Pump Unit Start-up. To ensure stable startup of molecular pump 1 and vortex pump, the entire startup process and system self-check of the pump unit need to be completed. Start vortex pumps a2 and b3. Before starting, check whether the first high-pressure solenoid valve 10, the second high-pressure solenoid valve 11, the third high-pressure solenoid valve 12, the fourth high-pressure solenoid valve 13, and the MFC flow controller 6 are closed to ensure that the pipeline connecting molecular pump 1 and the vortex pump is in a sealed state to prevent damage to molecular pump 1. When the reading of the high vacuum pressure sensor 9 on the pipeline is below 20 Pa, it means that the vacuum level of the pipeline meets the startup conditions of molecular pump 1. When the startup conditions are met, start molecular pump 1. When the speed of molecular pump 1 stabilizes, the vacuum pump unit startup is complete.

[0033] Step Two: The second high-pressure solenoid valve 11 is opened via PLC-controlled operation, connecting the nanodose detector cavity to the vortex pump. This process involves preliminary evacuation of the nanodose detector cavity, quickly bringing the internal vacuum level to below 20 Pa. When the low-vacuum pressure sensor 8 reading is ≤10 Pa, it indicates that the pipeline vacuum level has reached the limit of vortex pump a2, achieving pre-evacuation of the nanodose detector cavity. Completely removing all air from the nanodose detector cavity requires the use of a higher-speed molecular pump 1. At this point, the second high-pressure solenoid valve 11 is closed, and the third high-pressure solenoid valve 12 is opened, connecting the nanodose detector cavity to molecular pump 1. This enables precise evacuation of the nanodose detector cavity, extracting gas molecules more precisely. The higher pumping speed of molecular pump 1 maintains a lower vacuum level within the nanodose detector cavity.

[0034] Step 3: Due to leakage in the nanodose detector cavity and piping, as well as dead volume issues within the cavity, it is impossible to achieve an absolute vacuum. Therefore, gas replacement is necessary to minimize gaseous impurities within the cavity. When the low vacuum pressure sensor 8 reading reaches 1.2 Pa, the second high-pressure solenoid valve 11 is closed, and the inlet valves, first high-pressure solenoid valve 10, and second high-pressure solenoid valve 11 are opened to deliver nitrogen from gas cylinder A4 into the nanodose detector cavity. This allows nitrogen molecules to occupy space within the cavity, rapidly increasing the vacuum level to atmospheric pressure. When the low vacuum pressure sensor 8 reading is ≥10 Pa... -5 When Pa, the first high-pressure solenoid valve 10 and the second high-pressure solenoid valve 11 are closed to complete the nitrogen intake process.

[0035] Step 4: Repeat steps 2 and 3. By circulating nitrogen to purge the gas, the content of gases other than nitrogen inside the nanodose detector cavity can be controlled to decrease to below 0.1%. Repeat steps 2 and 3 16 times. Finally, maintain the nanodose detector cavity in a vacuum state and ensure that the first high-pressure solenoid valve 10, the second high-pressure solenoid valve 11, and the fourth high-pressure solenoid valve 13 are all in the closed state. The vacuum state refers to a vacuum degree ≤ 1.2 Pa.

[0036] Step 5: Introduce effective gas into gas cylinder B5 through the MFC flow meter to establish the relationship between vacuum pressure and propane gas concentration. Set the flow rate of the MFC flow meter to 0.02 mL / min. The PLC monitors the static vacuum gauge value in real time. Once the value equals the set value, the MFC flow meter and the first high-pressure solenoid valve 10 are closed. After the pressure equalization of the nanodose detector cavity is completed, the actual vacuum degree inside the nanodose detector cavity is guaranteed to deviate from the set value by 5%, thus achieving the ionization reaction conditions of the nanodose detector. The 5% deviation is caused by the dead volume inside the nanodose detector cavity and the adsorption of gas molecules by deposited grease, which is a normal phenomenon. The correspondence between the actual vacuum degree and the set value deviation is determined by fitting. Based on the fitted correspondence between the actual vacuum degree and the set value deviation, the vacuum degree is precisely controlled, thus achieving precise control of the gas density inside the nanodose detector cavity.

[0037] Following steps one through five, the vacuum level of the nanodose detector cavity is controlled at approximately 3 Torr to ensure accurate and stable measurement signals from the nanodose detector. The readout signals of ionization avalanche events are counted using the nanodose detector, and the correlation between nanodose biometrics and macroscopic water absorption dose is established based on the readout signal count results.

[0038] When the vacuum level of the nanodose detector cavity is less than 0.1 Pa, the ionization avalanche event cannot occur due to the extremely low content of gas molecules inside the nanodose detector cavity.

[0039] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A vacuum system for precise control of gas density in a nanodose detector, characterized in that: Includes a vacuum pump unit, a vacuum level detection unit, and a logic control unit; The vacuum pump unit consists of a molecular pump, a vortex pump a, a vortex pump b, a second high-pressure solenoid valve, and a third high-pressure solenoid valve. The inlet of vortex pump a is connected to the second high-pressure solenoid valve via a vacuum pipe; the inlet of vortex pump b is connected to the outlet of the molecular pump via a vacuum pipe; and the third high-pressure solenoid valve is connected to the inlet of the molecular pump via a vacuum pipe. The other sides of the second and third high-pressure solenoid valves are connected to the outlet below the nanodose detector via a KF interface. Vortex pump a is used for pre-evacuation to ensure that the vacuum level inside the cavity drops below 20 Pa. Since the vacuum level needs to be below 10 Pa when the molecular pump starts, vortex pump b is installed as a secondary pump for the molecular pump to ensure that the pressure on both sides of the inlet and outlet of the molecular pump is less than 10 Pa to complete the start-up of the molecular pump. The coordinated use of the pump unit ensures the stable operation of the molecular pump. If the molecular pump is started directly at atmospheric pressure, it will cause damage to the pump blades and thus damage the molecular pump. The vacuum detection unit includes a low vacuum pressure sensor, a high vacuum pressure sensor, and a static vacuum pressure sensor. The vacuum detection unit is used to detect the vacuum level inside the pipeline and cavity. The low vacuum pressure sensor is installed on the sixth vacuum pipeline between the bottom outlet and the third high-pressure solenoid valve, and is used to detect the vacuum level of the sixth vacuum pipeline in real time. The high vacuum pressure sensor is installed on the seventh vacuum pipeline between the molecular pump and the third high-pressure solenoid valve, and is used to detect the vacuum level in the seventh pipeline to determine whether the pipeline vacuum level has reached the molecular pump start-up condition. The static vacuum pressure sensor is connected to the middle outlet of the nanodose detector and is used to detect the static vacuum level inside the nanodose detector cavity. The vacuum detection unit ensures the detection of the vacuum level in the pipeline and cavity, and works with the PLC logic controller in the logic control unit to determine the current vacuum level status and control the high-pressure solenoid valve to operate. This, in turn, coordinates with the MFC flow controller to deliver effective gas, achieving the preset vacuum level conditions and gas concentration. The pipelines refer to the sixth and seventh vacuum pipelines. The logic control unit includes a PLC controller, an MFC flow meter, gas cylinder A, gas cylinder B, a fourth high-pressure solenoid valve, and a first high-pressure solenoid valve. The fourth high-pressure solenoid valve is connected to gas cylinder A and to the first high-pressure solenoid valve. One end of the MFC flow meter is connected to gas cylinder B, and the other end is connected to the first high-pressure solenoid valve. The other side of the first high-pressure solenoid valve is connected to the air inlet at the top of the cavity. Additionally, the PLC controller is connected to the communication interface of the first high-pressure solenoid valve, the second high-pressure solenoid valve, the third high-pressure solenoid valve, the fourth high-pressure solenoid valve, the MFC, and the static vacuum pressure sensor. The PLC logic controller collects branch vacuum values ​​and the status and flow rate of the MFC flow meter. Through serial communication and multi-device collaborative control, the PLC logic controller determines the current vacuum level and controls the high-pressure solenoid valve to operate. This, in turn, coordinates with the MFC flow controller to deliver effective gas, ensuring a constant concentration of sensitive gas inside the nanodose detector cavity and a negative pressure environment to achieve the measurement environment of the nanodose detector, thus meeting the preset vacuum level and gas concentration requirements. The relationship between vacuum pressure and propane gas concentration is established.

2. The vacuum system for precise gas density control of a nanodose detector as described in claim 1, characterized in that: The working method is as follows: Step 1: Start the vacuum pump unit; To ensure the stable start-up of the molecular pump and vortex pump, the entire start-up process and system self-check of the pump unit need to be completed; Start vortex pump a and vortex pump b. Before starting, check whether the first high-pressure solenoid valve, the second high-pressure solenoid valve, the third high-pressure solenoid valve, the fourth high-pressure solenoid valve, and the MFC flow controller are closed to ensure that the pipelines connecting the molecular pump and the vortex pump are in a sealed state to prevent damage to the molecular pump; Start the molecular pump when the start-up conditions are met, and the vacuum pump unit start-up is complete when the molecular pump speed is stable; Step 2: The second high-pressure solenoid valve is opened under PLC coordinated control, completing the connection between the nanodose detector cavity and vortex pump a. This process is a rough evacuation process for the nanodose detector cavity, quickly bringing the vacuum level inside the nanodose detector cavity to below 20Pa. When the low vacuum pressure sensor value is ≤10Pa, it indicates that the pipeline vacuum level has reached the limit of vortex pump a, achieving pre-vacuuming of the nanodose detector cavity. To completely remove the air from the nanodose detector cavity, a molecular pump with a stronger pumping speed is required. At this time, the second high-pressure solenoid valve is closed and the third high-pressure solenoid valve is opened, completing the connection between the nanodose detector cavity and the molecular pump, achieving precise evacuation of the nanodose detector cavity, and more precisely extracting gas molecules. The molecular pump has a stronger pumping speed, keeping the nanodose detector cavity at a lower vacuum level. Step 3: Due to leaks in the nanodose detector cavity and pipelines, as well as dead volume issues within the nanodose detector cavity, it is impossible to evacuate the nanodose detector cavity to an absolute vacuum. In this case, gas replacement is required to minimize gas impurities inside the nanodose detector cavity. When the low vacuum pressure sensor reading reaches 1~3 Pa, the third high-pressure solenoid valve is closed, and the first and fourth high-pressure solenoid valves are opened to deliver nitrogen from gas cylinder A into the nanodose detector cavity. This allows nitrogen molecules to occupy space inside the nanodose detector cavity, thereby rapidly increasing the vacuum level inside the cavity to atmospheric pressure. Step 4: Repeat steps 2 and 3. By circulating nitrogen to wash the gas, the content of other gases besides nitrogen inside the nanodose detector cavity can be controlled to decrease to below 0.1%. Repeat steps 2 and 3 15 to 20 times. Finally, keep the nanodose detector cavity in a vacuum state and keep the first high-pressure solenoid valve, the second high-pressure solenoid valve, and the fourth high-pressure solenoid valve in the closed state. The vacuum state refers to a vacuum degree ≤1.2 Pa. Step 5: Introduce effective gas into gas cylinder B through the MFC flow meter to establish the relationship between vacuum pressure and propane gas concentration; set the flow rate of the MFC flow meter to 0.01~0.1 mL / min; the PLC monitors the value of the static vacuum gauge in real time, and closes the MFC flow meter and the first high-pressure solenoid valve when the value equals the set value, waiting for the pressure equalization of the nanodose detector cavity to be completed, thus ensuring that the actual vacuum degree in the nanodose detector cavity deviates from the set value within 5%, thereby ensuring accurate and stable measurement signals; the correspondence between the actual vacuum degree and the set value deviation is determined by fitting, and the vacuum degree is precisely controlled according to the fitted correspondence between the actual vacuum degree and the set value deviation, that is, to achieve precise control of the gas density in the nanodose detector cavity.

3. The vacuum system for precise control of gas density in a nanodose detector as described in claim 2, characterized in that: The deviation within 5% is due to the dead volume inside the nanodose detector cavity and the adsorption of gas molecules by deposited grease.

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