Precision Detection Test Method for Trace Water in Lunar Soil under the Coupling of Equivalent Vacuum and Ultra-Low Temperature

Abstract

The present invention relates to a precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature, belonging to the technical field of extraterrestrial celestial body sampling and detection. Aiming at the technical bottleneck that the existing lunar soil trace water detection technology on the earth is interfered by water vapor, the solution steps include: placing the lunar soil in the sample chamber inside the nuclear magnetic probe; using liquid nitrogen refrigeration to adjust the temperature of the sample chamber to simulate the ultra-low temperature environment of the moon; adjusting the humidity condition of the sample chamber and collecting the nuclear magnetic signal amount of lunar soil under different humidities; establishing a relationship curve between different humidities and the increment of nuclear magnetic signals; when there are differences in the signal amounts, taking the nuclear magnetic signal per unit humidity under steady-state conditions as the benchmark to deduct the signal increment caused by environmental humidity, and obtaining the nuclear magnetic signal data under the equivalent vacuum environment; and inverting the signal amount under the equivalent vacuum to obtain the lunar soil water content. Compared with the existing technology, the present invention solves the key technical problems in the detection of extraterrestrial samples in the earth environment, and provides an important means for lunar water resource detection and the analysis of sampling return samples.

Description

Technical Field

[0001] The present invention relates to the technical field of extraterrestrial celestial body sampling and detection, and particularly to a precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature. Background Art

[0002] The lunar surface is under extreme space environmental conditions, and its environmental characteristics are essentially different from those of the Earth. Due to the lack of protection of the atmosphere, the lunar surface is directly exposed to cosmic space, resulting in an extreme fluctuation characteristic of the thermal environment. The highest temperature during the lunar day reaches 127 °C, while the lowest temperature at night can drop to -183 °C. This drastic temperature fluctuation has a significant impact on the physical and chemical properties of lunar surface substances (such as soil). Through satellite remote sensing, lunar soil sampling and analysis, it can be inferred that the forms of water occurrence on the moon include structural water in the mineral lattice and bound water on the particle surface. Among them, the lunar soil samples collected by Chang'e-5 show that the water content is less than 120 ppm (that is, about 120 grams of water in 1 ton of lunar soil). This indicates that there is trace water in the lunar soil. However, there are two key challenges in detecting trace water in lunar soil under the Earth's environment: First, the water content of the lunar soil sample itself is extremely low, and the water vapor in the air (humidity condition) in the Earth's environment inevitably interacts with the sample, and this interference will seriously affect the repeatability and accuracy of the experimental results; Second, the traditional water content measurement methods (such as time domain reflectometry, differential scanning calorimetry, etc.) have obvious limitations: the measurement error is large, and the heat generated during the test may interfere with the test results. In contrast, the low-field nuclear magnetic resonance technology has significant advantages: this method does not need to destroy the sample, can maximize the preservation of the original state of the soil sample, and can accurately obtain the true data of the water content in lunar soil under different temperature and humidity conditions. However, the existing equipment for detecting trace water in lunar soil, such as low-field nuclear magnetic resonance, is often distributed in different humidity regions. For example, when detecting the water content of lunar soil in Shanghai, the air humidity is relatively high, and vice versa, when conducting tests in the northwest region, the air humidity is relatively low. And due to the vacuum environment of the moon, most tests also need to rely on professional equipment of the aerospace institute to carry out tests. This not only greatly increases the research cost but also raises the technical threshold of the research. In order to give full play to the versatility of the low-field nuclear magnetic resonance moisture detection instrument, it is urgent to propose a precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature. Summary of the Invention

[0003] The object of the present invention is to propose a test method that can overcome environmental interference and achieve precise detection of trace water in lunar soil, so as to solve the problem of the influence of water vapor in the Earth's environment on the detection results. The core innovation of this method lies in the first proposed concept of "equivalent vacuum" - this system does not preset a fixed vacuum degree threshold, but realizes the intelligent discrimination of the test environment by establishing a dynamic correlation between the humidity gradient and the increment of nuclear magnetic resonance (NMR) signal: when the increment of NMR signal under specific humidity conditions approaches zero, it is determined that the current environment reaches the equivalent vacuum state, and the in-situ inversion analysis of the water content in lunar soil can be directly carried out; if there is a significant signal shift, the environmental moisture interference is deducted through a humidity compensation algorithm, and finally precise detection data of trace water equivalent to the lunar surface conditions are obtained. It breaks through the dependence on traditional mechanical vacuum pumping or high-precision and sophisticated equipment in aerospace, but relies on relatively common low-field NMR moisture detection instruments.

[0004] The relative characteristic of the "equivalent vacuum" concept is reflected in its environmental adaptation mechanism: under the conditions of an Earth laboratory, if the NMR response characteristics of lunar soil samples in different humidity regions (such as arid regions in northern China and humid regions in southern China) show a high degree of consistency, it indicates that the humidity fluctuation in the Earth's environment has not caused substantial interference to the test results, and at this time the test environment can be regarded as an equivalent vacuum. On the contrary, when cross-regional signal differences are detected, the reference humidity conditions corresponding to the signal stability threshold are identified, and by calculating the signal correction amount under different humidity gradients, the measured data in the Earth's environment are finally corrected to the equivalent vacuum reference to ensure the comparability of the lunar soil moisture content measurement results with the in-situ measurement values on the moon.

[0005] To achieve the above object, the present invention proposes a precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature, including the following steps:

[0006] S1: Place the lunar soil in the sample chamber of the NMR probe;

[0007] S2: Adjust the temperature of the sample chamber by liquid nitrogen refrigeration to simulate the ultra-low temperature environment on the lunar surface;

[0008] S3: Adjust the humidity conditions of the sample chamber and collect the NMR signal amounts of lunar soil under different humidity conditions;

[0009] S4: Establish a quantitative relationship curve between different humidity gradients and the increment of NMR signal;

[0010] S5: When there are differences in the increment of NMR signal, based on the NMR signal per unit humidity under steady-state conditions, deduct the signal increment caused by environmental humidity to obtain the NMR signal data in the equivalent vacuum environment;

[0011] S6: Invert and calculate the trace water content of the lunar soil sample according to the NMR signal data in the true equivalent vacuum environment.

[0012] Further, in step S1, the lunar soil sample is selected from real lunar soil or a lunar soil simulant that meets the standards. The materials of the lunar soil simulant include at least one of volcanic ash, basalt, and ilmenite; the sample chamber is made of a fluorine-free material, including polyetheretherketone (PEEK) or polyimide. The sample chamber is cylindrical in shape, with a diameter of 5 mm and a height of 20 mm, geometrically matching the NMR probe.

[0013] Further, in step S2, the temperature of the sample chamber is reduced to the target temperature of -183°C by liquid nitrogen refrigeration to freeze the temperature of the lunar soil body, so as to simulate the ultra-low temperature environment on the moon; the lowest temperature of the liquid nitrogen refrigeration is -193°C, so as to simulate the real temperature (-183°C, the temperature at night on the moon) experienced by the lunar soil; whether the soil body reaches the target temperature is judged according to the reading of the temperature sensor in the middle of the soil body in the sample chamber.

[0014] Further, in step S2, the liquid nitrogen refrigeration realizes precise temperature control through the PID temperature control algorithm, with a temperature control accuracy of ±0.1°C, and thermocouples are arranged every 1 cm along the axial direction of the sample chamber for temperature monitoring to ensure that the overall temperature of the sample uniformly reaches the target value.

[0015] Further, in step S3, the humidity adjustment adopts a staged pressure reduction method. Using a humidity regulator, with 10%RH as the gradient unit, the humidity in the chamber is gradually reduced from 100%RH to 0%RH, and a humidity meter is used to obtain the real humidity condition; combined with a molecular pump group, the pressure in the chamber is gradually reduced from atmospheric pressure to the order of 10-3 Pa; the NMR signal quantity includes the overall T2 spectrum and the layered T2 spectrum of the lunar soil sample under ultra-low temperature conditions, which are collected in real time by a low-field nuclear magnetic resonance device using the CPMG sequence.

[0016] Further, in step S3, the T2 spectrum and the layered T2 spectrum of the lunar soil sample under ultra-low temperature conditions at different humidity conditions are collected in real time by a low-field nuclear magnetic resonance device. The specific method for collecting the NMR signal quantity of the lunar soil at different humidity conditions is: fixing the temperature of the lunar soil sample at a constant -183°C, and collecting the overall T2 spectrum and the layered T2 spectrum of the sample at different humidity conditions (from 0%RH to 100%RH) corresponding to this temperature; the overall T2 spectrum and the layered T2 spectrum respectively refer to the overall T2 spectrum of the soil sample and the layered T2 spectrum at different heights of the soil sample.

[0017] Further, in step S4, the quantitative relationship curve is established through a controllable humidity experiment from 0%RH to 100%RH, which is used to calibrate the interference of water vapor in the Earth's environment on the nuclear magnetic detection signal. This curve is obtained under the coupled environment of equivalent vacuum and ultra-low temperature, which truly matches the actual environment of lunar soil. If the slope of the curve is zero (i.e., the increment of the nuclear magnetic signal does not change with humidity), it indicates that the lunar soil sample has the characteristic of completely blocking water vapor adsorption, that is, the humidity condition will not affect the detection of trace water in the lunar soil sample. At this time, the nuclear magnetic signal directly reflects the true water content, and the water content is directly inverted from the nuclear magnetic signal quantity. If the curve is linear or non-linear, it means that water vapor is adsorbed / absorbed by the lunar soil, resulting in the detection signal including the contribution of additional moisture in the Earth's environment. At this time, the environmental humidity interference is eliminated by the reference subtraction method.

[0018] Further, the reference subtraction method specifically measures the increment of the nuclear magnetic signal of lunar soil in adjacent humidity intervals with equal gradients through low-field nuclear magnetic resonance experiments in the reference humidity range. . If the signal increment in adjacent humidity intervals is the same, at this time, the nuclear magnetic signal increment strictly corresponds to the water vapor adsorption amount caused by humidity change. By calculation, the nuclear magnetic signal quantity per unit humidity can be obtained, and then the nuclear magnetic signal quantity under other humidity conditions is subtracted by the signal increment corresponding to the unit humidity, and the true nuclear magnetic signal data in the equivalent vacuum environment can be obtained, which is the net signal quantity of the inherent moisture of the lunar soil sample.

[0019] Further, in step S5, the steady state means that during the adjustment process of adjacent humidity intervals with equal gradients, when the relative deviation of the nuclear magnetic signal caused by three consecutive humidity changes ≤ 0.1%, it is considered that the system reaches a stable state. At this time, the nuclear magnetic signal increment strictly corresponds to the water vapor adsorption amount caused by humidity change, and the water content of the lunar soil body remains constant; in the stable state, the nuclear magnetic signal increment and the humidity difference show a linear relationship with equal gradient changes; when determining the nuclear magnetic signal increment corresponding to a certain humidity range, the nuclear magnetic signal quantity under other humidity conditions is subtracted by the signal increment caused by the environmental humidity, and the true nuclear magnetic signal data in the equivalent vacuum environment can be obtained.

[0020] Further, step S6 is specifically as follows:

[0021] Normalize the nuclear magnetic signal data in the true equivalent vacuum environment to obtain the nuclear magnetic signal quantity of the lunar soil sample per unit mass in the equivalent vacuum environment;

[0022] Use a standard sample to pre-calibrate the water content corresponding to the nuclear magnetic signal quantity per unit of the nuclear magnetic resonance equipment and record it as the calibration coefficient;

[0023] Based on the nuclear magnetic signal quantity per unit mass of the lunar soil and the calibration coefficient, the trace water content corresponding to the lunar soil sample per unit mass is inversely analyzed to achieve high-precision quantitative analysis of the trace water content of the lunar soil sample.

[0024] Furthermore, the inversion calculation formula for the trace water content in lunar soil samples is as follows:

[0025] ;

[0026] In the formula, w 1 is the water content of the lunar soil sample; w 0 is the water content corresponding to the unit signal quantity collected by the used nuclear magnetic resonance equipment; is the unit signal quantity collected by the used nuclear magnetic resonance equipment; F 1 is the nuclear magnetic signal quantity corresponding to the unit mass of the lunar soil.

[0027] Based on the data collected under ultra-low temperature and equivalent vacuum environment, the present invention establishes a relationship curve between different humidities and the increment of nuclear magnetic signals, scientifically evaluates the influence degree of environmental humidity on the detection of trace water in lunar soil, and obtains the nuclear magnetic signal data under equivalent vacuum environment by deducting the signal increment caused by environmental humidity. Using the water content corresponding to the unit signal quantity of different nuclear magnetic resonance equipment itself, the nuclear magnetic signal data collected under the true equivalent vacuum environment is solved and further inverted to obtain the trace water content in lunar soil, which not only provides important technical support for future human lunar immigration and deep space base construction, but also lays a key guarantee for the exploration and development of extraterrestrial celestial body resources.

[0028] Compared with the prior art, the advantages of the present invention are as follows:

[0029] 1. Through extreme environment simulation, dynamic signal calibration, precise interference stripping and high-precision inversion model, the present invention realizes the interference-free and high-precision detection of trace water in lunar soil in the earth laboratory for the first time, solves the two major problems of "uncontrollable earth environment interference" and "insufficient detection accuracy" that have long troubled the industry, and provides key technical support for lunar resource development and deep space exploration.

[0030] 2. The present invention accurately reproduces the extreme environment of ultra-low temperature and equivalent vacuum on the moon in the laboratory, solves the problem of earth environment interference that cannot be avoided in traditional detection, combines the dynamic humidity gradient experimental design, systematically calibrates the quantitative relationship between humidity and the increment of nuclear magnetic signals, quantifies the influence of earth environment humidity on the detection results, and provides necessary environmental conditions for subsequent interference elimination and precise detection.

[0031] 3. The present invention adopts the reference deduction method. Taking the nuclear magnetic signal per unit humidity under steady-state conditions as the reference, it gradually strips the signal increment caused by environmental humidity, and for the first time realizes the complete elimination of the interference of terrestrial water vapor. Combining with the nuclear magnetic signal acquisition technology at ultra-low temperature, it significantly improves the sensitivity and spatial resolution of the detection of trace water in lunar soil, effectively solves the technical problem that it is difficult for the existing technology to distinguish the humidity interference of the terrestrial environment from the accurate detection and characterization of trace water in lunar soil, and provides reliable technical support and scientific basis for the assessment of lunar water resource distribution, the analysis of sampling return samples, and the construction of a manned lunar base.

[0032] 4. The present invention does not rely on special aerospace equipment, has the adaptability of general-purpose equipment, can realize standardized detection in different regions, and greatly reduces costs.

[0033] 5. In the process of calculating the trace water in lunar soil, the present invention adopts the pre-calibration technology. By pre-calibrating the calibration coefficient between the nuclear magnetic signal amount and the water content with a standard sample, it solves the compatibility problem of signal amount differences between different devices; combined with the inversion calculation formula model, it converts complex signal data into intuitive water content values, simplifying the process of quantitative analysis of trace water.

[0034] 6. The method of the present invention organically combines cross-field technologies such as nuclear magnetic resonance technology, liquid nitrogen refrigeration, vacuum control, and dynamic humidity control to form a systematic solution, providing a general method framework for the exploration of extraterrestrial celestial body resources. This method is not only applicable to lunar soil, but also can be extended to the detection of trace water in other extraterrestrial samples such as Mars and asteroids, and has broad application prospects for deep space exploration. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic flow chart of a method for accurately detecting trace water in lunar soil under a coupled environment of equivalent vacuum and ultra-low temperature provided by an embodiment of the present invention;

[0036] Figure 2 It is a schematic diagram of a relationship curve in which the nuclear magnetic signal increment does not change with humidity (the slope is zero) in an embodiment of the present invention;

[0037] Figure 3 It is a schematic diagram of a relationship curve in which the nuclear magnetic signal increment changes linearly with humidity in an embodiment of the present invention;

[0038] Figure 4 It is a schematic diagram of a relationship curve in which the nuclear magnetic signal increment changes non-linearly with humidity in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be further described below.

[0040] This embodiment proposes a precise detection method for trace water in lunar soil under an equivalent vacuum and ultra-low temperature coupling environment, as Figure 1 shown. This method includes the following steps:

[0041] S1: Place the lunar soil in the sample chamber inside the nuclear magnetic probe;

[0042] In this embodiment, according to the provisions of the "Standard for Geotechnical Test Methods" (GBT 50123-2019), the lunar soil samples are selected from real lunar soil or lunar soil simulants that meet the standards (JSC-1A or CAS-1, earth materials for simulating lunar soil). The materials of this lunar soil simulant include at least one of volcanic ash, basalt, and ilmenite;

[0043] Among them, JSC-1A is a lunar soil simulant developed by NASA's Johnson Space Center, processed from volcanic ash in Arizona, USA, similar to the lunar mare basalt soil brought back by the Apollo mission. Its composition (vitreous particles, plagioclase, etc.) and particle characteristics (particle size range 0.01–1000 μm, average about 100 μm, similar to lunar fine particle dust) can highly simulate real lunar soil.

[0044] CAS-1 is a low-titanium basalt lunar soil simulation material developed by the Chinese Academy of Sciences. The raw materials are from the basalt of the Changbai Mountain volcanic group in Jilin Province, China. Its mineral composition is highly consistent with the low-titanium lunar mare soil obtained by the Apollo mission (including minerals such as plagioclase, pyroxene, and olivine), and its particle size is similar to that of JSC-1A.

[0045] In addition, the sample chamber is precisely machined from polyether ether ketone (PEEK) or polyimide materials. The shape of the sample chamber is designed as a cylinder with a diameter of 5 mm and a height of 20 mm, geometrically matching the nuclear magnetic probe. The chamber body of the sample chamber is a hollow structure, and the central area of the chamber is the sample area. The soil sample is loaded through a polytetrafluoroethylene mold (cylindrical). The area outside the mold is empty and used to circulate the liquid to reduce the temperature of the soil sample. The working temperature range of the cylindrical sample chamber is -269°C to 200°C. Place the prepared sample into the sample chamber, and then place the sample chamber into the nuclear magnetic resonance probe.

[0046] S2: Adjust the temperature of the sample chamber through liquid nitrogen refrigeration to simulate the ultra-low temperature environment on the lunar surface;

[0047] In this embodiment, the liquid nitrogen freezing controller is connected to both sides of the sample chamber through the inlet and outlet pipelines, and the liquid nitrogen refrigeration system (cold end) and the resistance heating system (hot end) are connected and started to pre-cool to the target temperature (-183 °C). Determine whether the soil body reaches the target temperature according to the readings of the temperature sensors in the middle of the lunar soil body in the sample chamber. Set the PID temperature control algorithm to ensure the temperature control accuracy of ±0.1 °C. Calibrate the thermocouple (type T or K, accuracy ±0.05 °C) to ensure data reliability. Start the anti-frosting system to prevent frosting at the cold end from affecting temperature control. Enter the target temperature of -183 °C in the control software, select the linear gradient mode, and the system automatically calculates the heat flux distribution and coordinates the regulation through the liquid nitrogen cold source (cold end) and the ceramic heating sheet (hot end) in the liquid nitrogen freezing controller. A thermocouple is arranged every 1 cm along the axis of the sample chamber (a total of N, depending on the sample length), fixed on the sample surface or embedded in the chamber wall. Use a standard temperature source (such as a platinum resistance thermometer) to calibrate all thermocouples in-situ to ensure data consistency. When the temperature stabilizes at -183 °C, it is considered that the ultra-low temperature environment has been created.

[0048] S3: Adjust the humidity condition of the sample chamber and collect the lunar soil nuclear magnetic signal amounts under different humidity conditions;

[0049] In this embodiment, a humidity meter is used to obtain the real humidity condition, and the humidity is adjusted by the staged pressure reduction method. The specific method is as follows:

[0050] 1) Connect the high-precision humidity regulator to the sample chamber through a sealed interface to ensure the airtightness of the system; then set the initial humidity in the sample chamber to 100% RH (relative humidity) to simulate the Earth's atmospheric environment; use the staged pressure reduction method, with a gradient unit of 10% RH, gradually reduce the humidity in the chamber. When the humidity drops below 10% RH, switch to the molecular pump group and gradually reduce the pressure in the chamber from normal pressure (101.325 kPa) to the order of 10-3 Pa; during this process, record the change curves of the humidity (accuracy ±0.1% RH) and pressure (accuracy ±0.1 Pa) in the chamber in real time.

[0051] 2) Turn on the low-field nuclear magnetic resonance (NMR) instrument, preheat the magnet to a stable state, load the special probe for lunar soil samples, select the CPMG sequence, adjust NMR parameters such as the 90° and 180° pulse widths and amplitudes, echo time, number of echoes, number and thickness of slice layers, etc., and use the low-field NMR instrument to collect the overall T2 spectrum and layered T2 spectrum of lunar soil samples at different humidities under normal temperature and ultra-low temperature conditions. Among them, the layer thickness can be determined according to the equipment accuracy and research needs. Specifically: According to the spatial resolution of the NMR instrument (usually 0.1 - 1 mm), ensure that the layer thickness ≥ the minimum resolvable scale (such as 3 mm). In order to capture the change of water content gradient, a layer thickness of 5 - 10 mm is preferably recommended (to capture the change of water content gradient). In order to focus on analyzing the change of solid-liquid interface effect, a thin layer of 3 - 5 mm is preferably selected. The layer position is symmetrically layered with the center plane of the probe as the reference (such as ±10 mm, ±20 mm). Or determine the high signal gradient area through pre-scanning and densify the layering (such as with an interval of 3 mm).

[0052] 3) Open the nuclear magnetic resonance imaging software, adjust parameters such as resonance frequency, sampling bandwidth, echo position, etc., and collect nuclear magnetic resonance images of lunar soil samples under normal temperature and ultra-low temperature conditions at different humidities as a whole and in different directions (XYZ).

[0053] S4: Establish a quantitative relationship curve between different humidity gradients and the increment of nuclear magnetic resonance signal;

[0054] In this embodiment, a quantitative relationship curve between the increment of nuclear magnetic resonance signal and humidity is established through a controllable humidity experiment (0%RH - 100%RH gradient) to calibrate the interference of water vapor in the earth environment on the detection signal. This curve is obtained under an equivalent vacuum and ultra-low temperature coupling environment and can truly match the actual environment of lunar soil. If the increment of nuclear magnetic resonance signal does not change with humidity (the slope is zero), as Figure 2 shown, it indicates that the lunar soil sample has the characteristic of completely blocking water vapor adsorption, that is, the humidity condition does not affect the detection of trace water in the lunar soil sample. At this time, the nuclear magnetic resonance signal directly reflects the true water content, and the water content is directly inverted based on this nuclear magnetic resonance signal quantity.

[0055] S5: When the nuclear magnetic resonance signal response shows humidity dependence, adopt a dynamic reference correction strategy: Use the nuclear magnetic resonance response amount per unit humidity change under steady-state conditions as the reference to subtract the signal increment caused by environmental humidity, so as to decouple the intrinsic nuclear magnetic resonance signal of trace water in lunar soil under an equivalent vacuum environment;

[0056] Among them, the steady-state condition means that during the adjustment process of adjacent humidity intervals with equal gradients, when the relative deviation of the nuclear magnetic resonance signal caused by humidity changes for three consecutive times ≤ 0.1%, it is considered that the system reaches a stable state. At this time, the increment of the nuclear magnetic resonance signal strictly corresponds to the water vapor adsorption amount caused by humidity changes, and the water content of the lunar soil body remains constant. In the stable state, the increment of the nuclear magnetic resonance signal has a linear relationship with the humidity difference (equal-gradient change). When determining the increment of the nuclear magnetic resonance signal corresponding to a certain humidity range, the nuclear magnetic resonance signal amount under other humidity conditions is deducted from the signal increment caused by the environmental humidity to obtain the true nuclear magnetic resonance signal data in an equivalent vacuum environment.

[0057] In this embodiment, as Figure 3 and Figure 4 shown, when the magnetic signal increment changes linearly or non-linearly with humidity, it indicates that water vapor is adsorbed / absorbed by the lunar soil, resulting in the detection signal containing the contribution of moisture in the additional earth environment. At this time, it is necessary to eliminate the environmental humidity interference through the reference subtraction method and subtract the water vapor entering the soil.

[0058] The specific method of this reference subtraction method is as follows:

[0059] 1) Calibrate the reference increment

[0060] Under the reference humidity range, measure the increment of the nuclear magnetic resonance signal of the lunar soil through low-field nuclear magnetic resonance experiments . For example: if the signal increases when the humidity rises from 10% to 20% .

[0061] Verification in other humidity ranges: In the range of 20% → 30%, the signal increment is still . Then the signal amount corresponding to the unit humidity (1%) is / 10. If the measured increment deviates from , it is necessary to check the systematic error, first check the system drift (temperature / magnetic field stability), then verify the sample homogeneity, and if necessary, expand the calibration humidity range to reconstruct the response model.

[0062] 2) Subtract the influence of air water vapor

[0063] Record the nuclear magnetic resonance signal of each humidity point (such as 15%, 25%) as (measured value). According to the calibrated signal amount of the unit humidity ( / 10%), calculate the trace water signal of the lunar soil corresponding to the current humidity. The formula for subtracting the nuclear magnetic resonance signal of water vapor in the air is: = - (current humidity × / 10).

[0064] Example: = 1000, the nuclear magnetic resonance signal amount corresponding to 1% humidity is 100, the nuclear magnetic resonance signal amount at 25% humidity is 4000, and the lunar soil trace water signal = 4000 - (25 * 100) = 1500.

[0065] When the humidity drops to the equivalent vacuum condition (the relative deviation of the nuclear magnetic resonance signal caused by three consecutive humidity changes ≤ 0.1%, subtracting the unit nuclear magnetic resonance signal difference of water vapor in the air), the true nuclear magnetic resonance signal data under the equivalent vacuum environment can be obtained. This remaining signal is the net nuclear magnetic resonance signal of the inherent water content characteristics of the lunar soil sample, realizing the accurate detection of the trace water in the lunar soil sample itself.

[0066] S6: Invert and calculate the trace water content of the lunar soil sample according to the nuclear magnetic resonance signal data under the true equivalent vacuum environment.

[0067] In this embodiment, the specific operations are as follows:

[0068] 1) Normalize the nuclear magnetic resonance signal data under the true equivalent vacuum environment obtained through the above steps to obtain the nuclear magnetic resonance signal amount of the lunar soil sample per unit mass under the equivalent vacuum environment;

[0069] 2) Use a standard sample with a known water content (such as pure water or water-containing minerals) to pre-calibrate the water content corresponding to the unit nuclear magnetic resonance signal amount of the nuclear magnetic resonance equipment and record it as the calibration coefficient;

[0070] For example: When calibrating with pure water, the nuclear magnetic resonance signal amount corresponding to 100% water content (1 g water / g sample) is 1000, then the water content corresponding to unit signal amount 1 is 0.1% (i.e., the calibration coefficient k = 0.1% / unit signal);

[0071] 3) Based on the nuclear magnetic resonance signal amount per unit mass of the lunar soil and the calibration coefficient, inversely analyze to obtain the trace water content corresponding to the lunar soil sample per unit mass, realizing the high-precision quantitative analysis of the trace water content of the lunar soil sample.

[0072] The inversion calculation formula for the trace water content of the lunar soil sample is:

[0073] ;

[0074] In the formula, w 1 is the water content of the lunar soil sample; w 0 is the water content corresponding to the unit signal amount collected by the used nuclear magnetic resonance equipment; is the unit signal amount collected by the used nuclear magnetic resonance equipment; F 1 is the nuclear magnetic resonance signal amount per unit mass of the lunar soil.

[0075] In this embodiment, when the nuclear magnetic resonance signal amount of the lunar soil sample is 50, then the water content of this lunar soil sample 。

[0076] Through the method of this embodiment, the interference-free and high-precision detection of trace water in lunar soil has been successfully achieved in the Earth laboratory.

[0077] The above are only the preferred embodiments of the present invention and do not impose any limitation on the present invention. Any person skilled in the art, within the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification and other changes to the technical solution and technical content disclosed by the present invention, which are all within the content of the technical solution of the present invention and still fall within the protection scope of the present invention.

Claims

1. A precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature, characterized in that, It includes the following steps: S1: Place the lunar soil in the sample chamber inside the nuclear magnetic probe; S2: Adjust the temperature of the sample chamber through liquid nitrogen refrigeration to simulate the ultra-low temperature environment on the lunar surface; S3: Adjust the humidity condition of the sample chamber and collect the nuclear magnetic signal amounts of the lunar soil under different humidity conditions; S4: Establish a quantitative relationship curve between different humidity gradients and the increment of nuclear magnetic signals; in step S4, the quantitative relationship curve is established through a controllable humidity experiment from 0%RH to 100%RH, which is used to calibrate the interference of water vapor in the earth environment on the nuclear magnetic detection signal; if the slope of the curve is zero, directly invert the water content with the nuclear magnetic signal amount; if the curve is linear or non-linear, eliminate the environmental humidity interference through the reference subtraction method; The specific method of the reference subtraction method is as follows: in the reference humidity range, measure the increment of nuclear magnetic signals ΔS0 of the lunar soil in adjacent humidity intervals with equal gradients through low-field nuclear magnetic resonance experiments; if the signal increments in adjacent humidity intervals are the same, at this time the increment of nuclear magnetic signals strictly corresponds to the water vapor adsorption amount caused by humidity change, and the nuclear magnetic signal amount per unit humidity can be obtained through calculation, and then subtract the signal increment corresponding to the unit humidity from the nuclear magnetic signal amounts under other humidity conditions to obtain the true nuclear magnetic signal data in an equivalent vacuum environment, which is the net signal amount of the inherent moisture of the lunar soil sample; S5: When there are differences in the increment of nuclear magnetic signals, subtract the signal increment caused by environmental humidity based on the nuclear magnetic signal per unit humidity under steady-state conditions to obtain the nuclear magnetic signal data in an equivalent vacuum environment; in step S5, the steady-state condition means that during the adjustment process of adjacent humidity intervals with equal gradients, when the relative deviation of the nuclear magnetic signals caused by continuous 3 humidity changes ≤ 0.1%, it is considered that the system reaches a stable state, at this time the increment of nuclear magnetic signals strictly corresponds to the water vapor adsorption amount caused by humidity change, and the water content of the lunar soil body remains constant; under the stable state, the increment of nuclear magnetic signals has a linear relationship with the humidity difference in an equal-gradient change; when determining the increment of nuclear magnetic signals corresponding to a certain humidity range, subtract the signal increment caused by environmental humidity from the nuclear magnetic signal amounts under other humidity conditions to obtain the true nuclear magnetic signal data in an equivalent vacuum environment; S6: Invert and calculate the trace water content of the lunar soil sample based on the nuclear magnetic signal data in the true equivalent vacuum environment and the calibration coefficient of the nuclear magnetic signal amount and the water content calibrated in advance.

2. The precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature according to claim 1, characterized in that, In step S1, the lunar soil sample is selected from real lunar soil or a lunar soil simulant meeting the standard, and the material of the lunar soil simulant includes at least one of volcanic ash, basalt and ilmenite; the sample chamber is made of a fluorine-free material, including polyether ether ketone (PEEK) or polyimide, and the shape of the sample chamber is cylindrical, with a diameter of 5 mm and a height of 20 mm, geometrically matching the nuclear magnetic probe.

3. The precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature according to claim 1, characterized in that In step S2, the temperature of the sample chamber is reduced to the target temperature of -183°C through liquid nitrogen refrigeration to freeze the temperature of the soil body to simulate the ultra-low temperature environment on the moon; judge whether the soil body reaches the target temperature according to the reading of the temperature sensor in the middle of the soil body in the sample chamber.

4. The precise detection test method for trace lunar water under the coupling of equivalent vacuum and ultra-low temperature according to claim 1, characterized in that, In step S2, the liquid nitrogen refrigeration realizes precise temperature control through the PID temperature control algorithm, with a temperature control accuracy of ±0.1 °C. Thermocouples are arranged every 1 cm along the axial direction of the sample chamber for temperature monitoring to ensure that the overall temperature of the sample evenly reaches the target value.

5. The precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature according to claim 1, characterized in that In step S3, the humidity adjustment adopts a step-by-step pressure reduction method. Using a humidity regulator, with 10%RH as the gradient unit, the humidity in the chamber is gradually reduced from 100%RH to 0%RH; combined with a molecular pump group, the pressure in the chamber is gradually reduced from atmospheric pressure to the order of 10 - 3 Pa magnitude; the nuclear magnetic signal quantity includes the overall T2 spectrum and the layered T2 spectrum of the lunar soil sample under ultra-low temperature conditions, which are collected in real time by a low-field nuclear magnetic resonance device using the CPMG sequence.

6. The precise detection test method for trace water in lunar soil under the coupling of equivalent vacuum and ultra-low temperature according to claim 5, characterized in that, In step S3, the specific method for collecting the nuclear magnetic resonance signal quantity of lunar soil under different humidity conditions is as follows: fix the temperature of the lunar soil sample at a constant -183 °C, and collect the overall T2 spectrum and the layered T2 spectrum of the sample under different humidity conditions at this temperature; the overall T2 spectrum and the layered T2 spectrum respectively refer to the overall T2 spectrum of the soil sample and the layered T2 spectrum at different heights of the soil sample.

7. The precise detection test method for trace lunar water under the coupling of equivalent vacuum and ultra-low temperature according to claim 1, characterized in that Step S6 is specifically as follows: Normalize the nuclear magnetic resonance signal data in the real equivalent vacuum environment to obtain the nuclear magnetic resonance signal quantity of the lunar soil sample per unit mass in the equivalent vacuum environment; Use a standard sample to pre-calibrate the water content corresponding to the unit nuclear magnetic resonance signal quantity of the nuclear magnetic resonance equipment, and record it as the calibration coefficient; Based on the nuclear magnetic resonance signal quantity per unit mass of the lunar soil and the calibration coefficient, perform inverse analysis to obtain the trace water content corresponding to the lunar soil sample per unit mass.

8. The precise detection test method for trace lunar water under the coupling of equivalent vacuum and ultra-low temperature according to claim 7, characterized in that, The inverse calculation formula for the trace water content of the lunar soil sample is: ； In the formula, w 1 is the water content of the lunar soil sample; w 0 is the water content corresponding to the unit signal quantity collected by the used nuclear magnetic resonance equipment, that is, the calibration coefficient; is the unit signal quantity collected by the used nuclear magnetic resonance equipment; F 1 is the nuclear magnetic signal quantity corresponding to the unit mass of lunar soil.

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