Analysis method for non-destructive measurement of stable metal isotope ratio in lunar soil

Abstract

The application belongs to the technical field of lunar soil measurement, and particularly relates to an analysis method for non-destructive measurement of stable metal isotope ratio in lunar soil, which comprises the following steps: step S1, placing a sample (2) to be measured into a reactor (1) to perform irradiation operation, wherein the sample (2) comprises a lunar soil sample and an earth sample, and the earth sample comprises standard substance and neutron fluence rate parameter monitoring material; step S2, after the irradiation operation is completed, cooling is performed, and then a HPGe detector (4) is combined with a digital multi-channel spectrometer (7) to perform radioisotope activity measurement on the activated lunar soil sample; step S3, after the radioisotope activity measurement is completed, the ratio of stable isotope A and stable isotope B in the lunar soil sample is calculated; and step S4, the difference between the isotope ratio of the lunar soil sample and the isotope ratio of the earth sample is calculated. The non-destructive isotope ratio measurement method provided by the application is complementary to the mass spectrometry method such as ICPMS, and more nuclear technology scientific data is provided for non-destructive analysis of lunar soil.

Description

Technical Field

[0001] This invention belongs to the field of lunar soil measurement technology, specifically relating to an analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil. Background Technology

[0002] The origin of the Moon is a core question in lunar research. While the Moon shares the same chemical elements as Earth, its evolution differs significantly from Earth's, resulting in different abundances and distributions of chemical elements. Currently, it is generally believed that the Moon was formed by a collision between an impactor and Earth; however, this process lacks sufficient constraints. With the development of high-precision metal isotope analysis techniques, metal isotope composition has become a crucial tool for studying the origins of planetary materials in the solar system, the genetic connections between different celestial bodies, and various planetary formation processes. Isotope anomalies have become a fingerprint for studying the origins of planetary materials, providing important constraints on the Moon's formation. Isotope anomalies are typically represented by measuring the ratios of different stable isotopes of the same element in a sample.

[0003] Existing methods for measuring stable isotope ratios primarily utilize mass spectrometry techniques such as inductively coupled plasma mass spectrometry (ICPMS). While these methods offer high accuracy, they require chemical dissolution of the sample. They employ ion optics and electromagnetic principles to separate isotopes according to their mass-to-charge ratio, thus determining their mass and relative abundance – a destructive analytical method. Given the preciousness and unique nature of lunar soil samples, non-destructive, high-precision analytical methods are generally preferred. Neutron activation analysis (NAA), using a reactor as a neutron source, is a nuclear analysis method based on nuclear reactions. Specifically, it involves bombarding the sample with reactor neutrons, causing multiple elements (at least one stable isotope of each element) to generate radioactive nuclides through nuclear reactions. By measuring the energy and intensity of the characteristic rays emitted by these nuclides, qualitative and quantitative analysis of the corresponding elements is performed. Neutron activation analysis (NAA) is a method for determining elemental abundance by measuring isotopes and offers advantages such as multi-element analysis, non-destructive nature, high sensitivity, and high accuracy. Summary of the Invention

[0004] The purpose of this invention is to provide a non-destructive neutron activation analysis method for measuring the stable metal isotope ratios in precious lunar soil. This method is fundamentally different from mass spectrometry and can complement each other's advantages.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is an analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil, comprising the following steps: Step S1, placing the sample to be tested into a reactor for irradiation, wherein the sample includes a lunar soil sample and an Earth sample, and the Earth sample includes standard materials and neutron flux rate parameter monitoring materials.

[0006] Step S2: After the irradiation operation is completed, the sample is cooled, and then the activity of radioactive isotopes is measured on the activated lunar soil sample using an HPGe detector combined with a digital multichannel spectrometer.

[0007] Step S3: After the radioactive isotope activity measurement is completed, calculate the ratio of stable isotope A to stable isotope B in the lunar soil sample.

[0008] Step S4: Calculate the difference between the isotopic ratio of the lunar soil sample and the isotopic ratio of the Earth sample.

[0009] Furthermore, in step S1, before the irradiation operation, the lunar soil sample and the Earth sample need to be packaged into a 1cm×1cm×0.1cm cube shape using high-purity polyethylene or aluminum foil; the standard material and the neutron flux rate parameter monitoring material include iron wire and zirconium sheet; the duration of the irradiation operation is set according to the half-life of the radioactive isotope to be measured generated by the activation of stable isotopes in the lunar soil sample and the neutron flux of the reactor.

[0010] Furthermore, in step S1, the lunar soil sample and the Earth sample are placed in an irradiation box made of polyethylene / polytetrafluoroethylene and transported to the reactor through a pneumatic transmission pipeline for the irradiation operation.

[0011] Furthermore, in step S2, the cooling time and measurement time need to be selected based on the radioactivity of the lunar soil sample and the half-life of the radioactive isotope to be measured.

[0012] Furthermore, in step S2, the irradiation box containing the lunar soil sample and the Earth sample is transferred to a lead glove box for cooling via a pneumatic transfer pipe, and the lunar soil sample and the Earth sample are removed from the irradiation box after cooling.

[0013] Furthermore, in step S2, the lunar soil sample and the Earth sample are placed in a lead-shielded chamber on an HPGe detector whose efficiency and other parameters have been calibrated in advance for radioactive isotope activity measurement.

[0014] Furthermore, in step S2, the HPGe detector needs to be inserted into a Dewar flask, and the liquid nitrogen stored in the Dewar flask is used to keep the HPGe detector at a liquid nitrogen temperature during operation.

[0015] Furthermore, in step S3, based on the peak area of ​​the characteristic gamma-ray peak of the radioactive isotope, as well as the reactor neutron flux, irradiation time, cooling time, measurement time, and detector efficiency, the ratio of stable isotope A and stable isotope B of the same element in the lunar soil sample is calculated using the first formula.

[0016] First formula: R = θ A / θ B ={P / (σγεSDC)} A / {P / (σγεSDC)} B

[0017] In the formula,

[0018] R is the ratio result;

[0019] P is the intensity of the characteristic gamma-ray peak of a radioactive isotope, measured in seconds (s). -1 ;

[0020] θ A It is the abundance of the stable isotope A;

[0021] θ B It is the abundance of the stable isotope B;

[0022] σ is the effective cross section for nuclear reactions between stable isotopes and neutrons, measured in cm. 2 ;

[0023] γ is the branching ratio of the characteristic gamma-ray peak of a radioactive isotope;

[0024] ε is the detection efficiency of the characteristic gamma-ray peak of a radioactive isotope.

[0025] S is the saturation factor, S = 1 - exp(-0.693t) i / T);

[0026] D is the decay factor, D = exp(-0.693t) d / T);

[0027] C is the measurement factor, C = [1 - exp(-0.693t]). c / T)] / (0.693t c / T);

[0028] t i It is the duration of the irradiation operation;

[0029] t d This refers to the cooling time;

[0030] t c The measurement time of the activity of the radioactive isotope;

[0031] T is the half-life of the radioactive isotope to be measured.

[0032] Furthermore, in step S4, the isotopic ratios in the lunar soil sample are compared with those in the Earth sample using the second formula.

[0033] Second formula: δ(‰)=(R) sa / R sd -1)×1000

[0034] In the formula,

[0035] The δ value represents the comparison result;

[0036] R sa This indicates the isotope ratio values ​​in the lunar soil sample;

[0037] R sd This indicates the isotope ratio of the Earth sample.

[0038] Furthermore, in step S2, a computer is connected to the digital multichannel spectrometer to set the relevant operating parameters of the digital multichannel spectrometer and to collect and record the characteristic gamma-ray energy spectra of radioactive isotopes measured by the digital multichannel spectrometer.

[0039] The beneficial effects of this invention are as follows:

[0040] 1. The non-destructive isotope ratio measurement method proposed in this invention can complement mass spectrometry methods such as ICPMS, providing more nuclear technology scientific data for the non-destructive analysis of precious lunar soil in my country.

[0041] 2. This invention utilizes the nuclear reaction between different stable isotopes of the same metallic element and neutrons to produce different radioactive isotopes. By measuring the intensity of the characteristic gamma rays of the generated radioactive isotopes, the ratio of different stable isotopes of the same element in the sample can be calculated, providing a non-destructive new measurement method for studying the stable isotope ratio in precious lunar soil samples. This method can also be applied to the measurement and research of stable isotope ratios in extraterrestrial samples such as asteroids and Mars.

[0042] 3. Non-destructive: No chemical treatment of the sample is required, completely avoiding the weakest link in the trace analysis traceability chain, namely the possible loss or contamination of the analyte during the sample dissolution process.

[0043] 4. This method is based on the nuclear properties of the elements in the sample and is not affected by the physical form of the sample or the chemical valence state of the elements. It has a different source of uncertainty than all other analytical methods based on atomic spectroscopy and chemical properties. The difference in isotope ratios or isotope anomalies can provide important constraints on the origin of the moon.

[0044] 5. This method measures the isotope ratios in lunar soil samples, which differs from conventional mass spectrometry methods used for isotope analysis of sample characterization (such as secondary ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS)). Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the apparatus and process used in the analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil, as described in a specific embodiment of the present invention.

[0046] In the diagram: 1-Reactor (neutron source), 2-Sample, 3-Lead chamber glove box, 4-HPGe detector, 5-Dewar flask, 6-Lead shielded chamber, 7-Digital multichannel spectrometer, 8-Computer. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0048] The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil provided by this invention includes the following steps:

[0049] Step S1: The sample 2 to be tested is placed in the reactor 1 for irradiation. The sample 2 includes lunar soil samples and Earth samples. The Earth samples include standard materials and neutron flux rate parameter monitoring materials.

[0050] Step S2: After the irradiation operation is completed, the sample is cooled, and then the activity of radioactive isotopes is measured on the activated lunar soil sample using the HPGe detector 4 combined with the digital multichannel spectrometer 7.

[0051] Step S3: After the radioactive isotope activity measurement is completed, calculate the ratio of stable isotope A to stable isotope B in the lunar soil sample.

[0052] Step S4: Calculate the difference between the isotopic ratio of the lunar soil sample and the isotopic ratio of the terrestrial sample.

[0053] In step S1, before the irradiation operation, the lunar soil sample and the Earth sample need to be encapsulated in a cube shape of about 1cm×1cm×0.1cm using high-purity polyethylene or aluminum foil; the standard material and neutron flux rate parameter monitoring materials include iron wire and zirconium sheet; the duration of the irradiation operation is set according to the half-life of the radioactive isotope to be measured generated by the activation of stable isotopes in the lunar soil sample and the neutron flux of reactor 1.

[0054] In step S1, lunar soil samples and Earth samples are placed in irradiation boxes made of polyethylene / PTFE and transported to reactor 1 via pneumatic transfer pipes for irradiation. Reactor 1 typically utilizes heavy nuclei. 235Uranium fission is a neutron source that produces neutrons through a chain reaction within the reactor. This is because neutron flux is high (typically, the thermal neutron flux within the reactor is >1 × 10⁻⁶). 12 cm -1 s -1 It has good uniformity and is mainly used as an irradiation source for neutron activation analysis.

[0055] In step S2, the cooling time and measurement time need to be selected based on the radioactivity of the lunar soil sample and the half-life of the radioactive isotope to be measured.

[0056] In step S2, the irradiation box containing the lunar soil sample and the Earth sample is transferred to the lead chamber glove box 3 via a pneumatic transfer pipe for cooling. After cooling, the lunar soil sample and the Earth sample are removed from the irradiation box. The lead chamber glove box 3 is mainly used for the storage and cooling of samples after irradiation, as well as for operations such as disassembling the irradiation box and taking samples.

[0057] In step S2, lunar soil samples and Earth samples are placed in a lead-shielded chamber 6 and placed on a pre-calibrated HPGe detector 4 with parameters such as efficiency to measure radioactive isotope activity. The lead-shielded chamber 6 is mainly used to shield the background radiation from the natural environment surrounding the HPGe detector 4.

[0058] In step S2, the HPGe detector 4 needs to be inserted into the Dewar flask 5. The liquid nitrogen stored in the Dewar flask 5 keeps the HPGe detector 4 at liquid nitrogen temperature during operation. Because the thermal noise (leakage current) is too high at room temperature, it needs to be cooled to -196°C to achieve optimal operating conditions. The HPGe detector (high-purity germanium detector) is a nuclear radiation detector made of germanium crystal, mainly used to measure the characteristic gamma (γ) rays of radioactive isotopes in samples. HPGe can be stored at room temperature, but should be at liquid nitrogen temperature during operation. Its gamma-ray detection efficiency is not as high as that of scintillator crystals such as sodium iodide (NaI(Tl)) detectors, but because HPGe has a high energy resolution (e.g., 1.80 keV @ 1332 keV), it plays an absolutely crucial role in high-precision quantitative elemental analysis applications. The digital multichannel spectrometer 7 is mainly used to collect the energy and intensity of gamma rays generated after neutron activation of the sample measured by the HPGe detector 6.

[0059] In step S3, based on the peak area of ​​the characteristic gamma-ray peak of the radioactive isotope (such as the radioactive isotope...) 49 The characteristic gamma peak energy of Ca is 3084 keV, and the radioactive isotope 47 The characteristic γ peak energy of Ca is 1297 keV. Parameters such as reactor 1 neutron flux, irradiation time, cooling time, measurement time, and detector efficiency were used to calculate the ratio of stable isotope A and stable isotope B of the same element in the lunar soil sample using the first formula.

[0060] First formula: R = θ A / θ B ={P / (σγεSDC)} A / {P / (σγεSDC)} B

[0061] In the formula,

[0062] R is the ratio result;

[0063] P is the intensity of the characteristic gamma-ray peak of a radioactive isotope, measured in seconds (s). -1 ;

[0064] θ A It is the abundance of the stable isotope A;

[0065] θ B It is the abundance of the stable isotope B;

[0066] σ is the effective cross section for nuclear reactions between stable isotopes and neutrons, measured in cm. 2 ;

[0067] γ is the branching ratio of the characteristic gamma-ray peak of a radioactive isotope;

[0068] ε is the detection efficiency of the characteristic gamma-ray peak of a radioactive isotope.

[0069] S is the saturation factor, S = 1 - exp(-0.693t) i / T);

[0070] D is the decay factor, D = exp(-0.693t) d / T);

[0071] C is the measurement factor, C = [1 - exp(-0.693t]). c / T)] / (0.693t c / T);

[0072] t i This refers to the duration of the irradiation operation on the lunar soil sample;

[0073] t d It is the cooldown time;

[0074] t c It is the measurement time of radioactive isotope activity;

[0075] T is the half-life of the radioactive isotope to be measured.

[0076] In step S4, the isotope ratios in the lunar soil sample are compared with those in the Earth sample (standard materials and neutron flux parameter monitoring materials) using the second formula.

[0077] Second formula: δ(‰)=(R) sa / R sd -1)×1000

[0078] In the formula,

[0079] The δ value represents the comparison result;

[0080] R sa This indicates the isotope ratio in the lunar soil sample.

[0081] Finally, examples illustrate the specific applications of this invention:

[0082] Stable isotopes of the metallic element Ca in lunar soil samples 46 Ca / 48 Taking the Ca ratio measurement as an example, the specific implementation method is as follows:

[0083] (a) 48 Ca isotope measurement methods and procedures:

[0084] (1) Milligram-level lunar soil samples were packaged into small cubes of approximately 1cm × 1cm × 0.1cm using high-purity polyethylene and placed in reactor 1 for neutron irradiation. The preparation methods for standard materials and neutron flux rate monitoring materials (such as iron, manganese, and zirconium sheets) were the same as those for lunar soil samples.

[0085] (2) The sample, standard, and neutron parameter monitoring material are placed together in a polyethylene / PTFE irradiation box and transported to reactor 1 via a pneumatic transfer pipe for irradiation. The reactor neutron flux is 2*10 13 cm -1 s -1 calculate. 48 Ca through 48 Ca(n,γ) 49 Radioactive nuclides produced by Ca nuclear reactions 49 The half-life of Ca is 8.718 minutes, so the irradiation time is 2 minutes.

[0086] (3) When the irradiation of the sample is finished, the sample 2 is transferred to the lead chamber glove box 3 through the pneumatic transfer pipe for cooling for 10 minutes.

[0087] (4) After cooling, the sample is placed on the HPGe detector 4, whose efficiency and other parameters have been pre-calibrated, inside the lead-shielded chamber 6. 49 The characteristic gamma rays of Ca are measured, with a characteristic gamma peak energy of 3084 keV; the measurement time is selected according to the peak intensity, such as 10 minutes.

[0088] (5) After the measurement is completed, save the characteristic γ-ray spectrum of the sample and record it. 49Parameters such as the area of ​​the characteristic γ peak of Ca, measurement time, and cooling time.

[0089] (b) 46 Ca isotope measurement methods and procedures:

[0090] (1) Take milligram-level lunar soil samples and encapsulate them in high-purity aluminum foil into small cubes of approximately 1cm*1cm*0.1cm. Place these cubes into reactor 1 for neutron irradiation. The preparation methods for standard materials and neutron flux rate monitoring materials (such as iron, manganese, and zirconium sheets) are the same as those for lunar soil samples.

[0091] (2) Place the sample, standard, and neutron parameter monitoring material together in an aluminum irradiation box and then place it into reactor 1 for neutron irradiation. The reactor neutron flux is set at 2*10 13 cm -1 s -1 calculate. 46 Ca through 46 Ca(n,γ) 47 Radioactive nuclides produced by Ca nuclear reactions 47 The half-life of Ca is 4.536 days, so the irradiation time is chosen to be 24 hours.

[0092] (3) After the sample irradiation is completed, the sample is transferred to the lead chamber glove box 3 for cooling, which takes 4-6 days.

[0093] (4) After the sample has cooled, place it in the lead-shielded chamber 6 onto the HPGe detector 4, whose efficiency and other parameters have been calibrated in advance. 47 The characteristic gamma rays of Ca are measured, with a characteristic gamma peak energy of 1297 keV; the measurement time is selected according to the peak intensity, such as 1 hour.

[0094] (5) After the measurement is completed, save the sample. 47 The characteristic gamma spectrum of Ca, recording the energy spectrum 47 Parameters such as the area of ​​the characteristic γ peak of Ca, measurement time, and cooling time.

[0095] (6) Calculate the stable isotopes in the sample according to Formula 1. 48 Ca and stable isotopes 46 Ca ratio.

[0096] (7) To calculate the difference between the isotopic ratios of lunar soil samples and terrestrial samples, the Ca isotopic ratio (Rc) in the lunar soil samples should be used. sa The ratio of Ca isotopes between the sample and the Earth sample (standard) (R) sd The comparison is performed using formula (2), and the comparison result is expressed as the δ value.

[0097] Table 1 Examples of measurable stable metal isotope parameters in lunar soil

[0098]

[0099] The device described in this invention is not limited to the embodiments described in the specific implementation. Other implementation methods derived by those skilled in the art based on the technical solution of this invention also fall within the scope of technical innovation of this invention.

Claims

1. An analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil, comprising the following steps: Step S1: The sample (2) to be tested is placed in the reactor (1) for irradiation. The sample (2) includes lunar soil sample and Earth sample. The Earth sample includes standard material and neutron flux parameter monitoring material. Step S2: After the irradiation operation is completed, the sample is cooled, and then the activity of radioactive isotopes is measured on the activated lunar soil sample using an HPGe detector (4) combined with a digital multichannel spectrometer (7). Step S3: After the radioactive isotope activity measurement is completed, calculate the ratio of stable isotope A to stable isotope B in the lunar soil sample. Step S4: Calculate the difference between the isotope ratio of the lunar soil sample and the isotope ratio of the Earth sample; In step S3, based on the peak area of ​​the characteristic gamma-ray peak of the radioactive isotope and the neutron flux, irradiation time, cooling time, measurement time, and detector efficiency of the reactor (1), the ratio of stable isotope A and stable isotope B of the same element in the lunar soil sample is calculated using the first formula; Range:R=θ A / θ B ={P / (σγε SDC)} A / {P / (σγε SDC)} B In the formula, R is the ratio result; P is the intensity of the characteristic gamma-ray peak of a radioactive isotope, measured in seconds (s). -1 ; θ A It is the abundance of the stable isotope A; θ B It is the abundance of the stable isotope B; σ is the effective cross section for nuclear reactions between stable isotopes and neutrons, measured in cm. 2 ; γ is the branching ratio of the characteristic gamma-ray peak of a radioactive isotope; ε is the detection efficiency of the characteristic gamma-ray peak of a radioactive isotope. S is the saturation factor, S = 1 - exp(-0.693t) i / T); D is the decay factor, D = exp(-0.693t) d / T); C is the measurement factor, C=[1-exp(-0.693t c / T)] / (0.693t c / T); t i It is the duration of the irradiation operation; t d This refers to the cooling time; t c The measurement time of the activity of the radioactive isotope; T is the half-life of the radioactive isotope to be measured; In step S4, the isotopic ratios in the lunar soil sample are compared with those in the Earth sample using the second formula. Second formula: δ(‰) = (R) sa / R sd -1)×1000 In the formula, The δ value represents the comparison result; R sa This indicates the isotope ratio values ​​in the lunar soil sample; R sd This indicates the isotope ratio of the Earth sample.

2. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 1, characterized in that: in In step S1, before the irradiation operation, the lunar soil sample and the Earth sample need to be packaged into a 1cm×1cm×0.1cm cube shape using high-purity polyethylene or aluminum foil; the standard material and the neutron flux rate parameter monitoring material include iron wire and zirconium sheet; the duration of the irradiation operation is set according to the half-life of the radioactive isotope to be measured generated by the activation of stable isotopes in the lunar soil sample and the neutron flux of the reactor (1).

3. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 2, characterized in that: in In step S1, the lunar soil sample and the Earth sample are placed in an irradiation box made of polyethylene / polytetrafluoroethylene and sent to the reactor (1) through a pneumatic transmission pipeline for the irradiation operation.

4. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 1, characterized in that: in In step S2, the cooling time and measurement time need to be selected based on the radioactivity of the lunar soil sample and the half-life of the radioactive isotope to be measured.

5. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 3, characterized in that: in In step S2, the irradiation box containing the lunar soil sample and the Earth sample is transferred to the lead glove box (3) for cooling via a pneumatic transfer pipe, and the lunar soil sample and the Earth sample are removed from the irradiation box after cooling.

6. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 4, characterized in that: in In step S2, the lunar soil sample and the Earth sample are placed in the lead-shielded chamber (6) and the HPGe detector (4) with the efficiency parameters pre-calibrated for radioactive isotope activity measurement.

7. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 4, characterized in that: in In step S2, the HPGe detector (4) needs to be inserted into the Dewar flask (5) and the liquid nitrogen stored in the Dewar flask (5) is used to keep the HPGe detector (4) at the liquid nitrogen temperature when it is working.

8. The analytical method for non-destructive measurement of stable metal isotope ratios in lunar soil as described in claim 1, characterized in that: in Step S2 also includes connecting a computer (8) to the digital multichannel spectrometer (7) to set the relevant operating parameters of the digital multichannel spectrometer (7) and to collect and record the characteristic gamma-ray energy spectrum of radioactive isotopes measured by the digital multichannel spectrometer (7).

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