CN104865283B - Mineral stantardless argon-argon dating method - Google Patents

Abstract

The invention discloses a method for standard-free argon-argon dating of minerals, which includes: wrapping mineral samples with aluminum foil to form a sample sheet, attaching a high-purity nickel sheet to the front and back of the sample sheet, and then putting them into an accelerator neutron source for Neutron irradiation; detect the irradiated nickel sheet, determine the value of the neutron reaction cross section σ, and calculate the neutron flux N of the neutron source of the accelerator; load the irradiated mineral sample into the rare gas measurement system, heat and melt the sample , use the rare gas mass spectrometer to measure the Ar isotope content of the mineral sample, and get the required ⁴⁰ Ar*/ ³⁹ Ar _K through data calculation; the neutron flux N, reaction cross section σ and ⁴⁰ Ar*/ ³⁹ Ar _K into the formula Calculation of argon-argon ages for mineral samples. The invention solves the problem that traditional argon-argon dating must use standard samples of known age to correct the J value related to the neutron flux of the ²³⁵ U fission reactor, etc., and can realize argon-argon dating without using any geological standard samples, The accuracy of the Ar-Ar dating method has been improved.

Description

A method for standard-free argon-argon dating of minerals

technical field

The invention relates to the technical field of argon-argon dating, in particular to a method for standard-free argon-argon dating of minerals.

Background technique

The basic principle of argon-argon dating is as follows: according to the law of radioactive decay, as long as the content of radioactive parent and radioactive factor body is measured, the mineral age can be calculated according to the following formula 1:

Formula 1

Among them, t is the age of the mineral, λ is the total decay constant of ⁴⁰ K, λ=5.543(±0.010)×10 ^-10 a ^-1 , λ _e and λ′ _e are the two branches that decay to ⁴⁰ Ar at ⁴⁰ K Decay constant, λ _e ＝0.572(±0.004)×10 ^-10 a ^-1 , λ′ _e ＝0.0088(±0.0017)×10 ^-10 a ^-1 , ⁴⁰ K is the content of radioactive precursor, ⁴⁰ Ar* is the content of radioactive composition The content of the factor body.

Transforming formula 1 can get:

Formula 2

According to the nuclear reaction ³⁹ K(n,p) ³⁹ Ar,

³⁹ Ar _K ＝ ³⁹ KΔ∫Φ(E)σ(E)dE Formula 3

Among them, ³⁹ Ar _K is ³⁹ Ar produced by neutron irradiation at ³⁹ K, Δ is the irradiation time, Φ(E) is the neutron flux with energy E, and σ(E) is the neutron with energy E Reaction cross section.

According to the above formula, and let

Formula 4

but:

Formula 5

Equation 5 is the basic equation for argon-argon dating. According to formula 5, to measure the argon-argon age of the sample, only the J value and ⁴⁰ Ar*/ ³⁹ Ar _K are needed.

Based on formula 5, the existing argon-argon dating method is to place the mineral sample to be tested and the standard sample at intervals, seal them in a quartz tube and put them in a ²³⁵ U fission reactor for neutron irradiation to transform ³⁹ K into ³⁹ Ar.

From Equation 5, we can get,

Formula 6

Correct the J value of the irradiation parameter by measuring the ⁴⁰ Ar*/ ³⁹ Ar _K of the standard sample with known age, and then measure the ⁴⁰ Ar*/ ³⁹ Ar _K of the sample to be tested respectively, and calculate the J value corrected by the standard sample according to formula 5 The argon-argon age of the sample was measured.

It can be seen from the above description that the current argon-argon dating method must correct the J value of the sample to be tested by measuring the standard sample with known age. However, with the change of fuel cycle and sample position, there are many unknown parameters, and it is difficult to obtain an accurate J value, resulting in a large J value gradient. The J value of the measured sample position will also introduce a large error, thus affecting the dating accuracy.

Contents of the invention

(1) Technical problems to be solved

In view of this, the main purpose of the present invention is to provide a method for carrying out standard sample-free argon-argon dating of minerals, to solve the problem that traditional argon-argon dating must use standard samples of known age to correct and ²³⁵ U fission reactor. The problem of J value related to sub-flux and so on can achieve the purpose of argon-argon dating without using any geological standard samples, and improve the accuracy of argon-argon dating method.

(2) Technical solution

In order to achieve the above object, the present invention provides a method for argon-argon dating without standard samples on minerals, the method comprising:

Step 1: Wrap the mineral sample with aluminum foil to form a sample sheet, and paste a high-purity nickel sheet on the front and back of the sample sheet, and then put it into the neutron source of the accelerator for neutron irradiation;

Step 2: Detect the irradiated nickel sheet, determine the value of the neutron reaction cross section σ, and calculate the neutron flux N of the accelerator neutron source;

Step 3: Load the irradiated mineral sample into the rare gas measurement system, after heating and melting the sample, measure the Ar isotope ( ⁴⁰ Ar, ³⁹ Ar, ³⁶ Ar) content of the mineral sample with a rare gas mass spectrometer, and obtain the required ⁴⁰ Ar*/ ³⁹ Ar _K ;

Step 4: Substitute the neutron flux N, reaction cross section σ and ⁴⁰ Ar*/ ³⁹ Ar _K of the accelerator neutron source into the formula Calculation of argon-argon ages of mineral samples;

where t is the argon-argon age of the mineral sample, λ is the total decay constant at ⁴⁰ K, λ=5.543(±0.010)×10 ^{- 10} a ^-1 , λ _e and λ′ _e are the ⁴⁰ K decay to ⁴⁰ Ar The decay constants of the two branches of λ _e ＝0.572(±0.004)×10 ^-10 a ^-1 _x λ′ _e ＝0.0088(±0.0017)×10 ^-10 a ^-1 , ⁴⁰ Ar*/ ³⁹ Ar _K is radioactive The ratio of ⁴⁰ Ar to ³⁹ Ar _K produced by neutron irradiation from ³⁹ K, N is the neutron flux of the accelerator neutron source, σ is the reaction cross section, and is the integral function of the neutron energy spectrum.

In the above scheme, the step 1 includes: taking a mineral sample, wrapping the mineral sample into a 1cm×1cm sample sheet with aluminum foil, pasting a 1 mm thick high-purity nickel sheet on the front and back of the sample sheet, and then putting it into the neutron source of the accelerator The neutron irradiation is carried out as close as possible to the bull's-eye, and the place where the accelerator neutron source is as close as possible to the bull's-eye is less than 3.5cm away from the bull's-eye.

In the above solution, the high-purity nickel sheet is a nickel sheet with a mass percentage >99.99%.

In the above scheme, the irradiated nickel sheet is detected in step 2, and the nuclear data measurement of the neutron generator is completed by using an ultra-low background high-purity germanium gamma spectrometer.

In the above scheme, the value of the mineral sample reaction cross section σ is determined in step 2. Since the neutron energy of the accelerator neutron source is single and 2.45 MeV, the mineral sample reaction cross section σ is 0.18 barn (b), where 1b= ^10-24 cm ² .

In the above scheme, the calculation of the neutron flux N of the accelerator neutron source in step 2 is based on the nuclear reaction ⁵⁸ Ni(n,p) ⁵⁸ Co to complete the calculation of the neutron flux.

In the above scheme, the irradiated mineral sample described in step 3 is loaded into the rare gas measurement system, which includes a high temperature furnace, a gas purification system and a Helix MC Plus rare gas mass spectrometer, wherein the high temperature furnace is used to melt the sample , the gas released by melting is purified by a gas purification system to remove active gases, and then the Ar isotope is measured with a Helix MCPlus rare gas mass spectrometer.

In the above scheme, the determination of the Ar isotope content of the mineral sample by the rare gas mass spectrometer in step 3 is to use the Helix MC Plus rare gas mass spectrometer to measure the content of Ar isotopes ( ⁴⁰ Ar, ³⁹ Ar, ³⁶ Ar).

In the above scheme, the required ⁴⁰ Ar*/ ³⁹ Ar _K is obtained through data calculation in step 3, and all ³⁹ Ar is produced by ³⁹ K through neutron irradiation, so ³⁹ Ar _K is the measured ³⁹ Ar, ⁴⁰ Ar*= ⁴⁰ Ar- ⁴⁰ Ar _a , wherein ⁴⁰ Ar _a is air ⁴⁰ Ar, ⁴⁰ Ar _a =295.5× ³⁶ Ar.

(3) Beneficial effects

As can be seen from the foregoing technical solutions, the present invention has the following beneficial effects:

1. The method for standard-free argon-argon dating of minerals provided by the present invention does not need to use a standard sample of known age to correct the J value determined by parameters such as neutron flux and reaction cross section at the position of the sample to be tested , only need to measure the neutron flux and ⁴⁰ Ar*/ ³⁹ Ar _K , thereby avoiding the influence of the J value gradient caused by the sample position and other reasons, and solving the problem that the traditional argon-argon dating must be corrected and The neutron flux of the ²³⁵ U fission reactor and other related J value problems have achieved the goal of argon-argon dating without using any geological standard samples. This method will make argon-argon dating from "relative dating" (sample age It is obtained by comparing the age of the standard sample) to "absolute dating" (no standard sample is required).

2. The method for standard-free argon-argon dating of minerals provided by the present invention does not need to use a standard sample of known age to correct the J value determined by parameters such as neutron flux and reaction cross section at the position of the sample to be tested , only the neutron flux and ⁴⁰ Ar*/ ³⁹ Ar _K need to be measured, thereby avoiding the influence of the J value gradient caused by the sample position and other reasons, thus improving the accuracy of the argon-argon dating method.

Description of drawings

Fig. 1 is a flowchart of a method for standard-free argon-argon dating of minerals according to an embodiment of the present invention.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

In order to be able to accurately calculate the neutron flux, in the embodiment of the present invention, a high-purity (mass percentage>99.99%) nickel sheet consistent with the size of the sample is pasted on the front and back of the mineral sample to be tested, with a thickness of about 1 mm, so as to accurately calculate the neutron flux. flux and eliminate the influence of neutron flux gradient, thereby improving the accuracy of dating.

In the embodiment of the present invention, the accelerator adopts a deuterium-deuterium (DD) accelerator neutron source that only produces 2.45 MeV single-energy neutrons to irradiate the mineral sample, and then the neutron hits the mineral sample to convert ³⁹ K into ³⁹ Ar, which satisfies the following formula,

Formula 7

The reaction cross section σ is the ratio of the number of reaction nuclei per unit area per unit time (for example: ³⁹ Ar _K ) to the number of incident particle neutrons N and the number of target nuclei (for example: ³⁹ K), and the unit is barn (b,1b=10 ^{- 24 cm2} ).

According to Equation 7, it can be known that

Formula 8

Among them, 1.255×10 ^-4 is the ⁴⁰ K/ ³⁹ K value in nature, N is the neutron flux, σ is the reaction cross section, which is the integral function of the neutron energy spectrum.

From formula 1, formula 7, formula 8 can get,

Formula 9

where t is the argon-argon age of the mineral sample, λ is the total decay constant at ⁴⁰ K, λ=5.543(±0.010)×10 ^{- 10} a ^-1 , λ _e and λ′ _e are the ⁴⁰ K decay to ⁴⁰ Ar The decay constants of the two branches of , λ _e ＝0.572(±0.004)×10 ^-10 a ^-1 , λ′ _e ＝0.0088(±0.0017)×10 ^-10 a ^-1 , ⁴⁰ Ar*/ ³⁹ Ar _K is radioactive The ratio of ⁴⁰ Ar to ³⁹ Ar _K produced by neutron irradiation from ³⁹ K, N is the neutron flux of the accelerator neutron source, σ is the reaction cross section, and is the integral function of the neutron energy spectrum.

Therefore, in order to accurately determine the age of the sample, it is only necessary to measure ⁴⁰ Ar*/ ³⁹ Ar _K , N and σ.

In order to accurately measure the neutron flux N and the reaction cross section σ, the embodiment of the present invention adopts the neutron nuclear reaction of nickel ( ⁵⁸ Ni(n,p) ⁵⁸ Co) which is closest to the nuclear reaction ³⁹ K(n,p) ³⁹ Ar monitor. Since the accelerator neutron source used produces neutrons with a single energy, the reaction cross section is constant, and the content of ³⁹ K and the age of the sample can be calculated by combining the accurate neutron flux recorded by the nickel sheet and the content of ³⁹ Ar, realizing the standard-free argon-argon dating.

Based on the above analysis, Fig. 1 shows a flow chart of a method for standard-free argon-argon dating of minerals according to an embodiment of the present invention, the method comprising:

Step 1: Wrap the mineral sample with aluminum foil to form a sample sheet, and paste a high-purity (>99.99%) nickel sheet on the front and back of the sample sheet, and then put it into the neutron source of the accelerator for neutron irradiation;

Step 2: Detect the irradiated nickel sheet, determine the value of the neutron reaction cross section σ, and calculate the neutron flux N of the accelerator neutron source;

Step 3: Load the irradiated mineral sample into the rare gas measurement system, after heating and melting the sample, measure the Ar isotope ( ⁴⁰ Ar, ³⁹ Ar, ³⁶ Ar) content of the mineral sample with a rare gas mass spectrometer, and obtain the required ⁴⁰ Ar*/ ³⁹ Ar _K ;

Step 4: Substitute the neutron flux N, reaction cross section σ and ⁴⁰ Ar*/ ³⁹ Ar _K of the accelerator neutron source into the formula Calculation of argon-argon ages of mineral samples;

where t is the argon-argon age of the mineral sample, λ is the total decay constant at ⁴⁰ K, λ=5.543(±0.010)×10 ^{- 10} a ^-1 , λ _e and λ′ _e are the ⁴⁰ K decay to ⁴⁰ Ar The decay constants of the two branches of , λ _e ＝0.572(±0.004)×10 ^-10 a ^-1 , λ′ _e ＝0.0088(±0.0017)×10 ^-10 a ^-1 , ⁴⁰ Ar*/ ³⁹ Ar _K is radioactive The ratio of ⁴⁰ Ar to ³⁹ Ar _K produced by neutron irradiation from ³⁹ K, N is the neutron flux of the accelerator neutron source, σ is the reaction cross section, and is the integral function of the neutron energy spectrum.

Among them, step 1 includes: taking a mineral sample, wrapping the mineral sample into a 1cm×1cm sample piece with aluminum foil, pasting a 1mm-thick high-purity nickel piece on the front and back of the sample piece, and then putting it into the neutron source of the accelerator as close as possible to the center of the target. neutron irradiation, where the neutron source of the accelerator is as close to the bull's-eye as possible, and the distance from the bull's-eye is less than 3.5cm.

Detect the irradiated nickel sheet as described in step 2, and use the ultra-low background high-purity germanium gamma spectrometer to complete the nuclear data measurement of the neutron generator, mainly the ⁵⁸ Co.

Determine the value of the neutron reaction cross section σ as described in step 2. Since the neutron energy of the accelerator neutron source is single and 2.45 MeV, the neutron reaction cross section σ of the neutron source is 0.18 barn (b), where 1b = 10 ⁻²⁴ cm ² .

The calculation of the neutron flux N of the accelerator neutron source described in step 2 is to complete the calculation of the neutron flux according to the nuclear reaction ⁵⁸ Ni(n,p) ⁵⁸ Co, and the specific calculation process is as follows:

According to the nuclear reaction ⁵⁸ Ni(n,p) ⁵⁸ Co, then

Formula 10

Among them, σ is the reaction cross section, N is the neutron flux of the accelerator neutron source, ⁵⁸ Co is the atomic number of ⁵⁸ Co measured by the high-purity germanium gamma spectrometer, and ⁵⁸ Ni is the atomic number of ⁵⁸ Ni in the high-purity nickel sheet. The reaction cross section σ is constant because the single neutron energy is constant, the number of ⁵⁸ Co atoms can be measured by a gamma spectrometer, and the number of ⁵⁸ Ni atoms can be calculated from the mass, purity and molar mass of a high-purity nickel sheet, which can be calculated by inserting it into formula 10 The neutron flux N of the accelerator neutron source.

As described in step 3, the irradiated mineral samples are loaded into the rare gas measurement system, which includes a high temperature furnace, a gas purification system and a Helix MC Plus rare gas mass spectrometer. As described in step 3, use a rare gas mass spectrometer to measure the Ar isotope content of the mineral sample. The specific measurement process is as follows: use a high-temperature furnace to melt the sample, and the gas released from the melting is purified by a purification system to remove active gases, etc., and then use Helix MC Plus The noble gas mass spectrometer was used to measure Ar isotopes, and the Faraday cup and electron multiplier equipped on the rare gas mass spectrometer were used to receive ⁴⁰ Ar, ³⁹ Ar, and ³⁶ Ar signals respectively, and the contents of ⁴⁰ Ar, ³⁹ Ar, and ³⁶ Ar were measured.

The required ⁴⁰ Ar*/ ³⁹ Ar _K is obtained through the data calculation described in step 3. All ³⁹ Ar is produced by neutron irradiation at ³⁹ K, so ³⁹ Ar _K is the measured ³⁹ Ar, ⁴⁰ Ar*= ⁴⁰ Ar- ⁴⁰ Ar _a , where ⁴⁰ Ar _a is air ⁴⁰ Ar, ⁴⁰ Ar _a =295.5× ³⁶ Ar, therefore, ⁴⁰ Ar*/ ³⁹ Ar _K ＝( ⁴⁰ Ar-295.5* ³⁶ Ar)/ ³⁹ Ar; specific calculation In this case, the measured ⁴⁰ Ar, ³⁹ Ar, and ³⁶ Ar are brought into the above formula to calculate the ⁴⁰ Ar*/ ³⁹ Ar _K value.

The method for standard-free argon-argon dating of minerals provided by the embodiment of the present invention does not need to use a standard sample of known age to correct the J value determined by parameters such as neutron flux and reaction cross section at the position of the sample to be tested. Only the neutron flux and ⁴⁰ Ar*/ ³⁹ Ar _K need to be measured, thereby avoiding the influence of the J value gradient caused by the sample position and other reasons, thus improving the accuracy of the argon-argon dating method.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (9)

1. A method for carrying out standard sample-free argon-argon dating to minerals, characterized in that the method comprises: Step 1: Wrap the mineral sample with aluminum foil to form a sample sheet, and paste a high-purity nickel sheet on the front and back of the sample sheet, and then put it into the neutron source of the accelerator for neutron irradiation; Step 2: Detect the irradiated nickel sheet, determine the value of the neutron reaction cross section σ, and calculate the neutron flux N of the accelerator neutron source; Step 3: Load the irradiated mineral sample into the rare gas measuring system, heat and melt the sample, measure the Ar isotope ⁴⁰ Ar, ³⁹ Ar and ³⁶ Ar content of the mineral sample with a rare gas mass spectrometer, and obtain the required ⁴⁰ Ar through data calculation Ar*/ ³⁹ Ar _K ; Step 4: Substitute the neutron flux N, reaction cross section σ and ⁴⁰ Ar*/ ³⁹ Ar _K of the accelerator neutron source into the formula Calculation of argon-argon ages of mineral samples; Where, t is the argon-argon age of the mineral sample, λ is the total decay constant at ⁴⁰ K, λ=5.543(±0.010)×10 ^-10 a ^-1 , λ _e and λ′ _e are the ⁴⁰ K decay to ⁴⁰ Ar The decay constants of the two branches of , λ _e ＝0.572(±0.004)×10 ^-10 a ^-1 , λ′ _e ＝0.0088(±0.0017)×10 ^-10 a ^-1 , ⁴⁰ Ar*/ ³⁹ Ar _K is radioactive The ratio of ⁴⁰ Ar to ³⁹ Ar _K produced by neutron irradiation from ³⁹ K, N is the neutron flux of the accelerator neutron source, σ is the reaction cross section, and is the integral function of the neutron energy spectrum. 2. The method for standard-free argon-argon dating of minerals according to claim 1, wherein said step 1 comprises: Take the mineral sample, wrap the mineral sample into a 1cm×1cm sample sheet with aluminum foil, and paste a 1mm thick high-purity nickel sheet on the front and back of the sample sheet, and then put it into the neutron source of the accelerator as close as possible to the bull's-eye for neutron irradiation. The place where the accelerator neutron source is as close as possible to the bull's-eye is less than 3.5cm away from the bull's-eye. 3. The method for standard-free argon-argon dating of minerals according to claim 1 or 2, characterized in that the high-purity nickel sheet is a nickel sheet with a mass percentage>99.99%. 4. the method for carrying out standard sample-free argon-argon dating to mineral according to claim 1, is characterized in that, described in step 2 detects the nickel sheet after irradiation, adopts ultra-low background high-purity germanium gamma spectrometer Completion of nuclear data measurements for neutron generators. 5. The method for carrying out standard sample-free argon-argon dating to minerals according to claim 1, characterized in that, the value of determining the reaction cross section σ of the mineral sample described in step 2, due to the single neutron energy of the accelerator neutron source and is 2.45 MeV, so the mineral sample reaction section σ is 0.18 barnes (b), where 1 barnes=10 ⁻²⁴ cm ² . 6. the method for carrying out standard sample argon-argon dating to mineral according to claim 1, it is characterized in that, the neutron flux N of calculating accelerator neutron source described in step 2 is based on nuclear reaction ⁵⁸ Ni(n ,p) ⁵⁸ Co to complete the calculation of neutron flux. 7. The method for standard-free argon-argon dating of minerals according to claim 1, characterized in that, in step 3, the irradiated mineral samples are loaded into a rare gas measurement system, and the rare gas measurement system includes High-temperature furnace, gas purification system and Helix MC Plus rare gas mass spectrometer, in which a high-temperature furnace is used to melt the sample, and the gas released by melting is purified by the gas purification system to remove active gases, and then the Helix MC Plus rare gas mass spectrometer is used for Ar isotope analysis. Measurement. 8. The method for standard-free argon-argon dating of minerals according to claim 1, characterized in that, the Ar isotope content of the mineral sample measured with a rare gas mass spectrometer described in step 3 is to utilize Helix MC Plus rare The content of Ar isotopes ⁴⁰ Ar, ³⁹ Ar and ³⁶ Ar was determined by gas mass spectrometer. 9. The method for standard-free argon-argon dating of minerals according to claim 1, characterized in that, in step 3, the required ⁴⁰ Ar*/ ³⁹ Ar _K is obtained through data calculation, and all ³⁹ Ar It is produced by neutron irradiation at ³⁹ K, so ³⁹ Ar _K is the measured ³⁹ Ar, ⁴⁰ Ar*= ⁴⁰ Ar- ⁴⁰ Ar _a , where ⁴⁰ Ar _a is air ⁴⁰ Ar, ⁴⁰ Ar _a = 295.5× ³⁶ Ar .