AU2020100872A4 - An Ar-Ar Dating Method Without Standards For Minerals - Google Patents
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Abstract
This invention discloses an Ar-Ar dating method without standards for minerals, which
includes: wrap the mineral sample with aluminum foil into a sample piece, attach a
high-purity nickel sheet on the front and back of the sample piece respectively, and then put
it into the accelerator neutron source for neutron irradiation; detect the irradiated nickel
sheet to determine the value of the neutron reaction cross section a, and calculate the
neutron flux N of the accelerator neutron source; load the irradiated mineral sample into the
noble gas measurement system, melt sample, and measure the Ar isotope content of the
mineral sample with the noble gas mass spectrometer, and obtain the required 40Ar*/ 39ArKby
calculating the data; substitute the neutron flux N, reaction cross section and 40Ar*/ 39Ar
of the accelerator neutron source into the formula
t 1( A
4 0Ar* N-a t n 1 + A ArK to calculate the Ar-Ar age of the mineral
sample. This invention solves the problem that the standard sample with known age must be
used in the traditional Ar-Ar dating to correct the J-value related to the neutron flux of 2 3 U
fission reactor, realizes the Ar-Ar dating without any geological standard sample, and
improves the precision of the Ar-Ar dating method.
Descriptions with Drawings
Wrap the mineral sample with aluminum foil into a sample piece, attach a high-purity j
nickel sheet on the front and back of the sample piece respectively, and then put it
into the acceleratorneutron source forneutron irradiation
Detect the irradiated nickel sheet, determine the value of the neutron reaction section
u, and calculate the neutron flux N of the accelerator neutron source
Load the irradiated mineral sample into the noble gas measurement system, melt sample,
measure the Ar isotope (Ar, 39Ar, 34 Ar) content of the mineral sample with the noble gas
mass spectrometer, and obtain the required 4Ar*/3#ArK by calculating the data
Substitute the neutron flux N, reaction cross section a and 4Ar*/39Ar of the 4
accelerator neutron source into the formula to calculate the Ar-Ar age of the mineral
sample
Figure 1
1/1
Description
Descriptions with Drawings
Wrap the mineral sample with aluminum foil into a sample piece, attach a high-purity j nickel sheet on the front and back of the sample piece respectively, and then put it into the acceleratorneutron source forneutron irradiation
Detect the irradiated nickel sheet, determine the value of the neutron reaction section u, and calculate the neutron flux N of the accelerator neutron source
Load the irradiated mineral sample into the noble gas measurement system, melt sample, measure the Ar isotope (Ar, 39Ar, 34 Ar) content of the mineral sample with the noble gas mass spectrometer, and obtain the required 4Ar*/3#ArK by calculating the data
Substitute the neutron flux N, reaction cross section a and 4Ar*/39Ar of the 4 accelerator neutron source into the formula to calculate the Ar-Ar age of the mineral sample
Figure 1
1/1
Descriptions
An Ar-Ar dating method without standards for minerals
Technical Field This invention relates to the technical field of Ar-Ar dating, in particular to an Ar-Ar dating method without standards for minerals.
Background Technology The basic principle of Ar-Ar dating is as follows: According to the law of radioactive decay, as long as the content of radioactive parent and radiogenic daughter is measured, the mineral age can be calculated according to the following Formula 1:
1 A Ar t-- 1 + Formula1
Where t is mineral age, X is the total decay constant of4 0 K,X=5.543(±0.010)x10 10 a-1
, ,eand 2'eare the decay constants of the two branches of 4'K decaying into 40Ar respectively, 2e =0.572( ± 0.004) x 10 a-1 , l'e=0.0088( ±0.0017) x 10-10 a-1, 4 0K is the content of radioactive parent, 4 0Ar* is the content of radiogenic daughter.
The following formula can be obtained by deforming Formula 1:
*OAr* = K l[(exp t) - 1] Formula 2
According to 39 39 K (n,p) Ar in the nuclear reaction,
9ArK = 9KA f0(E)o-(E)dE Formula 3
Where 3 9Ar is the 39 Ar generated by neutron irradiated 39K, A refers to the irradiation
time, 0(E) refers to the neutron flux with energy of E, and o(Erefers to the neutron
reaction cross section with energy of E.
According to the above formulas, set
J= ,Af O(E)u(E)dE Formula 4 : TnK
Then: t- +1 ) Formula 5
Formula 5 is the basic equation of Ar-Ar dating. According to Formula 5, only J-value and Ar*/ 3 Arof 4 9 the sample are needed to determine its Ar-Ar age.
Based on Formula 5, the existing Ar-Ar dating method is to place the mineral sample to be measured at a distance with the standard sample, seal them in a quartz tube and put the tube into the 2 5 U fission reactor for neutron irradiation to convert 39K into 39Ar.
From Formula 5, we get
(exp At)-1 - 40 40 ' Ar /1 ArK Formula 6
Correct the irradiation parameter J-value of4 0 Ar*/ 39Arof the standard sample with known age, measure the 4 0Ar*/ 3 9 ArKof the sample to be measured respectively, and calculate the Ar-Ar age of the sample to be measured in combination with the J-value corrected by the standard sample according to Formula 5.
It can be seen from the above description that the standard sample with known age must be measured by existing Ar-Ar dating method to correct the J-value of the sample to be measured. However, due to the change of fuel cycle and sample position as well as the large number of unknown parameters, it is difficult to get the accurate J-value, resulting in a large J-value gradient. Even if the J-value of the position of the standard sample is accurately measured, the J-value of the position of the sample to be measured calculated by fitting the J-value of the position of the standard sample will also introduce a large error, thus affecting the dating accuracy.
Invention Summary (I) Technical problems to be solved
In view of this, the main purpose of this invention is to provide an Ar-Ar dating method without standards for minerals to solve the J-value problem related to the neutron flux of 235U fission reactor which must be corrected by the standard sample with known age by traditional Ar-Ar dating, so as to accomplish the purpose of Ar-Ar dating without using any geological standard sample, and improve the precision of Ar-Ar dating.
(II) Technical proposal
In order to achieve the above purpose, this invention provides an Ar-Ar dating method without standards for minerals, which includes:
Step 1: wrap the mineral sample with aluminum foil into a sample piece, attach a high-purity nickel sheet on the front and back of the sample piece respectively, and then put it into the accelerator neutron source for neutron irradiation;
Step 2: detect the irradiated nickel sheet, determine the value of the neutron reaction cross section a, and calculate the neutron flux N of the accelerator neutron source;
Step 3: load the irradiated mineral sample into the noble gas measurement system, melt sample, measure the Ar isotope ( 4 0Ar , 3 9Ar 36Ar) content of the mineral sample with the noble gas mass spectrometer, and obtain the required 40Ar*/ 3 9 ArKby calculating the data;
40 Step 4: substitute the neutron flux N, reaction cross section a and Ar*/ 3 9 ArK of the
1 X 40Ar* N-a i accelerator neutron source into the formula t = -In 1 + -A . x to
calculate the Ar-Ar age of the mineral sample.
Where t is the Ar-Ar age of the mineral sample, k is the total decay constant of 40K, k=5.543(±0.010)xO-1 a- 0 1 , Xe and 'eare the decay constants of the two branches of 4 0K decaying into 40Ar respectively, Xe =0.572(±0.004)x10-1o a-', 'e=.0088(±0.0017)X10-10 a- 1, 40Ar*/ 39ArKis the ratio of radiogenic 40Ar to 39 ArtK produced by neutron irradiated 3 9K, N is the neutron flux of accelerator neutron source, a is the reaction cross section, which is the integral function of neutron energy spectrum.
In the above proposal, the described Step 1 includes: take the mineral sample, wrap the mineral sample into a lcmxlcm sample pieces with aluminum foil, attach a 1mm thick high-purity nickel sheet on the front and back of the sample piece respectively, and then put it into the accelerator neutron source as close as possible to the bull's eye for neutron irradiation, in which the accelerator neutron source is as close as possible to the bull's eye and is less than 3.5cm away from the bull's eye.
In the above proposal, the described high-purity nickel sheet is a nickel sheet with a mass percentage>99.99%.
In the above proposal, the nuclear data of the neutron generator is measured by the detected and irradiated nickel sheet described in Step 2 by ultra-low background HPGe y spectrometer.
In the above proposal, since the neutron energy of the accelerator neutron source is 2.45 MeV, the determined value of the reaction cross section a of the mineral sample in the described Step 2 is 0.18 barn (b), where 1b=10-24cm 2 .
In the above proposal, the calculation of the neutron flux N of the accelerator neutron source in the described Step 2 is a calculation of the neutron flux based on the nuclear 58 reactionsNi (n, p) Co.
In the above proposal, the noble gas measurement system loading irradiated mineral sample as described in Step 3 includes a high temperature furnace, a gas purification system and a Helix MC Plus noble gas mass spectrometer, in which the sample is melted in the high temperature furnace, the released gas by melting is purified by the gas purification system to remove the active gases, and then the Ar isotope is measured by the Helix MC Plus noble gas mass spectrometer.
In the above proposal, the determination of Ar isotope content of the mineral sample by noble gas mass spectrometer described in Step 3 is the determination of Ar isotopes 4(0 Ar, 39Ar and 3 6Ar) by Helix MC Plus noble gas mass spectrometer.
In the above proposal, all 3 9Ar in the required 40Ar*/ 39ArK obtained by data calculation described in Step 3 are generated by 39K irradiated by neutron. Therefore, 39ArK is the determined 3 9Ar, 4 OAr*= 4OAr 4 Ara, in which 4 Ara is air40 Ar, 4 0Ara=295.5x3 6 Ar.
(III) Beneficial effects
It can be seen from the above technical proposal that this invention has the following beneficial effects:
The Ar-Ar dating method without standards for minerals provided by this invention requires only measuring the neutron flux and 40Ar*/ 39ArK instead of correcting with the standard sample with known age, the J-value determined by the neutron flux, reaction cross section and other parameters at the position where the sample to be measured with known age is located, so it avoids the influence of the J-value gradient caused by the sample position and other reasons, solves the problem that the standard sample with known age must be used in the traditional Ar-Ar dating to correct the J-value related to the neutron flux of 2 3 5 U fission reactor, and achieves the goal of Ar-Ar dating without using any geological standard sample. This method will make Ar-Ar dating changing from "relative dating" (the sample age is obtained by comparing with the standard sample age) to "absolute dating" (no standards are required).
The Ar-Ar dating method without standards for minerals provided by this invention requires only measuring the neutron flux and 40Ar*/ 39ArK instead of correcting, with the standard sample with known age, the J-value determined by the neutron flux, reaction cross section and other parameters at the position where the sample to be measured with known age is located, so it avoids the influence of the J-value gradient caused by the sample position and other reasons, thus improves the precision of Ar-Ar dating method.
Description with Drawings Figure 1 is a flow chart of the Ar-Ar dating method without standards for minerals according to an embodiment of this invention.
Detailed Description of the Presently Preferred Embodiment
In order to make the purpose, technical proposal and advantages of this invention clearer, this invention will be further detailed with reference to the specific embodiment and the attached figures.
In order to be able to accurately calculate the neutron flux, the embodiment of this invention is to attach a high-purity (mass percentage > 99.99%) nickel sheet with the same size as the sample on the front and back of the mineral sample to be measured, in which the thickness of the nickel sheet is about 1mm, so as to accurately calculate the neutron flux and eliminate the effect of the neutron flux gradient, thus improving the dating precision.
In the embodiment of this invention, the deuterium-deuterium (D-D) accelerator neutron source which only generates 2.45MeV single energy neutrons is used to irradiate the mineral sample, then the neutrons hit the mineral sample to convert 3 9K into 39Ar, which meets the following formula, "9Ark '-= Formula 7
The reaction cross section c is the ratio of the number of reaction generated nuclei per unit time per unit area (e.g. 39Ar) to the number of neutrons of the incident particle and the number of target nuclei (e.g. 3 9K), and the unit is barn (b, lb=10-2 4cm 2).
According to Formula 7,
40K = 1.255 x 10-4 X "ArK Formula 8 N-a
Where 1.255x10-4 is the 40 K/ 39K value in nature, N is the neutron flux, a is the reaction cross section, and is the integral function of the neutron energy spectrum.
According to Formula 1, Formula 7 and Formula 8, 40 A Ar* N-a t In 1 + Formula 9 X0+, ArK 1.255xlt0 4 /
Where t is the Ar-Ar age of the mineral sample, ) is the total decay constant of 4K, ?,=5.543(±0.010)x0-1 a- 0 1, le and 'eare the decay constants of the two branches of 4 0K 40 decaying into Ar respectively, le =0.572(±0.004)x10-10 1a- , 'e=0.0088(±0.0017)X10-10 a- 1, 40Ar*/ 3 9 ArK is the ratio of radiogenic 4 0Ar to 39 ArK produced by neutron irradiation on 39K, N is the neutron flux of accelerator neutron source, a is the reaction cross section, which is the integral function of neutron energy spectrum.
Therefore, to accurately determine the sample age, only 40Ar*/ 39ArK, N and a need to be determined.
In order to accurately measure the neutron flux N and reaction cross section a , the embodiment of this invention adopts the neutron nuclear reaction ( 8Ni (n, p)5 8 Co) of nickel closest to the nuclear reaction 39 K(n,p) 39Ar to detect. Since the adopted accelerator neutron source generates single energy neutron and the reaction cross section is constant, the 39K content and sample age can be calculated with reference to the precise neutron flux and 39Ar content recorded by the nickel sheet, and the Ar-Ar dating without standards can be realized.
Based on the above analysis, Figure 1 shows the flow chart of the Ar-Ar dating method without standards for minerals in accordance with the embodiment of this invention, which includes:
Step 1: wrap the mineral sample with aluminum foil into a sample piece, attach a high-purity (>99.99%) nickel sheet on the front and back of the sample piece respectively, and then put it into the accelerator neutron source for neutron irradiation;
Step 2: detect the irradiated nickel sheet to determine the value of the neutron reaction cross section y, and calculate the neutron flux N of the accelerator neutron source;
Step 3: load the irradiated mineral sample into the noble gas measurement system, melt sample, measure the Ar isotope ( 4 0Ar, 39 Ar, 36Ar) content of the mineral sample with the noble gas mass spectrometer, and obtain the required 40Ar*/ 39 ArKby calculating the data;
0 3 9 Step 4: substitute the neutron flux N, reaction cross section a and 4Ar*/ ArK of the
1 A 40Ar* N-c accelerator neutron source into the formula t In (1 + -Arx .2x _to
calculate the Ar-Ar age of the mineral sample;
Where t is the Ar-Ar age of the mineral sample, , is the total decay constant of 40K, X=5.543(±0.0l0)x10-1 0a- 1, le and X'e are the decay constants of the two branches of 4 0K decaying into 40Ar respectively, ke =0.572(±0.004)x10-1 a-1 , ;'e=0.0088(±0.007)x10-10 a- 1, 4 0Ar*/ 3 9 ArKis the ratio of radiogenic 40 Arto 39ArK produced by neutron irradiated 39K, N is the neutron flux of accelerator neutron source, a is the reaction cross section, which is the integral function of neutron energy spectrum.
In which Step 1 includes: take the mineral sample, wrap the mineral sample into a lcmx 1cm sample pieces with aluminum foil, attach a 1mm thick high-purity nickel sheet on the front and back of the sample piece respectively, and then put it into the accelerator neutron source as close as possible to the bull's eye for neutron irradiation, in which the accelerator neutron source is as close as possible to the bull's eye and is less than 3.5cm away from the bull's eye.
The nuclear data of the neutron generator is measured by the detected and irradiated nickel sheet described in Step 2 by ultra-low background HPGe y spectrometer. The nuclear data mainly refers to the 5 8 Co generated by the nickel sheet after neutron irradiation by the neutron source.
Since the neutron energy of the accelerator neutron source is single and 2.45 MeV, the determined value of the reaction cross section a of the neutron in the described Step 2 is 0.18 barn (b), where 1b=10-2 4cm 2
. In the above proposal, the calculation of the neutron flux N of the accelerator neutron source in the described Step 2 is a calculation of the neutron flux based on the nuclear reaction 5 8Ni (n, p) 58 Co.The specific calculation process is as follows:
According to the nuclear reaction 5 8Ni 58 (n, p) Co, (Y= $Zo Formula 10
Where a is the reaction cross section, N is the neutron flux of the accelerator neutron source, 5 8Co is the number of 58 Co atoms measured by HPGe y spectrometer, and 58Ni is the number of 58Ni atoms of high-purity nickel sheet. Because the neutron energy of reaction cross section a is a single constant, the number of 58 Co atoms can be measured by the y spectrometer, the number of 5 8Ni atoms can be calculated by the mass, purity and molar mass of the high-purity nickel sheet. The neutron flux N of accelerator neutron source can be calculated by substituting the above values into Formula 10.
The noble gas measurement system loading irradiated mineral sample as described in Step 3 includes a high temperature furnace, a gas purification system and a Helix MC Plus noble gas mass spectrometer. The specific measurement process for measuring the Ar isotope content of the mineral sample by the noble gas mass spectrometer as described in Step 3 is as follows: melt the sample with a high temperature furnace, purify the released gas through the purification system, remove the active gases, etc., and then measure the Ar isotope by Helix MC Plus noble gas mass spectrometer, receive 4 0Ar, 39Ar and 36 Ar signals by the Faraday cup and the electron multiplier equipped on the noble gas mass spectrometer respectively, and measure the content of 4 0Ar, 39Ar and 3 6Ar.
All 3Ar in the required 40Ar*/ 3 9 ArKobtained by data calculation described in Step 3 are generated by 39K irradiated by neutron. Therefore, 39ArK is the determined 39Ar,
Ar*= 40 Ar-4OAra, in which 4 0Ara is air 4 0Ar, 4 0Ara=295.5X 36 Ar. Therefore, Ar*/ 39 ArK=(4 Ar-295.5*36Ar)/39Ar; the specific calculation is to substitute the measured
Ar, Ar and Ar into the above formula to get the 4 0Ar*/ 3 9ArK value. 39 36
The Ar-Ar dating method without standards for minerals provided by the embodiment of this invention requires only measuring the neutron flux and 40Ar*/ 3 9 ArK instead of correcting, with the standard sample with known age, the J-value determined by the neutron flux, reaction cross section and other parameters at the position where the sample to be measured with known age is located, so it avoids the influence of the J-value gradient caused by the sample position and other reasons, thus improves the precision of Ar-Ar dating method.
The specific embodiment described above further details the purpose, technical proposal and beneficial effect of this invention. It should be understood that the above is only a specific embodiment of this invention and is not used to limit this invention. Any modification, equivalent substitution, improvement, etc. made under the spirit and principle of this invention shall be included in the protection scope of this invention.
Claims (9)
1.An Ar-Ar dating method without standards for minerals, which is characterized in that the method includes:
Step 1: wrap the mineral sample with aluminum foil into a sample piece, attach a high-purity nickel sheet on the front and back of the sample piece respectively, and then put it into the accelerator neutron source for neutron irradiation;
Step 2: detect the irradiated nickel sheet to determine the value of the neutron reaction cross section a, and calculate the neutron flux N of the accelerator neutron source;
Step 3: load the irradiated mineral sample into the noble gas measurement system, melt sample, measure the Ar isotope (4 0Ar, 39 Ar, 36 Ar) content of the mineral sample with the noble gas mass spectrometer, and obtain the required 40Ar*/ 39 ArKby calculating the data;
Step 4: substitute the neutron flux N, reaction cross section a and 4 0Ar*/ 3 9 Arx of the 1 A Ar* Nu accelerator neutron source into the formula t In 9 + ±A+X' 3 ArK 1.255xio0-4
to calculate the Ar-Ar age of the mineral sample;
Where t is the Ar-Ar age of the mineral sample, X is the total decay constant of 40K, =5.543(±0.0l0)x10-10a-1, e and X'e are the decay constants of the two branches of 4 0K decaying into 40Ar respectively, Ae =0.572(±0.004)x10-1a-1, 'e=0.0088(±0.0017)X10-10 a- 1, 4 0Ar*/ 3 9 ArKis the ratio of radiogenic 4 0Ar to 39ArK produced by neutron irradiation on 39K, N is the neutron flux of accelerator neutron source, a is the reaction cross section, which is the integral function of neutron energy spectrum.
2.The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that the described Step 1 includes:
Take the mineral sample, wrap the mineral sample into a lcmx lcm sample pieces with aluminum foil, attach a 1mm thick high-purity nickel sheet on the front and back of the sample piece respectively, and then put it into the accelerator neutron source as close as possible to the bull's eye for neutron irradiation, in which the accelerator neutron source is as close as possible to the bull's eye and is less than 3.5cm away from the bull's eye.
3.The Ar-Ar dating method without standards for minerals according to Claim 1 or 2, which is characterized in that the described high-purity nickel sheet is a nickel sheet with a mass percentage>99.99%.
4.The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that the nuclear data of the neutron generator is measured by the detected and irradiated nickel sheet described in Step 2 by ultra-low background HPGe y spectrometer.
5.The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that since the neutron energy of the accelerator neutron source is 2.45 MeV, the determined value of the reaction cross section a of the mineral sample in the described Step 2 is 0.18 barn (b), where 1b=10-24 cm 2
. 6.The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that the calculation of the neutron flux N of the accelerator neutron source in the described Step 2 is a calculation of the neutron flux based on the nuclear reaction 5 8Ni (n, p) 58 Co.
7.The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that the noble gas measurement system loading irradiated mineral sample as described in Step 3 includes a high temperature furnace, a gas purification system and a Helix MC Plus noble gas mass spectrometer, in which the sample is melted in the high temperature furnace, the released gas by melting is purified by the gas purification system to remove the active gases, and then the Ar isotope is measured by the Helix MC Plus noble gas mass spectrometer.
8.The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that the determination of Ar isotope content of the mineral sample by noble gas mass spectrometer described in Step 3 is the determination of Ar isotopes 4(0 Ar, 39Ar and 36Ar) by Helix MC Plus noble gas mass spectrometer.
9. The Ar-Ar dating method without standards for minerals according to Claim 1, which is characterized in that all 39Ar in the required4 Ar*/ 39ArK obtained by data calculation described in Step 3 are generated by 39K irradiated by neutron. Therefore, 39ArK is the determined 9Ar, 4 OAr*=4 OAr- 4 Ara,in which 4 0Arais air4 0 Ar, 4 0 Ara=295.5x3 6 Ar.
Descriptions with Drawings 2020100872
Figure 1
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN112462037A (en) * | 2020-11-20 | 2021-03-09 | 核工业北京地质研究院 | Method for determining age of ore by radioactive cause abnormal lead under non-closed system |
| CN113325458A (en) * | 2021-08-02 | 2021-08-31 | 中国科学院地质与地球物理研究所 | Combined measurement method for argon-argon age and cosmic exposure age of extra-terrestrial sample |
| CN114646998A (en) * | 2022-03-24 | 2022-06-21 | 西北核技术研究所 | High-flux fast neutron energy spectrum measuring system and method based on gas activation |
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2020
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Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112462037A (en) * | 2020-11-20 | 2021-03-09 | 核工业北京地质研究院 | Method for determining age of ore by radioactive cause abnormal lead under non-closed system |
| CN112462037B (en) * | 2020-11-20 | 2022-10-21 | 核工业北京地质研究院 | Method for determining age of ore by radioactive cause abnormal lead under non-closed system |
| CN113325458A (en) * | 2021-08-02 | 2021-08-31 | 中国科学院地质与地球物理研究所 | Combined measurement method for argon-argon age and cosmic exposure age of extra-terrestrial sample |
| CN113325458B (en) * | 2021-08-02 | 2021-10-29 | 中国科学院地质与地球物理研究所 | Combined measurement method for argon-argon age and cosmic exposure age of extra-terrestrial sample |
| CN114646998A (en) * | 2022-03-24 | 2022-06-21 | 西北核技术研究所 | High-flux fast neutron energy spectrum measuring system and method based on gas activation |
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