JP5930460B2 - K-Ar dating apparatus and method - Google Patents

Description

  The present invention relates to a K-Ar dating apparatus and method for measuring the age when rocks are solidified by detecting K and Ar present in rocks, and more particularly, mainly in the field of solid earth science. K-Ar dating device and method capable of providing a portable device so as to be useful for elucidating the formation history of the moon and planets in the field of space and planetary science as well as (e.g. identifying the volcanic eruption age) About.

  Conventionally, there is known a dating method for measuring the solidification age of rocks using the decay of radioactive elements contained in rocks, and this measurement method is mainly used for research in the field of solid earth science (for example, volcanic eruptions). Used to identify the age).

  In recent years, with the goal of elucidating the formation history of the moon and planets in the space and planetary science field, further miniaturization of devices for carrying out dating methods has been demanded. For example, the formation date of the lunar surface has been estimated based on the dating results of rock samples brought back by the Apollo project, but in recent years, the non-uniformity of the lunar surface has become clear. The importance of performing dating not only in very limited areas where rock samples were collected but also at more points has increased. Other planets do not collect rock samples, and little information about the age is available. On the other hand, bringing back rock samples from the moon and planets is not only very technically difficult even now, 40 years after the Apollo project, but it is also difficult to send a large dating device to the field. And a lightweight dating device is needed.

One of the rock dating methods is the K-Ar dating method. The K-Ar dating method makes use of the fact that ⁴⁰ K contained in rocks decays to ⁴⁰ Ar by electron capture. Before the rock solidifies, ⁴⁰ Ar is degassed and does not remain inside, but after solidification, it accumulates inside. Since the half-life of this decay is known to be 1.25 billion years, if the ratio of ⁴⁰ K and ⁴⁰ Ar present in the rock is known, the solidification age of the rock can be specified.

  In the conventional K-Ar dating method, K is quantified by wet analysis, and Ar is quantified by degassing mass spectrometry. Further, a method has been devised in which K is measured by laser induced breakdown spectroscopy (Laser Induced Brea Kdown Spectroscopy, hereinafter referred to as “LIBS method”) and Ar is measured by degassing analysis.

  However, in any of the above methods, since K and Ar are measured by different methods, it is necessary to prepare a sample for each measurement, and the nonuniformity of the amount of K and Ar in the sample is measured. It was a source of error. Further, an ultra-high vacuum measuring device for Ar degassing analysis and a K wet analyzing device are required, and there is a problem that the entire device becomes large. This increase in the size of the entire apparatus makes it very difficult to apply the K-Ar dating method to the space and planetary science fields as described above.

  On the other hand, a technique for measuring the content of each of a plurality of components (elements) contained in a target substance using the LIBS method is known (for example, see Patent Document 1). However, such well-known technical literature does not disclose or suggest how to measure K and Ar at the same time, and is portable so as to facilitate application to the space planetary science field. No consideration has been given to how to implement a dating device.

JP2007-3510

  The present invention has been made in view of the above-mentioned facts, and by making it possible to measure K and Ar contained in the same rock sample using the LIBS method, the measurement error due to the difference in the samples is reduced, and various It is an object of the present invention to provide a portable dating device that enables the dating of rocks at various rock collecting points.

In order to solve the above problems, the present invention provides a K-Ar dating apparatus for measuring the age when a rock sample is solidified by detecting K and Ar contained in the rock sample. A pulse laser device for condensing a pulse laser beam on a surface; a toroidal concave diffraction grating arranged to disperse light emitted from a substance evaporated and excited from the rock sample by the pulse laser beam; and the toroidal Ar detector arranged to detect the Ar emission line contained in the first order light of the concave concave diffraction grating, and K detection arranged to detect the K emission line contained in the 0th order light of the toroidal concave diffraction grating And a container.
Preferably, the Ar detector detects an Ar emission line having a wavelength of 107 nm in a vacuum ultraviolet region. In order to detect this Ar emission line, for example, it is preferable to include a microchannel plate (MCP) capable of spatial decomposition and a multi-anode. As a result, the Ar emission line can be clearly separated from the O emission line having the wavelength of 104 nm, which is the closest to the emission line of the element having a high abundance in the rock, and the detection efficiency can be greatly improved. Preferably, the K detector includes a band pass filter and a photodiode for detecting a K emission line having a wavelength of 767 nm. Preferably, the number of grooves of the toroidal concave diffraction grating is at least 1200 gr / mm. In order to further reduce the size of the K-Ar dating device, it is preferable to increase the number of grooves of the toroidal concave diffraction grating, more preferably 4800 gr / mm, which is the maximum number of grooves that can be created at the present time. . Of course, the present invention does not limit the upper limit of the number of grooves to this value.

  Another aspect of the present invention is a method for measuring the age when a rock sample is solidified by detecting K and Ar contained in the rock sample, wherein a pulse laser beam is applied to the surface of the rock sample. By condensing, the surface material of the rock sample is evaporated and excited, and the emission of the surface material is dispersed with a toroidal concave diffraction grating, and the Ar emission line included in the primary light of the diffracted light of the toroidal concave diffraction grating And detecting the K emission line included in the 0th order light of the diffracted light of the toroidal concave diffraction grating, and the amounts of K and Ar contained in the rock sample based on the detected K emission line and the light quantity of the Ar emission line Is configured to include each process.

  Preferably, in the Ar emission line detection step, an Ar emission line having a wavelength of 107 nm is detected in a vacuum ultraviolet region using a microchannel plate (MCP) and a multi-anode. Preferably, in the K emission line detecting step, a K emission line having a wavelength of 767 nm is detected using a bandpass filter and a photodiode.

To measure the age of rocks, and the ratio of the ⁴⁰ K initial abundance of the total amount of K, from the amount of K contained in the rock samples identified, to estimate the amount of ⁴⁰ K contained in the rock sample You may further provide the process. Moreover, you may further provide the process of estimating the quantity of Ar contained in the identified rock sample as the quantity of ⁴⁰ Ar contained in the rock sample. ^In the process of estimating the amount of ⁴⁰ Ar, in order to identify the amount of ⁴⁰ Ar in the rock that originates from the decay of ⁴⁰ K, it is assumed that the amount of Ar mixed in from the atmosphere is the same in the same sample, and the amount of K present is Correction is made to remove the ⁴⁰ Ar amount mixed from the atmosphere from the measured value by measuring at a plurality of points in different samples and drawing an isochron (isochron).

  According to the K-Ar dating apparatus and method of the present invention, in order to enable simultaneous measurement of the amount of K and the amount of Ar contained in the same part of the sample by the LIBS method, a toroidal concave diffraction grating is used. Of the diffracted light, the Ar emission line is detected from the primary light and the K emission line is detected from the 0th order light, so that the measurement accuracy can be greatly improved and the apparatus can be greatly reduced in size to be portable. An apparatus can be realized.

  That is, in the conventional K-Ar dating, in order to measure the amount of K by wet analysis and to measure the amount of Ar by degassing analysis, it is necessary to divide the sample. It was. Since the dating apparatus and method of the present invention measure the amount of K and the amount of Ar in the same sample, this error factor can be removed, and the measurement accuracy is greatly improved.

  In addition, the Ar-Ar dating method and the method of measuring K by the LIBS method and measuring Ar by degassing analysis also require a large measuring device with a high degree of vacuum for degassing analysis. Conventional K-Ar dating requires a large device, and it has been difficult to measure near the point where a rock sample is collected. In the present invention, since degassing analysis is not used and both K and Ar are detected using the LIBS method, the demand for the degree of vacuum is not high, and the apparatus can be significantly downsized. Furthermore, by using a toroidal concave diffraction grating for spectroscopy, the number of parts can be reduced, and also from this point, the size of the apparatus can be reduced.

  Thus, according to the present invention, the dating device can be transported by a passenger car or a lunar planetary lander (and a lunar planetary exploration vehicle mounted thereon). This not only shortens the time from sample collection to measurement, but also makes it possible to select a sample to be taken back to the laboratory for precise measurement. On the lunar surface, dating is performed on samples brought back by the Apollo project, but the non-uniformity of the lunar surface has been revealed since then, and dating at more points is required. Has been. However, after the Apollo project, there is no plan to take samples from the moon surface, and it is very difficult to take further samples at present. In addition, the sample has never been brought back from the planet surface. The device of the present invention is composed of parts that have been used in space and is small and lightweight, so it can be mounted on a planetary lander, and it is possible to perform dating on-site without taking back the sample. Therefore, a great contribution to lunar and planetary science is expected.

FIG. 1 is a schematic diagram showing the configuration of a K-Ar dating apparatus according to an embodiment of the present invention. FIG. 2 is a diagram showing a light collection distribution on the detection surface of Ar emission lines and O emission lines by a toroidal diffraction grating provided in the K-Ar dating apparatus of the present invention. FIG. 3 is a schematic diagram showing dimensions of a K-Ar dating apparatus according to an embodiment of the present invention.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a K-Ar dating apparatus 1 according to an embodiment of the present invention. The K-Ar dating device 1 detects the K and Ar present in the rock using the LIBS method (Laser Induced Breakdown Spectroscopy) and measures the ratio of these elements to determine the age when the rock solidifies. Is to measure. The LIBS method is a method in which high energy electrons generated by evaporation and ionization of a part of a measurement sample by pulse laser irradiation drop into an excited state through recombination with bremsstrahlung and cations. This is an elemental analysis method for analyzing the type and amount of elements contained in a measurement sample by measuring the intensity and wavelength of the spectral light unique to the atoms emitted when the excited atoms are deexcited.

  As shown in FIG. 1, the K-Ar dating device 1 includes a pulse laser device 3 that focuses pulse laser light on the surface of a measurement sample (rock sample) 2, and a substance that is evaporated and excited from the surface of the sample 2. The toroidal concave diffraction grating 5 arranged to split the emitted light 4 and the Ar emission line having a wavelength of 107 nm contained in the primary light 6 of the toroidal concave diffraction grating 5 are arranged to be detected in the vacuum ultraviolet region. Detector 8 (a combination of a microchannel plate (MCP) and a multi-anode) and a bandpass filter 9 arranged to detect a K emission line having a wavelength of 767 nm contained in the 0th-order light 7 of the toroidal concave diffraction grating 5 And a photodiode 10. The pulse laser device 3 includes a condensing optical system (not shown). At least the toroidal concave diffraction grating 5, the detector 8, the bandpass filter 9 and the photodiode 10 constitute a spectroscopic unit 11. The installation location of the spectroscopic unit 11 and the rock sample 2 is arranged in a vacuum chamber (not shown). Since the pulse laser device 3 can be used in the atmosphere, the pulse laser device 3 may not be disposed in the vacuum chamber.

The measurement method using the K-Ar dating device 1 will be described below.
(1) The pulsed laser beam is focused on the surface of the rock sample 2 to evaporate and excite the surface material.
(2) The luminescence 4 of the surface material is dispersed by the toroidal concave diffraction grating 5 and the primary light 6 of the diffracted light is detected by a detector 8 that combines a spatially resolvable microchannel plate (MCP) and a multi-anode. . Here, the number of grooves of the diffraction grating 5 is set to 4800 gr / mm, and Ar light and other element emission lines (oxygen or the like) are separated. FIG. 2 shows a light collection distribution of each bright line on the detection surface of the detector 8 (the length of the scale bar on the lower right corresponds to 0.340 mm in the figure). As shown in FIG. 2, the Ar emission line in the vacuum ultraviolet region has an emission line with a wavelength of 107 nm and an emission line with a wavelength of 105 nm, and an O emission line with a wavelength of 104 nm exists adjacent to the Ar emission line with a wavelength of 105 nm. As apparent from FIG. 2, since the bright line having the wavelength of 107 nm is sufficiently separated from the O bright line having the wavelength of 104 nm, the Ar bright line having the wavelength of 107 nm is separated and detected in order to identify the amount of Ar. It becomes possible.
(3) The K emission line (767 nm) included in the 0th order light 7 out of the diffracted light of the toroidal concave diffraction grating 5 is transmitted through the band pass filter 9 and guided to the detector (photodiode) 10. Since the intensity of the emission lines of other main elements is very weak near the K emission line wavelength, the zero-order light of the diffraction grating can be detected by a combination of the bandpass filter 9 and the photodiode 10 that transmits only the vicinity of 767 nm.
(4) The amount of K and Ar is identified from the light quantity of the K emission line and the Ar emission line.

As described above, in the present invention, the K emission line in the near infrared region and the Ar emission line in the vacuum ultraviolet region can be simultaneously measured from the same location of the sample, and the amount of K and the amount of Ar contained in the sample can be quantified. .
(5) Estimate the amount of ⁴⁰ K and ⁴⁰ Ar contained in the rock from the amount of K and the amount of Ar identified, and measure the age at which the rock solidified from the ratio of ⁴⁰ K / ⁴⁰ Ar To do.

^Since the initial abundance of ⁴⁰ K is 0.0117% of the total amount of K, the amount of ⁴⁰ K contained in the rock sample is estimated by multiplying the amount of K identified in step (4) above by 0.0117%. be able to. In addition, the detected Ar is estimated to be ⁴⁰ Ar for most samples on the earth. Thus, ⁴⁰ K and ⁴⁰ Ar can be estimated by measuring the amounts of K and Ar. On the other hand, when the rock solidifies when exposed to the atmosphere, ⁴⁰ Ar contained in the atmosphere is mixed in the rock. Therefore, to identify the amount of ⁴⁰ Ar from the ⁴⁰ K decay in the rock, It is necessary to estimate the amount of ⁴⁰ Ar mixed in. Since it is considered that the amount of Ar mixed from the atmosphere is the same in the same sample, the amount of ⁴⁰ Ar mixed from the air is measured by using the K-Ar dating device 1 and measuring at a plurality of points having different K abundances. Can be corrected to remove from the measured value. That is, in the different samples but formation age is the same amount each measurement point K, ⁴⁰ when the amount of ⁴⁰ Ar relative to the amount of K is plotted for each measurement point arranged in a straight line (the straight line isochron (isochrones )), The slope of this isochron reflects the age of rock formation. If there is no contamination of ⁴⁰ Ar from the atmosphere, the amount of ⁴⁰ Ar relative to the amount of ⁴⁰ K is proportional to the intercept 0. Therefore, if at least one point is measured, the slope of the isochron can be obtained. The rock formation age can be determined based on the above. However, if the amount of ⁴⁰ Ar from the atmosphere cannot be ignored, the amount of ⁴⁰ Ar relative to the amount of ⁴⁰ K is aligned on a straight line, but the isochron at this time depends on the amount of ⁴⁰ Ar from the atmosphere. Will have an offset intercept. Therefore, in this case, accurate dating based on the true ⁴⁰ Ar amount originating from ⁴⁰ K decay is possible by removing the ⁴⁰ Ar amount mixed from the atmosphere by calculating the slope of the isochron from multiple measurement points. It becomes.

  In the conventional method in which K is performed by wet analysis and Ar is degassed, it is necessary to prepare a sample for each measurement, and the nonuniformity of the K amount and Ar amount in the sample is an error factor. Further, in order to perform Ar degassing measurement, an ultra-high vacuum measuring device is required, and a K wet analysis device is also required, so the device is large and expensive. In the present invention, since the amount of K and Ar in the same part of the sample can be measured, an error due to non-uniformity in the sample does not occur and the purpose is to obtain the K / Ar ratio. There is no need to obtain it, and the detection accuracy is greatly improved. The Ar emission line (107 nm) requires a vacuum device because the air permeability is very low, but the cost and size of the device are low because the demand for vacuum is lower than the vacuum device used in degassing analysis. Relatively small. In recent years, relatively inexpensive and small pulse lasers are commercially available, and the size of the apparatus is also expected to be reduced.

  Al-Ar dating methods have already been devised to avoid errors due to non-uniformity in the amount of K-Ar in the sample, but this requires neutron emission to the sample using a nuclear reactor, The cost of the device is very high. Moreover, although the method of quantifying K by the LIBS method and quantifying Ar by gas mass spectrometry has been devised, the apparatus becomes large for gas mass spectrometry. Compared to these methods, the dating device of the present invention is advantageous in that it is small and inexpensive.

  The above is the embodiment of the present invention. However, the present invention is not limited to the above-described example, and can be arbitrarily modified within the scope of the present invention. For example, in the above example, Ar is detected by a detector 8 that combines a spatially resolvable microchannel plate (MCP) and a multi-anode. However, if Ar emission lines can be detected separately from other emission lines, other elements can be detected. Detection means can also be used. Further, K is detected by a combination of the bandpass filter 9 and the photodiode 10, but this is not limited to this example as long as the K bright line can be detected separately from other bright lines.

  Due to the miniaturization of the K-Ar dating device according to the present invention, the K-Ar dating device 1 is carried by a passenger car (ordinary vehicle), a lunar planetary landing machine, and a lunar planetary exploration vehicle capable of traveling on the surface of the lunar planet. It becomes possible. As shown in FIG. 3, the spectroscope unit 11 can be accommodated in a size of 25 mm × 25 mm × 65 mm, and a vacuum chamber having an inner diameter of about 150 mm including the place where the rock sample is installed is sufficient. The laser and the condensing optical system can be used in the atmosphere, and only fit in a size of 50 mm x 50 mm x 150 mm. With this device, dating can be performed almost simultaneously with the collection of rock samples. In general, since the distance between the laboratory where the measuring apparatus is installed and the place where the rock is collected is long, the measurement efficiency can be improved by shortening the time. Further, if the sample according to the present invention is used to make a simple measurement at a rock collecting point and a sample is carefully selected and an important sample is brought back to a distant laboratory to perform a precise measurement, the measurement efficiency can be improved.

1 K-Ar dating device 2 Measurement sample (rock sample)
3 Pulse laser device 4 Light emitted from the material evaporated from the surface of the sample 2 5 Toroidal concave diffraction grating 6 First order light of toroidal concave diffraction grating 5 7th order light of toroidal concave diffraction grating 5 8 Ar Detector 9 Bandpass filter 10 Photodiode 11 Spectrometer

Claims (7)

A K-Ar dating device for measuring the age when the rock sample is solidified by detecting K and Ar contained in the rock sample,
A pulse laser device that focuses pulse laser light on the surface of the rock sample;
A toroidal concave diffraction grating arranged to disperse light emitted from a substance evaporated and excited from the rock sample by the pulsed laser light;
An Ar detector arranged to detect an Ar emission line included in the primary light of the toroidal concave diffraction grating;
A K detector arranged to detect K emission lines contained in the 0th order light of the toroidal concave diffraction grating;
A K-Ar dating device.   The K-Ar dating apparatus according to claim 1, wherein the Ar detector includes a microchannel plate (MCP) and a multi-anode for detecting an Ar emission line having a wavelength of 107 nm in a vacuum ultraviolet region.   The K-Ar dating apparatus according to claim 1, wherein the K detector includes a bandpass filter and a photodiode for detecting a K emission line having a wavelength of 767 nm.   The K-Ar dating apparatus according to any one of claims 1 to 3, wherein the number of grooves of the toroidal concave diffraction grating is at least 1200 gr / mm. A method for measuring the age when the rock sample solidifies by detecting K and Ar contained in the rock sample,
By condensing the pulse laser beam on the surface of the rock sample, the surface material of the rock sample is evaporated and excited,
Spectral emission of the surface material with a toroidal concave diffraction grating,
Detecting Ar emission lines contained in the primary light of the diffracted light of the toroidal concave diffraction grating,
Detecting K emission lines included in the 0th order light of the diffracted light of the toroidal concave diffraction grating;
A method comprising the steps of identifying the amounts of K and Ar contained in the rock sample based on the detected amounts of K emission lines and Ar emission lines.   6. The method according to claim 5, wherein in the Ar emission line detection step, an Ar emission line having a wavelength of 107 nm is detected in a vacuum ultraviolet region using a microchannel plate (MCP) and a multi-anode.   The method according to claim 5 or 6, wherein, in the step of detecting a K emission line, a K emission line having a wavelength of 767 nm is detected using a bandpass filter and a photodiode.