KR20100045616A - Separating method for calcium isotope - Google Patents

Abstract

PURPOSE: A method for separating calcium isotopes is provided to separate the calcium isotopes of high productivity and high purity, which is different with a laser photoionization method or an electro-magnetic separation method. CONSTITUTION: A method for separating calcium isotopes includes the following steps: selectively light-pumping a target isotope by irradiating a continuous wave laser to the calcium isotopes having a plurality of the isotopes; exciting the target isotope which is photo-pumped; and photonizing the excited target isotope to irradiate the pulse laser to the excited calcium isotope. A pulse repetition rate of the laser exciting the target isotope is 10 kHz or more.

Description

Calcium isotope separation method {SEPARATING METHOD FOR CALCIUM ISOTOPE}

The present invention relates to a method for isolating calcium isotopes, and more particularly, using laser to separate target calcium isotopes from other isotopes, as well as lower cost and higher productivity than conventional methods. It is about a method.

In nature, calcium has six kinds of isotopes. Among them, ⁴⁰ Ca having a mass number of 40 is representative of calcium isotopes, and is present in nature of about 96.941%. Calcium isotope otherwise there can be mentioned the ^{42 Ca, 43 Ca, 44 Ca} , 46 Ca and ⁴⁸ Ca, ⁴² Ca is 0.647%, ⁴³ Ca is 0.135%, ⁴⁴ Ca is 2.086%, ⁴⁶ Ca is 0.004% And ⁴⁸ Ca is present at about 0.187%.

Of these isotopes, particularly ⁴² Ca, ⁴³ Ca, ⁴⁶ Ca and ⁴⁸ Ca, are commercially very useful, especially calcium isotopes enriched in tens of percent of the ⁴⁸ Ca are tracers for human metabolism and osteoporosis. It is used as a tracer and, when concentrated to more than 90%, is used for super-heavy elements in accelerator nuclear physics studies. In particular, double beta decay experiments to determine the mass and properties of neutrinos require about tens to hundreds of kilograms of ⁴⁸ Ca.

However, as mentioned above, since the abundance of calcium isotopes, especially ⁴⁸ Ca, present in nature is only 0.187%, it is difficult to utilize the usefulness of ⁴⁸ Ca unless a separate operation of extracting it from other isotopes is performed. Do.

Enriching isotopes consists of separating isotopes from other isotopes and accumulating them. Among other things, the separation of target isotopes from other isotopes requires very demanding and delicate conditions because the nature of each isotope does not differ significantly.

Electromagnetic methods are the only separation techniques that have been used commercially for the separation of calcium isotopes. U.S. Patent No. 6,559,402 relates to the improvement of the calcium isotope separation process by an electromagnetic method, which is characterized by the motion trajectory of a single energy calcium ion beam passing through a spatially uniform magnetic field. It uses the principle of spatial separation. The electromagnetic method is a technology developed in the middle of the 20th century, but can be applied to various elements. In general, it is pointed out that a small amount of production per unit time, high separation cost is a big disadvantage.

In addition, although not about calcium, ionized metal ions, unlike neutral metal atoms, are focused on being affected by electromagnetic force under an electric field, and selectively ionize only the target isotopes by applying a laser to the atomic beam of the metal element. Atomic vapor laser isotope separation (AVLIS) technology for extracting ions of a target isotope from the flow of atomic vapor by applying an electric field to the atomic beam containing the ionized target isotope is an electromagnetic method It is emerging as an alternative to overcome the disadvantages. This method has been demonstrated for a variety of elements, including U.S. Patent Nos. 4,793,907, 5,202,005, and the patents for the separation of mercury (Hg), gadolinium (Gd), and erbium, Er, and the like. 5,443,702 is proposed.

However, in order to determine whether laser isotope separation is technically feasible for a particular element, it is necessary to examine in detail various problems including selective photoionization pathways. That is, the generation of atomic beams, the effective collection of photoion products, and the like, including isotope shift, energy, lifetime, and transition probability of each level associated with the photoionization pathway. Knowledge of atomic spectroscopy constants is required.

In the AVLIS method using a conventional photoionization method, a method of selectively ionizing only a desired isotope using a conventional pulse laser is performed by irradiating an atomic beam with a pulse laser of a frequency (wavelength) for selectively ionizing a target isotope. This is how the element can be directly ionized. When using this method, it is necessary to increase the output of the laser in order to increase the amount of isotope separated per hour, but in the case of increasing the output of the laser, especially in the case of pulsed laser, The so-called power broadening phenomenon is likely to occur in which the frequency range is widened. According to the power broadcasting phenomenon, even if the center frequency of the laser pulse is adjusted to the target frequency, the frequency is widened by the spread of the frequency, so that a laser including a frequency suitable for ionization of other isotopes other than the target isotope is irradiated. do.

Therefore, by using the laser with increased power, not only the target isotope but also other isotopes can be ionized together, and the purity of the separated isotope can be lowered. Therefore, in order to increase the purity, the power of the laser should be maintained below a certain level so as not to be ionized to other isotopes by power broadcasting, which leads to a decrease in productivity since the amount of separation of the target isotopes ionized per unit time is reduced. .

In order to solve this problem, Korean Patent No. 0478533 has been proposed by the inventors of the present invention. The Korean Patent No. 0478533 relates to a method for separating thallium isotopes. According to the method, ²⁰³ Tl can be separated by selective photopumping of a target isotope and photoionization with an infrared laser.

Although the method is effective for the separation of target isotopes, specific light pumping conditions and photoionization conditions cannot be uniformly determined for each target element.

In particular, the isotope separation by laser is very rarely proposed or attempted because calcium atoms have very small isotope transport. In addition, a three-stage photoionization method for selectively ionizing calcium isotopes using a narrow linewidth continuous oscillation laser has been proposed by AK Pulhani et al. (Journal of Physics B. 35, 3677 (2002)). It is intended for analysis and because of its very low ionization efficiency, it cannot be used for the separation of large amounts of calcium isotopes. A method of producing ⁴⁰ Ca using a 423 nm wavelength laser and a 389 nm continuous oscillation laser for the purpose of ion capture has been proposed by DM Lucas et al. (Physical Review A 69, 012711 (2004)). Because of this very low, it is impossible to use for the separation of large amounts of calcium isotopes.

One aspect of the present invention is to solve the problems of the prior art as described above, according to one aspect of the present invention is provided a method for separating calcium isotopes with high productivity and high purity.

The calcium isotope separation method of the present invention for solving the above problems is a method of selectively separating target isotopes from a calcium atom beam (steam) having a plurality of isotopes, continuous oscillation (cw) laser on the calcium atom beam Selectively optically pumping the target isotope by irradiating; Irradiating the calcium atomic beam with a pulsed laser to excite the light pumped target isotope; And photoionizing the excited target isotope by irradiating another pulsed laser beam to the excited calcium atom beam.

At this time, constitutes the excitation level and the resonance of the continuous emission type laser is 36731.62cm ^-2, it is also metastable level of an optical pumping by the continuous-wave laser is preferably 21849.63 cm ^-1 levels.

Also, the pulse repetition rate of the laser for exciting the optical pumping the target isotope is at least 10kHz, to the excitation level of the calcium isotope where in the laser of any one of 40537.89cm ^-1 cm ^-1 and 35818.71 is effective.

Further, when the excitation level of the calcium isotope excited by the pulsed laser is 40537.89 cm ^-1 , the pulse repetition rate of the laser for photoionizing the excited target isotope is 10 kHz or more, and the autoionization level by the laser is It is advantageous that it is 59463.5 cm ^-1 .

When the excitation level of the calcium isotope excited by the pulsed laser is 35818.71 cm ^-1 , the pulse repetition rate of the laser for photoionizing the excited target isotope is 10 kHz or less, and the autoionization level by the laser is It is good to be 60078.8 cm ^-1 .

In addition, it is effective to separate an ionized target isotope by applying an electric field to the atomic beam containing the photoionized target isotope.

According to the present invention, calcium isotopes can be separated in high purity with high productivity, which is difficult to implement in the conventional electromagnetic separation method or the conventional laser photoionization method.

Hereinafter, the present invention will be described in detail.

The present invention selectively optically pumps only isotopes of various isotopes (so-called, isotope selective optical pumping, ISOP), and then irradiates a laser to the optically pumped excited isotopes to target only isotopes. After photoionization, it is characterized in that it is separated in the presence of an electric field.

Optical pumping refers to an operation of changing an atom from a ground state to an excited state using light, and the present invention uses selective optical pumping to excite only a target isotope. In order to derive the appropriate conditions for selective optical pumping, it is necessary to first identify which of the many possible excitation states is advantageous for the next step, photoionization, and to excite only the target isotopes without exciting other isotopes. It is necessary to decide in advance whether light should be used.

Ca atom is a _{^{4 1 S 0 (0 cm -1}} ) levels of the metastable level _{^{4 1 D 2 (21849.6 cm -1}} ) floor level to the low energy region than _{^{30000 cm -1, 4 3 P 0}} (15157.9 cm - ¹ ), a relatively simple electronic structure with only 4 ³ P ₂ (15315.9 cm ^-1 ) levels, and the 4 ¹ P ₁ (23652.3 cm ^-1 ) and 4 ³ P ₁ (15210.1 cm ^-1 ) levels. Have In addition, the electric dipole moment between the ground level 4 ¹ S ₀ and the excitation level 4 ¹ P ₁ is relatively high, the lifetime of the 4 ¹ P ₁ level is very short, 19 ns, and 4 ¹ P ₁ Since the branching ratio from the level to the metastable level 4 ¹ D ₂ level is larger than the branching ratio to the floor level, it is very advantageous for light pumping. That is, such a case will be readily shifted to ⁴¹ D ₂ level because calcium atoms ⁴¹ do not stay in the P ₁ there is presented branching to the ground level or a ⁴¹ D ₂ level, a high branching ratio of a 4 ¹ D ₂ level .

Thus, in the case to remove the case of calcium target isotope from other isotopes it is preferred to herein as 4 _¹ P ¹ level from the bottom level of 4 ¹ S ₀ level in the optical pumping stage. As described above, since the 4 ¹ P ₁ level is easily branched to the 4 ¹ D ₂ level which is a metastable level, it is advantageous for optical pumping. The lifetime of the 4 ¹ D ₂ level at which the target isotope is photopumped is 2.3 ms long enough to be used in subsequent photoionization steps. In other words, for example, when the calcium atom beam is generated at 900 ° C., the average speed of the atomic beam is about 800 m / s. Since the metastable level 4 ¹ D ₂ calcium atom has a lifetime of 2.3 ms, it is about 1.8 years. It will be a long time to move the meter. Therefore, it is possible to move a sufficient distance to be irradiated to the additional laser beam for the light ionization step after the light pump.

The photopumped calcium atoms are then ionized to an ionization level through a higher intermediate level by additional laser irradiation.

With reference to Figure 1 showing an embodiment of the present invention will be described in more detail the above process. As already explained, the calcium atom at the 4 ¹ S ₀ level, which is the bottom level (| 1> level), is excited to the 4 ¹ P ₁ level (| 2> level) by laser irradiation. At this time, in the case of ⁴⁸ Ca, the wavelength of the laser required here corresponds to 272.16 nm. Therefore, in order to excite ⁴⁸ Ca, it is preferable to use the laser containing the said laser corresponding to 272.16 nm among lasers. Herein, a laser including 272.16 nm of the lasers is a concept that includes a laser having a wavelength distribution having a wavelength distribution of 272.16 nm even if the peak of the wavelength distribution curve is not located at 272.16 nm. It should be noted. Of course, the most preferred laser is one whose peak of the wavelength distribution curve is substantially 272.16 nm. In addition, in the case of pulsed lasers, since the power broadcasting effect is large, there is a possibility that optical pumping may be performed to other isotopes during optical pumping. Therefore, it is preferable to use a continuous oscillation (cw) laser to improve the optical pumping selectivity. . The continuous oscillation laser is also advantageous in that it can continuously supply optical energy to the target isotope atoms in the ground state.

The excited calcium atoms of the 4 ¹ P ₁ level (| 2> level) are then transferred to the 4 ¹ D ₂ level (| 3> level) or to the bottom level (| 1> level). 1> level, the calcium atoms can be excited by the laser irradiation back to the 4 ¹ P ₁ level (| 2> level), so that the productivity can be optically pumped.

Thereafter, a process of re-exciting the optically pumped calcium atom is necessary as an intermediate step for photoionization. In one aspect of the present invention, the optically pumped 4 ¹ D ₂ level (| 3> level) calcium atom is then pulsed. By the intermediate level ¹ F ₃ (40537.89 cm ^-1 ) level (| 4> level). At this time, since the Einstein A value of the ¹ D ₂ → ¹ F ₃ transition line is 10 ⁸ s ⁻¹ , calcium atoms can be easily excited even with a low power laser. In addition, the energy state of the already-pumped calcium atoms in the ¹ D ₂ level is very different from the energy states of the calcium atoms in the bottom level, so even if a power-broadcasting phenomenon occurs due to a pulsed laser, the atoms are located at the bottom level. It has no effect. Therefore, the target isotope of a calcium atom can be isolate | separated favorably. The wavelength of the laser used at this time is preferably 534.95nm.

Calcium atoms excited to the ¹ F ₃ level is then photoionized through additional laser irradiation. Calcium atoms excited to the ¹ F ₃ level may be photoionized in two forms (| 5> or | 5 'levels). One of them is to ionize to an automatic ionization level (| 5> level, 59463.5 cm ^-1 ) using a pulsed laser with a wavelength of 528.24 nm on the calcium atom excited in the previous step as shown in FIG. The other one is ionization to an automatic ionization level (59344.1 cm ⁻¹ ) using a pulse laser corresponding to 531.59 nm. Both are effective for ionizing the excited calcium atoms, and among them, a method of photoionizing at an automatic ionization level using a pulse laser having a wavelength corresponding to 528.24 nm is more preferable (Fig. 1 (a)). The photoionization cross-sectional area of the excitation level for the 528.24nm wavelength is about 4 × 10 ^-16 , which means that the excited atoms can be ionized by 80% or more when using a laser having an output density of 3mJ / pulse / cm ² per pulse. .

Another embodiment of the present invention will be described with reference to FIG. 2. The method shown in FIG. 2 is an energy state of the excitation level and the photoionization level that is different from that of FIG. 1, which corresponds to another preferred case of selectively ionizing calcium isotopes. As illustrated in FIG. 2, the process of selectively pumping calcium atoms of the 4 ¹ S ₀ level to the 4 ¹ D ₂ level is the same as the embodiment described with reference to FIG. 1. Therefore, the description of the optical pumping process is omitted.

Thereafter, the present embodiment uses the ³ F ₃ level (35818.71 cm ^-1 ) rather than the ¹ F ₃ level as the excitation level. The ³ F ₃ level also has the advantage that the Einstein A value of the ¹ D ₂ → ³ F ₃ transition line is very large, such as 10 ⁸ s ^−1, so that calcium atoms can be easily excited even with a low power laser. Thereafter, the calcium atoms of the excitation level are finally photoionized by additional laser irradiation. There are two types of lasers irradiated for photoionization, and as shown in FIG. 2, an automatic ionization level (| 5> level, 60077.8 using a pulse laser having a wavelength of 412.10 nm for the calcium atom excited in the previous step is shown. ionizing to cm ^-1 ), and the other is ionizing to an autoionization level (58829.6 cm ^-1 ) using a pulse laser corresponding to 434.46 nm. Both are effective for ionizing the excited calcium atoms, and among them, a method of photoionizing at an automatic ionization level using a pulse laser having a wavelength of 412.10 nm is more preferable (Fig. 2 (a)). The photoionization cross-sectional area of the excitation level for the wavelength of 412.10 nm is about 4 × 10 ^-16 , which means that the excited atoms can be ionized by 80% or more when using a laser having an output density of 3mJ / pulse / cm ² . .

Therefore, the calcium isotope separation method of the present invention comprises the steps of selectively optically pumping the target isotope by irradiating a continuous oscillation (cw) laser to the calcium atomic beam; Irradiating the calcium atomic beam with a pulsed laser to excite the light pumped target isotope; And irradiating another pulsed laser beam to the excited calcium atom beam to photoionize the excited target isotope.

As described above, the photo-ionized calcium isotope is moved toward the electrode where the attractive force acts by the electric field formed around the final isolator.

Hereinafter, a pretreatment method of forming an atomic beam including the calcium isotope will be described with reference to FIG. 3.

First, a process of heating calcium metal 1 to obtain calcium vapor (calcium atom beam) 3 is necessary. In this case, in order to obtain a sufficient amount of calcium atom beam, the heating temperature is preferably 600 ° C. or higher. On the other hand, when the heating temperature is too high, the density of calcium atoms is excessively increased, which is not preferable, so the upper limit of the heating temperature is 900. It is preferable to set it as ° C. The obtained calcium vapor (3) is floated to the top due to the low density, the continuous oscillation (cw) laser for light pumping the target isotope as shown in FIG. 4) is irradiated, and then a process of exciting and photoionizing a target isotope photocatalyst of the calcium atom beam is performed by pulsed lasers 5 and 6 installed above the laser irradiation unit.

At this time, the calcium atoms contained in the calcium atom beam are not maintained at a constant distance with respect to the direction in which the laser 4 is irradiated (horizontal direction on the drawing), but also move in the horizontal direction. Doppler effects occur between atoms. Such a Doppler effect produces an effect such that a wavelength deviating from the wavelength range of the laser light actually irradiated is irradiated to the atomic beam and thus has a distribution of 4 ¹ S ₀ → 4 ¹ P ₁ transition lines as shown in FIG. 4. for each calcium isotope s of ⁴⁸ Ca different isotopes, in particular ⁴⁸ Ca without isotope move (shift) is not large, from the number of the content it is ⁴⁴ Ca not there is a possibility that a large extent optically pumped by the extended wavelength range, . Thus, for example, when ⁴⁸ Ca is separated, it is important that ⁴⁴ Ca is not optically pumped together. In order to do so, it is necessary to reduce the movement of atoms in the direction of laser irradiation to minimize the Doppler effect.

To this end, it is effective to limit the divergence angle of the atomic beam, and as shown in FIG. 3, it is more preferable to limit the width of the atomic beam using the atomic beam collimator 2.

In order to allow the ⁴⁴ Ca to be separated from the ⁴⁸ Ca, it is effective that the Doppler line width of the atomic beam is about 200 MHz or less, and optically pumped using a single frequency continuous oscillation laser. 5 shows a 4 ¹ S _0- > 4 ¹ P ₁ transition line as a light pumping transition line for a calcium atom beam having a Doppler line width of 100 MHz, and a continuous oscillation laser having a power of 5 W has a diameter of 10 mm. This is the result of calculating the isotopic selectivity of the light pumping process when it has a Gaussian intensity shape. When ⁴⁸ Ca of 0.187% of natural component is used, it can be seen that the optical pumping selectivity of metastable level of 92.9% can be obtained. Considering that the isotope shift is around 1 GHz, it can be seen that the isotope selectivity of the present invention is very high.

Therefore, the calcium isotope separation method of the present invention preferably further includes a step of limiting the Doppler line width to 200 MHz or less with respect to the generated atomic beam as a pretreatment step.

^When the method devised in the present invention is applied to commercial calcium isotope separation such as ⁴⁸ Ca isotope and aims to produce several kilograms / year per year, the continuous oscillation type optical pumping laser is capable of output of about 500 mW. If the pulse laser is about 5W, the ionization pulse laser is about 50W, and the pulse repetition rate is 10 kHz or more.

1 is a conceptual diagram showing a phenomenon of transition of the level of the target isotope in one preferred embodiment of the present invention,

2 is a conceptual diagram showing a phenomenon of transition of the level of the target isotope in another preferred embodiment of the present invention;

3 is a schematic diagram illustrating a process for separating target isotopes in the method of the present invention;

Figure 4 is a graph, and showing a transition line at 4 ₀ ¹ S of the isotope of calcium as 4 _¹ P ¹ state to the target in the present invention

FIG. 5 is a graph showing the abundance of each isotope when ⁴⁸ Ca isotopes are separated by limiting the Doppler line width of the atomic beam to 100 MHz in one embodiment of the present invention.

Claims (6)

A method for selectively separating target isotopes from a calcium atom beam (vapor) having multiple isotopes, Selectively optically pumping a target isotope by irradiating a continuous oscillation (cw) laser to a calcium atom beam; Irradiating the calcium atomic beam with a pulsed laser to excite the light pumped target isotope; And, Irradiating the excited calcium laser beam with another pulsed laser to photoionize the excited target isotope; Calcium isotope separation method comprising a. The method of claim 1, wherein the continuous emission laser forms the excitation level and the resonance of 36731.62cm ^-2, also metastable level by the pumping light and continuous emission laser, characterized in that calcium levels 21849.63 cm ^-1 Isotope separation method. The method of claim 1, wherein the pulse repetition rate of the laser for exciting the optical pumping targeted isotope is at least 10 kHz, the excitation level of the calcium isotope where in any of the laser 40537.89cm ^-1 cm ^-1 and 35818.71 Calcium isotope separation method characterized in that one. The pulse repetition rate of a laser for photoionizing the excited target isotope is 10 kHz or more, when the excitation level of the calcium isotope excited by the pulse laser is 40537.89 cm ^-1 . Auto-ionization level by the separation method of calcium isotope, characterized in that 59463.5 cm ^-1 . The pulse repetition rate of the laser for photoionizing the excited target isotope is at least 10 kHz when the excitation level of the calcium isotope excited by the pulse laser is 35818.71 cm ^-1 . Auto-ionization level by the separation method of calcium isotope, characterized in that 60078.8 cm ^-1 . The method of claim 1, wherein the ionized target isotope is separated by applying an electric field to an atomic beam including the photoionized target isotope.

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