JP4953274B2 - Isotope separation method using molecular rotation period difference - Google Patents

Description

  The present invention relates to a method for separating isotopes, and uses a difference in molecular rotation period between molecules containing a target isotope and molecules containing a non-target isotope in an isotopically low-purity source gas. Thus, the target isotope is separated and recovered.

There are multiple isotopes in the majority of naturally occurring elements. The separated and concentrated isotopes are used as raw materials for tracers, medical test reagents, nuclear fuels and the like. Recently, ²⁸ Si, one of the isotopes of silicon, was isolated and highly concentrated (99.86%) single crystal was found to have higher thermal conductivity than natural isotope constituent materials, It attracted attention as a candidate for next-generation semiconductor substrate materials. As described above, the physical properties of a substance obtained by highly enriching a single isotope may be greatly different from those of a natural abundance ratio, and there is an increasing interest in materials with controlled isotopes.

  Centrifugation, gas diffusion, distillation, etc. have been put to practical use as isotope separation / concentration methods. However, since the enrichment obtained by one separation / concentration operation is extremely low, a large number of separation / concentration operations are required to obtain a highly enriched isotope, and the separation / concentration facility becomes large-scale. . As a result, facility construction and operation costs become enormous. Therefore, the isotopes to which these methods can be applied are limited to those that are in demand to meet their costs.

  As a separation / concentration method for other isotopes, an electromagnetic mass separation method is used. In principle, this method can be applied to all isotopes, and a high concentration can be obtained by a single separation / concentration operation. However, since it is necessary to perform separation and concentration under the condition of maintaining a high vacuum, only a very small amount of highly concentrated isotopes can be produced. Since the equipment construction and operation costs are large relative to the production amount, highly enriched isotopes are very expensive, and development of a method capable of producing highly enriched isotopes relatively inexpensively is expected.

As a candidate for this, research and development of molecular laser isotope separation methods are underway. This method uses a shift in molecular vibration spectrum caused by the difference in molecular frequency between the molecule containing the target isotope and the molecule containing the non-target isotope in the raw material, so-called isotope shift. Is subjected to infrared multiphoton decomposition, and the target isotope is separated and recovered. A highly enriched isotope can be obtained by a single separation / concentration operation (see, for example, Patent Document 1). Furthermore, a highly concentrated isotope can be produced in large quantities by performing separation and concentration operations while continuously supplying raw materials.
JP 2003-53153 A

  The molecular laser isotope separation method has a problem that the efficiency of separating and concentrating isotopes decreases as the mass of the target element increases. This is because the molecular frequency decreases as the mass of the target element increases. For example, in the case of an equinuclear diatomic molecule, the molecular frequency ν is inversely proportional to the 1/2 power of the mass of the atom, as shown in the following equation.

Formula 1

Here, π is the circumference ratio, k is the molecular spring constant, and m is the mass of the atom.

This decrease in molecular frequency with increasing mass has two adverse effects in molecular laser isotope separation. That is, a decrease in the difference in molecular frequency due to different isotopes and an increase in the proportion of vibrationally excited molecules in the raw material. A decrease in molecular frequency difference results in a decrease in isotope shift in the molecular vibration spectrum, and an increase in the proportion of vibrationally excited molecules results in an increase in molecular vibration spectrum width. As a result, it becomes difficult to photolyze only the molecule containing one isotope. Therefore, when the target of the molecular laser isotope separation method is a heavy element, the efficiency of isotope separation / concentration decreases.
[Means for solving problems]

  In order to solve the above problems, the present invention utilizes the difference in rotation period of molecules containing isotopes of heavy elements for the separation and enrichment of isotopes for heavy elements. Specifically, the present invention relates to a highly efficient method for separating and concentrating heavy element isotopes based on multiple pulse irradiation of a pulse laser (multiple irradiation by a pulse laser), which is a pulse that is linearly polarized on a source gas. By irradiating a laser beam, only molecules oriented in the same direction in the source gas are vibrated or electronically excited; a pulsed laser beam that is further linearly polarized in accordance with the molecular rotation period including the target isotope or non-target isotope 1 It is characterized by preferentially inducing a photochemical reaction of a molecule containing a target isotope or a non-target isotope by irradiation with a pulse or a plurality of pulses.

  Further, it is characterized in that the photochemical reaction proceeds stably and efficiently by controlling the time interval of the laser light irradiation using the difference in the distance through which the pulse laser light passes. This photochemical reaction is, for example, photolysis of a source gas by irradiation with a pulsed laser beam, and reacting an isotope atom generated at that time or a molecule containing the same with other atoms or molecules to convert the isotope atoms. A chemical reaction that produces a stable compound.

  The linearly polarized pulse laser used in the present invention is a laser beam in which the vibration of the photoelectric field is limited to one specific direction. For example, as described above, when the source gas is irradiated with a linearly polarized pulsed laser beam. Pulsed laser light that can vibrate or electronically excite only molecules oriented in a specific direction in the gas, and that is further linearly polarized in accordance with the molecular rotation period of the molecule containing the target isotope or the molecule containing the non-target isotope , The molecule containing the target isotope or non-target isotope can be preferentially photolyzed to separate the isotope. In the present invention, in the pulse laser light irradiation system of FIG. 1, a laser beam from a near-infrared pulse laser is passed through a polarizing element to form a linearly polarized pulse laser.

  The molecular rotation period increases as the mass of the target element increases. For example, in the case of a homonuclear diatomic molecule, the molecular rotation period T is proportional to the mass of the atom as represented by the following equation.

Formula 2

Here, r is the distance between atoms, and η is the Planck constant. Therefore, the molecular rotation period difference ΔT between a homonuclear diatomic molecule formed of atoms of mass m and a homonuclear diatomic molecule formed of atoms of mass (m + Δm) is expressed as follows: It is proportional only to the difference Δm.

Formula 3

That is, it does not depend on the mass m. Therefore, even when the target element becomes heavy, the molecular rotation period difference does not change. Also in the vibrationally excited molecules, the molecular rotation period does not change so much, so the influence of the increase in the proportion of vibrationally excited molecules is small. Therefore, in the present invention, even in the case of isotope separation / concentration for heavy elements, the separation / concentration efficiency is dependent on the difference in molecular rotation period that is less affected by the mass of the element itself. It is possible to suppress the decrease.

  Hereinafter, for the embodiment of the present invention, using iodine bromide IBr as a raw material, using an example in which bromine 79 and bromine 81 are separated by irradiating and decomposing linearly polarized pulsed laser light by two pulses, This will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an isotope separation / concentration apparatus according to the present invention, and FIG. FIG. 3 is a schematic diagram showing the behavior of iodine bromide molecules containing bromine 81, and FIG. 3 is a prediction diagram of the photolysis probability change of bromine 79 or iodine bromide containing bromine 81 with respect to the irradiation time difference between the two pulse lasers.

  As shown in FIG. 1, the isotope separation / concentration device consists of two device systems. First, in the raw material supply / separation / recovery and isotope composition ratio measurement system, the raw material gas is supplied to the reactor, and the gas after irradiation with pulse laser light is recovered, separated, and the isotope composition ratio is measured. On the other hand, in the pulse laser beam irradiation system, two-pulse irradiation of linearly polarized pulse laser beam is performed.

  First, in the raw material supply and separation / recovery and isotope composition ratio measurement system, the reactor is sufficiently evacuated using an evacuation apparatus, and then iodine bromide vapor and buffer gas as raw material gases are collected in a reaction vessel. As the buffer gas, a gas such as acetylene that reacts with atoms generated by photolysis of the source gas to produce a stable compound is used.

  Next, the reactor is transferred to the pulse laser beam irradiation system. The iodine bromide molecule in the ground electronic state has a photoelectric field oriented in a direction perpendicular to the molecular axis, and its energy is excited to the first electronic excited state by absorbing one photon near 780 nm in wavelength. Furthermore, by absorbing another photon of a photon of the same energy, it is excited to a dissociative electronically excited state and decomposes into an iodine atom and a bromine atom. Near-infrared pulsed laser light whose output wavelength is adjusted to around 780 nm is converted into vertically polarized laser light by passing through a polarizing element, and then split into two by passing through a half mirror, and the two laser lights pass through. After setting the difference in distance, the so-called optical path difference, it is returned to the same axis through the half mirror again. By adjusting this optical path difference, it is possible to stably and accurately control the time interval between two continuous linearly polarized pulsed laser light irradiations. If these two continuous linearly polarized pulsed laser beams are introduced into the reactor, as shown in FIG. 2, a group of first electronically excited molecules with molecular axes aligned in the horizontal direction is formed by the first pulse irradiation. Is done. The alignment state of the excited molecules collapses as the excited molecules rotate, but after the molecular rotation period, the excited molecules are aligned again. By performing second pulse irradiation in accordance with the time, iodine bromide molecules having the same molecular rotation period, that is, molecules containing the same bromine isotope atoms can be preferentially photolyzed.

The optical path difference is adjusted on the one hand by reflecting the laser beam transmitted through the half mirror with a fixed reflecting prism, irradiating the half mirror again and reflecting it in a right angle direction, and then introducing it into the reactor. The laser beam irradiated in the right angle direction is reflected by adjusting the distance with a movable reflecting prism, and is again transmitted through the half mirror and introduced into the reactor, thereby reducing the optical path difference of the laser light between these two optical paths. adjust.

  As shown in FIG. 3, a molecule containing bromine 79 is expected to realign in about 390 picoseconds, whereas a molecule containing bromine 81 is expected to realign in about 400 picoseconds. Therefore, if the optical path difference after dividing the laser light into two by the half mirror is set to about 11.7 centimeters, molecules containing bromine 79 can be preferentially photolyzed. On the other hand, if it is set to about 12.0 centimeters, the molecule containing bromine 81 can be preferentially photodegraded.

The reactor irradiated with the pulse laser beam is returned to the raw material supply, separation and recovery, isotope composition ratio measurement system. Isotope is separated and concentrated by isolating compounds with bromine atoms and buffer gas and unreacted iodine bromide using a distillation apparatus. When the iodine bromide molecule containing bromine 81 is preferentially photodegraded, bromine 81 is concentrated in the bromine atom and buffer gas compound, and bromine 79 is concentrated in the unreacted iodine bromide.
[The invention's effect]

According to the isotope separation method of the present invention, since the rotation period difference that is not easily affected by the mass of the target element is used, even when the mass of the target element increases, the decrease in the efficiency of isotope separation / concentration is suppressed. It is possible.

As explained based on the examples, according to the isotope separation method of the present invention, a photochemical reaction on a molecule containing a specific isotope is preferentially advanced by irradiating the source gas with a plurality of pulses of a pulse laser. Therefore, it is possible to efficiently perform isotope separation and concentration regardless of the mass of the target element.

According to the isotope separation method of the present invention, the photochemical reaction efficiency for molecules containing a specific isotope is increased by adjusting the time interval of pulsed laser light irradiation, or the photochemical reaction of molecules containing other isotopes. Efficiency can be reduced.

According to the isotope separation method of the present invention, it is possible to realize continuous pulsed laser light irradiation having a stable and highly accurate time interval by utilizing the optical path difference of laser light.

  According to the isotope separation method of the present invention, a linearly polarized pulsed laser beam is used as the irradiation laser beam, thereby increasing the photochemical reaction efficiency for a molecule containing a specific isotope, or a molecule containing another isotope. It is possible to reduce the photochemical reaction efficiency.

The present invention is used for separating and concentrating naturally occurring isotope elements to produce raw materials such as tracers, medical test reagents, and nuclear fuels. In addition, ²⁸ Si, which is one of the two isotopes of silicon, is separated and highly concentrated (99.86%) single crystal has a higher thermal conductivity than natural isotope constituent materials. Although it is raised as a candidate for a substrate material, the physical properties of a substance enriched with a single isotope may differ greatly from those of natural abundances. Used for production.

1 is a schematic view of an isotope separation / concentration apparatus according to an embodiment of the present invention. In the multiphoton decomposition reaction of iodine bromide molecules by irradiation with two pulses of linearly polarized laser light, which is an example of the present invention, two pulses of linearly polarized laser light were irradiated in accordance with the rotation period of iodine bromide molecules containing bromine 81 It is the schematic diagram which showed the behavior of the iodine bromide molecule | numerator containing the bromine 79 and the bromine 81 at the time. Photodegradation probability of iodine bromide containing bromine 79 or bromine 81 with respect to the irradiation time difference between two pulse lasers in a multiphoton decomposition reaction of iodine bromide molecules by irradiation with two pulses of linearly polarized laser light, which is an example of the present invention It is a prediction figure of a change.

Claims (2)

By irradiating the source gas with pulsed laser light, only molecules oriented in the same direction in the source gas are vibrated or electronically excited.
Further, one or more pulses of pulsed laser light are irradiated in accordance with the rotation period of the molecule containing the target isotope or non-target isotope, and the molecule containing the target isotope or non-target isotope is preferentially photolyzed and the isotope is obtained. A method of separating the body,
A method for separating isotopes, characterized in that a time interval of pulsed laser light irradiation is controlled by adjusting a difference in distance through which laser light passes.   2. The isotope separation method according to claim 1, wherein linearly polarized laser light is used as the pulsed laser light to be irradiated.

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