JP5542341B2 - Nanocluster - Google Patents

Description

  The present invention relates to spherical nanoclusters.

  In order to apply to cancer treatment, neutron source, ion beam probe, etc., a method of generating an ion beam by irradiating a target with an ultra-high intensity / ultra-short pulse laser has been studied.

For example, there has been proposed a flat plate target that can efficiently use the energy of the irradiated laser beam for ion acceleration. This flat plate target has a relatively high electron density front part that functions to transmit the energy of the incident laser light to the electrons to form a plasma state, and receives a large amount of energy to enter the plasma state. And a rear stage portion of relatively low electron density that gradually accelerates toward the rear end direction. In this example, the load on the laser beam generator is reduced and the size can be reduced, and the ion acceleration portion can also be reduced (see, for example, Patent Document 1).
JP 2008-198566 A (page 3-4, FIG. 1)

  However, the ion beam used for cancer treatment or the like needs to be monochromatic (single energy). However, in the conventional ion beam generator using the flat plate target as described above, there is no detailed description regarding monochromatization. . In addition, there is a problem that the efficiency is lowered by the monochromatic process. Furthermore, in order to make the ion beam monochromatic, it is necessary to form an ion accelerating electric field using electrons that are spatially uniform or symmetric, but in principle, it is difficult to realize with a flat plate target.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a nanocluster that can be used as a target that can efficiently generate a monochromatic ion beam.

The invention according to claim 1 is a nanocluster used as a target for generating an ion beam by laser irradiation, wherein the first atomic layer in which the first atoms are present at the first density, and A multilayer having a second atomic layer and a third atomic layer in which second atoms are present at a second density, the third atomic layer being sandwiched between the first atomic layer and the second atomic layer The structure forms a sphere having a size smaller than the wavelength of the laser as a whole, and the first atom and the second atom are hydrogen isotopes or carbon .

  According to such a configuration, the generation efficiency of the monochromated ion beam is the third efficiency with respect to the acceleration efficiency ratio, the charge mixture ratio, and the total volume of the nanocluster when the two types of atoms constituting the nanocluster are ionized. It depends on the ratio of the volume of the layer and the ratio of the distance from the center point of the nanocluster outside the third atomic layer to the sphere diameter of the entire nanocluster.

The invention according to claim 2 is a nanocluster used as a target for generating an ion beam by laser irradiation, and has a first atom and a second atom which are different from each other, and the first atom is a nanocluster. Present at a density having a first dependence on the distance from the central point of the second, and the second atom is present at a density having a second dependence on the distance from the central point of the nanocluster, structure der forming a laser small size spherical than the wavelength of the is, wherein the first atom and the second atom is characterized isotope or carbon der Rukoto hydrogen.

  According to such a configuration, the generation efficiency of the monochromated ion beam is determined by calculating the outermost diameter of the nanocluster in the calculation of the acceleration efficiency ratio when the two types of atoms constituting the nanocluster are ionized and the charge mixture ratio. Depends on the exponent of multiplying the ratio of the distance from the center point to the outermost diameter of the nanocluster by the charge mixing ratio in, and the charge mixing ratio at the outermost diameter of the nanocluster.

According to the nanocluster of claim 1 , it is possible to achieve a monochromaticity to a desired degree by optimizing the charge mixing ratio of two kinds of atoms constituting the nanocluster and the thicknesses of the first to third atomic layers. It is possible to efficiently generate the ionized beam.

According to the nanocluster according to claim 2 , the acceleration efficiency ratio of two kinds of atoms constituting the nanocluster, the charge mixing ratio at the outermost diameter of the nanocluster, and the distance from the center of the nanocluster of the two kinds of atoms By optimizing the density dependence on the ion beam, it is possible to efficiently generate an ion beam that is monochromatic to a desired degree.

  Hereinafter, spherical nanoclusters (hereinafter referred to as nanospherical clusters) according to the present invention will be described with reference to the drawings. The nanospherical cluster according to the present invention has a configuration in which at least two kinds of atoms aggregate on the order of several thousand to several tens of thousands, and as a result, form a spherical mass having a size of several nanometers to several hundred nanometers.

The nanospherical cluster as described above is irradiated with an ultrahigh intensity / ultrashort pulse laser that has been rapidly growing in recent years, for example, a laser having an irradiation intensity of 10 ¹⁶ to 10 ¹⁷ W / cm ^{2 at} a wavelength of 1 micron. At this time, the electrons are explosively scattered in the vacuum, and as a result, an ion sphere much heavier than the electrons is left behind in the center. These ions are accelerated in the outer direction by the strong coulomb repulsive force, causing a coulomb explosion. By this Coulomb explosion, an ion beam is generated. The present invention relates to optimization of the density ratio and density coordination of two types of atoms in a nanospherical cluster when used as a target for generating such an ion beam.

  Each embodiment of the present invention is based on theoretical calculation, and uses two parameters that characterize the following system, namely, an acceleration efficiency ratio α and a charge mixture ratio β. In the following embodiments, the nanospherical cluster will be described as having two types of atoms 1 and 2.

  In addition, since the size of the nanosphere cluster according to each embodiment of the present invention is sufficiently shorter than the wavelength of the laser to be irradiated, a sufficiently strong laser electric field acts on the atoms in the center of the nanosphere cluster, It is possible for atoms to ionize. Furthermore, since atoms 1 and 2 are atoms with small atomic numbers such as hydrogen isotopes and carbon, an ion cluster sphere that is almost completely ionized can be formed by selecting an optimum laser intensity and cluster size. it can. Therefore, in the nanosphere clusters according to the following embodiments, the ion density of the particles 1 and the particles 2 may be considered as the atomic density of the nanosphere clusters before laser irradiation.

Here, when two kinds of atoms are ionized, the ion species that becomes the background of relatively weak acceleration when the atoms 1 are ionized particles 1, and the ion species that the monochromatic and strong acceleration is desired is ionized by the atoms 2. This is particle 2. Further, the acceleration efficiency ratio α and the charge mixing ratio β are expressed as follows by the charge Z _x , the ion mass m _x , and the ion density _nx (number of particles per unit volume) (x = 1, 2). .
(1) Acceleration efficiency ratio α = (Z ₂ / m ₂ ) / (Z ₁ / m ₁ )
(2) Charge mixing ratio β = (Z ₂ n ₂ ) / (Z ₁ n ₁ + Z ₂ n ₂ )

As in the above equation (1), the acceleration efficiency ratio α is a function of each ion mass, and indicates a relative ease of acceleration. The charge mixing ratio β gives the mixing density ratio of each ion. Each ion has a charge Z and the Coulomb repulsive force must be taken into consideration, and the acceleration efficiency ratio α and the charge mixture ratio β are defined to include charges. In the following embodiments, an optimal target design was performed using these parameters. In the following embodiments, α = 2 (particle 1 was fixed as C ⁺⁶ (m ₁ = 12, Z ₁ = 6), and particle 2 was fixed as H ⁺ (m ₂ = 1, Z ₂ = 1).

Also, 1% monochromatic accuracy required for cancer treatment was used as a fixed parameter. The monochromatic accuracy required for general applications such as a neutron generation source and an ion beam is about 5%. An expression showing the range of energy E when the monochrome accuracy is t% is shown below.
(3) (1-t / 100) E _max ≦ E ≦ E _max

The monochromatic efficiency η ₁ that is an index of monochromatization is expressed by the following equation, where N _{2 (t%)} is the number of particles 2 in the range of the monochromatic accuracy t%, and N _{2 (Total)} is the total of the particles 2. Defined.
(4) Monochromatic efficiency η ₁ = N _{2 (t%)} / N _{2 (Total)}

The generation efficiency η ₂ is defined as follows because the charge mixing ratio β is the ratio of the particles 2 to the particles ₁ and is multiplied by the monochromatic efficiency η ₁ to become the efficiency with respect to the total number of particles.
(5) Production efficiency η ₂ = βη ₁

The nanospherical cluster according to the first embodiment of the present invention will be described below with reference to FIGS. The nanospherical cluster according to the first embodiment is a target in which two kinds of atoms are mixed at a uniform density with respect to the distance r from the center of the nanospherical cluster. In FIG. 1, the horizontal axis represents the distance r and the vertical axis represents the ion density n. As shown in FIG. 1, regardless of the distance r, the particles 1 are present at a density n ₁ and the particles 2 are present at a uniform density n ₂ .

In the case of generating an ion beam by irradiating a laser with this nanosphere cluster as a target, using the above equations (1) to (5), the acceleration efficiency ratio α = 2 and the monochromatic accuracy 1% are fixed parameters. The result of calculating the production efficiency η ₂ of FIG. 2, the horizontal axis is the charge mixing ratio beta, the vertical axis is the generation efficiency eta _2. As shown in FIG. 2, the maximum generation efficiency η ₁ can be obtained by setting the charge mixing ratio β to about 30%. At this time, the monochromatic efficiency η ₁ is about 32%, and the generation efficiency η ₂ is about 9%.

As described above, in the nanospherical cluster according to the first embodiment of the present invention, the atoms 1 are uniformly distributed with the density n ₁ and the atoms 2 with the density n ₂ , respectively, and are spherical as a whole. This is the structure. When generating an ion beam using such a nanospherical cluster as a target, by optimizing the charge mixing ratio β of the particle 1 in which the atom 1 is ionized and the particle 2 in which the atom 2 is ionized, for example, 1% is desired. It is possible to maximize the generation efficiency η ₂ of the ion beam monochromatized to the monochromatic accuracy of.

Next, a nanospherical cluster according to a second embodiment of the present invention will be described with reference to FIGS. The nanospherical cluster according to the second embodiment is a target having a sandwich structure in which accelerated particles 2 are sandwiched between layers of particles 1. In FIG. 3, the horizontal axis represents the distance r from the center of the nanospherical cluster, and the vertical axis represents the ion density n. As shown in FIG. 3, the nanosphere cluster according to the second embodiment has a first atomic layer in which particles 1 are distributed at a density n ₁ from a distance r = 0 to r = r ₁ , and the particles 1 have a density. The particles 2 are distributed at a density n ₂ sandwiched between the second atomic layer and the first atomic layer and the second atomic layer from the distance r = r ₂ to r = r ₀ distributed at n ₁ It has a third atomic layer from a distance r = r ₁ to r = r ₂ . That is, the first atomic layer, spheres of a radius _{r 1} of the particles 1 are distributed at a density _{n 1,} the second atomic layer, a spherical shell of a radius _r 2 ~r ₀ particles 1 are distributed at a density _{n 1,} the third atomic layer is configured that the radius r ₁ ~r ₂ of spherical shell particles 2 are sandwiched between the first atomic layer and the second atomic layer is distributed at a density n _2.

FIG. 4 shows the result of calculating the generation efficiency η ₂ using the above formulas (1) to (5) in the case of generating an ion beam by irradiating laser with this nanosphere cluster as a target. In FIG. 4, the horizontal axis represents the distance ratio r ₂ / r ₀ , and the vertical axis represents the generation efficiency η ₂ . In FIG. 4, the parameter p is the ratio of the particles 2 to the entire third atomic layer, and can be up to about 30%. The parameter p is expressed by the following equation.
_{(6) p = (4πr 2} 3 / 3-4πr 1 3/3) / (4πr 0 3/3)

Further, the parameter p and the charge mixing ratio β have a correlation and are expressed by the following expression.
(7) Charge mixing ratio β = (1 + Z ₁ / Z ₂ × r ₀ ³ / (r ₂ ³ −r ₁ ³ ) × (1-p) n ₁ / n ₂ ) ⁻¹

As shown in FIG. 4, the maximum generation efficiency η ₂ can be obtained by setting the radius r ₂ outside the third atomic layer of the particle 2 to be accelerated to 85% of the outermost radius r ₀ of the target. At this time, the radius r ₁ inside the third atomic layer of the particle 2 varies depending on the volume to be inserted (depending on the parameter p). Further, when p = 0.2 (β = 0.04), the monochromatic efficiency η ₁ is about 5% and the generation efficiency η ₂ is about 2%.

As described above, the nanospherical cluster according to the second embodiment of the present invention has a multilayer structure in which atoms 1 and 2 each form a layer structure, and the layer of atoms 2 is sandwiched between layers of atoms 1. None, it forms a spherical shape as a whole. In particular, the first atomic layer of atom _{1 has} a spherical shape with a radius r ₁ , and the third atomic layer of atom 2 and the second atomic layer of atom 1 each have a spherical shell shape to form a multilayer structure. Yes.

In this nanospherical cluster, the generation efficiency η ₂ of the generated ion beam is such that the acceleration efficiency ratio α, the charge mixing ratio β, and the particle 2 with respect to the deposition of the whole nanospherical cluster. Depending on the ratio p of the layer deposition and the ratio r ₂ / r ₀ of the outer diameter of the third atomic layer to the sphere diameter of the whole nanospherical cluster. That is, by optimizing the charge mixture ratio β and the ratio r ₂ / r ₀ , it is possible to maximize the generation efficiency η ₂ of an ion beam having a desired monochromization rate, such as 1%.

A nanospherical cluster according to a third embodiment of the present invention will be described with reference to FIGS. The nanospherical cluster according to the third embodiment is a target having a structure in which the particles 1 and the particles 2 are mixed with a density gradient with respect to the distance r from the center of the nanospherical cluster. In FIG. 5, the horizontal axis represents the distance r from the center of the nanospherical cluster, and the vertical axis represents each ion density n. As shown in FIG. 5, the density of the particles 1 has a first dependency that decreases with a constant gradient from the density n _1a to the density n _1b with respect to the distance r, and the density of the particles 2 is reversed. The distribution has a second dependency that increases from zero to n ₂ in proportion to the distance r.

FIG. 6 shows the result of calculating the generation efficiency η ₂ using the above formulas (1) to (5) in the case of generating an ion beam by irradiating laser with this nanosphere cluster as a target. 6, the horizontal axis index [nu, the vertical axis is the generation efficiency eta _2. Here, the index ν is the distance r dependence of the density of ions to be accelerated, the charge mixing ratio β (r), the charge mixing ratio β ₀ is the value of the charge mixing ratio β at the outermost radius of the nanospherical cluster, and the distance r ₀ is The outermost radius (10 to 100 nm) of the nanospherical cluster target is defined as an index used when expressed by the following formula (8).
(8) Charge mixing ratio β (r) = β ₀ × (r / r ₀ ) ^ν

As shown in FIG. 6, when the acceleration efficiency ratio α = 2 as described above, the maximum generation efficiency η ₂ when the charge mixing ratio β ₀ = 0.6 and the distance r ₀ = 0.75 is obtained. The monochromatic efficiency η ₁ is about 77% and the generation efficiency η ₂ is about 37%. The generation efficiency eta ₂ of about 37% indicates that 37% of all particles are present in a single color accuracy of 1% of the energy region. If this target structure can be realized, it is possible to dramatically improve the monochromatic ion beam generation efficiency.

  As described above, in the nanospherical cluster according to the third embodiment, the density of atom 1 has a dependency that decreases with respect to the distance r from the center point of the nanocluster, and the density of atom 2 is the nanocluster. It is configured to increase in proportion to the distance r from the center point of, and as a whole has a spherical shape. In this nanospherical cluster, by optimizing the dependence of the particle 1 and the particle 2 on the charge mixing ratio β and the distance r from the center of the nanospherical cluster, an ion beam with a desired monochromatic accuracy such as 1% can be generated. Efficiency can be maximized.

  In particular, if the nanospherical cluster according to the third embodiment in which the generation efficiency of an ion beam with a monochromatic accuracy of 1% is 37% can be realized, the generation efficiency of a monochromatic ion beam can be dramatically improved. The nanosphere cluster according to the third embodiment can be realized, for example, by utilizing a method of adjusting the density ratio of two kinds of atoms in terms of time.

  The nanocluster according to the present invention is not limited to the above-described embodiments, and various modifications and improvements can be made within the scope described in the claims. For example, in addition to the above embodiment, in a spherical nanocluster having at least two types of atoms, deformation such as changing the dependence of the ion density distribution on the distance to another function maximizes the generation efficiency by optimizing the parameters. It is within the scope of the present invention as long as the same effects can be obtained.

  As described in detail above, by using the nanosphere cluster according to the present invention as a target for generating a monochromatic ion beam by laser irradiation, the monochromatic accuracy, the generation efficiency, and the like can be greatly improved as compared with the conventional target.

  The nanocluster of the present invention can be applied to cancer treatment, neutron generation source, ion beam probe and the like.

The figure showing the density distribution of the particle | grains 1 and the particle | grains 2 in the nanosphere cluster by 1st Embodiment. The figure which shows the ion beam production efficiency using the nanosphere cluster by 1st Embodiment. The figure showing the density distribution of the particle | grains 1 and the particle | grains 2 in the nanosphere cluster by 2nd Embodiment. The figure which shows the ion beam production efficiency using the nanosphere cluster by 2nd Embodiment. The figure showing density distribution of particle 1 and particle 2 in a nanosphere cluster by a 3rd embodiment. The figure which shows the ion beam production efficiency using the nanosphere cluster by 3rd Embodiment.

t: Monochromatic accuracy α: Monochromatic efficiency β: Charge mixing ratio η ₁ : Monochromatic efficiency η ₂ : Generation efficiency

Claims (2)

Nanoclusters used as targets for ion beam generation by laser irradiation,
A first atomic layer in which first atoms are present at a first density, and a second atomic layer;
A third atomic layer in which second atoms are present at a second density;
Have
The third atomic layers, the first atomic layer and a multilayer structure sandwiched the second atomic layer, and form a smaller spherical than the wavelength of the laser as a whole, the said first atom The second atom is a nanocluster characterized by being an isotope of hydrogen or carbon . Nanoclusters used as targets for ion beam generation by laser irradiation,
Having a first atom and a second atom different from each other;
The first atoms are present at a density having a first dependence on distance from a center point of the nanocluster;
The second atoms are present in a density having a second dependence on the distance from the center point of the nanocluster;
Overall Ri structures der forming a laser small size spherical than the wavelength of, wherein the first atom and the second atom, nanoclusters, wherein the isotope or carbon der Rukoto hydrogen.

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