JP2006172898A - Laser plasma x-ray generating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an X-ray generating device which also has the characteristics of short-pulse high luminance point-like light source by generating characteristic X-rays which have been used by generating them, using a conventional X-ray tube by means of a laser plasma X-ray generation device. <P>SOLUTION: A far-infrared pulse laser beam is made to enter a target, consisting of a low atomic number substance, thereby high-speed electrons caused by resonance absorption phenomenon is generated; and this high-speed electrons are made to collide with a high atomic number substance, thereby characteristic X-rays of short-pulse high luminance point-like light source is generated. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser plasma X-ray generator that generates plasma by irradiating a target with laser light and generates X-rays from the plasma.

  A laser that generates high-temperature plasma by ionizing and ionizing a substance by focusing and irradiating the substance with intense pulsed laser light, and generates X-rays in the process of bremsstrahlung (braking radiation) in plasma or recombination of ionized plasma The plasma X-ray generator can be applied to industrial applications as a high-intensity point X-ray source with an extremely short pulse, and is disclosed in Patent Documents 1 to 12. The applicant of the present application has also filed Japanese Patent Application Nos. 2002-288426 and 2003-178081 as related patents.

  In these disclosed conventional examples, the bremsstrahlung in plasma or the recombination process of ionized plasma is used as the principle of X-ray generation, and the spectrum of X-rays generated in bremsstrahlung continues in a wide wavelength range. In the recombination process of ionized plasma, it becomes a continuous spectrum, and it becomes a line spectrum of a specific wavelength or an assembly of multiple line spectra. However, the main wavelength region is determined by the temperature of the plasma as a dominant factor, and is usually below several keV. It is difficult to generate hard X-rays having a high photon energy in the soft X-ray region.

  On the other hand, hard X-rays having photon energy of several keV or more are used in imaging apparatuses using X-rays that have been used for a long time, such as X-ray imaging and nondestructive inspection. A high-speed electron beam accelerated by an electrode to which a high voltage is applied collides with the solid surface, and the high-speed electrons repel the inner core electrons in the atoms, and other electrons in the same atom transition to the generated vacancies. An X-ray tube that generates characteristic X-rays by emitting photons having an energy difference at that time has been used. This X-ray tube has been widely used because it can supply a sufficient amount of X-rays with a simple structure. However, since it has lower luminance and longer pulse time width than a laser plasma X-ray source, the light source size (X-rays) The precision measurement that requires high spatial resolution and high time resolution is limited.

  Also, a method of generating a hard X-ray by accelerating electrons by this electric field by condensing laser light with an ultra-high intensity ranging from 10 15 to 20 20 per square centimeter. However, in order to obtain such a high-intensity laser, a very large laser used for nuclear fusion research that reaches as many as kilojoules per pulse, or a special type called chirped pulse amplification (CPA). An apparatus that generates femtosecond ultrashort pulse laser light using a laser amplifying apparatus is required, which is large and complicated.

Further, Patent Documents 13 to 15 below describe a method for generating X-rays by causing a laser beam to collide with an electron beam. None of these use laser plasma, and are different in principle from the present invention. Although it has the feature that the wavelength of monochromatic pulse X-rays can be taken out continuously, there is a drawback that an electron beam accelerator is required in addition to the laser device.
JP 2004-37324 A JP 2001-135877 A JP 2001-15296 A JP 2001-357997 A JP 2001-357998 A JP 2003-257698 A JP-A 63-304596 JP-A-1-137543 JP-A-8-236292 JP-A-10-208998 Japanese Patent Laid-Open No. 11-160499 JP 2001-682969 A JP 2003-288999 A JP 2002-43664 A JP 11-264899 A

  An object of the present invention is to generate a characteristic X-ray that has been used in an X-ray tube in the past by generating it in a laser plasma X-ray generator, thereby causing a problem of a low-intensity long pulse that has been a drawback of the X-ray tube. In a laser plasma X-ray generator to provide characteristic X-rays of a short-pulse high-intensity point light source.

  In the present invention, a resonance absorption phenomenon is used as a mechanism for generating fast electrons necessary for generating characteristic X-rays in a laser plasma. Resonance absorption is a phenomenon that occurs when laser light enters a plasma with a density gradient at a specific angle, and the frequency of the electron plasma wave that is generated when the laser light propagates through a plasma with a non-uniform density. And the electric field of the laser beam resonate to generate a remarkably large electric field in the plasma. When electrons in the plasma are accelerated by this electric field, they become high-speed electrons (about several hundred keV) having a remarkably large velocity, unlike the electrons in plasma in a normal thermal equilibrium state.

  This phenomenon occurs when far-infrared rays with a long wavelength, such as a carbon dioxide laser that can give a large amount of energy to the resonance absorption region without being attenuated to near the critical density that the laser beam reaches, are used as a laser. This is particularly noticeable.

  However, there has never been an apparatus for generating characteristic X-rays using a carbon dioxide laser. The reason is that, in order to generate characteristic X-rays like an X-ray tube, a long-wave far-infrared laser is resonated even though it is necessary for high-speed electrons to collide with a low-temperature material that is not ionized. This is because if the irradiation is performed until a density gradient suitable for absorption is generated, the temperature of the target rises, and ionization and ionization cannot generate X-rays having the same wavelength as the X-ray tube.

Therefore, in the present invention, low atomic number substances such as polyethylene and polystyrene (substances with a primitive number below F (fluorine) in the periodic table of elements, mainly H (hydrogen), C (carbon), N (nitrogen), O High oxygen number materials (in the periodic table of elements) that efficiently generate high-speed electrons by irradiating laser light onto (oxygen) and other materials that do not contain high atomic number materials) , By collision with a substance having an atomic number of Ti (titanium) or higher, mainly Ti (titanium), Cu (copper), Au (gold), W (tungsten), Pb (lead), etc. It is characterized by generating hard X-rays having a specific wavelength (stopping hard X-rays having a short wavelength as the atomic number increases).
Specifically, the following means are adopted.
(1) In a laser plasma X-ray generator,
In a laser plasma X-ray generator that generates X-rays using plasma generated by condensing and irradiating a target material with pulsed laser light, a far-infrared laser is provided as a pulsed laser, A substance is provided at a predetermined position, and fast electrons are generated by a resonance absorption phenomenon in the interaction between the laser beam generated by the infrared laser and the substance, and the generated fast electrons blow off the inner core electrons in the atom, and are generated. It is characterized in that another electron in the same atom is transferred to the vacancy and a photon having an energy difference at that time is emitted to generate characteristic X-rays.
(2) In the laser plasma X-ray generator described in (1) above, the substance is a low atomic number substance and a high atomic number substance, and laser light is focused on the low atomic number substance to generate fast electrons. The generated high-speed electrons collide with the high atomic number substance to generate X-rays having a specific wavelength.
(3) In the laser plasma X-ray generator described in (1) or (2) above, a carbon dioxide laser device is used as means for generating far-infrared laser light.

  According to the present invention, the same wavelength as that of an X-ray tube that has been widely used so far, using a carbon dioxide laser that has already been widely used for industrial purposes, without using a large and complicated ultra-high intensity laser. This X-ray can be realized as a short-pulse high-intensity point light source that is a feature of a laser plasma X-ray source.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of one embodiment of the apparatus of the present invention. Reference numeral 1 denotes a far infrared pulse laser device such as a carbon dioxide laser, which generates a far infrared pulse laser beam A having a wavelength of about 10 μm. The laser light A is focused and irradiated onto a target 3 made of a low atomic number material such as polyethylene by a lens 2. The lens 2 is made of a material such as sodium chloride suitable for the laser wavelength, or may be configured to condense the laser light A on the target 3 using different optical means such as a gold-plated concave mirror instead of the lens. Is possible. The target 3 is placed at the condensing point of the laser beam, but is not perpendicular to the laser beam and is arranged so that the laser beam is incident obliquely. This is because the density gradient of the plasma generated by the laser beam is generally perpendicular to the target surface, whereas the resonance absorption phenomenon occurs only when the laser beam is incident obliquely with respect to this density gradient. by. There is an optimum value for the target installation angle from the gradient of the density gradient and the laser wavelength, but the density gradient varies depending on the laser intensity, pulse width, target material, etc., so it can be used experimentally or for numerical simulations of plasma, etc. The angle at which high-speed electrons are generated most efficiently is set by this means.

  The fast electrons B generated by the resonance absorption collide with the second target 4 made of a high atomic number material such as copper, which is disposed in the very vicinity of the target 3, and blow off the inner core electrons of the atoms constituting the target 4. As a result, other electrons in the same atom transition to the vacancies generated, and photons having an energy difference at that time are emitted, thereby generating characteristic X-rays C. High-speed electrons have a certain degree of directivity. However, since the luminance of X-rays obtained by divergence decreases when propagating over a long distance, it is desirable that the target 3 and the target 4 be arranged close to each other.

  Although not explicitly shown in FIG. 1, it is possible to generate a conventional laser plasma X-ray such as in Patent Documents 1 to 12 if the target is placed in a vacuum or a low-pressure gas atmosphere as necessary to avoid the influence of the atmosphere. Similar to what is done on the device. Further, when the target is repeatedly and continuously irradiated with the pulse laser, the target 3 is destroyed due to the plasma, and the scattered matter from the target 3 is deposited on the target 4, so that the generation amount of X-rays gradually decreases. In order to avoid this, it is necessary to replace the target periodically. To that end, the target is made to have a long and thin structure in the direction perpendicular to the paper surface, and it moves with time, or it is rotated as a disk or cylinder. Or it can be set as the structure similar to the target of the conventional laser plasma X-ray generator, such as supplying while winding what was wound up as a wire form.

  FIG. 2 shows a second embodiment of the present invention. The difference from Example 1 is that the target 3 made of a low atomic number substance and the target 4 made of a high atomic number substance are not arranged close to each other independently, but rather a very thin low atomic number substance on the high atomic number substance 7. This is a point using a target 5 having a laminated structure with 6 attached thereto. Fast electrons due to resonance absorption are generated not only in the direction of the surface of the target but also in the direction of the back surface. By making the layer of the low atomic number substance 6 very thin, the generated fast electrons penetrate through this layer 6. Then, the high atomic number substance 7 can be reached. When fast electrons collide with a high atomic number substance, characteristic X-rays C are generated based on the same principle as in the first embodiment. The generated X-rays must pass through the layer 6, but the thickness of the layer 6 is thin and the X-ray absorption coefficient is low because it is a low atomic number material, so that it can be easily transmitted. Since the layer 6 is converted to plasma and disappears by irradiation with a pulsed laser beam, it is necessary to prepare a new target for each pulse. However, unlike the first embodiment, if the layer structure is used, only one target is required. It is rather easy to process into a plate shape, a disk shape, a cylindrical shape, a tape shape, a wire shape or the like and continuously supply.

  In the above embodiment, the laser device 1 is a carbon dioxide gas laser or the like. However, the laser device 1 is not limited to this as long as it is a long-wavelength pulse laser that easily causes resonance absorption. A laser device may be used.

  Also, the target is a solid, but this is limited to this as long as it can generate a density gradient necessary for resonance absorption and can arrange a low atomic number substance and a high atomic number substance adjacent to each other. However, other target supply methods disclosed as laser plasma X-ray generators such as liquids, gas jets, and clusters can also be applied.

  Although it has the same wavelength as the characteristic X-ray with the X-ray tube that has been widely used so far, it also has the characteristics of a short pulse high-intensity point light source of a laser plasma X-ray generator, and is widely used as an industrial laser Since the carbon dioxide laser that is used can be used in a pulsed manner, this enables high time-space-resolved measurement that was impossible with an X-ray tube.

It is the schematic block diagram which showed the implementation method of the laser plasma X-ray generator of this invention. It is the schematic block diagram which showed the implementation method of the laser plasma X-ray generator of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Barus laser apparatus 2 Condensing lens 3 Target made of low atomic number substance 4 Target made of high atomic number substance 5 Target made by stacking low atomic number substance on high atomic number substance 6 Low atomic number substance layer 7 High atomic number substance Layer A Laser light B High-speed electron C Characteristic X-ray

Claims (3)

  In a laser plasma X-ray generator that generates X-rays using plasma generated by condensing and irradiating a target material with pulsed laser light, a far-infrared laser is provided as a pulsed laser, A substance is provided at a predetermined position, and fast electrons are generated by a resonance absorption phenomenon in the interaction between the laser beam generated by the infrared laser and the substance, and the generated fast electrons blow off the inner core electrons in the atom, and are generated. A laser plasma X-ray generator characterized in that a characteristic X-ray is generated by causing other electrons in the same atom to transition to a vacancy and emitting photons having an energy difference at that time.   2. The laser plasma X-ray generator according to claim 1, wherein the substance is a low atomic number substance and a high atomic number substance, and the low atomic number substance is focused and irradiated with laser light to generate fast electrons. A laser plasma X-ray generator, characterized in that high-speed electrons collide with the high atomic number substance to generate X-rays having a specific wavelength.   3. The laser plasma X-ray generator according to claim 1, wherein a carbon dioxide laser device is used as means for generating far-infrared laser light.

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