JP2007265917A - X-ray tube and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray tube with a stable minute beam dimension and to provide its control method. <P>SOLUTION: The X-ray tube includes; a cathode releasing an electron beam, an anode accelerating the electron by voltage applied between it and the cathode; a first control grid installed between the cathode and the anode and carrying out current-control of the electron beam; a second control grid installed between the first grid and the anode and carrying out focus-control of the electron beam; and a target connected electrically with the anode and generating X-ray by collision of the accelerated electron beams. Spacing between an end part on the anode side of the second control grid and the cathode is two times or more but three times or less than the spacing between the end part and the anode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an X-ray tube and a control method thereof.

  An X-ray inspection apparatus equipped with an X-ray tube is widely used as a diagnostic instrument for medical or industrial purposes. The parameters that determine the resolution of an X-ray inspection apparatus vary depending on the object to be inspected and the application, but in general, the electron optical system, X-ray optical system, acceleration voltage, electron beam current amount, image processing control, etc. is there. Among these, it is particularly important to reduce the electron beam size in order to improve the resolution. However, since the electron beam size varies depending on the acceleration voltage and the amount of beam current, there is a problem that a constant minute beam size cannot always be obtained.

On the other hand, a cathode electrode that generates an electron beam, a grid electrode that controls the electron beam, a focus electrode that collects the electron beam, an anode target that generates X-rays by collision of the electron beam collected by the focus electrode, A bias voltage generator for generating a bias voltage to be applied between the cathode electrode and the grid electrode, a tube voltage generator for generating a tube voltage to be applied to the anode target, and a focus voltage divided from the tube voltage. An X-ray generator provided with a voltage dividing unit that applies this focus voltage to a focus electrode is disclosed (Patent Document 1). This X-ray generator can suppress the influence of voltage fluctuations on the formation of the focus of the electron beam.
JP 2003-163098 A

  The present invention provides an X-ray tube having a stable microbeam size and a control method thereof.

  According to an aspect of the present invention, the cathode is provided between the cathode that emits an electron beam, the anode that accelerates the electron by a voltage applied between the cathode, the cathode, and the anode. A first control grid that controls the current of the beam, a second control grid that is provided between the first control grid and the anode and that controls the convergence of the electron beam, and is electrically connected to the anode and accelerated. A target that generates X-rays by the collision of the electron beam, and an interval between the end on the anode side of the second control grid and the cathode is twice as large as an interval between the end and the anode. Thus, an X-ray tube characterized in that it is three times or less is provided.

  Moreover, according to another aspect of the present invention, in the X-ray tube described above, a maximum convergence half angle of the electron beam on the target is 22 milliradians or more between the first control grid and the anode. And an X-ray tube further comprising an aperture limiting element for shaping the electron beam so as to be 28 milliradians or less, and at least one of the first control grid and the second control grid. Provided is a method for controlling an X-ray tube, wherein a necessary electron beam current is obtained by compensating for a decrease in the electron beam current due to the aperture limiting element by increasing a current angular density by changing an applied voltage. Is done.

  According to the present invention, an X-ray tube having a stable micro beam size and a control method thereof are provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view showing a main part of an X-ray tube according to a first embodiment of the present invention.
2A is a schematic diagram illustrating an electron beam profile in a region X in FIG. 1, and FIG. 2B is a schematic diagram illustrating an electron beam profile in a region Y in FIG.

  The X-ray tube 5 of the present embodiment has a structure in which the cathode 10 and the target 40 are provided opposite to each other on both end faces of a vacuum vessel 30 having a substantially cylindrical hollow structure.

  The first control grid 15, the second control grid 25, and the anode 35 are provided in this order toward the target 40 that is disposed to face the main surface of the cathode 10. The second control grid 25 includes a first component 19, a second component 21, and a third component 22.

  In the X-ray tube 5 of the present embodiment, the distance L1 between the third component 22 of the second grid 25 and the cathode 10 is smaller than the distance L2 between the third component 22 and the anode 35 (L2> L1). The first control grid 15, the second control grid 25, and the anode 35 are provided substantially parallel to the main surface of the cathode, respectively.

  Each of these members has an axially symmetric hole (opening) structure surrounding the periphery of the optical axis connecting the cathode 10 and the target 40.

  The dimension of the vacuum vessel 30 is, for example, 100 millimeters. The dimensions of the cathode 10 and the target 40 are, for example, 20 millimeters.

  The thickness in the direction substantially perpendicular to the main surface of the cathode 10 is the largest for the second component 21, and decreases in the order of the anode 35, the first component 19, the third component 22, and the first control grid 15.

  That is, the thickness of the second component 21 is, for example, 10 millimeters, the anode 35 is, for example, 2 millimeters, the first component 19 and the third component 22 are, for example, 1 millimeter, and the first control grid 15 is, for example, 0.2. It is a millimeter. In addition, openings are provided in the main surfaces of the first control grid 15, the second control grid 25, and the anode 35, respectively.

  As will be described later, these openings serve to control the beam dimensions while allowing the electron beam emitted from the cathode 10 toward the target 40 to pass therethrough. The opening diameter of the opening is the largest in the anode 35, and decreases in the order of the second component 21, the first component 19, the third component 22, and the first control grid 15. That is, the opening diameter of the anode 35 is, for example, 15 millimeters, the second component 21 is, for example, 10 millimeters, the first component 19 and the third component 22 are each, for example, 8 millimeters, and the first control grid 15 is, respectively. For example, 0.5 millimeter.

  The cathode 10 has a role of generating an electron beam 45. The anode 35 is electrically connected to the target 40 and serves to guide the electron beam 45 radiated from the cathode to the target 40.

  As the material of the cathode 10, for example, a flat cathode impregnated with barium oxide can be used.

  A negative voltage is applied to the first control grid 15, and a positive voltage is applied to the third component 22 of the second control grid 25. These have a role of reducing the beam size of the electron beam 45. A positive voltage is applied to the first component 19 and the second component 21 of the second control grid 25. These electrodes have a role of focusing the electron beam 45. Thereby, the space between the third component 22 and the anode 35 serves as an electrostatic lens. Incidentally, the voltage applied to the first control grid 15 is, for example, minus 100 volts, and the voltages applied to the first component 19 and the third component 22 are each 1 kilovolt, for example, and are applied to the anode 35. The voltage is, for example, 50 kilovolts.

  Further, the target 40 has a role of receiving an electron beam 45 emitted from the cathode and generating X-rays in the direction opposite to the cathode 10 with the anode 35 interposed therebetween. For this target 40, for example, tungsten (W) or the like can be used. The material of the target 40 may be selected according to a desired X-ray wavelength.

FIG. 3 is a graph showing the relationship between the applied voltage of the second control grid and the applied voltage of the first control grid with respect to the beam current of the electron beam.
Here, the horizontal axis is the applied voltage (volt) of the second control grid, and the vertical axis is the applied voltage (volt) of the first control grid 15. The beam current of the electron beam 45 is (a) 10 micrometers, (b) 20 micrometers, and (c) 30 micrometers. Although this beam current is a negative applied voltage, it is displayed as a positive voltage.

  In any beam current, it can be seen that the voltage applied to the second control grid 25 and the first control grid 15 has a relationship that the beam current decreases as the first control voltage is increased. In this way, a desired beam current can be obtained by controlling the voltage applied to the second control grid 25 or the first control grid 15.

Next, the operation of the X-ray tube 5 of this embodiment will be described.
A filament (not shown) is provided on the opposite side of the cathode 10 from the target 40. An electric current is passed through the filament to generate heat. Then, the cathode 10 is heated, and an electron beam 45 is emitted from the surface of the cathode 10 toward the target 40 as shown in FIG. Thereafter, the emitted electron beam 45 travels in the direction a, the first control grid 15, the second control grid 25 composed of the first component 19, the second component 21 and the third component 22, the anode 35, , Each of which passes through the openings provided in order and collides with the target 40. Then, X-rays are emitted in the direction opposite to the cathode 10 with the target 40 interposed therebetween.

  At this time, as shown in FIG. 2A, the electron beam 45 emitted from the cathode 10 is converged at the opening of the first control grid 15. Then, the electron beam 45 is diverged by the first component 19 and the second component 21 of the second control grid 25. Subsequently, the electron beam 45 is converged at the opening of the third component 22 of the second control grid 25. In this way, the electron beam 45 is imaged on the target 40 as shown in FIG. Here, a point P (see FIG. 1) where the beam dimension of the electron beam 45 is maximized is formed on the second component 21.

Next, the operation concept of the X-ray tube 5 will be described by replacing it with an optical system.
FIG. 4 is a schematic diagram in which the operation concept of the X-ray tube of the first embodiment is replaced with an optical system.
A target 49 is provided so as to face the main surface of the emission light source 46. An optical axis 44 is formed in a direction substantially perpendicular to the main surface of the emission light source 46. Then, a first lens 47 and a second lens 48 are provided in this order toward a target 49 arranged to face the emission light source 46. These lenses are provided substantially parallel to the main surface of the emission light source 46.

  Here, the emission light source 46 has a role of emitting light. This corresponds to the cathode 10 in FIG. The first lens 47 has a role of collecting light. This corresponds to the first control grid 15 in FIG. The second lens 48 has a role of collecting light. This corresponds to the third component 22 of the second grid 25 of FIG.

  First, the light emitted from the emission light source 46 enters the first lens 47. This light is collected by the first lens 47 and crosses over at point C. The crossover light is converted into diverging light and enters the second lens 48. The light condensed by the second lens 48 forms an image on the target 49.

  Here, a value obtained by dividing the distance B2 between the second lens 48 and the target 49 by the distance A2 between the second lens 48 and the first lens 47 is represented by an optical system reduction magnification m. The angle formed by the optical axis 44 and the tangent line of light is defined as a convergence half angle α.

The focal diameter D2 of the light imaged by the target 49 is determined by the optical system reduction magnification m and the convergence half angle α. That is, the focal diameter D2 corresponds to the product of the focal diameter D1 condensed through the first lens 47 and the optical system reduction magnification m (D2 = D1 × m). However, an actual optical system also includes “blur aberration” which is an optical aberration. The “blur aberration” includes, for example, spherical aberration Cs, chromatic aberration Cc, and the like. The focal diameter including the blur aberration in the focal diameter D2 is defined as D3. The focal diameter D3 including the blur aberration can be calculated using Equation 1.

^{D3 = {(D1 × m)} 2 + (Cs × α 3/2) 2 + (Cc × α × ΔV / V) 2} 0.5 ( Equation 1)

Here, V is an acceleration voltage of the electron beam 45, and ΔV is a fluctuation voltage.
From Equation 1, it can be seen that the focal diameter D3 including the blur aberration is mainly determined by the balance between the optical system reduction magnification m and the convergence half angle α. It can also be seen that the influence on the focal diameter D3 is large.

  That is, in this embodiment, since the distance L1 between the third component 22 and the cathode 10 is smaller than the distance L2 between the third component 22 and the anode 35 (L2> L1), the optical system reduction magnification m is increased. However, the effect of reducing optical aberration is superior. Therefore, as shown in FIG. 2B, the convergence half angle α can be reduced. That is, a minute beam size can be obtained.

  On the other hand, even when the distance L1 between the third component 22 and the cathode 10 is smaller than the distance L2 between the third component 22 and the anode 35 (L1> L2), an aperture limiting element is provided on the electron beam 45, and the first By controlling the beam current with the control grid 15 or the second control grid 25, a fine beam size can be obtained.

  FIG. 5A is a schematic cross-sectional view showing the main part of the X-ray tube according to the second embodiment of the present invention, and FIG. 5B is a schematic view showing the electron beam profile in the region B of FIG. FIG.

  FIG. 6A is a schematic cross-sectional view showing a first comparative example, and FIG. 6B is a schematic view showing an electron beam profile in the region B of FIG.

First, the comparative example will be described first.
As shown in FIG. 6A, the basic structure of this comparative example is the same as that of the first embodiment described above with reference to FIG. However, the distance L1 between the third component 54 and the cathode 10 is not less than two times and not more than three times the distance L2 between the third component 54 and the anode 35 (L1 = (2-3) × L2). Therefore, the optical system reduction magnification m obtained by dividing the distance B between the anode 35 and the point P by the distance A between the cathode 10 and the point P can be reduced.

  Here, when the distance L1 between the third component 54 and the cathode 10 is not less than 2 times and not more than 3 times the distance L2 between the third component 54 and the anode 35, D2 = D1 × m is the smallest.

  However, as described above, since the influence of the blur aberration becomes strong, as shown in FIG. 6B, the beam size S becomes larger than that of the first embodiment described above with reference to FIG.

  In contrast, as shown in FIG. 5A, the basic structure of the present embodiment is the same as that of the first comparative example described above with reference to FIG. However, the opening limiting element 50 is provided in the second component 51 of the second control grid 25. Thus, by providing the aperture limiting element 50, the influence of the blur aberration can be extremely reduced. Therefore, as shown in FIG. 5B, for example, a minute beam dimension of about 2 micrometers can be obtained.

At this time, it is preferable that the maximum convergence half angle α _max is not less than 22 milliradians and not more than 28 milliradians. By setting the maximum convergence half angle α _max to this value, the beam size S can be reduced. Further, a large than the maximum convergence half alpha is 22 milliradians, 28 If it is smaller than mrad, Equation ^{1 (Cs × α 3/2} ) and (Cc × α × ΔV / V ) is the smallest.

  Here, the opening diameter of the opening limiting element 50 can be set to, for example, φ1 mm. Further, the radiation conditions of the electron beam 45 of the present embodiment are, for example, a beam current of 30 microamperes and an acceleration voltage of 40 kilovolts.

  In this embodiment, the beam current may be reduced by the aperture limiting element. However, as described above with reference to FIG. 3, the beam current can be prevented from being deteriorated by controlling the beam current by changing the voltage using the first control grid or the second control grid.

FIG. 7 is a graph showing the relationship between the convergence angle and the current angular density when the target is irradiated with an electron beam.
Here, the horizontal axis is the convergence half angle α (milliradian), and the vertical axis is the current angular density (ampere / steradian). Line (a) represents the first example, line (b) represents the second example, and line (c) represents the first comparative example.

  The F-line of current angular density is a “threshold value” required to convert the electron beam 45 into X-rays. The intersection of the current angular density F line and the (a) line is a point G, and the intersection of the (c) line is a point I. The electron beam characteristic on the target obtained from the second embodiment is a point H. Here, in the second embodiment, the case where the beam current is not controlled even if the aperture limiting element is used is represented by a point H ″. Moreover, the convergence half angle α at each point corresponds to the maximum beam size after conversion to X-rays.

  In any of the examples and comparative examples, it can be seen that the current angular density decreases as the convergence half angle α increases. That is, it can be seen that the blur aberration is large due to the influence of the convergence half angle at the point I where the line (c) and the line F in the first comparative example intersect.

On the other hand, at the point G where the line (a) of the first example and the F line intersect, it can be confirmed that the convergence half angle is smaller than that of the first comparative example. Further, according to the second embodiment, while using the aperture limiting element 50, the current change due to this is controlled by the first control grid 15 or the second control grid 25. Thereby, for example, a convergence half angle of 22 milliradians or more and 28 milliradians or less can be obtained while maintaining a high current density.
However, as shown by the point H ″, it can be seen that the convergence half angle α becomes large if current control is not performed even if the aperture limiting element is provided.

Next, the relationship between the radiation condition of the electron beam 45 and the beam size will be described.
FIG. 8A is a schematic cross-sectional view showing the main part of the X-ray tube according to the third embodiment of the present invention, and FIG. 8B is a schematic view showing the electron beam profile in the region B of FIG. FIG.

  As shown in FIG. 8A, the basic structure of this embodiment is the same as that of the second embodiment described above with reference to FIG. However, the radiation conditions of the electron beam 45 of this embodiment are, for example, a beam current of 30 microamperes and an acceleration voltage of 125 kilovolts. That is, the acceleration voltage in this embodiment is larger than that in the second embodiment described above with reference to FIG.

  However, as shown in FIG. 8B, according to the present embodiment, it can be seen that a minute beam size of about 2 micrometers can be obtained. This is presumably because the beam size of the electron beam 45 becomes smaller due to the increase in acceleration voltage.

FIG. 9 is a graph showing the relationship between the acceleration voltage and the beam size in the second embodiment of FIG.
Here, the horizontal axis is the acceleration voltage (kilovolt), and the vertical axis is the beam size (micrometer). (A) As for the line condition, an aperture limiting element 50 was provided, and an electron beam 45 was emitted with a beam current of 30 microamperes. The line (b) was provided with an aperture limiting element 50 and the beam current was 10 microamperes. The line (c) has a beam current of 30 microamperes without the aperture limiting element 50, and the line (d) has a beam current of 10 microamperes without the aperture limiting element 50. When the aperture limiting element 50 is provided, the current control is performed as described above.

  As shown in the lines (c) and (d), the beam size when the aperture limiting element 50 is not provided is larger than when the aperture limiting element 50 for the (a) line and the (b) line is provided. I understand that. In addition, when the aperture limiting element 50 is not provided from the lines (c) and (d), it can be confirmed that the beam size is easily affected by the beam current and the acceleration voltage.

  On the other hand, as shown in the lines (a) and (b), it can be confirmed that the aperture limiting element 50 is less affected by the beam current and the acceleration voltage. Therefore, by providing the aperture limiting element 50 and controlling the beam current in this manner, a stable minute beam size can be obtained.

Next, the aperture limiting element 50 that can be used in this embodiment will be described.
FIG. 10 is a schematic diagram showing a first specific example of an aperture limiting element that can be used in the second embodiment.

  The aperture limiting element 50 of this specific example is provided with one plate in a direction substantially perpendicular to the central axis 52 of the electron beam. The thickness of this plate is 0.5 millimeters, for example. An opening 51 is provided in this plate. The opening 51 has a cross section facing the optical axis. That is, these cross sections are provided substantially parallel to the optical axis.

  The electron beam 45 passing through the aperture limiting element 50 diverges while traveling in the direction a. The electron beam 45 smaller than the aperture diameter of the aperture limiting element 50 reaches the target. A part of the electron beam that has passed through the opening 51 may collide with the end of the cross section 68 on the target side, and a scattered beam 65 may be generated. However, since the scattered beam 65 is very small, it often does not affect the beam size.

FIG. 11 is a schematic diagram showing a second specific example of the aperture limiting element that can be used in the second embodiment.
A plurality of aperture limiting elements 50 of the present embodiment are stacked in a direction substantially perpendicular to the central axis 52 of the electron beam. Each opening limiting element 50 is arranged so that the opening diameter of the opening 51 becomes larger toward the anode 35. Therefore, the opening ends of the opening limiting element 50 each have a knife edge shape. Here, the aperture limiting elements 50 are provided at equal intervals. This interval ΔZ is, for example, 1 millimeter.

  Moreover, the opening diameter of the opening part 51 becomes large at equal intervals. This interval ΔR is, for example, 30 micrometers. Therefore, according to the aperture limiting element 50 of the present embodiment, a stable fine beam diameter can be obtained while suppressing the scattered beam 65.

As described above, according to the present embodiment, the distance L1 between the third component 53 and the cathode 10 is set to be not less than twice and not more than three times the distance L2 between the third component 54 and the anode 35, and the aperture limiting element 50 is used as the second component. The beam current is controlled by the first control grid or the second control grid for the current change by the aperture limiting element 50. As a result, an X-ray tube having a stable minute beam size in which the maximum convergence half angle α _max is 22 milliradians or more and 28 milliradians or less is obtained.

  The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. For example, the X-rays 70 emitted from the X-ray tube of the present embodiment emit in substantially the same direction a as the electron beam 45 as shown in FIG. 12, but the present invention is not limited to this. For example, as shown in FIG. 13, X-rays 70 radiated in the same direction a as the electron beam 45 may be reflected by, for example, an X-ray mirror 75 and reflected in the direction b.

  Further, according to the present embodiment, as shown in FIG. 5, one aperture limiting element is provided on the electron beam. However, the present invention is not limited to this, and a plurality of aperture limiting elements may be provided on the electron beam. A similar effect can be obtained.

  Further, in the X-ray tube of the present invention, the elements such as the arrangement of the third component, the shape of the aperture limiting element, and the aperture diameter thereof may be appropriately modified by those skilled in the art. Insofar as the gist is included, it is included in the scope of the present invention.

It is a schematic cross section showing the principal part of the X-ray tube which is 1st Example of this invention. 2A is a schematic diagram illustrating an electron beam profile in a region X in FIG. 1, and FIG. 2B is a schematic diagram illustrating an electron beam profile in a region Y in FIG. It is a graph showing the relationship between the applied voltage of the 2nd control grid with respect to the beam current of an electron beam, and the applied voltage of a 1st control grid. It is the schematic diagram which replaced the operation | movement concept of the X-ray tube of 1st Example with the optical system. FIG. 5A is a schematic cross-sectional view showing the main part of the X-ray tube according to the second embodiment of the present invention, and FIG. 5B is a schematic view showing the electron beam profile in the region B of FIG. FIG. FIG. 6A is a schematic cross-sectional view showing a first comparative example, and FIG. 6B is a schematic view showing an electron beam profile in the region B of FIG. It is a graph showing the relationship between a convergence angle when a target is irradiated with an electron beam and current angular density. FIG. 8A is a schematic cross-sectional view showing the main part of the X-ray tube according to the third embodiment of the present invention, and FIG. 8B is a schematic view showing the electron beam profile in the region B of FIG. FIG. FIG. 6 is a graph showing the relationship between acceleration voltage and beam size in the second embodiment of FIG. 5. It is a schematic diagram showing the 1st specific example of the aperture limiting element which can be used for 2nd Example. It is a schematic diagram showing the 2nd specific example of the aperture limiting element which can be used for 2nd Example. It is a cross-sectional schematic diagram showing the advancing direction of the X-ray obtained from the 1st Example of FIG. It is a cross-sectional schematic diagram showing the other advancing direction of the X-ray obtained from the 1st present Example of FIG.

Explanation of symbols

 5 X-ray tube, 10 cathode, 15 first control grid, 19 first part, 21, 51, 53 second part, 22, 52, 54 third part, 25 second control grid, 30 vacuum vessel, 35 anode, 40, 49 target, 44 optical axis, 45 electron beam, 46 emission light source, 47 first lens, 48 second lens, 50 aperture limiting element, 51 aperture, 52 central axis of electron beam, 65 scattered beam, 68 cross section, 70 X-ray, 75 X-ray mirror

Claims (5)

A cathode that emits an electron beam;
An anode that accelerates the electrons by a voltage applied to the cathode;
A first control grid provided between the cathode and the anode and current-controlling the electron beam;
A second control grid that is provided between the first control grid and the anode and controls convergence of the electron beam;
A target electrically connected to the anode and generating X-rays by collision of the accelerated electron beam;
With
The X-ray tube according to claim 1, wherein an interval between the end on the anode side of the second control grid and the cathode is not less than 2 times and not more than 3 times an interval between the end and the anode.   An aperture limiting element is further provided between the first control grid and the anode to shape the electron beam so that the maximum convergence half-angle of the electron beam on the target is not less than 22 milliradians and not more than 28 milliradians. The X-ray tube according to claim 1, wherein: The aperture limiting element has a plurality of apertures each having a knife edge at the tip,
The X-ray tube according to claim 2, wherein the plurality of apertures are arranged such that an opening diameter thereof becomes larger as it is closer to the anode.   The said opening limiting element is provided between the said edge part by the side of the said target of the said 2nd control grid, and the said edge part by the side of the said anode of the said 2nd control grid, The Claim 2 or 3 characterized by the above-mentioned. X-ray tube as described. Using the X-ray tube according to any one of claims 2 to 4,
By changing the voltage applied to at least one of the first control grid and the second control grid to increase the current angular density, the necessary electron beam is compensated for the decrease in the electron beam current by the aperture limiting element. A method for controlling an X-ray tube, comprising obtaining an electric current.

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