JP2002008572A - X-ray tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray tube which can produce a high-definition perspective image through micronization of an X-ray tube focus in a wide range of tube voltage. SOLUTION: Electron emitted from a thermionic cathode 1 is drawn out by a control grid electrode 2, which electron beam is accelerated by a perforated positive electrode 3. A motor with a decelerator 25 is operated to move the perforated positive electrode 3 to a driving direction 26 through screw parts 22, 21, with which a crossover diameter of the electron beam is diminished, and the electron beam is introduced into a vacuum pipe 12. Then, a direction is adjusted by a convergence coil 4, and the beam is irradiated at a target by converging it with the convergence coil. An X-ray tube focus is micronized by moving the perforated positive electrode 3 to and fro according to the tube voltage, and a high-definition perspective image can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an X-ray tube,
In particular, the present invention relates to an X-ray tube for adjusting a distance between a cathode and an anode corresponding to an acceleration voltage in order to converge an electron beam on a target through a hole of a perforated anode.

[0002]

2. Description of the Related Art Techniques for observing a fine internal structure by a nondestructive inspection method are required in various fields. For example, for the development of semiconductor packaging, mounting inspection, and quality assurance, internal defects and the like are examined using an X-ray tube having a minute focus. Such an X-ray tube is generally called a “microfocus X-ray tube”, and is hereinafter referred to as a microfocus X-ray tube. This microfocus X-ray tube converges an electron beam starting from a hot cathode in an evacuation vessel by an electron lens, and strikes a minute region having a diameter of 1 μm to 200 μm on a target made of a heavy element material such as tungsten. It utilizes the generated X-rays. Among the microfocus X-ray tubes, those whose focal dimensions can be miniaturized are of the so-called open type.
The open type microfocus X-ray tube includes a vacuum vessel opening / closing mechanism and a vacuum exhaust pump, and has a feature that a hot cathode and a target material can be exchanged. For this reason, in an open-type microfocus X-ray tube, the focal length is reduced by raising the temperature of the hot cathode of the cathode at the expense of the life of the hot cathode or the target, or by reducing the focal length under high tube voltage and high tube current conditions. Can be miniaturized. The open type microfocus X-ray tube is further classified into two types called a transmission type and a reflection type. In the transmission type, the electron beam and the output X-ray are located on opposite sides when viewed from the target surface, whereas in the reflection type, the electron beam and the output X-ray are located on the same side when viewed from the target surface. The mechanism for converging an electron beam to a minute region on a target and reducing the focal size of X-rays is the same for both the transmission type and the reflection type. As an example, FIG.
2 shows a cross-sectional structure of a transmission-type microfocus X-ray tube of the open type. This X-ray tube is a high-voltage insulator 14
A hot cathode 1 of a cathode portion held by a cathode, a control grid electrode 2 functioning as an electrostatic lens for shaping electrons emitted from the hot cathode 1 into a beam, and a perforated anode 3 for accelerating electrons
A deflection coil 4 that passes through a central hole of the perforated anode 3 and adjusts the direction of the electron beam 8 in the vacuum pipe 12; and a focusing coil stored in a yoke 6 that focuses the adjusted electron beam 8. 5 and a target 7 which collides with the converged electron beam 8 and emits X-rays, and each part is vacuum-tightly connected to each other by an O-ring (not shown), and includes a turbo pump and a rotary pump (not shown). ) And a vacuum vessel 11 connected to a two-stage vacuum pump 13. In the cathode section, a high-voltage cable is inserted into a high-voltage insulator 14, and a negative high voltage is applied to the hot cathode 1 composed of a Wehnelt electrode and a filament. The exterior of the target 7, the yoke 6, the vacuum pipe 12, and the vacuum vessel 11 at the anode section is kept at the ground potential. When a voltage is applied from a high-voltage cable (not shown) to the hot cathode 1 and a current flows and is heated, thermoelectrons are emitted, extracted by the control grid electrode 2, and accelerated toward the perforated anode 3. The electron lens 8 formed by the control grid electrode 2 and the perforated anode 3 forms a crossover 9 in which the electron beam 8 is converged in a region having a cross-sectional diameter of 10 to 500 μm. Then, the electron beam 8 passes through the center hole of the perforated anode 3 and the deflection coil 4
, The direction is adjusted. The electron beam 8 having a small diameter is converged by the converging coil 5 and enters the target 7.

The target 7 has a thickness T = aluminum of aluminum.
It is mounted inside an X-ray transmission window of about 0.5 mm. As the target 7, for example, tungsten having a thickness of about 5 μm is used, or a target material is formed directly on the X-ray transmission window. When the electron beam 8 enters the target 7, X-rays 10 are emitted there. The radiation direction is directional, and the X-ray 10
I use. The X-ray condition of the X-ray tube is such that the tube voltage is 5-1.
In the case of a microfocus X-ray tube at 60 kV and a tube current of about 0.3 mA, the electron beam 8 on the target 7
Is approximately the value obtained by multiplying the diameter of the crossover 9 by the magnification of the electron lens, and usually the value is 0.3 to 200 μm as the focal dimension. It is important to reduce the diameter of the crossover 9 to make the X-ray tube finely focused. In order to reduce the crossover diameter of the electron beam in the microfocus X-ray tube, it is necessary to increase the electric field intensity between the anode and the cathode. This is because, when the electric field intensity between the anode and the cathode is small, the electron emission area of the hot cathode increases due to the space charge effect, and the crossover of the electron beam also increases. FIG. 4 and FIG. 5 schematically show this phenomenon. In the case of a low electric field (FIG. 4B), the emitted electron density on the surface of the hot cathode 1 is reduced by the space charge effect, and the electron emission area is smaller than that in the case of a high electric field (FIG. 4A) even at the same tube current. It gets bigger. When the electron emission area on the cathode surface increases, the cross-sectional area of the electron beam 8 increases, and the diameter of the crossover 9 also increases. As a result, in the case of a low electric field (FIG. 5B), the case of a high electric field ( FIG.
The size of the X-ray focal point becomes larger than in (a).

[0004]

In the conventional open-type microfocus X-ray tube, the distance between the anode and the cathode is fixed to a distance having an insulation strength that can withstand the rated tube voltage. When the tube is used at a low tube voltage, the electric field intensity between the anode and the cathode becomes small, and the diameter of the crossover 9 of the electron beam 8 becomes large due to the effect of space electricity. However, there is a problem that the size of the X-ray focal point 20 becomes large as compared with the above.

The present invention has been made in view of such circumstances, and provides an X-ray tube capable of miniaturizing the size of an X-ray focal point in a wide tube voltage range regardless of whether the voltage is high or low. The purpose is to do.

[0006]

In order to achieve the above object, an X-ray tube according to the present invention comprises a hot cathode filament, a control grid electrode, and a perforated hole having a central through-hole for accelerating an emitted electron beam. An anode, an electromagnetic lens that focuses the accelerated electron beam on the target, and a target are provided. In an X-ray tube that emits X-rays by transmitting or reflecting from the target, the position of the perforated anode is set with respect to the cathode. And moving means for moving back and forth. The moving means moves the perforated anode so that the distance from the cathode increases as the tube voltage increases.

Further, the X-ray tube of the present invention comprises a hot cathode filament, a control grid electrode, a perforated anode having a central through-hole for accelerating the emitted electron beam, and applying the accelerated electron beam to a target. In an X-ray tube provided with an electromagnetic lens for focusing and a target and emitting X-rays from the target, any one of a plurality of types of perforated anodes or cathodes having different shapes so that the distance between the perforated anode and the cathode is variable Or, it is provided with attaching / detaching means for attaching / detaching both.

The X-ray tube of the present invention is constructed as described above, and the position of the perforated anode is moved back and forth from the outside of the X-ray tube with respect to the cathode to open the X-ray tube. Since a plurality of types of perforated anodes having different shapes can be attached to and detached from the perforated anode, the intensity of the electric field between the cathode and the anode is changed, that is, the size of the X-ray focal point is reduced in a wide range of the tube voltage. Can be used.

[0009]

FIG. 1 shows an embodiment of an X-ray tube according to the present invention.
This will be described with reference to FIG. FIG. 1 is a view showing a cross section of a part of a perforated anode 3 and a control grid electrode 2 of a cathode of an X-ray tube according to the present invention. The X-ray tube includes a hot cathode 1 and a control grid electrode 2 constituting a cathode, a perforated anode 3 having a drive mechanism constituting an anode, and a deflection coil 4 for adjusting the direction of an accelerated electron beam 8. The convergence coil 5 (not shown) for converging the electron beam 8 collides with the converged electron beam 8 and X
And a target 7 (not shown) for generating a line. The X-ray tube of the present invention has the same electrode configuration as the conventional X-ray tube shown in FIG. 3, but the conventional perforated anode 3 is fixed, whereas the X-ray tube of FIG. The hole anode 3 has a drive mechanism that can move back and forth. The driving mechanism connected to the perforated anode 3 includes a mechanism that allows the perforated anode 3 to move back and forth with respect to the control grid electrode 2. A motor 25 with a speed reducer is provided on the outside of the vacuum vessel 11 near the perforated anode 3, and a gear 23 is mounted inside the vacuum vessel 11 of the drive shaft. The gear 23 meshes with the gear 24 of the perforated anode 3 provided inside, and rotates the perforated anode 3. When the perforated anode 3 rotates, the screw portion 22 of the perforated anode 3 rotates along the screw portion 21 of the vacuum pipe 12 and moves back and forth in the driving direction 26 of the perforated anode 3. By attaching this drive mechanism, without opening the vacuum vessel 11,
The distance between the perforated anode 3 and the control grid electrode 2 can be changed from the outside.

Next, another embodiment of the X-ray tube of the present invention is shown in FIG.
This will be described with reference to FIG. FIG. 2 is a view showing a cross section of a portion of the perforated anode 3 and the control grid electrode 2 of the cathode of the X-ray tube of the present invention. (A) shows a case in which the distance between the perforated anode 3a and the control grid electrode 2 is increased and the device is used in a high electric field. (B) shows a case in which the distance between the perforated anode 3b and the control grid electrode 2 is reduced and used in a low electric field. This X-ray tube includes a hot cathode 1 and a control grid electrode 2 constituting a cathode, a detachable perforated anode 3a (or 3b) constituting an anode, and a perforated anode 3a (or 3).
b) The vacuum vessel 11 having a mechanism for opening the vacuum vessel lid 11b and the vacuum vessel main body 11a via the hinge 27 to replace the vacuum vessel lid, the inside of the perforated anode 3a (or 3b) and the vacuum holding the same. Screw fitting portion 28 provided on pipe 12
A deflection coil 4 for adjusting the direction of the accelerated electron beam 8 and a converging coil 5 for converging the electron beam 8
(Not shown), and a target 7 (not shown) that generates an X-ray by collision with the converged electron beam 8. The X-ray tube includes a mechanism that can open the vacuum vessel lid 11b from the vacuum vessel main body 11a around the hinge 27. Then, perforated anodes (3a, 3b) having different sizes and shapes can be screwed into the screw fitting portions 28 of the vacuum pipe 12 and mounted on the vacuum vessel lid 11b. As a result, the distance between the perforated anode 3a (or 3b) and the control grid electrode 2 can be changed, and the electric field strength between the two can be changed. FIG. 2A shows an example of a case where the two are used at a high voltage with a distance therebetween.
This is an example in which the diameter of is reduced. (B) shows an example in which the distance between the two is reduced and the device is used at a low voltage, and conversely, the diameter is reduced and the diameter of the crossover 9 of the electron beam 8 is reduced. In the present embodiment, an example was shown in which the distance from the anode to the cathode tube was made variable by attaching and detaching perforated anodes having different shapes, but the same applies to the method of attaching and detaching several types of cathodes having different shapes. The effect is obtained. When attaching and detaching the cathode, a method of attaching and detaching an assembly in which the cathode and the control grid electrode are integrated is desirable.
This is due to the fact that in an X-ray tube, the relative position between the tip of the cathode and the control grid electrode must be adjusted with an accuracy of about ± 0.1 mm.

Although the insulation strength of the vacuum is about 10 kV / mm, in practice, for safety, the distance from the control grid electrode 2 to the perforated anode 3 is usually determined based on the insulation strength of 11 kV / mm. I have. The rated tube voltage of the X-ray tube shown in FIG. 3 is, for example, 160 kV, and the distance from the control grid electrode 2 that withstands the rated tube voltage to the perforated anode 3 is 18 mm.
Generally, in an industrial X-ray tube, the user changes the tube voltage at any time according to the material and thickness of the sample to be viewed.
It is very often possible in practice to use a tube voltage that is significantly lower than the rated voltage. For example, an x-ray tube with a rated tube voltage of 160 kV is often used at a tube voltage of 30 kV. In the case of this tube voltage of 30 kV, it is sufficient for the insulation to have a distance of 3.3 mm or more from the control grid electrode 2 to the perforated anode 3. When the position of the perforated anode 3 is brought closer to the control grid electrode 2 using the driving mechanism of the X-ray tube shown in FIG. 1 of the present invention, the electric field intensity between the control grid electrode 2 and the perforated anode 3 increases, so that the electron The diameter of the beam crossover can be reduced. If the distance between the control grid electrode 2 and the perforated anode 3 is too small, the effect of the electrostatic lens created by the depression of the control grid electrode 2 becomes weak. Distance between is 3.3
mm, the crossover 9 of the electron beam 8,
Consequently, the position of the perforated anode 3 is adjusted so that the focal spot size of the X-ray tube is minimized. The tube voltage and the distance from the control grid electrode 2 to the perforated anode 3 are merely examples. The X-ray tube of the present invention is a micro focus X-ray tube.
It is, of course, possible to apply it as a wire tube.

Further, the X-ray tube according to the present invention includes an X-ray tube called “transmission type” in which X-rays pass through the target 7 and are output to the opposite side of the electron beam irradiation surface, The present invention can be applied to an X-ray tube called “reflection type” that is output to the electron beam irradiation surface side. Further, the X-ray tube of the present invention uses the perforated anode 3 having a predetermined shape described in FIG. 1 to move it, and the perforated anode (3a, 3b) having a different shape previously described in FIG. ) Are prepared in advance, and the anode is replaced with an anode having an optimal shape according to the tube voltage to be used. The latter can be easily realized as a so-called open-type microfocus X-ray tube in which a hot cathode which is a consumable can be replaced.

[0013]

The X-ray tube of the present invention is constructed as described above, and the user presumes the X-ray tube voltage depending on the material and thickness of the sample to be seen through, and sets the X-ray tube container. The anode is moved to an appropriate position by a mechanism provided outside of the anode, or the X-ray tube is opened, and an anode with a different shape is attached to and detached from the anode. Of the electron beam, the diameter of the crossover of the electron beam can be reduced, and the electron beam can be focused on a minute area on the target. Therefore, the focal point of the X-ray tube can be reduced in a wide range of the tube voltage, whereby a high-definition fluoroscopic image can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of an X-ray tube of the present invention.

FIG. 2 is a view showing another embodiment of the X-ray tube of the present invention.

FIG. 3 is a diagram showing a structural cross section of a conventional X-ray tube.

FIG. 4 is a diagram for explaining the spread of an electron emission surface of a hot cathode of an X-ray tube.

FIG. 5 is a diagram for explaining the spread of an electron beam crossover of an X-ray tube.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Hot cathode 2 ... Control grid electrode 3, 3a, 3b ... Perforated anode 4 ... Deflection coil 5 ... Focusing coil 6 ... Yoke 7 ... Target 8 ... Electron beam 9 ... Crossover 10 ... X-ray 11 ... Vacuum container 11a ... Vacuum container body 11b Vacuum container lid 12 Vacuum pipe 13 Vacuum pump 14 High voltage insulator 15 Electron 16 Electric field 17 Electron traveling direction 18 Repulsive force between electrons 19 Electron emission surface 20 X-ray focal point DESCRIPTION OF SYMBOLS 21 ... Screw part 22 ... Screw part 23 ... Gear 24 ... Gear 25 ... Motor with reduction gear 26 ... Drive direction 27 ... Hinge 28 ... Screw fitting

Claims (2)

[Claims] 1. A hot cathode filament, a control grid electrode,
A perforated anode that accelerates the emitted electron beam and has a through hole in the center, an electromagnetic lens that focuses the accelerated electron beam on the target, and a target are provided, and X-rays are emitted by transmitting or reflecting from the target. X-ray tube
An X-ray tube comprising moving means for moving the position of the perforated anode back and forth with respect to the cathode. 2. A hot cathode filament, a control grid electrode,
An X-ray tube that has a perforated anode having a through hole at the center thereof for accelerating an emitted electron beam, an electromagnetic lens for converging the accelerated electron beam on a target, and a target, and emitting X-rays from the target An X-ray tube, comprising: a detachable means for attaching and detaching one or both of a plurality of types of perforated anodes and cathodes having different shapes so that a distance between the perforated anode and the cathode can be changed.