JP2011034734A - Field emission electron source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve long life of a field emission electron source through restraint of deterioration of electron emission part thereof. <P>SOLUTION: In the field emission electron source emitting electrons with application of an electric field, a conductive material 4 is arranged around an emitter 1 emitting an electron flow to a target 3 so that a distance between the target 3 and the emitter 1 is to be shorter than that between the target 3 and the conductive material 4, and a voltage approximately same to one applied between the target 3 and the emitter 1 is applied between the target 3 and the conductive material 4. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a field emission electron source that emits electrons when an electric field is applied.

  2. Description of the Related Art Conventionally, field emission electron sources that are used in electron microscopes, electron beam tubes, X-ray tubes, and the like and emit electrons when an electric field is applied are known. An electron flow (emission current) J emitted from a cold cathode (emitter) of a field emission electron source is expressed by the following formula (1) where F is a field strength, φ is a work function, and A and B are constants. ).

J = A (F ² / φ) exp [−Bφ ^3/2 / F] (1)
According to equation (1), a large emission current J can be obtained by lowering the work function φ.

  A field emission X-ray tube is an example of the use of the field emission electron source. This is to generate X-rays by applying a strong electric field to an emitter made of a field emission type electron material to emit electrons and irradiating the X-ray generation target with electrons. In such an X-ray tube, in order to emit electrons from the emitter, a high voltage must be applied between the X-ray generation target and the emitter to give a sufficient electric field strength to cause the emitter to emit electrons. .

In this type of X-ray tube, as schematically shown in FIG. 8, an electron source including an emitter 1 and a voltage source 2 and an X-ray target 3 are enclosed in a glass tube, and the inside is maintained in a high vacuum state. Configured to do. By applying a high voltage _VET from the voltage source 2 between the X-ray target 3 and the emitter 1 and applying a strong electric field to the tip of the emitter 1, electrons emitted from the emitter 1 are irradiated onto the target 3. X-rays are generated from the target 3. At this time, when the emitted electrons collide with the target 3, the gas attached to the target 3 is ionized, and the ionized gas comes toward the emitter 1.

  In the conventional field emission type X-ray tube, the strongest electric field is applied to the emitter among the constituent members, and ions generated as described above are accelerated at a high voltage and collide with the emitter, thereby emitting electrons from the emitter. The part disappears and its ability to emit electrons is lost. Therefore, in order to extend the lifetime of the emitter, it is necessary to avoid ion collision.

  That is, the lifetime of the field emission X-ray tube as described above depends on the degree of disappearance of the electron emission portion due to ion collision. As shown in FIG. 9, the ions move along the lines of electric force, and the electric lines of force are concentrated at the tip of the emitter 1. Therefore, the collision frequency of ions against the electron emitting member to which a strong electric field is applied is low. It becomes higher than the applied member. For this reason, the lifetime of the electronic material which forms the conventional field emission part is short, and this has become an obstacle to extending the lifetime of an X-ray tube or the like.

  An object of the present invention is to provide a field emission electron source capable of realizing a long life by suppressing deterioration of an electron emission portion in the field emission electron source as described above.

  According to a first aspect of the present invention (first invention), in a field emission electron source that emits electrons when an electric field is applied, a conductive material is disposed around an emitter that emits an electron current to the target. The distance between the target and the conductive material is shorter than the distance between the target and the conductive material, and the same voltage is applied between the target and the conductive material between the target and the emitter. Is applied.

  According to the first aspect of the invention, the conductive material is disposed so that the distance between the target and the conductive material is shorter than the distance between the target and the emitter. The electric field strength of the material is strong, and ions generated at the X-ray generation target are directed toward the conductive material rather than toward the emitter. For this reason, the ion bombardment frequency at the tip of the emitter is much smaller than that of a conventional field emission electron source. Thereby, the lifetime of the field emission electron source is extended, and an X-ray tube or the like having a lifetime that can be put to practical use is obtained.

  According to a second aspect (second invention) of the present invention, in a field emission type electron source that emits electrons when an electric field is applied, a conductive material is disposed around an emitter that emits an electron current to the target. The distance between the target and the conductive material is shorter than the distance between the emitter and the emitter so that the electric field applied to the conductive material is larger than the electric field applied to the emitter. And an adjustment power source for applying a voltage between the emitter and the conductive material within a range in which the relationship between the electric field strengths is not reversed.

  According to the second invention, the conductive material is disposed so that the distance between the target and the conductive material is shorter than the distance between the target and the emitter. The electric field strength of the material is strong, and ions generated at the X-ray generation target are directed toward the conductive material rather than toward the emitter. For this reason, the ion bombardment frequency at the tip of the emitter is much smaller than that of a conventional field emission electron source.

  In addition, by providing an adjustment power source for applying a voltage between the emitter and the conductive material so that the relationship of the electric field strength is not reversed, the potential difference between the emitter and the conductive material can be finely adjusted. By adjusting the degree to which the potential of the conductive material is made lower than the potential of the emitter, the amount of ions drawn into the conductive material can be adjusted, thereby further reducing or minimizing the collision of ions with the emitter. Can be adjusted. Thereby, the lifetime of the field emission electron source is extended, and an X-ray tube or the like having a lifetime that can be put to practical use is obtained.

  In the field emission electron sources of the first and second inventions, the conductive material is preferably a material having a work function higher than that of the emitter.

  According to the present invention, since the electric field strength of the conductive material is stronger than that of the emitter, if the work function of the conductive material is low, the electron emission from the conductive material may be superior to the electron emission from the emitter. However, by using a material having a work function higher than that of the emitter as the conductive material, it is possible to prevent electron emission from the conductive material from being dominant over electron emission from the emitter.

  The conductive material is preferably a material that does not cause sputtering due to ion collision, and is preferably a material having a large atomic number or a material having strong covalent bonding. This is because the ions generated in the field emission electron source have energy of several tens of keV, so that further ions can be generated when sputtering occurs.

(A) is a longitudinal cross-sectional view of the field emission electron source according to the embodiment of the present invention, and (B) is a plan view thereof. The figure which shows the circuit structure which applies a high voltage between an emitter and an electroconductive material, and a target with the field emission type electron source of FIG. The figure which shows the flow of the ion by the electric force line which goes to an emitter and an electroconductive material with the field emission type electron source of FIG. The figure which shows the circuit structure which applies a high voltage between the emitter and electroconductive material of another embodiment, and a target. The figure which shows the circuit structure of embodiment which provided the power supply for adjustment between the emitter and the electroconductive material in the field emission type electron source of FIG. (A) shows the configuration of an X-ray tube using the electron source shown in FIG. 2 as an embodiment of the present invention, and (B) is an X-ray tube having the same shape as (A) and has no conductive material. FIG. The graph which shows the time change of the emission current from the target in each X-ray tube shown to FIG. 6 (A) (B). 1 is a schematic configuration diagram of a conventional field emission X-ray tube. The figure which shows the flow of the ion by the electric force line which goes to an emitter with the structure of FIG.

  Hereinafter, embodiments of the present invention will be described.

  FIG. 1 shows the structure of a field emission electron source according to an embodiment of the present invention. This is a structure in which a cylindrical conductive material 4 is arranged in alignment with the emitter 1 above the rod-shaped emitter 1. That is, the emitter 1 is disposed at a concentric position below the circular hole 5 at the center of the conductive material 4. As shown in FIG. 2, the conductive material 4 is disposed such that the distance between the target 3 and the conductive material 4 is shorter than the distance between the target 3 and the emitter 1.

  According to this structure, when an electric field is applied in the direction of the arrow in the figure, the electric field strength of the conductive material 4 becomes stronger than that of the emitter 1. That is, in the field emission electron source of FIG. 1, the emitter 1 is surrounded by the conductive material 4, and the conductive material 4 is arranged so that the electric field strength applied to the emitter 1 is stronger than that of the emitter 1.

FIG. 2 shows a circuit configuration for applying a high voltage _VET between the emitter 1 and the conductive material 4 and the target 3 in the field emission electron source of FIG. Here, when the high voltage _VET is applied between the emitter and the target in a state where the emitter 1 and the conductive material 4 are at the same potential, the lines of electric force are concentrated on the tip of the emitter 1 as shown in FIG. Alleviated. Since ions flow to the conductive member 4 to which a higher electric field strength is applied, the amount of ions flowing into the emitter 1 is reduced.

  The number of lines of electric force varies depending on the distance between the target and the emitter, the applied voltage, the geometrical positional relationship between the emitter 1 and the conductive material 4, and the like.

  In the field emission electron source according to the present invention, the conductive material surrounding the emitter is preferably a material having a higher work function φ than the emitter. According to the present invention, since the electric field strength of the conductive material is stronger than that of the emitter, if the work function of the conductive material is low, the electron emission from the conductive material can be dominant over the electron emission from the emitter. However, such a possibility can be eliminated by using a material having a work function higher than that of the emitter as the conductive material.

  The conductive material is preferably a material that does not cause sputtering due to ion collision, and is preferably a material having a large atomic number or a material having strong covalent bonding. This is because ions have energy of several tens of keV, so that further ions can be generated by sputtering.

  The field emission electron source of the present invention is not limited to the structure of FIG. 1, and may have a structure in which a plurality of needle-like conductive materials 4 are arranged around the upper portion of the emitter 1 as shown in FIG. 4. In this case, the emitter 1 is arranged at the center position of a circle made of the conductive material 4 arranged on the upper circumference thereof. The conductive material 4 is disposed such that the distance between the target 3 and the conductive material 4 is shorter than the distance between the target 3 and the emitter 1.

  Therefore, also in the field emission electron source having this structure, when the electric field is applied in the direction of the arrow in the figure, the electric field strength of the conductive material 4 becomes stronger than that of the emitter 1. For this reason, since ions flow to the conductive material 4 to which a higher electric field strength is applied, the amount of ions flowing into the emitter 1 is reduced, and the disappearance of the electron emission portion in the emitter 1 is suppressed.

  As described above, in the field emission electron source of FIGS. 2 and 4, the emitter 1 is surrounded by the conductive material 4, and the conductive material 4 is closer to the target 3 than the distance between the target 3 and the emitter 1. By arranging the distance between the conductive material 4 to be short, the electric field strength applied to the conductive material 4 becomes stronger than the electric field applied to the emitter 1. For this reason, ions generated at the X-ray generation target are directed to the conductive material 4 more than toward the emitter 1, so that the ion bombardment frequency at the tip of the emitter 1 is higher than that of a conventional field emission X-ray tube. Overwhelmingly smaller. Thereby, a field emission X-ray tube having a useful life can be obtained.

  As another embodiment, in the field emission electron source of FIG. 2 or FIG. 4, the amount of ions drawn into the conductive material can be adjusted according to the frequency of ion collision with the emitter. FIG. 5 shows an example of the configuration.

  This is to cope with the case where ion collision with the emitter causes a problem even in the configuration in which the emitter 1 and the conductive material 4 have the same potential as shown in FIGS. Specifically, for example, in the field emission electron source of FIG. 2, a variable for adjustment that provides a potential difference between the emitter 1 and the conductive material 4 and between the emitter 1 and the conductive material 4 as shown in FIG. 5. A power source 6 is connected.

  The power source 6 does not reverse the relationship of the electric field strength that the electric field strength applied to the conductive material 4 is stronger than the electric field strength applied to the emitter 1 (that is, the electric field strength applied to the emitter 1 is The voltage is adjusted to be applied within a range that does not become stronger than the electric field strength applied to the conductive material 4. By adjusting the degree to which the potential of the conductive material 4 is made lower than the potential of the emitter 1 in this way, the amount of ions drawn into the conductive material 4 can be adjusted, thereby further reducing the collision of ions with the emitter. Fine adjustments can be made.

  As shown in FIG. 6A, an electron-emitting device having a carbon nanostructure was used for the emitter 1 in the X-ray tube using the field emission electron source shown in FIG. 2, and stainless steel was used as the conductor material 4. The opening diameter of the cylindrical conductor material 4 was 8 mm, and the distance from the upper surface to the tip of the target 3 and the emitter 1 was 1 mm. These were enclosed in the glass tube 6, and the inside was kept in a high vacuum state. For comparison, an X-ray tube not provided with the conductive material 4 as shown in FIG. 6B was also prepared.

  In each X-ray tube of FIGS. 6A and 6B, a voltage of 10 kV was applied between the emitter 1 and the target 3, and the emission current from the target in each X-ray tube was measured. The result is shown in FIG.

  In the graph of FIG. 7, the emission current is substantially constant between 0.6 and 0.7 [mA] in the X-ray tube of FIG. 6 (A), and the emission current is smaller than 0.5. It is the X-ray tube in FIG. 6B that decreases from [mA]. As shown in this graph, the lifetime of the X-ray tube in which the conductive material 4 in FIG. 6A is arranged exceeds 100 hours (actually, 2000 hours or more is possible), whereas FIG. The life of the X-ray tube (not including the conductive material) shown in (4) was about 40 to 50 hours.

  Accordingly, it can be seen that there is a significant difference in deterioration in the X-ray tube shown in FIGS. That is, in the X-ray tube of FIG. 6 (A), it was demonstrated that the provision of the conductive material 4 significantly improves the lifetime as compared with the X-ray tube of FIG. 6 (B).

  As described above, the case where the present invention is used for an X-ray tube has been described. However, the present invention is applicable to all devices using a field emission electron source, for example, an electron beam source of an electron beam tube in addition to an X-ray tube. Suitable for electron source of scanning part, scanning electron microscope and transmission electron microscope, electron source part of electron beam excitation type fluorescent lamp, electron beam source part of electron beam microprobe analyzer, electron beam source part of cathode luminescence device, etc. Used for.

  DESCRIPTION OF SYMBOLS 1 ... Emitter, 2 ... Voltage source, 3 ... Target, 4 ... Conductive material, 5 ... Hole, 6 ... Control power supply, 7 ... Glass tube.

Claims (4)

In a field emission electron source that emits electrons when an electric field is applied,
A conductive material is disposed around an emitter that emits an electron current to the target, such that a distance between the target and the conductive material is shorter than a distance between the target and the emitter, A field emission electron source, wherein the same voltage as that applied between the target and the emitter is applied between the target and the conductive material. In a field emission electron source that emits electrons when an electric field is applied,
A conductive material is disposed around an emitter that emits an electron current to the target, such that a distance between the target and the conductive material is shorter than a distance between the target and the emitter, The electric field applied to the conductive material is configured to be stronger than the electric field applied to the emitter, and the electric field strength relationship is not reversed between the emitter and the conductive material. A field emission electron source comprising an adjustment power source for applying a voltage in a range.   3. The field emission electron source according to claim 1, wherein the conductive material is a material having a work function higher than that of the emitter.   4. The field emission electron source according to claim 1, wherein the conductive material is a material that does not cause sputtering due to ion collision.

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