JP5927665B2 - Field emission X-ray generator - Google Patents

Description

  The present invention relates to an X-ray generator using a field emission cold cathode device.

  An X-ray tube using a field emission type cold cathode device (hereinafter referred to as a “field emission type X-ray tube”) can emit electrons at room temperature, has a small amount of heat generation, and facilitates high performance even with a small size. For this reason, it is expected as a high-power light source to replace the conventional filament type (heated filament is used as an electron emission source) X-ray tube. However, the amount of electron emission of the electron-emitting device (cold cathode device) that is an electron source tends to increase exponentially as the applied voltage rises in the range above the operating voltage.

  For example, in an X-ray tube in which a tube current of 300 μA (current flowing between a cold cathode element and a grounded target electrode inside the X-ray tube) is generated when an operating voltage of −13.5 kV is applied, it is −14.5 kV, which is higher than the operating voltage. When a voltage is applied, the tube current becomes 3000 μA, and the tube current rapidly increases 10 times. Such a sudden increase in current tends to induce a local discharge phenomenon in the tube, and this discharge phenomenon causes damage to the cold cathode device and the target that are the electron emission source, deterioration of the vacuum level in the tube, As a result, the X-ray output is reduced in the rated voltage range, that is, the life is shortened. According to the knowledge of the inventors, the discharge due to the electric charge accumulated on the inner wall surface of the glass tube containing the cold cathode element can be cited as a variation factor of the tube voltage causing the abnormal discharge.

  This will be described in detail with reference to FIG. A conventional field emission X-ray tube 101 has a casing 102 that is grounded and has an open upper surface, and one end of a glass tube 104 is supported in the casing 102 via an insulator 103. . The glass tube 104 is accommodated in the housing 102. A cold cathode element 105 is provided on the insulator 103 side of the glass tube 104, and a cylindrical metal electrode 106 is provided around the cold cathode element 105. The cold cathode element 105 is connected in parallel with the metal electrode 106, and a negative voltage is applied to these electrodes from a DC power source 107. A target 108 made of a metal material with good conductivity is formed on the inner surface of the other end side of the glass tube 104. The target 108 is joined to a window 109 that is at a ground potential, and is exposed to the outside. The window 109 is made of a material having a function of emitting X-rays generated by the target 108 to the outside, and is made of beryllium having excellent X-ray transmission, for example. The glass tube 104 is hermetically sealed, and a fixing member (hereinafter referred to as “fixing member”) for fixing the glass tube 104 to an appropriate housing 102 between the glass tube 104 and the upper surface end of the housing 102. 110 is spanned. With this configuration, a space S is formed between the housing 102 and the glass tube 104.

  When a predetermined high voltage, for example, -13.5 kV, is applied to the cold cathode element 105 to operate the field emission X-ray tube 101, electrons are emitted from the cold cathode element 105 toward the target 108, and the window 109 Emits X-rays.

  At this time, since an electric field is formed between the metal electrode 106 having the same potential around the cold cathode element 105 and the grounded casing 102, some electrons are supplied toward the casing 102 by the electric field. The However, a glass tube 104 that is an insulator exists between the metal electrode 106 and the housing 102, and electrons emitted toward the housing 102 are accumulated on the inner surface of the glass tube 104. . This accumulation of electrons continues until there is no potential difference between the glass tube 104 and the metal electrode 106. In other words, when the potential becomes the same (in this case, -13.5 kV), the increase due to the accumulation of electrons on the inner wall surface of the glass tube 104 disappears, and an equilibrium state is reached.

  As a result, there was a potential difference of -13.5 kV between the cold cathode element 105 and the housing 102 immediately after the operation, whereas in the equilibrium state after the operation, the air layer between the glass tube 104 and the housing 102. A potential difference of −13.5 kV is generated in S, and the risk of occurrence of local discharge is increased. In particular, electric field concentration is more likely to occur near the lower end A of the glass tube 104, and is likely to be a starting point for discharge generation. When this discharge occurs, an abrupt change in electric field is generated in the periphery, and the voltage applied to the cold cathode element 105 is also affected and changed. Due to the change in the applied voltage, particularly when the applied voltage increases (as an absolute voltage, for example, increases from −13.5 kV to −14 kV), the amount of electron emission increases rapidly, and this overcurrent causes cooling. This causes damage to the electron emission surface of the cathode element 105 and causes a significant deterioration in life.

  In this regard, it has been proposed to fix an electric resistance film having a high resistance value on the outer surface of an insulating member inside an X-ray tube employing a conventional thermoelectron method as an electron source (Patent Document 1). .

JP 2009-245806 A

  However, the conventional technique described in Patent Document 1 has a problem of discharge inside the X-ray tube, and how to deal with the discharge generated between the casing 102 and the glass tube 104 as described above. It is unclear whether it corresponds. The prior art has a limitation on the resistance value of the electric resistance film, and is limited to those having a resistance value higher than that of the conductive portion of the X-ray tube and lower than that of the insulating portion. I could not say. Furthermore, in the prior art, there is no description as to how the discharge phenomenon occurs, and there is no description as to why the discharge can be prevented by applying the resistance film. Therefore, the specific effect of the countermeasure cannot be understood, and how to apply to the local discharge phenomenon generated between the glass tube 104 and the casing 102 as described above, and when applied. There is no suggestion about the effect.

  The present invention has been made in view of such a point, and an X-ray generator using a field emission cold cathode element accommodates the cold cathode element without applying such a high resistance film. The object is to prevent the micro discharge between the glass tube and the casing located outside thereof, thereby extending the life of the cold cathode element and the X-ray generator more than before and realizing stable operation.

In order to achieve the above object, the present invention provides a cold cathode device in which one end of a glass tube is supported in a grounded casing through an insulating member, and electrons are emitted to the one end of the glass tube. And a target for generating X-rays by irradiation of electrons emitted from the cold cathode device and a window for emitting X-rays generated by the target to the outside on the other end side of the glass tube. A field emission type X-ray generator comprising: a peripheral edge portion of a lower end portion of the glass tube, and a periphery from the peripheral edge portion to the middle of the side surface of the glass tube; and the glass tube The end portion of the insulating material covering the middle of the side surface of the material has a shape curved outwardly and convexly .

  According to the present invention, the peripheral surface of the one end portion, which is likely to be the starting point of discharge due to electric field concentration, is covered with the insulating material, so that an increase in electric field strength in the air can be prevented, and charge accumulation can be suppressed. The occurrence of discharge as described above can be prevented.

In the present invention, it is sufficient to cover at least the peripheral surface of the end of the glass tube that is likely to be the starting point of discharge due to electric field concentration.

The surface of the glass tube that is not covered with the insulating material and the end of the insulating material may be covered with a conductor (electric conductor), and the conductor may be grounded.

  Furthermore, the surface of the insulating member covering the surface of the glass tube may be covered with a conductor, and this conductor may be grounded. The conductor in such a case includes, for example, a mesh made of a conductor, a wire net, a conductive coil, and a conductor film.

  As the conductor, for example, a conductor film made of silver paste, nickel paste, gold paste, palladium paste, carbon paste, or a combination thereof can be proposed.

  According to the present invention, it is possible to prevent discharge between a glass tube containing a cold cathode element and a housing located outside the glass tube without applying a high resistance film, and to generate a cold cathode element and X-rays. It is possible to extend the life of the apparatus than before and to realize stable operation.

It is explanatory drawing which showed typically the side surface cross section which shows the outline of a structure of the field emission type | mold X-ray generator concerning embodiment. In the field emission X-ray generator of FIG. 1, it is explanatory drawing which showed typically the side surface cross section of the example which coat | covered the surface of the insulating material with the conductor. It is explanatory drawing which showed typically the side surface cross section of the example which covered all the surrounding surfaces of the glass tube with the insulating material, and coat | covered the surface of this insulating material with the conductor. In the field emission X-ray generator of FIG. 1, it is explanatory drawing which showed typically the side surface cross section of the example which coat | covered the glass tube surface which is not covered with the insulating material with the conductor. It is a graph which shows the time-dependent change of the tube current in the field emission type | mold X-ray generator concerning Embodiment and the field emission type | mold X-ray generator concerning a prior art. It is explanatory drawing which showed typically the side surface cross section which shows the outline of a structure of the field emission type | mold X-ray generator concerning a prior art.

  Embodiments of the present invention will be described below. FIG. 1 shows an outline of the configuration of a field emission X-ray generator 1 according to an embodiment. The field emission X-ray generator 1 is grounded and the inside of a substantially cubic housing 2 whose upper surface is open. Further, one end of the glass tube 4 is supported via the insulating material 3, and the glass tube 4 is accommodated in the housing 2.

  The insulating material 3 is provided between the lower end portion 4a of the glass tube 4 and the bottom surface portion 2a of the housing 2, and the insulating material 3 covers the peripheral edge portion 4b of the lower end portion 4a of the glass tube 4, The periphery of the glass tube 4 is covered to the middle. It should be noted that the range covered with the insulating material 3 can be achieved by covering the range of at least 3 mm from the lower end 4a to the upper end of the glass tube 4 according to the knowledge of the inventors. The material of the insulating material 3 is made of, for example, silicone rubber.

  A cold cathode element 5 is provided on the insulating material 3 side of the glass tube 4, and a cylindrical metal electrode 6 is provided around the cold cathode element 5. The cold cathode element 5 and the metal electrode 6 are connected in parallel, and a negative voltage is applied to these electrodes from a DC power source 7 via a high voltage cable. The penetration portion of the high-voltage cable with respect to the insulating material 3 is on the lower end 4 a side of the glass tube 4.

  On the inner surface of the upper end portion of the glass tube 4, a target 8 made of a metal material having good conductivity such as tungsten or copper is formed. A window portion 9 having a ground potential is joined to the target 8, and the window portion 9 is exposed to the outside. The window 9 is made of a material having a function of emitting X-rays generated at the target 8 to the outside, and is made of beryllium having excellent X-ray transmission, for example.

  The glass tube 4 is configured to be airtight, and a fixing member 10 is provided between the glass tube 4 and the upper surface end of the housing 2. With this configuration, a space S is formed between the housing 2 and the glass tube 4.

  The field emission X-ray generator 1 according to the embodiment has the above-described configuration. When a predetermined high voltage, for example, -13.5 kV is applied to the cold cathode element 5, the cold cathode element 5 causes the target to Electrons are emitted toward 8, X-rays are generated by collision of electrons at the target 8, and X-rays are emitted from the window portion 9.

  At this time, since an electric field is still formed between the metal electrode 6 having the same potential around the cold cathode element 5 and the grounded casing 2, some electrons are directed toward the casing 2 by the electric field. As described above, electrons are accumulated on the inner surface of the glass tube 4. However, in the present embodiment, the peripheral portion 4b of the lower end portion 4a of the glass tube 4 that has conventionally been concentrated in electric field and is liable to generate discharge is also covered with the insulating material 3. The material 3 can be formed, and the risk that the peripheral edge 4b becomes the starting point of discharge in the space S can be reduced. Therefore, it is possible to prevent the cold cathode element 5 from being damaged and deteriorated due to the discharge, to extend the life of the X-ray generator 1 itself, and to realize a stable operation.

  In order to avoid electric field concentration, it is preferable that the end portion 3a of the insulating material 3 covering the middle of the side surface of the glass tube 4 has a shape curved outwardly as shown in FIG. .

  In the above embodiment, the peripheral edge 4b of the lower end 4a of the glass tube 4 is covered with the insulating material 3, and the periphery of the glass tube 4 is covered to the middle of the side surface to suppress the occurrence of discharge. As shown in FIG. 2, after the insulating material 3 is formed as described above, the surface of the insulating material 3 may be further covered with a conductor 21. The conductor 21 is grounded. For grounding, it is sufficient to electrically connect the casing 2 which is already grounded.

  As a result, a positive charge having a polarity opposite to that of electrons accumulated in the wall surface of the glass tube 4 is induced (inductive charge), and the strong electric field formed outside by the induced charge can be reduced to almost zero. . As a result, the aforementioned discharge phenomenon can be reliably prevented. On the other hand, on the wall surface of the glass tube 4, a potential difference of −13 kV is generated between the inner surface and the outer surface, but the discharge breakdown voltage of the glass material having a thickness of about 1 to 2 mm is about 20 to 30 kV / mm. By using a glass wall of 1 mm or more, discharge due to this potential difference can be reliably prevented.

  As the conductor 21, for example, a conductive material such as a metal mesh, a wire mesh, or a conductive coil can be used. The material and thickness are not particularly limited as long as the material has a performance capable of quickly forming an induced charge by grounding. Moreover, it may not be a uniform film, but may be a wire mesh, or simply a conductive wire wound at a predetermined interval. In such a case, it is desirable that the distance between adjacent conductive lines is 2 mm or less. This is because if this distance is too long, the electric field formed between the housing 2 and the housing 2 gradually increases.

  From this point of view, the conductor 21 may be a coating made of a conductor film made of any one of silver paste, nickel paste, gold paste, palladium paste, carbon paste, or a combination thereof instead of the above-described wire mesh or the like. preferable.

  Furthermore, the region covered with the conductor 21 may be expanded not only to the surface of the insulating material 3 but also to the surface of the glass tube 4 not covered with the insulating material 3.

  Furthermore, as shown in FIG. 3, the region covered by the insulating material 3 may be extended to the entire periphery of the side surface of the glass tube 4 as well as the peripheral edge portion 4 b of the lower end portion 4 a of the glass tube 4. Thus, the portion of the wall surface of the glass tube 4 exposed in the space S may be zero, and the dielectric breakdown strength of the surface of the glass tube 4 may be improved over all. Also in such a case, the surface of the insulating material 3 having an enlarged covering region may be covered with the conductor 21 and the conductor 21 may be grounded. Thereby, the discharge between the glass tube 4 and the housing 2 can be made zero, and the strength of the glass tube 4 itself can be improved.

  As described above, by covering the peripheral edge 4b of the lower end 4a of the glass tube 4 with the insulating material 3, the risk that the peripheral edge 4b becomes the starting point of the discharge in the space S is reduced, and the cooling caused by the discharge is reduced. The cathode element 5 can be prevented from being damaged and deteriorated, and the life of the X-ray generator 1 itself can be extended as compared with the conventional one, but the portion of the glass tube 4 not covered with the insulating material 3 is covered with the conductor 21. It may be.

FIG. 4 shows the X-ray generator 1 having such a configuration. In this example, the portion of the glass tube 4 that is not covered with the insulating material 3 is compared to the example shown in FIG. The conductor 21 is grounded. In addition, when the boundary part between the conductor 21 and the insulating material 3 is located on the surface of the glass tube 4 in the part covered with the conductor 21, there is a possibility of causing electric field concentration in the part. Therefore, the boundary portion between the conductor 21 and the insulating material 3 is preferably located on the surface portion of the insulating material 3 as shown in FIG. 4 that is not located on the surface of the glass tube 4. That is, the portion covered with the conductor 21 is a portion that is not covered with the insulating material 3 in the glass tube 4 and a portion that is the entire circumference of the end portion 3 a of the insulating material 3 and is not in contact with the surface of the glass tube 4. It is good that it is. In addition, since the housing | casing 2 is already earth | grounded, the said conductor 21 should just be electrically connected with the housing | casing 2 through the window part 9, for example.

  By adopting such a configuration, it is possible to prevent damage and deterioration of the cold cathode element 5 due to discharge more reliably than in the example shown in FIG. it can. Further, by covering the portion of the glass tube 4 that is not covered with the insulating material 3 with the conductor 21, the heat of the glass tube 4 can be released to the outside through the conductor 21. It can do by suppressing that the inside stores heat. Therefore, also from this point, the lifetime of the glass tube 4 and the cold cathode element 5 in the glass tube 4 can be extended.

  Further, when a cubic body or a square housing having a horizontal cross section is used as the housing 2, the distance between the housing 2 and the surface of the cylindrical glass tube 4 is not uniform. That is, the distance between the center of the side surface of the housing 2 and the surface of the glass tube 4 is the shortest, and dielectric breakdown occurs in this portion, and electric discharge is likely to occur. Therefore, when a casing having a square horizontal cross section is used as the casing 2, it is preferable to cover the peripheral surface of the glass tube 4 with the conductor 21 in order to prevent the occurrence of discharge at the portion.

  Furthermore, when there is no convex portion that causes electric field concentration at the periphery of the lower end portion of the glass tube 4, the distance between the center of the side surface of the housing 2 and the surface of the glass tube 4 is the shortest. There is a high possibility that a discharge will occur. In such a case, the entire peripheral surface of the glass tube 4 is covered with the conductor 21 without covering the peripheral edge 4b of the lower end portion 4a of the glass tube 4 with the insulating material 3, and the conductor 21 is grounded. Good.

  Next, two field emission X-ray generators 1 according to the embodiment shown in FIG. 2 and two conventional X-ray tubes shown in FIG. 6 were manufactured, and these four continuous lighting tests were simultaneously performed. The results will be described. In the field emission X-ray generator 1 used for this test, a conductive film made of silver paste was used as the conductor 21 and the thickness thereof was about 20 μm.

  In this test, an ON-OFF (about 1 minute) -ON operation, which is predicted to increase the probability of occurrence of a phenomenon that is a technical problem, was performed about every 10 minutes. The rated applied voltage to each of the cold cathode devices 5 and 105 was set to a higher potential −14 kV so that the effectiveness of the present invention can be more clearly evaluated. Further, as a means for observing the micro discharge phenomenon, focusing on the fact that the electron emission characteristics of the cold cathode elements 5 and 105 tend to deteriorate due to the occurrence of the micro discharge, a method of continuously monitoring the electron current is employed. Adopted. The result of carrying out such a test for 150 hours is shown in FIG. In FIG. 4, X1 and X2 respectively show the results of two units of the field emission X-ray generator 1 according to the embodiment, and Y1 and Y2 respectively show the results of two units of the conventional X-ray tube. Yes.

  According to this, the initial tube current varies slightly from 490 to 500 μA, but a clear difference is recognized when changes in the tube currents are compared. That is, in both the field emission X-ray generators 1 according to the embodiment, the tube current is very stable with almost no change for 150 hours. On the other hand, in both conventional X-ray tubes, a rapid decrease is observed at a plurality of locations. Y1 is reduced by about 30 μA and Y2 is reduced by about 60 μA with respect to the initial current. Degradation of electron emission performance is obvious. In addition, in the conventional X-ray tube, it is estimated that the steep drop seen at 5 to 6 locations is caused by the local discharge phenomenon. Further, the decrease width tends to increase as the number of times increases.

  It has been confirmed that X1 and X2 of the field emission X-ray generator according to the embodiment used in the test can be continuously lit and maintain a stable output for 3000 hours.

  The present invention is useful for an X-ray generator using a field emission cold cathode device.

DESCRIPTION OF SYMBOLS 1 Field emission type X-ray generator 2 Case 2a Bottom face part 3 Insulating material 4 Glass tube 4a Lower end part 4b Peripheral part 5 Cold cathode element 6 Metal electrode 7 DC power supply 8 Target 9 Window part 10 Fixing member 21 Conductor S Space

Claims (4)

One end of the glass tube is supported in a grounded casing through an insulating member, and a cold cathode element that emits electrons is provided on the one end side in the glass tube, and the other end of the glass tube A field emission type X-ray generator having a target for generating X-rays by irradiation of electrons emitted from the cold cathode device and a window for emitting X-rays generated by the target to the outside. And
The periphery of the lower end portion of the glass tube and the periphery from the periphery to the middle of the side surface of the glass tube are covered with an insulating material, and the insulating material is covered to the middle of the side surface of the glass tube. The field emission type X-ray generator , wherein the end portion has a shape curved outwardly and convexly . One end of the glass tube is supported in a grounded casing through an insulating member, and a cold cathode element that emits electrons is provided on the one end side in the glass tube, and the other end of the glass tube A field emission type X-ray generator having a target for generating X-rays by irradiation of electrons emitted from the cold cathode device and a window for emitting X-rays generated by the target to the outside. And
In the glass tube, at least one peripheral edge surface is covered with an insulating material, the surface of the glass tube not covered with the insulating material and the end of the insulating material are covered with a conductor, and the conductor is grounded. A field emission X-ray generator. The field emission type X-ray generator according to claim 1 or 2, wherein the surface of the insulating material is covered with a conductor, and the conductor is grounded. 4. The field emission type according to claim 2, wherein the conductor is a conductor film made of any one of silver paste, nickel paste, gold paste, palladium paste, and carbon paste, or a combination thereof. X-ray generator.

Images