According to one embodiment, an X-ray tube includes an elongated anode target, a cathode, and a vacuum envelope. The cathode includes an electron emission source and a converging electrode including a trench portion. The trench portion includes a closest inner circumferential wall, an upper inner circumferential wall, and a lower inner circumferential wall. The electron emission source projects towards a opening of the trench portion from a boundary between the closest inner circumferential wall and the upper inner circumferential wall.

FIELD

Embodiments described herein relate generally to an X-ray tube.

BACKGROUND

X-ray tubes are used for X-ray image diagnosis, non-destructive inspection and the like. The X-ray tubes include a stationary anode type and a rotating anode type, which can be selected according to use. An X-ray tube comprises an anode target, a cathode and a vacuum envelope. The anode target is configured to emit X-ray by incidence of an electron beam.

The cathode comprises a filament coil and an electron converging cup. The filament coil is configured to emit electrons. A high tube voltage in the range of several tens to several hundreds of kilovolts (kV) is applied between the anode target and the cathode. In this manner, the electron converging cup can act an electron lens and converge an electron beam emitted towards the anode target. The electron converging cup comprises a trench portion in which the filament coil is accommodated. The trench portion comprises an upper inner circumferential wall and a lower inner circumferential wall located on an opposite side to the anode target with respect to the upper inner circumferential wall and having dimensions smaller than those of the upper inner circumferential wall.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided an X-ray tube comprises:

an anode target configured to radiate X-rays by incidence of an electron beam;

a cathode comprising an elongated electron emission source configured to emit electrons, and a converging electrode including a trench portion accommodating the electron emission source, and configured to converge the electron beam towards the anode target through an opening of the trench portion as the electrons are emitted from the electron emission source, and

a vacuum envelope accommodating the anode target and the cathode,

wherein the trench portion comprises:

a closest inner circumferential wall extending linearly in a depth direction of the trench portion, having dimension shorter than dimension of the electron emission source in the depth direction of the trench portion, and facing the electron emission source with a narrowest gap between the closest inner circumferential wall and the electron emission source over an entire circumference of the electron emission source in width direction of the electron emission source,

an upper inner circumferential wall located on an opening side of the trench portion with respect to the closest inner circumferential wall and having a shape widening in the width direction further from the closest inner circumferential wall, and

a lower inner circumferential wall located on an opposite side to the upper inner circumferential wall with respect to the closest inner circumferential wall and having a shape widening in the width direction further from the closest inner circumferential wall, and

the electron emission source projects towards the opening of the trench portion from a boundary between the closest inner circumferential wall and the upper inner circumferential wall.

An X-ray tube assembly according to the first embodiment will now be described in detail with reference to accompanying drawings. In this embodiment, the X-ray tube assembly is of the rotating anode type.

As shown inFIG. 1, the X-ray tube assembly comprises a rotating anode X-ray tube1, a stator coil2serving as a coil to generate a magnetic field, a housing3to accommodate the X-ray tube and the stator coil, and insulating oil4filled in the housing as a coolant.

The X-ray tube1comprises a cathode (cathode electron gun)10, a sliding bearing unit20, an anode target60and a vacuum envelope70. A control unit5of an X-ray apparatus (not shown) in which an X-ray tube assembly is mounted, is electrically connected to the cathode10.

The sliding bearing unit20comprises a rotor30, a fixed shaft40serving as a fixed member and a liquid metal lubricant (not shown) as a lubricant, and thus employs sliding bearing.

The rotor30is formed into a cylindrical shape, one end of which is blocked. The rotor30extends along a central axis of rotation thereof. In this embodiment, the axis of rotation is the same as a tube axis al of the X-ray tube1, and will be described as the tube axis al hereinafter. The rotor30is rotatable around the tube axis al. The rotor30comprises a joint member31located at one end thereof. The rotor30is formed of a material such as iron (Fe) or molybdenum (Mo).

The fixed shaft40is formed to have a cylindrical shape having dimensions smaller than those of the rotor30. The fixed shaft40is provided coaxially with the rotor30, and extends along the tube axis al. The fixed shaft40is engaged with an internal part of the rotor30. The fixed shaft40is formed of a material such as Fe or Mo. One end of the fixed shaft40is exposed to the outside of the rotor30. The fixed shaft40rotatably supports the rotor30.

The liquid metal lubricant is applied so that it fills the space between the rotor30and the fixed shaft40.

The anode target60is disposed along the tube axis al such that it faces the other end of the fixed shaft40. The anode target60comprises an anode main body61and a target layer62provided partially on an outer surface of the anode main body61.

The anode main body61is secured to the rotor30via the joint member31. The anode main body61has a disk-like shape and is made of a material such as Mo.

The anode main body61is rotatable around the tube axis al. The target layer62is formed into a ring-like shape. The target layer62comprises a target surface S which faces the cathode10in the direction along the tube axis al with an interval therebetween. In the anode target60, a focal spot is formed on the target surface S when an electron beam is made incident on the target surface S, and then X-ray is radiated from the focal spot.

The anode target60is electrically connected to a terminal91via the fixed shaft40, the rotor30and the like.

As shown inFIGS. 1, 2 and 3, the cathode10comprises one or more electron emission sources and the electron converging cup15as a converging electrode. In this embodiment, the cathode10comprises a first filament coil11, a second filament coil12and a third filament coil13, each serving as an electron emission source. The first to third filament coils11to13are arranged in the direction of rotation of the anode target60at intervals. The first filament coil11and the third filament coil13are each disposed on an inclined surface. The first to third filament coils11to13are formed of a material, a main component of which is tungsten.

The first to third filament coils11to13and the electron converging cup15are electrically connected to terminals81,82,83,84and85.

The electron converging cup15comprises one or more trench portions configured to accommodate filament coils (electron emission sources), respectively. In this embodiment, the electron converging cup15comprises three trench portions (a first trench portion16, a second trench portion17and a third trench portion18) in which the first to third filament coils11to13are respectively accommodated.

A current (filament current) is supplied to the first to third filament coils11to13, and thus, the first to third filament coils11to13emit electrons (thermoelectrons).

A relatively positive voltage is applied to the anode target60from the terminal91via the fixed shaft40, the rotor30and the like. Conversely, a relatively negative voltage is applied to the first to third filament coils11to13and the electron converging cup15from the terminals81to84and terminal85.

An X-ray tube voltage (referred to as tube voltage hereinafter) is applied between the anode target60and the cathode10, and therefore the electrons emitted from the first to third filament coils11to13are accelerated and made incident on the target surface S as electron beam.

The electron converging cup15is configured to converge the beam of electrons emitted from the first to third filament coils11to13towards the anode target60through openings16ato18aof the first to third trench portions16to18.

As shown inFIG. 1, the vacuum envelope70is cylindrical. The vacuum envelope70is formed of a combination of insulating materials such as glass and ceramics, metals, etc. In the vacuum envelope70, the diameter of a portion thereof which faces the anode target60, is larger than that of another portion facing the rotor30. The vacuum envelope70comprises an opening71. The opening71is tightly attached to one end of the fixed shaft40in order to maintain the vacuum-tightness of the vacuum envelope70. The vacuum envelope70fixates the fixed shaft40. In the vacuum envelope70, the cathode10is mounted on an inner wall thereof. The vacuum envelope70is sealed, and accommodates the cathode10, the sliding bearing unit20, the anode target60, etc. The inside of the vacuum envelope70is maintained in a vacuum state.

The stator coil2is provided to surround the vacuum envelope70while facing a side surface of the rotor30. The stator coil2has a ring-like shape. The stator coil2is electrically connected to the terminals92and93(not shown) and driven via these terminals.

The housing3comprises an X-ray transmitting window3aconfigured to transmit X-rays to a vicinity of the target layer62facing the cathode10. The housing3accommodates the X-ray tube1and the stator coil2, and is further filled with the insulating oil4.

The control unit5is electrically connected to the cathode10via the terminals81,82,83,84and85. The control unit5is configured to drive one of the first to third filament coils11to13, or two or more of the first to third filament coils11to13, or to apply a voltage to the electronic convergence cup15so that the potential of the electronic convergence cup15may become lower than the potential of a filament coil.

Next, the X-ray radiating operation of the above-described X-ray tube assembly will now be described.

As shown inFIGS. 1 to 3, when the X-ray tube assembly is in operation, first, the stator coil2is driven via the terminals92and93, and thus generates a magnetic field. That is, the stator coil2produces a rotating torque to be applied to the rotor30. With this structure, the rotor rotates, and the anode target60rotates therewith.

Next, the control unit5supplies a current to at least one of the first to third filament coils11to13to be driven, via the respective ones of the terminals81to84. A relatively negative voltage is applied to the filament coils to be driven. A relatively positive voltage is applied to the anode target60via the terminal91.

Since the tube voltage is applied between the filament coil (cathode10) and the anode target60, the electrons emitted from the respective filament coil are converged and accelerated and collide with the target layer62. In other words, an X-ray tube current (referred to as the tube current hereinafter) flows from the cathode10to a focal spot on the target surface S.

The target layer62radiates X-rays by the incidence of the electron beam, and the X-rays radiated from the focal spot are transmitted to the outside of the housing3through the X-ray transmission window3a. Thus, X-ray imaging is performed.

Next, the structure of the X-ray tube assembly of an example according to the embodiment and the structure of an X-ray tube assembly of a comparative example will now be described. The X-ray tube assemblies of the example and comparative example are manufactured similarly except for the trench portions of the electron converging cup15. The first to third trench portions16to18are formed to be similar to each other, and therefore only the first trench portion16will be considered in the following description.

As shown inFIGS. 12 and 13, an opening16aof the first trench portion16has a rectangular shape having sides in a first direction da, which extends from the first filament coil11, and sides in a second direction db, which orthogonally crosses the first direction da. The depth direction of the first trench portion16is a third direction dc, which orthogonally crosses the first direction da and the second direction db.

The first trench portion16comprises an upper inner circumferential wall51and a lower inner circumferential wall52.

The upper inner circumferential wall51is located on the side of the opening16aof the first trench portion16, that is, an upper section of the first trench portion16. The upper inner circumferential wall51is formed into a rectangular frame shape to have the same dimensions as those of the opening16ain a plane in the first direction da and the second direction db.

The lower inner circumferential wall52is located on the opposite side to the electron beam emitting direction with respect to the upper inner circumferential wall51, that is, a lower section of the first trench portion16underneath the upper inner circumferential wall51. The lower inner circumferential wall52is formed into a rectangular frame shape to have dimensions smaller as those of the upper inner circumferential wall51in a plane in the first direction da and the second direction db.

In this comparative example, the diameter of the first filament coil11is defined as OSDa, the width of the upper inner circumferential wall51in the second direction db as L1a, the depth of the upper inner circumferential wall51(that is, the length from the furthermost end of the upper inner circumferential wall51from the opening16ato the opening16ain the third direction dc) as D1a, the width of the lower inner circumferential wall52in the second direction db as L2a, the fd value, which indicates the projection of the first filament coil11towards the opening16afrom the boundary between the upper inner circumferential wall51and the lower inner circumferential wall52, is defined as fda. The gap between the first filament coil11and the lower inner circumferential wall52in the second direction db is defined as Ya.

As shown inFIG. 4and alsoFIGS. 2 and 3, the opening16aof the first trench portion16has a rectangular shape having sides in the first direction da and sides in the second direction db. The depth direction of the first trench portion16is the third direction dc.

The first trench portion16comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52.

The closest inner circumferential wall53is shorter than a dimension (diameter) of the first filament coil11in the third direction dc. The closest inner circumferential wall53is formed into a rectangular frame shape. The closest inner circumferential wall53faces the first filament coil11in the width direction of the first trench portion16along the second direction db with a narrowest gap.

The upper inner circumferential wall51is located on the nearer side to the opening16aof the first trench portion16than the closest inner circumferential wall53. The upper inner circumferential wall51is formed into a rectangular frame shape to have the same dimensions as those of the opening16ain a plane in the first direction da and the second direction db, and also dimensions larger than those of the closest inner circumferential wall53. The upper inner circumferential wall51in a plane in the second direction db and the third direction dc extends linearly in the third direction dc. The upper inner circumferential wall51has a shape widening further from the closest inner circumferential wall53in the width direction (the second direction db).

The lower inner circumferential wall52is located on the opposite side to the upper inner circumferential wall51with respect to the closest inner circumferential wall53. The lower inner circumferential wall52is formed into a rectangular frame shape to have dimensions larger than those of the closest inner circumferential wall53in the second direction db. The lower inner circumferential wall52in a plane in the second direction db and the third direction dc extends linearly in the third direction dc. The lower inner circumferential wall52has a shape widening further from the closest inner circumferential wall53in the width direction (the second direction db).

In this example, the diameter of the first filament coil11is defined as OSDb, the width of the upper inner circumferential wall51in the second direction db as L1b, the depth of the upper inner circumferential wall51(that is, the length from the furthermost end of the upper inner circumferential wall51from the opening16ato the opening16ain the third direction dc) as D1b, the width (minimum width) of the closest inner circumferential wall53along the second direction db as L3b, the depth of the closest inner circumferential wall53(that is, the length from the furthermost end of the closest inner circumferential wall53from the opening16ato the opening16ain the third direction dc) as D3b, the width (maximum width) of the lower inner circumferential wall52in the second direction db as L2b, the depth of the lower inner circumferential wall52(that is, the length from the furthermost end of the lower inner circumferential wall52from the opening16ato the opening16ain the third direction dc) as D2b, the fd value, which indicates the projection of the first filament coil11towards the opening16afrom the boundary between the upper inner circumferential wall51and the closest inner circumferential wall53, is defined as fdb. The gap between the first filament coil11and the closest inner circumferential wall53in the second direction db is defined as Yb.

Next, the results of comparison and contrast between the example and comparative example in terms of the dimensions of the first trench portion16and the first filament coil11will now be provided.
OSDb=OSDa
Yb=Ya+X
L1a≦L1b≦L1a+2·0.75 mm·X
L3b=L2a+2·X

Further, the dimensions of the first trench portion16of this example satisfy the following relationships:
1.5·L3b≦L2b≦2.0·L3b
D1b<D3b<D1b+0.5 mm

X represents the expansion of the gap between the first filament coil11and the first trench portion16in the second direction db.

The dimensions of the first trench portion16and the first filament coil11of the example are as follows.

Here, the present inventors conducted a computer simulation of electron beam trajectory by using the X-ray tube assembly according to the embodiment and another computer simulation of electron beam trajectory by using the X-ray tube assembly according to the comparative example. In these simulations, only the first filament coil11of the first to third filament coils11to13was driven. Therefore, the focal spot formed on the target surface S was a single focal spot. The simulations were carried out under the same conditions.

First, the procedure and results of the simulation of electron beam trajectory by using the X-ray tube assembly according to the embodiment will be described.

As shown inFIGS. 5 and 6, only the first filament coil11was driven for emitting electrons. Electrons emitted from the first filament coil11were made incident on the target surface S of the anode target60as an electron beam. The electron beam was converged by the effect of the electric field produced by the first trench portion16of the electron converging cup15.

Then, the main focal spot formed by the electrons emitted from the upper surface (on the anode target60side) of the first filament coil11and the sub-focal spot formed by the electrons emitted from the side surface of the first filament coil11are made to substantially coincide with each other in position and dimensions.

The results of the electron density distribution in the focal spot were as shown inFIG. 7. The region where the electron density is at maximum was indicated as 100%.FIG. 7shows an electron density distribution when the target surface S was viewed from a direction vertical to the tube axis al.

The width of the effective focal spot Fb in a direction dd along the direction of rotation of the anode target60was 0.552 mm. The length of the effective focal spot Fb in a direction de along the tube axis al was 1.004 mm. Note that in order be in conformity with IEC standards, it suffices if the width of the effective focal spot Fb is 0.75 mm or less, and the length of the effective focal spot Fb is 1.1 mm or less.

Next, the procedure and results of the simulation of electron beam trajectory by using the X-ray tube assembly according to the comparative example will be described.

As shown inFIG. 13, only the first filament coil11was driven for emitting electrons. Electrons emitted from the first filament coil11were made incident on the target surface S of the anode target60as an electron beam. The electron beam was converged by the effect of the electric field produced by the first trench portion16of the electron converging cup15.

Then, the main focal spot formed by the electrons emitted from the upper surface (on the anode target60side) of the first filament coil11and the sub-focal spot formed by the electrons emitted from the side surface of the first filament coil11are made to substantially coincide with each other in position and dimensions.

FIG. 14shows an effective focal spot Fa formed on the target surface S. The width of the effective focal spot Fa in the direction dd along the direction of rotation of the anode target60was 0.753 mm, which was larger than that of the example. The length of the effective focal spot Fa in the direction de along the tube axis al was 1.040 mm, which was slightly larger than that of the example.

Next, the example and the comparative example will now be compared and contrasted with each other in the emission of the electron beam.

FIGS. 6 and 13show the results of the example and comparative example. As shown, there are some cases in the example that electrons released from the side surface of the filament coil11collide with the closest inner circumferential wall53or were bent by the electric field produced by the inner circumferential wall53, so that the electrons did not reach the anode target. On the other hand, in the comparative example, electrons released from the side surface of the filament coil were bent by the electric field produced by the lower inner circumferential wall52but they reached the anode target. Thus, in the example, the electrons released from the side surface of the filament coil do not contribute to the formation of the focal spot. In contrast, in the comparative example, the electrons, whose direction was bent by the lower inner circumferential wall, reach an undesired outer portion of the main focal spot on the target surface S, to make a sub-focal spot, and thus the focal spot does not fit in the desired size.

Next, the example and comparative example will be compared and contrasted in the state of focal spot.

As shown inFIGS. 7 and 14, a substantially rectangular focal spot was obtained in the example although slight sub-focal spots were observed, whereas in the comparative example, there were strong sub-focal spots, which makes it no longer possible to maintain a square focal spot.

According to the X-ray tube assembly having the above-described structure of the example according to the first embodiment, the X-ray tube1comprises an anode target60configured to radiate X-rays by incidence of an electron beam, a cathode10comprising an electron converging cup15, and a vacuum envelope70accommodating the anode target60and the cathode10.

The electron converging cup15comprises filament coils configured to emit electrons (first to third filament coils11to13) and trench portions (first to third trench portions16to18) in which the first to third filament coils are respectively accommodated. The electron converging cup15is configured to converge an electron beam towards the anode target60through an opening of the trench portions (openings16ato18a) as the electrons are emitted from each of the respective filament coils.

Each of the trench portions (first to third trench portions16to18) comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52. The closest inner circumferential wall53has a dimension shorter than a dimension of the respective filament coil in the depth direction of the trench portion (third direction dc), and faces the filament coil11with a narrowest gap between the closest inner circumferential wall53and the filament coil11over an entire circumference of the filament coil11in the width direction of the trench portion (or the electron emission source). The upper inner circumferential wall51is located on the opening side of the trench portion than the closest inner circumferential wall53, and has a shape widening in the width direction further from the closest inner circumferential wall53. The lower inner circumferential wall52is located on the opposite side to the upper inner circumferential wall51with respect to the closest inner circumferential wall53, and has a shape widening in the width direction further from the closest inner circumferential wall53.

With the above-described structure, the X-ray tube assembly of the example can obtain such advantages as listed in the following.

(1) As for the X-ray tube assembly of the comparative example, there is no effective means to make the electron density distribution within a focal spot uniform and make a focal spot having desirable dimensions simultaneously, whereas for the X-ray tube assembly of the example, there is such effective means. Further, in the X-ray tube assembly of the example, the X-ray tube1can be formed so that the sub-focal spot fits inside the main focal spot, or more preferably, if possible, the position and dimensions of the main focal spot substantially coincide with those of the sub-focal spot.

Since each trench portion comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52, an electron beam can be reliably converged even if the space between the filament coil and the trench portion (closest inner circumferential wall53) is made larger than that of the comparative example. Further, with the closest inner circumferential wall53, it is possible to make it difficult for the electrons emitted from the side surface of the filament coil to reach the anode target, and thus the electron density distribution of sub-focal spots can be suppressed at low level.

(2) As for the X-ray tube assembly of the comparative example, there is no effective means to suppress a sub-focal spot and increase the dimensions of the lower inner circumferential wall simultaneously, whereas for the X-ray tube assembly of the example, there is such effective means.

A focal spot of the same dimensions can be obtained between when the gap Ya is set to about 0.15 mm in the comparative example and when the gap Yb is set to about 0.485 mm in the example. That is, the dimensions of a focal spot can be reduced by further decreasing the gap Yb.

Here, when the gap Yb is set to 0.2 mm or more, or more preferably, 0.3 mm or more, the dimensions of a focal spot can be reduced while preventing filament touch and the occurrence of electric breakdown between the filament coil and the electron converging cup15.

(3) As for the X-ray tube assembly of the comparative example, there is no effective means to suppress a sub-focal spot and obtain a focal spot of desirable dimensions simultaneously, whereas for the X-ray tube assembly of the example, there is such effective means.

As described above, each trench portion comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52. By appropriately setting the dimensions of these, it is possible to suppress sub-focal spots and obtain a focal spot of desirable dimensions without adjusting the gap between the anode target60and the cathode10. In other words, it is possible to obtain a focal spot having a uniform electron density distribution therewithin and desirable dimensions while maintaining a voltage durability between the anode target60and the cathode10.

(4) As for the X-ray tube assembly of the example, it is possible to make the electron density distribution uniform within a focal spot and obtain a focal spot of desirable dimensions without curving the upper inner circumferential wall51. Therefore, the design and processing costs can be reduced as compared to the case where the upper inner circumferential wall51should be curved.

As described above, it is possible to realize an X-ray tube1which can make the electron density distribution uniform within a focal spot and obtain a focal spot of desirable dimensions, and also an X-ray tube assembly comprising such an X-ray tube1.

An X-ray tube assembly according to the second embodiment will now be described in detail. In this embodiment, the structural members other than those which will be particularly discussed are identical to those of the first embodiment, and therefore they are designated by the same reference numbers and the detailed descriptions therefor will be omitted.

As shown inFIG. 8, the first trench portion16comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52. The closest inner circumferential wall53is formed into a substantially rectangular frame shape. The lower inner circumferential wall52is formed to pierce through the electron converging cup15in the first direction da. A cross section of the lower inner circumferential wall52in a plane in the second direction db and third direction dc has an ovally rounded rectangle. Here, the ovally rounded rectangle has two parallel lines with equal length, and two semi-circles with an equal radius.

Next, the processing of the lower inner circumferential wall52will now be described.

The lower inner circumferential wall52can be processed using, for example, a ball end mill. For example, the rotating shaft of the ball end mill is set in the first direction da, and the material is processed while being fed in the first direction da and the second direction db. Thus, the processing cost can be reduced as compared to the case where the discharge process is required (that is, the lower inner circumferential wall52is formed to have a rectangular frame shape). It is alternatively possible that a drill through-hole is made in the electron converging cup15in the same direction in advance before the ball end milling process.

According to the X-ray tube assembly having the above-described structure of the second embodiment, the X-ray tube1comprises an anode target60configured to radiate X-rays by incidence of an electron beam, a cathode10comprising an electron converging cup15, and a vacuum envelope70accommodating the anode target60and the cathode10.

Each of the trench portions (first to third trench portions16to18) comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52. The cross section of the lower inner circumferential wall52in a plane in the second direction db and third direction dc may have an ovally rounded rectangle. In this case as well, a similar advantageous effect to that of the first embodiment can be obtained by adjusting the dimensions of the lower inner circumferential wall52.

The lower inner circumferential wall52is formed by making a through-hole to extend in the first direction da in the electron converging cup15. Thus, the lower inner circumferential wall52can be formed merely by making the through-hole, and no such a process of blocking the through-hole is required later. Therefore, the processing cost of the lower inner circumferential wall52can be reduced as compared to the first embodiment previously described.

Accordingly, it is possible to realize an X-ray tube1which can make the electron density distribution uniform within a focal spot and obtain a focal spot of desirable dimensions, and also an X-ray tube assembly comprising such an X-ray tube1. Further, the above-described X-ray tube1can prevent the occurrence of both filament touch and electric breakdown between the filament coils and electron converging cup15at the same time.

Next, a modified example of the X-ray tube assembly according to the second embodiment will now be described.

As shown inFIG. 9, the upper inner circumferential wall51is formed to be multistage. In this example, the upper inner circumferential wall51is of a two-stage. Each stage of the upper inner circumferential wall51is formed to have a rectangular frame shape. The stage on the nearer side to the closest inner circumferential wall53formed into a shape widening further from the closest inner circumferential wall53in the width direction (second direction db). The stage on the nearer side to the opening16ain the upper inner circumferential wall51is formed to have the same dimensions as those of the opening (opening16a) in a plane in the first direction da and the second direction db into a shape widening further from the stage on the nearer side to the closest inner circumferential wall53in the width direction (second direction db).

In this case as well, a similar advantageous effect to that of the second embodiment can be obtained by adjusting the dimensions of the upper inner circumferential wall51. Further, with the multistage structure of the upper inner circumferential wall51, this example exhibited such an advantage that the electron density distribution can be made uniform within a focal spot and a focal spot of desirable dimensions can be obtained.

Next, another modified example of the X-ray tube assembly according to the second embodiment will now be described.

As shown inFIG. 10, the upper inner circumferential wall51is formed to have a curved surface shape. More specifically, a cross section of the upper inner circumferential wall51has a curved surface shape in a plane in the second direction db and the third direction dc.

In this case as well, a similar advantageous effect to that of the second embodiment can be obtained by adjusting the curved surface shape of the upper inner circumferential wall51. Further, with the curved surface structure of the upper inner circumferential wall51, this example exhibited such an advantage that the electron density distribution can be made uniform within a focal spot and a focal spot of more desirable dimensions can be obtained.

Next, an X-ray tube assembly according to the third embodiment will now be described in detail. In the embodiment, the structural members other than those which will be particularly discussed are identical to those of the first embodiment, and therefore they are designated by the same reference numbers and the detailed descriptions therefor will be omitted.

As shown inFIG. 11, the lower inner circumferential wall52has a curved surface shape. A cross section of the lower inner circumferential wall52has such a curved surface shape as a part of a circle in a plane in the second direction db and the third direction dc. The lower inner circumferential wall52is formed into a shape widening further from the closest inner circumferential wall53in the width directions (the first direction da and the second direction db) in a plane in the first direction da and the second direction db. The lower inner circumferential wall52can be processed, for example, in the following manner. The rotating shaft of the ball end mill is set in the third direction dc, and the material is processed while being fed in the first direction da and the third direction dc.

An insulating member100is secured to the electron converging cup15. The insulating member100is placed to face the lower inner circumferential wall52. In this embodiment, the insulating member100is formed of ceramics and brazed to the electron converging cup15. The insulating member100is configured to support each respective filament coil (first to third filament coils11to13) and regulate (secure) the position of the respective filament coil.

According to the X-ray tube assembly having the above-described structure of the third embodiment, the X-ray tube1comprises an anode target60configured to radiate X-rays by incidence of an electron beam, a cathode10comprising an electron converging cup15, and a vacuum envelope70accommodating the anode target60and the cathode10.

Each of the trench portions (first to third trench portions16to18) comprises a closest inner circumferential wall53, an upper inner circumferential wall51and a lower inner circumferential wall52. The cross section of the lower inner circumferential wall52in a plane in the second direction db and third direction dc may have a curved surface shape. In this case as well, a similar advantageous effect to that of the first embodiment can be obtained by adjusting the dimensions of the lower inner circumferential wall52.

The lower inner circumferential wall52can be processed using a ball end mill. Therefore, the processing cost of the lower inner circumferential wall52can be reduced as compared to the first embodiment previously described.

As described above, it is possible to realize an X-ray tube1which can make the electron density distribution uniform within a focal spot and obtain a focal spot of desirable dimensions, and also an X-ray tube assembly comprising such an X-ray tube1. Further, the above-described X-ray tube1can prevent the occurrence of both filament touch and electric breakdown between the filament coils and electron converging cup15at the same time.

It should be noted that the embodiments and modifications discussed here are presented merely examples, and are not intended to limit the scope of each embodiment. These novel embodiments can be carried out in various modifications, and they may be subjected to various omissions, replacements and variations as long as the essence of the embodiments remains. These embodiments and modifications naturally fall within the scope of the embodiments and are covered by the embodiments recited in the claims as well as their equivalencies.

For example, each of the trench portions (first to third trench portions16to18) may further comprises one or more other upper inner circumferential walls located on the respective opening (openings16ato18a) side than the closest inner circumferential wall53and having dimensions larger than those of the closest inner circumferential wall53, and/or one or more other lower inner circumferential walls located on the opposite side to the upper inner circumferential walls51with respect to the closest inner circumferential wall53and having dimensions larger than those of the closest inner circumferential wall53.

Each of the trench portions (first to third trench portions16to18) may further comprise one or more other closest inner circumferential walls shorter than a dimension of the respective filament coil (electron emission source) in the depth direction of the trench portion (third direction dc), and faces the filament coil with a narrowest gap between said other closest inner circumferential walls and the filament coil over an entire circumference thereof in the width direction of the electron emission source.

The upper inner circumferential wall51may be formed into a squarish, a circular or an ovally rounded rectangle.

The cross section of the lower inner circumferential wall52in a plane in the second direction db and third direction dc may have the shape of a circle, an ovally rounded rectangle or a portion thereof.

The first to third filament coils11to13may be of different types from each other, or they may differ from each other in properties (electron emission amount). For example, the dimensions of a respective one of the filament coils may be varied to change the dimensions of the focal spot.

The number of filament coils (electron emission sources) and trench portions provided in the cathode10is not limited to 3, but the structure may be modified in various ways to have 1, 2 or 4 or more of coils or trench portions.

The electron emission sources may be modified in various ways, and for example, any type of thermoelectron emission source can be employed. Further, such a thermoelectron emission source may not be a filament coil. An electron emissive material may be made of a material comprising, for example, lanthanum boride (LaB6) as a main component.

The X-ray tube assemblies of these embodiments are not limited to those described above, but may be modified in various ways. Thus, the embodiments are applicable to various types of X-ray tube assemblies, such as a stationary anode X-ray tube assembly.