Patent Publication Number: US-7720199-B2

Title: X-ray tube and X-ray source including same

Description:
TECHNICAL FIELD 
   The present invention relates to an X-ray tube taking out X-rays generated wherein toward an exterior, and an X-ray source in which the X-ray tube and a power supply unit are configured integrally. 
   BACKGROUND ART 
   X-rays are electromagnetic waves that are highly transmitted through objects and are frequently used for nondestructive, noncontact observation of internal structures of objects. As a conventional X-ray irradiation apparatus applicable to such fields, an X-ray tube, described in Patent Document 1 indicated below, is known. An X-ray generating unit of the X-ray tube described in Patent Document 1 has a tubular casing that houses a target, and an exhaust pipe, put in communication with an internal space, is mounted to the casing (see FIG. 4, etc., of Patent Document 1). In manufacturing the X-ray tube, vacuum is drawn from the internal space of the casing via the exhaust pipe. After vacuum drawing, the exhaust pipe is closed and the internal space that houses the target is put in a vacuum state (state of being depressurized to a predetermined degree of vacuum). 
   Patent Document 1: U.S. Pat. No. 6,229,876 
   DISCLOSURE OF THE INVENTION 
   Problems that the Invention is to Solve 
   The present inventors have examined the conventional X-ray tubes, and as a result, have discovered the following problems. That is, in the conventional X-ray tube, the exhaust port for drawing vacuum is formed in an inner wall surface of the casing onto which the exhaust pipe is mounted, and at an edge of the exhaust port, a corner portion with a sharp tip is present at a boundary with the casing inner wall. When a high potential difference is generated across the casing and an anode during driving of the X-ray tube, an electric field across the casing and the anode may become disrupted due to an influence of the corner portion. A possibility of discharge occurring across the casing and a tip of the anode thus increases due to the presence of the corner portion that is inevitably formed due to forming of the exhaust port. However, in the conventional X-ray tube, no measures are taken to suppress such discharge and there was a possibility of destabilization of the X-ray output due to such discharge. 
   The present invention has been developed to eliminate the problems described above. It is an object of the present invention to provide an X-ray tube having a structure for effectively suppressing discharge at a tip of an anode that is irradiated with electrons to generate X-rays, and to provide an X-ray source including the X-ray tube. 
   Means for Solving the Problems 
   An X-ray tube according to the present invention irradiates X-rays generated at an X-ray target to an exterior by making electrons emitted from an electron gun be incident on the X-ray target of an anode. The X-ray tube comprises a casing, an irradiation window (X-ray emission window) disposed on the casing; an exhaust port, and a shielding structure. The casing defines an internal space housing a tip of the anode that is irradiated with electrons. The irradiation window is disposed on the casing defining the internal space, in order to take out the X-rays generated at the X-ray target to the exterior of the casing. The exhaust port is prepared for vacuum drawing of the internal space and is disposed at an inner wall surface of the casing. In particular, the shielding structure is disposed in the internal space of the casing so as to hide the exhaust port from the tip of the anode. 
   Here, as a first aspect, the shielding structure preferably includes a shielding member comprised of a conductive material and having an inner side surface that faces the tip of the anode, and an outer side surface opposing the inner side surface. 
   In the X-ray tube having the above-described structure, the exhaust port is disposed at the inner wall surface of the casing. A corner portion with a sharp tip is thus formed as a boundary between an edge of the exhaust port and the inner wall surface of the casing. The present X-ray tube is thus provided with a structure, with which the exhaust port is hidden from the tip of the anode by the shielding member. Thus, in this X-ray tube, disruption of an electric field across the anode and the edge of the exhaust port during driving is alleviated and discharge at the tip of the anode is suppressed effectively. 
   In order to exhibit the above action effectively, the shielding member is preferably disposed between the tip of the anode and the exhaust port in a state of being separated by a predetermined distance from the inner wall surface at the exhaust port side of the casing. In addition, at least the inner side surface of the shielding member that faces the tip of the anode preferably has an area larger than an opening area of the exhaust port. In this configuration, the edge of the exhaust port (the corner portion with the sharp tip) can be covered reliably. Also, during manufacture of the X-ray tube, vacuuming of the internal space can be performed using a gap between the shielding member and the inner wall surface at the exhaust port side as a passage for air. 
   The shielding member may also be disposed in the internal space in a state of being separated by a predetermined distance from an inner wall surface at the irradiation window side of the casing. In this configuration, during manufacture of the X-ray tube, vacuuming of the internal space can be performed using a gap between the shielding member and the inner wall surface at the irradiation window side as a passage for air. 
   The shielding member may be provided with a plurality of through holes each communicating between the inner side surface facing the tip of the anode and the outer side surface opposing the inner side surface. In this case, at the tiem of vacuuming the internal space during manufacture of the X-ray tube, the through holes serve as passages for air from the internal space and vacuum drawing can thus be performed efficiently. 
   The shielding member may be a part of the casing that extends from an inner wall surface of the casing to the internal space. In this case, the inner side surface of the shielding member that opposes the tip of the anode is matched with the inner wall surface of the portion of the casing. In this configuration, the surface of the shielding member and the inner wall surface of the casing can be made smoothly continuous with respect to each other. Disruption of the electric field is thus alleviated and the discharge at the tip of the anode can be suppressed further. 
   The shielding member may have a plurality of through holes each putting the inner side surface and the outer side surface in communication, and be disposed so that the inner side surface facing the tip of the anode is matched with the inner wall surface of the casing. In this case, because the exhaust port is closed by the shielding member, the shielding member is required to have the plurality of through holes that serve as passages for air during vacuum drawing. In the X-ray tube, because the shielding member that closes the exhaust port is formed flush to the inner wall surface of the casing at which the exhaust port is formed, a corner portion with a sharp tip does not appear at the edge of the exhaust port and disruption of the electric field across the tip of the anode and the exhaust port is alleviated. As a result, the discharge at the tip of the anode is suppressed effectively. Because the plurality of communicating holes formed in the shielding member serve as passages for air, vacuum drawing of the internal space during manufacture can also be carried out without any problem. 
   Also, in the X-ray tube according to the present invention, the shielding structure may be realized according to a second aspect that differs from the first aspect described above. Specifically, the casing may be constituted of a first anode housing portion and a second anode housing portion, and an inner tubular member may be disposed as the shielding structure in the internal space of the casing. The first anode housing portion is a hollow member comprised of a conductive material, the first anode housing portion surrounding the tip of the anode that has the exhaust port disposed at an inner wall surface thereof and having the irradiation window. The second anode housing portion defines an internal space for housing the anode together with the first anode housing portion, by being joined to the first anode housing portion. The inner tubular member that is the shielding structure of the second mode is a hollow member disposed in the internal space of the casing so as to surround at least the tip of the anode and, by a part thereof being positioned between the inner wall surface of the first anode housing portion and the tip of the anode in a state of being separated by a predetermined distance from the inner wall surface of the first anode housing portion, functions to hide the exhaust port from the tip of the anode. 
   In the X-ray tube having the above-described shielding structure of the second aspect, the exhaust port, disposed at the inner wall surface of the first anode housing portion, is hidden from the tip of the anode by the inner tubular member, at least a part of which is positioned between the tip of the anode and the inner wall surface of the first anode housing portion. Thus, in this X-ray tube, even when a corner portion appears as a boundary between the edge of the exhaust port and the inner wall surface of the first anode housing portion, disruption of the electric field across the anode and the edge of the exhaust port during driving is alleviated by the inner tubular member. Also, because discharge at the tip of the anode is suppressed effectively, destabilization of X-ray output of the X-ray tube is suppressed. During manufacture of the X-ray tube, vacuuming of the internal space can be performed using a gap between the inner tubular member and the inner wall surface of the first anode housing portion as a passage for air. 
   Even when the above-described inner tubular member is employed as the shielding structure according to the second mode, a gap is preferably formed between an end of the inner tubular member and an inner wall surface at the irradiation window side of the first anode housing portion. In this configuration, during manufacture of the X-ray tube, vacuum drawing of the internal space can be performed using the gap between the inner tubular member and the inner wall surface at the irradiation window side of the first anode housing portion as a passage for air. 
   The inner tubular member preferably has a plurality of through holes disposed at least at a part positioned between the inner wall surface of the first anode housing portion and the tip of the anode. In this case, because the through holes themselves serve as passages for air from the internal space during vacuum drawing of the internal space during manufacture, the vacuum drawing can be performed efficiently. 
   In the X-ray tube according to the present invention, the first anode housing portion preferably has a head comprised of a conductive material, and the second anode housing portion having a bulb comprised of an electrically insulating material and a connecting portion comprised of a conductive material, the connecting portion being joined to an end of the bulb and to the head of the first anode housing portion. In this configuration, the inner tubular member has a shape that extends toward the second anode housing portion side in the internal space so as to hide a joined portion of the bulb and the connecting portion from the anode. That is, in this X-ray tube, discharge occurs comparatively readily across the anode and the joined portion of the bulb comprised of the electrically insulating material, and the connecting portion comprised of the conductive material. Thus, in this X-ray tube, the joined portion is hidden from the anode by employment of the inner tubular member with the above-described structure. Disruption of the electric field across the joined portion and the anode is thus alleviated and the discharge across the joined portion and the anode is suppressed effectively. As a result, destabilization of the X-ray output of the X-ray tube is suppressed. 
   In the X-ray tube according to the present invention, the second anode housing portion preferably has a bulb comprised of an electrically insulating material, and the first anode housing portion has a head comprised of a conductive material, and a connecting portion comprised of a conductive material, the connecting portion being disposed at an end of the head and joined to the bulb of the second anode housing portion. The inner tubular member preferably has a shape that extends toward the second anode housing portion side in the internal space so as to hide a joined portion of the bulb and the connecting portion from the anode. In the X-ray tube with this structure, discharge occurs comparatively readily across the anode and the joined portion of the bulb comprised of the electrically insulating material, and the connecting portion comprised of the conductive material. Thus, in this X-ray tube, the joined portion is hidden from the anode by employment of the inner tubular member with the above-described structure. Disruption of the electric field across the joined portion and the anode is thus alleviated and the discharge across the joined portion and the anode is suppressed effectively. As a result, destabilization of the X-ray output of the X-ray tube is suppressed. 
   The inner tubular member may have a loopback portion, at which an end at the second anode housing portion side is looped back into a round shape. In this case, it is preferable that a tip of the loopback portion is joined to the first anode housing portion and a through hole is formed in the loopback portion. In this configuration, because the second anode housing portion side end of the inner tubular member has the round shape, a corner portion with a sharp tip is not formed. Disruption of the electric field across the end and the anode is thus suppressed effectively. As a result, discharge across the end and the anode is suppressed and destabilization of the X-ray output of the X-ray tube can be suppressed. Also, in this case, a space is formed in a region surrounded by the looped back inner tubular member and the first anode housing portion. However, because the through hole formed in the loopback portion serves as a passage for air during vacuum drawing of the internal space in the manufacture of the X-ray tube, retention of air in this space is prevented. 
   Furthermore, an X-ray source according to the present invention comprises the X-ray tube with the above-described structure (X-ray tube according to the present invention), and a power supply unit supplying a voltage for generating X-rays at the X-ray target toward the anode at which the X-ray target is disposed. 
   The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will be apparent to those skilled in the art from this detailed description. 
   Effects of the Invention 
   In accordance with the X-ray tube according to the present invention, by employment of a special shielding structure inside the casing, discharge at the tip of the anode is suppressed effectively. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an arrangement of a first embodiment of an X-ray tube according to the present invention; 
       FIG. 2  is a vertical sectional view of the X-ray tube according to the first embodiment shown in  FIG. 1 ; 
       FIG. 3  is a horizontal sectional view of the X-ray tube according to the first embodiment shown in  FIG. 1 ; 
       FIG. 4  is a perspective view of an arrangement of a first modification example of the X-ray tube according to the first embodiment; 
       FIG. 5  is a sectional view of the X-ray tube shown in  FIG. 4  (first modification example of the X-ray tube according to the first embodiment); 
       FIG. 6  is a perspective view of an arrangement of a second modification example of the X-ray tube according to the first embodiment; 
       FIG. 7  is a sectional view of the X-ray tube shown in  FIG. 6  (second modification example of the X-ray tube according to the first embodiment); 
       FIG. 8  is a perspective view of an arrangement of a third modification example of the X-ray tube according to the first embodiment; 
       FIG. 9  is a sectional view of the X-ray tube shown in  FIG. 8  (third modification example of the X-ray tube according to the first embodiment); 
       FIG. 10  is a perspective view of an arrangement of a second embodiment of an X-ray tube according to the present invention; 
       FIG. 11  is an exploded perspective view of the X-ray tube according to the second embodiment shown in  FIG. 10 ; 
       FIG. 12  is a sectional view of the X-ray tube according to the second embodiment shown in  FIG. 10 ; 
       FIG. 13  is a sectional view taken across a central axis of an exhaust tube of the X-ray tube according to the second embodiment shown in  FIG. 10 ; 
       FIG. 14  is a sectional view of a vicinity of a mounting portion of the exhaust tube of the X-ray tube according to the second embodiment shown in  FIG. 10 ; 
       FIG. 15  is a sectional view of an arrangement of a first modification example of the X-ray tube according to the second embodiment; 
       FIG. 16  is a sectional view of principal portions of a second modification example of the X-ray tube according to the second embodiment, that is, a modification example of the X-ray tube shown in  FIG. 15  (first modification example of the X-ray tube according to the second embodiment); 
       FIG. 17  is a sectional view of an arrangement of a third modification example of the X-ray tube according to the second embodiment; 
       FIG. 18  is an exploded perspective view of an arrangement of an embodiment of an X-ray source according to the present invention; 
       FIG. 19  is a sectional view of an internal structure of the X-ray source according to the embodiment; and 
       FIG. 20  is a front view for describing actions of the X-ray source (including the X-ray tube according to the embodiment) incorporated in an X-ray generating apparatus of a nondestructive inspection apparatus. 
   

   DESCRIPTION OF THE REFERENCE NUMERALS 
     1 A,  1 B,  1 C,  1 D,  2 A,  2 B,  2 C,  2 D . . . X-ray tube;  3  . . . electron gun;  5  . . . anode;  5   a  . . . anode tip;  9  . . . body portion (second anode housing portion);  9   a  . . . bulb;  9   b  . . . connecting portion;  9   c  . . . fused portion (joined portion);  13  . . . head (first anode housing portion);  14  . . . electron gun housing unit;  15  . . . irradiation window;  17 ,  57  . . . exhaust port;  19 ,  59  . . . exhaust port side inner wall surface;  25 ,  61 ,  63 ,  65  . . . shielding member;  29  . . . irradiation window side inner wall surface;  31 ,  33 ,  35  . . . inner tubular member;  31   d  . . . loopback portion;  31   e  . . . free end of loopback portion;  31   f  . . . through hole;  31   k  . . . communicating hole;  58  . . . inner wall surface;  61   a ,  63   a  . . . shielding member surface;  63   f,    65   f  . . . communicating hole; R . . . internal space; d 1 , d 2 , d 3 , d 4 , S 1 , S 2  . . . gap;  100  . . . X-ray source;  102  . . . power supply unit;  102 A . . . insulating block;  102 B . . . high voltage generating unit;  102 C . . . high voltage line;  102 D . . . socket;  103  . . . first plate member;  103 A . . . screw insertion hole;  104  . . . second plate member;  104 A . . . screw insertion hole;  105  . . . fastening spacer member;  150 A . . . screw hole;  106  . . . metal tubular member;  106 A . . . mounting flange;  106 B . . . relief surface;  106 C . . . insertion hole;  108  . . . conductive coating;  109  . . . fastening screw;  110  . . . high voltage insulation oil; XC . . . X-ray camera; SP . . . sample plate; P . . . observation point; and XP . . . X-ray generation point. 
   BEST MODES FOR CARRYING OUT THE INVENTION 
   In the following, embodiments of an X-ray tube and an X-ray source, including the X-ray tube according to the present invention will be explained in detail with reference to  FIGS. 1 to 20 . In the description of the drawings, identical or corresponding components are designated by the same reference numerals, and overlapping description is omitted. 
   First Embodiment 
   First, a first embodiment of an X-ray tube according to the present invention will be explained with reference to  FIGS. 1 to 3 .  FIG. 1  is a perspective view of an arrangement of the first embodiment of the X-ray tube according to the present invention.  FIG. 2  is a vertical sectional view of the X-ray tube according to the first embodiment shown in  FIG. 1 .  FIG. 3  is a horizontal sectional view of the X-ray tube according to the first embodiment shown in  FIG. 1 . 
   As shown in  FIGS. 1 to 3 , the X-ray tube  1 A makes electrons, emitted from an electron gun  3 , be incident on a target  5   d , which is an electron incidence portion (X-ray generating portion) disposed at a tip  5   a  of an anode  5  in vacuum, and irradiates X-rays, generated as a result of the incidence of electrons, to an exterior. The X-ray tube  1 A includes a glass bulb  9 , holding the rod-like anode  5  in an insulated state, and an X-ray generating unit  11 , housing the anode tip  5   a  and generating X-rays. 
   The X-ray generating unit  11  has a head  13 , which is a metal casing that houses the anode tip  5   a , and substantially the entirety of the anode  5  is housed in a sealed internal space R, defined by the head  13  and the bulb  9 , in a state of being insulated from the head  13 . An inclined surface  5   c  is disposed at an end surface of the anode tip  5   a , and on the inclined surface  5   c  is disposed the target  5   d  that generates X-rays with a desired energy upon the incidence of electrons. The anode tip  5   a  is surrounded by an inner wall surface  19  of the head  13  forming a cylindrical surface coaxial to the anode  5 . The electron gun  3  is housed in an electron gun housing unit  14 , mounted onto the head  13 , and a tip of the electron gun  3  is directed toward the anode tip  5   a.  That is, an axial line of the electron gun  3  and an axial line of the anode  5  are made substantially orthogonal to each other so that the electrons emitted from the electron gun  3  are made incident on the target  5   d  on the inclined surface  5   c , formed so as to face the electron gun  3 . Furthermore, at an end at the anode tip  5   a  side of the head  13  is disposed a circular irradiation window  15  (X-ray emitting window) comprised of a material of high X-ray transmittance for transmitting the X-rays generated at the target  5   d  and thereby irradiating the X-rays to the exterior. 
   In order to put the internal space R in a vacuum state (a state of being decompressed to a predetermined degree of vacuum), an exhaust port  17 , for evacuating air inside the internal space R, is disposed at the inner wall surface  19  of the head  13 . On the other hand, an exhaust tube  21 , put in communication with the internal space R via the exhaust port  17 , is mounted on an outer wall surface of the head  13 . In manufacturing the X-ray tube, by performing vacuum drawing of the internal space R via the exhaust port  17  and the exhaust tube  21  and thereafter closing the tube opening by squashing the exhaust tube  21 , etc., the internal space R is sealed in a vacuum state. In this process, the exhaust port  17  is left open to the internal space R even after completion of assembly of the X-ray tube. 
   In the X-ray tube  1 A, a base end  5   b  (high voltage application portion) of the anode  5 , exposed from the bulb  9 , is connected to a high voltage supply circuit. During driving, a high voltage of approximately 100 kV is applied from the high voltage supply circuit to the anode  5  via the base end  5   b . When the electrons emitted from the electron gun  3  in this state become incident on the target  5   d , X-rays are generated from the target  5   d  by the incidence of electrons. The generated X-rays are transmitted through the irradiation window  15  and irradiated to the exterior. 
   Because the high voltage is thus applied to the anode  5  during driving, a high potential difference arises across the anode  5  and the head  13 , which is the metal casing. In particular, because the anode tip  5   a  is housed so as to be surrounded by the head  13 , there is a problem of discharge occurring across the anode tip  5   a  and the inner wall surface  19  of the head  13 . Here, at an edge of the exhaust port  17 , formed in the inner wall surface  19 , a corner portion with a sharp tip is present as a boundary with the inner wall surface  19 . An electric field across the anode  5  and the head  13  is disrupted due to an influence of the corner portion, and consequently, there is an especially high possibility of discharge occurring across the edge of the exhaust port  17  and the anode tip  5   a . Because when the discharge occurs, problems, such as destabilization of an X-ray output of the X-ray tube  1 A, occur, the discharge must be suppressed. 
   Thus, in the X-ray tube  1 A, in order to suppress the discharge across the edge of the exhaust port  17  and the anode tip  5   a , a special shielding structure (first mode) is employed. That is, a partitioning-screen-like shielding member  25 , hiding the exhaust port  17  from the anode tip  5   a , is disposed between the anode tip  5   a  and the exhaust port  17 . The shielding member  25  is a flat plate member comprised of a conductive material, the shielding member  25  being processed to a rectangular shape and having an area larger than an open aperture of the exhaust port  17 . The shielding member  25  has two opposing sides fixed to the inner wall surface  19  and is disposed so as to cover the exhaust port  17  across a gap d 1  from the inner wall surface  19  at a central portion. The shielding member  25  extends very close to an inner wall surface  29 , on which the irradiation window  15  is disposed, so that a small gap d 2  is formed between the shielding member  25  and the inner wall surface  29 . By the shielding member  25 , the edge of the exhaust port  17  is prevented from being viewed from the anode tip  5   a.    
   In the X-ray tube  1 A, by such a shielding member  25  being disposed, disruption of the electric field across the anode tip  5   a  and the edge of the exhaust port  17  is alleviated. Discharge across the anode tip  5   a  and the edge of the exhaust port  17  is thus suppressed. Also, by the gaps d 1  and d 2 , an interior of the exhaust tube  21  and the internal space R are put in communication, and because the gaps d 1  and d 2  function as passages for air, vacuum drawing of the internal space R via the exhaust port  17  can be performed without any problem during manufacture. Although vacuum drawing will take some time, the shielding member  25  may be disposed so that the gap d 2  is not formed. In this case, vacuum drawing can be performed using just the gap d 1  as a passage for air. The shielding member  25  is not limited to being a flat plate member and may be a curved plate member with a curvature larger than that of the inner wall surface of the head  13 . 
   (First Modification Example of the X-ray Tube According to the First Embodiment) 
   Subsequently, a first modification example of the X-ray tube according to the first embodiment will be explained with reference to  FIGS. 4 and 5 .  FIG. 4  is a perspective view of an arrangement of the first modification example of the X-ray tube according to the first embodiment.  FIG. 5  is a sectional view of the X-ray tube  1 B shown in  FIG. 4 . 
   The X-ray tube  1 B, shown in  FIGS. 4 and 5 , differs from the X-ray tube  1 A of the first embodiment in a shielding member structure that hides an exhaust port  57  from the anode tip  5   a . In the X-ray tube  1 B, the exhaust port  57  is positioned at an inner wall surface  59  formed by digging into a part of an inner wall surface  58  in a direction of an outer wall surface of the head  13 . A shielding member  61  for hiding the exhaust port  57  from the anode tip  5   a  is disposed between the exhaust port  57  and the anode tip  5   a . The shielding member  61  has an inner side surface  61   a , facing the anode tip  5   a  and being matched with the inner wall surface  58  (and being practically a part of the head  13  in the present modification example), and has a rectangular shape with an area larger than the open aperture of the exhaust port  57 . The shielding member  61  is disposed so that a gap d 3  is formed across from the exhaust port  57 . The shielding member  61  extends very close to an inner wall surface  29 , on which the irradiation window  15  is disposed, so that a small gap d 4  is formed between the shielding member  61  and the inner wall surface  29 . By the shielding member  61 , the edge of the exhaust port  57  is prevented from being viewed from the anode tip  5   a.    
   The shielding member  61  and the exhaust port  57  with the above-described structure is prepared by carving out a region of rectangular parallelepiped shape sandwiched between the shielding member  61  and the inner wall surface  59  in the head  13  while leaving the shielding member  61  and thereafter forming the exhaust port  57  and the gap d 4 . Or, the inner wall surface  59  may be formed by digging into the inner wall surface  58  and, after forming the exhaust port  57  in the inner wall surface  59 , installing the shielding member  61  as a separate member so that its inner side surface is matched with the inner wall surface  58 . 
   In the X-ray tube  1 B, by the provision of the shielding member  61 , disruption of an electric field across the anode tip  5   a  and the exhaust port  57  is alleviated. Discharge across the anode tip  5   a  and the edge of the exhaust port  57  can thus be suppressed. Also, by the gaps d 3  and d 4 , the interior of the exhaust tube  21  and the internal space R are put in communication, and because the gaps d 3  and d 4  function as passages for air, vacuum drawing of the internal space R via the exhaust port  57  can be performed without any problem during manufacture. Also, by the inner side surface  61   a  of shielding member  61  being matched with the inner wall surface  58  that surrounds the anode tip  5   a , the inner side surface  61   a  of the shielding member  61  is made smoothly continuous with the inner wall surface  58 . In this configuration, disruption of the electric field around the target tip  5   a  due to the shielding member  61  can thus be minimized. 
   (Second Modification Example of the X-ray Tube According to the First Embodiment) 
   Subsequently, a second modification example of the X-ray tube according to the first embodiment will be explained with reference to  FIGS. 6 and 7 .  FIG. 6  is a perspective view of an arrangement of the second modification example of the X-ray tube according to the first embodiment.  FIG. 7  is a sectional view of the X-ray tube  1 C shown in  FIG. 6 . 
   The X-ray tube  1 C, shown in  FIGS. 6 and 7  differs from the X-ray tube  1 B of the second embodiment in a structure of a shielding member  63 . The shielding member  63  is a mesh-like conductive member provided with a plurality of through holes  63   f  and has the same shape as the above-described shielding member  61 . The shielding member  63  is formed so that an inner side surface  63   a , facing the anode tip  5   a , is matched with the inner wall surface  58  that surrounds the anode tip  5   a.    
   Even in accordance with the shielding member  63 , by making the through holes  63   f  fine, disruption of the electric field across the anode tip  5   a  and the edge of the exhaust port  57  is alleviated in similar to the shielding member  61  in the X-ray tube  1 B. Discharge across the anode tip  5   a  and the edge of the exhaust port  57  can thus be suppressed effectively with the X-ray tube  1 C as well. Because in the process of vacuum drawing of the internal space R during manufacture not only the gaps d 3  and d 4  but the through holes  63   f  also function as passages for air, smooth vacuum drawing is enabled. As a hole diameter of the through holes  63   f,  0.1 to 1 mm is preferable for alleviating the disruption of the electrical field and performing smooth vacuum drawing. 
   (Third Modification Example of the X-ray Tube According to the First Embodiment) 
   A third modification example of the X-ray tube according to the first embodiment shall now be described with reference to  FIGS. 8 and 9 .  FIG. 8  is a perspective view of an arrangement of the third modification example of the X-ray tube according to the first embodiment.  FIG. 9  is a sectional view of the X-ray tube  1 D shown in  FIG. 8 . 
   The X-ray tube  1 D, shown in  FIGS. 8 and 9 , differs from the X-ray tube  1 A of the first embodiment in a structure of a shielding member that hides the exhaust port  17  from the anode tip  5   a . The shielding member  65  is a mesh-like conductive member, provided with a plurality of through holes  65   f  and disposed so as to close the exhaust port  17  while an inner side surface, facing the anode  5 , is matched with the inner wall surface  19 . 
   In the shielding member  65 , because an end portion does not appear at the inner wall surface  19  at the edge of the exhaust port  17 , disruption of the electric field across the anode tip  5   a  and the edge of the exhaust port  17  is alleviated. Discharge across the anode tip  5   a and the edge of the exhaust port  17  can thus be suppressed. Also, the interior of the exhaust tube  21  and the internal space R are put in communication by the plurality of through holes  65   f , provided in the shielding member  65 , and the through holes  65   f  function as passages for air. Vacuum drawing of the internal space R via the exhaust port  17  can thus be performed without any problem during manufacture. As a hole diameter of the through holes  65   f,  0.1 to 1 mm is preferable for alleviating the disruption of the electrical field and performing smooth vacuum drawing. 
   The present invention is not restricted to the above-described first embodiment and modification examples thereof and can be modified variously. For example, although the target  5   d  is disposed as a separate member on the inclined surface  5   c  of the anode  5 , the anode  5  and the target  5   d  can be configured integrally so that a part of the inclined surface  5   c  constitutes the target. Also, although the anode  5  has a shape having the inclined surface  5   c  disposed at the tip of a cylindrical column, other shapes can be provided at the tip of the anode  5  by any of various types of carving. In this case, even if a corner-like portion is present at the tip of the anode, discharge across the anode tip and the exhaust port can be suppressed effectively by the shielding member. 
   Second Embodiment 
   Next, an arrangement of a second embodiment of an X-ray tube according to the present invention will be explained with reference to Vs.  10  to  14 .  FIG. 10  is a perspective view of the arrangement of the second embodiment of the X-ray tube according to the present invention.  FIG. 11  is an exploded perspective view of the X-ray tube  2 A according to the second embodiment shown in  FIG. 10 .  FIG. 12  is a sectional view of the X-ray tube  2 A according to the second embodiment shown in  FIG. 10 .  FIG. 13  is a sectional view taken across a central axis of an exhaust tube of the X-ray tube  2 A according to the second embodiment shown in  FIG. 10 .  FIG. 14  is a sectional view of a vicinity of a mounting portion of the exhaust tube of the X-ray tube  2 A according to the second embodiment shown in  FIG. 10 . 
   As shown in  FIGS. 10 to 13 , in similar to the X-ray tube  1 A according to the first embodiment, the X-ray tube  2 A makes electrons, emitted from the electron gun  3 , be incident on the target  5   d , which is the electron incidence portion (X-ray generating portion) disposed at the tip  5   a  of the anode  5  in vacuum, and irradiates X-rays, generated as the result of the incidence of electrons, to the exterior. The X-ray tube  2 A includes a body portion (second anode housing portion)  9 , holding the rod-like anode  5  in an insulated state, and the head (first anode housing portion)  13 , which is the metal casing that surrounds the anode tip  5   a.  The body portion  9  is constituted of a bulb  9   a  comprised of glass, which is an electrically insulating material, and a connecting portion  9   b  connecting the bulb  9   a  and the head  13 . One end side of the bulb  9   a  is open and the other end side holds the anode  5 . At the open side of the bulb  9   a , one end of the cylindrical connecting portion  9   b , which is comprised of metal, is joined by fusing. An outwardly extending flange is disposed at the other end of the connecting portion  9   b , and the connecting portion  9   b  is welded to the head  13  at this flange. That is, the bulb  9   a  and the head  13  are connected via the connecting portion  9   b.  By the bulb  9   a , the head  13 , and the connecting portion  9   b  that are thus connected, the sealed internal space R is defined. Substantially the entirety of the anode  5  is housed inside the internal space R in a state of being insulated from the head  13  and the connecting portion  9   b . The inclined surface  5   c  is disposed at the anode tip  5   a , and on the inclined surface  5   c  is disposed the target  5   d  that generates the X-rays with the desired energy upon the incidence of electrons. 
   As another example, the first anode housing portion may be configured by integrally disposing the tubular connecting portion  9   b , for fusing with the bulb  9   a , at an end of the head  13 . In this case, the bulb  9   a  constitutes the second anode housing portion. 
   The head  13  has inner wall surfaces  19  and  20 , constituting cylindrical surfaces coaxial to the anode  5 , and the anode tip  5   a  is surrounded by the inner wall surfaces  19  and  20 . The electron gun housing unit  14 , housing the electron gun  3 , is mounted to a mounting hole  13   a , formed so as to penetrate through a side wall of the head  13 . The electron gun  3  is positioned while the axial line of the electron gun  3  and the axial line of the anode  5  are made substantially orthogonal to each other. That is, the tip of the electron gun  3  is directed toward the anode tip  5   a  so that the electrons emitted from the electron gun  3  are made incident on the target  5   d  on the inclined surface  5   c , formed so as to face the electron gun  3 . Furthermore, at the end at the anode tip  5   a  side of the head  13 , which is the metal casing, is disposed the circular irradiation window  15  (X-ray emitting window) comprised of a material of high X-ray transmittance for transmitting the X-rays generated at the target  5   d  and thereby irradiating the X-rays to the exterior. 
   In order to put the internal space R in a vacuum state (a state of being decompressed to a predetermined degree of vacuum), the exhaust port  17 , for evacuating air inside the internal space R, is disposed at the inner wall surface  19  of the head  13 . Furthermore, the exhaust tube  21 , put in communication with the internal space R via the exhaust port  17 , is mounted on the outer wall surface of the head  13 . In manufacturing the X-ray tube, by performing vacuum drawing of the internal space R via the exhaust port  17  and the exhaust tube  21  and thereafter closing the tube opening by squashing the exhaust tube  21 , etc., the internal space R is sealed in a vacuum state. In this process, the exhaust port  17  is left open to the internal space R even after completion of assembly of the X-ray tube. Although, in the present embodiment, the exhaust port  17  is formed at an inner wall surface  19  position diagonally in front of the mounting hole  13   a , the exhaust port  17  may be formed at any position of the inner wall surface  19  or  20 . 
   In the X-ray tube  2 A, the base end  5   b  (high voltage application portion) of the anode  5 , exposed from the bulb  9 , is connected to the high voltage supply circuit. During driving, the high voltage of approximately 100 kV is applied from the high voltage supply circuit to the anode  5 , including the target  5   d , via the base end  5   b . When the electrons emitted from the electron gun  3  in this state become incident on the target  5   d , X-rays are generated from the target  5   d  by the incidence of electrons. The generated X-rays are transmitted through the irradiation window  15  and irradiated to the exterior. In similar to the first embodiment, the terms, “upper,” “lower,” etc., are used with the irradiation window  15  side being the upper side and the base end  5   b  side of the anode  5  being the lower side in the description of the second embodiment as well. 
   Because the high voltage is thus applied to the anode  5  during driving, a high potential difference arises across the anode  5  and the head  13 . In particular, the anode tip  5   a  is housed so as to be surrounded by the head  13 . There is thus a problem of discharge occurring across the anode tip  5   a  and the inner wall surface  19  of the head  13 . Here, as shown in  FIG. 14 , at the edge of the exhaust port  17 , formed in the inner wall surface  19 , an abrupt corner portion  17   e  appears at a boundary between an inner wall surface  21   a  of the exhaust tube  21  and an end surface  21   b  of the exhaust tube  21  and an abrupt corner portion  17   f  appears at a boundary between the exhaust port  17  and the inner wall surface  19 . The electric field across the anode  5  and the head  13  is disrupted due to influence of the corner portions  17   e  and  17   f . Consequently, there is an especially high possibility of discharge occurring across the edge of the exhaust port  17  and the anode tip  5   a.  Because when the discharge occurs, problems, such as destabilization of the X-ray output of the X-ray tube  2 A, occur, the discharge must be suppressed. 
   Thus, in the X-ray tube  2 A, in order to suppress the discharge across the edge of the exhaust port  17  and the anode tip  5   a , a special shielding structure (second mode) is employed. That is, an inner tubular member  31  is disposed between the inner wall surface  19  of the head  13  and the anode tip  5   a . The inner tubular member  31  is a conductive member comprised of metal and has a thickness thinner than the head  13 , the inner tubular member  31  having a cylindrical shape that surrounds the anode tip  5   a . By the provision of such an inner tubular member  31 , in the X-ray tube  2 A, the exhaust port  17  is hidden from the anode tip  5   a . That is, the edge of the exhaust port  17  is prevented from being viewed from the anode tip  5   a.    
   The inner wall surface  20 , coaxial to the inner wall surface  19  of the head  13  and constituting a cylindrical surface slightly smaller in diameter than the inner wall surface  19 , is formed below the inner wall surface  19 . On the other hand, an outer diameter of the inner tubular member  31  is set substantially equal to an inner diameter of the head  13  at the inner wall surface  20 . By an outer wall surface  31   a  of the cylindrical portion  31  contacting the inner wall surface  20  across its entire periphery, the cylindrical portion  31  is disposed so as to be coaxial to the anode  5  and the inner wall surface  19  of the head  13 . By this positional relationship, a small gap S 1  is formed between the outer wall surface  31   a  of the inner tubular member  31  and the inner wall surface  19  of the head  13 . Furthermore, the inner tubular member  31  extends very close to the inner wall surface  29 , on which the irradiation window  15  is disposed, so that a small gap S 2  is formed between an upper end  31   b  of the inner tubular member  31  and the inner wall surface  29 . By the above structure, the internal space R is put in communication with the interior of the exhaust tube  21  via the gaps S 1  and S 2 , and in the process of vacuum drawing of the internal space R, the gaps S 1  and S 2  function as passages for air. 
   A lower end  31   c  side of the inner tubular member  31  protrudes from a lower end of the head  13  and extends below a fused portion (joined portion)  9   c  of the bulb  9   a  and the connecting portion  9   b . By this structure, the inner tubular member  31  is made present between the fused portion  9   c  and the target  5 . The fused portion  9   c  is thus hidden from view from the anode  5  by the inner tubular member  31 . The lower end  31   c  of the inner tubular member  31  is looped back into a round shape with a curved surface and a free end  31   e  of a loopback portion  31   d  facing the bulb  9   a  side is joined by brazing to a lower end surface  13   c  of the head  13 . 
   Because the lower end  31   c  of the inner tubular member  31  is thus looped back into the round shape, a corner portion does not appear at the lower end of the inner tubular member  31 . Disruption of an electric field across the inner tubular member lower end  31   c  and the anode  5  is thus suppressed, and discharge across the lower end  31   c  of the inner tubular member and the anode  5  can be suppressed effectively. Also, by the lower end  31   c  of the inner tubular member being looped back, a small space Q, surrounded by the looped back inner tubular member  31  and the lower end surface  13   c  of the head  13 , is formed. Through holes  31   f , for putting the small space Q in communication with the internal space R are thus formed in the loopback portion  31   d . The through holes  31   f  thus serve as passages for air during vacuum drawing of the internal space R and retention of air in the small space Q is prevented. 
   In the inner tubular member  31 , an insertion hole  31   h  is formed at a position corresponding to the electron gun  3 , and a tip  3   a  of a housing container that houses the electron gun  3  is inserted into the insertion hole  31   h  and becomes exposed at the anode tip  5   a  side. A pair of flat portions  31   p , parallel to the axial line of the electron gun  3 , are formed on the inner tubular member  31 . The flat portions  31   p  are positioned symmetrically so as to sandwich the insertion hole  31   h  in between and have shapes that bulge toward the anode tip  5   a  side from an inner wall surface  31   j . The flat portions  31   p  function as electrodes for putting the electric field, via which the electrons emitted from the electron gun  3  reach the target  5   d , into a desired state. 
   In the X-ray tube  2 A, by the provision of the above-described inner tubular member  31 , disruption of the electric field across the anode tip  5   a  and the edge of the exhaust port  17  is alleviated. Thus, discharge across the anode tip  5   a  and the edge of the exhaust port  17  is suppressed. As a result, in the X-ray tube  2 A, destabilization of the X-ray output due to discharge is suppressed and stable X-ray irradiation is enabled. Also, by the gaps S 1  and S 2 , the interior of the exhaust tube  21  and the internal space R are put in communication, and because the gaps S 1  and S 2  function as passages for air, vacuum drawing of the internal space R via the exhaust port  17  can be performed without any problem during manufacture of the X-ray tube  2 A. 
   Also, rear sides of the flat portions  31   p  are processed to shapes that are recessed from the outer wall surface  31   a . Thus a comparatively wide space, corresponding to the amount of recess from the outer wall surface  31   a , is formed between the inner wall surface  19  of the head  13  and the rear side of each flat portion  31   p . Because the exhaust port  17  is positioned in the comparatively wide space between the inner wall surface  19  and the rear side of one of the flat portions  31   p  so as to face the rear side of the flat portion  31   p , the passage of air is made good by the space and vacuum drawing of the internal space R via the exhaust port  17  during manufacture of the X-ray tube  2 A is thereby facilitated. 
   In assembling the inner tubular member  31  onto the head  13 , positioning in a direction of extension of the anode  5  is enabled by contacting of the tip  31   e  of the loopback portion with the lower end surface  13   c  of the head  13 . The positioning in a surface orthogonal to the direction of extension of the anode  5  is performed by making the outer wall surface  31   a  of the inner tubular member  31  contact the inner wall surface  20  of the head  13 . By such positioning of the inner tubular member  31  by the two surfaces of the inner wall surface  20  and the lower end surface  13   c  of the head  13 , the gaps S 1  and S 2 , which put the internal space R and the interior of the exhaust tube  21  in communication, can be formed with good precision. 
   The inner tubular member  31  is a separate member from the head  13 , and because the inner tubular member  31  can be prepared independently, the inner wall surface  31   j  that is smooth and high in precision is obtained. That is, because in comparison to directly subjecting the head  13  to processing for hiding the exhaust port  17  from the anode tip  5   a , it is easier to smoothen the inner wall surface  31   j  that faces the anode tip  5   a , the discharge across the anode tip  5   a  and the inner tubular member  31  can be suppressed effectively. 
   Also at the bulb  9   a  of the X-ray tube  2 A, a boundary between an insulating member and a conductive member is formed at the fused portion  9   c . Discharge to the anode  5  thus occurs comparatively readily. However, the above-described inner tubular member  31  extends to the bulb  9   a  side and the fused portion  9   c  of the bulb  9   a  and the connecting portion  9   b  is hidden from the anode  5  by the inner tubular member  31 . By this structure, disruption of an electric field across the fused portion  9   c  and the anode  5  is suppressed, and discharge across the fused portion  9   c  and the anode  5  is suppressed effectively. 
   Because, in the X-ray tube  2 A having the shielding structure of the second mode, the discharge at the anode  5  can be suppressed effectively, destabilization of the X-ray output due to the discharge is suppressed (stable X-ray irradiation can be performed). 
   (First Modification Example of the X-ray Tube According to the Second Embodiment) 
   Subsequently, a first modification example of the X-ray tube according to the second embodiment shall now be described with reference to  FIG. 15 .  FIG. 15  is a sectional view of an arrangement of the first modification example of the X-ray tube according to the second embodiment. 
   As shown in  FIG. 15 , the X-ray tube  2 B (first modification example of the X-ray tube according to the second embodiment) has an inner tubular member  33  in place of the inner tubular member  31  of the X-ray tube  2 A. In the inner tubular member  33 , a part that protrudes below the lower end surface  13   c  of the head  13  extends below the fused portion  9   c  of the bulb  9   a  and the connecting portion  9   b  and is formed to be thicker than the other portions. By such a thick portion  33   d , the fused portion  9   c  is hidden from view from the anode  5 . Furthermore, a lower end  33   c  of the thick portion  33   d  is rounded into a round shape to suppress discharge to the anode  5 . 
   In assembling the inner tubular member  33  onto the head  13 , positioning in the direction of extension of the anode  5  is performed by contacting of a step  33   e  of the thick portion  33   d  with a lower end surface  13   f  of the head  13 . By such positioning of the inner tubular member  31  by the two surfaces of the inner wall surface  20  and the lower end surface  13   f  of the head  13 , the gaps S 1  and S 2 , which put the internal space R and the interior of the exhaust tube  21  in communication, can be formed with good precision with the inner tubular member  33  as well. In the X-ray tube  2 B, the exhaust tube  21  is disposed at a position at which it opposes the electron gun  3 . 
   The same actions and effects as those of the X-ray tube  2 A can be exhibited by the above-described X-ray tube  2 B as well. 
   (Second Modification Example of the X-ray Tube According to the Second Embodiment) 
   On the other hand,  FIG. 16  is a sectional view of principal portions of a second modification example of the X-ray tube according to the second embodiment, that is, a modification example of the X-ray tube  2 B shown in  FIG. 15 . As shown in  FIG. 16 , in the X-ray tube  2 C (second modification example of the X-ray tube according to the second embodiment), a plurality of through holes  31   k , each of a diameter smaller than that of the exhaust port  17 , may be formed at a position of the inner tubular member  31  in front of the exhaust port  17 . Or, at a position in front of the exhaust port  17 , a mesh-like member, having a plurality of through holes, position in front of the exhaust port  17 , a mesh-like member, having a plurality of through holes, may be fitted onto the inner tubular member  31 . Because with such a structure, not only the gaps S 1  and S 2  but the through holes  31   k  also serve as passages for air, vacuum drawing can be performed efficiently in performing vacuum drawing of the internal space R. 
   (Third Modification Example of the X-ray Tube According to the Second Embodiment) 
   Subsequently, a third modification example of the X-ray tube according to the second embodiment shall now be described with reference to  FIG. 17 .  FIG. 17  is a sectional view of an arrangement of the third modification example of the X-ray tube according to the second embodiment. 
   As shown in  FIG. 17 , the X-ray tube  2 D (third modification example of the X-ray tube according to the second embodiment) has an inner tubular member  35  in place of the inner tubular member  31  of the X-ray tube  2 A. The inner tubular member  35  has a cylindrical shape with a diameter slightly less than the inner diameter of the head  13  at the inner wall surface  19  and is positioned between the inner wall surface  19  of the head  13  and the anode tip  5   a  so as to surround the anode tip  5   a . The inner tubular member  35  is positioned by a step  13   b,  formed below the inner wall surface  19  of the head  13 . By the provision of the inner tubular member  35 , the exhaust port  17  is hidden from the anode tip  5   a , and the edge of the exhaust port  17  cannot be viewed from the anode tip  5   a.    
   An inner wall surface  35   j  of the inner tubular member  35  is formed so as to be matched with the inner wall surface  13   c  of the head  13 . A corner portion thus does not appear at a boundary between the inner wall surface  35   j  of the inner tubular member  35  and the inner wall surface  13   c  of the head  13 , and discharge across the anode  5  and either of the inner wall surface  35   j  and the inner wall surface  13   c  is suppressed. 
   Also, the head  13  has an annular wall portion  13   e  that extends below the fused portion  9   c  of the bulb  9   a  and the connecting portion  9   b  inside the internal space R. By the annular wall portion  13   e , the fused portion  9   c  is hidden from view from the anode  5 . Furthermore, a lower end  13   d  of the annular head  13  is rounded into a round shape to suppress discharge to the anode  5 . 
   The same actions and effects as those of the X-ray tube  2 A can be exhibited by the above-described X-ray tube  2 D as well. 
   The present invention is not restricted to the above-described second embodiment and modification examples thereof and can be modified variously. For example, although the inner tubular member  31  is provided with the flat portions  31   p , the flat portions  31   p  may be omitted. Also, although the bulb  9   a  and the head  13  are joined via the connecting portion  9   b , the bulb  9   a  and the head  13  may be joined together directly. Also, although the target  5   d  is disposed as a separate member on the inclined surface  5   c  of the anode  5 , the anode  5  and the target  5   d  can be made integral so that a part of the inclined surface  5   c  constitutes the target. Also, although the anode  5  has a shape having the inclined surface  5   c  disposed at the tip of a cylindrical column, other shapes can be provided at the tip of the anode  5  by any of various types of carving. In this case, even when a corner-like portion is present at the tip of the anode, discharge across the anode tip and the exhaust port can be suppressed effectively by the inner tubular member  31 . 
   An X-ray source  100  according to the present invention, to which an X-ray tube with any of the above-described structures (an X-ray tube according to the present invention) is applied, shall now be described with reference to  FIGS. 18 and 19 .  FIG. 18  is an exploded perspective view of an arrangement of an embodiment of the X-ray source according to the present invention.  FIG. 19  is a sectional view of an internal structure of the X-ray source according to the embodiment. Although any of the X-ray tubes  1 A to  1 D according to the first embodiment and the X-ray tubes  2 A to  2 D according to the second embodiment can be applied to the X-ray source  100  according to the present invention, for the sake of simplicity, all X-ray tubes applicable to the X-ray source  100  shall be expressed simply as “X-ray tube  1 ” in the description that follows and in the relevant drawings. 
   As shown in  FIGS. 18 and 19 , the X-ray source  100  includes a power supply unit  102 , a first plate member  103 , disposed at an upper surface side of an insulating block  102 A of the power supply unit  102 , a second plate member  104 , disposed at a lower surface side of the insulating block  102 A, four fastening spacer members  105 , interposed between the first plate member  103  and the second plate member  104 , and an X-ray tube  1 , fixed above the first plate member  103  via a metal tubular member  106 . The power supply unit  102  has a structure, with which a high voltage generating unit  102 B, a high voltage line  102 C, a socket  102 D, etc., (see  FIG. 19 ), are molded inside the insulating block  102 A comprised of an epoxy resin. 
   The insulating block  102 A of the power supply unit  102  has a short, rectangular column shape, with the mutually parallel upper surface and lower surface of substantially square shapes. At a central portion of the upper surface is disposed the cylindrical socket  102 D, connected to the high voltage generating unit  102 B via the high voltage line  102 C. An annular wall portion  102 E, positioned concentric to the socket  102 D, is also disposed on the upper surface of the insulating block  102 A. A conductive coating  108  is applied to peripheral surfaces of the insulating block  102 A to make a potential thereof the GND potential (ground potential). A conductive tape may be adhered in place of coating the conductive coating. 
   The first plate member  103  and the second plate member  104  are members that, for example, act together with the four fastening spacer members  105  and eight fastening screws  109  to clamp the insulating block  102 A of the power supply unit  102  in the vertical direction in the figure. The first plate member  103  and the second plate member  104  are formed to substantially square shapes that are larger than the upper surface and the lower surface of the insulating block  102 A. Screw insertion holes  103 A and  104 A, for insertion of the respective fastening screws  109 , are formed respectively at four corners of the first plate member  103  and the second plate member  104 . A circular opening  103 B, surrounding the annular wall portion  102 E that protrudes from the upper surface of the insulating block  102 A, is formed in the first plate member  103 . 
   The four fastening spacer members  105  are formed to rectangular column shapes and are disposed at the four corners of the first plate member  103  and the second plate member  104 . Each fastening spacer member  105  has a length slightly shorter than an interval between the upper surface and the lower surface of the insulating block  102 A, that is, a length shorter than the interval by just a fastening allowance of the insulating block  102 A. Screw holes  105 A, into each of which a fastening screw  109  is screwed, is formed at upper and lower end surfaces of each fastening spacer member  105 . 
   The metal tubular member  106  is formed to a cylindrical shape and has a mounting flange  106 A formed at a base end thereof and fixed by screws across a sealing member to a periphery of the opening  103 B of the first plate member  103 . A peripheral surface at a tip of the metal tubular member  106  is formed to a tapered surface  106 B. By the tapered surface  106 B, the metal tubular member  106  is formed to a tapered shape without any corner portions at the tip. An opening  106 C, through which a bulb  7  of the X-ray tube  1  is inserted, is formed in a flat, tip surface that is continuous with the tapered surface  106 B. 
   The X-ray tube  1  includes the bulb  7 , holding and housing the anode  5  in an insulated state, an upper portion  9   c  of the head  9 , housing the reflecting type target  5   d  that is made electrically continuous with and formed at an inner end portion of the anode  5 , and an electron gun housing unit  11 , housing the electron gun  15  that emits an electron beam toward an electron incidence surface (reflection surface) of the target  5   d . A target housing unit is formed by the bulb  7  and the head  9 . 
   The bulb  7  and the upper portion  9   c  of the head  9  are positioned so as to be matched in tube axis, and these tube axes are substantially orthogonal to a tube axis of the electron gun housing unit  11 . A flange  9   a , for fixing to the tip surface of the metal tubular member  106 , is formed between the bulb  7  and the upper portion  9   c  of the head  9 . A base end  5   a  (portion at which a high voltage is applied from the power supply unit  102 ) of the anode  5  protrudes downward from a central portion of the bulb  7  (see  FIG. 19 ). 
   An exhaust tube is attached to the X-ray tube  1 , and a sealed vacuum container is formed by interiors of the bulb  7 , the upper portion  9   c  of the head  9 , and the electron gun housing unit  11  being depressurized to a predetermined degree of vacuum via the exhaust tube. 
   In the X-ray tube  1 , the base end  5   a  (high voltage application portion) is fitted into the socket  102 D molded in the insulating block  102 A of the power supply unit  102 . High voltage is thereby supplied from the high voltage generating unit  102 B and via the high voltage line  102 C to the base end  5   a . When in this state, the electron gun  15 , incorporated in the electron gun housing unit  11 , emits electrons toward the electron incidence surface of the target  5   d , X-rays, generated by the incidence of the electrons from the electron gun  15  onto the target  5   d,  are emitted from an X-ray emission window  10 , fitted into an opening of the upper portion  9   c  of the head  9 . 
   Here, the X-ray source  100  is assembled, for example, by the following procedure. First, the four fastening screws  109 , inserted through the respective screw insertion holes  104 A of the second plate member  104 , are screwed into the respective screw holes  105 A at the lower end surfaces of the four fastening spacer members  105 . And by the four fastening screws  109 , inserted through the respective screw insertion holes  103 A of the first plate member  103 , being screwed into the respective screw holes  105 A at the upper end surfaces of the four fastening spacer members  105 , the first plate member  103  and the second plate member  104  are mutually fastened while clamping the insulating block  102 A in the vertical direction. A sealing member is interposed between the first plate member  103  and the upper surface of the insulating block  102 A, and likewise, a sealing member is interposed between the second plate member  104  and the lower surface of the insulating block  102 A. 
   A high voltage insulating oil  110 , which is a liquid insulating substance, is then injected into an interior of the metal tubular member  106  from the opening  106 C of the metal tubular member  106  that is fixed above the first plate member  103 . The bulb  7  of the X-ray tube  1  is then inserted from the opening  106 C of the metal tubular member  106  into the interior of the metal tubular member  106  and immersed in the high voltage insulating oil  110 . In this process, the base end  5   a  (high voltage application portion) that protrudes downward from the central portion of the bulb  7  is fitted into the socket  102 D at the power supply unit  102  side. The flange  9   a  of the X-ray tube  1  is then fixed by screwing across the sealing member onto the tip surface of the metal tubular member  106 . 
   In the X-ray source  100 , assembled by the above process, the annular wall portion  102 E, protruded from the upper surface of the insulating block  102 A of the power supply unit  102 , and the metal tubular member  106  are positioned concentric to the anode  5  of the X-ray tube  1  as shown in  FIG. 19 . Also, the annular wall portion  102 E protrudes to a height of surrounding and shielding the periphery of the base end  5   a  (high voltage application portion), which protrudes from the bulb  7  of the X-ray tube  1 , from the metal tubular member  106 . 
   In the X-ray source  100 , when a high voltage is applied to the base end  5   a  of the X-ray tube  1  from the high voltage generating unit  102 B of the power supply unit  102  and via the high voltage line  102 C and the socket  102 D, the high voltage is supplied to the target  5   d  via the anode  5 . When in this state, the electron gun  15 , housed in the electron gun housing unit  11 , emits electrons toward the electron incidence surface of the target  5   d , housed in the upper portion  9   c  of the head  9 , the electrons become incident on the target  5   d . The X-rays that are thereby generated at the target  5   d  are emitted to the exterior via the X-ray emission window  10 , fitted onto the opening of the upper portion  9   c  of the head  9 . 
   Here, in the X-ray source  100 , the metal tubular member  106 , housing the bulb  7  of the X-ray tube  1  in a state of being immersed in the high voltage insulating oil  110 , is protruded from and fixed above the exterior of the insulating block  102 A of the power supply unit  2 , that is, the first plate member  103 . A good heat dissipating property is thus realized, and heat dissipation of the high voltage insulating oil  110  inside the metal tubular member  106  and the bulb  7  of the X-ray tube  1  can be promoted. 
   The metal tubular member  106  has a cylindrical shape with the anode  5  disposed at the center. In this case, because the distance from the anode  5  to the metal tubular member  106  is made uniform, an electric field formed in a periphery of the anode  5  and the target  5   d  can be stabilized. The metal tubular member  106  can thus effectively discharge charges of the charged high voltage insulating oil  110 . 
   Furthermore, the annular wall portion  102 E, protruded on the upper surface of the insulating block  102 A of the power supply unit  102 , surrounds the periphery of the base end  5   a  (high voltage application portion), protruding from the bulb  7  of the X-ray tube  1 , and thereby shields the base end  5   a  from the metal tubular member  106 . Abnormal discharge from the base end  5   a  to the metal tubular member  106  is thus prevented effectively. 
   The X-ray source  100  has the structure with which the insulating block  102 A of the power supply unit  102  is clamped between the first plate member  103  and the second plate member  104  that are fastened to each other via the four fastening spacer members  105 . This means that conductive foreign objects that can induce discharge and charged foreign objects that can induce disruption of electric field are not present inside the insulating block  102 A. Thus, in the X-ray source  100  according to the present invention, unwanted discharge phenomena and electric field disruptions in the power supply unit  102  are suppressed effectively. 
   Here, the X-ray source  100  is incorporated and used, for example, in an X-ray generating apparatus that irradiates X-rays onto a sample in a nondestructive inspection apparatus, with which an internal structure of the sample is observed in the form of a transmission image.  FIG. 20  is a front view for describing actions of an X-ray source (including the X-ray tube according to the embodiment) that is incorporated, as a usage example of the X-ray source  100 , in an X-ray generating apparatus of a nondestructive inspection apparatus. 
   The X-ray source  100  irradiates X-rays to a sample plate SP, positioned between an X-ray camera XC and the X-ray source  100 . That is, the X-ray source  100  irradiates X-rays onto the sample plate SP through the X-ray emission window  10  from an X-ray generation point XP of the target  5   d , incorporated in the upper portion  9   c  of the head  9  that protrudes above the metal tubular member  106 . 
   In such a usage example, because the shorter the distance from the X-ray generation point XP to the sample plate SP, the greater the magnification factor of the transmission image of the sample plate SP taken by the X-ray camera XC, the sample plate SP is normally positioned close to the X-ray generation point XP. Also, to observe the internal structure of the sample plate SP three-dimensionally, the sample plate SP is inclined around an axis orthogonal to a direction of irradiation of the X-rays. 
   If, when an observation point P of the sample plate SP is to be observed three-dimensionally upon being brought close to the X-ray generation point XP while inclining the in  FIG. 20 , corner portions, such as indicated by alternate long and two short dashes lines, are left at a tip of the metal tubular member  106  of the X-ray source  100 , the observation point P of the sample plate SP can be made to approach the X-ray generation point XP only up to a distance, with which the sample plate SP contacts a tip corner portion of the metal tubular member  106  that is, only up to a distance at which a distance from the X-ray generating point XP to the observation point P becomes D 1 . 
   On the other hand, in the X-ray source  100 , with which the tip of the metal tubular member  106  is configured to have a tapered shape without a corner portion by the provision of the tapered surface  106 B as shown in  FIGS. 18 and 19 , the observation point P of the sample plate SP can be made to approach the X-ray generation point XP to a distance, with which the sample plate SP contacts the tapered surface  106 B of the metal tubular member  106  as indicated by solid lines  FIG. 20 , that is, to a distance at which the distance from the X-ray generating point XP to the observation point P becomes D 2 . Consequently, the transmission image of the observation point P of the sample plate SP can be magnified further and nondestructive inspection of the observation point P can be performed more precisely. 
   The X-ray source  100  according to the present invention is not restricted to the above-described embodiment. For example, although a cross-sectional shape of an inner peripheral surface of the metal tubular member  106  is preferably circular, a cross-sectional shape of an outer peripheral surface of the metal tubular member  106  is not restricted to being circular and may be a rectangular shape or other polygonal shape. In this case, the peripheral surface of the tip of the metal tubular member can be formed to be an inclined surface. 
   The insulating block  102 A of the power supply unit  102  may have a short, cylindrical shape, and the first plate member  103  and the second plate member  104  may correspondingly have disk shapes. The fastening spacer members  105  may have cylindrical shapes and the number thereof is not restricted to four. 
   The structure of the X-ray tube  1  may be a structure with which the electron gun is disposed inside the bulb  7 . 
   From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 
   INDUSTRIAL APPLICABILITY 
   The X-ray tube according to the present invention can be applied as an X-ray generating source in various X-ray imaging apparatuses that are frequently used for nondestructive, noncontact observations.