Abstract:
There is provided a radiation generating apparatus having a simple structure and capable of shielding unnecessary radiation, cooling a target, reducing the size and weight of the apparatus, and achieving higher reliability, and a radiation imaging apparatus having the same. A transmission type radiation tube is held inside a holding container filled with a cooling medium. The transmission type radiation tube includes an envelope having an aperture, an electron source arranged inside the envelope so as to face the aperture of the envelope, a target unit for generating a radiation responsive to an irradiation with an electron emitted from the electron source, and a shield member for shielding a part of the radiation emitted from the target unit. The cooling medium contacts at least a part of the shield member.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a radiation generating apparatus applicable to non-destructive X-ray imaging or the like in the fields of medical devices and industrial equipment, and a radiation imaging apparatus having the radiation generating apparatus. 
       BACKGROUND ART 
       [0002]    A radiation tube (radiation generating tube) accelerates electrons emitted from an electron source to high energy and irradiates a target with the accelerated electrons to generate radiation such as X-rays. The radiation generated at this time is emitted in all directions. In light of this, a container holding the radiation tube or the circumference of the radiation tube is covered with a shield member (radiation shielding member) such as lead so as to prevent unnecessary radiation from leaking outside. Thus, it has been difficult to reduce the size and weight of such a radiation tube and a radiation generating apparatus holding the radiation tube. 
         [0003]    Japanese Patent Application Laid-Open No. 2007-265981 discloses a transmission type multi X-ray generating apparatus for shielding unnecessarily emitted X-rays by arranging shields each on an X-ray emission side and an electron incident side of the target. 
         [0004]    It has been difficult for such a target (anode)-fixed type transmission type radiation tube to generate high-energy radiation because the target has a relatively low heat radiation. The X-ray generating apparatus disclosed in Japanese Patent Application Laid-Open No. 2007-265981 is configured such that the target is bonded to the shield member, which allows heat generated in the target to be transferred to and dissipated through the shield member, thereby suppressing an increase in temperature of the target. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL1: Japanese Patent Application Laid-Open No. 2007-265981 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    However, a conventional transmission type radiation tube is configured such that the shield member is placed inside a vacuum chamber, which limits a region for transferring heat from the shield member to outside the vacuum chamber. Accordingly, the heat radiation of the target is not necessarily sufficient, leading to a problem in achieving a balance between a target cooling capability and a compact lightweight apparatus. 
       Solution to Problem 
       [0007]    It is an object of the present invention to provide a radiation generating apparatus which is small in size, light in weight, excellent in heat radiation, and high in reliability, and a radiation imaging apparatus having the same. 
         [0008]    In order to achieve the above object, a radiation generating apparatus according to the present invention comprises: a holding container; a transmission type radiation tube arranged in the holding container; and a cooling medium filling between the holding container and the transmission type radiation tube, wherein the transmission type radiation tube includes an envelope having an aperture, an electron source arranged in the envelope, a target unit arranged at the aperture, for generating a radiation responsive to an irradiation with an electron emitted from the electron source, and a shield member arranged at the aperture so as to surround the target unit for shielding a part of the radiation emitted from the target unit, wherein at least a part of the shield member contacts the cooling medium. 
       Advantageous Effect of Invention 
       [0009]    The present invention is configured such that a shield member is bonded to a target unit and at least a part of the shield member contacts a cooling medium so that heat generated in the target unit is transferred to the shield member, through which the heat is transferred to the cooling medium for quick heat dissipation. Further, a thermal insulating member is interposed between the target unit and the cooling medium, thereby suppressing deterioration of the cooling medium due to local overheating because heat transfer from a surface of the target unit to the cooling medium is controlled. This can provide a radiation generating apparatus having a simple structure and capable of shielding the unnecessary radiation and cooling the target. Further, the size of a member for shielding the unnecessary radiation can be reduced, and thus reduction in size and weight of the entire radiation generating apparatus can be achieved. Furthermore, suppression of deterioration of the cooling medium due to overheating allows the pressure resistance of the cooling medium to be maintained for a long period of time, thus enabling a more highly reliable radiation generating apparatus to be provided. 
         [0010]    Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a schematic view of a radiation generating apparatus of the present invention. 
           [0012]      FIGS. 2A ,  2 B,  2 C,  2 D, and  2 E are schematic views illustrating a configuration around a target unit of the present invention. 
           [0013]      FIG. 3  is a configuration view of a radiation imaging apparatus using the radiation generating apparatus of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0014]    Hereinafter, embodiments of the present invention will be described using drawings, but the present invention is not limited to these embodiments. Further, the radiation for use in the radiation generating apparatus of the present invention includes not only X-rays but also neutron radiation and γ radiation. 
         [0015]      FIG. 1  is a schematic view of the radiation generating apparatus (X-ray generating apparatus) of the present invention. A transmission type radiation tube  10  (hereinafter referred to as an X-ray tube) is held inside a holding container  1 . The remaining space inside the holding container  1  holding the X-ray tube  10  therein is filled with a cooling medium  8 . The holding container  1  includes thereinside a voltage control unit  3  (voltage control unit) having a circuit board, an isolation transformer, and the like. A cathode control signal, an electron extraction control signal, an electron beam converging control signal, and a target control signal are applied from the voltage control unit  3  to the X-ray tube through terminals  4 ,  5 ,  6 , and  7  respectively to control X-ray generation. 
         [0016]    The holding container  1  may have a sufficient strength as a container and is made of metal, plastics, and the like. The holding container  1  may include a radiation transmission window  2  made of glass, aluminum, beryllium, and the like as the present embodiment. When the radiation transmission window  2  is provided, the radiation emitted from the X-ray tube  10  is radiated outside through the radiation transmission window  2 . 
         [0017]    The cooling medium  8  may have electrical insulation. For example, an electrical insulating oil can be used which serves as an insulating medium and a cooling medium for cooling the X-ray tube  10 . A mineral oil, a silicone oil, and the like are preferably used for the electrical insulating oil. The other available examples of the cooling medium  8  may include a fluorine series electric insulator. 
         [0018]    The X-ray tube  10  includes an envelope  19 , an electron source  11 , a target unit  14 , and a shield member  16 . The X-ray tube  10  further includes an extraction electrode  12  and a lens electrode  13 . An electric field generated by the extraction electrode  12  causes electrons to be emitted from the electron source  11 . The emitted electrons are converged by the lens electrode  13  and are incident on the target unit  14  to generate radiation. The X-ray tube  10  may further include an exhaust pipe  20  like the present embodiment. When the exhaust pipe  20  is provided, for example, the inside of the envelope  19  is exhausted to vacuum through the exhaust pipe  20  and then a part of the exhaust pipe  20  is sealed, thereby enabling the inside of the envelope  19  to be vacuum. 
         [0019]    The envelope  19  is provided to maintain vacuum inside the X-ray tube  10  and is made of glass, ceramics, and the like. The degree of vacuum inside the envelope  19  may be about 10 −4  to 10 −8  Pa. The envelope  19  may include thereinside an unillustrated getter to maintain the degree of vacuum. The envelope  19  further includes an aperture. The shield member  16  is bonded to the aperture. The shield member  16  has a path communicating with the aperture of the envelope  19 . The target unit  14  is bonded to the path to hermetically seal the envelope  19 . 
         [0020]    The electron source  11  arranged inside the envelope  19  so as to face the aperture of the envelope  19 . A hot cathode such as a tungsten filament and an impregnated cathode or a cold cathode such as a carbon nanotube can be used as the electron source  11 . The extraction electrode  12  is arranged near the electron source  11 . The electrons emitted by an electric field generated by the extraction electrode  12  are converged by the lens electrode  13  and are incident on the target  14  to generate radiation. An accelerating voltage Va applied to between the electron source  11  and the target  14  is different depending on the intended use of the radiation, but is roughly about 40 to 120 kV. 
         [0021]    As illustrated in  FIG. 2A , the target unit may include a target  14  and a transmission plate  15 . The transmission plate  15  supports the target  14  and transmits at least a part of the radiation generated in the target  14 . The transmission plate  15  is arranged in a path of the shield member  16  communicating with the aperture of the envelope  19 . The material forming the transmission plate  15  preferably has sufficient strength to support the target  14 , absorbs less radiation generated in the target  14 , and has high thermal conductivity so as to quickly dissipate heat generated in the target  14 . For example, diamond, silicon nitride, aluminum nitride, and the like can be used. In order to satisfy the above requirement for the transmission plate  15 , the thickness of the transmission plate  15  is appropriately about 0.1 mm to 10 mm. The transmission plate  15  may be integrally formed with the target  14 . 
         [0022]    The target  14  is arranged on a surface (inner surface side) of the transmission plate  15  facing the electron source side. The material forming the target  14  preferably has a high melting point and a high radiation generation efficiency. For example, tungsten, tantalum, molybdenum, and the like can be used. In order to reduce the radiation absorbed when the generated radiation passes through the target  14 , the thickness of the target  14  is appropriately about 1 μm to 20 μm. 
         [0023]    The shield member  16  shields a part of the radiation emitted from the target  14 . The shield member  16  is arranged in the aperture of the envelope  19  so as to surround the target unit  14 . The shield member  16  is connected to the target unit  14  over the entire periphery thereof, but may not be necessarily connected over the entire periphery thereof depending on the arrangement relation between the shield member  16  and the target unit  14 . The shield member  16  has a path communicating with the aperture and the transmission plate  15  is bonded to the path. The target  14  may not be connected to the path. The shield member  16  may include two shield members (a first shield member  17  and a second shield member  18 ) of a tubular shape such as a cylinder like the present embodiment. 
         [0024]    The first shield member  17  has a function of shielding the radiation scattered toward the electron source side of the target  14  when the electrons are incident on the target  14  and the radiation is generated. The first shield member  17  has a path communicating with the aperture of the envelope  19 . The electrons emitted from the electron source  11  pass through a path of the first shield member  17  communicating with the aperture of the envelope  19  and the radiation scattered toward the electron source side of the target  14  is shielded by the first shield member  17 . 
         [0025]    The second shield member  18  has a function of shielding unnecessary radiation of the radiation passing through the transmission plate  15  and emitted therefrom. The second shield member  18  has a path communicating with the aperture of the envelope  19 . The radiation passing through the transmission plate  15  passes through a path of the second shield member  18  communicating with the aperture of the envelope  19 , and the unnecessary radiation is shielded by the second shield member  18 . 
         [0026]      FIGS. 2A to 2E  are schematic views around the target unit  14 . In the present embodiment, as illustrated in  FIGS. 2A to 2E , the sectional area of the path of the second shield member  18  can gradually increase toward the opposite side of the electron source from the transmission plate  15  (the more away from the transmission plate  15 , the more the area increases). The reason for this is that the radiation passing through the transmission plate  15  is radially radiated. 
         [0027]    Further, in the present embodiment, it is preferable that between the electron source side from the transmission plate  15  and the opposite side of the electron source from the transmission plate  15 , the center of gravity of the opening of the path on each side matches (the center of gravity of the opening of the path of the first shield member  17  matches the center of gravity of the opening of the path of the second shield member  18 ). More specifically, as illustrated in  FIGS. 2A to 2E , the opening of the path of the first shield member  17  and the opening of the path of the second shield member  18  are preferably arranged on the same straight line perpendicular to the surface on which the target of the transmission plate  15  is placed with the transmission plate  15  interposed therebetween. This is because in the present embodiment, the target  14  irradiated with electrons to generate radiation and the radiation passing through the transmission plate  15  is emitted. 
         [0028]    The material forming the shield member  16  (the first shield member  17  and the second shield member  18 ) preferably has a high radiation absorption rate and a high thermal conductivity. For example, a metal material such as tungsten and tantalum can be used. In order to sufficiently shield unnecessary radiation and prevent an unnecessary increase in size around the target, the thickness of the first shield member  17  and the second shield member  18  is appropriately 3 mm to 20 mm. 
         [0029]    An anode grounding system and a neutral grounding system may be used as the voltage control unit for use in the radiation generating apparatus of the present embodiment, but the neutral grounding system is preferably used. The anode grounding system is such that assuming that an accelerating voltage applied between the target  14  and the electron source  11  is Va[V], the voltage of the target  14  serving as the anode is set to ground (0[V]) and the voltage of the electron source  11  is set to −Va[V]. In contrast to this, the neutral grounding system is such that the voltage of the target  14  is set to +(Va−α)[V] and the voltage of the electron source  11  is set to −α[V] (where Va&gt;α&gt;0). Any value in the range of Va&gt;α&gt;0 may be set to α, but Va/2 is preferable. The use of the neutral grounding system can reduce the absolute value of the voltage with respect to ground and can shorten the creeping distance. Here, the creeping distance means a distance between the voltage control unit  3  and the holding container  1 , and a distance between the X-ray tube  10  and the holding container  1 . A reduction in the creeping distance can reduce the size of the holding container  1 , which can reduce the weight of the cooling medium  8  by the reduced size, thus leading to a further reduction in size and weight of the radiation generating apparatus. 
       First Embodiment 
       [0030]      FIG. 2A  illustrates a configuration around the target unit  14  of the present embodiment. The target  14  is in a mechanical and thermal contact with the first shield member  17  and the second shield member  18  directly or through the transmission plate  15 . A surface of the transmission plate  15  on the opposite side (outer surface side) of the electron source and the second shield member  18  form a part of an outer wall of the envelope  19  and is located inside the holding container  1  in a direct contact with the cooling medium  8 . Consequently, the heat generated when electrons are incident on the target  14  is dissipated from the surface of the transmission plate  15  on the opposite side of the electron source to the cooling medium  8  and at the same time is quickly dissipated to the cooling medium  8  through the second shield member  18  as well. Thus, an increase in temperature of the target  14  is suppressed. 
         [0031]    Thus, the present embodiment can extremely improve the target cooling effects. 
         [0032]    The radiation generating apparatus of the present embodiment may be configured such that the shield member  16  includes only the second shield member  18 . In this case, the heat generated when electrons are incident on the target  14  is dissipated from the surface of the transmission plate  15  on the opposite side of the electron source to the cooling medium  8  and at the same time is quickly dissipated to the cooling medium  8  through the second shield member  18  as well. Thus, an increase in temperature of the target  14  is suppressed. Note that another shielding member (for example, a shielding member made of a lead plate and covering a part of the outer wall of the envelope  19 ) is required on the electron source side of the target  14  to shield the scattered radiation but the shielding member does not need to cover the entire surface of the radiation tube, thus enabling reduction in size and weight of the radiation generating apparatus. 
       Second Embodiment 
       [0033]    In the first embodiment, the transmission plate directly contacts the cooling medium, and thus the heat generated in the target causes a sharp local increase in temperature of a portion of the cooling medium contacting the transmission plate. The local increase in temperature causes a convective flow of the cooling medium, which causes a turnover of the cooling medium on the surface of the transmission plate, but a part thereof exceeds a decomposition temperature (generally about 200 to 250° C. for the electrical insulating oil), which may decompose (deteriorate) the cooling medium. Advancement of decomposition of the cooling medium reduces the pressure resistance of the cooling medium, which has caused a problem such as discharge due to long time driving. 
         [0034]      FIG. 2B  illustrates a configuration around the target unit  14  of the present embodiment. 
         [0035]    A thermal insulating member is provided on an inner surface side of the shield member  18  so as to prevent a direct contact between the transmission plate  15  and the cooling medium  8 . The thermal insulating member is a space  22  formed by the transmission plate  15  and a cover plate  21  provided in an end portion of a protrusion portion of the shield member  18 . The cover plate  21  is bonded to the second shield member  18 . The cover plate  21  is preferably made of a material having a low radiation absorption rate such as diamond, glass, beryllium, aluminum, silicon nitride, and aluminum nitride. In order to provide the cover plate  21  with enough strength as a substrate and reduce radiation absorption, the thickness of the cover plate  21  is preferably about 100 μm to 10 mm. 
         [0036]    The material forming the heat insulating space  22  preferably has lower thermal conductivity than those of the materials forming the second shield member  18 , low radiation absorption rate, and high heat resistance, and vacuum or a gas is suitable. Examples of the gas may include air, nitrogen, an inert gas such as argon, neon, and helium. The pressure of the gas forming the heat insulating space  22  may be atmospheric pressure, but may be preliminarily set to be lower than the atmospheric pressure because the gas expands by the heat generated in the target when radiation is generated. The pressure of the gas forming the heat insulating space  22  is proportional to the absolute temperature, and thus based on the assumed temperature, a pressure at formation may be set thereto. The X-ray tube  10  of the present embodiment may be formed by bonding or welding the cover plate  21  to the second shield member  18  in a vacuum or gaseous atmosphere. 
         [0037]    According to the present embodiment, except the inner surface side of the shield member  18 , the shield member  18  directly contacts the cooling medium  8 ; and on the inner surface side of the shield member  18 , the thermal insulating member  22  having a lower thermal conductivity than that of the second shield member  18  is formed between the transmission plate  15  and the cooling medium  8 . Accordingly, the heat generated in the target  14  is transferred to the second shield member  18 , through which the heat is transferred to the cooling medium  8  to be quickly dissipated therefrom. Thus, an increase in temperature of the target  14  is suppressed and at the same time the heat transfer from the transmission plate  15  to the cooling medium  8  is suppressed, thereby suppressing deterioration of the cooling medium  8  due to local overheating. 
         [0038]    When the thermal insulating member  22  is vacuum, as illustrated in  FIG. 2C , a hole (communication hole)  23  is provided in the first shield member  17  and the second shield member  18 , and through the hole, the inside of the envelope  19  may be adapted to communicate with the inside of the thermal insulating member  22 . When the communication hole  23  is provided, the X-ray tube  10  of the present embodiment can be formed in such a manner that after the cover plate  21  is bonded to the second shield member  18 , the inside of the envelope  19  and the inside of the thermal insulating member  22  are exhausted at the same time through the exhaust pipe  20 , and the exhaust pipe  20  is sealed. 
       Third Embodiment 
       [0039]      FIG. 2D  illustrates a configuration around the target unit  14  of the present embodiment. The thermal insulating member interposed between the transmission plate  15  and the cooling medium  8  is made of a solid thermal insulating member  24 . The other components may be the same as the components of the second embodiment. 
         [0040]    The material forming the thermal insulating member  24  preferably has lower thermal conductivity than those of the material forming the second shield member  18 , low radiation absorption rate, and high heat resistance. Examples of the material may include silicon oxide, silicon nitride, titanium oxide, titanium nitride, titanium carbide, zinc oxide, aluminum oxide, and the like. The thermal insulating member  24  may be formed by a film formation method in which any of the above materials is subjected to sputtering, deposition, CVD, sol-gel, or other processes on a surface of the transmission plate  15 ; or in such a manner that a substrate made of any of the above materials is attached or bonded to the surface of the transmission plate  15 . In order to suppress the heat transfer between the transmission plate  15  and the cooling medium  8  and reduce the radiation absorption rate, the thickness of the thermal insulating member  24  is preferably in the range of 10 μm to 10 mm. 
         [0041]    According to the present embodiment, the thermal insulating member  24  is formed mainly by film formation. Thus, the manufacturing process can be simplified and the manufacturing costs can be reduced. 
       Fourth Embodiment 
       [0042]      FIG. 2E  illustrates a configuration around the target unit  14  of the present embodiment. The present embodiment is configured such that a thermal insulating member  25  is formed not only between the transmission plate  15  and the cooling medium  8  but also between an inner wall of a path of the second shield member  18  and the cooling medium  8 . The material and the film formation method of the thermal insulating member  25  are the same as those of third embodiment. 
         [0043]    The present embodiment can suppress the heat transfer to the cooling medium  8  not only from the transmission plate  15  but also from a relatively high temperature portion of the second shield member  18  near the transmission plate  15 . Thus, the present embodiment can further suppress the deterioration of the cooling medium  8  due to overheating. 
       Fifth Embodiment 
       [0044]      FIG. 3  is a configuration view of a radiation imaging apparatus of the present embodiment. The radiation imaging apparatus includes a radiation generating apparatus  30 , a radiation detector  31 , a signal processing unit  32 , an apparatus control unit  33 , and a display unit  34 . As the radiation generating apparatus  30 , the radiation generating apparatus according to one of the first to fourth embodiments is used. The radiation detector  31  is connected to the apparatus control unit  33  through the signal processing unit  32 . The apparatus control unit  33  is connected to the display unit  34  and the voltage control unit  3 . 
         [0045]    The process of the radiation generating apparatus  30  is integratedly controlled by the apparatus control unit  33 . For example, the apparatus control unit  33  controls radiation imaging by the radiation generating apparatus  30  and the radiation detector  31 . The radiation emitted from the radiation generating apparatus  30  passes through an object  35  and is detected by the radiation detector  31 , in which a radiation transmission image of the object  35  is taken. The taken radiation transmission image is displayed on the display unit  34 . Further, for example, the apparatus control unit  33  controls driving of the radiation generating apparatus  30  and controls a voltage signal applied to the X-ray tube  10  through the voltage control unit  3 . 
         [0046]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
         [0047]    This application claims the benefit of Japanese Patent Applications No. 2010-275619, filed Dec. 10, 2010, and No. 2010-275621 filed Dec. 10, 2010, which are hereby incorporated by reference herein in their entirety.