Patent Application
20130235975
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.
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.
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.
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.
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.
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.
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.
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.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
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.
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.
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.
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.
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.
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.
As illustrated in
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.
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.
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.
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.
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
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.
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>α>0). Any value in the range of Va>α>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.
Thus, the present embodiment can extremely improve the target cooling effects.
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.
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.
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.
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.
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.
When the thermal insulating member 22 is vacuum, as illustrated in
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.
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.
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.
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-275619 | Dec 2010 | JP | national |
| 2010-275621 | Dec 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/075645 | 11/1/2011 | WO | 00 | 5/9/2013 |
