X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
1998-02-12
2002-11-26
Porta, David P. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Target
C378S143000
Reexamination Certificate
active
06487275
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an X-ray tube generating an X-ray by irradiating an electron beam, an anode of an X-ray target of an X-ray tube and an X-ray apparatus using the X-ray tube and, more particularly, to a medical X-ray tube and a medical X-ray apparatus which is required to be high in load resistivity and high in brightness and definition of an image.
BACKGROUND ART
In an X-ray generating apparatus for industrial use or medical use, an X-ray is generated by irradiating thermal electrons emitted from an cathode onto an anode target. An X-ray generating metal for the anode target (hereinafter, referred to as “X-ray target”) used is tungsten (W) or a tungsten alloy which has a high X-ray generating efficiency and a high melting point.
An X-ray tube for medical use is required to produce a high definition image of a medical examination portion and to have a higher X-ray output compared to a common X-ray tube. Since most part of energy of an electron beam is converted into heat when an X-ray is generated, the X-ray target is heated to high temperature.
Further, a high power X-ray tube is so constructed that the X-ray target is rotated during electron beam irradiation in order to prevent the X-ray target from overheating. Therefore, the X-ray tube is required to have a high heat resistance and a high strength during rotation. A method for coping with this problem is disclosed, for example, in Japanese Patent Application Laid-Open No.58-59545. In the method, a tungsten or tungsten alloy layer is formed onto the surface of a molybdenum or molybdenum alloy base plate through a chemical deposition method or the like. This method has an advantage in better bonding ability between the surface of the molybdenum alloy base plate and the tungsten alloy layer and accordingly in a high thermal conductivity. A method of manufacturing an X-ray target is also disclosed in Japanese Patent Application Laid-Open No. 57-176654. In the method, a tungsten or tungsten alloy layer is successively laminated onto the surface of a molybdenum or molybdenum alloy base plate through a chemical deposition method or the like, and then the laminated X-ray target is annealed to improve the adhesive force. The X-ray tubes using such X-ray targets have a better load resistivity compared to an X-ray tube having a conventional X-ray generating metal, and can withstand a longtime and continuous use.
As the progress of an X-ray apparatus with computer processing such as a X-ray CT apparatus for medical use, an X-ray tube is required to cope with a high resolution processed image. Further, it is required that the X-ray tube can withstand a long-time and continuous use. In order to do so, it is necessary to increase input power to the X-ray tube to increase the amount of X-ray radiation. In addition to this, in order to obtain a high resolution image, it is important to converge an electron beam from a cathode small, that is, to increase the brightness by small focusing and large current density. Therefore, it is required that the X-ray target can withstand a large heat load on the electron irradiation surface. To these requirements, the method of Japanese Patent Application Laid-Open No.58-59545 has a problem in that the surface of the X-ray generating metal made of a tungsten alloy is roughed and the X-ray generating efficiency is decreased as it is used long time.
On the other hand, the method of Japanese Patent Application Laid-Open No.57-176654 has a disadvantage in that the process of manufacturing the target is complex and accordingly its manufacturing cost may be increased.
DISCLOSURE OF INVENTION
An object of the present invention is to provide an X-ray tube which is high in brightness and high in resolution, and can withstand continuous long-time use, that is, can withstand a high heat load, and to provide an X-ray apparatus such as an X-ray CT apparatus capable of obtaining a more clear image using the X-ray tube.
The object of the present invention can be attained by providing an X-ray tube generating an X-ray from a metal surface by irradiating an electron beam, wherein at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises an X-ray generating metal having an average crystal grain diameter not larger than 30 &mgr;m, preferably not larger than 10 &mgr;m, on the surface of a base plate made of a metal. The “average crystal diameter” here means a minor axis when the crystal grain is flat. The crystal grain diameter may be obtained by taking a picture of a polished surface using an optical microscope or an electron microscope, and calculating through an image processing method or measuring crystallographically using an X-ray. In these cases, although the crystal grain diameter is apt to be measured smaller in a case of using the X-ray, it is sufficient that the measured average crystal grain diameter is within the above range whichever method is chosen.
It is preferable that the X-ray generating metal having an average crystal grain diameter not larger than 30 &mgr;m is composed of two or more layers. The “two or more layers” means that the composition of each layer may be different, or a boundary may be simply formed between layers. For example, in a case of forming an X-ray generating metal layer through the chemical vapor deposition method, by stopping to supply the process gas for a while during forming a layer and then starting to supply the process gas, a boundary is formed and two layers can be observed. In film forming through chemical vapor deposition, seed crystals are firstly formed on a base plate and then crystals grow based on the seed crystals to form a film. When supply of the process gas is stopped for a while, crystal growth is stopped at that time. When supply of the process gas is started again, seed crystals are newly formed. In such a way, two or more layers of metal films can be formed even if the composition of each of the layers is the same. The most convenient way to judge whether two or more layers are formed is to polish a cross section of the X-ray target and observe it by a microscope.
Further, it is preferable that, in the X-ray tube, the X-ray generating metal having an average crystal grain diameter not larger than 30 &mgr;m is composed of two or more layers containing tungsten and rhenium, and tungsten concentration in the layer in contact with the metal base plate is higher than tungsten concentration in the surface layer of the electron irradiating surface. A preferable X-ray generating metal is a substance having a larger atomic number which has a higher X-ray generating efficiency, but it is required to have a higher melting point. Although tungsten is generally used as an element to satisfy these requirements, rhenium is added as an alloy element since tungsten itself is low in high temperature strength and accordingly is unsuitable for practical use.
It is also preferable that the thickness of the X-ray generating metal layer is not larger than 200 &mgr;m.
It is preferable that the X-ray generating metal layer described above has a tungsten alloy layer in the side of the base plate.
Further, the present invention provides an X-ray tube in which at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises two or more layers of alloy layers on the surface of a metal base plate. The definition of “two or more layers” is the same as described above.
Furthermore, the present invention provides an X-ray tube generating an X-ray from a metal surface by irradiating an electron beam in which at least a part of an electron irradiating surface of an anode target of the X-ray tube comprises an X-ray generating layer having a columnar crystal structure on the surface of a metal base plate. The “columnar crystal structure” hear means a crystal structure in which directions of crystals (directions of longitudinal axis of the crystals) are oriented in nearly the same direction and the aspect ratio of the crystal is approximately more than 5.
Further, the present invention provides
Baba Noboru
Doi Motomichi
Kozono Yuzo
Maeda Kunihiro
Seki Masatoshi
Antonelli Terry Stout & Kraus LLP
Porta David P.
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