Patent Application
20140140480
The present invention relates to a radiation generating tube which generates a radiation by irradiating a target with electrons, and can be applied to an X-ray photography and the like, and to a radiation imaging apparatus using the same.
A radiation generating tube, which is used as a radiation source, generates a radiation by emitting electrons from an electron source within the radiation generating tube (radiation emitting source) which is kept at a vacuum, and making the electrons collide against the target which is formed from a metal material such as tungsten having a large atomic number.
Incidentally, in order to efficiently generate electrons from an electron source and to extend the life of the radiation generating tube, it is necessary to keep the inside of the radiation generating tube at a vacuum for a long period of time. Japanese Patent Application Laid-Open No. 2000-306533 discloses a technique for keeping the inside of the radiation generating tube at a vacuum by brazing the perimeter of the target to a target holding ring of the radiation generating tube (X-ray tube), in a radiation generating tube that uses a transmitting type target.
In addition, in the radiation generating tube, the radiation generated in the target is emitted to all directions, and accordingly, it is general to shield an unwanted radiation other than a radiation required for photography, by providing a radiation shielding member. Japanese Patent Application Laid-Open No. 2007-265981 discloses a structure in which the radiation shielding member is arranged in an electron incident side and a radiation extraction side of the transmitting type target.
A known method for keeping the radiation generating tube at a vacuum and shielding the unwanted radiation is to shield the radiation which diffuses in an unwanted direction by forming an electron passing hole and a radiation passing hole with the radiation shielding member in the perimeter of the target. Furthermore, a structure is known in which a connected portion of the shielding member and the target has airtightness by brazing the target to the radiation shielding member.
However, when the target is bonded to the radiation shielding member with a brazing technique for the purpose of vacuum sealing, an excessive brazing filler flows out and occasionally flows even onto the inner surface of the electron passing hole provided in the radiation shielding member. The amount of the used brazing filler is previously determined and is used according to the size of a gap between the radiation shielding member and the target, but if the parts have dimensional tolerance, the excessive brazing filler flows out. If the brazing filler has flowed onto the inner surface of the electron passing hole, an electron beam may directly irradiate the brazing filler. The brazing filler has a lower melting point than the target and the peripheral radiation shielding member, and accordingly is melted when being irradiated with the electron beam, and a vacuum degree inside the radiation generating tube tends to be easily lowered. In addition, when the vacuum degree inside the radiation generating tube is lowered, thermal electrons become difficult to be generated from the electron source, and the quantity of the radiation which can be extracted decreases. Thus, a problem is that the radiation generating tube consequently becomes difficult to be continuously used.
For this reason, an object of the present invention is to provide a radiation generating tube which has such a structure that a brazing filler is not directly irradiated with an electron beam, thereby suppresses the lowering of a vacuum degree inside the radiation generating tube, can generate a stable quantity of radiation, and can be continuously used for a long period of time, and to provide a radiation imaging apparatus using the same.
According to an aspect of the present invention, a radiation generating tube comprises: an electron emitting source; a target spaced from the electron emitting source, for generating a radiation responsive to an irradiation with an electron emitted from the electron source; and a tubular shielding member having an electron passing hole, wherein the electron passing hole has an electron incident aperture at one end thereof and has a target supporting surface supporting the target at the other end thereof, wherein the target supporting surface is connected through a brazing filler to a periphery of a surface of the target at a side on which the electron is incident, and an opening size of the other end of the electron passing hole is larger than an opening size of the electron incident aperture at the one end thereof.
According to a further aspect of the present invention, a radiation generating tube comprises: an electron emitting source; a target spaced from the electron emitting source, for generating a radiation responsive to an irradiation with an electron emitted from the electron source; and a tubular shielding member having an electron passing hole, wherein the electron passing hole has an electron incident aperture at one end thereof and has a target supporting surface supporting the target at the other end thereof, wherein the target supporting surface is connected through a brazing filler to a periphery of a surface of the target at a side on which the electron is incident, and the tubular shielding member has an annular recess which is arranged along the target supporting surface, and is recessed in a direction of increasing a distance from the target supporting surface.
The radiation generating tube according to the present invention has such a structure that even if a brazing filler has flowed onto and deposits on the inner surface of an electron passing hole provided in a radiation shielding member, the brazing filler which has flowed out is not directly irradiated with the electron beam. Thereby, the brazing filler which has flowed out is not melted by being directly irradiated with the electron beam, and accordingly the radiation generating tube can suppress the lowering of the vacuum degree in its inside, and can suppress the lowering of a radiation quantity when having been continuously used for a long period of time.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Embodiments suitable for a radiation generating tube according to the present invention will be illustratively described in detail below with reference to the drawings. However, the materials, dimensions, shapes, relative arrangements and the like of the components which are described in the following embodiments do not limit the scope of the present invention only to those, unless otherwise specified.
A structure of a radiation generating tube of a first embodiment will be described below with reference to
The radiation generating tube 19 is structured so as to have a radiation emitting source 1 and a driving power source 16 arranged in an envelope 17 having the radiation extraction window 18, and so as to have a left space in the envelope 17 filled with an insulating oil 15.
The radiation emitting source 1 includes an electron emitting source 3, the anode 10, a getter 12 and a vacuum chamber 6.
The electron emitting source 3 includes an electric-current introduction terminal 4 and an electron emitting portion 2. An electron emission mechanism of the electron emitting source 3 may be an electron emitting source which can control the amount of electrons to be emitted from the outside of the vacuum chamber 6, and a hot cathode electron emitting source, a cold cathode electron emitting source and the like can be appropriately applied to the electron emission mechanism. The electron emitting source 3 is electrically connected to the driving power source 16 arranged in the outside of the vacuum chamber 6 so that the amount of electrons to be emitted and an ON/OFF state of the electron emission can be controlled through the electric-current introduction terminal 4 which is arranged so as to penetrate the vacuum chamber 6.
The electrons which have been emitted from the electron emitting portion 2 are converted into an electron beam 5 having an energy of approximately 10 keV to 200 keV by a not-shown drawing grid and a not-shown accelerating electrode, and is enabled to be incident on a target 9 which is arranged so as to oppose to the electron emitting portion 2. The drawing grid and the accelerating electrode can also be housed in an electron gun tube of the hot cathode. Alternately, it is also possible to add a correcting electrode for adjusting the position of a spot to be irradiated with the electron beam and the astigmatism of the electron beam, to the electron emitting source 3, and then to connect the electron emitting source 3 to a not-shown correcting circuit which is arranged in the outside.
The anode 10 includes a radiation shielding member 7 and a target 9 including a target substrate 9a and a target film 9b.
Typically, a metal material having an atomic number of 24 or more can be used for the target film 9b. The metal material can be used which has larger thermal conductivity and a higher melting point. Specifically, the usable metal materials can be a metal material such as tungsten, molybdenum, chromium, copper, cobalt, iron, rhodium and rhenium, or an alloy material thereof. The value of the thickness of the target film 9b is 1 μm to 15 μm, although the optimal value varies depending on an accelerating voltage, because an infiltration depth of the electron beam into the target film 9b, in other words, a region in which the radiation is generated, varies depending on the accelerating voltage. The above described thickness of the target film 9b can be specified as a thickness in a normal direction with respect to the surface of the target substrate 9a in a side having the target film 9b provided thereon.
The target film 9b can be integrated with the target substrate 9a by sputtering, vapor deposition or the like. As another method, the target film 9b can be integrated with the target substrate 9a by separately preparing a thin film of the target film 9b having a predetermined thickness with a rolling or polishing process, and diffusion-bonding the target film 9b to the target substrate 9a under a high temperature and a high pressure.
A covering form of the target film 9b for the target substrate 9a of the present invention is not limited to a form (coverage of 100%) in which the target film 9b covers the entire one main surface, as in
The target substrate 9a needs to have high transmissivity for radiation and adequate thermal conduction and withstand vacuum sealing. For instance, diamond, silicon nitride, silicon carbide, aluminum carbide, aluminum nitride, graphite, beryllium or the like can be used for the target substrate 9a. The target substrate 9a can more desirably contain any one of the diamond, the aluminum nitride and the silicon nitride which have smaller transmissivity for radiation than that of the aluminum and larger thermal conduction than that of the tungsten, as a main ingredient. The thickness of the target substrate 9a may satisfy the above described functions, and can be 0.3 mm or more and 2 mm or less, though varying depending on the material. Particularly, the diamond is more excellent because of having extremely higher thermal conductivity than that of other materials, having also high transmissivity for radiation and being easy to keep the vacuum. However, these materials largely lower the thermal conductivity as the temperatures rise, and accordingly the target substrate 9a needs to suppress the rise of the temperature as much as possible. The above described thickness of the target substrate 9a can be specified as a thickness in the normal direction with respect to the coated surface of the target substrate 9a in a side having the target film 9b provided thereon.
The radiation shielding member 7 has the electron passing hole 8 having an opening formed in each of both ends. Electrons are incident from one end (opening in end of electron emitting source 3 side) of the electron passing hole 8, and irradiate the target 9 provided in the other end side (opposite side of electron emitting source 3) of the electron passing hole 8 to generate the radiation. The electron passing hole 8 serves as a passage for guiding the electron beam 5 therethrough into an electron beam irradiation region (radiation generating region) of the target film 9b, in a side closer to the electron emitting source 3 than the target 9, and serves as a passage for emitting the radiation therethrough to the outside, in a side closer to the radiation extraction window 18 than the target 9. Unwanted radiations in a radiation to be emitted to the electron emitting source 3 side from the target film 9b and a radiation to be emitted to the radiation extraction window 18 side from the target film 9b are shielded by the inner wall of the electron passing hole 8, respectively. The electron passing hole 8 has a circular shape in a plan view which is viewed from the radiation extraction window 18 side, as is illustrated in
The material which can be used for the radiation shielding member 7 may be a material which can shield a radiation generated at 30 kV to 150 kV. The usable materials are, for instance, molybdenum, zirconium, niobium or the like in addition to tungsten and tantalum, or an alloy material thereof.
The radiation shielding member 7 and the target 9 can be bonded to each other with a brazing technique or the like. When the member and the target are bonded with the brazing technique, it is important to keep the inside of the vacuum chamber 6 at a vacuum state. A brazing filler for brazing can be appropriately selected according to the material, a heat-resistant temperature and the like of the radiation shielding member 7. When the temperature of the target substrate 9a becomes specially high, for instance, a brazing filler metal such as Cr—V series brazing material, Ti—Ta—Mo series brazing material, Ti—V—Cr—Al series brazing material, Ti—Cr series brazing material, Ti—Zr—Be series brazing material and Zr—Nb—Be series brazing material can be selected as a brazing filler of a high-melting-point metal. When vacuum airtightness is regarded as more important, brazing materials of Au—Cu series brazing material, nickel series brazing material, brass series brazing material, silver series brazing material, palladium series brazing material and the like can be used as a brazing filler for high vacuum equipment.
The vacuum chamber 6 can be formed from glass, ceramics or the like, and the inside of the vacuum chamber 6 becomes an internal space 13 which has been evacuated (decompressed). The internal space 13 may have such a vacuum degree that electrons can fly at least a distance between the electron emitting source 3 and the target film 9b which emits the radiation, as a mean free path of the electrons, and the vacuum degree of 1×10−4 Pa or less can be applied to the internal space. It is possible to appropriately select the vacuum degree in consideration of the electron emitting source to be used, the operating temperature and the like. In the case of the cold cathode electron emitting source, the vacuum degree can further be set at 1×10−6 Pa or less. In order to keep the vacuum degree, it is also possible to arrange the getter 12 in the internal space 13, or to place the getter 12 in a not-shown auxiliary space which communicates with the internal space 13.
The structure of the anode 10 will be described in detail below with reference to
When the target 9 is brazed to the radiation shielding member 7, a not-shown metallization layer is previously provided in the perimeter of the target 9. As for the metallization layer, a paste is prepared, for instance, by adding a resinous bonding material and a dispersion medium to a metallization composition powder which contains a compound containing at least one element selected from Ti, Zr or Hf as an active metal component. After that, the metallization layer is obtained by applying the paste onto a portion to be metallized, and baking the applied paste at a predetermined temperature.
Next, an active brazing filler metal is deposited onto the metallized surface in the perimeter of the target 9. For instance, a Ti-containing silver-copper brazing filler can be used. The target 9 onto which this active brazing filler metal has been deposited is set in the target supporting portion 7a which is provided in the radiation shielding member 7 that has been previously formed so as to have a predetermined dimension, and is baked at a predetermined temperature for a predetermined period of time. These baking conditions vary depending on the type of the active brazing filler metal. The above described Ti-containing silver-copper brazing filler can be held at approximately 850° C. for approximately 100 seconds.
When the target 9 is bonded to the radiation shielding member 7 with a brazing technique, the inside of the vacuum chamber 6 needs to be kept vacuum-airtight, and accordingly this brazing is extremely important. Because of this, it is ideal that the brazing filler melts when having been baked, and the target 9 and the radiation shielding member 7 can adhere to each other without forming a gap therebetween. However, certain variations result in occurring in the actual target 9, in the radiation shielding member 7, and even in the thicknesses of the metallization layer and the brazing filler, and accordingly a larger amount than an assumed average value of the brazing filler needs to be provided in the perimeter of the target 9, in order to keep at least the vacuum airtightness. Because of this, the brazing filler occasionally flows out along the inner surface of the electron passing hole 8 which has been provided in the radiation shielding member 7, when the brazing filler has been baked.
In the present embodiment, the width d1 of a sectional area in the opening in the opposite side of the end in the electron incident side of the electron passing hole 8 is set so as to be larger than the width d2 of a sectional area in the end in the electron incident side of the electron passing hole 8, in other words, an opening size d2 of an electron incident aperture. It is general to set the diameter of the electron beam so as to be smaller than d2. Because of this, even when the brazing filler flows from the target supporting surface 7b and is deposited on the inner surface of the electron passing hole 8, the radiation generating tube can prevent the electron beam from directly colliding with the deposited brazing filler.
In the structure of
On the other hand, as is illustrated in
Thus, with the radiation generating tube according to the present embodiment, the anode 10 configured as shown in
A second embodiment will be described below with reference to
The radiation generating tube of the present embodiment is provided with an annular recess 7d which is formed along the target supporting surface 7b and is recessed in a direction from the target supporting surface 7b toward the above described electron incident aperture, that is in a direction of increasing a distance from the target supporting surface 7b, or decreasing a distance from the electron incident aperture, which is a point different from that in the first embodiment. Except this point, the radiation generating tube of the present embodiment employs the same members as those in the first embodiment, and has the same structure as in the first embodiment.
In the structure of
Thus, with the radiation generating tube according to the present embodiment, the anode 10 configured as shown in
A third embodiment will be described below with reference to
The radiation generating tube of the present embodiment is provided with an annular recess 7e that is formed along a target supporting surface 7b and is recessed in a direction from the target supporting surface 7b toward the above describe electron incident aperture, which is a point different from that in the first embodiment. Except for this point, the radiation generating tube of the present embodiment employs the same members as those in the first embodiment, and has the same structure as in the first embodiment.
In the structure of
Thus, with the radiation generating tube according to the present embodiment, the anode 10 configured as shown in
A fourth embodiment will be described with reference to
The radiation generating tube of the present embodiment has two step shapes provided from a target supporting surface 7b toward the end in the electron incident side of an electron passing hole 8, in other words, toward an electron incident aperture, which is a point different from that in the first embodiment. Except for this point, the radiation generating tube of the present embodiment employs the same members as those in the first embodiment, and has the same structure as in the first embodiment.
In the structure of
Thus, with the radiation generating tube according to the present embodiment, an anode 10 configured as shown in
The electron passing hole 8 may have a sectional area of which width discontinuously decreases step by step from the target supporting surface 7b toward the end in the electron incident side of the electron passing hole 8, in other words, toward the electron incident aperture.
A fifth embodiment will be described reference to
The radiation generating tube of the present embodiment has a declining portion 7g provided from a target supporting surface 7b to the end in the electron incident side of an electron passing hole 8, which is a point different from that in the first embodiment. Except for this point, the radiation generating tube of the present embodiment employs the same members as those in the first embodiment, and has the same structure as in the first embodiment.
In the structure of
Thus, with the radiation generating tube according to the present embodiment, the anode 10 configured as shown in
A sixth embodiment of the present invention is a radiation imaging apparatus (not-shown) using a radiation generating tube. The radiation imaging apparatus of the present embodiment includes a radiation generating tube, a radiation detector, a signal processing portion, a device control portion and a display portion. The radiation detector is connected to the device control portion through the signal processing portion, and the device control portion is connected to the display portion and a voltage control portion. The radiation generating tube according to the first to fifth embodiments can be used as the radiation generating tube.
The processing in the radiation generating tube is generally controlled by the device control portion. The device control portion controls, for instance, a radiation imaging operation by the radiation generating tube and the radiation detector. The radiation which has been emitted from the radiation generating tube is detected by the radiation detector through a subject, and a radiation transmission image of the subject is taken. The taken radiation transmission image is displayed on the display portion. The device control portion also controls, for instance, the driving of the radiation generating tube, and controls a voltage signal applied to a radiation emitting source, through the voltage control portion.
Thus, the radiation imaging apparatus according to the present embodiment can suppress the lowering of the vacuum degree and the lowering of the radiation quantity originating in the lowering of the vacuum degree, similarly to that in the first to fifth embodiments.
An exemplary embodiment of the present invention will be described below with reference to
A high-pressure synthesis diamond was prepared as a target substrate 9a. A high-pressure and high-temperature diamond has a disk shape (columnar shape) with a diameter of 5 mm and a thickness of 1 mm. An organic matter existing on the surface of the diamond was removed beforehand with a UV-ozone asher.
A titanium layer was formed beforehand on one surface with a diameter of 1 mm of this diamond substrate with a sputtering method which used Ar as a carrier gas, and then a tungsten layer with a thickness of 8 μm was formed as a target film 9b. Thus, a target 9 was obtained.
A metallization layer which used titanium as an active metal component was formed in the perimeter of this target 9, and a brazing filler formed of silver, copper and titanium was applied on to the metallization layer.
On the other hand, tungsten was prepared as a radiation shielding member 7, and a target supporting portion 7a, a target supporting surface 7b, a brazing filler receiving portion 7c and an electron passing hole 8 were formed, as illustrated in
The target 9 on which the brazing filler was provided was set in the radiation shielding member 7 which had been worked into the above described shape, and was baked at 850 (C. Thus, the anode 10 was produced.
Next, the anode 10 formed by integrating the target 9 with the radiation shielding member 7 was positioned so that an impregnation type thermoelectron gun having an electron emitting portion 2 opposed to the electron emitting source 3 and so that an electron beam 5 was incident on the electron passing hole 8, as illustrated in
Thus, five pieces of radiation emitting sources were produced, and were used as radiation emitting sources of Exemplary Embodiments Nos. 1 to 5, respectively.
A target 9 similar to that of the above described exemplary embodiment was produced by using a radiation shielding member 7 illustrated in
<Measurement of Vacuum Degree and Radiation Quantity>
The internal pressures of both of the radiation emitting source in the exemplary embodiment and the radiation emitting source in the comparative example were monitored, and while the internal pressures were monitored, simultaneously the radiation quantities were measured with a semiconductor type of dose meter. The radiation emitting source was driven with an accelerating voltage of 100 kV and an electric current of 5 mA, for an irradiation period of time of 10 msec and a stopping period of time of 90 msec. The diameter of the electron beam was determined to be 1.7 mm from the size of a focal diameter which was separately measured.
Table 1 illustrates changes of pressures and radiation quantities of each of the radiation emitting sources of Exemplary Embodiments Nos. 1 to 5 and each of the radiation emitting sources of Comparative Examples Nos. 1 to 5.
In Exemplary Embodiments No. 2, No. 4 and No. 5, the internal pressures slightly increase. The internal pressures of Exemplary Embodiments No. 1 and No. 3 largely increase compared to those of Exemplary Embodiments No. 2, No. 4 and No. 5. In all of Exemplary Embodiments Nos. 1 to 5, the radiation quantities were the same as those in the beginning, and the lowering of the radiation quantities according to the number of pulse generation times was not observed.
On the other hand, in Comparative Examples No. 2 and No. 4, the internal pressures slightly increase. The internal pressures of Comparative Examples No. 1, No. 3 and No. 5 largely increase. In Comparative Examples No. 2 and No. 4, the lowering of the radiation quantities according to the number of pulse generation times was not observed, but in Comparative Examples No. 1, No. 3 and No. 5, the radiation quantities decreased as the number of the pulse generation times increased.
From the above described result, it is assumed that the used brazing filler did not flow to the inner surface of the electron passing hole 8 of the radiation shielding member 7, in Exemplary Embodiments No. 2, No. 4 and No. 5 and Comparative Examples No. 2 and No. 4. On the other hand, it is assumed that the brazing filler flowed out, in Exemplary Embodiments No. 1 and No. 3 and Comparative Examples No. 1, No. 3 and No. 5. It is assumed that the electron beam directly collided with the brazing filler, accordingly the internal pressure increased, and the amount of generated thermoelectrons decreased, which caused the lowering of the radiation quantity, in Comparative Examples No. 1, No. 3 and No. 5. In Exemplary Embodiments No. 1 and No. 3, the electron beam did not collide with the brazing filler; accordingly, even though the internal pressure increased, the degree was not so much as to cause the lowering of the radiation quantity; and the effect was shown.
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 Application No. 2011-189108, filed Aug. 31, 2011, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-189108 | Aug 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/068543 | 7/17/2012 | WO | 00 | 12/19/2013 |
