JPS6333261B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an X-ray generator that generates X-rays by impacting a target with an electron beam. Typically, X-rays are generated by bombarding a metal target with an electron beam. Since 99% of this electron beam is converted into thermal energy, it is necessary to irradiate it with a strong electron beam, but in that case the target will melt and be damaged. Therefore,
Conventionally, a cylindrical anticathode was configured to be rotated while being cooled with water, and an electron gun was installed opposite the anticathode to irradiate a powerful electron beam to a target placed around the cylindrical anticathode. A rotating anticathode X-ray generator is used to generate X-rays. However, when the electron beam becomes powerful (about 1A),
There is a high probability that the residual gas in the tube will be ionized by the electron beam, and the generated ions and electrons may further ionize the residual gas, causing a discharge.
For this reason, conventional X-ray generators have the drawback that stable operation cannot be expected, and attempts have been made to solve the above-mentioned drawbacks by improving the degree of vacuum within the X-ray tube. The purpose of the present invention is to create an X-ray generator capable of stably generating high-output X-rays without causing discharge even at low degrees of vacuum, without following the conventional idea of achieving stable operation by increasing the vacuum. Our goal is to provide the following. In order to achieve such an object, the present invention provides an X-ray generator that generates X-rays by impacting an electron beam on a target, including an electron beam accelerating section and an electron beam receiving the impact accelerated by the accelerating section. The electron beam focusing section is provided between the electron beam accelerating section and the target and converging the electron beam accelerated by the accelerating section. A small hole is provided at a position facing the target to allow the focused electron beam to pass through, and a plurality of holes are provided to prevent substances generated by the impact of the electron beam from flowing from the target into the electron beam acceleration section. It is characterized by being equipped with an exhaust system. The present invention will be described in detail below with reference to the drawings. First, we will consider why stable operation cannot be achieved even if the inside of the X-ray tube is evacuated to a high vacuum. Since the surface of the target provided around the rotating anticathode is cooled during one rotation and irradiated with the electron beam, gases adsorbed during cooling are released by the electron beam irradiation. That is, since the target surface needs to be sufficiently cooled during one rotation, the cooled target surface has the function of adsorbing the residual gas in the tube and transporting it to the electron beam irradiation section. Furthermore, considering that 20 to 40% of the electrons that irradiate the metal target become recoil electrons that scatter inside the tube and heat the tube wall, it is extremely difficult to maintain the electron beam irradiation section in a high vacuum. can be,
Moreover, long-term aging is required to achieve this. In contrast, in the present invention, the anticathode section and the electron beam acceleration section are separated, and an electron beam focusing lens and an orifice (small hole) are provided between them, so that stable operation can be achieved even at a relatively low degree of vacuum. ,
The structure is such that only the acceleration section is kept in a high vacuum. FIG. 1 shows an embodiment of the X-ray generator of the present invention. Here, 1 is an electron gun and 2 is an anode, and both constitute an electron beam accelerating section 3. electron gun 1
The generated electrons are accelerated by the voltage applied between the anode 2 and the anode hole 4 formed in the anode 2.
The liquid is injected into a duct 5 having a length of, for example, 1.5 m. The emitted electron beam travels through the duct 5 while being focused by the electron lens systems 6-1, 6-2, and 6-3. Lens system 6-1
～The electron beam exiting from 6-3 is a small hole (orifice)
7 and a target 9 provided on the anticathode 8.
is irradiated. Here, the size of this small hole 7 is narrowed down to the same size as the spread of the electron beam, and the front and back of this small hole 7 are evacuated by oil diffusion pumps 10 and 11, and the target is irradiated with the beam. The configuration is such that the gas generated at 9 does not flow into the electron beam accelerating section 3. On the other hand, the electron beam accelerator 3 is communicated only through the duct 5 and the anode hole 4, is almost independently evacuated by the oil diffusion pump 12, and is always maintained at a high vacuum. That is,
12 is an oil diffusion pump for exhausting the electron beam accelerator 3. In addition, in the case of a microfocus X-ray generator with high focal brightness but low output, an electron lens is arranged between the anticathode 8 and the electron gun 1 to focus the electron beam on the anticathode 8. is commercially available, but the electron lens in this case is used to focus the electron beam, and therefore does not have a small hole.
In contrast, in the present invention, the electron lens systems 6-1 to 6-3 are used to increase the distance between the electron gun 1 and the target 9. Next, the anticathode used in the X-ray generator according to the present invention will be explained. As mentioned above, in order to impact a target with an electron beam and generate powerful X-rays, it is necessary to irradiate the target with a stronger electron beam. However, 99% of these electrons are converted into thermal energy, so conventionally, to prevent the target from rising in temperature and melting, a cylindrical rotating anticathode was rotated with cooling water flowing through its cavity. Measures are being taken to prevent melt damage. A conventional rotating anode cathode is constructed as shown in FIG. 2, for example, and the cooling water flows into the target 15 as shown by the arrow through a cooling water partition plate 14 provided inside the rotating anode cathode 13. It's getting old. In this case, there are two types: one in which the partition plate 14 is fixed and does not rotate, and one in which it rotates together with the rotating anode cathode 13. In the former case, a sufficient flow rate of cooling water can be ensured, but water resistance is added to the rotating anticathode 13, and a powerful electric motor is required to drive the rotating anode. On the other hand, in the latter case, the rotating anode cathode 1
3 can be easily rotated, but the disadvantage is that the pressure of the cooling water must be extremely high. The reason for this, as will be explained later, is that the cooling water
It is thought that this is because the centrifugal force of the centrifugal force is blocking the flow. In the present invention, the rotating anode cathode is configured as follows so that the cooling water rotates together with the rotating anode cathode 13 and prevents centrifugal force from being applied to the cooling water. First, the principle of the rotating anticathode with enhanced cooling effect in the present invention will be explained based on FIG. When the cooling water discharged from the center of the rotating anode cathode is made to travel straight to the circumference without rotating, it travels on the rotating partition plate 14 tracing a spiral curve trajectory as shown in FIG. 3. . The required amount of cooling water is W
Assuming cm ² /sec, the velocity of the cooling water going from the center to the circumference is a function of the distance r from the center, and is given by V=W/2πrd (1). Here, d is the distance between the partition plate 14 and the rotating target side plate 15A (see FIG. 2). If the spiral curve on the partition plate 14 rotating at an angular velocity ω is expressed in polar coordinates (r, θ), the conditions for making the cooling water go straight to the circumference are dr=Vdt=Vdθ/ω=W/2πrd dθ/ω. , from this the spiral curve is It is expressed as That is, it is sufficient to provide a cooling water guide given by equation (2) on the partition plate 14. On the back side of the partition plate 14, a spiral guide with the same shape and in the opposite direction is provided so that water can similarly flow straight from the circumference to the center. FIG. 4 shows an example of the configuration of a rotating anode in the X-ray generator of the present invention, in which a band-shaped electron beam 16 is attached to a target 15 provided on the outer periphery of the rotating anode 13.
is irradiated to generate X-rays 17. Cooling water guides 18, 18' are provided protrudingly on the cooling water partition plate 14,
This partition plate 14 is rotated together with the anticathode 13. 19 is a cooling water return path, and the cooling water flows in the direction of the illustrated arrow. Here, the guides 18, 18' on the cooling water partition plate 14 are formed as shown in FIG.
In other words, it is difficult to provide a spiral guide on the partition plate 14 as shown in FIG.
In this example, the linear guide 18 is arranged in the tangential direction of the spiral curve. Such a guide 18 acts as a web, but does not act as a mere pump. The guide 18 functions to prevent rotation of the cooling water. That is, in order to perform a pumping action, it is desirable that the angle between the guide 18 and the diametrical direction be approximately 45 degrees, but in this embodiment, the angle is increased in proportion to the rotational speed.
As a result of the experiment, it is necessary to increase the angle with the rotation speed according to the above equation (2). For example, for a rotating anode with a diameter of 40 cm, when d = 10 mm, the angle becomes = 45° at 1000 rpm and = 90° at 2000 rpm. It turned out that it was necessary. Furthermore, the X-ray generator according to the present invention will be explained in more detail. First, the electron beam accelerator 3 is composed of an electron gun 1 and an anode 2, as shown in FIG. (usually arranged around a hot cathode, and used by applying an appropriate negative or positive voltage to the hot cathode) and an anode 2.
In order to converge the thermoelectrons generated from the electron gun 1 as much as possible and allow them to pass through the anode hole 4,
It is preferable to form them in a concentric fan shape as shown in FIG. The anode hole 4 has a size of, for example, 7×25 cm ² in accordance with the size of the emitted electron beam source. In this case as well, some electrons collide around the anode hole 4, so the anode 2
Preferably, the anode 2 is made of copper, and cooling water is circulated through the anode 2 through a pipe 22 to cool the anode 2 with water. On the other hand, since it is necessary to move the optical axis of the electron beam within its meridian plane (a plane perpendicular to the optical axis), the anode 2 and electron gun 1 are coaxially integrated so that they can be operated from outside the vacuum chamber. . FIG. 7 shows a perspective view of the electron gun 1 and anode 2 shown in FIG. 6, where the filament of the electron gun 1 has a spiral shape as shown in FIG. Manufactured.

[Table] The saturated emission current density i _s of the filament is i _s = 2A ₀ T ² exp (-e/kT) A ₀ : 6×10 ⁵ A/cm ² (electron emission constant of tungsten) k: Boltzmann constant: 4.54 eV (work function) T: temperature e: electron charge, and beam current I is given by I=i _s ·B (B: electron emission area). Since the filament of this device shown in Table 1 has a B of 150 mm ² , I = 150 i _s . As shown in FIG. 8, the electron beam focusing section includes an electron beam duct 5 and, for example, three electron lenses 6-.
1, 6-2 and 6-3. The electron lens systems 6-1 to 6-3 are arranged as anticathodes to prevent the electron beam emitted from the electron beam accelerator 3, which is arranged apart from the anticathode 8 (see FIG. 1), from colliding with the duct 5. This is to propagate up to 8 and create a focused focus (approximately 1 x 10 mm ² ) on a metal target 9 provided on the anticathode 8 . In this device, the appropriate lens current for creating the above-mentioned narrow focus differs depending on the degree of divergence of the emitted electron beam. In the case of an electron beam accelerating section having the shape and arrangement shown in Fig. 8, the first, second and third electron lenses 6-1, 6-2 in the case of an anode voltage of 65 kV and an electron beam current of 1 A.
and 6-3 are shown in Table 2 below.

[Table] Next, the specific structure of the anticathode 13 is shown in FIG. The rotating shaft 31 of the anticathode 13 is supported by a bearing 33 attached to a support plate 32 and a bearing 35 on a tube wall 34. Further, the vacuum inside the tube is maintained by the oil seal portion 36. The above rotation mechanism can ensure a rotation speed of approximately 3000 rpm or more. Next, the metal used for the target 15 provided on the anticathode 13 is related to the desired wavelength of X-rays.
Generally, molybdenum is used to obtain X-rays with a wavelength of about 1 Å. Molybdenum has a high melting point (2600°C), which is advantageous because it allows a considerably large load capacity, but it has the disadvantage of poor corrosion resistance and short life. In this example device, niobium is used as the target metal to generate stronger X-rays than molybdenum even when loaded with the same capacity. It is known that the intensity N of X-rays generated per irradiated electron is given by the following equation. N=K ₀ (E ₀ −Ex) ^1.63 K ₀ : Coefficient (related to atomic number) E ₀ : Irradiation electron energy Ex : X-ray excitation voltage The results of comparing K ₀ and Ex for molybdenum and niobium are as follows. It will look like Table 3.

[Table] The Ex of niobium is slightly smaller than that of molybdenum, and the K ₀ is about 10% larger. Therefore, with the same load capacity, X-rays that are at least 10% more intense can be obtained. On the other hand, the maximum load capacity must be determined so that the rise in surface temperature of the target due to electron beam irradiation does not exceed the melting point of the target metal. The surface temperature of the target is Tm−To=Wd/c 2δ/πD+(2W/π)(2δ/Dnch) ^1/
2 W: Unit area of incident electron beam, energy per unit time d: Thickness of the anticathode c: Thermal conductivity of the anticathode 2δ: Width of the electron beam D: Diameter of the anticathode h: Heat capacity of the anticathode n: It is given by the number of rotations per unit time of the anticathode. Here, it is impossible to increase D from a structural standpoint, so in order to suppress the increase in target surface temperature due to electron beam irradiation, it is important to increase n, c, and h.
Comparing niobium with molybdenum, there is not much difference in heat capacity, but the thermal conductivity of niobium increases as the temperature rises, as shown in FIG. When the temperature exceeds 1400℃, the thermal conductivity exceeds that of molybdenum, and the cooling effect increases. This allows niobium targets to be operated at approximately the same load as molybdenum targets. In addition, niobium has excellent corrosion resistance and is not eroded by water, which can significantly improve its service life. According to the present invention, the distance between the electron beam accelerating section and the target is increased, an electron beam focusing lens and a small hole are provided between the electron beam accelerating section and the target, and a plurality of exhaust holes are provided between the electron beam accelerating section and the target. By evacuating the vicinity of the target using the device, the electron beam irradiation section on the target can accelerate the electron beam even in a low vacuum, and the electron beam can be focused by the electron beam focusing lens, making it possible to use a large filament. can. Therefore, according to the present invention, a target can be stably irradiated with a large current electron beam, and a high output X-ray source can be obtained. As a result, it is possible to easily realize a high-intensity electron beam source, which has been an unsolved problem in the past, when manufacturing a large-current X-ray generator. Due to the above-mentioned effects, a high-output X-ray generator that was conventionally considered difficult to realize [for example, tube voltage of 60 kV, tube current of about 2 A, X-ray focal spot size (cross section of electron beam on target surface) of 1× 10mm] can be easily achieved. Furthermore, in order to prevent melting and damage of the target due to the impact of strong electron beams, conventional high-speed rotating anticathodes are designed to have a cylindrical shape and rotate at high speed while passing cooling water through the cylindrical cavity. In this case, mechanical strength is required to withstand high-speed rotation, so if the partition plate is fixed, it is not possible to install a support between both sides of the rotating anode, and it is extremely difficult to ensure mechanical strength. Have difficulty. Also,
Conventionally, the method of rotating partition plates required high water pressure and mechanical strength to match the water pressure. In the X-ray generator of the present invention, when cooling the anticathode, a cooling water guide is provided protrudingly on the partition plate, and the inflow and outflow cooling water flow is rotated as the anticathode rotates. By doing this, it became possible for the first time to manufacture a large-diameter, high-speed rotating anticathode without requiring high water pressure, and thus a high-output X-ray generator was realized.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing one embodiment of the X-ray generator of the present invention, FIG. 2 is a cross-sectional view of a rotating anode used in a conventional X-ray generator, and FIG. 3 is a cross-sectional view of a rotating anode used in the X-ray generator of the present invention. An explanatory diagram of the principle of a rotating anode cathode, FIG. 4 is a sectional view showing one embodiment of the rotating anode cathode in the X-ray generator of the present invention, and FIG. 5 is a diagram showing the rotating anode cathode provided in the X-ray generator of the present invention FIG. 6 is a side view showing one embodiment of the cooling water partition plate, FIG. 6 is a sectional view showing a detailed configuration example of the electron beam accelerating section in the X-ray generator of the present invention, and FIG. 7 is a diagram showing the electron gun and anode shown in FIG. 6. FIG. 8 is a sectional view showing a detailed configuration example of the electron lens system in the X-ray generator of the present invention; FIG. 9 is a perspective view of a portion;
The figure is a sectional view showing a detailed configuration example of an anticathode in the X-ray generator of the present invention, and FIG. 10 is a characteristic curve diagram showing the relationship between temperature and thermal conductivity of niobium and molybdenum. 1... Electron gun, 2... Anode, 3... Electron beam accelerator, 4... Anode hole, 5... Duct, 6-1,
6-2, 6-3...electronic lens system, 7...small hole,
8... Anticathode, 9... Target, 10, 11,
12...Oil diffusion pump, 13...Rotating anticathode, 1
4... Partition plate, 15... Target, 15A...
...Target side plate, 16...Electron beam, 17...X
Line, 18, 18'... Cooling water guide, 19... Cooling water return path, 20... Wehnelt, 21... Vacuum container, 22... Cooling water pipe, 31... Rotating shaft, 32... Pipe wall, 33...Bearing, 34...
...Pipe wall, 35...Bearing, 36...Oil seal section.

Claims (1)

[Claims] 1. In an X-ray generation device that generates X-rays by impacting an electron beam on a target, the an electron beam converging section provided between an electron beam accelerating section and the target and converging the electron beam accelerated by the accelerating section;
A small hole through which the focused electron beam passes is provided at a position facing the target of the electron beam converging section, and substances generated by the impact of the electron beam from the target are directed to the electron beam accelerating section. An X-ray generator characterized by comprising a plurality of exhaust devices for preventing the inflow of X-rays.