JPH06162972A - X-ray tube provided with transmission-type anode - Google Patents

Abstract

PURPOSE: To produce an X-ray tube capable of utilizing the electric power more effectively than the currently used X-ray tube with reflection type anode. CONSTITUTION: An X-ray tube, being collided with electrons 3 during working, comprises the target layer 1 composed of one or more metals with large atomic numbers, and the transmission anode being composed of the carrier layer 2 to be connected to the target layer and consisting of one or more materials with small atomic numbers. Maintaining the angles formed between the electron incidence direction and a part of X-ray transmitting the carrier layer after using outside the X-ray tube, within the ranges of 10 deg. to 40 deg., the intensity of the X-ray can be increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a target layer composed of a metal having a large atomic number of 1 or 2 or more, which is bombarded by electrons in an operating state, and a target layer bonded to the target layer and having a small atomic number of 1 or 2. The present invention relates to an X-ray tube having a transmission type anode composed of a carrier layer made of the above substances.

[0002]

2. Description of the Related Art X-ray tubes of this type are disclosed, for example, in German Published Specification No. 2,729,833 and US Pat. No. 2,0.
90,636, U.S. Pat. No. 3,894,239.
Known by the issue. Contradictory conditions are imposed on the thicknesses of the two layers. On the one hand, the target layer is preferably as thick as possible so that the incident electrons can be converted into a high proportion of X-ray quanta. On the other hand, this layer should be as thin as possible in order to minimize the attenuation of the X-ray quanta generated there.
The carrier layer must, on the one hand, be thin enough to minimize the attenuation of the emitted X-rays, and on the other hand sufficient to ensure mechanical stability and dissipation of the thermal energy generated in the target layer. Must be thick.

Because of these conflicting conditions, this type of X
The wire tube is difficult to use in practice, and particularly difficult to use in the voltage range of 50 to 500 kV, which is important in medical and industrial inspection. For these purposes, the use of an X-ray tube with an anode in which the X-rays emerge from the anode on the side where the electrons are incident. Therefore, these anodes are hereinafter referred to as "reflective anodes".

In an X-ray tube, only a small amount of applied electron energy is X in the voltage range up to 500 kV.
Converted to ray radiation. Other energies raise the temperature of the anode. Outside the X-ray tube, only part of the X-rays generated are used as an effective radiation beam.

[0005]

DISCLOSURE OF THE INVENTION The object of the present invention is 50
An X-ray tube having an operating voltage in the range of kV to 500 kV and electron energy applied to the operation of the X-ray tube producing more effective X-rays as compared to that of an X-ray tube having a reflective anode. To configure.

[0006]

This object is achieved by setting the angle θ between the incident direction of electrons and the direction of X-rays of an effective radiation beam emitted through the carrier layer in the range of 10 ° to 40 °. It The invention is based on the recognition that the intensity of X-rays depends largely on the angle enclosed by the X-rays emitted with respect to the direction of the electrons. If the attenuation by the target is not taken into account, the apparent maximum intensity appears at the outer surface of the cone whose central axis is formed by the direction of the electron beam producing the X-rays. The opening angle of this cone depends on the operating voltage and becomes smaller as the operating voltage increases. For an operating voltage of 60 kV, half the value of the opening angle of the cone of maximum intensity is about 40 °, which is 500 kV.
The operating voltage is about 10 °.

The invention takes advantage of the fact that the angle between the effective radiation beam, ie the part of the X-radiation outside the X-ray tube, and the direction of the generation of the electrons which generate the X-rays is properly chosen. It is a thing. Generally, the effective radiation beam has a nonzero aperture angle in at least one direction. The angle between the X-rays at the center of the effective radiation beam and the direction of incidence of the electrons is chosen as described in the claims.

In the transmission anode X-ray tubes known to date, the effective radiation beam is usually along the extension of the electron path, ie at an angle of zero. However,
There is also a transmission anode type X-ray tube in which the angle θ is deviated from 0. For example, according to U.S. Pat. No. 3,894,239, a rotating anode X-ray having a transmissive anode in which an electron beam is incident substantially perpendicular to a target layer tilted about 80 ° with respect to a radiation exit window. The tube is known. The continuous spectrum of control radiation generated in the target layer is
It is attenuated at a substantially greater rate than the fluorescent radiation generated in the target layer.

Further, German Published Specification No. 2,729,
As shown in FIG. 7 of No. 833, it has an annular anode in which X-rays are generated by two groups of cathodes distributed around the anode, the cathodes extending in the source on both sides of a central plane. An X-ray tube located at is known. As a result,
An angle θ of 45 ° is obtained each time. However, in all of the cited documents, the X-radiation ranges from 15 ° (at high voltage) to 4 °
It does not take advantage of the fact that it becomes particularly strong in the angular range between 0 ° (at low voltage).

Finally, according to WO092 / 03837, electrons are incident on the anode at an angle of 10 ° (rather than the usual 70 ° -90 °) and the effective radiation beam is 5 ° to 15 ° to the anode. X-ray tubes are known which have a reflective anode which spreads at an angle of °. However, the radiation exit window tends to be strongly heated by scattered electrons. In the embodiment of the present invention, which is very important for X-ray output,
When the weight w per unit surface area of the target layer is expressed in g / cm ² , at least the following formula is satisfied: w = 1.08 · 10 ⁻⁶ · (A / Z) ^2.5 · U ^1.6 · cos
β Here, A is the atomic weight of the metal of the target layer, Z is its atomic number, U is the kV value of the rated operating voltage of the X-ray tube, and β is the angle between the electron incident direction and the vertical line to the target layer. Represent. Taking an X-ray tube having a tungsten target layer as an example, the weight per unit surface area is 0.017 g /
cm ² , thickness 8.6 μm (operating voltage U = 100 kV
And β = 0).

The present invention can be used with different x-ray tubes for different applications. In a preferred embodiment of the invention, the X-ray tube is constructed as a rotating anode X-ray tube and the target layer (for example made of tungsten and / or rhenium),
It is located on the surface of a frusto-cone enclosing an angle to the direction of the X-rays used outside the X-ray tube, the angle being smaller than the angle between said direction and the direction of the incident electrons. The anode is then shaped in a dish-like shape symmetrical about the axis of rotation, the inner surface of which is provided with the target layer facing the electron-emitting electron source and its effective radiation beam. Radiate from its outer surface, preferably at an angle of 90 ° to the axis of rotation.

The present invention will be described in detail below with reference to the drawings.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENT The transmission anode shown in FIG. 1 has a target layer 1 made of a metal having a high atomic number, which is provided on a carrier layer 2 made of a substance having a low atomic number. The target layer 1 can be made of, for example, tungsten or rhenium, or an alloy thereof. Other suitable metals for the target layer are platinum or thorium. The carrier layer 2 consists of graphite or beryllium, the thickness of which on the one hand has a suitable mechanical strength and, on the other hand, the attenuation of the X-radiation to be as low as possible.

The arrow 3 indicates an electron beam which is incident on the target layer 1 at an angle β with respect to the vertical line. X-radiation is generated that propagates conically around the point of incidence. However, according to theoretical and experimental investigations, if the attenuation due to the target layer is not taken into consideration, a cone with a predetermined opening angle θ (its apex is at the point of incidence of electrons and its axis of symmetry is the electron beam direction) X-rays propagating on the surface of) have the strongest intensity. The upper boundary 4a and the lower boundary 4b of the cone are shown in FIG. The half angle of the cone opening angle θ depends on the operating voltage and follows approximately the following table.

[0015]

[Table 2]

Therefore, the X-ray tube must be constructed such that the direction of the effective radiation beam coincides with one X-ray lying on the conical surface. The X-rays produced in the target layer travel at different angles with respect to the plane of that layer, the figure showing the smallest angle α ₁ and the largest angle α ₂ . These angles are represented by the following equations.

Α ₁ = 90 ° −β−θ (1) α ₂ = 90 ° −β + θ (2) The optimum weight per unit surface area of the target layer for the radiation output is given by the following relational expression. To be w = 1.08 · 10 ⁻⁶ · (A / Z) ^2.5 · U ^1.6 · cos β (3) Here, A is the atomic weight of the metal of the target layer, and Z is the atomic number thereof. β represents the incident angle of electrons, that is, the angle between the direction of the incident electron beam 3 and the vertical line to the target layer. When the target layer is made of an alloy of two or more metals, the weight per unit surface area of the target layer is calculated according to formula (3) w for each metal of the alloy, and the calculated value is the ratio of each metal. Weighted according to.

If the emission direction of the radiation is selected according to the table and the thickness of the target layer calculated according to equation (3) is used, the intensity of the X-radiation of the effective X-ray beam is
This is much higher than that in an X-ray tube having the same tube voltage value and the same tube current value and having a reflecting anode that makes the angle between the incident direction of the electron beam and the emitting direction of the radiation about 90 °. The increase in intensity increases with increasing voltage. However, these strength advantages are diminished if the X-ray tube is operated at voltages other than the rated voltage.

FIG. 2 shows an embodiment of a rotary anode type X-ray tube having a transparent anode according to the present invention. The X-ray tube has a glass envelope 5 in which a cathode device 6 and an anode device 7 are arranged. The anode device is constituted by a transparent anode 2 which is connected to the rotor 8 in a known manner and is rotatably journalled inside the X-ray tube. The rotor is rotated by a stator provided outside the glass envelope and not shown in FIG.

The transparent anode comprises a carrier member 2 made of graphite and open towards the cathode device 6 in the form of a dish. A target layer 1 made of rhenium is provided on the carrier member 2 in the region of the transmission type anode where the electron beam 3 from the electron emitter provided in the cathode device 6 collides. If this X-ray tube is used for computer tomography and is made for an operating voltage of 150 kV and the electron beam 3 is incident on the layer at an angle of 40 ° with respect to the vertical direction, the unit surface area of this layer is According to the formula (3), the weight is 0.024 g / cm ² . This is achieved with a 11.5 μm thick rhenium layer.

The X-ray tube is mounted inside the housing and in FIG. 2 only the wall 10 of a part of the housing is shown on the right side thereof. The walls of the housing are lined with an X-ray absorbing material of sufficient thickness, such as lead.
For example, a radiation exit window 11 made of an X-ray transparent material such as aluminum is provided only at the level of the target layer, so that the effective radiation is emitted only in that area. Effective radiation propagates perpendicular to the axis of rotation and at an angle of 30 ° to the direction of the electron beam. In the case of CT examination, a substantially flat, fan-shaped radiation beam is shaped by the radiation window in a direction perpendicular to the plane of FIG. The direction of maximum dimension of the X-ray exit window also extends perpendicular to the plane of the drawing.

While the present invention has been described as being used for medical purposes in a rotating anode X-ray tube having a glass jacket, the present invention can also be used in other embodiments. For example, a stationary anode can be used instead of a rotating anode. Instead of an X-ray tube consisting of a glass jacket, it is also possible to use an X-ray tube with a metal jacket whose cathode and / or anode are connected via an insulating material. The X-ray tube can also be used for non-destructive inspection in the industrial field. For these purposes, tube voltage 2
Particularly high efficiency is obtained in the range of 00 to 500 kV.

[Brief description of drawings]

FIG. 1 is a principle view of a part of a transmissive anode.

FIG. 2 is a rotary anode type X having a transmission type anode according to the present invention.
It is a figure which shows a line tube.

[Explanation of symbols]

 1 Target Layer 2 Carrier Layer 3 Electron Beam 4 X-ray 5 Enclosure 6 Cathode Device 7 Anode Device 8 Rotor 10 Housing Wall 11 Radiation Exit Window

Claims (4)

[Claims] 1. Colliding with electrons in an operating state,
What is claimed is: 1. An X-ray tube having a transmission type anode comprising a target layer made of a metal having a large atomic number of 1 or 2 or more, and a carrier layer bonded to the target layer made of a substance having a small atomic number of 1 or 2 or more, An X-ray tube characterized in that an angle θ between an incident direction of electrons and a direction of X-rays in an effective radiation beam emitted through a carrier layer is in a range of 10 ° to 40 °. 2. The angle θ and the rated operating voltage of the X-ray tube have at least approximately the following relationships: The X-ray tube according to claim 1, wherein 3. When the weight per unit surface area of the target layer is expressed in g / cm ² , at least about the following relationship: w = 1.08 · 10 ⁻⁶ ((A / Z) ^2.5 · U ^1.6 · cos)
β Here, A is the atomic weight of the metal of the target layer, Z is the atomic number thereof, U is the rated operating voltage of the X-ray tube in kV, and β is the angle formed by the electron incident direction and the direction perpendicular to the target layer. The X-ray tube according to claim 1 or 2, characterized in that 4. A rotary anode type X-ray tube, wherein the target layer (1) is on a frustoconical surface surrounding an angle (α ₁ ) with respect to the direction of the X-ray used outside the X-ray tube. 4. The arrangement according to claim ₁ , wherein the angle (α ₁ ) is smaller than the angle (θ) between the X-ray direction and the electron incident direction. X-ray tube.