EP0584871B1 - X-ray tube with anode in transmission mode - Google Patents

Description

The invention relates to an X-ray tube with a transmission anode, which comprises a target layer made of one or more metals with a high atomic number hit by electrons in the operating state and a carrier layer made of one or more substances with a low atomic number connected to the target layer.

Such X-ray tubes are known - for example from DE-A-27 29 833, from US-A-20 90 636 and from US-A-3 894 239. There are conflicting requirements for the thickness of the two layers. On the one hand, the target layer should be as thick as possible in order to convert the impinging electrons as high as possible into X-ray quanta. On the other hand, this layer must be as thin as possible in order to weaken the X-ray quanta generated in it as little as possible. On the one hand, the carrier layer must be thin enough to weaken the emerging X-rays as little as possible and, on the other hand, thick enough to ensure the mechanical stability and the dissipation of the thermal energy generated in the target layer.

Probably because of these contradicting requirements, these X-ray tubes - at least for a voltage range between 50 and 500 kV, which is important for medical, but also for industrial examinations - have hardly found their way into practice. For this purpose, X-ray tubes with anodes are used, in which the X-rays are emitted from the side of the anode on which the electrons strike. These anodes are therefore also referred to below as reflection anodes.

With all X-ray tubes, only a small part of the electrical energy applied is converted into X-rays in the voltage range up to 500 kV; the rest of the energy used leads to the heating of the anode. Only a small fraction of the X-rays generated is used outside the X-ray tube as a useful beam.

The object of the present invention is to design an X-ray tube of the type mentioned at the outset, whose operating voltage is in the range between 50 kV and 500 kV, in such a way that with the electrical energy applied to operate the X-ray tube, more X-radiation is generated in the useful radiation beam than in the case of an X-ray tube Reflection anode. This object is achieved by the measures specified in the claim.

The invention is based on the knowledge that the intensity of the X-rays is very dependent on the angle that the emitted X-rays form with the direction of the electrons. Neglecting the weakening by the target results in a pronounced maximum intensity on the lateral surface of a cone, the central axis of which is formed by the direction of the electron beam generating the X-rays. The opening angle of this cone depends on the operating voltage, and the smaller the higher the operating voltage, the smaller it is. For an operating voltage of 60 kV, half the opening angle of the cone with the maximum intensity is approx. 40 °, and for an operating voltage of 500 kV approx. 10 °.

The invention takes advantage of this knowledge by determining the angle between the light beam, i.e. selects the part of the X-ray radiation used outside the X-ray tube, and the direction of incidence of the electrons generating the X-ray radiation accordingly.

As a rule, the useful beam has an aperture angle that differs from zero at least in one direction. In this case, the angle between an X-ray beam in the center of the useful beam and the direction of incidence of the electrons must be chosen as specified in the claim.

In the previously known x-ray tubes with a transmission anode, the useful beam generally runs in the extension of the electron path, ie the angle θ is zero.

However, there are also x-ray tubes with a transmission anode in which the angle θ is different from zero. From US-A-3 894 239 a rotating anode X-ray tube with a transmission anode is known, in which an electron beam strikes approximately perpendicularly on a target layer which is inclined by approximately 80 ° with respect to the radiation exit window. is. This is intended to weaken the continuous brake radiation spectrum generated in the target layer considerably more than the fluorescence radiation generated in the target layer.

Furthermore, FIG. 7 of DE-A-27 29 833 describes an X-ray tube with an annular anode, in which the X-radiation is generated by means of two groups of cathodes distributed over the circumference of the anode, which are arranged on both sides of a central plane running through the radiator . This results in an angle θ of 45 °.

Neither of these publications takes advantage of the fact that the X-rays are particularly intense in an angular range between 15 ° (at high tube voltages) and 40 ° (at low tube voltages).

Finally, from WO-A-92/03837 an X-ray tube with a reflection anode is known, in which the electrons strike the anode at an angle of 10 ° (instead of usually 70 ° - 90 °) and in which the useful beam is at an angle of 5 ° - 15 ° with respect to the anode. However, the radiation exit window can heat up considerably due to scattered electrons.

wobei A die relative Atommasse und Z die Ordnungszahl des Metalls der Targetschicht ist, U die Betriebsspannung in kV, für die die Röntgenröhre ausgelegt ist, und β der Winkel ist, den die Einfallsrichtung der Elektronen mit der Normalen auf die Targetschicht einschließt. Für eine Röntgenröhre mit einer Targetschicht aus Wolfram ergibt sich daraus für eine Betriebsspannung U = 100 kV eine Masse pro Flächeneinheit von 0,017 g/cm² bzw. eine Dicke von 8,6 µm (für β=0°).In an embodiment of the invention it is provided that the weight w of the target layer per unit area, which is essential for the X-ray yield - expressed in grams / cm ² - at least approximately meets the relationship: $${\text{w = 1.08 x 10}}^{\text{-6}}{\text{· (A / Z)}}^{\text{2.5}}{\text{· U}}^{\text{1.6}}\text{· Cosβ,}$$

where A is the relative atomic mass and Z is the atomic number of the metal of the target layer, U is the operating voltage in kV for which the X-ray tube is designed, and β is the angle which is the direction of incidence of the electrons with the normal to the target layer. For an x-ray tube with a target layer made of tungsten, this results in a mass per unit area of 0.017 g / cm ² or a thickness of 8.6 μm for an operating voltage U = 100 kV (for β = 0 °).

The invention can be used in different X-ray tubes for different applications. According to a preferred development of the invention, it is provided that it is designed as a rotating anode X-ray tube and that the target layer (for example made of tungsten and / or rhenium) lies on the lateral surface of a truncated cone which encloses an angle with the direction of the X-rays used outside the X-ray tube , which is smaller than the angle that exists between this direction and the direction of the incident electrons. The anode is in the form of a bowl which is symmetrical with respect to its axis of rotation, the inner surface of which is provided with the target layer and faces the electron-emitting electron source and the useful beam of rays is preferably emitted from the outer surface at an angle of 90 ° to the axis of rotation.

Fig. 1

eine Prinzipzeichnung eines Teils einer Transmissionsanode und

Fig. 2

eine Drehanoden-Röntgenröhre mit einer erfindungsgemäßen Transmissionsanode.

The invention is explained in more detail below with reference to the drawings. Show it

Fig. 1

a schematic diagram of part of a transmission anode and

Fig. 2

a rotating anode X-ray tube with a transmission anode according to the invention.

The transmission anode shown in FIG. 1 comprises a target layer 1 made of a metal with a high atomic number, which is applied to a carrier layer 2 made of a material with a low atomic number. The target layer 1 can consist, for example, of tungsten or rhenium or of an alloy of these metals; other metals suitable for the target layer 1 are platinum or thorium. The carrier layer 2 can consist of graphite or beryllum and have such a thickness that, on the one hand, there is sufficient mechanical stability and the X-ray radiation, if possible is weakened little.

The arrow 3 denotes an electron beam which strikes the target layer 1 at an angle β with the normal. This creates X-rays that spread on a sphere around the point of impact. However, theoretical and experimental investigations have shown that neglecting the weakening by the target layer, the X-rays, which spread on the surface of a cone (with its tip in the electron impingement point and its axis of symmetry parallel to the electron beam direction) with a certain opening angle θ, have the greatest intensity . The upper limit beam 4a and the lower limit beam 4b of this cone are shown in FIG. 1. Half the opening angle θ of this cone depends on the operating voltage, whereby the table applies approximately: U / kV 60-100 100-150 150-200 200-350 350-500 θ 40 ° - 35 ° 35 ° - 30 ° 30 ° - 25 ° 25 ° - 20 ° 20 ° - 15 °

$${\text{α}}_{\text{2}}\text{= 90° - β + θ}$$

Therefore, the x-ray tube must be designed so that the direction of the useful beam coincides with the direction of one of the beams on the cone jacket. The X-rays generated in the target layer can run at different angles to the layer planes, the drawing showing the smallest angle α ₁ and the largest angle α ₂ . The equations apply to these angles $${\text{α}}_{}$$$${}_{\text{1}}\text{= 90 ° - β - θ}$$

$${\text{α}}_{\text{2nd}}\text{= 90 ° - β + θ}$$

The optimal mass of the target layer per unit area for the radiation yield is calculated approximately according to the relationship $${\text{w = 1.08 x 10}}^{\text{-6}}{\text{· (A / Z)}}^{\text{2.5}}{\text{· U}}^{\text{1.6}}\text{· Cosβ}$$

A is the relative atomic weight and Z is the atomic number of the metal from which the target layer is made. β is the angle of incidence of the electrons, ie the angle that the direction of the electron beam 3 forms with the normal to the target layer. If the target layer consists of an alloy of two or more metals, the mass of the target layer per unit area is calculated by using each metal of the alloy calculates the value w according to equation (3) and the calculated values are weighted according to the respective alloy proportion.

If the radiation exit direction is selected in accordance with the table and the thickness of the target layer is dimensioned in accordance with equation (3), the intensity of the X-ray radiation in the useful beam bundle is significantly greater than for an X-ray tube with a reflection anode, with the angle between the same tube voltage and the same tube current Electron incidence direction and beam exit direction is approx. 90 °. The greater the tube voltage, the more pronounced the increase in intensity. - However, if the X-ray tube is operated at a voltage other than that for which it is designed, these intensity advantages decrease.

2 shows a rotating anode X-ray tube with a transmission anode according to the invention as an exemplary embodiment. The x-ray tube comprises a tube bulb 5 made of glass, in which a cathode arrangement 6 and an anode arrangement 7 are located. The anode arrangement comprises a transmission anode 2 which is fastened in a known manner to a rotor 8 which is rotatably mounted in the interior of the X-ray tube. The rotor is driven by a stator arranged outside the glass bulb and not shown in FIG. 2.

The transmission anode comprises a carrier body 2 made of graphite and has a bowl or plate shape which is open towards the cathode arrangement 6. In the region of the transmission anode which is swept by the electron beam 3 from an electron emitter fastened to the cathode arrangement 6, a target layer 1 made of rhenium is applied to the carrier body 2. If the X-ray tube is intended for the purposes of computer tomography and is accordingly designed for an operating voltage of 150 kV and if the electron beam 3 strikes the layer at an angle of 40 ° with the normal direction, then the mass of this layer, based on the unit area, is according to equation (3) 0.024 g / cm ² . This is achieved by a 11.5 µm thick rhenium layer.

The X-ray tube is located inside a housing, of which part of the housing wall 10 is shown in FIG. 2 only on the right side. The housing wall comprises a lining made of an X-ray absorbing material, for example lead of sufficient thickness. Only at the level of the target layer is a radiation exit window 11 made of a material transparent to the X-rays, e.g. made of aluminum, so that useful radiation can only escape in this area. The useful radiation then runs perpendicular to the axis of rotation at an angle of 30 ° to the direction of the electron beam. When used for CT examinations, an almost flat fan-shaped bundle of rays is masked out perpendicular to the plane of the drawing in FIG. 2 through the radiation exit window. In this case, the main direction of expansion of the radiation exit window likewise runs perpendicular to the plane of the drawing.

Although the invention was explained above on the basis of a rotating anode X-ray tube with a glass bulb intended for medical examinations, the invention can also be used in other embodiments. For example, a fixed anode can be used instead of a rotating anode. Instead of an X-ray tube with a glass bulb, an X-ray tube with a metal bulb can also be used, in which the cathode and / or anode are connected to the metal bulb via insulators. The X-ray tube can also be used for non-destructive examinations in the industrial sector; In the range of tube voltages (200 - 500 kV) used for this purpose, the efficiency is particularly high.

Claims (4)

1. An X-ray tube, comprising a transmission anode which comprises a target layer which is struck by electrons in the operating condition and consists of one or more metals of high atomic number, and also comprises a carrier layer which is connected to the target layer and consists of one or more substances of low atomic number,
   characterized in that the angle θ between the direction of incidence of the electrons and the direction of the central ray of the useful x-ray beam emitted through the carrier layer is between 10° and 40°.
2. An X-ray tube as claimed in Claim 1, characterized in that the angle θ and the rated operating voltage U of the X-ray tube satisfy at least approximately the following relation: U/kV 60-100 100-150 150-200 200-350 350-500 θ 40°-35° 35°-30° 30°-25° 25°-20° 20°-15°
3. An X-ray tube as claimed in any one of the preceding Claims,
   characterized in that the weight w of the target layer per unit of surface area, expressed in g/cm², at least approximately satisfies the relation: $${\text{w = 1.08 · 10}}^{\text{-6}}{\text{· (A/Z)}}^{\text{2.5}}{\text{· U}}^{\text{1.6,}}\text{cosβ}$$

   

   where A is the relative atomic weight and Z is the atomic number of the metal of the target layer, U is the rated operating voltage of the X-ray tube in kV, and β is the angle enclosed by the direction of incidence of the electrons relative to the normal to the target layer.
4. An X-ray tube as claimed in any one of the preceding Claims,
   characterized in that it is constructed as a rotary-anode X-ray tube, and that the target layer (1) is situated on the surface of a truncated cone which encloses an angle (α₁) relative to the direction of the X-rays used outside the X-ray tube, which angle is smaller than the angle θ existing between said direction and the direction of the incident electrons.

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