RU2117358C1 - X-ray tube revolving anode - Google Patents

Abstract

FIELD: X-ray engineering; large X-ray tubes used for medical diagnostics. SUBSTANCE: uniform heating of substrate and reduced stresses in substrate and target are attained by using variable geometry and/or material composition and properties of material. Substrate layer thickness H in vicinity of focusing track is minimum (E₁H $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$ /E₂)^0,5 and maximum 10(E₁H $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ /E₂)^0,5, where H₁ is target thickness, E₁ and E₂ are Young moduli of target and substrate layer, respectively; width of this region amounts to at least size of focusing track. EFFECT: improved service life and reliability of revolving anode. 7 cl, 2 dwg

Description

 The invention relates to x-ray technology, and more particularly to rotating anodes of x-ray tubes used in medical diagnostics.

 A rotating anode of an X-ray tube is known that contains a graphite substrate and a target of a refractory material or alloy, while in the graphite substrate, in order to reduce thermal stresses caused by a significant difference in the linear expansion coefficients of the substrate and target materials, recesses are made in the form of a blind hole system distributed over its surface, filled with target material [1].

 However, this anode design is quite difficult to manufacture, the consumption of expensive tungsten increases, in addition, high thermal stresses appear in the target, increasing with decreasing target thickness. Ultimately, this adversely affects the life of the anode and its reliability.

 The closest technical solution to the invention by its technical essence - the prototype, is a rotating anode of an X-ray tube, in which to prevent cracking of the target on the target surface a large number of slots are formed that are symmetrical about the axis of symmetry of the anode [2].

 However, the known design still does not allow to sufficiently get rid of the stresses arising in the target. In addition, with such a design, the power of the X-ray tube is proportional to the area of the slits in the target, and this design is also difficult to manufacture.

 The present invention is to increase the service life and increase the reliability of the rotating anode, as well as reducing its cost.

The problem is solved in that in the rotating anode of the x-ray tube made of a substrate and placed on its target surface, the substrate is made variable in geometric dimensions and / or material composition and material properties, and the thickness of the substrate layer H in the focal track area is not less than 0 , 1 • (E ₁ H $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ / E ₂ ) ^0.5 and not more than 10 • (E ₁ H $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ / E ₂ ) ^0.5 , where H ₁ is the thickness of the target, E ₁ and E ₂ are the elastic moduli of the target and the substrate layer, respectively, and the width of this zone is not less than the focal track.

 The substrate in the area of the focal track can be made with a cavity filled with a material whose boiling point is lower than the melting temperature of the substrate material, for example, Na, Li, and the distance between the target and the cavity is chosen equal to H.

 In addition, the target can only be placed in the area of the focal track, exceeding the width of the latter on both sides by no less than the thickness of the target.

 The target can be made of a single crystal, in addition, the density of the substrate material in the area of the focal track can be higher than the average density of the substrate material.

 The substrate can also be made of a material having greater ductility than the target material.

 In addition, the substrate can be made of a material with a decreasing radius coefficient of thermal expansion.

 As already mentioned, due to the difference in the linear expansion coefficients of the substrate and target materials, thermal stresses arise that adversely affect the life of the anode and its reliability. These stresses depend on the ratio of the thicknesses of the target and the substrate. With an increase in the ratio of the thickness of the substrate to the thickness of the target, the stresses in the target increase, but in the substrate, and vice versa. As a rule, in rotating anodes of X-ray tubes, the thickness of the target is significantly less than the thickness of the substrate, therefore, the stresses in the target at high temperatures are high enough, which can lead to cracking of the target. Thickening of the substrate is used to better remove heat from the focal spot region of the anode. However, during short-term exposures, the substrate does not have time to heat up, and in the intervals between exposures, cooling due to radiation leads to a significant redistribution of the temperature field in the anode, which allows one to choose operating modes that do not lead to overheating of the substrate without increasing its thickness. The implementation of the substrate is variable in geometric dimensions or material composition and properties of materials can reduce the temperature drops of a substantially unevenly heated anode and, accordingly, thermal stresses that occur in the anode during operation.

 Choosing the optimal ratio of the target and coating thicknesses, it is possible to select the minimum target thickness for the given operating modes of the x-ray tube, which simplifies the manufacture of the anode and reduces its cost due to the lower consumption of the target.

Computational and experimental studies show that the level of stresses that occur during operation of the anode substantially depends on the thickness of the target and the substrate in the focal track zone, as well as on the elastic moduli of the target E ₁ and substrate E ₂ . With a focused target thickness H ₁ and a substrate thickness less than 0.1 • (E ₁ H $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ / E ₂ ) the ^0.5 level of thermal stresses in the substrate will significantly exceed the corresponding level of stresses in the target, which is not desirable, since the target material is usually selected from a material with a higher tensile strength than the substrate material. In addition, thinning of the substrate in the region of the focal track can lead to overheating of the anode at sufficiently high powers and long exposures. When the thickness of the substrate is more than 10 • (E ₁ H $\genfrac{}{}{0ex}{}{\text{2}}{\text{1}}$ / E ₂ ) ^{0.5 a} high level of stresses in the target leads to its destruction during thermal cycling during operation. If the width of the substrate zone, in which the thickness of the substrate is performed according to the indicated ratios, is less than the focal track, the effect of the selected substrate thickness in this zone on the target voltage level is insignificant.

 The execution of a cavity filled with a material in a substrate, the boiling point of which is less than the melting temperature of the substrate material, can reduce the working temperature of the focal track, since when the temperature of the focal track zone is close to the boiling point of the material filling the cavity, the cavity acts like a heat pipe when the anode rotates ( during the rotation of the anode, the liquid is pressed against the outer wall of the cavity, and the evaporation products enter the inner wall), which allows cooling the focal track to get close to equal omernomu temperature distribution over the volume of the anode. As shown by calculation and experimental studies, when a cavity is performed in the anode substrate, the stresses arising in the target depend mainly not on the total thickness of the substrate, but on the distance between the target and the cavity.

 The application of the target layer only in the area of the focal track leads to a decrease in the consumption of the target material, in addition, this allows better heat removal from the focal spot region, using a more heat-conducting material for the substrate than the target material. This also reduces thermal stresses in the target and the substrate of the anode, which increase with increasing radius of contact of the target with the substrate. Exceeding the width of the target of the focal spot on each side by less than the thickness of the target can lead to overheating of the substrate.

 The execution of the target from a single crystal allows to increase the resource of the anode, since a single crystal material has greater plasticity and strength than polycrystalline. In addition, due to the channeling effect, a single-crystal target makes it possible to obtain x-rays of higher intensity.

 Computational and experimental studies show that large temperature differences and, correspondingly, a high level of stresses are realized only in the zone of the focal track, while thermal stresses in the remaining volume of the anode substrate are significantly lower. Therefore, to facilitate the anode and reduce the cost of its manufacture, the average density of the substrate material can be lower than the density of the material in the zone of the focal track (for example, the substrate can be made porous in the entire volume except the zone of the focal track).

 As the thickness of the target decreases, studies show that the plastic characteristics of the target approach the corresponding characteristics of the substrate, therefore, if the substrate is made of a material with greater plasticity than the target material, the focal track of the anode can withstand more thermal cycles to failure than the anode made entirely of material the target.

 As a result of pulsed heating of the anode during its operation, the base surface in the zone of the focal track substantially overheats relative to its central part. For example, with an anode diameter of 100 mm, the substrate of which is made of MH6 molybdenum alloy with a thickness of 5 mm and with a heat flux power of 13.5 kW, the radial temperature difference exceeds 500 K. Therefore, tensile stresses arise in the central part of the anode.

 When the central part of the substrate is made of an alloy, the CTE of which is 5% higher than the CTE of the MH6 alloy, the level of tensile stresses in the anode decreases by more than 15%, which ensures anode resistance to thermal loads.

 The drawing shows the proposed rotating anodes of the x-ray tube. They contain a substrate 1 on which a target 2 with a focal track 3 is placed. The drawing (view a) also shows a cavity 4 filled with a material with a low boiling point.

Concrete example
In a molybdenum substrate, the diameter of which is 120 mm, the thickness of the central part is 29 mm, and the angle of the conical working surface 12, a toroidal cavity 20 mm wide and 15 mm high is mechanically formed, the thickness of the bridge between the cavity and the working surface is 3.5 mm. Half of the volume of the cavity was filled with sodium. The diameter of the Central hole for the rotor shaft is 10 mm, after which a layer of a tungsten target with a thickness of 1.5 mm is applied (for example, from the vapor-gas phase) to the working surface of the anode. As shown by calculation and experimental studies, during operation of the anode in continuous mode with a heat flux rate of 9 kW, exposure time 12 with a break between exposures of 20 s, a break time between series of 12 s, the temperature of the focal track of such a design is more than 50 K below the temperature a similar anode without a cavity, while the stress level in the target is 20% lower than the voltage level in the anode without a cavity, which leads to an almost twofold increase in the anode resource.

 The inventive design provides in comparison with the prototype increased operational reliability and availability. The achieved reduction in operating voltages allows the use of the anode in high-power tomographs.

Claims (7)

1. The rotating anode of the x-ray tube made of a substrate and a target placed on its surface, characterized in that the substrate is made variable in geometric dimensions and / or material composition and properties of materials, and the thickness of the substrate layer H in the focal track area is not less
0.1 • (E ₁ H ₁ ² / E ₂ ) ^0.5
and no more
10 • (E ₁ H ₁ ² / E ₂ ) ^0.5
where H ₁ is the thickness of the target;
E ₁ and E ₂ are the elastic moduli of the target and the substrate layer, respectively,
and the width of this zone is not less than the size of the focal track.  2. The anode according to claim 1, characterized in that the substrate in the area of the focal track is made with a cavity filled with a material whose boiling point is lower than the melting temperature of the substrate material, for example, Na, Li, while the distance between the target and the cavity is chosen equal to H.  3. The anode according to claim 1, characterized in that the target is placed only in the area of the focal track, exceeding the width of the latter on both sides by no less than the thickness of the target.  4. The anode according to claim 1, characterized in that the target and / or substrate is made of a single crystal.  5. The anode according to claim 1, characterized in that the density of the substrate material in the area of the focal track is higher than the average density of the substrate material.  6. The anode according to claim 1, characterized in that the substrate is made of a material having greater ductility than the target material.  7. The anode according to claim 1, characterized in that the substrate is made of a material with a decreasing radius coefficient of thermal expansion.