EP1636818A1

EP1636818A1 - X-ray generator tube comprising an orientable target carrier system

Info

Publication number: EP1636818A1
Application number: EP04741818A
Authority: EP
Inventors: André Gabioud
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2003-06-20
Filing date: 2004-06-17
Publication date: 2006-03-22
Anticipated expiration: 2024-06-17
Also published as: EP1636818B1; WO2004114353A1; US20070064873A1; US7302044B2; FR2856513A1

Abstract

The invention relates to an X-ray generator tube comprising a target carrier system which enables the incline of the radiating surface of the target to be regulated in relation to the electronic beam in order to determine the intensity of the X-ray emission and the resolution of the tube according to the desired application. For high-energy applications requiring a cooling circuit, the arrangement of said cooling circuit also enables the geometry thereof to be essentially improved in order to essentially increase its efficiency. The invention further relates to a plurality of arrangements of the cooling circuit, and to the method for producing the same.

Description

X-RAY GENERATOR TUBE WITH TARGET ASSEMBLY

ADJUSTABLE

The field of the invention is that of X-ray generating tubes. The invention relates more particularly to the arrangement of the emitting surfaces which are at the source of the X-ray.

The operating principle of an X-ray generator tube 10 is exposed in FIG. 1. It mainly comprises a vacuum enclosure 6 comprising at one of these ends a cathode block 4 carried by an insulator 3 and at the other end an anode block 2. The anode block 2 comprises a target holder assembly 1 comprising a flat metal surface said target 9 disposed opposite the cathode block. The electron beam 7 coming from the cathode is accelerated under the action of very high electric voltages greater than 10 kVolts and strikes the target 9 in a focusing zone O where the electrons lose their kinetic energy. There follows a significant release of heat and an emission 8 of X-rays (symbolized by the arrows in FIG. 1). X-rays pass through the wall of the anode block at privileged locations 5 called windows.

The release of heat causes very intense localized heating at the target. In the case of tubes operating at high power, the rise in the temperature of the target is such that it could lead to the destruction of the target by fusion. Also, in this case, the release of heat is evacuated by a cooling circuit 60 passing through the target holder 1 under the target 9. In order to optimize the distribution of the X-ray in space in direction and intensity, the target 9 is inclined at an angle α relative to the mean direction of the electron beam 7.

The production of a target holder assembly therefore has two main constraints. On the one hand, the angle of inclination a must be adapted to the use and on the other hand, the cooling circuit must allow sufficient removal of the calories due to the impact of the electron beam. In known X-ray tubes, the target holder assembly generally has the shape of a shouldered cylinder as shown in FIGS. 2, 3 and 4. The axis of this cylinder is parallel to the direction of the electron beam .

A cutaway of the cylinder inclined at an angle α constitutes the target subjected to the beam.

When the power is low, a cooling circuit is not necessary. In this case illustrated in FIG. 2, the target holder assembly is connected to the anode block so that the calories are first transmitted to the periphery of the anode block by conduction through the various metal parts of the target holder assembly and of the anode block (internal white arrows in Figure 2) then evacuated to the outside by convection (external white arrows in Figure 2).

When the power emitted is greater, the previous provision is no longer sufficient. In these cases, a coolant circulation conduit which may be, for example, water or oil is required to extract calories from the target. The entry and exit of this fluid are in the part opposite the target of the target holder assembly. FIG. 3 illustrates a first embodiment of the cooling duct arranged inside the target holder assembly. It comprises a single tube 60 passing under the surface of the target and which best matches said surface. FIG. 4 illustrates a second embodiment of a coaxial type conduit. It comprises an inlet tube 60 located in the axis of the cylinder of the target holder, an internal cavity 61 which conforms as best as possible to the interior of the target holder and an outlet tube 62 connected to the internal cavity. This arrangement optimizes the exchange surface between the cooling fluid and the target holder assembly.

However, these different types of cooling circuits have drawbacks. In particular, these conduits have elbows which cause changes of direction for the fluid. These generate, at the level of the heat exchange surfaces with the target carrier assembly, zones where the speed of the fluid is almost zero and where the heat exchanges are therefore very low. In addition, these changes in direction induce pressure losses which can prove prohibitive when it is desired to increase the fluid flow rate in order to increase the possibility of heat dissipation. When an electron beam strikes the surface of the target at an incidence α corresponding to the angle of inclination of the target, the X-ray is emitted in all directions of space as shown in Figure 5. L the emission intensity indicator depends on the angle θ made by the direction of the radiation with the normal N at the surface of the target (dotted perimeter of FIG. 5). This indicator has a maximum for θ zero and tends to 0 when 0 tends to 90 degrees. It is not possible to use all the X-ray emitted and only part is recovered through a transmission window which defines a limited solid angle of emission. This window is necessarily located outside the path of the electron beam. If it is desired to recover a large part of the radiation emitted, the angle of inclination α must then be sufficiently large.

However, the angle of inclination also conditions the geometric resolution of the emission source X as illustrated in FIGS. 6 and 7. An electron beam 7 with a circular section of diameter, section also called finesse, falls on an inclined target of an angle α with respect to the direction of incidence. This beam will generate X-ray radiation. In a given emission direction, the X-ray radiation, passing through a diaphragm 11 of very small diameter, then has a divergence β. This divergence β is proportional to the angle α as shown in FIGS. 6 and 7. This divergence β conditions the resolution of the X-ray generator tube and the sharpness of the images observed. Indeed, if a screen 12 is placed in the X-ray, the image of the diaphragm is no longer almost punctual but has a certain dimension directly proportional to the divergence β. Consequently, to obtain small image sizes, that is to say high resolutions, it is necessary to reduce the angle of inclination α.

The angle of inclination α is necessarily the result of a compromise between, on the one hand the energy of the X-ray radiation and on the other hand, the resolution of the tube. Depending on the applications, the tube designers are thus led to propose, for the same configuration of tubes, different versions of target-holder assemblies in which the inclinations of the target vary. The study, implementation and management of these different variants generate additional costs and additional delays which can be important, given the complexity of the room and the materials used.

The invention proposes to replace these different variants by a single assembly making it possible to adjust the angle of inclination of the target. The arrangement of the part also makes it possible to improve the geometry of the cooling circuit in order to significantly increase its efficiency. On the other hand, the various mechanical parts do not involve complex machining.

More specifically, the subject of the invention is an X-ray generator tube comprising an electron gun emitting an electron beam, an anode block comprising a target holder assembly having a plane surface called the target on which the electron beam is focused in a spot. focusing (O), characterized in that the target holder assembly has an axis of revolution substantially perpendicular to the mean direction of the electron beam and passing through the plane of the target.

Advantageously, the target-holder assembly is of generally cylindrical shape with circular section, the target being located in a plane passing through the axis of revolution of the cylinder and the anode block comprises a housing of also generally cylindrical shape in which is housed said target holder assembly so that the axis of revolution of the target holder assembly passes through the focusing spot.

For applications requiring significant X-ray radiation, advantageously, the target holder assembly comprises at least one internal main coolant circulation duct passing through the target holder assembly in a direction substantially parallel to its axis of revolution and passing under the target to cool it.

The invention will be better understood and other advantages will appear on reading the description which follows given without limitation and thanks to the appended figures among which:

• Figure 1 shows a sectional view of an X-ray generator tube comprising a target holder assembly according to the prior art. • Figure 2 shows a sectional view of an anode block comprising a target holder assembly without cooling circuit according to the prior art.

• Figure 3 shows a sectional view of an anode block comprising a target holder assembly comprising a first type of cooling circuit according to the prior art.

• Figure 4 shows a sectional view of an anode block comprising a target holder assembly comprising a second type of cooling circuit according to the prior art. • Figure 5 shows the X-ray emission indicator.

• Figures 6 and 7 represent [influence of the angle of inclination of the target on the resolution of the tube.

• Figure 8 shows a perspective view of the target holder assembly according to the invention. • Figure 9 shows a front view and a side view of the target holder assembly according to the invention.

• Figure 10 shows a sectional view of a target holder assembly according to the invention comprising a coolant circulation duct. • Figure 11 shows a perspective view of the part of the conduit located under the target.

• Figure 12 shows a perspective view of a set of cylindrical secondary conduits with circular section placed under the target. • Figure 13 shows a sectional front view and a side view of the target holder assembly comprising cylindrical secondary conduits with circular section.

• Figure 14 shows a perspective view of a set of secondary cylindrical conduits of triangular section placed under the target.

• Figure 15 shows a perspective view of a set of cylindrical secondary conduits in the shape of an arch placed under the target. • Figure 16 shows a sectional front view and a sectional side view of the target holder assembly comprising cylindrical secondary conduits of triangular section.

The heart of the invention is to make the angle of inclination of the target adjustable on the mean direction of the electron beam while maintaining the focusing of the beam on the target. There are different possible mechanical arrangements.

By way of nonlimiting example, the target holder assembly 1 has the general shape shown in the perspective view of FIG. 8. This figure represents a target holder assembly 1 without a coolant circulation duct. The target holder assembly generally has the shape of a cylinder of revolution. The central part of this cylinder includes machining. In this machined part, half of the cylinder has been removed in order to define a planar surface 9 which constitutes the surface of the target. Thus, the target is in a plane passing through the axis 20 of the cylinder so that when the cylinder rotates about its axis, the center of the target always occupies a fixed position. FIG. 9 represents a front view and a sectional profile view of the target holder assembly 1 mounted in the anode block 2. This comprises a cylindrical recess of diameter substantially equal to that of the target holder assembly so that said assembly 1 can rotate without play in the anode block. The axis of revolution of this cylinder is substantially perpendicular to the mean direction of the electron beam and this axis passes through the focusing spot of the electron beam 7 as indicated in FIG. 8. This arrangement makes it possible to optimize the diameter of the focusing spot O. Under these conditions, when the target holder assembly is rotated in the anode block, the target surface tilts at a variable angle α and the focusing of the electron beam on the target is preserved . To position the target at a particular angle α, there are different possible methods using, for example, a suitable tool which are not the subject of this invention and which are known to those skilled in the art. Once this inclination has been adjusted, the target-holder assembly is brazed in the anode block in order on the one hand to maintain this inclination and on the other hand to ensure the vacuum tightness of the assembly, sealing necessary for the operation of the electron gun. This provision is very advantageous insofar as the machining operations of the various parts (target holder assembly and anode block) are simple and can be carried out with great precision.

For high-power tubes requiring a coolant circulation duct, the above arrangement is particularly suitable for installing said duct. By way of example, FIG. 10 represents a sectional view of a target-holder assembly of the type of that of FIGS. 8 and 9 comprising a coolant circulation pipe 60. This passes right through 'target carrier assembly along its axis of revolution and passes under the target 9. The exchange of calories is mainly in the area below the target called exchanger. This geometry which does not have mechanical bends ensures good transfer of the coolant through the target holder assembly, greater than that of the devices according to the prior art. Sleeves 63 arranged at the ends of the duct ensure its connection with the circuits for the arrival and discharge of the coolant.

The design of the exchanger conditions the efficiency of the coolant circulation duct. It results from a compromise between optimal efficiency and acceptable mechanical complexity. In a first type of embodiment presented in the perspective view of FIG. 11, the exchanger mainly consists of two plane walls parallel to each other and separated by a thickness e. The first wall is located under the target and parallel to it. Consequently, the water circulates in the exchanger in the form of a sheet of thickness e (parallel arrows in FIG. 11). This exchanger has reduced performance given its limited exchange surface. It is possible to improve its efficiency by using it in two-phase mode, the quantities of heat absorbed by the phase changes, for example when the liquid water passes in the form of vapor, thus making it possible to improve the efficiency of the circuit of cooling.

To improve the efficiency of the exchanger, it is also possible to increase the area of the exchange surface. The perspective view of FIG. 12 shows a first embodiment of an exchanger with a large exchange surface. In this embodiment, the exchange surface consists of a plurality of secondary conduits 64 of cylindrical shape and generator parallel to the axis of revolution of the target holder assembly. The conduits 64 are separated from a wall of thickness P and have a diameter d. Typically, the diameter d is between 0.8 millimeters and 3 millimeters and the thickness P must be less than d. This optimizes the exchange surface which is, in this case, much greater than that illustrated in Figure 1 1. Figure 13 shows two views of the target holder assembly comprising an exchanger according to the previous arrangement. In this case, the conduit 60 has at its ends two cylindrical bores 65 and in the area of the exchanger a plurality of secondary conduits 64 according to the arrangement of FIG. 12, each of these conduits opening into the cylindrical bores 65. When the 'the target holder assembly is oriented as indicated in the side view, the exchanger assembly follows the inclination of the target. The machining of the conduit can be done simply by drilling through one of the ends of the cylinder. However, drilling holes of small diameter, typically less than 1.5 millimeters in materials such as copper can be difficult over long lengths, typically greater than 10 times the diameter. In this case, it is possible to replace the process for producing the exchanger by conventional machining with the process comprising the following production steps:

• Production of a first mechanical assembly 1 of generally cylindrical shape comprising a main conduit 65 passing through said first assembly in a direction substantially parallel to its axis of revolution and in its central part a recess comprising a flat surface

101, the main conduit 65 opening into this recess.

• Production of a second mechanical assembly 102 comprising a flat upper surface and a lower surface comprising identical grooves 103. This second set can be of generally cylindrical shape.

• Assembly of the second set in the recess of the first set so that the grooves 103 are placed opposite the planar surface 101 of the recess, the upper surface of the second set constituting the target 9, all of the grooves of the second set and surface plane of the recess constituting as many secondary conduits forming the exchanger.

The final shape of the conduits depends on the initial shape of the grooves, thus making it possible to configure the desired exchange surface. By way of example, FIGS. 14 and 15 show two forms of grooves 103.

In Figure 14, the grooves are V-shaped and the final section of the conduits is triangular. In FIG. 15, the grooves are in the form of an arch and the final section of the conduits is in the form of an inverted D. FIG. 16 represents a front view in section and a profile view in section showing the arrangement of the target holder assembly 1 comprising the mechanical assembly 102 in the anode block 2. In this arrangement, the ends of the conduit can also have adapter sleeves 63.

Claims

1. X-ray generator tube (10) comprising an electron gun (4) emitting an electron beam (7), an anode block (2) comprising a target holder assembly (1) having a plane surface (9) called a target on which the electron beam (7) is focused in a focusing spot (O), characterized in that the target holder assembly (1) has an axis of revolution (20) substantially perpendicular to the mean direction of the electron beam ( 7) and passing through the plane of the target (9).

2. Tube (10) according to claim 1, characterized in that the target holder assembly (1) is of generally cylindrical shape with circular section, the target (9) being located in a plane passing through the axis of revolution (20) of the cylinder and that the anode block (2) comprises a housing also of generally cylindrical shape in which is housed said target-holder assembly (1) so that the axis of revolution (20) of the holder assembly target goes through the focus spot.

3. Tube (10) according to claim 2, characterized in that the target holder assembly (1) comprises at least one internal coolant circulation duct (60) passing through the target holder assembly in a direction substantially parallel to its axis of revolution and passing under the target to cool it.

4. Tube (10) according to claim 3, characterized in that the conduit (60) comprises a central part called an exchanger placed under the target and formed of several secondary conduits (64) of cylindrical shape and generatrix parallel to the axis of revolution of the target holder assembly.

5. Tube (10) according to claim 4, characterized in that the section of the secondary conduits is circular.

6. Tube (10) according to claim 5, characterized in that the secondary conduits have a diameter greater than the thickness of the wall separating them.

7. Tube (10) according to claim 4, characterized in that the section of the secondary conduits is triangular or in the form of an arch.

8. Method for making an anode block assembly for an X-ray tube (10), comprising the following steps: • Making a target holder assembly (1) having a flat surface (9) called a target having a axis of revolution (20) passing through the plane of the target (9);

• Production of an anode block (2) comprising a housing;

• Positioning of the target holder assembly (1) in the housing of the anode block (2) so that the axis of revolution (20) is substantially perpendicular to the mean direction of the electron beam (7) of emission of the tube (10);

• Adjustment of the angle of inclination α of the target (9) on said mean direction by rotation of the axis (20); • Fixing of the target holder assembly (1) in the anode block (2).

9. A method of producing an anode block assembly according to claim 8 comprising a target holder assembly (1) according to claim 4, characterized in that the step of producing the target holder assembly comprises the sub- following steps:

• Production of a first mechanical assembly of generally cylindrical shape comprising a main conduit (66) passing through said first assembly in a direction substantially parallel to its axis of revolution and in its central part a recess comprising a flat surface

(101), the main conduit (66) opening into this recess.

• Production of a second mechanical assembly (102) comprising a flat upper surface and a lower surface comprising identical grooves (103). Assembling the second assembly (102) in the recess of the first assembly so that the grooves (103) are placed opposite the flat surface (101) of the recess, the upper surface of the second assembly constituting the target, l 'set of grooves of the second set and the flat surface of the recess constituting as many secondary conduits forming the exchanger.