US3165658A - Directly-cooled x-ray tube anode - Google Patents

Description

Jan. 12, 1965 M. J. ZUNICK DIRECTLY- COOLED X-RAY TUBE ANODE Filed March 51, 1961 INVENTOR. MICHAEL J. ZUNICK "WflW ATTORNEY fer.

United States Patent 3,165,658 DHKECTLY-CGOLED X-RAY TUBE ANGDE Michael J. Zunich, Huhertus, Win, assignor to General Electric Company, a corporation of New York Filed Mar. 31, 1961, Ser. N 9?,897 Claims. (Cl. 3l330) This invention generally relates to X-ray generator tubes, and more particularly, to an X-ray target or anode construction.

The X-ray generator here involved is conventional in respect that it comprises an electron emitter, such as a hot cathode, which develops a sharply focused electron beam that impinges on a metal anode target to generate X-rays. It is well known that the efficiency of X-ray generation by this phenomena is low, and it is safe to say that in all cases more than ninety percent of the electron beam energy is converted into heat which must be dissipated from the target. If the heat developed at the concentrated focal spot is not conducted away rapidly enough, the target is likely to be perforated or roughened and rendered incapable of producing X-radiation from a sharp point or line source as is required in radiographic and analytical techniques.

One prior art approach to dissipating heat from the target in stationary anode tubes has been to insert the X-ray generating target material into a massive metallic heat sink that provides a heavy cross-section for conducting heat and presents a heat exchange surface to a fluid coolant. A common construction is one where a tungsten target is set in the vacuum exposed face of a cup-like anode member so the inside of the cup may be bathed with coolant on a side away from the vacuum enclosure of the X-ray tube envelope. Embedding the X-ray target in a cast copper support contributes toward solving the problem of heat conduction, but it raises other perplexing problems which are not easily solved. For instance, the copper must be cast in vacuum under carefully controlled conditions, and even then discrete grains of copper, whose size is greater than the thickness of the casting, are usually formed. These grains are separated by perceptible boundaries in the nature of minute fractures that allow gases to pass into the tube envelope and destroy its vacuum, thereby rendering the tube inefficient for production of X-rays. The condition is aggravated by the fact that it is desirable to make the section of the cup immediately underneath the target insert as thin as possible in order that the target may be in close proximity with the fluid coolant for obtaining optimum heat trans- In such a thin section, it is not uncommon for the grain boundaries to extend directly from the water immersed surface to the vacuum surface without being overlapped by any grains which would intervene and close the boundaries or lengthen their path to the evacuated region.

Aside from the difliculty of obtaining sound copper castings, an anode structure of the type just described presents another problem. Copper is frequently incompatible for joining or brazing directly with the material out of which the tube envelope is made so it requires interposition of another material that is compatible with both the copper and the envelope. This further increases the number of parts in the X-ray tube and, obviously, increases the cost and the likelihood of leaks developing in the additional joints. Moreover, joining a series of metal parts creates additional interfaces which reduce heat conductivity regardless of how intimately the parts are fused or brazed.

Accordingly, it is an object of this invention to overcome the above noted disadvantages by providing an X-ray tube target assembly that can-be efficiently cooled without jeopardizing gas-tight integrity of the tube enveope. A further object is to provide an X-ray generator water or gas.

3,165,658 Fatented Jan. 12, 1965 tube that can be constructed without resorting to hydrogen or vacuum casting techniques. Another object is to simplify the tube construction by eliminating intervening materials that were formerly necessary for obtaining compatibility between various parts that are to be joined or sealed and to eliminate interfaces that impede the flow of heat. Other general objects are to provide a new X-ray tube that is simple in its construction, eflicient, small, in expensive and rugged. I

Achievement of the foregoing objects and other more specific objects will appear from time to time throughout the course of the ensuing specification.

In general terms, a preferred embodiment of the present invention is characterized by an evacuated metal envelope on which is sealingly mounted at one end a glass part that supports a cathode assembly within the envelope. At the other end of the envelope there is provided a novel anode construction which presents an X-ray generating target to an electron beam emanating from the cathode when a voltage is applied between it and the anode structure. The target support or adapter is usually made of a metal different than the target in the form of a cup that may be turned from solid stock or drawn from a metal sheet. By techniques which are later described in more detail, the target material can be directly joined with the adapter which can be directly joined with the envelope. It is preferable that theadapter be hollow on the nonevaeuated side to enable bathing it with coolant such as One side or face of the target insert is exposed directly to the coolant so as to obtain optimum heat transfer.

A preferred embodiment of the invention will now be described in greater detail in conjunction with the drawing in which:

FIG. 1 is a side elevational assembly view of the X-ray tube with parts broken away to expose the interior thereof;

FIG. 2 is a fragmentary view of an X-ray tube showing an alternative form of the novel target assembly;

FIG. 3 illustrates another form of target assembly embodying the principles of the invention;

FIG. 4 is a detail cross-sectional view of part of a target assembly suitable for use in the tube illustrated in FIG. 1;

FIG. 5 is a front view of the assembly shown in FIG. 4; and,

FIG. 6 is an enlargement of a part of FIG. 1 illustrating how the X-ray target is assembled with its supporting adapter before being heated to flow the brazing wire ring which is shown in circular cross section.

FIG. 1 illustrates a grounded anode X-ray tube assembly comprising an envelope 10 constituting a hollow cylindrical tube of No. 304 or 347 stainless steel having a wall thickness of about .065 inch, for example. Other grades of stainless steel or other materials having different wall thicknesses may be suitable in some cases. Within envelope 10, there is provided a conventional hot cathode emitter 11 mounted in insulating relationship with respect to a metal focusing cup 12 whose details are omitted because they are well known to those making X-ray tubes.

Cathode cup 12 is supported in spaced relationship from the metal envelope lit by means of a re-entrant annular support 13 which may be of glass or other insulating material. To connect glass part 13 with stainless steel envelope 10, it is necessary to first weld on the envelope a U-shaped annular adapter 15 which may be made of Kovar or other suitable material that is compatible for joining with stainless steel and glass. To achieve this, envelope 10 has its outside diameter reduced as at 14 to provide a seat for U-shaped ring 15. With the ring 15 deposited as shown, and before the glass is attached, the

ring and tube envelope 149 may be joined by a weld as at 16. The welding process here employed is commonly j) known as tungsten inert gas welding, abbreviated TIG, and is adapted for being carried on in ambients of various gases such as helium, argon and carbon dioxide.

Before ring is located and before weld in is made, the ring is wetted with a small glass bead 17 at its edge which is to join the glass part 13. Glass part 13 is then attached at head 17 by conventional glass joining procedures.

At the left end of the tube assembly in FIG. 1, there is shown the conventional lead wires 13 which supply power for heating cathode emitter '11. Also extending into the broken away part of tube Ill) is a post 15 to which may be attached one side of a high voltage source, the other side being grounded and thereby connected with the X-ray tube anode so as to accelerate electrons from the cathode for generating X-rays;

X-rays generated within evacuated envelope It? are transmitted to the exterior for doing useful radiography or analysis through thin metal windows Zll which are preferably of low atomic number metal such as beryllium. The windows may be fastened over apertures in envelope 1%? by any means. In the region of the tube wall ill in which the holes for setting the windows are formed, the wall is flattened and drilled with a small hole about which there remains a flat shoulder. In the present design, it is deemed advisable to set the beryllium window on the internal flanges of a cup washer and let its external flanges bear upon the flat shoulder. Another washer, with an opening as large as a desired window, may be set on the beryllium and the assembly brazed vacuum tight to envelope 1%. Of course, the joints between the beryllium window and its supporting washer should also be joined with suitable brazing material so as to insure vacuum tightness. The details of the window assembly and means for joining it to a metal tube envelope are known in the art of building X-ray tubes and need not be elaborated on here.

Attention is now invited to the Xray target assembly shown at the right hand of the generator in FIG. 1. It is seen to comprise a metal disk or button 21 of a high atomic number element like tungsten on which electrons emanating from cathode 11 impinge in a sharply focused spot or line to generate X-rays that emerge through the windows 2t Target 21 is supported on a conical adapter 22 which in this case may be made of No. 304 or No.347 stainless steel. Adapter 22 may be formed by turning it from a flaw-free billet of stainless steel or it may be advantageously drawn as will be further discussed later. Adapter 22 has an opening 23 which is surrounded by a pair of step like annular counterbores 24 and 25, see

FIG. 6 for enlarged details. The annular bearing surface defined by counterbore 24 supports tungsten target disk 21 in close alignment with the axis of envelope it).

The second shoulder 25 allows enough free space around the periphery of tungsten target 21 to enable placing a ring of brazing wire ill around the target for brazing target 21 and adapter 22 before assembling with the envelope It). The brazing wire may be thirty-thousandths inches in diameter and constitute eighty percent nickel and twenty percent manganese. When the wire is in place, after suitably cleaning the surfaces, the adapter 22 and target 21 are placed in a hydrogen or vacuum furnace whose temperature can be raised at least up to 1300 C. in which case the Wire flows and fuses to effect a bond between tungsten target 21 and stainless steel adapter 22 along the shoulder 24, 25. To enhance fluxing and assure a vacuum tight bond, it is desirable to pre-plate or clad tungsten 21 with a coating of nickel between one-half thousandth and sixty-thousandths inch thick, for instance, in the region where the braze joint occurs.

It will be observed in FIG. 1 that adapter 22 is provided at its wide end with a pair of lips in and 27 that are conterminous with the end of envelope 1%. These are for facilitating welding of the adapter to adjacent parts. When the adapter is fitted flush into the end of envelope 10 as shown, it may be joined therewith at 26 l by the TlG welding or fusion process mentioned earlier. Thus, in this case a direct stainless steel to stainless steel bond is effected Without resorting to use of an intervening member that would have to be mutually compatible for brazing with adapter 22 and envelope 19 and that would create undesirable interfaces.

A coolant chamber 28 is defined inside of adapter 22 by closing it with an annular element 29 also provided with a lip 3% which facilitates welding it with lip 27 on the adapter. Frojecting inte rally from annular closure elementZ? is a fluid deflector element 31 through which projects a cooling water inlet tube 32 that may be brazed to element 29 by any suitable water-tight means. Water is exhausted from chamber 28 by an exit tube 33 which is also secured in 29 by brazing.

Thus, it is seen that coolant admitted through inlet tube 32 is directed straight at the face of tungsten target insert 21 so as to effect optimum heat transfer therefrom. Because of the mushroom shape of deflector 31, the coolant is caused to iiow away from target 21 along the interior periphery of adapter 2?. whence it is discharged through tube 33. The maximum temperature gradient is obtained between the target 21 and the cooling water,

where it is most needed, and a subsiding temperature from a billet as is adapter 22 in FIG. 1.

gradient is obtained along the wall of adapter 22 to transfer heat which is conducted from the target to the adapter. A typical quantity of cooling water used in practice is nine pints per minute at twenty pounds per square inch.

The tungsten target 21 may be made of various thicknesses to suit conditions. If it has a heavy cross section, it is more adapted to conduct heat away from the focal spot in a lateral direction to thereby increase the area of maximum heat transfer, whereas, if it is thinner, heat generated at the focal spot travels a shorter path to the coolant which is directly impinging on its face.

Because of conditions under which steel billets are worked during manufacture, an adapter 22 turned from such billet may have its metal crystals oriented in parallel with each other and theraxis of the adapter. Such unidirectionally oriented crystals may have grain boundaries that will allow minute gas penetration during the life of the tube. There may also be axial flaws in the raw stock in some cases. To avoid this possibility, FIG. 2 shows an alternative form of target adapter 42 which is drawn from a sheet of stainless steel rather than being turned The advantage of this is that all of the grains are oriented in a single direction so that when the material is formed they always lie in parallelism with each other along the adapter rather than across its section. This reduces the possibility of leaks through grain boundaries in thin sections. In the FIG. 2 construction, target 21 may be similarly brazed in adapter 42 as was described in connection with the previous embodiment. Likewise, a water cooling chamber 28 may be defined by welding a stainless steel closure member 29 to the adapter and tube 10 at the edges of each of them.

An alternative target assembly is shown in FIG. 3. This embodiment is particularly well adapted to use in a tube where the X-ray exit windows 20 are desired to be located closer to the end of the envelope 10 for facilitating location of associated instruments in an X-ray diffractometer, for example. In this construction there is provided an adapter ring 50 that terminates in radially extending flanges 51 which allow joinder with the end of tube envelope 19 such as by a TIG weld at 52. The adapter may be provided with an opening 53 for admitting cooling water in a fashion similar to that described in connection with the previous embodiments. -In this case, however, the target of tungsten or other suitable material 54 is concave-convex or cup shaped and adapted to fit over a suitable upstanding flange 55 on adapter 50. If the target cup 54 is tungsten, it may be brazed at 56 to stainless steel adapter. 59 by means such as described above. This involves heating the adapter and target 54 assembly to 1300 C. so as to melt a ring of brazing wire that may be interposed between the lip 55 and the plane of adapter 50. Again the brazing wire is preferably eighty percent nickel and twenty percent manganese. When heated as mentioned, the brazing wire flows and fuses with the surface of adapter 50 to form a vacuum and 7 water tight bond of high mechanical stren th.

The target assemblies shown in FIGS. 2 and 3 may be advantageously employed in diffraction tubes and high energy radiographic tubes where it is desired to reduce the overall length of the tube as much as possible or where 21 or 54 are to be used as transmission targets. That is,

plications, it is desirable to assure that none of the radiation produced at the surfame of the X-ray generating target be intercepted by its edges before passing through windows 20. For this reason, it may be desirable to shape the target in accordance with FIG. 4 where a target 21' is shown to be chamfered at its edges. The charnfered edges may be in substantial parallelism or inclined at the same angle as the wall of adapted 22. Thus, it will be seen that even though radiation is produced in a line 45 as is evident from the face view in FIG. 5, the target will not intercep any appreciable portion of it.

In tubes designed for X-ray analysis work, various metals may be substituted for the tungsten of target 21 in FIG. 1 and for the adapter 22, envelope 10, and brazing wire For example, target 21 may be chromium, copper or silver brazed to a stainless steel adapter 22 with a copper-gold or copper-silver alloy brazing wire 40. The target 21 may be of iron, cobalt or nickel and the adapter 22 of stainless steel brazed with copper wire 40. Non-corrosive metals like titanium, Monel or nickel may be used for envelope 1% and adapter 22, in which case these parts may be joined directly by TIG welding and the target 21 may be brazed with a wire depending upon the target metal as in the case of a stainless steel adapter. It is occasionally desirable to develop X-radiation whose spectrum is characteristic of a target made of gold, platinum or other precious metal. In these cases it would be unnecessarily costly to make the whole target 21 of the precious metal. This can be obviated, without sacrij ficing the concept of the direct target cooling, by brazing a thin wafer or depositing a layer of the expensive metal on a common metal target, such as copper or steel, in the region where X-rays are produced by the impinging electron beam.

In summary, it will be seen that a simplified X-ray target assembly has been described which allows direct joining of compatible material and which oflers optimum heat transfer characteristics by virtue of allowing coolant to directly impinge upon the target insert. Moreover, the problem of tube leakage is minimized where the invention permits avoiding the use of a vacuum cast enclosure member and its attendant inclination to leak along grain boundaries.

Although an embodiment of the invention using pre- 6 I ferred components and materials has been described, this is to be considered illustrative rather than limiting, for various materials may be substituted and the invention may take other forms. Accordingly, what is embraced by the invention is to be determined by construing the claims which follow.

It is claimed: 7

1. An X-ray generator tube comprising:

(a) an evacuated envelope at least a portion of which is metal,

(b) a cathode structure mounted in one end of said envelope for emitting an electron beam toward the other end of the envelope,

(0) a hollow adapter means having a wide end the edge of which is welded directly to the edge that constitutes the end of the metal envelope,

(d) said adapter means also having an apertured narrower end that terminates remote from the wide end in the direction of the cathode structure,

(e) a target element constituting a disk of metal sealed over the aperture to close the same and to present one of its sides toward the cathode and its other side directly to the inside of the hollower adapter means,

(f) means for applying coolant fiuid directly to said.

(g) and an X-ray transmissive window in the metal envelope in a region that would be intersected by extending the plane of the target element and which window is spaced from the welded joint between the adapter and envelope. I

2. The invention set forth in' claim 1 wherein said adapter means comprises a hollow truncated cone, said hollow truncated cone and said other side of the target element defining a coolant fluid chamber, the small diameter end of said cone having said aperture and shoulder means around it providing a seat for said target.

3 The invention set forth in claim 2 wherein said adapter means is provided with a pair of concentric lip means extending axially in a direction remote from said beam, one of said lip means being conterminous with and welded to said envelope, a closure element inserted in and having a part conterminous with the other of said lip means and welded thereto, said closure elementdefining a coolant chamber with said adapter means.

4. The invention set forth in claim 1 wherein said en-' velope and adapter means are stainless steel and said target metal is selected from the class consisting of copper, silver, chromium, tungten, iron, cobalt and nickel.

5. The invention set forth in claim 1 wherein said envelope and adapter means are selected from the class consisting of stainless steel, titanium, nickel and Monel.

References ited in the file of this patent UNITED STATES PATENTS Zunick et al May 8, 1962

Claims (1)

1. AN X-RAY GENERATOR TUBE COMPRISING: (A) AN EVACUATED ENVELOPE AT LEAST A PORTION OF WHICH IS METAL, (B) A CATHODE STRUCTURE MOUNTED IN ONE END OF SAID ENVELOPE FOR EMITTING AN ELECTRON BEAM TOWARD THE OTHER END OF THE ENVELOPE, (C) A HOLLOW ADAPTER MEANS HAVING A WIDE END THE EDGE OF WHICH IS WELDED DIRECTLY TO THE EDGE THAT CONSTITUTES THE END OF THE METAL ENVELOPE, (D) SAID ADAPTER MEANS ALSO HAVING AN APERTURED NARROWER END THAT TERMINATES REMOTE FROM THE WIDE END IN THE DIRECTION OF THE CATHODE STRUCTURE,

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