US1733744A - Composite x-ray target - Google Patents

Description

Gm. 29, 1929. J, W. MARDE N ET AL coMPosITE x-RAY TARGET Filed Jan. 16'. 1928 Patented Oct. 29, 1929 'UNITD s'rA'ra JOHN WESLEY 'MABDEN AND CLAYTON TRIDL ULREY, OE EAST ORANGE, AND LOUIS F. EHBKE, OF NEWABK, NEW JERSEY, ASSIGNORS T PANY, A CORPORATION QF PENNSYLVANIA l 1 WESTINGHOUSE LAMP COM- COMPOSI'I'E X-BAY 'TARGET Application flled January 16, 1926. Serial No. 81,645.

This invention relates to X-ray targets and more particularly to a composite X-ray target composed at least partially of thorium or uranium,i. e., metals whose oxides are irre- 5 ducible by hydrogen.

It has been customary in the manufacture of com osite X-ray targets consisting of a body o 'copper and a tungsten insert or button to melt the copper around the tungsten insert. Considerable difliculty, however, is encountered in the manufacture of composite targets in which the insert is thorium or uranium for the reason, in'the case ofthorium, it alloys with the copper and forms an intermetallic compound which has the formula ThCu4. This compound'is very hard and brittle and when the button is heated the thorium breaks loose from the copper. This alloy A is a poor conductor of heat and prevents the proper heat conduction from the focal spot or button. In addition, owing to the difi'erence in expansion of the two metals, the copper having a co-eflicient of expansion which is about 30 per cent greater than that of thorium, there is danger of permanent separation of the metals when the target is heated beyond the temperature at which the copper will restore itself to its original Shape.

Thorium and uranium are more suitable than tungsten or Platinum for X-ray purposes since they possess superior ray-radiating properties since the atomic weights of thorium and uranium aregreater. The melting points of thorium and uranium are however, less than that of tungsten.

Owing to the high heat-concluctivity of such metals as copper, silver and gold, it is preferable to insert a thorium button dlrectly into such metals, but owing to the alloying effect between thorium and these metals and the difference 'of co-efi'icient of expansion therebetween as mentioned above, it is diflicult toform a composite target thereof. w

T his diificulty has been overcome, by the present invention, by the provision of an intermediate material between the thorium orI uranium and copper which material Will unite satisfactorily to the co per without the formation of an alloy and which Will also wet the thorium or uranium so as to forma perfect bond therewith. The material which has been found to be most satisfact'ory for this.

intermediate bonding agent is tantalum, although molybdeum has also been found to be suitable for this purpose. The invention contemplates, however, the use of other bonding materials such as tungsten, or in fact any material which lhas a heat-conductivty as high or higher than thorium and with which both thorium andl uranium and copper form an intimate contact.

Therefore, the main characteristics requisite for a composite target developed by the i present invention are z (1) that the thorium or uranium intimately unites or is held to a backing of copper or other high heat-conductivity metal by means of an intermediate bonding vmaterial which is ca able of surface-holding 'the dther materials; 2) the use of an intermediate bonding material in which the co-efficient of expansion is not utilized in any way for holding the thorium or uranium button in place or for making inti'mate contact; (3)

the utilizationz as the intermediate bonding material of a metal having approximately the same mass as thethorium or uranium button but having a smaller mass than the the elimination of any welding compounds or brazmg mixtures in producing the union between the metals forming the composite- Another object of the invention is to prohacking' metal to which it is united' and (4) vide a composite target consisting o f a thorium or uranium button or disk intimately united to al backing material such as copper in such'manner that the contact therebetween is maintained when the thorium or uranium is heated to near its melting point.

Another object of the invention is to provide a method of joining thorium or uranium to a body of good heat-conductivity to form a serviceable X-ray target or anode.

A further object of the invention is to provide a composite target having a thorium or uranium button'or disk united to a body of large mass by means of an intermediate body having the same mass as. the thorium or uranium button but which is capable of forming an intimate union with the backing material and the thorium or uranium.

A still further object of the invention is to provide a composite target for X-ray tubes and the like having a thoriumor uranium button ordisk joined to a material of good heatconductivity b an intermediate body of metal having a co-e 'cient of expansion which is not a factor in maintaining the contact.

A more specific object of the invention is to provide a compositetarget for X-ray tubes and the like consisting of a thorium button or disk united to a copper backing'by a metal ;o such as tantalum or molybdenum4 A still more specific object of the invention is to provide a composite target for X-ray tubes and the like eonsisting of a thorium or uranium button which is relatively thin and 85 which is united to a copper backin by means of a relatively thin disk of tanta um.

- Other objects of invention will become apparent as the following description is read, taken in conjunction with accompanying drawings in which:

Fig. 1 is a crosssectional view of a button or disk of compressed thorium or uranium powder temporarily secured by means of Wire to the intermediate bonding material such as tantalum, prior to being heat-treated.

Fig. 2 is a in Flg. 1.

Fig.' 3 is an elevation-al view, partly broken away; of. a high-vacuum high-frequency inplan' view of the material shown ;o duction furnace showing a'thorium or uranium button on a disk of tantalum in position to be heat-treated. w

Fig. 44: is a crosssectional view of the disk of thorium as it appears after the preliminary heat-treatment.

Fig; 5 is a crosssectional view of a disk of thorium on the intermediate bonding material ready to receive a second heat-treatment.

Fig. 6 is a crosssectional view of a thorium or uranium button on a disk of tantalum, the

former being fused thereto, said view showing these metalsl after they have received the second heat-treatment.

Fig. 7 is an elevational view,.partly in crosssection, showing the method pursued in comwhich the partially finished target shown in Fig. 8 is inserted and in which the copper is placed for the purpose of melting the same around and to the intermediate bonding material.

Fig. 10 is a crosssectional view of a composite X-ray target embodying the present invention and in which the thorium or uranium button has been reduced in thickness.

Fig. 11 is a view similar to Fig. 10 but in which the shank of the target is composed of one piece of copper or a similar good heatconducting metal, and

Fig. 12 is a crosssectional view of a composite target in which the thorium or uranium button -is shown directly united to a massive body of tantalum or the like.

A composite'target manufactured in accordance with present invention may be produced by forming a button 14 from a good grade of thorium or uranium powder. This button is formed by pressing the same into al disk of the desired diameter and of relatively small mass. After being formed, the pressed button is placed upon a disk of tantalum or sten or molybdenum wire 16 of small diameter.

The next step in the process is to heat-treat the thorium so as to sinter the same. This is accomplished by inserting the tantalum disk with the thorium or uranium button 14 thereon in a high-vacuum furnace 17 containing a high frequency induction coil 18 said furnace. bein more fully described and illustrated in atent 1,480,301. After the furnace has been evacuated to a high degree, the'tantalum disk and thorium or uranium. button are heated slowly until the adsorbed and absorbed gases are driven ofl from the thorium or uranium, the vacuum. pump connected to the furnace-being kept in operation throughout the .heating of the material. After the ases have been umped away and the thorium thoroughly egasified,' the temperature is raised to just below the melting point of thorium or uranium by 'circulating eddy currents in the materials by hi h-frequency' induction. It will be foun that durin the heat-treatment operation just mentione thelthorium or uranium becomes distorted and as'sumes a concavo-convex form 19, being incontact with the tantalum around' its marginal edges, this being due probably to the shrinkage of the thorium or uranium on the tantalum disk. VTheappearance of the materials at this time is illustrated in Fig. 4.

'i trode 31.

similar to' that shown in Fig. 5; thereafter the two disks are replaced in the furnace and, after evacuation, the heating is continued until the thorium or uranium begins to melt and until the tantalum disk and the thorium or uranium button have the appearance 22 in the furnace as illustrated in Fig. 6. It appears that the thorium or uranium during this second heat-treatment melts at the center first and the liquid thorium or uranium welds the major portion of the button lon to the tantalum disk into a uniform solid mass.

After cooling, the thorium Oruranium and tantalum, which rare now welded or' fused together, are removed from the furnace and 'this composite body subjected to pressure by means vof a hardened steel plunger 23 which is forced against the button. This produces an effect corresionding to that obtained by mechanical wor ing so that the thorium or uranium is in a very compact state.

After the compression step, the composite body of thorium or uranium and tantalum is placed in a lathe and turned down to the shape 24 illustrated in Fig. 8.4 This composite body is then inserted into a graphite container 25 (Fig. 9) having an opening 26 in the bottom thereof of a size equal to that of the button with the edges 27 of the tantalum disk resting upon the marginal edges of the opening. In order to 'provide a tight joint between the tantalum disk and the bottom of the crucible, it is customary to place allittle fiake of graphite or aquadag in the joint.` Previously vacuum-fused copper 28 isplaced on top of the tantalum disk 27 so as to very nearly fill the crucible 25. This 'crucible with its contents is then placed in the vacuum furnace, illustrated in Fig. 3, and after a good vacuum has been obtained, high frequency 'a currents are again circulated through the primary coil of the furnace which results in the copper being melted. The copper is maintained in the molten condition for a period of two minutes or longer in order to aiford an opportunity for any gas bubbles to ,escape therefrom. After cooling, the crucible with its-contents is removed from the furnace. The copper hacking and the tantalum with the thorium and uranium adhering thereto is removed from the crucible and shaped into an desirable form such as that illustrated in ig. 10; In this view the copper backing has been turned'down and a threaded shank 29 provided thereon; this shank being threaded into a copper elec- Previous to mounting the composite target in the electrode 81, it is desirable' to first reduc'e the thickness of the vthorium or uranium button, inasmuch as these metals must be very thin since their heat-conductivity is low as compared to copper and the amount of heat which flows through a plate thereof under a given temperature difl'erence between its faces is inversely proportional to the thickness of the plate.

The quantity of heat which flows through the plate can be expressed by the following equation z KA' T z) K=Thermal conductivity of the material;

A=the effective area of'focal spot T1 the temperature of hotter side of plate i T2=the temperature of the cool side of plate;

d=the thickness of the plate. In the case of a thorium X-ray target button in copper I-I, which is the quantity of heat flowi'ng through the button, should be as large as possible. The surface area of the button is practically limited by the size of the focal spot. Letting T1'correspond to the melting point of thorium .and T2 the melting point of copper, K being a Constant for thorium, then the only factor which can be varied is (Z, that is the thickness of the thorium. Substituting the values for these factors. that is the values for the melting point of thorium, copper, and a constant for thorium, as well as the effective area of the focal spot, it will be found that the thickness (Z in order to obtain the i maximum quantity of heat which may fiow through the plate of thorium should be made at least as small as .015" or less. ing explanation is made with reference to the thickness of the focal spot in order to emphasize the importance of making the thorium focal spot as thin as is mechanically possible and practical under normal usage.

The power which can be dissipated at lthe focal spot is limited by the thermal conductivity of the metal, at the focal spot, when the target is 'one having a heat capacity large enough so that the average temperature rise of the target is small in comparison with that of the focal'spot, after a single exposure of short duration. The metal at the focal spot can be melted before the rest of the targethas risen in temperature any appreciable amount. The

temperature of the focal' spot fiuctuates with.

fluctuations of the voltage. The average power which can be dissipated by a target depends upon the form 'of the voltage applied to the tube. The greatest average power can be dissipated when the tube is supplied with'unij- The forego- 'sheet of tantalum' or molybdenum they will be firmly united thereto and form an intimatc w bond'therewith.

metals;

' which the composite target is inserted, it is obvious that an entire shank of solid copper 32,

such as illustrated in F ig. 11, may be formed by simply enlarging the size of graphite crucible 25 inwhich the copper is melted and joinedto the tantalum.

Another modification of the invention is illustrated in Fig. 12 in which the thorium or uranium button is united to a large mass of tantalum 33 which latter may be provided with a threaded shank 34 which in turn may be screwed into a copper rod 35.

The advantage of using tantalum or molybdenum or. a similarmaterial as an intermediate bonding material has been set forth above.`

To reiterate, this advantage resides in the satisfactory .bond which these metals make With both thorium or uranium and copper. It is preferable to use tantalum as an intermediate onding material as more satisfactory results have been vobtained by use thereof, although i molybdenum has been employed for such purposes. It might seem that where the intermediate bonding material such as tantalum or molybdenum 1s employed that owing to the fact that these metals have a co-eflicient of exvpansion which is lower than either of the metals to which it is united, that the same Would not firmly adhere to these metals when the target is heated and cooled. It has been found, however, in actual practice that no separation takes place between these metals and that the firm contact is maintained so that the heat conductivity at all times remains unimpaired.

As explained above, the Steps actually followed in producing the composite body forming the subject of this inventioninvolve two separate heat-treatments to 'oin the thorium or uranium to the tantalum acking, it being apparent that this was e'ssential in' view of the fact that thefirst heattreatment resulted in the thorium. ur uranium taking the 'form 19 shown in Fig. 4. However, it is thought that vthis second heat-treatment can readily beeliminated by previously Shaping the button `so`tl1at the upper surface thereof is concave, similar in appearance to that shown in Fig. 5, thus providing for uniform shrinkage through the mass of thorium` orl uranium As explained previously, the con'-v` metal. cavo-convex fform orl shape 'of the 'thorium or uranium illustrated in Fig. 4 is a result of thel metal 'at the center of the buttony- 1 shrinking away from the tantalum bonding' material. By previously Shaping the tho- .rium or uranium button, as above explained, the shrmkage may be made to take place' ma more uniform manner with the result that the second heat-treatment may be elim- A inated.

Modifications of the present invention may occur to those skilled in the art, howev'er, such modifications are contemplated as fall defined bythe appended claims.

What is claimed is: i

1. A composite target for X-ray tubes and the like comprising abutton of a rare refractory metal of high atomic weight, totally fused to a hacking material, the heat-conductivity of which is at least as good as that of copper.

2. A composite X-ray target comprising a horium button intimately united to tantaum. o

3. A composite X-ray target comprising a fused thorium. insert intimately united. to tantalum, said tantalum being intimately united to a hacking material of good heatconductivity. o

4; A composite X-ray target comprising a hacking material, the heat-conductivity of which is'at least as good as that of`*copper,

having welded thereto a dense coherent body of thorium.

5. A composite body for X-ray tubes comprisin a hacking material of good heat-conductivity having welded thereto a thin disk of thorium, the thickness of said disk being not greater than about .015 of an inch.

6. An` X-ray target comprising a hacking material of good heat-conductivity and of large mass,,an intermediate bonding material of tantalum of relatively small mass and a button of good X-ray emitting material welded to said intermediate bonding material. p

7. An X-ray target comprising a button of thorium welded to a hacking materialof tantalum. a

- 8. An X-ray target comprising a large mass of copper and a button of thorium, the copper being joined to the t-horium by tantalum as an intermediate hondingmaterial.

9. The method of manufacturing composite X-ra targets employing a button of a metal of i gh atomic weight whichl comprises joining said metal to an intermedate bonding material which' is wet by the button within the scope of the present invention as material and then melting copper in a vacuum o to the intermediate bonding material.

10. The method of uniting thorium to copper to form a composite X-ray target which comprises fusing thorium to tantalum and thereafter melting copper in a vacuum to the tantalum.

In testimony whereof, We have hereunto Asubscribed 'our names this 15th day of J anuary, 1926.

JOHN WESLEYMARDEN. CLAYTON TRIDLE ULREY. LOUIS F. EHRKE.

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