US2168780A - X-ray tube - Google Patents

Description

Aug. 8, 1939 D. E. oLsHEvsKY X-RAY TUBE Original Filed Dec.

3mm M fw LParental Aug. `e, 1939 ,UNITED STATES PATENT GFFICE Divided and this application August 10. 1937, Serial No. 158,287

8 Claims.

This invention relates to X-ray tubes and especially to a tube for the generation and transmission of X-rays wherein the rays may be more readily controlled as to direction and be generated at a point close to the outer surface of the tube so that the object to be examined may be placed in a position close to the target or source of the rays. 'Ihe present application is a division of my co-pending application, Serial No. 500,583, flied December 6, 1930 (Patent No. 2,090,636).

In the past it has been the usual practice to generate X-rays by the discharge of electrons from a heated lament against a target which is opaque to the passage of the X-rays and reflects these rays through the walls of the tube. Usually the surface of the target is disposed at an angle of about to the electron beam, and the X-rays generated by the stoppage of the electrons are reflected through the side of the bulb at substantially a right angle to the path of the electrons. In such instances it will be apparent that the source of the X-rays is some little distance from the Wall of the tube and hence, as the rays radiate in'all directions from the target, the beam of the rays will spread considerably before the rays pass through the wall of the tube where they may be applied to the object to be examined. The result is that the rays which emerge through the wall of the tube cover a much greater area than is necessary or desirable, but where the rays are reected, it is impossible to entirely overcome disadvantage.

To most effectively generate X-rays, it is necessary to employ a target of a hard metal having a relativelyhigh .atomic number. Such metals are, however, more or less opaque to the passage of the rays, and hence a target of such metal, unless extremely thin, will not permit the X-rays to pass therethrough, but on the other hand the rays will be reflected from the surface of such a target.

At least one attempt has been made in the past to use a relatively thin target of a metal having a high atomic number and which would not only generate the X-rays, but would allow the transmission thereof through the target, but such attempt has not been attended with great success, on account of the fact that the impinging of the electrons on the target tends to heat the latter excessively, and, as the target must be very thin to permit the passage of the rays, the dissipation of the heat therefrom is substantially negligible.

One object of the present invention is to pro- (Cl. Z-149) vide an X-ray tube which will overcome the disadvantages referred to above and will provide for the generation of X-rays at a point adjacent the outer surface of the tube.

Another object of the invention is the provision of an X-ray tube which will employ a target of such a nature that, while the X-rays will be eficiently generated, at the same time the target will be transparent to such rays, and will also be of such a character that it will not become overheated.

A still further object of the invention is the provision of an X-ray tube having a target con sisting of two or more elements distributed in such a way as to insure superior conditions` as to X-ray generation, X-ray transmission and heat dissipation.

More specifically the invention relates to an X-ray tube having at one end thereof a multipart target which shall consist partly of a metal having a relatively high atomic number, and partly of a metal having a lower atomic number, the metals being alloyed or compounded, the first-named metal being relatively thin and adapted to generate the X-rays, and the lastnamed metal serving as a support for the first and being of such a nature as will permit the passage of the rays and also allow of ecient cooling of the target by ordinary heat conduction to the other parts of .the tube, by direct radiation, or by being cooled by the circulation of Water therethrough.

To these and other ends, the invention consists in the novel features and combinations of parts to be hereinafter described and claimed.

'I'he figure of the drawing is a sectional view of an X-ray tube embodying my improvements.

To illustrate one preferred embodiment of my invention, I have shown in the drawing a tube I0 having therewithin either a source of electrons or a source of positive rays. This source may be a heated filament, a cold point, a gaseous discharge or other suitable means. In the embodlliment illustrated there is employed an electronic lament II to which are connected the usual positive and negative leads I2 and I3 to heat the filament, and also one terminal of a high tension circuit I4, the other terminal I5, of which is connected to the base of the tube, designated generally by the numeral I6, at the central portion of which in the form shown is a target which will be described more particularly hereinafter. The tube I0, as shown, may preferably be formed, of glass to which is sealed a substantially cylindrical member I1 preferably 56 made of some metal such as copper. for example, so that a tight seal may be made between it and the glass portion of the tube. The cylindrical portion Il serves to loin the glass portion oi the tube to the base II, and in this instance is shown as formed integrally with the base. 'Ihe member i1 is provided with an annular shoulder Ill to which is threadedly secured an electrostatic shield Il which serves to prevent an electrostatic ileld being set up between the iilament and the wall of the tube. The electronic iilament Il, which is the source of electrons, may preferably be formed of tungsten, but variation in this respect may, of course, be resorted to, and any material used which will serve as the source of electrons.

The base I6 is provided with an opening at its central portion, and in this opening is secured a target I8, which will in the present instance be made of an alloy of some metal of lower atomic number, such as forged aluminum, and a metal having a high atomic number, such as molybdenum, silver, tungsten or gold. 'I'he joint I9 between the alloy comprising the target and the base IB will preferably be tightly sealed. Also, in the form shown, the base is preferably provided with water passages i6 closed by the plates lb..

It will be apparent that with this construction the target member Il can be removed from the base I6, and replaced with another target if desired. 'I'his will, of course, break the vacuum within the tube, and if the target is removed, it will be necessary to restore the vacuum, which may readily be done by connection with a pump for this purpose.

The target, which in this instance is an, alloy, is constructed of two or more metals, each of which has its particular properties and particular functions to be performed. It will, as stated, contain a metal of low atomic number, which will comprise a principal portion of the target, and a metal of high atomic number to generate the X-rays. The metal of low atomic number must be one which is relatively transparent to the passage of the X-rays, so that it may have sui?- ilcient thickness to act as a support and a carrier for the X-ray-generating portion of the target, and not impede to any great degree the passage of the rays. It will be understood that metals having a low atomic number usually also have a low melting point, and as it is necessary to keep this metal at a comparatively low temperature, it will be found desirable to arrange for the cooling of the metal by water or other uid, as described.

At the central portion of the target, depressions and 2| are formed so as to leave a relatively thin part 22 through which the generated X-rays may easily pass. As the metal of high atomic number present as a part of the alloy comprising the target is relatively opaque to the passage of X-rays, a relatively small quantity of this metal is employed. It will be present, however, in a sufficient quantity to cause the generation of the X-rays, but not to prevent the passage of the X-rays through the alloy of the target; that is, it will not be suillcient to render the target as a whole opaque.

It will, of course, also be understood that considerable variation in the metals employed in the alloy `may be resorted to, depending upon the character of the rays which it is desired to ob tain, assome metals are relatively transparent to rays of certain wave lengths. but will not permit the e of rays of' longer wave length,

and there will, at certain positions in the scale of atomic numbers, be metals which might be used either for the generation of X-rays or for the transmitting part of the target.. For instance, I may use for the metal of low atomic 5 number, constituting the greater portion of the body ci' the target, lithium, beryllium, boron, carbon, magnesium, aluminum, silicon, iron, nickel. cobalt, copper or silver, and for the X-ray-generating portion of the target I may use copper, chromium, molybdenum, silver, platinum, tungsten or gold. When two or more of these metals are alloyed together, as described, they will not only form a satisfactory target transparent to X-rays, but the two metals will be in intimate metal-to-metal heat-conducting relation by mutual molecular adhesion, so that the heat which is generated when the electrons strike the metal of high atomic number will be carried away by the main portion of the target, which in turn contacts closely with the base i6 of copper or similar material, this base in turn having provision for being cooled by a circulating iiuid.

It will also be seen that, owing to the depres sions 20 and 2i, the more transparent central part of the target will be limited as to area, so that the X-rays will emerge from the base of the tube through a relatively small opening, and substantially at the point of generation of the rays, so that any object to be examined may be placed close to the source of the rays. Also the entire tube may be shielded so as to protect the user from the harmful effects of the rays, and for this purpose I have provided the shield 25, which may be of lead or rubber or other suitable material. As shown, this shield may be of substantially cylindrical form, and may be provided with a base 26 having an internally threaded opening 21 in which may be screwed a portion of the base of the tube.

The shield may be closed at the end opposite the target by means of an insulating member 28 which may be formed of lead-glassor other suitable insulating material. If desired, the shield 2i and closure member 28 may both be made of the same material which will be both an electric insulator and opaque to X-rays. The closure member 2l is provided with an opening in which is received the member 29 carrying the positive and negative low tension leads 30 and 3i. These leads are connected respectively to the contact members 32 and 33, which are resilient so as to be urged into close contact with the members 34 and 35 on the tube.

It will be understood that the member 29 makes a tight connection with the surrounding portions` of the closure member 2li, and the interior of the shield 25 may be iilled with oil or a suitable insulating medium to prevent the passage oi current from the high` tension lead to the wall of the shield. It will also'be apparent, that with this construction, the parts of the shield including the contact members 32 and 3l may be assembled and thereafter the tube may be placed therein and screwed in place by means of the threaded connection 2l so as to provide for a ready and convenient assembly of the parts. It will be understood that, when the tube is screwed into place, the contact members 34 and l5 thereon will engage the members 32 and 33.`

The outer portion of the base may be threaded. as shown at I6, to provide for the connection thereto of any suitable apparatus which it may be desired to use in connection with the tube.

It will be observed that the target opening in u aioa'z'so the base is cone-shaped in form so' as to provide4 Ia conical seat for bthey target, `and thusv assist in providing for a tight seal'between the target and j vthe base; The depressions and ,2i are -also conical in form so as to providea relatively thin portion adjacentthe center of the target readily transparent to X-rays, and also to limit this-portion to a relatively small area so that the rays will be confined adjacent the point at which they -to the point at While I have shown and described a preferred Vembodiment of my invention, it will be understood that it is not to be limited to all of the details shown, but is capable of fmodication and variation within the spirti of the invention and within the scope of the appended claims.

What I claim is:

l. An X-ray tube having an envelope, a source of electrons, and a target, the latter being a part of said envelope and emitting as useful radiation X-rays having passed through the target, said target being relatively transparent to X-rays, and consisting of an alloy of a plurality of metals, one of said metals having a relatively high atomic weight and serving as an X-ray generating substance, and the other having a relatively low atomic weight and serving as an X-ray transmitting substance, and the amount of said second metal present in the alloy, being greater than that of the first metal.

2. An X-rayv tube having an nvelope, a source of electrons, and a target, the l ter being a part of said envelope and emitting as useful radiation X-rays having passed through the target, said target being relatively transparent to X-rays, and consisting of an alloy of a plurality of metals, one of said metals having a relatively high atomic Weight and serving as an X-ray generating substance, and the other having a relatively low atomic weight equal to or less than that of silver, andserving essentially as an X-ray transmitting substance, and the proportion of said lower atomic weight metal being greater than that of the metal of higher atomic weight.

3. An X-ra/y tube having an envelope comprising a'target and a source of electrons and emitting as useful radiation X-rays having passed through the target, said target being transparent to permit the passage of X-rays therethrough and consisting of an alloy having tungsten as a component of heavy atomic weight, the remainder of the alloy being composed of vmetal of relatively low atomic weight, such as beryllium, aluminum or copper, the proportion of said low atomic weight metal being greater than that of tungsten.

4. An X-ray tube having an envelope and a target and a source of electrons, said tube emitting as useful radiation X-rays passed through the target, said target being transparent to permit the passage of X-rays therethrough and consisting'of an alloy of a plurality of metals, one of the constituent metals having a relatively high atomic weight andserving as an X-ray-generat-` ing substance and another metal having a relatively low atomic weight equal to, or less than,

that of silver and serving essentially as an X-raytransmitting and heat-conductingY substance.

7 5..v An X-ray tube emitting as useful radiation X-rays having passed through the target, said tube having an envelope comprising a target and a source of electrons, said target being composed of aplurality of metals in the form of an alloy, one of said metals having a relatively high atomic weight and serving as an X-ray-gejnerating substance and another having a. relatively low atomic weight and serving mainly as an X-ray-transmitting substance, as a carrier for theA heavy atomic weight component, and as a heat-conductor, and

said alloy target beingremovably secured' in the base of the envelope.

6. An-X-ray tube-having an envelope, a source of electrons and a target and emitting as useful radiation X-rays having passed through the target, said target being a part of the envelope and transparent to permit the passage of the X-rays therethrough and being composed of a plurality of metals in the form of an alloy, one of saidmetals having a relatively high atomic weight and serving as an X-raysgenerating substance.

and another having a relatively low atomic weight and serving mainly as an X-ray-transmitting substance, as a carrier for the heavyatomic weight component and as a heat conductor, and said target being formed independently of the remainder of the envelope.

'7. An X-ray tube having an envelope, a source of electrons, and a target, the latter being a part of said envelope and emitting as useful radiation only X-rays having passed through the target, said target being' relatively transparent to X-rays, and consisting of an intimate mixture of a plurality of metals, one of said metals having a relatively high atomic weight and serving as an X-ray-generatingsubstance, and the other having a. relatively low atomic weight equal to or less than that of silver, and serving essentially as an X-ray-transmitting substanceyand the proportion of said lower atomic weight metal being greater than that of the metal of higher atomic weight.

8. An X-ray tube having an envelope comprising a target and a/source of electrons and emitting as useful radiation X-rays having passed

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