US 2090636 A
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Description (OCR text may contain errors)
Patented Aug. 24, 1937 Ul'i'ED STATS it QFFWE 2 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 gen- 5 erated 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.
In the past it has been the usual practice to generate X-rays by the discharge of electrons from a heated filament 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 45 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 reflected, 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 relatively high 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 provide 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 efficiently 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 10 provision of. an X-ray tube having a target consisting of two or more elements distributed in such a way as to insure superior conditions as Y-ray generation, X-ray transmission and heat dissipation. 15
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, or being arranged in layers, the first-named metal being relatively thin and adapted to generate the X-rayaand the last-named 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 efficient 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.
In the drawing:
Fig. 1 is a sectional view of an X-ray tube em- 35 bodying my improvements;
Fig. 2 is a fragmentary sectional view of a modified form of X-ray tube;
Fig. 3 is an enlarged cross-sectional view of a modified form of target which may be employed 40 in the tube;
Fig. 4 is a transverse sectional view on line 44 of Fig. 3, and
Fig. 5 is a sectional view of a further modification of the target. 45
To illustrate one preferred embodiment of my invention, I have shown in Fig. l of the drawing a tube It? 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 5O discharge or other suitable means. In the embodiment illustrated there is employed an electronic filament H to which are connected the usual positive and negative leads l2 and L3 to heat the filament, and also one terminal of a 55 high tension circuit 14, the other terminal l5,
. the target and which will be described more particularly hereinaften- The tube ID, as shown in Fig. 1, may preferably be formed of glass to which is sealed a substantially, cylindrical member; l1.
preferably 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 l1 serves to join the glass portion of the tube to the base' or target I6, and may be formed integrally with the base. As
shown it is formed separately and welded to the base as shown at I 7; This portion I I is also provided With'an annular shoulder l'l to which is threadedly secured an electrostatic shield H which serves to prevent an electrostatic field being set up between the filament and the wall of the tube. The electronic filament I l, which is the source of the electrons, may'prefer'ably be formed of tungsten, but variation in this respect may, of course, be resorted to and any material used which will serve as a source of electrons. While in; the present description the X-rays' are regarded as being produced by the stoppage of electrons or negativelycharged particles, it will n e be understood that the invention is not limited in this-respect, but, on the otherhand, the X-rays *may be produced by the discharge and stoppage of positively charged particles or protons.
The tube, as shown, is closed at one end by the base or target l6, which is a multipart target and consists of two or more metals as will presentlyhe described, each of which has its par-;
7 ticular properties'and particular functions to be performed. The main body portion l8 of the target is relatively thick in a direction axially 40 of the tube, and is preferably made'of a metal having a relativelylow atomic number. This metal must be one which is relatively transparent to'the passage of the X-rays, so that it may have suflicient thickness to act as asupport. for the X-ray generating portion of the target, and not impede to any great degree the passage of the rays. 'Italso is preferably of sufiicient thickness so that it may be provided with water passages 18* formed in the targetand closed by plates I9 5 throughwhich a cooling fluid such as water may be circulated to keep the target from becoming heated to excess. It will be understood that metals having a low atomic number usually also have a low melting point and, as such a metal must be used in order that the X-rays will be efficiently transmitted, it is necessary. to keep; this metal at a comparatively low temperature so thatit will not melt. It will, therefore, be found desirable to arrange for the cooling of the metalby water'or other fluid.
It will be noted that depressions 20 and 2| are formed in the portion I8 of the target adjacent the central portion thereof so as toleave a relatively'thinpart 22 through which, as will be aph e5parentas the description proceeds, the X-rays will pass.
Incorderto efliciently generate X-rays, it is necessary that the electrons be propelled against a target comprised by a metal having a relatively high atomic. number, and one which is relatively opaque to the passage of the rays, except whena very thin sheet is used. I, therefore, place upon the inside of the portion 22 of the target avery thin layer 23 of a metal of higher atomic tinumber than the portion 18 of the target. At this point it may be stated that, while the portion l8 and the portion 23 of the target comprise metals of lower and higheratomic numbers respectively, considerable variation in the metals employed in Ithese portions of the target may be resorted to depending upon the characterof the rays whichit is desired to obtain, as some metals are relatively transparent to rays of certain wave lengthsbut will not permit the passage of rays of longer wave lengths, and there will, at certain positions in the scale of atomic numbers, be metals which might be used either for the generation ofX-rays 'or for the transmitting part of the target.
For instance, for the portion I8 beginning with the metals of lower atomic numbers,'I may use lithium, beryllium, boron, carbon, magnesium, aluminium, silicon, iron, nickel, cobalt, copper or silver, and for the X-ray generating portion of the target I may use copper, chromium, molybde num, silver, platinum, tungsten, or gold. I have found that a very satisfactory tube may be produced by the use of a layer of molybdenum,
for example, upon a base or supporting member V placed in the tube for this purpose. That is, as
originally made, no layer of hard metal 23 would be provided, but a special auxiliary filament of the desired metal would be placed in the tube, and the metal. from this filament would, during the action of the tube, be carried over and deposited upon the target.
Instead of being in layers, the target might consist of an alloy of metals having the desired characteristics as heretofore explained, but it will generally be found preferable to arrange the difierentmetals in layers asdescribed. ,In any event, the target will consist of' two or more elements distributed insuch a way as to insure superior conditions with respect to X-ray generation, X-ray transmission, and heat dissipation. As will be understood, the dissipation of the heat which is generated will be realized by heat conductivity to the body of the tube by radiation or by a transfer of the heat to a cooling fluid in contact with the conducting metal.
As 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, it will be apparent that any object to be examined may be placed close to the source of the rays. 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 made of lead or lead 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 formedof lead-glass or othersuitable insulating material. If desired, the shield 25 and closure member 28 may both be made of the same material which will be both an electric Also the entire tube maybe shielded insulator and opaque to X-rays. The closure member 28 is provided with an opening in which is received the member 29 carrying the positive and negative high tension leads 3!! and 3 l 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 on the tube.
It will be understood that the member 29 makes a tight connection with the surrounding portions of the closure member 28, and the interior of the shield 25 may be filled with oil or a suitable insulating medium to prevent the passage of 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 slleld includ ing the contact members 32 and 33 may be assembled and thereafter the tube may be placed therein and screwed in place by means of the threaded connection 27 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 3d and 35 thereon will engage the members 32 and 33.
The portion iii of the target may be threaded, as shown at 36, to provide for the connection thereto of any suitable apparatus which it may be desired to use in connection with the tube.
In Fig. 2 of the drawing I have shown a somewhat modified form of tube having a base designated generally by the numeral le wherein the outer portion 4!] is integral with the cylindrical sleeve ii and is preferably composed of copper so that an efficient seal may be made with the 7 glass tube Ill The central portion 42 of the base constitutes the target which will in this instance be made of some metal of lower atomic number such as forged aluminium, and the layer 53 may be any of the metals having a high atomic numher such as molybdenum, silver, tungsten and gold. In this instance the joint 44 between the aluminium and copper portions of the target may be tightly sealed. In this instance also, the base is provided with the water passages 45 closed by the plates d5.
It will be apparent that with this construction the target member 42 may readily be removed and replaced with another target if desired. This will, of course, break the vacuum within the tube, to and, if the target is removed, it will be necessary to restore the vacuum. In some instances, however, such tubes are connected with pumps for this purpose, and in any event such replacement may be made at the factory without having to disturb other parts of the structure.
In Figs. 3 and 4 of the drawing, I have shown in vertical and horizontal sectional views a target comprising a portion of a metal of low atomic number upon which is a layer of metal of higher atomic number 5!. Within the metal 58 is provided a number of transverse passages 52 for the circulation of a cooling fluid, as water for example. In this instance it will be seen that the X-rays will necessarily pass through the cool- 65 ing fluid, but as the drawing is considerably enlarged, these passages will be shallow in the direction of travel of the rays and they will not impede the rays to a great extent. Connections for the ingress and egress of the cooling fluid are shown 70 at 53 and 54. As the main portion of the target will usually be made of a soft metal, such metal will lend itself readily to the formation of the fluid passages by milling or other mechanical operations so that the target, as shown, may be 75 conveniently and economically manufactured.
It will be understood that the portion of the target shown in these figures is only the central portion at the bottom of the depression as shown in Figs. 1 and 2.
In Fig. 5 of the drawing I have shown a further modified form of target of which the main body portion Ell is formed of a metal of a low atomic number transparent to X-rays and provided with a depression El to provide a relatively thin portion 62 which must be traversed by the rays. Opposite the bottom of the depression 6! a slight protuberance B3 is formed on the portion til of the target, and with the exception of this protuberance, the inner surface of the target is covered with a layer 54 of a metal of higher atomic number of sufficient thickness to be opaque to X-rays. Upon the protuberance $3 is a thin layer or coating $5 of a metal of high atomic number to generate the X-rays and allow them to pass therethrough. It will be apparent that, with this construction, the target is opaque to the passage of X-rays, except at that portion covered by the thin layer 55 of heavy metal. This portion of the target may be made of the desired shape and size so as to assist in defining the beam to the desired extent. In other words, this portion of the target forms a slit which permits the X- rays to pass while the remainder of the target is opaque and prevents such passage. This will be found advantageous in the study of crystals and in other applications of X-rays where it is necessary or desirable to define closely the beam of the rays to the area desired for the particular work to be done. Complete definition of the beam may, of course, be obtained by placing additional slits axially of the tube and spaced from the slit above described.
It will be apparent that in all forms of the invention shown, the beam of X-rays is: more or less defined and is generated close to the outer surface of the tube. For this reason, the object to be examined under the rays may be placed close to the point of generation and thus the beam may be confined to the work at hand and not permitted to spread out to cover an unnecessarily large area. It will, of course, be understood that the outer depression in the soft metal portion of the target may be made relatively shallow, or in some instances, may be omitted entirely so that the object may be brought to a position immediately adjacent the target.
The target construction shown in Figs. 3 and 4 of the drawing will also be useful whether or not a cooling fluid is passed through the passages 52. Although no cooling fluid is used, these passages will reduce the thickness of the target where they are formed and it will, therefore, be more transparent to the X-rays, while at the same time the ribs formed between the channels will serve as conducting members to conduct the heat from the central portion of the target to the periphery thereof where it may be dissipated by the means shown in connection with Figs. 1 and 2. In some aspects of the invention, therefore, it is advantageous to provide a target of this character with depressions or channels upon its rear side to render the target more transparent to the X- rays and at the same time conduct heat to the periphery of the target.
While I have shown and described a preferred embodiment of my invention, it will be understood that it is not to be limited to the details shown but is capable of modification and variation within the spirit of the invention and the scope of the appended claims.
-What I claim is: V r e ,1. An X-ray tube having an envelope comprise ing a target and a source of electrons, said target being transparent to X-rays and comprising a relatively thick layer of heat-conducting metal of low atomic number and a relatively thin layer of an X-ray-g'enerating element, said thick layer having a multiplicity of small transverse grooves formed therein in the path of the-issuing X-rays in the path of the X-rays. t
2. An X-ray tube having an'envelope comprisfor increasing the transparency of the target 'thereto by presenting a decreased thickness of material to the passage of the rays, and means to conduct a cooling fluid to said grooves to obtain a relatively large surface contact between the fluid and the target, said thin metal layer being in intimate contact with the thick metallayer ing'a target and a source of electrons, said target 1 2,090, g V V 'being transparent to X-rays and being composed of a plurality of metals in intimate metal-tometal heat-conducting relation with each other, one of said metals having a relatively high atomic weight and serving as an X-ray generating sub-' "said target having a multiplicity of small transverse grooves formed therein in the path of the issuing X-rays for increasing the transparency of v the target thereto by presenting, a decreased thickness of material to'the passage of the rays,
and means to conduct a cooling fluid to said grooves to obtain a relatively large surface contact between the fluid and the target.
DIMITRY E. OLSHEVSKY.