US2679017A - X-ray tube - Google Patents

Description

May 18, 1954 Filed Dec. 26, 1950 T. H.'ROGERS ETAL X-RAY TUBE 2 Sheefs-Sheet 1 FIG. I

INVENTORS THOMAS H. ROGERS JOSEPH W. SKEHAN ATTORNEY May 18, 1954 'r. H. ROGERS ETAL 2,679,017

X-RAY TUBE Filed Dec. 26, 1950 2 Sheets-Sheet 2 FIG. 2

INVENTORS THOMAS H. ROGERS JOSEPH W. SKEHAN ATTORNEY Patented May 18, 1954 UNITED STATES PATENT OFFICE X-RAY TUBE Thomas H. Rogers, New Canaan, and Joseph W. Skehan, Stamford, Conn, assignors to Machlett Laboratories, Incorporated, Springdale, Conn., a corporation of Connecticut Application December 26, 1950, Serial No. 202,776

12 Claims. 1

This invention relates to an X-ray tube capable of producing a beam of X-raysof highly uniform intensity. The ability of this tube to produce a high intensity beam of soft X-rays is of particular advantage. New to the X-ray art is the position of the X-ray permeable window in the tubes end parallel to itstarget, thus permitting complete axial symmetry of the tube which, in turn, affords convenient and secure mounting of the tube.

This invention is an improvement upon the invention of C. B. l-Iorsley described in U. S. Patent No. 2,482,275. Horsley has taught the use of an X-ray target perpendicular to the tubes axis surrounded by a circular filament. Our invention is essentially a modification of the Horsley type construction presenting advances over his invention which make it particularly valuable for specialized uses, some of which are herein described.

Many X-ray applications require great uniformity of intensity across the X-ray beam. For instance, uniform intensity X-ray beams are beneficial in general for radiography in that they permit uniformity of exposures over the entire picture area. An extremely uniform beam over a relatively small cross section is required for some specialized uses, of which miororadiography is typical.

Perfect uniformity, oi course, represents an ideal condition probably not physically obtainable. Uniformity as a practical matter, however, may be achieved in varying degrees to suit the needs of various uses. X-rays emanate in all directions from the target thus giving a 180 solid angle of radiation coterminus with the target face. The intensity of this radiation is not uniform, but varies throughout the solid angle in characteristic patterns. Different degrees of uniformity may be obtained at various positions equidistant from the target by selection of different beams. At relatively low voltages, intensity is a maximum in the direction perpendicular to the target and falls off as the angle from the perpendicular is increased. The rate of decrease in intensity is not linear. In fact, in most cases intensity does not begin to fall off markedly until the angle from the perpendicular is in the order of 30 to 40. As the plane of the target is appreached, the change in intensity becomes quite severe even with small change in beam angle. Thus for any given size of beam, the most uniform possible beam at relatively low voltages is that one which includes those rays perpendi lar to the target at its center.

The physical arrangement of elements in prior art tubes has resulted in a beam of X-rays of non-uniform intensity. Since it has proved convenient in tube assembly to introduce the anode and the cathode from opposite ends of the envelope, X-ray tubes have commonly had their filaments and targets on the tube axis. As described above, a 180 solid angle of X-rays is produced at the target, but, because uncontrolled radiation would prove highly dangerous, most X-ray tubes permit the use of only a small part of the generated radiation. It has become common to remove this small beam of radiation through an X-ray permeable window in the side wall of the tube enclosure, and this window is commonly placed radially opposite the target of the X-ray tube. Thus it has become common for targets to be placed at an angle of 45 or more with the tube axis and an angle of 45 or less with the center of the X-ray beam passing through the window, and smaller angles with the beam have become widely used for specialized purposes, such as producing a line focal spot. Since the X-ray beam is normally several degrees wide, the fact that it is at a relatively large angle (at least 45) with the perpendicular accounts for the often severe change in intensity across its cross section.

Accordingly, it is a primary object of our invention to provide a structure which will produce a usable beam of X-rays of essentially uniform intensity across its cross section. Such uniformity of the X-ray beam is obtained by locating the X-ray permeable window in such a position that it permits the passage of those rays of greatest possible uniformity for any given cross section of beam. The location of the target on and perpendicular to the tubes axis of rotation makes possible the use of the most highly uniform beam available at relatively low voltages by the simple expedient of placing the X-ray permeable window on the tubes axis parallel to the target. The angular width of the X-ray beam is determined by the spacing between the target and the window and the diameter of the window. The construction of our tube makes possible the confining of the beam used to whatever small angle is necessary for the uniformity of intensity required in each particular application. It will be obvious that the smaller this angle the more nearly uniform the intensity. However, for many uses a beam 50 to wide will be entirely satisfactory and will demonstrate a uniformity of beam superior to that of most prior art tubes using a much smaller cone of radiation.

It is another object of our invention to pro duce an X-ray tube capable of producing high intensity X rays even at small applied voltages. The large cathode area as taught by Horsiey is of great advantage here because it enables production of an extremely large electron current between cathode and anode. Since X-ray intensity increases directly as the electron current, as well as directly as the square of the applied voltage, high intensity X radiation is made possible even at relatively low voltages by the large cathode. Another factor important to intensity is the distance between the X-ray source and the irradiated area. Intensity is effectively decreased per unit of the irradiated area the further this area is placed from the source because of the tendency for the X-rays to spread out. Since the further the irradiated area is removed from the source, the smaller the intensity per unit area, according to the inverse square law, it becomes important in many instances to place the K-ray windows as close as possible to the target. Our construction permits this minimum spacing be tween the target and the Window and also permits a minimum spacing between window and irradiated area by virtue of the planar construction of the window and its location in a protruding portion of the tube structure.

Another object of this invention is the production of a high intensity, uniform soft (long wave length) X-ray beam. It has been difficult to remove the easily attenuated soft X-rays from the vacuum envelope once produced, but, since the recent development of thin beryllium windows has alleviated this problem to a large ex.- tent, the importance of soft X-radiation in such fields as superficial therapy and radiography has grown. However, it is a difficult problem to generate soft iii-rays in the first place because for production of long wave lengths, without a larger amount of accompanying hard radiation, relatively small voltages are required. Using prior art tube constructions, such low voltages would produce a very low intensity X-ray beam. By our invention, however, high intensity irradiation by soft X-rays is possible because of the use of the Horsley type construction, which permits large electron currents, and because of the close spacing possible between the target and the irradiated area. X ray window and the irradiated area is especially important when soft X-rays are used because of the tendency for these long wave length X-rays to be attenuated by the air through which they must pass. This close spacing is possible because of the use of a planar window located in a protruding portion of the tube.

Other specialized uses for X-rays require monochromatic radiation. For instance, microradiography is aided by the use of X-rays having but one wave length. For complete analyses, work of this sort often requires several different wave lengths of X-rays which may be most easily obtained by employing several target materials. Since there is no Way at present to replace the targets in a sealed-off vacuum tube, it is necessary to have as many tubes as there are target materials. Furthermore, tubes have been awkward to replace so that, when used, much time has been consumed in changing from one target material to another. Thus, the great expense involved and the replacement difficulties have prevented wide spread development of special .monochromatic X ray techniques.

Close spacing between the It is therefore an object of our invention to furnish a small, light, compact tube of simple, inexpensive design. At the same time it is our object to make a tube which is easily replaceable. Our tube can often be made light and small because such relatively low voltages are applied to it in most instances that heavy, long insula tion members are unnecessary between cath= ode and anode. Our overall coaxial design, new to the X-ray art makes tube replacement especially easy. The coaxial ring type filament terminals employed make it possible to accurate- 1y position and to securely mount tubes quickly and easily. lhe tubes symmetry lends itself toease and accuracy in assembly and permits use of parts which may be easily and inexpensively manufactured. In addition our tube is exceptionally rugged because of its sturdy coconstruction which lends itself to large diameter metal to dielectric seals.

It is also our object to provide a small, compact X-ray tube housing which may be employed as a self-contained, self-excited unit. The smallness and lightness of our tube make it practical for use in compact apparatus. The concentric filament terminals of our tube make possible a socket arrangement which makes for case in changing tubes and which insures secure and accurate mounting of'the tube withinthe housing. This type of socket also lends itself to leakproof construction, thus preventing the escape of insulation oil where used. Insulation oil itself reduces the requirements of tube insulation in that it inhibits spark over. This socket also permits the tube window to be placed close to the irradiated object where necessary and at the same time insures the operator a maximum of visibility. The tube window is normally kept at ground potential for safetys sake, and, since it is mounted within a filament ring terminal, the filament must also be at ground. potential. Having the filament at ground potential is a major advantage in that it reduces the insulation necessary for the filament transformer, thereby substantially reducing its size and permitting its easy enclosure within the tube housing.

For a better understanding of this invention reference is made to the following drawings:

Fig. 1 shows an axial section of a preferred construction of our invention.

Fig. 2 illustrates an elevational view from above the tube pictured in Fig. 1 looking down toward the X-ray permeable window.

Fig. 3 illustrates partly in section and partlyin elevation a socket arrangement for mounting our tube in an X-ray shockproof housing with a tube shown in place therein.

Referring to Fig. 1 anode block it is of a gen! erally cylindrical shape having one end H re.- duced in diameter and terminating in planar target surface, i2- Various kinds of target-ma-v terial. may be used to produce-X-rays of differ-.- ence specific wave length. At its opposite end, heavy annular terminal i3 is aifiXed-to the base of block l0. Tubular insulator M, which; may be advantageously made of ceramic, is afiixedto terminal is by annular bracket 15. A similar annular bracket l6 joins its opposite end, toring filament terminal. member l1, Concentric with ring terminal I! is smaller ring terminal H3, the two terminals being separated by dielectric washer 19 which may beadvantageously made of ceramic. An annularbracket 20,,and indirectly terminal [8, rigidly support X-ray, permeable 5. Window 2| which is preferably constructed of beryllium. For a narrower beam of X-rays a smaller window in a non-permeable annular collar may be used or the window may be further removed from the target. In general, however, the minimum spacing between the target [2 and the window 2| which will not interfere with electron flow is desirable in order to make possible the use of a window of minimum diameter for any given solid angle of radiation.

Within the vacuum envelope is the filament 22 which is circular in shape and which is mounted coaxial with the target 12 but, in a preferred form, a little closer to the X-ray window 25 than is the target in order to simplify focusing of the electron beam. The spacing between the filament and the window is preferably somewhat smaller than the radius of the filament circle. The filament may be joined to terminal members by support and conductor posts 23, which are in turn connected alternately to terminal extension members 24 and 25. Filament 22 advantageously lies within or at the mouth of an annular channel having side walls 26 and 21. The cathode potential on the inner side wall 25 prevents electrons from being drawn from the filament directly to proximate points on the anode. The outer wall 21 extends beyond wall 2% and is flared inwardly above the filament. When at cathodepotential, wall 27 causes the electrons to be deflected into a funnel shaped path toward the focal spot on the target surface 12..

The channel members, since they are all at oathode potential, are advantageously connected as shown to the respective terminal members. It will be noted that the base of the annular channel formed by member 25 is also connected to one of the terminals as shown. It is necessary to terminate this base member 25 just short of inner side wall 25 in order to prevent shorting of the filament. Dielectric members it permit the insulated passage of alternate support posts '24 through the channel base 25 to member 24.

Exhaust of the tube may be conveniently accomplished by means of duct 29 through the anode block terminating in seal off means 30. The seal off means may be protected by cap 3|, if desired. For mounting convenience an annular collar 32 is often mounted around terminal I! atop bracket 16.

Fig. 2 illustrates the same tube from the window end without collar 32. Visible are annular bracket l6 and adjoining terminal H. The

ceramic washer is separating terminals l! and I8 may be seen, but terminal is itself is hidden by bracket 29. Beryllium window 2| is clearly visible. The target surface l2 and the surrounding filament 22 are indicated to show the relative positions of these elements. Also indicated are the insulation bushings 28.

Fig. 3 illustrates the tube in elevation within a possible housing mounting. As here shown spring fingers 35 affixed to conical socket mem ber 36 contact filament terminal l8. Since conical member 36 engages tubular member 31 which is in turn aflixed to housing 38, this terminal of the filament and consequently the beryllium window will be maintained at ground potential, as is the housing. Members 38 and 31 may have opposing shoulders which approach one another when the members 36 and 31 are advanced in threaded engagement. Upon one of these shoulders may be placed a dielectric block 39 which itself has a shoulder advantageously engaging collar 32. Between collar 32 and the shoulder of member 31 may be placed a de formable O-ring 40 which forms an oil-tight joint between tube and socket. Spring fingers M, or other contacting means, may be mounted such that contact is made with annular bracket l6 which will serve in this instance as the other filament terminal. These fingers are insulated from the rest of the socket and from the housing by dielectric washers 42 and 53 and held in place atop tubular socket member 31 by some means such as a series of screws 44 parallel to the tubes axis whose heads rest atop dielectric washer 42. When used in general applications there may be employed conical shield 45 which is conveniently afiixed in threaded engagement with socket 36. When soft radiation is employed however, it may be desirable to minimize the extension of 36 such that the X-ray window may be placed immediately adjacent the irradiated area.

The size and shape of the housing 38 is left to the individual application. but in every case the tube housing may be made extremely small because of the small tube size and the smallness of the enclosed auxiliary equipment, such as the filament transformer.

While the actual shape of the tube as illustrated is preferred, anymodification in the shape which does not materially change the overall rela tive position of the basic parts is within the scope and spirit of this invention. Thus despite the accompanying disadvantage of lower temperature for baking and outgassing the insulation herein described as the ceramic may be replaced by glass. Likewise the exact shape and arrangement of the terminals and the electron deflection members are not limited to that shown in the drawings. Obvious modifications in structure occurring to any one skilled in the art are considered to be within the scope of this invention.

We claim:

1. An X-ray tube comprising a vacuum envelope including a planar window permeable to. X-rays and a ring type cathode terminal coaxial with and surrounding said window and within the vacuum envelope an emitting cathode and an anode, said anode including an X-ray producing target surface parallel to said X-ray permeable window and coaxial with the ring type cathode terminal.

2. An X-ray tube comprising a vacuum envelope including a planar window permeable to X-rays and surrounding said window a ring type cathode terminal to which the window is sealed and within the vacuum envelope an emitting cathode and an anode, said anode including an X-ray producing target parallel to said X-ray permeable window and coaxial with the ring type cathode terminal.

3. An X-ray tube comprising a vacuum envelope including a pair of coaxial ring type filament terminals and within the vacuum envelope an emitting filament and an anode, said anode being coaxial with the ring filament terminals and including an X-ray producing target.

4. An X-ray tube comprising a vacuum envelope including a pair of coaxial ring type filament terminals and a planar X-ray permeable window and within the vacuum envelope an emitting filament and an anode, said anode being coaxial with the filament terminals and including an X-ray producing target parallel to said X-ray permeable window.

5. An X-ray tube comprising a vacuum envelope including a pair of coaxial ring type filament terminals an a planar X-ray permeable ing target parallel to said X-ray permeable win -v dow.

6. An X-ray tube comprising a vacuum envelope including a pair of coaxial ring type filivr.

ment terminals, a coaxial anode terminal, and a planar X-ray permeable window sealed to one of said terminals and within the vacuum envelope.

and emitting filament and an anode, said anode.

being coaxial with the filament terminals and including an X-ray producing target parallel to said X-ray permeable window.

7; An X-ray tube comprising a vacuum ene velope including a pair of coaxial ring type filament terminals, a coaxial anode terminal, and a planar X-ray permeable window sealed to one of said terminals and within the vacuum envelope a coaxial circular filament affixed to said filament terminals and a block type anode afiixed to the: anode terminal, said anode being coaxial with the filament and its terminals, and including an X-ray producing target parallel to said X-ray permeable window.

8. An X-ray tube comprising a vacuum envelope including a pair of coaxial ring type filament terminals, a coaxial anode terminal, and a planar X-ray permeable window sealed to one of said terminals and within the vacuum envelope a coaxial circular filament lying wholly within the 180 solid angle subtended by the target face on the window side and affixed to the filament terminals, said anode being coaxial with the filament and its terminals, and including an X-ray producing target parallel to said X-ray permeable window.

9. An X-ray tube comprising a vacuum envelope including a pair of coaxial ring type filament terminals, a coaxial anode terminal and a planar X-ray permeable window sealed to one of said terminals and within the vacuum envelope a coaxial circular filament affixed to said filament terminals, electron defiection means affixed to ment terminals, a coaxial anode terminal and a planar X-ray permeable window sealed to one of said terminals and Within the vacuum envelope a coaxial circular filament affixed to said filament terminals, an annular cathode-potential trough below said filament having parts aifixed to the terminals, that wall of the trough between the filament and the anode preventing direct flow of electrons from filament to anode and the other wall of the trough surrounding the filament and curving inward and above said filament to cause deflection of the electrons into a funnel-shaped path toward the X-ray target, and a block type anode afiixed to the anode terminal, said anode being coaxial with the filament and its terminals and including an X-ray producing target parallel to said X-ray permeable window.

11. An X-ray tube comprising a vacuum envelope consisting of a pair of coaxial ring type terminals, a coaxial anode terminal, a planar X-ray permeable window sealed to one of said filament terminals, a ceramic ring sealed between the filament terminals and a ceramic tube sealed between the outer filament ring and the anode terminal and within the vacuum envelope an emitting filament afiixed to the filament terminals and a block type anode ailixed to the anode terminal, said anode including an X-ray producing target parallel to said X-ray permeable window.

12. An X-ray tube comprising a vacuum envelope consisting of a pair of coaxial ring type filament terminals, a coaxial anode terminal, a planar X-ray permeable window sealed to one of said filament terminals, a ceramic ring sealed between the filament terminals and a ceramic tube sealed between the outer filament ring and the anode terminal and within the vacuum envelope a coaxial circular filament affixed to the filament terminals, an annular cathode potential trough below said filament having parts aiiixed to the terminals, that wall of the trough between the filament and the anode preventing direct fiow of electrons from filament to anode and the other wall of the trough surrounding the filament and curving inward and above said filament to cause deflection of the electrons into a funnel-shaped path toward the X-ray target and a block type anode affixed to the anode terminal, said anode being coaxial with the filament and its terminals and including an X-ray producing target parallel to said X-ray permeable window.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,183,871 Grimes May 23, 1916 1,626,465 Holst et al. Apr. 26, 1927 2,472,745 Frevel June 7, 1949 2,482,275 Horsley Sept. 20, 1949 2,496,003 Eaves Jan. 31, 1950 2,517,334 Murdock Aug. 1, 1950 2,569,872 Skehan et al. Oct. 2, 1951

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