US2952790A - X-ray tubes - Google Patents

Description

Sept. 13, 1960 Filed July 15, 1957 G. W. STEEN X-RAY TUBES 2 Sheets-Sheet 1 GOTTFRID W. STEEN AGENT INVENTOR.

Sept. 13, 1960 G. w. STEEN 2,952,790

X-RAY TUBES Filed July 15, 1957 2 Sheets-Sheet 2 INVENTOR. GOTTFRID W. STEEN AGENT ilnited States ?atent i X-RAY TUBES Gottfrid W. Steen, Stamford, Conn., assignor, by rnesne assignments, to Raytheon Company, a corporation of Delaware Filed July 15, 1957, Ser. No. 671,781

4 Claims. (Cl. 313-58) This invention relates to improvements in X-ray tubes and has particular reference to an X-ray tube having novel shielding means located adjacent the interelectrode space for protection of the glass envelope from bombardment by secondary and cold emission electrons.

Two serious problems are encountered in the operation of X-ray tubes; first, the variable biasing effect of wall charges on the flow of electrons through the interelectrode space which causes undesirable variations in the output radiation and, second, the rapid deterioration and short life of tubes when operated at relatively high voltages.

Charges are built up on the walls of a glass envelope .due to bombardment of the inner surface of the envelope by maximum Velocity electrons, both secondary electrons emanating from the target when bombarded by primary electrons, and cold emission electrons emanating from electrode structures subjected to high voltage gradients. Such bombardment of the envelope walls continues 'until the walls, due to the accumulation of electrons thereon, become charged to .a negative potential sufiiciently high to repel additional electrons.

This bombardment sometimes causes gas evolution and the accumulation of charge sometimes causes rupture of the glass envelope, thus considerably shortening the life of an otherwise eificiently operating tube.

Since, in high voltage tubes as normally constructed and .operated, charges which accumulate on the envelope walls tend to leak away'nearly as rapidly as the bombarding electrons arrive, it will be apparent that some time must elapse before the walls are sufliciently negatively charged to repel electrons. Furthermore, wall charges affect the electron flow between cathode and anode to a considerable extent. In fact, the output radiation may vary with time as much as 15%, which is definitely objectioriable since variations in radiation may adversely affect the operation being performed with the tube. This is particularly true when using the tube for time exposures such as in X-ray therapy.

Biasing of electron flow by wall charges varies with the magnitude of the wall charges, which in turn is dependent to a considerable degree upon the rate of leakage through the glass of the envelope. The resistivity of the glass to such leakage is dependent upon the temperature of the envelope, which temperature increases from approximately room temperature to an elevated equilibrium value after several minutes of tube operation. Thus, a lengthy warmup period of several minutes must be permitted before an X-ray tube can be operated with some degree of stability of output radiation.

In some types of high voltage X-ray tubes, the interelectrode space is encircled by a shield which is maintained at anode potential. In these tubes, While the envelope is fairly well protected from bombardment by secondary electrons, damage may be caused by cold emission :electrons which leave the anode structure during the inverse cycle.

Accordingly, it is a primary object of the present in- 2,952,790 Patented Sept. 13, 1960 vention to provide an X-ray tube structure wherein the envelope is protected from harmful high velocity electron bombardment.

Another object is to provide an X-ray tube which when operated when in a cold condition will substantially immediately produce relatively stable output radiation.

Another object is to provide a tube wherein the interelectrode space is enclosed by means adapted to be maintained at a potential substantially midway between the potentials applied to the cathode and anode respectively, whereby cold emission to the envelope wall is substantially reduced or eliminated and whereby a stable electrical field encloses the path taken by the flow of primary electrons between cathode and anode.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawings, wherein:

Fig. 1 is an elevational view partly in axial section of one type of X-ray tubeembodying the invention;

Fig. 2 is .a diagram illustrating schematically the electrical system embodied in connection with the invention as applied to the tube in Fig. 1;

Fig. 3 is a view similar to Fig. 1 of another type of .X-ray tube embodying the invention;

Fig. 4 is a diagram similar to Fig. 2 as applied to the tube of Fig. 3; and

Figs. 5 and 6 are fragmentary axial sectional views 'illustrating modifications in the shielding structure.

:Referring more particularly to the drawings, the tube illustrated in Fig. 1 embodies an evacuated envelope 10 having two glass portions 11 and 12 joined by a metal ring 13. Envelope portion 11 supports within it a cath- Ode structure 14 which embodies a metal body 15 within which is positioned an electron emitting filament 16. The filament is preferably recessed slightly within a depression ar cavity 17 formed in the inner end of body 15 so that electrons emanating from the filament will tend to be focussed by the side walls of the cavity 17 in a wellknown manner. The cathode structure, and particularly the filament 16, is adapted to be supplied with a suitable potential by means of conductors 18-19 which extend outwardly through the adjacent end of the envelope.

Envelope portion 12 supports within it an anode 20 which is provided with means such as pipe 21, which extends from its outer end outwardly of the envelope, by which a suitable potential may be applied.

The inner end of the anode 20 is directed toward the cathode and is provided with a recess or cavity 22 which terminates in a wall 23 in which is embedded a target 24, preferably of tungsten. The target is angled so as to obtain maximum electron loading with proper X-ray beam coverage and focal spot size. Electron bombardment of the target 24 by electrons liberated by the filament 16 generates X-radi'ation which passes outwardly of the cavity 22 through a window 25 carried in a wall surrounding the cavity, and through the glass of the envelope. The window is comprised of a disc of a material highly transparent to X-rays, such as beryllium or the like. Generation of X-rays produces anode heating, which may be dissipated by suitable means and methods such as by forcing cold fluid through conduits therein or by other selected means well known in the art whereby the target 24 is prevented from overheating.

When the filament 16 is heated by application of electrical energy through conductors 1819, and when the anode is simultaneously made positive with respect to the cathode, primary electrons are caused to flow from the filament 16, through the interelectrode space, onto the target 24 for generation of X-radiation.

In conventional tubes, when a cold tube is operated some primary electron emission and a considerable amount of secondary electron emission is attracted to the glass walls of the envelope 10. Such bombardment at maximum velocities may rupture the envelope, renderingthe tube faulty. Wall charges are built up which tend to' bias the electron flow and, consequently, affect the output of the tube. Since a relatively long period of time must elapse before the wall charge is built up to a potential which will repel bombardment, the biasing efiect will vary considerably during this warmup period and, consequently, the output radiation will vary during this period. The electrodes themselves are additional sources of electrons which bombard the envelope and consequently build up wall charges. In the normal operation of many tubes of the presently described char- --acter, each tube is mounted within a metal enclosure,

as indicated by dotted lines 26 in Fig. 2, which enclosure is connected to ground. The enclosure attracts cold emission electrons from the cathode structure, which electrons are intercepted by the glass walls of the envelope as they are accelerated toward the enclosure. On the inverse cycle such cold emission electrons are likewise drawn from the anode structure.

The above undesirable features are overcome in the present invention by a substantially tubular metal shield 27 which is coaxially mounted in the envelope in encircling relation to the interelectrode space and overlying the metal ring 13 and adjacent ends of the glass portions 11 and 12 of the envelope. Shield 27 is of a length to extend at least between the planes of the ends of the cathode and anode, and'preferably extends a substantial distance beyond the adjacent ends of both electrodes, thus intercepting most, if not all, of the secondary electrons from the target which spray out through the interelectrode space as well as the cold emission electrons from the electrode structures.

A further important feature of such a shield 27 is that it is located substantially at mid-potential; that is, it is electrically connected to and supported by the metal ring '13 to which is applied potential of an amount which is approximately midway between the potentials which are applied to the anode and cathode respectively. Thus, there is created a stable electric field about the interelectrode space. The addition to a conventional tube of a shield 27 will reduce or eliminate the damaging cold emission and the secondary electrons which bombard the envelope and create a stable field around the interelectrode space, and will further permit the tube to be operated at higher voltages with long life, with stable output, and without the lengthy warmup periods formerly required.

Referring to Fig. 2, a very schematic circuit is shown to illustrate the invention as applied to the so-called center grounded tube shown in Fig. 1. The filament 16 is heated by a transformer 29. A second transformer 28 has one side of its secondary winding connected to transformer 29 while its other side is connected to the anode for application of high voltage for electron driving purposes. Transformer 28 is tapped substantially midway of its secondary winding by a lead 30 which is grounded. The shield 27 and enclosure 26 are also connected to ground. Thus the shield is at a potential substantially midway between the potentials which are applied to the cathode and anode respectively.

Although an alternating current circuit is specifically described herein, it is to be understood that constant potential or rectified direct current circuits, or combinations of the two, can be used if desired with the present invention.

In Fig. 3 is shown an end grounded tube embodying the invention. In this tube the cathode structure 31 is mounted on a reentrant portion 32 of the glass envelope and includes a tubular metal cathode shield or hood 33. Within the shield 33 is located an emitter or filament 34 to which electrical energy is supplied by conductors 35-36 which penetrate the end of the en velope. The inner end of the shield 33 extends beyond the emitter 34 and is provided with a reentrant portion 37 which terminates in a bottom portion or platform 38 which is slotted to expose the emitter. This structure thus forms a focusing device whereby electrons from the emitter 34 are directed toward a target 39.

An anode 40 is supported within an opening 41 by any suitable means such as a metal ring structure 42 which is sealed at one end to a circumferential portion of the anode 40 and at its other end to the end of the envelope to close the opening therein.

The main body of the anode is located externally of the glass envelope and is provided with a deep recess or cavity 43 which opens into the interior of the envelope. Cavity 43 terminates in a wall in which is embedded the target 39, which is preferably formed of tungsten. X-radiation from the target 39 passes outwardly through a window 44 which is comprised of a disc of a material highly transparent to X-rays, such as beryllium or the like.

To aid in reducing secondary electron bombardment of the glass walls of the envelope, there is provided a metal anode shield 45 which is tubular in shape and extends coaxially of the tube toward the cathode from the inner end of the main body of the anode, to which it is connected.

Shield 45 is provided at its end nearest the cathode with an inwardly turned flange 46 which is curved so as to prevent the relatively sharp edges from being presented in such a manner as to increase cold electron emission. While one of the primary purposes of the shield 45 is to restrict secondary electron emission, a substantial amount of such emission still escapes into the interelectrode space where it may bombard the glass envelope.

End grounded tubes are operated within enclosures 47 as shown in the schematic diagram of Fig. 4. The anode 40 is grounded and the filament 34 is heated by a transformer 48. A second transformer 49 has one side of its secondary winding effectively grounded while the other side is connected to the cathode for application of high voltage for electron driving purposes.

In the normal operation of this tube, the metal enclosure 47 is connected to and maintained at anode (or ground) potential. This causes cold emission electrons to be drawn from the cathode shield 33 and accelerated toward the enclosure, whereupon they bombard the glass walls with undesirable results.

A substantially tubular shield 50, generally similar to shield 27 in Fig. 1, is mounted in encircling relation to the interelectrode space for the purpose of stabilizing the electric field around the interelectrode space and intercepting cold emission and secondary electrons before they contact the glass walls of the envelope. In the operation of this end-grounded tube, transformer 49 (Fig. 4) is tapped substantially midway of its secondary winding by a lead 51 which is connected to the shield 50 through a metal ring 52, in the envelope, which supports the shield 50. Thus the shield 50 is supplied with a potential substantially midway between the potentials which are applied to the cathode and anode respectively.

From the above, it will be understood that the invention may be applied to tubes where the anode is at ground potential or where the anode and cathode are maintained at potentials above or below ground. Generally, the shield should be maintained at a potential which is substantially midway between the potentials applied to the anode and cathode respectively. However, it may be desirable. in certain cases to apply a potential to the shield appreciably different from the mid-potential.

The center shield may take any of several shapes depending upon the type of tube in which it is used. For example the shield 27 in the tube of Fig. 1 consists merely of two tubular-shaped members 5354 having their adjacent ends provided with flanges 55-56 by which they are afiixed to a flat ring 57. This assembled unit is secured by brazing or the like to the inner side of ring 13. The ends of members 5354 may be turned outwardly as shown, if desired, in tubes Where cold emission is not a serious problem, such as where the center shield and the tube enclosure are at the same potential.

In Fig. 3 a somewhat different construction is shown. Supporting ring 52 is formed of two annular flanged members 5859 the flanges of which are sealed together and to an inner annulus 60. The shield 50 may be secured in any suitable manner directly to the annulus 60, and the ends thereof are turned inwardly away from the glass to reduce possibilities of cold emission.

To further increase the area of the shield for the purpose of still more efliciently intercepting secondary and cold emission electrons, and to isolate the anode and cathode sections of the tube and maintain a stable electric field even closer to the electron stream passing through the interelectrode space, the shield may be provided with an annular baifle. For this purpose shield 50 is formed of two tubular members 61-62 located in coaxial end-to-end relation, with their adjacent ends being bent inwardly to form flanges which are joined together to form a baflie 63 having a central opening through which passes the primary electron flow.

Another satisfactory shield structure is shown in Fig. 5 and comprises a pair of tubular members 64-65 which are brazed at their adjacent ends to a relatively heavy, fiat, washerlike member 66. Member 66 is sealed throughout its outer periphery to two kovar rings 67-68 which form the metal portion of the envelope. The inner peripheral portion of member 66 extends for a short distance toward the axis of the structure, with its inner edge having a curved bead 69 which is provided to eliminate the sharp edges which may become sources of cold emission.

In Fig. -6 modified shield structure is shown as embodying a baflle 70 which has an orifice 71 therein of approximately the same size as the openings in the cathode and anode structures 72 and 73. In Fig. 6 also, the shield 74 is shown as being of such a length that its ends lie in planes defined by the ends of the anode and cathode structures.

From the foregoing description it will be seen that novel shielding means has been provided in accordance with the objects of this invention. While the preferred embodiments of the invention have been shown and described and are pointed out in the annexed claims, it is to be understood that many changes may be made by those skilled in the art without departing from the spirit of the invention. Therefore, all matter shown or described is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. A high voltage X-ray tube comprising an envelope enclosing a single evacuated space having therein spaced cathode and anode electrode structures adapted to be maintained during operation of the tube at known difierent potentials whereby a stream of electrons may be driven through the interelectrode space from the cathode to the anode, said envelope comprising a pair of spaced dielectric bulb portions and an annular metal terminal encircling the interelectrode space and having opposite ends sealed to the adjacent ends of the respective bulb portions, said terminal being exposed on its outer surface to atmosphere external of the tube and on its inner surface to the evacuated space within the envelope, and a metal shield in the evacuated space within the envelope encircling the interelectrode space and overlying the adjacent ends of the electrode structures and the seals between the terminal and bulb portions, said shield being mounted directly upon the inner side of the terminal in rigid supporting and electrically conductive relation to the terminal, said terminal and shield being maintained during operation of the tube at a potential substantially midway between the potentials applied to the anode and cathode electrode structures for maintaining a stable electric field in the interelectrode space.

2. An X-ray tube substantially as set forth in claim 1 wherein the shield includes a longitudinally extending member of generally tubular shape, and an annular metal weblike member sealed at its outer periphery to the terminal and at its inner periphery to the longitudinally extending member, said member having an annular portion encircling the electron stream in predetermined spaced relation thereto.

3. An X-ray tube substantially as set forth in claim 2 wherein said annular portion is a bafile mounted on the inner surface of the longitudinally extending member.

4. An X-ray tube substantially as set forth in claim 2 wherein said annular portion is a portion of the longitudinally extending member which is of reduced diameter and which extends a controlled distance toward the center of the electron stream.

References Cited in the file of this patent UNITED STATES PATENTS 1,920,606 Stewart Aug. 1, 1933 1,927,475 Volkel Sept. 14, 1933 1,954,709 Niclassen Apr. 10, 1934 1,955,665 Alfter Apr. 17, 1934 1,980,017 Traub Nov. 6, 1934 2,024,332 Bouwers Dec. 17, 1935 2,113,334 Slack Apr. 5, 1938 2,405,477 Westendorp Aug. 6, 1946

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