US3751701A - Convergent flow hollow beam x-ray gun with high average power - Google Patents

Abstract

An X-ray generating tube characterized by grid means formed with narrow annular slits which serve to isolate the cathode from being influenced by the high voltage of the anode. According to one embodiment, a target formed as a hollow conical surface of revolution closed at one end forms an X-ray window at one end of the tube. According to another embodiment, the axis of the tube remains substantially unobstructed whereby X-rays generated from a conical surface of the type noted above are emitted at the end of the tube disposed opposite the target end. Another embodiment combines the advantages of the first named and last named embodiment by utilizing concentricly oriented coaxially disposed electron beam control elements formed with a narrow annular slit for passing electrons radially into the axial region of the tube.

Description

ilnite States Gralenski et a1.

:1 ate I 19] 1 Aug. 7, 1973 I CONVERGENT FLOW HOLLOW BEAM X-RAY GUN WITH HIGH AVERAGE POWER [73] Assignee: Watkins-Johnson Company, l alo Alto, Calif.

[22] Filed: Mar. 8, 1971 [21] Appl. N0.: 122,065

[52] 11.8. CI. 313/57, 313/59, 313/82 R, 313/240, 313/243, 313/300 [51] Int. Cl H0lj 35/14, HOlj 35/30 [58] Field of Search 313/57, 55, 59, 76, 313/82, 86, 87, 107.5, 296, 297, 300

[56] 1 References Cited UNITED STATES PATENTS 3,691,417 9 1972 Gralenski 313/57 TO RETURN 37 0 PIN (VIA PIN) Primary Examiner-Roy Lake Assistant Examiner-Darwin R. Hostetter Att0rney-Flehr, Hohbach, Test, Albritton & Herbert [57] ABSTRACT An X-ray generating tube characterized by grid means formed with narrow annular slits which serve to isolate the cathode from being influenced by the high voltage of the anode. According to one embodiment, a target formed as a hollow conical surface of revolution closed at one end forms an X-ray window at one end of the tube. According to another embodiment, the axis of the tube remains substantially unobstructed whereby X- rays generated from a conical surface of the type noted above are emitted at the end of thetubedisposed opposite the target end. Another embodiment combines the advantages of the first named and last named embodiment by utilizing concentricly oriented coaxially disposed electron beam control elements formed with a narrow annular slit for passing electrons radially into the axial region of the tube.

3 Claims, 9 Drawing Figures POWER SUPPLY PATENTEB 3.751.701

sum 1 or 4 TORETURN 37 PIN POWER SUPPLY INVENTORSI NICHOLAS M, GRALENSKI GEORGE WADA DAVID J.BATES WM KMMZ M,

ATTORNEYS murmu SIEEI 2 HF 4 T'II3 E| INVENTORS NIC HO LAS M. GRALENSKI GEORGE WADA DAVID J. BATES W zii WSRNEYS PAIENIEU 3.751.701

saw u 0F 4 TIE-' EI INVENTORSI NICHOLAS MpGRALENSKI GEORGE WADA DAVID J. BATES BYFM ,wmms-a ATTORNEYS CONVERGENT FLOW HOLLOW BEAM X-RAY GUN WITH HIGH AVERAGE POWER BACKGROUND OF THE INVENTION This invention pertains to an X-ray generating device or gun of a type having a beam of electrons for impinging upon a target material serving to generate ions which, in turn, form X-rays within an evacuated tube. More particularly, this invention pertains to an X-ray gun having improved means for control and employment of the electron beam in such gun.

Heretofore, X-ray tubes have been known in which a hollow electron beam is directed against a target material for generating ions. In operation, such devices have typically experienced difficulty in obtaining a sufficiently small electron impact area hereafter referred to as the"spot" on the target material. By use of a tiny spot, it is possible to obtain sharper radiograms as compared to those taken with a larger spot. However, when the spot is reduced in size substantially to a point source considerable heat is immediately generated on the surface of the target and damage can be caused.

In addition, under the foregoing and other circumstances it is usually advantageous to employ a relatively high beam intensity. The beam intensity is increased by increasing the difference in potential between the target and cathode whereby the target will appear to be quite positive with respect to the cathode. It has been observed that, in certain hollow beam X-ray tubes heretofore constructed, the presence of a substantial voltage differential between the cathode and target results in the target influencing the emissive characteristic of the cathode.

Accordingly, it is desirable that the beam intensity from the cathode be made independent of the voltage on the target.

As disclosed herein, a conically-shaped target has been provided with a hollow electron beam in a manner serving to obtain the advantages of an extremely small spot together with high beam intensity without danger of overheating the target.

In addition, as described herein, the manner of orientation of the target with respect to the cathode provides a gun arrangement with better control of secondary electrons emitted from the target in the course of bombarding the target.

SUMMARY OF THE INVENTION AND OBJECTS In general, there is provided an X-ray generating device of a type employing a target of material disposed to be struck by high energy electrons to develop X- rays. A cathode having an electron-emitting portion is adapted to be charged negatively with respect to the target to create a flow of electrons from the cathode portion to the target. A grid means disposed adjacent the cathode and across the flow of electrons is formed in a manner to include an annular slit therein coaxially disposed relative to the axis of the flow of electrons for passing the electrons therethrough to the target. The slit is sufficiently narrow and the cathode is spaced sufficiently behind the slit so as to inhibit the high potential of the target influencing electron emission from the cathode portion. Further, an annular control element encircles the flow of electrons and is coupled to means for varying the potential thereof so as to cause the flow of electrons to provide a cylindrical beam impinging upon a target with a varying diameter in response to variations in the potential applied to the control element.

According to one preferred embodiment, the electron beam impinges into the interior ofa closed conical surface forming the target whereby the apparent spot size is relatively small as represented by the apex of the conical surface, but the surface area constituting the electronic impact area remains relatively large and in immediate conjunction with a heat sink or other material of a type serving to readily absorb the heat of impact of the electron beam.

According to two other preferred embodiments of the invention, a window for emitting X-rays from within the tube is formed coaxially of the hollow beam, in one instance at the end of the tube opposite the target end, and in the other instance at the target end itself.

In general, it is an object of the present invention to provide an improved X-ray tube construction capable of handling high average power without sacrifice of the advantages achieved by utilization of a small spot size.

Another object of the invention is to provide an improved X-ray generating device wherein X-rays can emerge from the device from either end.

The foregoing and other objects of the invention will become more readily apparent from the following detailed description of preferred embodiments when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic side elevation section view of an X-ray tube according to the invention;

FIG. 2 is a transverse section view taken along the line 2-2 of FIG. 1;

FIG. 3 is a diagrammatic section view taken along 3-3 of FIG. 1;

FIGS. 4, 5 and 6 are a series of diagrams illustrating the functioning of apparatus of the type shown in FIG.

FIG. 7 is a side elevation section view of an X-ray to another embodiment of the invention;

FIG. 8 is a side elevation section view of a portion of an X-ray tube according to another embodiment of the invention;

FIG. 9 is an enlarged detail view diagrammatically illustrating a portion of another embodiment of an X-ray tube according to the invention adapted to be used in conjunction with apparatus of a type such as shown in the embodiment of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS An X-ray generating tube assembly 11 comprises an elongated evacuated envelope 12 of glass or ceramic material. At one end of envelope 12 a cylindricallyshaped metallic insert 13 of a type, such as Kovar, suitable for forming a metal seal with glass or ceramic as used in envelope 12, forms a re-entrant opening 14 into which a target terminal assembly 16 can be mounted and sealed.

Assembly 16 comprises a cylindrical body 17 or section of suitable material, such as tungsten, to be bombarded by electrons to form X-rays within envelope 12. Body 17 is formed with a planar face 18 disposed transversely of the axis of body 17 and oriented at an angle on the order of 76 thereto.

Body 17 is brazed inside of the metallic insert 13 and a suitably sized anode terminal rod 19 is brazed to the back of body 17. Rod 19 is preferably of a highly heat conductive material so as to quickly dissipate heat from body 17 and form a heat sink therefor.

During the glassing operation where envelope 12 is glass, the thinness and poor thermal conductivity of the Kovar insert 13 prevents heat from being excessively lost to other portions of assembly 16. At the same time, rod 19 can be highly heat conductive to allow a rapid heat drain from body 17 during tube operation. In addition, cooling fins or the like can be attached to rod 19 by suitable means, such as soldering, after assembly 11 is completely vacuum sealed.

Terminal rod 19 is adapted to be coupled to the positive terminal of a relatively high voltage source on the order, for example, of 40,000 volts or much higher so as to form target assembly as the anode of tube assembly 11. The other side of power supply 21 is directly coupled via line 22 to a cathode terminal 23 carried in the end of envelope l2. Cathode terminal 23 is electrically coupled to cathode assembly 24 now to be described.

A generally cylindrical metallic body 26 encloses a and support plug 27. Plug 27 is held in place within body 26 by means of a retaining lip 28 bent around the rear edge of plug 27 on one side and by means of the generally comically-shaped hollow, annular enclosure 29 on the other.

Coaxially of plug 27 a relatively thin, cylindrical metal thermally-insulating sleeve 31 is spot welded at its rear end to a projecting collar portion 29a of enclosure 29 and serves, as its other end, to support a heater housing 32 encircling the coils of a heater element 33. Heater 33 is connected by leads 34, 36, passing through plug 27 and subsequently by attachment to respective ones of pins 37, 38. Accordingly, a rivet 39 anchors the inner end of an L-shaped support leg 41 while a connecting strap 42 extends between leg 41 and pin 37. A six volt power supply 43 or other suitable low voltage source can be used to energize heater element 33.

The inner end of cathode assembly 24 carries an electron-emitting portion adapted to be charged negatively relative to target assembly 16 to form a flow of electrons from the cathode-emitting portion to the target. The electron-emitting portion of cathode assembly 24 comprises an annular oxide-coated surface portion in the form of the edge of lip 44 of the cathode element 46. Element 46 consists of a suitable metallic material upon which there is deposited an oxide coating of a highly electron-emitting material, such as strontium, calcium, or barium, or a mixture thereof, deposited around the edge of lip 44. The remainder of element 46 can be made of conventional cathode nickel.

Element 46 merges with housing 32 to form a heater a. A perforated triangular plate 47 is formed with a hole or circular opening encircling, in closely spaced relation, the end margin of housing 32. The outer three corners of plate 47 are secured, by suitable means, such as spot welding, to tabs 48 formed on the end of body 26.

As thus arranged, plate 47 provides rigidity to the distal end of housing 26 and further serves to align and retain withinrelatively close tolerances the position oflip 44 with its electron-emitting annular surface portions.

The edge of lip 44 forms the source ofa flow of electrons from cathode assembly 24 to target assembly 16.

Thus, the electron-emitting oxide-coated surface portion of lip 44 of cathode 46 is directed toward target assembly 16 in a manner to form a hollow annular electron beam 45.

For additional focusing of the beam onto target assembly 16, a grid element 49 is carried by insulators 51 attached to plate 47 and grid 49 in coaxial relation to the oxide-coated electron-emitting surface of lip 44.

Grid element 49 includes relatively broad surface portions disposed transversely to the axis of the electron beam and includes tapered surface portions serving to direct the emitted electrons as a hollow beam of electrons toward the target.

Thus, grid element 49 includes an annular opening or slit comprised of three arcuate sections 52 of a circle disposed coaxially of the center of grid element 49. Between each adjacent pair of arcuate circle sections 52, there remains a radial support 53 serving to hold the central ion shield 54 coaxially of grid 49.

Grid 49 influences the electron emission of beam 45 and serves to direct it as an annular hollow beam by means of the focusing action of the surfaces of grid 49 remote from cathode element 46. An outer annular surface 56 slopes inwardly toward the center of grid 49 at a Pierce angle, for example, on the order of 225. A second annular surface 57 tapers rearwardly and radially outwardly of the center of grid 49 at a similar Pierce angle. The two surfaces 56, 57 are spaced slightly to define the circular slit 50 coaxially of grid 49 through which electrons will be emitted from cathode element 46.

Means are provided for biasing grid 49 to cathode potential or slightly above, as desired.

Accordingly, a variable resistance 58 taps into line 22 to provide a variable voltage drop between the potential on line 22 (and hence cathode 24) and the potential of grid 49. The wiper 58a is directly connected via diode 59, poled in a direction to pass current to grid 49, via line 61 (entering envelope 12 through one of the pins disposed through the end of the envelops). Line 61 is directly attached to grid 49 as shown.

Accordingly, it is readily evident that grid 49 can be adjusted to the potential of cathode assembly 24 or arranged to a selected bias condition by the adjusting resistor 58.

Ideally, as shown in FIG. 4, beam 45 will form a substantially right cylindrical hollow beam when grid 49 has been biased to cathode potential. It has been also observed, however, that (FIG. 5) with the grid at slightly negative potential, there will be generated an electron beam with divergent sides with a cross-over point 45a located intermediate the grid and target.

Conversely, (FIG. 6) with grid 49 slightly positive, the electron beam 45 will be slightly divergent from each of the bounding edges of annular lip 44.

As now to be described, means have been provided for continuously varying the potential of an annular control element encircling the beam 45 so as to cause the beam to continuously vary in diameter in response to variations of the potential applied to the annular control element. In this way, a continuously varying convergent/divergent flow of electrons in a hollow beam gun will be maintained with high average power while avoiding problems attendant the use of high power concentrated on a small spot on the target.

Thus, an annular spot size control grid 66 is supported upon insulator rods 51 so as to align its central opening 67 coaxially of beam 45. Grid 66 is further electrically coupled via the semi-rigid strip 68 to an AC power supply 69 Connected via lead 71 passing outwardly and through the end of envelope 12 via a suitable pin connection (not shown). Power supply 69 operates at a suitable frequency whereby the electron flow of beam 45 will cyclically diverge and converge between large and small diameter annular hollow spots on the face-18 of body 17, in response to variations in potential applied to grid 66.

In addition to the foregoing, and as shown in FIG. 1, means for inhibiting the potential of target assembly 16 from influencing the electron emission from cathode portion 44 have been provided in the form of a pair of axially spaced grids 72, 73 formed respectively with narrow annular slits 74, 76 serving to pass the flow of electrons from the cathode to the target. Slits 74, 76 are sufficiently narrow and cathode portion 44 is disposed sufficiently behind slits 74, 76 so as to inhibit the potential of target 16 frominfluencing electron emission from cathode portion 44 when target 16 is at high voltage.

' Control grids 72, 73 may be pulsed via lead 77 coupled to pulse generator 62 so as to cause emission of electrons from cathode lip 44. Grids 72, 73 are of suitable metallic material, such as molybdenum, providing appropriate rigidity and electrical characteristics as are well known to those skilled in the art. The refractory nature of this material serves to protect cathode surface 44 from erosion caused by ion bombardment.

Grids 72, 73 are supported within envelope 12 by means of a cylindrical open-ended support jacket 78.

From the foregoing, it will'be readily evident that the embodiment shown in FIG. 1 can serve to provide relatively high average beam power while evenly distributing the electron energy of the beam over the target assembly 16. Accordingly, discrete locations of high intensity within the beam will be averaged out over the broad target surface so as to minimize danger of localized hot spots. The slits formed in control grids 72, 73 act to isolate the electron-emitting portions of cathode 46 from being influenced by the high voltage of target 16. Accordingly, means have been provided both for safe guarding against overheating of target assembly 16 and for obtaining high average beam power in a hollow beam X-ray tube.

'According to another embodiment as shown in FIG. 7, the further advantage of being able to obtain a very small electron impact area or spot whereby the sharpness and definition of radiographs becomes substantially improved has been provided as now to be described.

As shown in FIG. 7, an annular electron beam is generated from an annular cathode 82 having an electronemissive' portion 83, the cathode 82 being heated by a heater filament 84. v

Cathode 82 and heater 84 are supported by a relatively thin metal sleeve 86 disposed coaxially of an evacuated envelope 87 by suitable means.

Insulative stud supports 88 serve to support a focusing grid 89 and control grid 91 whereby the axis of X-ray tube 80 remains substantially unobstructed to the travel of electrons, ions, X-rays, etc.

A target 92 in the form of arelatively large tungsten block is supported at one end of tube 80 by means of zone 94 formed in the shape ofa closed conical surface oriented in a manner whereby the sides converge in a direction leading away from cathode 82.

By means of a type such as shown with respect to FIG. 1, electrical control of grid 91 can serve to provide a continuously varying diameter to the annular beam of electrons generated by cathode 82. In this way, the electrons will strike varying portions of zone 94 and, as arranged herein, by using the closed conical zone surface, it becomes possible to generate X-rays from a very small spot on the target.

Finally, an X-ray window 96 of material suitably pervious to X-rays, such as beryllium, is disposed in the opposite end of tube whereby X-rays may be discharged axially of tube 80.

In addition to the foregoing advantages of a structure as shown in FIG. 7, the conical target surface provides an increased efficiency in the utilization of the secondary electrons in that secondary electrons are readily generated from the conical surface of impact zone 94 so as to continue to strike other portions of zone 94 thereby generating additional X-rays.

As shown in FIG. 8, another embodiment of an X-ray gun 97 incorporates advantages and features of embodiments described above but with additional advantages of its own.

As noted above, certain advantages are obtained by means of the construction shown in FIG. 1. Certain additional advantages are obtained in the construction shown in FIG. 7. The advantages of both systems may be obtained upon use of the construction shown in FIG. 8 wherein a conical electron impact zone 98 is formed with sides which converge in a direction leading away from the cathode assembly 99. In addition, the annular electron beam 101 has been provided in a manner whereby-the high voltage of target 102 will not influence the emission of electrons from cathode assembly 99.

Thus, a cathode assembly 99 with its associated heater includes an electron-emitting portion 103 disposed to project radially inwardly of cathode assembly 99 and to direct electrons through a narrow slit 104 formed between the confronting edges of the two halves of a focusing grid element 106. In addition, control grids 107, 108 each formed with a slit 109, 11] respectively serve to function in the manner disclosed with respect to the embodiment shown in FIG. 1 whereby cathode 99 will not be influenced by a high potential on target 102. Accordingly, slit 104 and the slits 109, 111 are all sufficiently narrow and the cathode-emissive portion 103 is sufficiently removed from the foregoing slits that cathode 99 will not be influenced by the high potential of target 102. Suitable electrical lead throughs" and connections are made as before.

An X-ray window 112 of suitable material pervious to. the transmission of X-rays, such as beryllium is mounted coaxially of electron beam 101 and impact zone 98whereby a relatively high voltage can be plied to target 102 and X-rays projected outwardly from essentially a very small electron impact area or spot formed on' target 102. At the same time, the heat of impact of the electrons upon target 102 is dissipated over the relatively broad conical surface of the zone 98. v In addition, a heat conducting electrical insulating plug or seal 1 13 of beryllium oxide formed with annular grooves 114 at each end thereof for improved voltage control serves to dissipate heat generated by target 102. Further, cooling fins 116 can be provided to extend radially outwardly and spaced circumferentially about the perimeter of plug 113.

Finally, according to another embodiment of the invention, the target becomes the X-ray transmissive window. More particularly, the target comprises an X-ray productive layer of material such as a sufficiently thin layer of tungsten 117 formed within a closed conical surface forming an electron impact zone 118 whereby X-rays can pass through layer 117. A heat sink 119 in the form of a heat conductive material having X-ray transmissive qualities, e.g., beryllium, is mechanically supported to an end of the evacuated envelope 121 and immediately adjacent the target layer 117 whereby heat generated on target layer 117 can be readily dissipated while X-rays escape through layer 117 and heat sink 119. The dashed line 125 (FIG. 9) represents the degree of reduction in the size of heat sink 119 which may be accommodated with the reduction in beam power and target voltage as desired.

Accordingly, as shown in FIG. 9, an envelope 121 includes a cylindrical molybdenum sleeve 122 chosen for its thermal expansion coefficient corresponding substantially to that of glass as used in the end plug 123 of envelope 121. Sleeve 122 is suitably secured into the end of plug 123 in conventional style whereas interiorly of sleeve 122, a metal sleeve 124 such as may be provided from Monel metal is secured at the inner end of sleeve 122 to substantially rigidly support impact zone 118 coaxially of tube 121. As thus arranged, a suitable electron gun can be provided to generate a hollow beam for bombarding electrons upon the target surface 117 and, upon generating X-rays, the X-rays will be discharged through the relatively thin layer of target material and subsequently through the X-ray transmissive beryllium material of heat sink 119.

In this way, the X-ray window is disposed axially of the electron beam via heat sink member 119.

From the foregoing, it will be readily apparent that there has been provided an improved X-ray generating device solvingv a number of problems pertaining to the overheating of target material, limitations on high average power of the electron beam as heretofore encountered and in which the high resolution advantage of obtaining a minimum diameter spot so as to produce sharp radiograph images have been obtained.

We claim:

1. In an X-ray generating device of a type employing a target of material disposed to be struck by an annular beam of high energy electrons to develop X-rays, and having means for focusing the beam upon the target, said means including a grid disposed transversely to the axis of the beam and means for applying a charge to the grid for focusing the beam to form an annular hollow beam, the improvement comprising an indirectly heated cathode having an electron emitting portion adapted to be charged negatively with respect to said target to create a flow of electrons from said portion to said target, means for inhibiting the potential of said target from influencing electron emission from said cathode portion, the last named means comprising control grid means disposed between the first named said grid and said target and across said flow of electrons and formed to include an annular slit therein coaxially disposed relative to the axis of said flow for passing said flow of electrons therethrough to said target, said slit being sufficiently narrow and said cathode portion being spaced sufflciently therebehind so as to inhibit the potential of said target from influencing said electron emission from said cathode portion, an annular control element encircling said flow of electrons, and means coupled to said control element for varying the potential thereof to cause said flow of electrons to provide an annular beam impinging upon the target with a varying diameter in response to variations in said potential of the last named means.

2. In an X-ray generating device according to claim 1 wherein said cathode comprises an oxide-coated cathode.

3. In an X-ray generating device according to claim 1 comprising a pair of axially spaced control grids formed with narrow annular slits for passing said beam therethrough and serving to inhibit the potential of said targer from influencing the electron emission from said cathode, and a pulse generator coupled to said pair of grids for gating said beam to said target.

I t i t

Claims (3)

1. In an X-ray generating device of a type employing a target of material disposed to be struck by an annular beam of high energy electrons to develop X-rays, and having means for focusing the beam upon the target, said means including a grid disposed transversely to the axis of the beam and means for applying a charge to the grid for focusing the beam to form an annular hollow beam, the improvement comprising an indirectly heated cathode having an electron emitting portion adapted to be charged negatively with respect to said target to create a flow of electrons from said portion to said target, means for inhibiting the potential of said target from influencing electron emission from said cathode portion, the last named means comprising control grid means disposed between the first named said grid and said target and across said flow of electrons and formed to include an annular slit therein coaxially disposed relative to the axis of said flow for passing said flow of electrons therethrough to said target, said slit being sufficiently narrow and said cathode portion being spaced sufficiently therebehind so as to inhibit the potential of said target from influencing said electron emission from said cathode portion, an annular control element encircling said flow of electrons, and means coupled to said control element for varying the potential thereof to cause said flow of electrons to provide an annular beam impinging upon the target with a varying diameter in response to variations in said potential of the last named means.

2. In an X-ray generating device according to claim 1 wherein said cathode comprises an oxide-coated cathode.

3. In an X-ray generating device according to claim 1 comprising a pair of axially spaced control grids formed with narrow annular slits for passing said beam therethrough and serving to inhibit the potential of said targer from influencing the electron emission from said cathode, and a pulse generator coupled to said pair of grids for gating said beam to said target.

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