US3517195A - High intensity x-ray tube - Google Patents

Description

e. A. LEAVlTT HIGH INTENSITY X-RAY TUBE June 23, 1970 Filed July 2, 1968 INVENTOR. GEORGE A. LEA VITT BY ,ZA- d ATTORNEY United States Patent US. Cl. 250-90 Claims ABSTRACT OF THE DISCLOSURE A tube for generating relatively high intensity, uniform density beams of X-rays having, for example, currents of tens of amperes for hundreds of microseconds. The beam may be uniformly dispersed over a relatively large diameter, and if desired the large diameter beam may be collimated into many closely spaced clustered small spots having nearly comparable lesser intensities. The clustered spots facilitate X-ray sampling at locations of much closer spacing than is obtainable with a cluster of small X-ray tubes due to the limitations imposed by their physical size. Alternatively, the tube may be employed to produce a very intense X-ray beam of small spot size.

BACKGROUND OF THE INVENTION This invention was evolved in the course of, or under, Contract W-7405-ENG48 with the United States Atomic Energy Commission.

X-ray tubes have long been employed as diagnostic tools in a variety of applications. Commercially available tubes are typically of either the very high currentshort time interval flash variety productive of X-ray beam currents of the order of kiloamperes for time intervals of the order of fractions of microseconds, or the low current-long time interval standard type of tube productive of beam currents of the order of milliamperes for nearly continuous operation. Tubes operable in the area between these two categories, i.e., capable of producing medium beam currents of the order of tens of amperes for relatively long time intervals of the order of hundreds of microsoeconds have not been heretofore available. It is desirable that tubes of an intermediate category be provided since it would then be possible to obtain an X-ray suited to nearly any diagnostic requirement. In one application there is required a relatively long duration medium current X-ray beam uniformly dispersed over a relatively large beam diameter, e.g., 1 /2 inches, which may be collimated to form a plurality of clustered closely spaced small beam spots substantially equal in intensity to each other. The collimated spots are employable for X-ray sampling of locations of much closer spacing, and therefore with much higher resolution, than has been previously possible. More particularly, the usual practice has been to cluster a plurality of miniature X-ray tubes to thereby provide a cluster of small beam spots for viewing by a correspondingly clustered array of detectors. Although semiconductor X-ray detectors of extremely small size are available which may consequently be clustered in very closely spaced relationship, limitations are imposed on the closeness of the spacing by the physical size of the clustered X-ray tubes producing the beam spots viewed by the detectors. The problem may be readily overcome by collimating a uniform relatively large diameter medium intenity beam from a single tube into a cluster of very closeyl spaced beam spots matching the attainable closeness of the spacing of the semiconductor detectors.

Various previous attempts to provide an X-ray tube having the advantageous characteristics noted hereinbefore were unsuccessful. The flash type of X-ray tube is fice not suited to modification for operation at reduced currents and materially increased pulse lengths due to limitations imposed by the field emission cathode employed in this type of tube for current generation. Even when operated at medium currents, the cathode quickly fails by melting of the field emission points when the pulse length is increased to the extent desired. Modification and scaling up of the conventional low current-long time interval type of tube for pulsed operation with increased currents was also an unsatisfactory approach to the problem for several reasons. The thermionic emission employed in such tubes is limiting upon the quantity of electrons available for X-ray production. Consequently, in a conventional tube geometry employing a disc cathode coaxially spaced from a disc anode inclined to the tube axis, a relatively large diameter cathode is required to provide a sufficient electron emission density. It was found that the necessary emission densities could not be obtained with practicable associated heater designs and values of heater power. Moreover, it was found that the electron beam did not expand sufiiciently in a practicable distance between the cathode and anode to produce an X-ray beam having the desired relatively large diameter. Furthermore, the intensity of the X-ray beam generated with the inclined anode geometry was not uniform across the beam diameter. Accordingly, in order to provide an X-ray tube having the desired characteristics of medium beam current, relatively long pulse duration, and uniform intensity across a preferably large beam diameter, it was necessary to depart from conventional design.

SUMMARY OF THE INVENTION The present invention relates to an X-ray tube having a novel electrode geometry capable of producing a medium current, relatively long pulse length X-ray beam, preferably uniformly dispersed over a relatively large beam diameter, although the beam may be alternatively concentated into a more intense spot of smaller diameter. The invention is particularly well suited to the production of an X-ray beam having, for example, a ampere beam current for a microsecond pulse length and uniformly distributed across a diameter of 1 /2 inches. As an important feature of the invention, the electrode geometry is capable of being scaled up to produce beam currents of hundreds of amperes.

'In the accomplishment of the foregoing, an X-ray tube with novel electrode geometry in accordance with the present invention generally includes an anode coaxially disposed within an evacuated tube envelope in opposed spaded relationship to an X-ray transparent window thereof, and an annular directly heated cathode filament and concentric inwardly spaced annular electron accelerating grid coaxially disposed outwardly of the anode on the window side thereof. With heating current applied to the cathode filament to effect electronemission, and a positive potential applied to the anode and grid, electrons are accelerated radially inward from the filament and turned towards the anode for impingement uniformly over the surface thereof. The decelerative collisions of the electrons with the anode produce X-rays which are beamed coaxially through the window and have the desirable characteristics noted hereinbefore. The filament is preferably of sinewave configuration formed around a right cylindrical surface, while the grid is preferably provided as a cage of circumferentially spaced wires dis posed parallel to the filament axis in order to insure that a sufiicient quantity of electrons impinge the anode uniformly over its entire surface to produce an X-ray beam having the desired current and uniformity. The anode is preferably in the form of a circular disc in instances where a large beam diameter is desired. A conical anode is best employed in applications where a more intense beam of small spot size is required.

3 In accordance, with a further important aspect of the invention, means are incorporated in the tube to prevent breakdown between external electrode terminals thereof due to ionization of air at the external surface of the envelope by the emitted X-rays.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view, with portions broken away, of an X-ray tube in accordance with the present invention, as arranged to produce an X-ray beam of relatively large diameter.

FIG. 2 is a sectional view taken at line 22 of FIG. 1.

FIG. 3 is an exploded perspective view, with portions broken away, of the electrode geometry of the tube as modified to produce an X-ray beam of relatively small diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Considering now the invention in detail with reference to the illustrated forms thereof in the drawing, there will be seen to be provided an X-ray tube 11 including cathode filament 12, accelerating grid 13, and anode 14 arranged in a novel electrode geometry within an evacuated envelope 16 to produce an X-ray beam having a medium current of the order of tens of amperes and relatively long pulse duration of the order of hundreds of micros conds. The envelope is preferably of enlongated cylindrical configuration including a cylindrical cupped metallic can 17 at one end, a cylindrical cupped glass insulating member 18 at the opposite end, and an intermediate annular metallic coupling 19 coaxially interconnecting the open ends of the can insulating member in hermetically sealed relationship. The end wall 21 of the can is provided with a central circular window 22 of beryllium, or equivalent material that is transparent to X-rays. The end wall 23 of the insulating member is formed with a coaxially reentrant cylindrical indentation 24 for purposes subsequently described.

Within the can 17 and annular coupling 19 there is mounted a tubular cylindrical open ended metallic ground plane and heat shield 26 in close inwardly spaced coaxial relation thereto. The shield is formed with inwardly flared annular flanges 27 and 28 adjacent its opposite ends and extends longitudinally between a transverse plane substantially adjacent the sealed joint between insulating member 18 and coupling 19, and a transverse plane adjacent the end wall 21 of the can. The shield is additionally provided with an inwardly flared annular flange 29 at a location intermediate its ends encompassed by the peripheral wall of the can.

To facilitate mounting of the heat shield, a pair of diametrically opposed metallic lugs 31 are secured to the interior surface of the peripheral wall of the can 17 to project radially inward therefrom adjacent the end wall 21. A second pair of diametrically opposed lugs 32 similarly project radially inward from the peripheral wall of the can adjacent the joint between the can and the coupling 19 at positions circumferentially spaced 90 from the lugs 31. First and second pairs of diametrically opposed clips 33 and 34 are secured to the periphery of the heat shield at positions corresponding to the lugs 31 and 32. The clips are formed with eyes receiving screws 36 threadably engaging tapped bores provided in the lugs to thereby secure the heat shield coaxially within the envelope in the previously noted position. The clips and lugs also serve to electrically connect the heat shield to the can.

Considering now the novel electrode geometry of the X-ray tube 11 in detail, it is to be noted that the anode 14 is disposed within the envelope 16 in coaxially spaced relation to the window 22. More particularly, in one embodiment of the invention for producing an X-ray beam of relatively large diameter, the anode is of circular disc configuration and concentrically disposed within the inter- 4 mediate flange 29 of heat shield 26 in inwardly spaced relation, as shown in FIG. 1. Mounting of the anode in position is preferably accomplished by means of an elongated metallic sleeve 37 which coaxially traverses the indentation 24 of the insulating member 18 in hermetically sealed relation thereto and extends into the envelope to a location adjacent the flange 29. The proximal end of the sleeve relative to the flange is formed with a plug 38 having a central tap 39 extending thereinto. The tap is engaged by a threaded stud 41 coaxially projecting from the disc anode to thereby secure same in place. The sleeve serves as a pump out conduit, as well as an electrical terminal for applying high voltage to the anode. More particularly, at least one aperture 42 extends radially through the wall of the sleeve to communicate the sleeve bore with the interior of the envelope. A tubular member 43 of glass, or equivalent insulating material, is hermetically secured to the exterior end of the sleeve in spaced relation to the sealed joint between the sleeve and insulating member 18 such that an annular band 44 of the sleeve surface encompassed by the indentation 24 is exposed. The insulating tubular member 43 facilitates insulated communicable connection of the sleeve to a pump whereby the interior of the envelope may be evacuated to high vacuum dimensions. The exposed band 44 of the sleeve defines an electrical terminal for high volt age energization of the anode 14.

In further respect to the novel electrode geometry, the cathode filament 12 is directly heated and disposed in an annular region of space coaxially outward of the anode on the proximal side thereof relative to the window 22. The accelerating grid 13 is annular and concentrically disposed inwardly of the filament in coaxial outwardly spaced relation to the anode. The grid is electrically connected to the anode and is consequently at the same potential.

The filament is preferably of sinewave configuration formed around a right cylindrical surface such that the cycles of the sinewave are parallel to the axis of the envelope 16 and repeat in the circumferential direction. The sinewave filament provides an extended electron emissive surface capable of emitting copious quantities of electrons requisite to the production of X-ray beams of relatively high currents.

The grid is preferably provided as a cage of circumferentially spaced wires 46 extending longitudinally from the periphery of disc anode 17 towards the window 22. The free ends of the wires are best secured to an interconnecting ring 47 to prevent sagging of the wires into the filament during operation.

Support of the filament in operative position is best facilitated by means of insulated fastening elements 48 secured to the heat shield 26 at circumferentially spaced positions thereof to project radially inward into securance with the filament. The filament is provided with an opening so as to define a pair of ends for facilitating energization with heater current. In this regard, one end is preferably electrically connected to the heat shield, as indicated at 49. The other end of the filament is led through the heat shield, in insulated relationship thereto, into electrical connection with the conductor of a filament input terminal 51 mounted on the exterior of the can 17. A ground terminal 52 is also conductively mounted on the can exterior. Thus, the terminal 52 may be connected to ground and a filament supply (not shown) connected between the terminals 51 and 52 to ground the can and heat shield and energize the filament with heater current, thereby resulting in the thermionic emission of electrons therefrom.

The X-ray tube 11 further'includes a getter 53 preferably connected at one end to the interior surface of can 17, and at the other end to the conductor of a getter input terminal 54 mounted on the can exterior. Thus, a power supply may be connected between the terminals 52 and 54 to energize the getter, whereupon same func' tions in the usual maner to establish high vacuum conditions in the envelope 16.

In the overall operation of the X-ray tube 11, the envelope 16 is first evacuated to establish a high vacuum therein. This is accomplished by connecting the tubular insulating member 43 at the exterior end of the sleeve 37 to a vacuum pump capable of pumping the envelope interior to rough vacuum dimensions. The getter 53 is momentarily energized to deposit some of the getter material on the interior surface of the envelope for the purpose of adsorbing residual gas and thereby establishing the desired high vacuum. A high voltage pulsed power supply is connected to the exposed band 44 of sleeve 37 to apply high voltage pulses to the anode 14 and grid 13. Terminal 52 is connected to ground to thereby place the can 17 and heat shield 26 at ground potential. A filament supply is connected between terminals 51 and 52 to energize the filament 12 with heater current and thereby effect the generation of copious quantities of electrons. The electrons are accelerated from the filament radially inward and downward to impinge the anode 14 with a uniform distribution over its entire surface due to the electric field established by each pulse of positive potential on the grid and anode. The electrons suffer decelerative collisions with the anode productive of X-rays which are preferentially emitted in a slowly diverging relatively large diameter beam directed coaxially through the window 22. By virtue of the substantial quantity of electrons emitted from the extended surface of the filament, and their uniform distribution over the anode surface, the X-ray beam has a relatively high current which is substantially uniformly distributed across the beam diameter.

The X-ray tube 11 may be modified to produce a more intense beam of reduced diameter, in which case a conical anode 14' is employed in place of the disc anode 14, as shown in FIG. 3. More particularly, the anode 14' is secured to the end of sleeve 37 in inwardly spaced coaxial relationship to the grid 13 with the apex of the cone even with, or short of the ring 47. Securance of the conical anode 14 to the sleeve is preferably accomplished in a manner similar to that of the disc anode 14. A threaded stud 56 projects coaxially from the base of the conical anode for engagement in the tap 39. Although the grid wires 46 may be secured to the base of the anode 14', such wires are preferably secured to a washer 57 traversed by the stud 56 to thereby secure the grid structure in place.

As in the case of the disc anode 14, electrons emitted from the filament impinge the conical anode 14 with a uniform distribution over the surface thereof. However, the emission pattern of the conical anode is such as to concentrate the X-rays into an intense beam of small spot size transmitted coaxially through the window 22.

It is to be noted that a relaitvely long insulated path exists across member 18 between the exposed band 44 of sleeve 37, which is at high positive potential, and the metallic coupling 19, which is at ground potential. Still, there would normally be a tendency for breakdown to occur across the exterior surface of insulating member 18 due to ionization of a surface film of air by a portion of the X-rays emanating from the anode being directed through the insulating member. However, in accordance with another important feature of the present invention the foregoing problem is obviated by the provision of means for masking the insulating member from X-ray emission at the anode. More particularly, an inverted funnel shaped metallic collar 58 is preferably concentrically mounted upon the sleeve 37 at a position wherein the outwardly flared conical portion 59 of the collar is disposed in radially inwardly spaced relation to the rim of flange 27 of heat shield 26. In addition, the base edge of conical portion 59 is disposed within the insulating member 18 at a greater radius than that of the rim of flange 27. Thus, the collar 58in cooperation with the flange 27 block line-of-sight paths between the anode and insulating member while permitting free communication between the interiors of the heat shield and insulating member. Since the line-of-sight paths are blocked, the insulating member is masked from X-ray mission, and ionization breakdown between the coupling 19-and anode terminal band 44 is prevented. The collar also functions as a heat shield for protecting the seal between the sleeve 37 and indentations 24 of insulating member 18 against heat damage.

I claim:

1. An X-ray tube comprising an evacuated cylindrical envelope having an X-ray transparent window coaxially disposed in an end thereof, an anode disposed within said envelope in coaxially spaced relation to said window, an annular directly heated cathode filment coaxially disposed outwardly of said anode on the proximal side thereof relative to said window, filament supply means coupled to said filament for energizing same with heating current to thereby effect the emission of electrons therefrom, and means coupled to said anode for establishing an electron accelerating electric field extending between said filament and anode, whereby said electrons are accelerated to said anode for decelerative collisions therewith productive of X-rays directed coaxially through said window.

2. An X-ray tube according to claim 1, further defined by said filament being of sinewave configuration formed about a right cylindrical surface disposed coaxially outward from said anode.

3. An X-ray tube according to claim 1, further defined by said means for establishing an electron accelerating electric field comprising a tubular cylindrical metallic ground plane and heat shield coaxially mounted within said envelope in outwardly spaced encompassing relation to said filament, an annular accelerating grid concentrically disposed inwardly of said filament in coaxial outwardly spaced relation to said anode, means for applying ground potential to said ground plane and heat shield, and means for applying positive potential to said anode and said grid.

4. An X-ray tube according to claim 3, further defined by said filament being of sinewave configuration formed about a right cylindrical surface disposed coaxially outward from said anode, and said grid comprising a plurality of circumferentially spaced longitudinally extending wires electrically connected to said anode.

5-. An X-ray tube according to claim 4, further defined by said anode being of circular disc configuration.

6. An X-ray tube according to claim 4, further defined by said anode being of conical configuration and disposed with its apex substantially flush with the proximal ends of said grid wires relative to said window.

7. An X-ray tube according to claim 5, further defined by said envelope including a cupped cylindrical metallic can having said window centrally of the end wall thereof and a cupped cylindrical insulating member, said can and insulating member having their open ends coupled in coaxially opposed thermetically sealed relationship, said insulating member having a coaxially reentrant cylindrical indentation formed in the end wall thereof, means electrically connecting said ground plane and heat shield to said can and coaxially mounting said ground plane and heat shield within said can to extend between a transverse plane adjacent the coupling between said can and insulating member and a transverse plane adjacent said end wall of said can, a metallic sleeve coaxially traversing said indentation of said insulating member in hermetically sealed relation thereto and extending into said ground plane and heat shield to an intermediate position thereof, said sleeve having an exterior end portion encompassed by the peripheral wall of said indentation in inwardly spaced relation thereto, means securing said anode and grid in electrically conducting relation to the interior end of said sleeve, said exterior end portion of said sleeve defining a terminal for connection of a positive potential supply thereto and thereby comprising said means for applying positive potential to said anode and said grid, and means disposed within said envelope for blocking line-of-sight paths between said anode and insulating member to thereby mask the insulating member from X-ray emission at said anode and prevent ionization breakdown between said terminal and said can.

8. An X-ray tube according to claim 7, further defined by said filament being of si newave configuration formed about a right cylindrical surface disposed coaxialy outward from said anode and inwardly from said ground plane and heat shield, and said grid comprising a plurality of circumferentially spaced longitudinally extending wires secured in electrically conducting relation to said anode and having their free ends interconnected by a ring.

9. An X-ray tube according to claim 7, further defined by said means for blocking line-of-sight paths between said anode and insulating member comprising an inwardly flared annular metallic flange of said ground plane and heat shield substantially in said transverse plane adjacent the coupling between said can and insulating member, said flange being coaxially spaced from said anode in the direction of said insulating member, and an inverted funnel shaped metallic collar concentrically mounted upon said sleeve, said collar having an outwardly flared conical portion traversing the opening of said flange in radially inwardly spaced relation to the rim thereof with the base edge of said conical portion disposed within said insulating member at a greater radius than that of the rim of said flange.

10. An X-ray tube according to claim 9, further defined by said filament being of sinewaye configuration formed about a right cylindrical surface disposed coaxially out- Ward from said anode and inwardly from said ground plane and heat shield, and said grid comprising a plurality of circumferentially spaced longitudinally extending wires secured in electrically conducting relation to said anode and having their free ends interconnected by a ring.

No references cited.

JAMES W. LAWRENCE, Primary Examiner C. R. CAMPBELL, Assistant Examiner US. Cl. X.R.

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