US 3679927 A
An X-ray tube with a metal window and having increased power loading capabilities without undesirable overheating of the window, this being achieved by the use of a magnet which is arranged so that magnetic field lines pass over the window surface to reduce bombardment of the window with secondary or back-scattered electrons from the anode.
Claims available in
Description (OCR text may contain errors)
14 1 July 25, 1972 United States Patent Kirkendall 54] HIGHPOWERX-RAYTUBE 1,203,495 10/1916 Coolid e...........................313/155x  Inventor:
Thomas D. Kirkendall, Potomac, Md.
Primary ExaminerNathan Kaufman Th e Mame Laboratories Incorporated Attorney-Harold A. Murphy and Joseph D. Pannone ABSTRACT  Filed: Aug. 17, 1970 An X-ray tube with a metal window and having increased  Appl. No.:
the window, this being achieved by the use of a magnet which is arranged so that magnetic field lines pass over the window surface to reduce bombardment of the window with secondary or back-scattered electrons from the anode.
References Cited UNITED STATES PATENTS 3 Claims, 6 Drawing Figures Coolidge.................................313/59 WINDOW TEMPERATURE, C
WINDOW TEMPERATURE, C
PATENTEII I972 3,679,927
SIIEEI 2 [IF 2 80 I I I l I 50 KV CONST PoTENTIAL 5o Kwze mu MAXIMUM RATING I300 WATTS I20" '7 WITHOUT MAGNET o KV/26 mo 80- ANODE /NITH MAGNET I WINDOW X X I l I I I I o 200 400 600 800 I000 I200 I400 x x ANODE POWER, wATTs X X I I l I I 5 I0 I5 x x I ANODE CURRENT, mo TARGET CATHODE l I l l l )1 26 KV/50mcI 2s KV coNsT POTENTIAL I4o- X=MAGNETIC FIELD LINEs MAxIMuM RATING INTo THE PAPER I300 WATTS WITHOUT 6 |QO MAGNEy' ,1
26KV/26mc sos0- -/WITH 40- MAGNET O I I I J l I o 200 400 600 800 I000 I200 I400 AN ODE POWER, WATTS I I I l I I I I l l 5 IO I5 20 25 3O 35 4O 5O ANODE CURRENT, mu
HIGH POWER X-RAY TUBE BACKGROUND OF THE INVENTION X-ray tubes such as used in X-ray spectroscopy and various X-ray diffraction applications commonly employ a vacuumized casing or envelope which is at least partially made of metal and which includes a port or opening through which an output beam of X-rays is permitted to pass. In order to preserve the vacuum within the envelope, a window or filter of beryllium or other selected material which efficiently transmits X-radiation, particularly of relatively long wavelength, is sealed within the port and the X-ray beam passes through it.
It has been found that during operation of such a tube the window becomes heated to such an extent that it becomesv damaged, consequently resulting in loss of the tube. A major cause of such heating of the window is the effect of secondary electrons from the anode striking the window which may be at anode potential. The maximum power rating of an X-ray tube having a thin beryllium window on the order of 0.005 inch and 0.010 inch thick is limited severely by the high temperature operation of the window, even though the power dissipation capability of the anode target material may allow higher power loading. For example, the window of a tube operated at less than 1,000 watts will become heated to temperatures as high as about 225C, and, therefore, in most cases it has been assumed that 1,200-1,300 watts is a maximum safe loading. At such power levels the tube will perform at relatively low yield.
SUMMARY OF THE INVENTION The problem of excessive heating of X-ray tube windows is overcome in the present invention by the use of a magnet which is so arranged that the magnetic field lines pass over the window substantially parallel to it in order to reduce bombardment of the window by secondary or back-scattered electrons from the anode. The window is preferably a beryllium disc about 0.005 inch to 0.010 inch thick and the magnet may have a field strength of approximately 400 gauss, for example. The magnet may be any suitable permanent or electromagnetic type and, for example, may be a horseshoe type positioned externally around the anode housing with its pole pieces situated one on either side of the window.
As an example, when operating a platinum, molybdenum, chromium, tungsten or rhenium target tube with a 0.005 inch thick beryllium window at 50kv constant potential, at maximum ratings of 1,200-1,300 watts, it has been found that when using a magnet as described the window temperature is decreased to as little as 36 to 40 percent of the temperature when no magnet is used. This, therefore, permits a substantial increase in the maximum loading capability of the tube which was heretofore limited to 1,200-1,300 watts by the excessive heating'of the window. By using the magnet, the same tube may be safely operated at 2,500 watts (50kv/50ma), which is about double the power, with the temperature of the window not exceeding about 225C.
By using a magnet to reduce secondary electron bombardment of the window and resultant heating thereof, the window temperature is no longer the limiting factor in maximum power capabilities of the tube. Furthermore, with a magnet, in a spectrographic application where the X-ray tube is used for fluorescence analysis of a specimen or object, the yield of the tube for a broad range of elements shows a 50 to 100 percent increase when the tube is operated at 2,500 watts compared to operation at 1,200 watts without a magnet. Further in accordance with the invention the magnet may be moved to provide a magnetic field whlch may extend as desired with respect to other tube parameters such as the primary beam, for example, and thus to control focal spot location and size.
BRIEF DESCRIPTION OF THE DRAWINGS Other objectives and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein:
FIG. 1 is a fragmentary side elevational view of an X-ray tube embodying the invention and showing the electrode structures in axial section;
FIG. 2 is a horizontal sectional view taken on line 2-2 of FIG. 1 looking in the direction of the arrows;
FIG. 3 is a top plan view of the tube shown in FIG. 1 utilizing an electromagnet:
FIGS. 4 and S are graphs showing window temperature comparisons between tubes operated with and without a magnet; and
FIG. 6 is a diagrammatic illustration of the magnetic field lines in an X-ray tube according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The tube shown in the drawing comprises an evacuated envelope 10 having one end portion 11 formed with a reentrant portion 12 which supports a cathode structure 14 in a conventional manner. The cathode structure 14 includes the usual electron-emitting filament 16 which is located within a cathode head or focusing element 18 so that when suitable filament potential is applied across leads 20, which pass out through the reentrant portion 12 of the envelope, electrons will be emitted by the filament 16 and will flow through an aperture 22 in the head 18. The portion 24 of the envelope which encircles the cathode structure 14 is a hollow metal cylinder hermetically sealed by a metal ring 26 to one end of envelope portion 11. The opposite end of cylindrical envelope portion 24 is hermetically sealed to and supports and anode structure 28 which includes a hollow member 30 of copper or other suitable thermally and electrically conductive material.
One end of the cavity 32 within the anode member 30 is in communication with the cathode structure of the tube by way of an opening 34 in the inner end of the member. Within the other end of the cavity 32 is disposed a target-supporting block 36 having its exposed surface directed toward and inclined at a predetermined angle with respect to the cathode structure 14. Embedded in or otherwise suitably secured to the inclined surface of supporting block 36 is a target disc 38, of a material such as tungsten, molybdenum, chromium, rhenium or rhodium which generates copious characteristic X-radiation of desired wavelength when bombarded by electrons, as is well known. Opposite the inclined target 38 a flattened portion 39 of the side wall of the anode member 30 is provided with a window opening or port 40 through which the X-rays are allowed to pass outwardly of the tube. The opening 40 is closed by a window or filter 42 of beryllium which is hermetically sealed into the opening 40 as a metal bezel 44. Thus, the interior of the tube including the cavity 32 is maintained substantially gas free.
During operation of a tube of this character it has been found that the metal window 42 becomes extremely hot and often becomes damaged as a result. Such damage as cracking of the window or destruction of the hermetic seal destroys the vacuum within the tube and, therefore, becomes disastrous. A major cause of window heat is the effect of secondary electrons from the anode which strike the window which may be at anode potential. With a thin beryllium window 42 on the order of 0.005 inch and 0.010 inch thick, for example, it has been necessary to severely limit the maximum operating power levels of the tube to prevent overheating of the window, although the power dissipation capability of the tube, including the target material, will allow higher power loading. For example, in one tube run at 50kV constant potential with increasing anode current and utilizing a chromium target, the window assumed a temperature of about l60C at 1,300 watts anode power. This is illustrated in FIG. 4 by the line denoted without magnet. In the same tube operated at 25kv constant potential the window still assumes a temperature of nearly C at 1,300 watts anode power, as shown by line indicated as without magnet, in FIG. 5.
In accordance with this invention, the window temperature is considerably reduced so that the tube may be operated at much higher power levels without overheating the window. This is achieved by using a magnet of suitable magnetic flux density, such as, for example, about 400 gauss, to effectively change the trajectories of the back-scattered or secondary electrons to reduce bombardment of the window by such electrons. The magnet must be situated so that the magnetic field lines pass substantially parallel to the surfaces of the window. This is illustrated diagrammatically in the FIG. 6 example where the magnetic field lines indicated by x are directed into the paper and therefore parallel to the surfaces of the window and target and perpendicular to the axis of the tube and the flow of primary electrons from the cathode to the anode.
A permanent magnet 50 of the common horseshoe type may be used and, in the example illustrated, is placed around the external portion of the anode 30 with its pole pieces 52-54 situated one on either side of the window and perpendicular to it so that the magnetic field lines between the pole pieces pass over the surface of the window 42 and of the target 38 perpendicular to the axis of the tube and, consequently, to the How of primary electrons from the cathode to the anode.
An electromagnet 56 may also be used, as shown in FIG. 3, and would embody a horseshoe-shaped core within a suitable electric coil 58.
The magnet may, of course, be supported with respect to the anode by any suitable means. For instance, it -may be mounted directly upon the tube by mechanical means or may be supported separately by idenpendent means.
Referring again to FIGS. 4 and 5, it will be seen that the same tube operated at 50kv and 25kv respectively with a magnet as described will assume much lower window temperatures. For example, the line indicated as with magnet" in FIG. 4 shows that at 1,300 watts the window will be heated to a temperature of only about 1 l5C, while in the example of FIG. 5 the window reaches a temperature of only about 92C.
With magnets, these tubes have been successfully operated at 2,500 watts without excessive heating of the windows, in comparison to the previously described tubes without magnets which require maximum power levels of 1,200-1 ,300 watts.
With tubes embodying the present invention it has been found that certain other advantages have also been achieved because of the use of magnets in the manner described. For example, when the invention is used in a fluorescence spectrometer apparatus the yield of the tube for a broad range of elements shows a 50 percent to 100 percent increase in effciency when operated at 2,500 watts compared to operation of the same tube without a magnet at a maximum safe loading of about 1,200 watts. ln addition, the copper spectral impurity, which originates from additional secondary electron impingement upon the walls of the cavity, is reduced when the tube is operated with the magnet as described.
Although the foregoing description relates to an X-ray tube having a metal window, it has also been found that a magnet used as described will reduce wall charges which are often built up on glass envelopes by bombardment of secondary electrons. The magnet, further may be located within the tube structure and may be positioned so as to affect the primary electron beam flowing from the cathode to the anode whereby the location and size of the focal spot may be controlled. The magnet may also be of the so-called cylindrical" type which may surround the interelectrode space either within or outside the envelope, as long as the magnetic field lines extend substantially parallel to the metal window surface or the surface of the window portion of a glass envelope. Additionally, it is to be understood that the magnetic field density may be made to vary in accordance with the particular use to which the tube may be put.
From the foregoing it will be seen that all of the objectives and advantages of this invention have been achieved by employing, in an X-ray tube, magnet means as shown and described. However, it will be apparent that other modifications and changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompan ing claims. Therefore, all matter shown and described shoul be interpreted as illustrative and not in a limiting sense.
l Claim 1. An X-ray device comprising an X-ray tube including a housing having an anode made of non-magnetic material and a cathode therein, said anode having a cavity therein with its open end facing the cathode in predetermined spaced relation therewith, an X-ray generating target located within the cavity at the bottom thereof and having its effective surface inclined at an angle with respect to the axis of the tube, a port in a side wall of the anode through which a beam of X-rays from the target may pass, said cathode being positioned to direct electrons along said axis onto said surface of the target for the generation of X-rays therefrom, a metal window in said port, and magnet means positioned with respect to said window and producing a magnetic field within the anode having field lines extending transverse to the axis of the tube for reducing impingement on the window of electrons emitted or backseattered from the target.
2. An X-ray device as set forth in claim 1 wherein said magnet means comprises a magnet so located that the magnetic field lines pass over the interior surface of the window and the effective surface of the target substantially parallel thereto.
' 3. An X-ray device as set forth in claim 2 wherein said magnet comprises pole pieces located at opposite sides of the window and disposed in a plane perpendicular to said axis and the magnetic field lines also extend perpendicular to the flow of electrons between the cathode and target.
UNITED STATES PATENT OFFICE Patent No.
Invent0r(s) Thomas D. Kirkendall EDWARD M.FLE;TCHER,JR. Attesting Officer CERTIFICATE OF CORRECTION Dated July 25 1972 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Include under References Cited" 3, 567,983 to deVries- Signed and sealed this 6th day of February 1973.
ROBERT GOTTSCHALK Commissioner of Patents ORM PO1050(10-69) USCOMM-DC 60376-P69 UYS. GOVERNMENT PRINTING OFFICE: I969 0-365-334