US3549931A - X-ray transmissive window assembly - Google Patents

Description

United States Patent Victor E. DeLucia Los Angeles, Calif.

Dec. 4, 1968 Dec. 22, 1970 Torr Laboratories, Inc.

Los Angeles, Calif.

a corporation of California Continuation of Ser. No. 752,623, Aug. 14, 1968, now abandoned.

lnventor Appl. No. Filed Patented Assignee X-RAY TRANSMISSIVE WINDOW ASSEMBLY 7 Claims, 3 Drawing Figs.

US. Cl 313/55,

313/59, 313/110 Int. Cl. H01j 35/18 Field ofSearch 313/55, 57,

59, 1 10, 74, 330, 63, 94, (Cursory) [56] References Cited UNlTED-STATES PATENTS 2,340,362 2/1944 Atlee et a1. 313/59 2,361,897 10/1944 De Graaf. 313/57X 2,490,246 12/1949 Zunick 313/59X 2,604,229 7/1952 Schwarz (313/59UX) 3,419,741 12/1968 Legendre 313/59 Primary Examiner-James W. Lawrence Assistant Examiner-Palmer C. Demeo Attorney-Jessup & Beecher ABSTRACT: A high voltage X-ray tube is provided which includes a hooded cathode to prevent stray and secondary emission electron charges from collecting on the inner surface of the envelope of the tube, and by which improved operation is achieved through the focusing effect of the hood on the electron beam in the tube; and which includes an improved window for permitting X-rays to be passed through the envelope of the tube, and which is constructed as a separate subassembly.

PATENTED mm v 3.549531 i l i/ MAM AIraRN- i? X-RAY TRANSMISSIVE WINDOW ASSEMBLY This application is a continuation of eopending application Ser. No. 752,623 which was filed Aug. 14, 1968, and now abandoned BACKGROUND OF THE INVENTION It has been found in the past, especially with the high voltage type of X-ray tubes, that there is a tendency for electron charges to collect on the inner surface of the glass envelope of the tube. These charges are created by stray electrons from the electron beam, and by secondary emission electrons discharged from the target electrode.

In order to prevent the formation of the aforesaid electron charges, hooded anode X-ray tubes have been provided in the prior art. In such prior art X-ray tubes, a cylindrical electrically conductive hood" is mounted around the anode elec trode and is electrically connected thereto. The hood extends coaxially with the longitudinal axis of the X-ray tube to the cathode, and it surrounds the cathode out of electrical contact therewith. Since the anode electrode is established at a high positive voltage with respect to the cathode, the hood also is established at such a positive voltage. Therefore, the electron beam form the cathode enters a positive field which tends to diverge the electrons in the electron beam.

The prior art hooded anode X-ray tube has proven effective in preventing scattered electrons and secondary emission electrons from creating negative charges on the wall of the tube, which charges would adversely affect the operation of the tube. However, as mentioned above, there is a tendency for the electron beam from the cathode in the prior art hooded anode X-ray tube to diverge on its way to the anode, this being due to the effect of the electric field established by the hood. This divergence of the electron beam, in turn, creates a tendency for the resulting spot on the target portion of the anode electrode to become enlarged, and the divergence also causes the spot to exhibit irregularities throughout its area, so that it is not uniform.

For optimum X-ray performance of the tube, a small and uniform spot on the target electrode is required. For only then can a relatively large anode angle be used without excessive enlargement of the spot. It is desirable to utilize large anode angles in the X-ray tube with respect to the vertical axis. This is because the larger the anode angle, the more widely divergent X-ray beam will be produced. A widely divergent X-ray beam is desirable, since it enables relatively large workpieces to be X-rayed over their entire surfaces while positioned relatively close to the X-ray source for optimum intensity, and for minimum space requirements in the equipment.

In brief, it is axiomatic in an X-ray tube that the smaller the spot formed by the incident electron beam on the target electrode, and the more uniform the spot, the better the resolution capabilities of the X-ray tube. Also, spot uniformity serves to prolong the life of the target electrode since the formation of target deteriorating hot spots on the target electrode by the electron beam is obviated.

The X-ray tube of the present invention uses a hooded cathode to prevent the formation of electron charges on the wall of the tube, so that the proper operation of the tube is not impeded by such charges. The hooded cathode of the X-ray tube constructed in accordance with the concepts of the invention, however, does not adversely affect the electron beam. Instead, the hooded cathode of the X-ray tube of the invention has a converging effect on the electron beam, and it actually serves to enhance the operating characteristics of the tube.

The use of the hooded cathode in the X-ray tube of the invention produces a relatively small and uniform spot on the target electrode, this being achieved without the need for a small high temperature cathode emissive area, with its short life characteristics. The hooded cathode assembly of the invention permits large anode angles to be used in the X-ray tube. For example, a hooded cathode X-ray tube constructed in accordance with the concepts of the present invention can produce without difficulty a 0.2 millimeter uniform rectangular spot on the target electrode, and permits anode angles up to 50 to be used. This is to be compared with the usual prior art hooded anode X-ray tube in which the maximum anode angle is of the order of 22.

The improved X-ray tube of the invention also uses an improved window assembly which is sealed in the envelope of the X-ray tube, and which permits X-rays to pass freely through the envelope. This window, as will be described, involves a disc member of X-ray transmissive material, such as beryllium, and it is constructed as a separate subassembly The window subassembly can easily be mounted in the envelope of the X-ray tube, and can just as easily be demounted and reused in a different X-ray tube should the original tube fail to function or cease its normal operation.

The aforesaid construction of the beryllium window assembly, as will be described, is a distinct improvement on the typical prior art arrangement in which a Monel metal ring is usually silver soldered to a beryllium disc. The ring requires a large peripheral area of the disc due to the difficulty in attaching it to the disc securely by the silver solder, this being due to a tenacious oxide film on the disc. The result is an inefficient use of the available disc area for passing the X-rays. The prior art procedure also results in a poor vacuum joint, when the Monel ring is silver soldered, or otherwise attached to the envelope of the X-ray tube. Moreover, the resulting prior art window is not conveniently replaceable and cannot normally salvaged it the tube fails or ceases to function.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view showing an X-ray tube constructed to incorporate the concepts of the present invention;

FIG. 2 is a side view of a cathode hood used in the tube of FIG. 1, and of a window formed in the hood; and

FIG. 3 is a representation of a window subassembly which is mounted in the envelope of the tube of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The X-ray tube shown in FIG. 1 includes an envelope 10 formed of glass or other suitable vitreous or other material. The envelope has a generally elongated configuration, and it includes a cathode assembly 12 at one end, and an anode assembly 14 at the other end. The cathode assembly and anode assembly may be mounted within the envelope l0 and supported therein in any appropriate known. manner.

The cathode and anode electrodes are supported within the tube to be spaced from one another along the longitudinal axis, and the electron beam from the cathode is directed along that axis towards the target electrode. As is usual in the X-ray art, the anode electrode 14 is established at a relatively high voltage with respect to the cathode, so that the electron beam from the cathode may be accelerated towards the anode l4 and strike the target electrode 15 with a relatively high velocity.

In the practice of the present invention, an electrically conductive cylindrical shaped hood 16 formed of any appropriate electrically conductive metal is threaded or otherwise attached to the cathode 12. The hood 16 is coaxial with the longitudinal axis of the tube, and it extends over the forward portion of the anode 14 so as to enclose the active face of the target electrode 15. However, the hood 16, which is established at the potential of the cathode is spaced sufficient from the surface of the anode 14 to be out of electrical contact with the anode and to be adequately insulated therefrom.

In the construction of the present invention, the electrons from the cathode enter a negative electric field from the hood 16, rather than a positive electric field as is the case with the prior art construction. This negative electric field causes the electron beam to converge to a relatively small and uniform spot on the target 15 of the anode 14. The hood 16 prevents stray electrons and secondary emission electrons from establishing negative charges on the inner surface of the envelope, which charges adversely would affect the operation of the X-ray tube, as is well known.

The incidence of the electron beam on the target causes an X-ray beam to be emitted from the target, and this X-ray beam passes through a first window formed in the wall of the hood 16, and a second window assembly 22 which in mounted in the envelope lit). The fact that the spot on the target lS is relatively small and uniform means that the anode may have a relatively large angle with respect to the vertical, for example, so that the X-ray beam passed through the windows 2t) and 22 has a relatively wide divergence which is desirable, for the reasons discussed above.

As best shown in FIG. 2, for example, the window 24) formed in the hood 16 includes an aperture in the hood, and a thin disc of suitable X-ray transmissive material, such as beryllium, designated 24 in FIG. 2. The disc 24 is welded or otherwise attached to the rim of the window 20, and serves to prevent electrons from passing from the interior of the hood l6, and yet permits X-rays to pass freely through the wall of the hood.

The subassembly 22, as shown in FIG. 3, includes an open ended cylindrical member 36 which is inserted in a corresponding cylindrical member 30, the latter being sealed in the envelope lb, as shown in FIG. I, by usual glass sealing techniques. Both the cylinders 30 and 36 may be formed of Kovar or other suitable material having a coefficient of expansion matching that of the glass envelope or bulb ll of the X- ray tube. The cylindrical members 30 and 36 are dimensioned to have diameters so that the cylinder 36 may slip into the cylinder 30 with an easy but close fit.

A ring 32 which may, for example, be pure silver, or which may be composed of two separate materials is sealed in the open ended cylindrical member 36. The ring 32 may have a first portion 32a which is composed, for example, of stainless steel, and which projects radially beyond the periphery of the remaining portion of the composite ring. The stainless steel portion 324 is welded to the internal peripheral surface of the cylindrical member 36, by means for example, of an electron beam weld, so that the ring 32 may be supported within the cylindrical member 36. It is believed at this time that the monolithic pure silver ring is preferable to the composite ring, since it is compatible with the beryllium of the window; is adaptable to high temperature use; and does not exhibit any tendency to develop leaks, as sometimes occurs between the two portions of the composite ring.

The ring 32 may have a further portion 32b which is formed, for example, of aluminum. The portions 32:: and 3212 are metallurgically bonded together. The portion 32b has a peripheral axially extending lip portion which provides a suitable configuration for receiving the flat this disclike member 245. The disc member 24 is formed of suitable X-ray transmissive material such as pure beryllium.

In the construction of the subassembly, the disc 24 is snapped into place over the aluminum portion 32b of the composite ring 32. The disc is then welded to the portion 32b of the ring 32 by a clean weld, such as by an electron beam, so as to secure its periphery to the ring 32. This causes the disc firmly to be supported in the ring with a vacuum tight joint. The ring and disc are then inserted into the cylindrical member 36 and the stainless steel portion 32a of the ring is also electron beam welded, or otherwise attached to the inner peripheral surface of the cylindrical member in a vacuum tight joint. The stainless steel and the Kovar material are metallurgically compatible for this purpose.

After the subassembly of FIG. 3 has been completed, the cylinder 36 may be inserted into the cylinder 30 in the envelope ill as shown in FIG. I, and sealed to the cylinder 39, for example, by heliarc welding at the outer peripheral edges of the two cylinders. It will be appreciated that in the event that the tube of FIG. 2 fails to function or ceases to function, the beryllium window assembly 22 can be salvaged merely by cuttin the heliarc weld and removing the cylinder 36 from the cylinder 3% in the envelope of the tube, after which the assembly may easily be reused.

The invention provides, therefore, an improved X-ray tube in which the electron beam is controlled to define a small and uniform spot on the target electrode, without any tendency for electron charges to gather on the inner surface of the tube onvelope. The tube therefore, is capable of generating widely divergent X-rays.

The tube of the invention also includes an improved window subassembly which is constructed to facilitate the mounting of the window in the envelope of the tube, and also to make most efficient use of the available X-ray transmissive material. Also, the window assembly of the invention, as described, may be readily removed from the tube and reused it desired.

Iclaim:

ll. In an X-ray tube having a cooperating anode electrode and cathode electrode enclosed in a sealed envelope, the improvement comprising a window formed in the envelope to pass X-rays therethrough, said window including a first open ended cylindrical member sealed to said envelope and extending therethrough, said first open ended cylindrical member being formed of material having a coefficient of expansion matching that of the envelope; 2. second open ended cylindrical member formed of a material having a coefficient of expansion matching that of said first cylindrical member and having an outer diameter corresponding to the inner diameter of said first open ended cylindrical member, said second open ended cylindrical member being mounted in said first open ended cylindrical member in a close fit coaxial relationship therewith and sealed thereto; a ring mounted at one end of said second open ended cylindrical member and including a first axial portion having an outer diameter corresponding to the inner diameter of said second open ended cylindrical member and extending into said end of said second open ended cylindrical member, said first axial portion of said ring having an axially extending lip at the inner end thereof forming a peripheral shoulder, and said ring having a second axial portion extending radially as a flange over the peripheral edge of said second open ended cylindrical member at said end thereof and sealed to said peripheral edge; and a flat thin disclike member formed of X-ray transmissive material mounted on said shoulder and affixed to said lip portion of said first axial portion of said ring and extending thereacross.

2. In the X-ray tube of claim I, in which said first portion of said ring is formed of a first material, and said second portion of said ring is formed of a second material metallurgically bonded to said first material.

3. In the X-ray tube of claim 2, in which said first material of said ring is aluminum and said second material of said ring is stainless steel.

4. In the X-ray tube of claim I, in which said disclike member is welded to said first portion of said ring, and in which said second portion of said ring to said peripheral edge of said second open ended cylindrical member.

5. In the X-ray tube of claim I, in which said disclike member is formed of beryllium.

6. In the X-ray tube of claim 1, in which said ring has a unitary monolithic composition.

7. In the X-ray tube of claim 6, in which said ring is composed of pure silver.

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