US2256229A - X-ray tube and shield - Google Patents

Description

P 1941'- z. J. ATLEE arm, 2,256,229

X-RAY TUBE AND SHIELD Filed Feb. 12, 1940 Inventors 2 Then" Attorn ey.

object the provision of Patented Sept. 16, 1941 .X-RAY' TUBE AND SHIELD Zed J. Atlee, Elmhurst, and Jack T. -Wilson, Evanston, 'Ill., .assignors to General .Electric X-Ray Oorporation;"0hicago, 111., a corporation of New York Application February 12, --1940,=Seria l No. 318,516

. anelongated.tubular envelope I, which is prefer- 6 Claims.

The present-inventionrelates to a shielded X-ray tube construction, :and'has as a primary an effective shielding arrangement for X-ray tubesof the protuberant window type.

Tubes .of the characterabove referred to are widely used in difiraction crystallography and for. this purpose are required to be of such construction as to permit the ready egressof X-rays of a wide range of wave lengths Because of the latter requirementitis customary to provide in the region of desired ray emission one or more windows consisting of aspecial glass, such as so-called Lindemann glass, which-is highly permeable to. .X-radiations. Mechanical con-- siderations having to do with ease of manufacture make itexpedient toapply each of the window parts tothe extremity of a short tubulation extending outwardly from the main body of the tube envelope.

In the above, electrical difliculties are-frequently encountered due to the excessive potential gradients developed at the boundaries of the window parts. It is a particular-object of the present invention to minimize these difficulties by the provision of a shielding arrangement effective to preventthe occurrence of localized electrical stresses. This object. together with the additional desideratum of a compactand inexpensive construction, is realized .by the use of. a shielding "collar which is constituted of a synthetic resin having incorporated therein a substantial proportion of axconductive substance, such as graphite. the'shield'by the presence of the conductive ingredient causes it to serve a potential distributing function, while the dilution of this ingredient by the resinous'component maintains the resistivity of the shield'sufiiciently high to avoid danger of short-circuiting effects.

' The features which we desire to protect herein are pointed out with particularity in the appended claims. "The invention itself, together with fu'rther'objects and advantages thereof may' bestv be understood by reference to the following description taken in connection with the drawing, in which Fig. 1 is a side view in partial section of an X-ray tube suitably embodying the invention; Fig. 2 is a transverse section taken on line 2-2 of 'Fig. 3 ;Fig. 3 is a side elevation of the central portionof Fig. 1, and Fig; 4 is a circuit diagram illustrating schematically one mode of application of the invention.

Referring particularly toFig. 1 there is shownuse. of devices of the form described The partial conductivity imparted to threaded terminal base 6 ably constituted of. glass. 'At one end, the ena' reentrant stem, in-

a cathode structure at its inner extremity. Thestructure'referred to includes a filamentaryheater 3, a recessed focusing cup 4 and an apertured hood or shield 5 which prevents passage of electrons from'the cathode to the "walls of the tube. A screwserves to connect'the cathode to a supply of heating current and to a high potential operating source (not illustrated).

At the-end remote from the cathode the en- 7 'velope I encloses'an anode including particularly atarget l which is preferably formed of a material such as copper, iron or nickel. The target is backed up by a heavy mounting structure 8 which facilitates dissipation of-heat from the --target. Removal of heat may-be assisted by the provision of appropriate conduits, indicated at Sand In, for circulating a cooling fluid in the interior of the anode-structure.

In the use of the tube an appropriate potential from 15,000 to- 50,000 volts) is impressed between the anode and cathode to produce electron -fiow between them. The resultant electron stream is focussed'by the action of the cathode cup 4 into aconcentrated beam which strikes the target Tin a generally rectangular pattern as indicated at I2 in Fig. 2. As a result of this impact, X-rays are emitted 'by the target in all directions, the. density of such rays being greatest on lines which parallel the major dimension of the rectangle l2.

In order to favor. selective emission of the X-rays from the tube in desired directions, the tube is provided in the vicinity of the anode with a series of four circumferentially spaced windows, shown at l4, l5, l6 and IT. (The use of a number of windows is for the purpose of permitting the tube to be used simultaneously for several independent investigations, and the illustrated arrangement of four windows is to be taken as exemplary rather than limiting.) Each of these windows preferably includes a thin central portion constituted of a glass which is more readily permeable to X-rays of long wave length than the glass Which "containing a-high percentage of lithium). In

--order to permit the application of the Lindemann glass parts to the main structure of the envelope, it is customary to provide on the envelope outwardly projecting tubular extensions as indicated at l5, l5, I6 and I1. Lindemann glass is then used to close the outer extremities of these tubular sections so that each of the completed windows comprises a cylindrical or frustro-conical structure which is outwardly terminated by a fiat diaphragm of highly permeable glass. If desired, an intermediate region cons'isting of lime glass may be provided between the lead glass which constitutes the main body of the envelope and the Lindemann window in order to assure a good seal between them.

In the use of the tube thus formed, it is found that the presence of the more or less irregular projections provided by the window structures greatl increases the danger of puncture of the tube due to electrical causes. In particular, the necessity of using the tube in proximity to grounded structures (such as a crystallographic camera) which may be at a considerable difference of potential from the walls of the tube itself, leads to the creation of high potential gradients at the boundaries of the Lindemann diaphragms and the tubular extensions on which my are supported.

In order to overcome this difficulty our present invention utilizes a novel shielding arrangement for the central part of the tube. More specifically, it is proposed in this connection to encompass the windows and the parts of the tube in proximity thereto by a shielding structure of partially but not completely conductive character. To this end, use is made of an apertured collar which embraces the tube in the vicinity of the anode and which is composed of a synthetic resin having incorporated therein a substantial quantity of a conductive material such, for example, as graphite. As a particular example of a composition which may be used in this way, we may refer to a molded composition containing from 40 to 60 percent of a phenolic resin such, for example, as that sold under the trade name Bakelite, 20 to 30 percent wood flour, 20 to 30 percent graphite and 25 to 35 percent lead oxide. In this combination theuse of graphite'is deemed advantageous because of its ability to become thoroughly dispersed in the composition. Obviously, however other materials of similar characteristics may be substituted for this element. Wood flour is a common concomitant of synthetic resins in making molded parts, and leadoxide is a desirable constituent in the present connection because of its effect in increasing the impenetrability of the shield to X-rays.

In the drawing, a shielding collar of the type referred to is shown as comprising two separable parts 20 and 2! which are joined at their abutting edges by means of bolts 22. The composite collar thus formed is provided with four guadrangularl displaced openings 24 which extend radially and which are adapted to receive the protuberant window portions l4, l5, l6 and H. An electrical connection for maintaining the collar at a fixed potential is provided by the use of a metallic band 20 (Fig. 3), which is clamped to a shoulder provided for that purpose at one end of the collar.

It is the function of the shielding collar to distribute the electrical stress in the vicinity of the protuberant window parts and to prevent the existence of excessive gradients at the edges of these parts. The partial conductivity of the collar enables it to serve this function by maintaining all parts of the collar at a common potential so as to eliminate the possibility of high local gradients. On the other hand, in the event of a transient breakdown involving the shield, the relatively high resistance character of the shield composition serves to prevent excessive current flow which might result in destruction of the X-ray tube. For this latter purpose it is desirable that the resistivity of the shielding material be in a range between about 1,000 and about 3,000 ohm centimeters.

A typical mode of use of the invention is illustrated in Fig. 4, which shows schematically a circuit arrangement incorporating an X-ray tube ofthe type shown in Fig. 1. In the illustrated arrangement the anode target I, which is grounded, is connected to one terminal of a high voltage transformer 28. The cathode 3 is connected to the other terminal of the transformer 28 through a high voltage rectifier indicated at 29. The conducting band 20 which is secured to one end of the shielding collar 20 is grounded as indicated at 30 so that the collar is maintained at the same potential as the anode. It will be understood that other modes of connection of the shielding collar are possible and that its use is equally feasible in a system in which either the cathode or the center point of the high voltage winding is maintained at ground potential. Under some conditions it may even be advantageous to permit the collar to remain at a floating potential.

The utility of the invention is obviously not limited to a case in which the radiation windows are protuberant, as above described. On the contrary, the shielding collar might be applied with equal benefit even though the Lindernann glass parts were set flush with the main body of the envelope; provided. their presence gave rise to a condition of local potential stress.

What we claim as new and desire to secure b Letters Patent of the United States is:

1. In combination, an X-ray tube having an anode and a cathode, a vitreous envelope enclosing said anode and cathode, the envelope being provided in the vicinity of the anode with a vitreous window part which is highly permeable to X-rays and the presence of which gives rise to excessive local potential gradients at the boundaries of the window, and a shielding structure embracing the said envelope and having a radially extending opening in alignment with the said window part, said shielding structure comprising an insulating subs ance which incorporates a suiiicient amount of a conductive substance to impart partial conductivity thereto.

2. The combination of claim 1, wherein the shielding structure comprises a synthetic resin embodying a substantial proportion of graphite.

3. In combination, an X-ray tube having an anode and cathode, a vitreous envelope enclosing said anode and cathode, the envelope being provided with a vitreous window-bearing protuberance in the vicinity of the anode, and a shielding structure embracing the envelope in a region which includes the said protuberance, said shielding structure comprising a normally insulating substance which incorporates a sufficient amount of a conductive substance to impart partial conductivity thereto.

4. In combination, an X-ray tube having an anode and cathode, an elongated tubular vitreous envelope enclosingthe anode and cathode, said envelope being provided in the vicinity of the anode With a plurality of circumferentially spaced protuberances adapted to support vitreous Window parts which are highly permeable to X- rays, and a collar embracing the said envelope and having a plurality of radial openings for receiving the said protuberances, said collar comprising a synthetic resin which incorporates a substantial proportion of a conductive material. 5. The combination of claim 4, wherein the said collar comprises a phenolic resin containing from 10 to 40 percent by weight of graphite.

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