US3736457A

US3736457A - Radiation shielded electron discharge device

Info

Publication number: US3736457A
Application number: US00128056A
Authority: US
Inventors: W Cullen; F Holub; M Mccormick
Original assignee: General Electric Co
Current assignee: General Electric Co; INDIANA NATIONAL BANK
Priority date: 1971-03-25
Filing date: 1971-03-25
Publication date: 1973-05-29
Anticipated expiration: 1990-05-29

Abstract

An electron discharge device is provided with shields to protect against inadvertent emission of penetrating electromagnetic radiation. The positions and mounting of the shields is designed to provide shielding in all directions which is relatively undefeatable by inadvertent tampering by the user.

Description

United States Patent 1 1 Cullen et a1.

11] 3,736,457 1 May 29, 1973 RADIATION SHIELDED ELECTRON DISCHARGE DEVICE Inventors: William F. Cullen, Philpot, Ky.; Fred F. Holub, Schenectady, NY. Michael R. McCormick, Louisville; Ky.

General Electric Co., Owehsboro, Ky.

Filed: Mar. 25, 1971 Appl. No.: 128,056

Assignee:

11.8. CI ..3l3l3l3, 313/240 Int. Cl ..II0lj l/52, H0lj 19/40 Field of Search ..313/240, 313

References Cited UNITED STATES PATENTS 11/1960 Downing et a1. ..313/313 2/1941 Atlee et al. ..3l3/313 X 3,534,215 10/1970 Hughes et al ..313/313 3,603,716 9/1971 Koren ..3l3/3l3 X FOREIGN PATENTS OR APPLICATIONS 573,660 11/1945 Great Britain ..3l3/313 Primary Examiner-David Schonberg Assistant Examiner-Toby H. Kusmer Attorney-Nathan J'. Cornfeld, John P. Taylor, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT 9 Claims, 5 Drawing Figures Patented May 29, 1973 3,736,457

2 Sheets-Sheet 1 FIG.2.

INVENTORS: WILLIAM F. CULLEN, FRED F. HOLUB MICHAEL R. MCORMI THEIR ATTO NEY.

Patented May 29, 1973 3,136,457

2 Sheets-Sheet 2 fines.

INVENTORS;

WILLIAM F. CULLEN,

FRED F. HOLUB, MICHAEL R. MCCORMICK,

BY MK:

THEIR ATTOR Y.

diation in industrial equipment BACKGROUND OF THE INVENTION This invention relates to electron discharge devices. More particularly, thisinvention relates to means for mitigating the emission of undesirable electromagnetic radiation from electrondischarge devices.

Electron discharge device used in high voltage applications such as high-voltage rectifiers have been known to emit undesirable levels of penetrating electromagnetic radiation particularly when a failure modein the high voltage system, for example, in a television receiver, cause the high voltage in the device to rise above normal operating levels. The increased use of such rectifiers at highervoltages in color television has increased the need for protection against such radiation.

The usual approach to shielding from undesirable rais to employ a heavy metal such as lead. However, in a television receiver, the weightrequirements forbid the use of a large lead cage while insulation requirements prevent the use of metal such as lead in intimate contact with the walls of the tube. Furthermore, the shielding means must not be defeatable by the removal and reinsertion of the tube in the chassis by the user or other unskilled personnel.

The use of a non-metallic material as a jacket or shield for the tube is thus necessary. Unfortunately, however, most non-metallic materials do not, in general, provide sufficient radiation shielding, and furthermore, are, particularly in the case of organic materials, subject to degradation upon prolonged exposure to radiation. The use of certain fillers in, for example, plastic binders to increase the radiation shielding effect of the material has been suggested in the art such as US. Pat. No. 3,114,721 assigned to the assignee of this invention. Furthermore, in an article published by E.I. DuPont deNemours and Company entitled Hypalon Report No. 9, it is stated on page 10 that Hypalon 45, a chlorosulfonated polyethylene, is an excellent binder for radiation shielding and a subsequent table shows the high level of loading with litharge possible.

However, due to the degradation mentioned above, the use of such fillers is complicated by thereduction of plasticity resulting from the use of sufficiently high levels of such fillers to provide the desired amount of shielding coupled with the further increase in brittleness due to the radiation exposure. As an additional complicating factor, the addition of fillers having known radiation shielding properties can increase the flammability of a material.

SUMMARY It is therefore an object of the invention to provide a high voltage electron discharge device having radiation attenuation means intimately associated therewith which do not materially affect the electrical insulation properties of the device.

It is a further object of the invention to provide radiation inhibiting means for an electron discharge device having low flammability characteristics and high resistance to both thermaland radiation degradation.

These and other objects of the invention will be apparent from the specification and accompanying drawmgs.

In accordance with the invention a high voltage discharge device comprising an evacuated cylindrical glass envelope having a plurality of electrodes therein is surrounded by radiation shielding means including first shield means bonded thereto comprising elastomeric polymer means including a radiation attenuating filler and a sufficient amount of additional filler to impart flame resistance to the shielding means. Auxiliary shield means are also provided to shield against radiation emanating axially from the device.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical cross-sectional view of the invention.

FIG. 2 is an exploded view of a portion of FIG. 1.

F IG. 3 is a partially broken-away exploded view of an alternate embodiment of the invention.

FIG. 4 is a fragmentary, cross-sectional view of another embodiment of the invention.

FIG. 5 is a fragmentary, exploded view of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, an electron discharge device constructed in accordance with the invention is generally indicated at 2. The illustrated device is a high voltage rectifier comprising an evacuated cylindrical glass envelope 10 having a phenolic base 20 with a plurality of pins 22circularly mounted therein. Device 2 also contains an anode 40 and a cathode 30 which, in the illustrated embodiment, is an indirectly heated cathode. Anode 40 communicates with the exterior of envelope 10 via an anode cap 42 which passes through envelope l0 and to which envelope 10 is sealed. Cathode 30 is electrically attached to the appropriate pins 22 via leads 24 which pass through envelope 10.

In accordancewith the invention, a first radiation shield 50 surrounds glass envelope 10. Radiation shield 50, which will be described in more detail below, comprises a filled elastomeric material which is preferably bonded to envelope 10 so that it cannot be removed by the user.

Second and third radiation shields 60 and are mounted respectively adjacent the bottom and top of device 2 as viewed in FIGS. 1 and 2.

Radiation shields

60 and 70 provide auxiliary shielding along the axis of device 2 to provide, in conjunction with the primary shield 50, a complete shield which will intercept internally generated radiation regardless of direction.

In a preferred embodiment,

shields

60 and 70 comprise metal discs of nickel-plated steel of sufficient thickness to provide the desired shielding. It has been found that an effective thickness of about mils provides satisfactory shielding. Effective thickness is defined as the actual thickness of the shield (when nickelplated steel is used) plus the additional thickness of the metal parts to which it is assembled as will be more fully described below with respect to upper shield 70.

Other metals such as, for example, molybdenum or tungsten can be used. The particular thickness needed to provide an effective thickness comparable to the 80 mil thickness when nickel-plated steel is used will vary somewhat depending upon the density of the material. In any event the metal chosen must be consistent with good electron discharge device material selection such as is well known to those skilled in the art.

Disc-like shield 60 is mounted within envelope preferably immediately above the glass fillets 12 surrounding leads 24. Shield 60 is mounted generally coaxially with device 2 with the planes of the disc generally normal to the axis of device 2. Shield 60 may be retained in this position by welding to appropriate leads 24.

Shield 70, in the preferred embodiment illustrated in FIGS. 1 and 2, comprises a disc of about 60 mils thickness. Disc 70 is assembled between anode 40 and anode cap 42 as best seen in FIG. '2. The cylindrical sidewall 46 of anode 40 extends beyond end wall 44 to define a recess into which disc-like shield 70 is inserted. Anode cap 42 is then placed on top of shield 70 within the recess and rim 48 of anode cap 42 is welded to shield 70 and end wall 44. The end of sidewall 46 is then crimped over rim 48 at 49 as best seen in FIG. 1.

The effective thickness of radiation shield 70 is then the actual thickness of disc 70 plus the thickness of end wall 44 and the thickness of either rim 48 or end wall 41 of anode cap 42. In the illustrated embodiment disc 70 is about 60 mils thick; end wall 44 is about 10 mils thick; and rim 48 and end wall 41 are each about 10 mils thick to give a total effective thickness of about 80 mils.

Shields

60 and 70 are thus positioned within device 2 to act in conjunction with shield 50 to provide a complete three dimensional radiation shield about the internal, active elements of device 2. The additional shields are necessary to provide complete surroundment of the device which cannot be done with a unitary shield unless a cage of larger dimensions was used since some parts of the device must be accessible for external electric connections. Complete shielding of the device is believed necessary because the source and direction of the radiations are not completely known and, even if the direction of the source could be pinpointed, the scattering of the rays would still necessitate complete surrounding of the device to insure minimizing of the escape of any radiation. It should be understood, however, that the additional shields need not be mounted within the device although this is preferable from a standpoint of compactness.

Shield 50, as previously mentioned, is a filled elastomeric material bonded to glass envelope 10. The use of a filler is basically necessary to provide opaqueness to the radiation since organic materials are relatively transparent to electromagnetic radiation. The preferred filler material is lead oxide, other materials may be used, however, provided they possess the required radiation shielding properties and do not deleteriously effect the physical properties of the shield.

Shield 50, as best seen in FIG. 1, is generally cylindrical having an inner diameter generally conforming to the outer diameter of cylindrical envelope 10. Shield 50 extends downwardly, beyond the terminus of envelope 10, to the bottom of phenolic base 20. Phenolic base 20 is constructed of a slightly larger diameter than envelope 10 to provide a slight interference fit with shield 50 as well as to provide a surface to which shield 50 may be bonded. It should be noted here that base 20 has been described herein as phenolic but may be constructed of other insulative materials as well. In fact, it is within the scope of this invention to construct base 20 of a material which, either with or without fillers, will have radiation attenuation properties equivalent to shield 50 thus eliminating the need for shield 60.

The elastomeric binder material must be sufficiently ductile and deformable after compounding and forming into a sleeve to slip over the glass envelope. The binder material also must be sufficiently flame retardant to pass standardized tests for flame retardancy. The latter characteristic may be obtainable through the use, for example, of highly halogenated rubber compounds, blends of such compounds, with the aid of fillers or additives having fire retardant properties, or combinations of rubber compounds having flame retardant characteristics with flame retardant fillers or additives.

The binder material (as well as any fillers or additives) must also be sufficiently radiation resistant to withstand the amounts of radiation to which the shield may be exposed over the life of the tube. The binder must also be able to withstand prolonged exposure to heat without undue embrittlement.

Examples of elastomeric compounds which have been found to possess the above properties and thus are useful as binder materials include, for example, chlorinated polyethylene, chlorosulfonated polyethylene, and silicone rubber.

The elastomeric binder material may be further defined as a material having a glass transition temperature below zero centigrade; having a low initial viscosity to aid in the initial blending of the fillers into the binder; and curable to a higher viscosity wherein the material is still sufficiently ductile and deformable to permit mounting of a sleeve formed of the material onto the glass envelope of the electron discharge device.

As previously stated, the preferred filler material to impart the desired radiation shielding is lead oxide. Preferably, the amount of lead oxide used is about 2.5-3.0 parts by weight lead oxide per part binder. It has been found that this ratio provides sufficient binder to impart the desired ductile properties and sufficient lead oxide to provide a factor of about ten radiation attenuation per about 0040-0045 inch thickness. Other filler materials can be substituted for the lead oxide if they provide this amount of shielding without the use of such large amounts of filler as to impair the physical properties of the final product. In accordance with the invention a shield having a total thickness of about 0.125 inch and constructed of materials providing a radiation attenuation factor of about 10 per 0.040-0.045 inch thickness has been found to be sufficient to lower the radiation levels to below 0.1 milliroentgens per hour.

As previously stated, a fire-retardant filler material or additive may be used, if necessary, to impart further flame retardancy to the material. In accordance with the invention the final material must possess, in addition to the required radiation shielding and physical properties, sufficient flame retardancy with or without additional flame retardant additives to pass the SE-l test of the fire underwriters laboratories.

In accordance with a preferred embodiment of the invention, and as a specific illustration thereof, parts by weight of a chlorosulfonated polyethylene elastomer containing about 25-30 percent by weight chlorine and about l1.5 percent by weight sulfonyl chloride was blended with 275 parts by weight of PhD; 100 parts by weigh of A1 0 31-1 0; and about 25 parts by weight of chlorinated paraffin plasticizer. One part by weight Tetrone A (dipenta methylene-thiuramtetrasulfide) and 0.5 parts by weight MBTS (benzothiazyl disulfide) were added as curing accelerators, no additional catalyst being necessary in the particular system due to the presence of PbO which acts as a curing agent for chlorosulfonated polyethylene. About 20 parts by weight of Thermax, a carbon black pigment, was added to impart a grey color to the material.

The material was molded into a 0.125 inch thick shield and bonded to the glass envelope of an electron discharge device using a rubber contact cement. The electron discharge device was internally fitted with the axially mounted metal shields previously described. The device was operated at 42KV and radiation measurements taken to test the effectiveness of the primary shield. No emissions over 0.1 milliroentgens per hour (the limit of the sensitivity of the equipment) were measured.

Referring now to FIGS. 3-5 alternate embodiments are illustrated which relocate the secondary shields to positions outside envelope 10. In FIG. 3, shield 60 has been replaced by a disc 160 which may be constructed of the same material as primary shield 50. Disc 160, when constructed of the same material as shield 50, has a comparable thickness, e.g. about 0.125 inch and is provided with tiny holes 162 through which leads 24 may pass. Disc 160 is constructed of approximately the same diameter as base 20 on which it is seated. Leads 24 pass through holes 162 and are attached in conventional manner to pins 22 on base 20. Disc 160 may be molded with a central depression defining a concave central portion on the upper surface and a convex central portion on the lower surface to accommodate the exhaust tip of glass envelope without changing the overall shield thickness.

In FIG. 4 a construction is illustrated which is capable of replacing shield 70. In the illustrated embodiment, the top cap metal connector cup 174 and lead 176 are embedded in a molded insulated shield 170 which is constructed of the same material as shield 50 and to a comparable thickness. While FIG. 4 shows the shield lead as demountable, it may be integrally formed with shield 50 as a one piece assembly. The latter construction actually is preferred since the shielding cannot, then, be defeated by the user of the device. In FIG. 5, another construction is illustrated in which a metallic cap or thimble 270 is placed over anode cap 42. Thimble 270 is constructed of sufficient thickness to provide an effective total thickness of 80 mils as previously discussed. Thimble 270 is preferably non-detachably mounted to anode cap 42 after shield 50 has been installed. A conventional connector cup 274 properly sized to the outer diameter of thimble 270 is illustrated as providing external electrical contact to the anode Thus, the invention provides an electron discharge device which is completely shielded to eliminate the emanation of harmful amounts of electromagnetic radiation. The device is light enough to be removable from a television chassis yet durable to withstand prolonged exposure to both heat and radiation. The device is relatively fireproof and its radiation shielding properties are relatively undefeatable by inadvertent tampering by the user as might happen with removable shielding.

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

l. A high voltage electron discharge device comprising an evacuated cylindrical glass envelope having a plurality of electrodes therein and being surrounded by radiation shielding means intimately associated with said device comprising: first shield means bonded to said envelope comprising elastomeric polymer binder means including a radiation attenuating filler in an amount sufficient to provide a radiation attenuating factor of about 10 per 0.040-0.045 inch thickness of said first shield, and having flame retardant means therein; and second shielding means adjacent said envelope comprising a plurality of shield means coaxially mounted generally normal to the axis of said device including at least one shield mounted adjacent one end of said cylindrical envelope and at least one other shield mounted adjacent the opposite end of said envelope, said shields cooperating to provide a complete shield which will intercept internally generated radiation regardless of direction.

2. The device of claim 1 wherein said filler is lead oxide and the amount of filler in said first shield is about 2.5-3 parts by weight per part binder.

3. The device of claim 1 wherein said second shielding means comprise metallic discs mounted within said envelope and having an effective thickness of about mils and sufiicient diameter to provide, with said first shield, complete surroundment of said electrodes.

4. The device of claim 1 wherein said second shielding means includes at least one shield mounted externally of said envelope.

5. The device of claim 1 wherein one of said electrodes comprises an anode having an anode cap protruding from one end of said envelope and a series of leads protruding from an opposite end of said envelope and said second shielding means includes a shield for said anode cap and another shield in intimate contact with said leads protruding from said envelope, said shields being made of insulative material having radiation-attenuating properties similar to said first shield and being positioned contiguous to said first shield.

6. The device of claim 1 wherein said binder is selected from the class consisting of chlorinated polyethylene, chlorosulfonated polyethylene, and silicone rubher.

7. The device of claim 1 wherein said binder is chlorosulfonated polyethylene and said filler is lead oxide.

8. A high voltage electron discharge device comprising an evacuated cylindrical glass envelope having a cathode and an anode therein and being substantially surrounded by a first shielding means comprising a lead-oxide-filled chlorosulfonated polyethylene shield having fire-retardant means therein and having a loading ratio of about 2.5 to 3 parts by weight lead oxide per part binder and having a total thickness of about 0.125 inches; said device having second shielding means within said envelope comprising a first metallic disc having an effective thickness of about 80 mils coaxially positioned in said envelope adjacent one end of said cylinder and a second metallic disc having an effective thickness of about 80 mils positioned adjacent the opposite end of said cylinder, said discs being mounted normal to the axis of said device and. having sufficient diameter to intersect any radiation having a direction which would avoid intersection with said first shield.

9. The device of claim 8 wherein said anode is connected to an anode cap passing through one end of said cylindrical glass envelope and said cathode is connected to leads which pass through the opposite end of said envelope and into a base comprising an insulating ter approximately equal to the diameter of said envelope to provide a snug fit thereon and an interference fit with said base.

Claims

1. A high voltage electron discharge device comprising an evacuated cylindrical glass envelope having a plurality of electrodes therein and being surrounded by radiation shielding means intimately associated with said device comprising: first shield means bonded to said envelope comprising elastomeric polymer binder means including a radiation attenuating filler in an amount sufficient to provide a radiation attenuating factor of about 10 per 0.040-0.045 inch thickness of said first shield, and having flame retardant means therein; and second shielding means adjacent said envelope comprising a plurality of shield means coaxially mounted generally normal to the axis of said device including at least one shield mounted adjacent one end of said cylindrical envelope and at least one other shield mounted adjacent the opposite end of said envelope, said shields cooperating to provide a complete shield which will intercept internally generated radiation regardless of direction.

3. The device of claim 1 wherein said second shielding means comprise metallic discs mounted within said envelope and having an effective thickness of about 80 mils and sufficient diameter to provide, with said first shield, complete surroundment of said electrodes.

6. The device of claim 1 wherein said binder is selected from the class consisting of chlorinated polyethylene, chlorosulfonated polyethylene, and silicone rubber.

8. A high voltage electron discharge device comprising an evacuated cylindrical glass envelope having a cathode and an anode therein and being substantially surrounded by a first shielding means comprising a lead-oxide-filled chlorosulfonated polyethylene shield having fire-retardant means therein and having a loading ratio of about 2.5 to 3 parts by weight lead oxide per part binder and having a total thickness of about 0.125 inch; said device having second shielding means within said envelope comprising a first metallic disc having an effective thickness of about 80 mils coaxially positioned in said envelope adjacent one end of said cylinder and a second metallic disc having an effective thickness of about 80 mils positioned adjacent the opposite end of said cylinder, said discs being mounted normal to the axis of said device and having sufficient diameter to intersect any radiation having a direction which would avoid intersection with said first shield.

9. The device of claim 8 wherein said anode is connected to an anode cap passing through one end of said cylindrical glass envelope and said cathode is connected to leads which pass through the opposite end of said envelope and into a base comprising an insulating member having a series of parallel pins protruding therefrom; said base having a diameter slightly larger than said envelope and being contiguous with one end of said envelope; and said first shield extending from said anode cap to said base and having an inner diameter approximately equal to the diameter of said envelope to provide a snug fit thereon and an interference fit with said base.