US2479882A - Radioactive metal products and method for manufacturing - Google Patents

Description

Aug. 23, 1949. c. w. WALLHAUSEN ETAL 2,479,882

RADIOACTIVE METAL PRODUCTS AND METHOD FOR MANUFACTURING Filed March 14, 1946 FIG. I FIG. 2

RADIOACTIVE CORE RADIOACTIVE CORE SANDWICHED BETWEEN SEALING FILMS E IL , SEALING FILMS BACKING FIG. 3 FIG. 4 FIG. 5

SEALING FILM SEALING FILM SEALING SLEEVE RADIOACTIVE CORE BACKING mmumlllnlm RADIOACTIVE CORE BALM/E 'CORE WIRE FIG.6 FIG? FIG.8

SPLIT TUBE CHANNEL S W BAKING ANGULAR ANGULAR ANGULAR WIDTH I WIDTH WIDTH OF BEAM OF BEAM I OF B AM HEAV BACKING FIG IO RADIOACTIVE FOIL FIG. 9 n [I I M A ANGULAR WIDTH I OF EAM V i w A TRAVELLING WEB DEEP CHANNEL BACKING 0F sHEET MATERIAL sTATIc ELIMINATING DEvIcE SPACED FROM WEB BY 6M2! ATTORNEYS Patented Aug. 23, 1949 RADIOACTIVE METAL PRODUCTS AND METHOD FOR MANUFACTURING Clarence W. Wallhausen and Harry H. Dooley, Morristown, and Clayton 0. Carroll, Madison, N. J., assignors to United States Radium Corporation, New York, N. Y., a corporation of Delaware Application March 14,1946, Serial No. 654,460

Claims. 1

This invention relates to radioactive metal products, and is concerned especially (but not solely) with an improved radioactive metal product that emits alpha particles. Basically this product comprises a metallic core in which a radioactive substance is incorporated, and a sealing film over the core which prevents escape of gaseous products of the radioactive disintegration process but which advantageously is thin enough to be penetrable b alpha particles emitted in this process. The invention further provides a new method for making radioactive metal products of this character, and a device based on the new radioactive metal product for eliminating static charges.

Various proposals have been made heretofore for incorporating radioactive substances in metal products. In particular the patent to Alois Fischer No. 2,326,631 describes a metal foil in which a radioactive substance is incorporated, and specifically a metal foil capable of emitting alpha particles and efiicient in its use of the radioactive substance employed. Foils and extremely fine wires are especially advantageous forms of metal products which are intended to emit alpha particles, because the depth of penetration in metals of alpha particles having the average energy with which they are emitted by ordinary radioactive elements is very small. When the radioactive substance is uniformly distributed through the body of the metal product, the thickness of the product should be no greater than can be penetrated by an alpha particle as emitted by the substance; otherwise alpha particles originating deep in the metal will not be able to reach the surface and be emitted from the metal. In consequence efi'icient use will not be made of that portion of the radioactive substance distributed deep in the metal.

While thin foils and very fine wires are known to possess the above-indicated advantage of being efficient in the use of the radioactive substance when the foil or Wire is primarily intended to emit alpha particles, the very thinness of the foil or wire which leads to this advantage eontributes also to a substantial disadvantage. This disadvantage arises from the fact that in each of the well-known natural radioactive disintegration series, one of the early disintegration products is a radioactive gas. For example, radon, a gas, is the first disintegration product after radium in the uranium-radium series; thoron (an isotope of radon) is the fifth disintegration product after thorium in the thorium series; and actinon (another isotope of radon) is the third disintegration product after actinium in the uranium-actinium series. These radioactive gases all have short half-lives, but when they are produced by disintegration of their parent substance in very thin foils or extremely fine wire, at least a small percentage of some of them can and does escape beyond the surface of the foil or wire before disintegrating to the next product in the series. Escape of the radioactive gas is disadvantageous on two accounts. First, the escaped gas is highly poisonous and even Very dilute concentrations are dangerous to human and animal life. Second, the escaped gas is no longer able to contribute, by its own disintegration and the disintegration of its products, to the radioactivity of the foil or wire-the escaped gas thus represents a waste of radioactive energy which would not occur if the gas did not escape.

The present invention contemplates the provision of an improved radioactive metal product which retains the efilcient alpha-particle-emitting characteristics of thin foils but avoids the foregoing disadvantages of thin radioactive foils as heretofore used or proposed. The new radioactive metal product comprises a thin metallic core or base structure (for example, a metallic film, ribbon, or fine wire) having a radioactive alpha-particle-emitting substance intimately dispersed therein. A thin, substantially continuous sealing film substantially free of radioactive material is bonded to a surface of the core metal. The combined thickness of the core and. the sealing film is no greater (for a product that emits alpha particles) than can be penetrated by viz., is permeable to alpha particles having the maximum energy with which they are emitted by the radioactive substance dispersed in the core. If desired, this thin metal product may .be bonded to a backing sheet or strip, advantageously of metal, to provide it with mechanical support, and which may also act as an emanation shield, and to limit the angular width of the emitted beam of alpha particles. (The term emanation is used throughout this specification to denote all the products emitted in the course of the radioactive process, and not, as is often the case, to denote merely the gaseous disintegration products.)

The core of the new product most advantageously is a metal film and preferably a film of noble metal such as gold or platinum. A film of gold or other metal composed of pressed metal powder is especially satisfactory because such films are conveniently fabricated to incorporate, intimately dispersed, the radioactive substance.

The radioactive substance may be a radioactive element, but most conveniently is a salt of such element, for example, a salt of radium such as radium sulphate or radium bromide.

The sealing film serves primarily to prevent the escape of gaseous disintegration products -of the radioactive substance incorporated in the base structure. It may be composed of any material capable of forming a sufliciently thin film which yet is sufiiciently contmuous to perform this sealing function, viz., to be impermeable to the radioactive gas. Organic film-forming compounds, such as various plastic compositions, will serve as the sealing film material, but it is usually better to employ a material which is more resistant than organic compounds to emanations from radioactive substances. Non-metallic in organic films, such as a thin film of fused silver chloride, perform satisfactorily and may be used.

Generally, however, a metallic sealing film, and

preferably a noble metal sealing film of the same metal as that comprising the core, will be found most satisfactory.

When, as is generally the ease, the core is in the form of a thin ribbon or film, it is desirable to apply the sealing film to both surfaces of the base structure film. This is not always necessary however.- For example, if the film is to be mounted on a relatively heavy backin strip, it is not necessary, in order to prevent radon loss through the back, that a sealing film be applied to that surface of the core which is to be bonded to the backing; Nevertheless, if the backing strip is of metal (such as silver) different from that of the core (such as gold), it is preferable that a sealing sion of alpha particlesfrom the metal product;

For optimum alpha particle emission, the core and sealing film combined should be thin enough to be penetrable by alpha particles possessing the average energy with which they are emitted by the radioactive substance employed. The metal product will emit alpha particles, however, so long asthe sealing film alone is permeable to alpha particles, viz., is of a thickness less than that which can be penetrated by alpha particles having the maximum energywith which the radioactive substance emits them.

In the case of the metals that are most satisfactory for use in making the new' metal product, such as gold and other noble metals, the maximum thickness of the base structure and sealing film combined generally should not exceed about 5 microns, and advantageously does not exceed about 4 microns, with thecore contributing about to. 21/ microns to the total thickness and the sealing film contributing about /2 to 1 /2 microns.

By making the sealing film of a material which is substantially continuous and free of any radioactive substance, it is effective in preventing the escape outside the metal of any appreciable amount of gaseous disintegration products of the radioactive process, even though it be of thethickness necessary to permit the passage of alpha particles emitted in this process. Sealing films thicker than specified above also are erred 4 tive for preventing escape of radioactive gas, and may be employed if alpha-particle emission from the metal product is not required. For example, if a foil product that emits only beta particles and gamma radiationis desired, the sealing film, or the core, or'both, may be substantiall'ythicker 7 than above indicated, and the sealing film will still perform the useful function, with the abovestated advantages, of preventing escape of radioactive gas.

If a backing sheet or strip is employed to support the thin product, it is usually so heavy as to effectively prevent emission of alpha particles from the surface to which it is bonded. In such case thereforaif a sealing film is applied to both surfaces of the core film before bonding the resulting composite film to the backing, the thickness of the sealing film between the base structure and the backing is of no great consequence, and it may appreciably exceed the thickness indicated above. In such case only the thickness of the core film and the sealing film on its exposed side need be as thin as indicated for efficient alpha particle emission.

Any material capable of providing the degree of support desired for the thin metal foil product described above may be used as a backing. Silver is especially satisfactory for this purpose. Corrosion-resistant metals such as copper, nickel and stainless steel are also well suited for backing purposes, but other backing materials, metallic or nominetallic, may be used if desired. The thin composite radioactive foil may be secured to the backing by any conventional method, such, for example, as by soldering, brazing or welding, or by the use of an adhesive.

The invention provides a method for making thin radioactive metal products, as described above, preferably using metal powders. In accordance with this method, a metal powder such as gold powder and-a finely-divided, radioactive alpha-particle-emitting substance such as a radium salt are intimately mixed together. The radioactive substance advantageously constitutes about 1 to about 12% by weight of the mixation to a thickness no greater than can be penetrated by alpha particles having the maximum energy with which they are emitted by the radioactive substance. The layer of sealing metal may be bonded to the core compact by welding or i otherwise before the composite is reduced in thiekness, or the metal-working operation may be carried out at an elevated temperature high enough so that at the pressure applied the several layers of metal are firmly and permanently bonded together.

In an advantageous embodiment of this meth- 0d. a backing metal is applied and bonded to the composite of radioactive and sealing metals, preferably after the composite has been reduced substantially in thickness but before it has been reduced to its final thickness. The composite and backing metalsthen may be rolled together sufiiciently so as to reduce the composite component and other layers to the thicknesses specified.

The radioactive metal product herein described serves admirably as the fundamental element. of a device for eliminating static charges wherever their accumulation interferes with normal or desired operation. Such charges, for instance, accumulate on webs or sheets of paper in papermaking, -fabricating and -printing operations;

on travelling webs of cloth in the textile industry;

and on plastic materials in operations involving their manufacture and use. The accumulation of static charges may constitute a fire hazard, because the potential of the charge may'become sufiicient to cause a spark capable of igniting the inflammable material on which the charge accumulates, or of igniting other inflammable material nearby. Such static charges also often cause sheets of material to adhere to each other or to other objects.

A device for eliminating such static charges based on the radioactive metal product described herein may comprise a narrow strip of the radioactive metal product, advantageously supported by a backing strip and of a length about equal to the width of the web from which static charges are to be eliminated. The strip is mounted fairly close to the web, but is spaced therefrom by a short distance of the order of an inch or so, and it extends transversely across the web. Since the 15 'active metal foil is very small, and the cost of the metal therefore is notv a very large factor in the radioactive metal product emits alpha particles efiiciently, it is highly effective for ionizing the atmosphere between it and the web with safety,

and static charges tending to accumulate on the web 'areneutralized and discharged in consequence of this ionization.

If desired, the radioactive strip may be mounted in the hollow of a channel, semicylinder, or other shape which is thick enough to prevent the passage of alpha particles. In this manner the effective beam of emitted alpha particles may be confined to a particular direction (determined by the direction in which the open side of the channel faces). The angular width of the efiective beam may be controlled by suitable choice of the depth of the channel flanges relative to the width of the radioactive strip, and other factors afiecting the geometry of assembled structure.

The following'description of the manufacture of a, radioactive foil will afiord a better understanding of the invention, although it is understood that the invention is not specifically limited to the embodiments described below.

In the following description reference is made to the accompanying drawings, in which Figs. 1 to 5 show schematic cross-sections through various forms of the new radioactive metal product;

Figs. 6 to 9 show various forms of backing for limiting the angular width of the alpha particle beam emitted from the radioactive metalproduct; and

Fig. 10 shows diagrammatically the relation in which a static eliminating device according to the invention may be mounted relative to a travelling web of sheet material.

Generally, the first step in the manufacture of ciently long period of time. A more satisfactory method is to mix the radioactive substance,'in finely divided form, intimately with a metal powder of the composition desired.- Numerous metal powders are available and may be used, such as copper powder, nickel powder, tungsten powder,

or alloy powder such as brass powder. Ordinarily, however, a noble metal powder such as gold or platinum powder or a gold-platinum alloy powder is preferred. Noble metals are virtually unaffected by. the emanations of radioactive substances, they are highly resistant to corrosion and chemical attack, and they are very easily worked. While noble metal powders are costly, the amount used per unit area of the thin radiocost of the product.

For a metal product that emits alpha particles,

the radioactive substance employed is of course 'one which emits alpha particles in the course of its radioactive disintegration. A radioactive ele-. ment of this character may be employed as such, but production of the radioactive elements in elemental formisdifiicultand accordingly it is most 'convenientto employ a salt or other compound of a radioactive element.- Salts of radium, such as radium chloride, radium bromide, or radium sulphate, are readily prepared in finely divided form and are especially advantageous substances salts or other compounds of radioactive elements above uranium in the periodic table, such, for example, as salts of neptunium (atomic number 93) of atomic weight 237, an alpha-particleemitting element having a half-life of 2.25 10 years; salts of plutonium (atomic number 94) of atomic weight 238, an alpha-particle-emitting element having a half-life of 50 years; and salts of plutonium of atomic weight 239, an alphaparticle-emitting element having a half-life of 24,000 years.

Besides the foregoing elements, which are normally radioactive, it is possible to employ elements and compounds which are not normally radioactive but in which radioactivity has been artificially induced. For example, radioactive forms of carbon have been prepared and may be used as the radioactive substance, especially those forms that emit alpha particles. Also compounds such as common salt (sodium chloride) have been rendered radioactive, and such compounds (as well as compounds prepared from elements in which radioactivity has been artificially induced) may be employed as the radioactive substance.

The proportions of radioactive substance and metal powder used in preparing the mixture may be varied over wide limits, and in general will depend upon how intense a degree of radioactivity is desired. Ordinarily, however, the radioactive substance should constitute at least 1% by weight of the mixture. Where intense radioactivity is desired, as much as 12% by weight of the mixture may be made up of radioactive salt, and even higher percentages may sometimes be usdtvithinihe li mit of 'forming a' cohesive and workable compact'withf the metal powder;

' Purity; which reduces the ductilityvof the gold,'

and. itiisl therefore usually necessary to anneal.

The-mixture orymetal powder, and radioactive saltor other"'substance""are"subjected to high mechanical pressure of the order of 30 tons per square inch, to compact the powders and {forma" cohesive mass:v The'compressed mass; while '00- hesive'; is very fragile andit*therefore is next';

subjected to a sintering operation involving heat ing it-at 'about 95% 'C.'" for aboutpnehalf'hour.

When'welP-purifiedgolcl'powder isused', no harm: a

is doneto the pressed:mass" by' introducing *it directly into an ovenat the sintering temperature occluded gases -may be driven "01f. during sinter-' ing-ancl raise blisters on-the'pressedm'ass unless the sinte'ring operation is conducted carefully? In such case, it is best to heat thepressed mass very*slow-ly (over -aperiod 'of -fseverali"hours) "to the-final sin'tering temperature; and then-to'hold it at tliistemperature for A' to 2"hours'." Th"e":

compaeted'mass after sintering is quite, strongand maybe handled without-Tear --of breakage."

The sintered-compactdsnext-rolled -toreduce its thickness and increase its density.- After some reduction-in thickness-has been affectedy-it is wr-apped in asheet-of suitable-metal containing no radioactive substance-and the resulting s'andwich 'is further rolledtore'duce the thickness of both the siritered compact 'andthe -metal-"wrap Q ping, which-,' in the finished product, forms the the; core metall; Other metalsgthowever such as copperinickel; tin or; chromium, or alloys 811611225 sealing film;- Alternatively the'sinterectcompact;" before being rolled at-all, maybe'wrapped -in-- the metal:= and maybe-subjected to its first-loll brass, bronze or; platinumegold may: be employed successfullya. 7

Advantageeusly the;thickness; ofwthe ggoldxlor othermentally/rapping- ;sheetis substantially less;-

than i the thickness .of the sinterecl-compaot,abut

it must not be sothin that 'inlthelfinishecl prod-:-

uct .it does...not form :a continuous-sealing -,film impervious: .to the radioactive gasp A wrapping about the thickness of the compactgenerallyq is satisfactory,

Inlieu 0f mech'anic'a11y applying "the; sealing metal as a separate sheet,'by wrappingabout the. core or otherwise, it ma be. electrode'posit'edion, the radioactive core jmetal either immediately after sinteringor after preliminary rollingbf the core-following"sinteringi There is novparticular difficulty in applying the 'sealingme'talby ,electrodeposition, and any electroplating bath and technique suitable for the'sealin'g metal .to be applied (gold, platinum, nickeL'copper or chromium ior,

example), using soluble or insoluble ahode's,lma.y,

be employed; After'a 'layer of'sealing metalof desired thickness has been electrodeposited on v the core, it is sometimes, advantageous to, heat, theresulting metal product to an elevated temperature '(say 950 CI) for a isuffrcie'ntj period of time (say /2 hour) to eliminate'hydrogen which oftenaccumulates in undesirably large amounts in electrodeposited metal andgif not eliminated may make it difficult to roll the product.

The rolling operation may-be 'carrieid-out atj room temperature, orat-ahig-her temperature if desired.- The radioactive-substance-incorpo the productjperiodically during, rolling (genera ally;v after each. three or four ,passes through the rollsj when the "rolling operation is conducted; at Annealing is effectedlby heatingjthe product to about 800to 900 Cf; Thefl combinationof rolling andjannealingsteps con-I tributes to, firmly and permanently bonding the: outer layers of metal .to the core 1metal,.'esp,ecially when. the outer layers have been applied by sim-..

room, temperature) yi'w pping them about the oore metal. Rolling," with intermediate anne a1's',, may a be.

continued until the total thickness ofrthe product I. t has 'beenireduce'd "toiabout .002" inch. This, is av convenientthickness at which to apply the backing sheet or strip, Owing to.the extreme thin,-

ness-of the product in. its finished-form, a fairly, heavy "backing strip'alrnost always is necessary to provide supportfor handling. Silver is a particularly advantageous metal to employfor back-w ing purposes althoughpther ,metalssuch as cop,- per; 'nickeL'et'c m'ay be employed;if desired;

The.rolled'product" "advantageously is initially bonded. to the backing*metal by .welding under. pressure." .Fo'rthispurposethe foil and backing metalysuitably c1eaned,'are pressed together withv a force of, about. '5 tons. per .squ'a're inch while: heated-electrically or otherwise to' a Welding tern? perature'. After welding the rolled product to the backing metalythelassembly is ,further rolled to reduce'the radioactive component to there quired thinness. As above indicated, rolling should becontinued untilthe, total thickness of the radioactive metalgcoreand the exposed seal-' ingfilh'i" is no "greater than canbe penetrated by, alpha particles having the maximumenergy with iwhiclrtheyare emitted by the radioactiiveisube stance-employed; and "preferably to a thicknessslight enoughtto be penetrable by alpha particles. havin'g'the :averageenergy'with which such substahoeemitsfthem. The thickness of the backing metal, at the time-therolle'd radioactive-product, is gweidedtd it',"should"be such! that in. the final;

rolled"pr0duct thefbackingmetal is stillfisufii ciently thick (say.,about 0.010inch). to provide effective support. If the rolling operation reduces the backing metal to athicknessdessthan this,

a further 'layer 'of 'backing fmetaljojf adequate thickness may be applied by welding or otherwise.

The radioactive metal 'product madelasda scribed above isjillustrated -(without a backing) in Fig. 1, which shows the core of pressed metal powder and radioactive substance sandwiched between "the sealing films which completely sure round the core. throughf a structure of. thistype bonded to-a' back a wire structure 'iniwhichuthe radioactive :core

wire is enclosed ,in a'jlse'alihg sleeve substantially free of radioactive material.

Figs; 6 to 9 illustrate how the backin-glayer (or a second s'eparatejbackmg 'or'supportiiig element.)

may be configured to'.limit thean ular width-of f a beam'of alpha particles .emitted from a strip of radioactive :foil '(orf'from; a radioactive wire) made'as herein described." Fighdidrlexample',

illustrates 'hOiW'a'fiat backing'at least as wide as the-foil limits the angular width 'ofxthe' effective beam to"180" Figs/7 and 8 'show cha'nnel "shaped a rated in-the-sintered-compactconstitutes an im"-" backing elements'wf conventional" channel cross A, fragmentary, cross-section 7 web.

section as in Fig. '7 or in the form of a longitudinally sectionedtube as in Fig. 8) which limit the angular width of the alpha particle beam to something less than 180 A deep channel backing element as shown in Fig. 9 may be employed if a beam of narrow angular width is required. The

backing elements in all of the structures shown in Figs. 6 to 9 are, of course, thick enough to be impenetrable by alpha particles of even the maximum energy. with which they are emitted .from

the radioactive foil, so as to confine the effective emitted beam of these particles to the desired angular width.

Fig. 10 illustrates diagrammatically a staticeliminating device according to the invention (which advantageously consists essentially of a long narrow strip of the new radioactive metal product mounted on a backing and supporting element of the character shown in Figs. 6 to 9) mounted for eliminating staticcharges that accumulate on a web of travelling :sheet material. The static-eliminating device extends transversely "of the web, and preferably completely across its width. It is spaced an inch or so from the web, with the beam of emitted alpha particles directed toward the web. The alpha particles ionize the atmosphere between theradioactivefoil and the .web, and static charges accumulated on the web to mount the static eliminator with the exposed surface of the foil no farther than this from the It is evident thatinstead of the web of sheet material travelling over a stationary static eliminator, the static eliminator may be moved overa stationary sheet. It may be set up to be moved mechanically, or' it may be hand operated as a wand. It also may be of other physical form than a thin ribbonfor example, it may be in the form of a broad sheet, and may be flat or curved. The radioactive element may cover a continuous surface expanse of the backing structure, or it may be arrangedthereon as a series of spaced strips, triangles, squares, circles, or other shapes. The alpha particles emitted from the foil diverge unless confined by a shield of some sort, so even when the foil is discontinuous on the backing, the separate foil pieces may be arranged so that the alpha particle density in the atmosphere a short distance from the plane of the foil is substantially uniform. r

It is also evident that the web itself may be thin and continuous, as is usually the case with paper or sheet plastic material, or it may be relatively thick'and discontinuous, as are the rovings and bats encountered in textile mills.

Another advantageous use of the new product isto eliminate static charges on the parts of analytical and otherbalances, the accurate operation of which is affected by such charges. A foil or wire prepared in accordance with the invention, with or without a backing, may be mounted near the pans, or beam, or both, wherever objectionable static charges might accumulate, to'eliminate v 10 such charges. Numerous other similar uses of the new product are apparent.

Radioactive metal products prepared as above described with a thin, continuous sealing film substantially free of any radioactive substance over the radioactive core give off virtuallyno radioactive gas. Their use therefore will not result in contaminating the surrounding atmosphere with these highly poisonous gases. ,Moreover, by retention of the radioactive gas within the core, the radioactive effects of its own disintegration, and of the disintegration of its products, are preserved for use'in the environment in which the metal product is employed.

The efiect of the sealing film in retaining the gaseous disintegration product in the core makes the use of such film advantageous even in those instances where the thickness of the sealing film alone, or of the sealing film and metal core combined, is greater than can be penetrated by any alpha particles emitted by the radioactive substance; Such thick metal products. of course, will not emit alpha particles, but for uses wher betaparticle emission and gamma radiation are all that are-required of the foil, the sealingfilm, even though thicker than above specified, still is advantageous and performs the highly useful functions of preventing contamination of the surrounding atmosphere With a radioactive gas and preserving within the metal the radioactive energy of the gas and its disintegration products.

We claim: I

1. The method of making a radioactive metal product which is substantially proof againstleakage of radioactive gas which comprises, intimate- -'ly admixing a finely divided metal and a finely divided radioactive alpha-particle-emitting substance, pressin and sintering the resultingmixture into a compact, applying a sealing layer of metal substantiall free of radioactive material to one surface of said compact, bonding a relatively thick metal backing to the other surface of said compact, and rolling the resulting product until the thickness of said sealing layer is no greater than can be penetrated by alpha particles having the maximum energy with which they are emitted b said radioactive substance.

2. The method of making a radioactive metal product which is substantially proof against leakage of radioactive gas, which comprises, intimately admixing a metal powder and a finel divided radioactive alpha-particle-emitting substance, pressing and sintering the resulting mixture into a compact, sandwiching the compact between layers of a metal substantially free of radioactive material, rolling the sandwich to reduce the thickness thereof and to bond the several layers together, applying a relatively thick layer of backing metal to the resulting bonded sandwich, and rolling the sandwich and backing metal together until the compact and at least one of the layers of the sandwich component of a thickness no greater than can be penetrated by alpha par ticles .having the maximum energy with which they are emitted b said radioactive substance.

3. The. method of making a radioactive metal product which i substantially proof against leakage of radioactive gas, which comprises, intimately admixing a finely divided metal and a finely divided radioactive alpha-particle-emitting substance, pressing, sintering and forming the resulting mixture into a compact, wrapping said compact in a sheet of non-radioactive malleable metal of a thickness about one-tenth that of the compact to form a sandwich, rolling said sandsaid core film and constituting the alpha-particle-emitting front of said product, the combined thickness of said two films being no greater than can be penetrated by alpha particles having the maximum energy with which they are emitted by said radium compound, a backing and supporting member positioned at the back of said core film, said member being greatly thicker than said core and sealing films together and being of silver whereby radium may tend to migrate from said core film into said member, and a sealing layer of gold substantially free of radioactive material interposed between and bonded to said core film and to said backing member so as to concore film having a radioactive alpha-particlejemitting substance intimately dispersed therein, ialn effectively continuous radioactive-gas-sealing film of noble metal free of radioactive material and bonded to a first surface of said core film whereby radioactive gas is substantially prevented from escaping from said first surface of said core film, said sealing film being thin enough to be penetrable by alpha particles having the maximum energy with which they are emitted by said radioactive substance whereby alpha particles are free to escape from said core film through said sealing film, and a metallic backing element affixed to the second surface of said cOre film, said element being suificiently thick to support said core and sealing films and to be impenetrable by said emitted alpha particles and radioactive gas and having side flanges so deep as to limit the angular width of the beam of alpha particles emitted from said first surface to substantially less than degrees.

CLARENCE W. WALLHAUSEN. HARRY H. DOOLEY. CLAYTON C. CARROLL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,048,490 Bilstein July 21, 1936 2,264,683 Smith Dec. 2, 1941 2,266,738 Byler et al Dec. 23, 1941 2,300,923 Hornor 2. Nov. 3, 1942 2,326,631 Fischer Aug. 10, 1943 2,405,026 Feuer et a1. July 30, 1946 Certificate of Correction Patent No. 2,479,882

CLARENCE W. WALLHAUSEN ET AL.

It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 35, before viz. insert an opening parenthesis; line 36, after to insert a closing parenthesis; column 10, line 62, after component insert are; column 11, line 2, for oft he read of the; line 36, for amanates read emanates; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 10th day of January, A. D. 1950.

August 23, 1949 THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,479,882 August 23, 1949 CLARENCE W. WALLHAUSEN ET AL. It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 35, before viz. insert an opening parenthesis; line 36, after to insert a closing parenthesis; column 10, line 62, after component insert are; column 11, line 2, for oft he read of the; line 36, for amanates read emanates;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 10th day of January, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

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