US2760887A - Technique and apparatus for radioactive source preparation - Google Patents

Description

L. G. LEWIS Aug. 28, 1956 TECHNIQUE AND APPARATUS FOR RADIOACTIVE SOURCE PREPARATION Filed July 29, 1953 m 7 EEEEEE a I i 3/0 35 INVENTOR.

Lloyd 6. Lewis ATTORNEY United States Patent TECHNIQUE AND APPARATUS FOR RADIQ- ACTIVE SQURCE PREPARATION Lloyd G. Lewis, La Grange, illlL, assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana Application July 29, 1953, Serial No. 371,137

9 Claims. Cl. 117-93 This invention relates to apparatus and method for making radioactive sources. More particularly, the invention is concerned with an improved apparatus for forming a nearly uniform deposit of the radioactive material on a base sheet.

Conventionally, radioactive deposits are obtained by the evaporation of an aqueous solution of salt wherein only a very minor portion of the salt in solution is radioactive. It is desired to lay down a deposit which is uniform as to distribution of the radioactive salts and also uniform as to the distribution of the non-radioactive salts. With solutions of very high proportions of radioactive salts, the problem of uniform distribution is increased. Accordingly, it is a primary object of my invention to provide a technique and apparatus for producing uniform radioactive deposits from an aqueous solution. A specific object is to provide an apparatus adapted to produce such a radioactive source. Further, an important object of my invention is to deposit a major proportion of the total salts in solution on the base sheet. These and other objects of the invention will become apparent as the description thereof proceeds.

Briefly, in accordance with this invention, a nearly uniform deposit of radioactive material is obtained by evaporating a solution of radioactive salt on a thin base sheet while confined within a limited area on the sheet. A flat sheet of material, such as mica or metal foil, is clamped against one face of a block made from teflon or other material. The block contains a conical bore, the lower end of which is sealed by the sheet. Thus, the sheet seals off the lower end of the bore in the block and forms the bottom of a tapered cup in which the solution of radioactive material is initially placed.

The block and sheet are mounted on an annular resilient gasket supported on a turntable. A clamping plate is positioned over the top face of the block and bolted at its periphery to the turntable. The clamping plate is provided with a central opening which is in register with the top of the cup. The turntable, and hence the cup, is rotated about its central axis at a constant speed.

An air jet is mounted at a small angle of discharge to the axis of rotation and impinges on the wall of the cup at about its midpoint. This air jet, when so adjusted to strike the side of the cup near the meniscus surface of the liquid serves to reduce greatly the proportion of the radioactive material deposited on the sides of the cup.

This effect is due in part to the fact that the proper place-.

ment of the jet produces a wiping action on the wall surface and, in addition, produces a stirring action which prevents salt concentration from reaching saturation at the meniscus. I have found that the adjustment of the direction and strength of the air jet, so as to impinge upon the wall at a very small angle, results in .a nearly uniform deposit of the radioactive material over substantially only the bottom of the cup, i. e., on the base sheet. As much as 90% of the radioactive material is "ice deposited on the base sheet and only a minor portion remains on the Walls of the cup.

The rate of evaporation can be increased by adjustably I maining on the walls of the cup can be recovered and.

utilized in the source. This is done by removing the first sheet and placing a new or second base sheet against the bottom of the tapered bore and redissolving the wall deposit in a minimum volume of a suitable solvent and then repeating the evaporation technique as described above. The two sheets can then be laminated with the deposits face to face to produce a radioactive source with a minimum loss of the radioactive material originally placed in the cup.

For a more complete understanding of this invention and of the objects and advantages thereof, the invention will be described in connection with the accompanying drawing which illustrates a preferred embodiment of the apparatus.

Referring to the drawing, I provide a drying cup assembly 10 having a top cover plate. 11 about two inches across with a central port 12 in register with the upper end of the conical cavity 13 in the block 14. An annular rubber gasket 15 supports the mica sheet or gold foil 16 across the bottom of the conical cavity 13 in the block 14. The block 14 is preferably formed of teflon (polytetrafiuoethylene), such material having been found to facilitate the removal of the solution and deposits therefrom. The conical cavity 13 in the block 14 tapers from about one-half inch at the top face to about one quarter inch at the lower face and the block 14 is about 0.375 inch thick with an overall width of about 1.25 inches. This gives a wall slope of about 13.

The rubber gasket 15 is provided with a central opening 17 so as to prevent the flexing of the exposed portion of the base sheet 16 when the block 14 and the sheet 16 are clamped in place. This is done by the clamping plate 11 and the threaded pins 1% provided in the corners of the plate 11 and which thread into the turntable 19.

The turntable 19 is provided on its lower side with a boss 20 having a non-circular bore 21 adapted to receive the correspondingly shaped drive pin 22 of the speed reducer 23 which may be integral with the motor 24 as shown in the drawing. A suitable motor 24 is a H. P. Bodine motor producing 1800 R. P. M. with a gear reducer 23 to reduce the speed of drive shaft 22 to about R. P. M. The motor 24 is mounted by bolts 25a on a base 25, which provides a stable support for the entire assembly.

The drying cup assembly 10 is so arranged that the metallic foil or mica sheet 16 forms the bottom of the conically shaped cavity 13 which then becomes the solution holder. The walls of the cavity 13 have a steep taper sloping toward the base sheet 16 at an angle of about 5 to 15 with the vertical axis of the cup about which it is to be rotated.

After the block 14 and sheet 16 have been mounted as illustrated, the solution of radioactive salt is poured into place through the opening 12 in the clamping plate 11 and the solution slowly evaporated to dryness. During the drying operation, the drying cup assembly 10 is iet 26 should be sufiicient to form a dimple in the pool "a of liquid 2'7 in the cup 13, but not sufiicient to cause the air stream to break under the surface of the pool 27.

One suitable arrangement of the air jet 26 is illustrated in the drawing wherein the jet 26 comprises a inch 0. D. copper tubing 23 leading from a reducing valve 29. A support assembly for the jet tubing 28 comprises a segmented rod 30-31 which is adjustably threaded into the base 25 and secured by wing nut 32. The upper end of the rod 31 is provided with a flexible shaft 33 which supports a heat source assembly 3d including, for example, an infrared lamp 34a. keyed or indexed bayonet blade 35 extending axially from the lower end of rod 31a enters the upper end of rod 30 and permits ready removal of the jet assembly 26 and heat assembly 34 without the necessity for disturbing the adjustment of these assemblies with respect to the cup 13 when it becomes necessary to remove the block 14 and sheet 16.

The two lengths 31 and 31a of the upper rod support are joined by two sides of a four-way threaded pipe connector 36, the two lengths of tubing 28 and 28a also being coupled by the connector 36. An air supply 37 and a hose connection fitting 38 attached to the inlet end of the tubing 23a complete the assembly.

In operation, a quantity of solution containing sufficient inactive plus active material to produce a deposit of not more than 10 mg./cm. on the base sheet 16 is poured into the cup 13 Within the block 14 which has been bolted to the turntable 19. A typical solution in cludes 3.70 millicuries strontium 90 (in equilibrium with yttrium 90) chloride per milliliter of solution in weak HCl having a total solid content of 1.1 milligrams per milliliter.

The air jet 26 is adjusted so as to have a terminal length of about 2 inches thereof substantially parallel to the axis of the cup and so as to direct the stream of fiuid against the tapered wall of the cup 13. The air supply 37 is connected by hose connection 38 to the tubing 28a and the turntable l9 rotated at about 180 R. P. M. If heat is to be used to accelerate the drying operation, the rod support 31 and the flexible cable 33 are adjusted so as to place the heat source 34 in the proper position. When employing an infrared lamp of 250 watts (Westinghouse 115 volt, clear lens) a spacing of about 10 to 12 inches from the lens of lamp 34a to the surface of the liquid in the pool 27 will provide adequate heat without causing the liquid 27 to boil and spatter. The radioactive deposit upon the thin base sheet 16, resulting from evaporation of the pool of solution 27 in the manner described, is uniform across a circular area of the sheet 16 corresponding to the bottom dimension of the conical solution holder or cup 13. A typical base sheet 16 upon which the salt is deposited is made of gold leaf or mica about 0.004 inch thick and suitably about 1.25 inches in diameter.

It has been found advantageous to employ an air flow at the terminal of the jet 26 at a rate of at least about 0.02 and below about 0.03 C. F. M. A preferred rate is 0.025 C. F. M. and such a flow provides a rather wide path of impingement of the wall of the cavity 13 with the result that the air jet is continuously applied at the meniscus of the pool 27 in the cavity 13. With a inch 0. D. tubing 23 and an air supply at a pressure of about 3 to pounds per square inch at the reducing valve 29, a tubing length of about 2 feet gives a suitable air jet flow in the cup 13.

There are several effects of the stream of air provided by jet 26. One is to continuously agitate the pool 27 at the meniscus-wall interface in an unsymmetrical manner. This produces a turbulence within the pool 27 which prevents the formation of local super-saturated solutions which would result in the premature deposition of salts from the pool 27. On the other hand, the jet of air does not impinge as low as the junction of the mica sheet 16 with the bottom of the sloping wall of the cavity 13. Thus, there is a minor portion of salt which deposits on the lower edge 39 of the wall thereby providing a uniform coating or deposit across the entire exposed area of the sheet 16. If the air jet, on the other hand, is placed so as to displace the meniscus from the wall 39 and onto an exposed annular area of the sheet 16, then in that event the deposit is non-uniform and not reproducible.

When the salt has been evaporated to dryness by rotat ing the cup 13 at a constant speed, for example, at about 180 R. P. M. and by directing the jet of air as described, the jet and heating assembly may be temporarily removed by separating the unit at joint 35. A new sheet 16 is put in place and secured as before. A quantity of a suitable solvent is then introduced into the cup 13 to dissolve the portion of the salt which has accumulated on the wall of the cup 13. The procedure is then repeated with this dilute solution of redissolved salt whereby substantially all of the radioactive salt is placed on one of the two sheets 16. The two sheets are then arranged with the deposits face to face and assembled in a suitable casing.

The complete source comprises a pair of thin base sheets which enclose the radioactive deposit. The laminated unit is mounted in a split holder having regis tering apertures on opposite faces of the holder to provide a double-faced unit. One such source is described and claimed in co-pending application S. N. 311,734 filed September 26, 1952 and issued January 18, 1955, as U. S. 2,700,111.

Relative rotary motion of the pool of liquid 27 within the cup 13 and the stream discharged from jet 26 is essential, but can be attained by either rotating the pool as described in some detail above or by rotating the jet. In this latter case, the jet tube 26 is crank-shaped with the discharging end traveling within and about the cup and the inlet end being rotated through a centrally held bearing and fluid-connector means. Such bearing and fluid-connector means are well known and can be adapted by one skilled in the art for use with the tapered cup as described herein.

My invention has been described with reference to an apparatus specifically designed for the fabrication of radioactive sources. However, it should be understood that this is by way of illustration only and that the invention is not necessarily limited thereto. It is contemplated, therefore, that the drying cup assembly, including the turn-table, directed air jet and heat source, may be used in other systems Where it is desired to concentrate and evaporate solutions on a minimum area. In any event, other modifications of the apparatus and uses thereof will become apparent to those skilled in the art in view of my description and can be made without departing from the spirit of my invention.

What I claim is:

1. A method for preparing uniformly distributed solid deposits from aqueous solution thereof which comprises the steps of supporting a pool of such a solution on a base sheet transparent to beta rays, rotating said pool, and simultaneously directing a jet of air downwardly at the periphery of said pool and obliquely to the top surface thereof, whereby a top-to-bottom turbulence is imparted to the pool thereby avoiding any localized premature supersaturation of the solution.

2. A method for preparing uniformly radioactive solid deposits from an aqueous solution of mixed radioactive strontium chloride and non-radioactive strontium chloride which comprises the steps of disposing a pool of such a solution on a base sheet of mica transparent to beta rays, rotating said sheet and pool about a vertical axis at about R. P. M., simultaneously directing a jet of air downwardly at the peripheral boundary of said pool at a rate of about 0.025 C. F. M., whereby a topto-bottom turbulence is imparted to the pool thereby avoiding any localized premature supersaturation of the solution, and applying radiant heat to the top surface of said pool while rotating it and while directing the said jet of air.

3. In the method of evaporatively drying a solution of salts having a minor radioactive salt composition, the improvement which includes maintaining a pool of such solution within an open cup having sloping sides and a removable bottom, rotating said cup and pool in a substantially horizontal plane about the axis of said cup at about 180 R. P. M., downwardly jetting a stream of gasiform fluid to the periphery of said rotating pool within said cup, the center of said stream striking the said pool at the junction of the meniscus of the pool with the sides of the cup, applying radiant heat to the upper surface of the pool during such rotation and jetting, and continuing the application of heat and the jetting of the fluid until the solution is evaporated to dryness.

4. In the method of evaporatively drying a solution of salts having a radioactive content, the improvement which includes maintaining a volume of such solution in an initially deep pool of limited lateral extent within an open cup having sloping sides and a removable bottom, rotating said cup and pool in a substantially horizontal plane about the axis thereof at about 180 R. P. M., downwardly jetting a stream of gasiform fluid to the peripheral boundary of said rotating pool, applying radiant heat to the exposed upper surface of the pool during such rotation, and continuing said rotating, heating and jetting until substantially all of the solution is evaporated to dryness on said removable bottom.

5. An apparatus for evaporating to dryness a solution of mixed solids comprising in combination an open-ended cup comprising a tapered bore extending through a block, said bore having a taper of between about and degrees, and a removable sheet across the smaller bottom end of the said bore, the sloping walls of said bore and said sheet together forming a cup, a turntable supporting said cup, a resilient support between said turntable and the lower surface of said sheet, said support having an opening in register with the bottom'end of said cup, an apertured clamping plate across the top surface of said block for removably holding said block on said turntable with the said sheet and resilient support therebetween, said cup being arranged with its axis in alignment with the axis of rotation of said turuable, a jet tube means having its discharge end below the upper edge of said cup and arranged to impinge a fluid stream upon the tapered wall of said cup substantially above the bottom thereof, means for supporting said jet means in adjustable array with respect to the said cup, a radiant heating means disposed above said cup, and adjustable means for supporting said heating means.

6. An apparatus for evaporating to dryness a solution of mixed solids comprising in combination, an open-ended cup comprising a tapered bore in a block and a removable sheet across the lower end of the said bore, a turntable supporting said cup, a resilient support between said tumtable and said sheet, an apertured clamping plate across the top surface of said block, bolt means for removably holding said clamping plate on said turntable, the axis of said bore and of said turntable being in alignment, drive means for rotating said turntable, a jet tube means having its discharge end below the upper end of said cup and aranged to impinge a fluid stream upon the tapered bore of said cup adjacent the mid-point thereof, means for supporting said jet means in adjustable but fixed array with respect to said cup, a radiant heating means disposed above said cup, and adjustable means for supporting said heating means above said cup.

7. The apparatus of claim 6 wherein said tapered bore is in a block of polytetrafluorethylene and said removable sheet is transparent to beta rays.

8. The apparatus of claim 7 wherein the jet tube means is arranged to discharge substantially parallel to the axis of rotation of said cup.

9. The apparatus of claim 7 wherein themeans for supporting said jet and the adjustable means for supporting said heating means are unitary and are supported jointly by an upstanding rod having an indexed bayonet blade joint.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. 2. A METHOD FOR PREPARING UNIFORMLY RADIOACTIVE SOLID DEPOSITS FROM AN AQUEOUS SOLUTION OF MIXED RADIOACTIVE STRONTIUM 90 CHLORIDE AND NON-RADIOACTIVE STRONTIUM CHLORIDE WHICH COMPRISES THE STEPS OF DISPOSING A POOL OF SUCH A SOLUTION ON A BASE SHEET OF MICA TRANSPARENT TO BETA RAYS, ROTATING SAID SHEET AND POOL ABOUT A VERTICAL AXIS AT ABOUT 180 R.P.M., SIMULTANEOUSLY DIRECTING A JET OF AIR DOWNWARDLY AT THE PERIPHERAL BOUNDARY OF SAID POOL AT A RATE OF ABOUT 0.025 C.F.M., WHEREBY A TOPTO-BOTTOM TURBULENCE IS IMPARTED TO THE POOL THEREBY AVOIDING ANY LOCALIZED PREMATURE SUPERSATURATION OF THE SOLUTION, AND APPLYING RADIANT HEAT TO THE TOP SURFACE OF SAID POOL WHILE ROTATING IT AND WHILE DIRECTING THE SAID JET OF AIR.

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