Title:
A METHOD OF PREPARING A CALIFORNIUM-252 NEUTRON
United States Patent 3627691


Abstract:
A method of encapsulating a radioisotope wherein an open ended tube is used first to support a porous member for filtering a radioisotope precipitate from a solution and next for containing the precipitate on the porous member while heating to convert the radioisotope to a thermally stable form. The tube is then sealed at both ends providing a leaktight capsule to prevent loss of the radioisotope. This method is particularly applicable to the encapsulation of californium-252 for use as a point source of neutrons.



Inventors:
Boulogne, Alexander R. (Aiken, SC)
Faraci, Jean P. (Aiken, SC)
Application Number:
05/001484
Publication Date:
12/14/1971
Filing Date:
01/08/1970
Assignee:
ATOMIC ENERGY COMMISSION USA
Primary Class:
Other Classes:
250/432R, 264/.5
International Classes:
G21G4/06; (IPC1-7): C09K3/00; G21H5/00
Field of Search:
252/301.1 250
View Patent Images:
US Patent References:
3124538N/A1964-03-10Lewis
2830190Radioactive source1958-04-08Karp
2592115Neutron source1952-04-08Carroll



Primary Examiner:
Quarforth, Carl D.
Assistant Examiner:
Tate R. L.
Claims:
What is claimed is

1. A method of encapsulating a californium-252 radioisotope for use as a neutron source which comprises:

2. The method according to claim 1 wherein said capsule tube and impervious end plugs are a platinum-rhodium alloy and said porous members are pressed and sintered particulate stainless steel for retaining precipitate particles having dimensions in excess of about 10-20 microns.

3. The method according to claim 1 wherein said slurry additionally contains terbium oxalate.

4. The method according to claim 1 wherein said californium oxalate is calcined to californium oxide.

Description:
BACKGROUND OF THE INVENTION

The present invention was made in the course of or under a contract with the U.S. Atomic Energy Commission.

1. Field of the Invention

The present invention relates to the encapsulation of various radioisotopes in varying quantities but more particularly to the encapsulation of small quantities of rare and expensive radioisotopes to form point sources of radiation. For purposes of this application a point source shall be defined as one containing a small concentrated mass of radioisotope such that radiation emanating therefrom generally appears to originate at a point.

The spontaneous fission of only microgram quantities of 252 Cf provides a significant flux of neutron radiation. Consequently, an encapsulated small mass of this element has great value as a portable neutron source and may be employed in areas such as radiography, neutron activation analysis of unknown materials and mineral exploration. Point sources are especially useful for producing sharp images on neutron radiographs.

2. Description of Prior Art

Prior californium point sources have been prepared by coprecipitating 252 Cf(0H)3 with Fe(0H)3 from a purified solution. (International Journal of Applied Radiation and Isotopes 1969, Vol. 20, pp. 453-461, Pergamon Press.) In this prior process, the resulting gelatinous precipitate is centrifuged from the remainder of the solution and drawn into a pipette for transfer to a small platinum cone. The precipitate is dehydrated, the cone is mechanically folded to form a pellet and the Cf(0H)3 and Fe(0H)3 inside the pellet are calcined to form oxides. The pellet is then suitably encapsulated for utilization.

This prior process has several limitations and disadvantages. Radioisotopes, especially californium, with which this prior process is used are rare and costly elements. Unavoidable losses can occur as the gelatinous precipitate is transferred by pipette or as the cone containing the radioisotope is folded into a pellet. Precision in the quantity of radioisotope in the source is difficult to achieve because of these losses and because of the uncertainty in the amount of radioisotope in the gelatinous precipitate drawn into the transfer pipette. Moreover, the transfer and mechanical folding steps are cumbersome to perform with remote handling equipment within a shielded compartment.

SUMMARY OF THE INVENTION

In view of the limitations of the prior art it is an object of the present invention to provide an economical method of encapsulating a precise amount of a radioisotope.

It is also an object to provide a convenient method for remotely preparing a radiation source within the confines of a shielded compartment.

It is a further object to provide a method of preparing a neutron source containing californium isotopes.

In accordance with the present invention a method is provided for encapsulating a radioisotope to be used as a radiation source. An open ended tube or capsule is used to support a porous member for filtering a crystalline precipitate including the radioisotope from solution. The precipitate is calcined within the capsule to a more thermally stable form. The capsule is then sealed at both ends to complete the encapsulation of the radioisotope.

The limitations of the prior art are practically eliminated by the sequential use of the source capsule for the steps of filtration, calcination and encapsulation of the radioisotope.

The present method of encapsulation is particularly applicable to the preparation of point radiation sources of rare and expensive radioisotopes, such as isotopes of californium.

DESCRIPTION OF THE DRAWINGS

The present invention is illustrated in the following drawings wherein:

FIG. 1 is a simplified elevation view partially in cross section of one apparatus for precipitating and filtering a radioisotope.

FIG. 2 is a cross-sectional view of an encapsulated radioisotope.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 1, an assembled apparatus is shown for implementing the method of the present invention. A precipitator vessel 11 containing a slurry 13 of radioactive precipitate 37 and solution is provided with an opening 15 for admitting the process ingredients. A threaded outlet 21 permits slurry 13 to flow from the precipitator vessel 11.

A first transition member 19 is sealingly attached to precipitator vessel 11 at outlet 21. A tapered projection 27 is formed in transition member 19 at the end opposite the precipitator vessel 11. A passageway 23 extends from outlet 21 of precipitator vessel 11 through the tapered projection 27 of transition member 19.

A radiation source capsule 29, in the shape of an open ended tube, has a longitudinal passageway 31 with tapered openings 33 at both ends. Projection 27 of transition member 19 is tapered to correspond to and sealingly fit into mating relationship with an opening 33. A porous metal member 35 is fitted against a chamfer 36 in passageway 31 to serve as a filter for radioactive precipitate 37. Porous member 35 can be a pressed and sintered compact of particulate stainless steel designed to retain particles greater than about 10-20 microns in diameter.

A second transition member 39 connects the source capsule 29 with a filtrate receptacle 41. The second transition member 39 is similar to transition member 19 except that an extension tube or downspout 43 extends into receptacle 41 to below the level of a port or connection 47 provided for engaging a vacuum source. A passageway 45 is provided through transition member 39 and downspout 43 to allow filtrate 49 passing through porous metal member 35 to enter receptacle 41.

The assembled apparatus shown in FIG. 1 is normally disposed in a shielded compartment provided with ordinary remote handling devices or "slaves" to begin preparation of a radiation source. The assembly is initially arranged horizontally with opening 15 of precipitator vessel 11 in an upward position.

A solution containing a measured amount of the desired radioisotopes, such as 252 Cf, and possibly 254 Cf, is transferred to the precipitator vessel 11. A second ingredient, such as oxalic acid (H2 C2 O4), either in solution or pure form, is added in substantial stoichiometric excess to vessel 11 to precipitate the radioisotope as a crystalline compound, such as californium oxalate [Cf2 (C2 O4)3 ]. The hydrogen ion concentration in solution should be limited to allow the H2 C2 O4 to be in equilibrium with a substantial quantity of C2 O4 for reacting with californium. A carrier element, such as terbium, can also be added to the solution to ensure precipitation of substantially all of the radioisotope. Terbium is chemically compatible with californium and does not form undesirable radioisotopes on neutron bombardment.

The assembly is then rotated to a vertical position as shown in FIG. 1 to allow slurry 13 with the crystalline precipitate 37 to flow into contact with porous metal member 35. A suitable clamping device (not shown) is provided to support the assembly. As an alternative the assembly may be maintained in the vertical orientation shown with a valve between vessel 11 and capsule 29 to control the flow of slurry. A vacuum source is coupled to port 47 of the filtrate receptacle 41 to draw the filtrate solution 49 into receptacle 41. Crystalline precipitate 37 is thereby deposited on porous member 35.

FIG. 2 shows a fully encapsulated radiation source which is completed as follows. Capsule 29 is removed from the above-described assembly of FIG. 1 and placed in a suitably sized pressing device. A second porous metal member 51 is pressed into longitudinal passageway 31 in capsule 29 to a location just above the precipitate. A compartment 52 for containing the radioisotope is thereby defined. This second porous member 51, although not necessary for operability, is preferably included to retain the radioisotope while allowing volatiles to escape in the following heating step.

Capsule 29 with precipitate 37 is then heated to a sufficient temperature to convert the precipitate to a thermally stable form of the radioisotope. Gaseous products, for instance carbon dioxide and steam, pass through the porous members 35 and 51 and leave a dried and calcined product 53 such as californium oxide (Cf2 O3) within compartment 52.

Impermeable end plugs 55 are next pressed and welded into the tapered openings 33 of capsule 29. For safety and handling convenience, capsule 29 is placed in an outer housing or outer capsule 57 and sealed therein with an end closure 59 welded or otherwise permanently fixed in place. Outer capsule 57 is shown provided with a threaded end portion 61 for attachment to a utilization means.

EXAMPLE I

It was desired that a point source of neutron radiation be prepared containing 700 micrograms of 252 Cf. Three milliliters of 1M HNO3 containing 730.6 micrograms of 252 Cf, 4 ml. of distilled water and 0.5 ml. of 1M H2 C2 O4 were combined in a precipitator vessel. A white crystalline precipitate of Cf2 (C2 O4)3 began to form after about 12 minutes. After about 20 minutes of reaction time the crystalline precipitate was filtered on a porous stainless steel disk which was previously pressed into a platinum-rhodium source capsule. Ninety-two percent by weight of the 252 Cf introduced into the precipitator vessel was captured in the filtered precipitate. The filtrate, containing the remainder of the 252 Cf, was further treated with 500 micrograms of terbium nitrate [Tb(NO3)3 ] dissolved in 0.25 ml. 1M HNO3 and 0.5 ml. of 1M H2 C2 O4. Precipitate including both Tb2 (C2 O4)3 and 252 Cf2 (C2 O4)3 was allowed to form for about 50 minutes before again filtering through the porous stainless steel disk in the source capsule. Ninety-six and two-tenths percent of the 252 Cf in the original feed solution was recovered after this second filtration. The source contained 702.6 micrograms of 252 Cf which was within 0.4 percent of the desired quantity (700 micrograms). The filtrate containing the remainder of the 252 Cf was retained for further processing. A second porous stainless steel disk was pressed into the platinum-rhodium capsule above the oxalate precipitate. The capsule was then air dried at 200° C. in a quartz tube oven for about 30 minutes. The temperature was then slowly raised to 550° C. for about another 30 minutes to complete the conversion of 252 Cf2 (C2 O4)3 to 252 Cf2 O3. Platinum-rhodium end plugs were inert gas welded by thermal fusion into the openings at both ends of the capsule. After testing for leaks and decontaminating, the platinum-rhodium capsule was sealed inside a 304-L stainless steel outer capsule.

EXAMPLE II

A second encapsulated 252 Cf neutron source was prepared in substantial accordance with the method employed in example I. The precipitator vessel was loaded with 49.86 micrograms of 252 Cf in 3 ml. 0.1M HNO3 and with 500 micrograms of terbium in nitrate solution. After about 15-20 minutes the precipitate had formed and was processed as in Example I. 97.6 percent by weigh or 48.6 micrograms of the 252 Cf in the feed was deposited within the capsule. The lower original nitric acid concentration used in this example than used in example I provided more available oxalate ions for reaction and consequently a shorter reaction time was required.

Although the foregoing examples only describe the encapsulation of californium to form point sources, other radioisotopes such as curium, americium and plutonium isotopes may be encapsulated by the method of the present invention. Furthermore, radiation sources having greater amounts of radioisotopes than can be contained in point sources may be prepared.

A method is described herein for preparing a source of radiation. The method provides a high yield of expensive radioisotopes from the feed to the finished source. Precise quantities of the radioisotope are deposited into the source capsule to allow sources of exactly the desired intensity to be prepared. Few mechanical steps are required to facilitate remote performance of this method within a shielded compartment.