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WILLIAM J. MAYER and ROBERT L. ANDERSON Research Laboratories, General Motors Corp., Warren, Mich.
Make Y o u r O w n Tracers b y .
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Radioactive Parent-Daughter Separations
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NE O F THE IMPORTANT by-products of nuclear energy is the large-scale production of radioisotopes. These materials have played important roles as tracers in the fields of chemistry, medicine, and biology. Industrial applications, especially large-scale in-plant production uses such as some routine quality control and inspection testing procedures, could also benefit from the use of radioactive indicator isotopes.
Radioactive tracers for production use require
k Particular types of emitters p Correct energy of radiation k Suitable chemical form Short half life T h e use of short half-life isotopes minimizes inventory hold-up time of inspected material or scrap and also reduces contamination problems in the case of accidental spills. Problems associated with scheduling reactor production and with rapid transporation of radioactivity are presently so serious that use of reactor-produced activity in routine test procedures is precluded. There are two ways of circumventing the difficulties. One is to use a neutron source. This is a system that supplies a relatively small number of neutrons and can be used to activate selected materials. This type of system is most commonly used for the analysis of materials by neutron activation and the neutron flux is often too small to produce radioactive isotopes in usable amounts. The other way is to separate a shortlife daughter isotope from a long-lived parent. If this separation can be effected, a supply of short-lived material can be made available for routine plant applications. One such system has been studied extensively on a laboratory scale. I t involves the separation of the shortlived indium-1 13m, daughter isotope from the long-lived tin-113 parent. I n this particular system the tin-113 parent and indium-113m daughter were in a 1M hydrochloric acid solution and the separation was effected by Dowex 1, a n anion exchange resin. Dowex 1 was placed in a glass column, 20 cm. high and 5 mm. in diameter, to form a resin bed approximately 10 cm. high. After the tin-113 solution was placed
Large-scale in-plant production uses often require a continuing supply of short half-life tracers. This problem may be solved by storing a longer-lived parent element and repeatedly removing the short half-life daughter. Decay of tin-113 to indium-113%lends itself to this “milker” technique on the column, the indium-I 13m could be “milked” by allowing 1 M hydrochloric acid to flow through the column. T o widen the applicability of this system, it was necessary to place the indium-1 13m daughter product in an organic phase. I t was found that indium-1 13m could be extracted by 0.25M thenoyltrifluoroacetone (TTA) in benzene after first adjusting the p H of the solution to 3.5 with a saturated solution of sodium acetate. T h e indium-1 13m daughter can thereby be used in a n oil or organic system as well as in a n aqueous system. There are a number of radioisotope parent-daughter relationships that may be used for a source of short half-life radioactive material. A number of evaluation studies have been made and a commercial version of a n ion-exchange column milker has been described ( 4 ) . I n the present study, the tin-indium system was chosen because it afforded a source of gamma activity with a sui table half life-namely, 1.7 hours for the indium daughter and 112 days for the tin parent. Moreover, from a chemical viewpoint it seemed reasonable to assume that the separation of the tinindium could be effected.
Nuclear Properties Indium-113m is a decay product of tin-1 13 which is reactor-produced by the n,y reaction starting with tin-112 as the initial material with a natural abundance of about 1%. The tin-113 produced decays by electron capture with a half life of 112 days to indium113m which in turn decays to a ground state indium-113 by the emission of a photon with a n energy of 0.393 m.e.v. ( 2 ) . However, the transition from a n excited level of the nucleus (indium113rn) to a lower level of the same nucleus (indium-1 13) can also be accomplished by direct interaction with a bound atomic Conseelectron of the same atom.
quently, no photon is emitted to the detector. This means that a percentage of the indium activity that decays by this process is effectively lost to the detector. For indium-l13m, about 3570 is lost (5). This is a factor to be considered in producing indium-1 13m where a high specific concentration of gamma energy is desired.
Growth-Decay Relationship I t is sometimes desirable in a problem of this kind to calculate the growth of the daughter in the system when the daughter has been previously removed. A situation known as secular equilibrium exists when a radioisotope decays to a n isotope that is in turn radioactive, and in the case where the parent is much longer-lived than the daughter. T h e number of daughter atoms produced at any time t can be calculated from:
Where N z is the number of atoms of daughter; N1, the number of atoms of parent; A B , the decay constant of daughter (for indium-l13m, 4.07 X lo-’ hr.-l); and X I , the decay constant of hr.-l). parent (for tin-113, 2.58 X When t becomes long compared to the half life of the daughter this equation reduces to h2Nz = XIN1, and the growth of the daughter, XzAVz,equals the rate of decay of the parent, A,NI.
Chemical Properties and Chemical Separation
Two considerations are very important Chemical separation of daughter and parent Repeated milking of the daughter from the parent Ion exchange offers one of the best possible solutions for making the separation. T i n can be separated from indium on a VOL. 52, NO. 12
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DECEMBER 1960
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Dowex 1 type resin ( I ) in a n acid solution. When tin is dissolved in hydrochloric acid solution it forms a complex ion, SnCla=. This ion is strongly adsorbed by the Dowex resin. Indium probably does not form a complex chloride ion but exists as a cation in the acid solution which shows little tendency to adsorb o n the resin. A small glass column was designed, approximately 20 cm. in length and 5 mm. in diameter, to contain a resin bed of Dowex I about 10 cm. high. The Dowex 1 used throughout this study was 50 to 100 mesh and 8% cross-linked; it had a total capacity 3.4 =t0.3 meq. per dry gram, and a moisture content 39 to 45% by weight. This column was operated with approximately 0.1 mc. of tin-113 in 1M hydrochloric acid. The indium was analyzed by counting a 2.2-ml. portion of the eluate immediately after elution from the column and approximately three days later. The first few samples contained some residual activity that could be attributed to wall effects of the column or to some impurity that was present in the tin. Assuming all residual counts were tin in the bulk of the samples, it was apparent that indium was separated from the tin between approximately one part per thousand and five parts in ten thousand. A typical analysis of an eluate sample counted 3.07 X 106 c.p.m. in a well-type scintillation crystal immediately after elution and 1.99 X l o 3 c.p.m. after three days decay.
Distribution To study the separability of tin and indium quantitatively and to obtain d a t a for column design studies, the distribution of tin a n d indium between the Dowex 1 a n d the eluting solution was obtained. T h e distribution coefficient K ( d ) is defined as the concentration 01 the ion in the resin divided by the concentration of the ion in the eluting solution. The separation factor of two ions is then the ratio of the distribution coefficients for the individual ions. T o determine the tin distribution coefficients, small amounts of resin were accurately weighed (about 0.4 gram) in small 25-ml. screw cap bottles. The resin was equilibrated with 10 ml. of 1 M hydrochloric acid to which was added 5.12 X 106 c.p.m. of tin-113. The samples were shaken for 4 hours. Because of the difficulty in analyzing the resin for activity, aliquots of the aqueous phases were
Table I. Distribution Coefficients for Tin-1 13 and Indium-1 13rn (In 1.M HC1. K ( d ) : Sn-113, 353; In113m, 7.4. Separation factor ( a ) = 35317.4) Concn., C.P.1\1. Aqueous, ml. Resin, g. Sn-113 7.66 x 104 1.05 X 10s 8.26 X I O 4
2.81 x 107 3.23 X 107 2.90 X 107
K(d) 366 308 386
In-113m 9.44 x 108 9.85 X 10s 1.06 x 1 0 4
994
5.48 x 104 6.36 X l o 4 1.06 x 105
5.7 6.5 10.1
analyzed before and after equilibration, using a well-type sodium iodide (Tlactivated) scintillation detector. Distribution coefficients for indium were determined in the same way as for tin. The indium was obtained from a small column previously used for performance evaluation. Because of the short half life of indium, the samples were shaken for only ’/zhour (Table I). The average distribution coefficients of tin and indium show that a separation factor of 47.8 is obtained. Since a separation factor of 1.5 is usually sufficient to separate two ions in a small column, it is evident that tin and indium can be efficiently separated using Dowex 1 resin.
Table It.
(Organic phase: 5.2 ml.; 0.25-%I TTA in benzene. Aqueous phase: 5 . 2 ml.; acid soh. adjusted t o pH of 3.5 with saturated NaCzH302. Feed concn., 6.43 X 106 c.p.m./ ml.)
Extraction 1st 2nd 3rd
T h e usefulness of a milker process can be augmented if the daughter activity can be placed in organic media, because many applications call for radioactive tracers in oils. Experiments were conducted to extract the indium with organic compounds that could be dissolved in oil. There a r e several organic solvents for indiumnamely, %hydroxy quinoline, acetylacetone, a n d a chelating agent, thenoyltrifluoroacetone (TTA), dissolved in benzene ( 3 , 6). All of these reagents would extract tin from a n aqueous phase and, therefore, solvent extraction with these solvents was not suitable as a milker itself; rather, extraction would have to be carried out after the indium was TTA in eluted from the column. benzene was selected and, to obtain actual extraction data, a stepwise countercurrent laboratory extraction was performed using graduated separatory funnels. Indium-I 13m solution was milked from the small column described previously and then adjusted to a p H of 3.5, using saturated sodium acetate. This solution was placed in a graduated separatory funnel with the same volume of extractant ( T T A in benzene) and was shaken for 5 minutes. I t was assumed that equilibrium conditions were established in this time. Aliquots of both phases were taken for analysis (Table 11). The data indicate that 85y0 of the indium was extracted in the first extraction stage, and that 0.2% indium was left in the aqueous phase after three extractions.
Column Brea k-T hrough I n addition to the separation and extraction studies of the tin-indium system, experiments were carried out to determine the break-through of tin from the Dowex 1 resin. Three columns were made u p containing 1, 2, and 4 grams of resin. The columns were made of 5-mm. glass tubing approximately 10.5 inches long. The I-gram column gave a resin height of 5.5 cm.; the 2-gram column, 11 cm.; and the 4gram column, 22 cm. On each column was placed a total of 3.03 X lo7 c.p.m. of tin activity. The flow rates of the columns were 1 ml. per minute. Columns were gravity fed from the reservoirs containing 1.44 hydrochloric acid and the flow rates were adjusted by the stopcocks on the columns. The columns were sampled
INDUSTRIAL AND ENGINEERING CHEMISTRY
Phase Concn., C.P.M./XIl. Organic Aqueous 5.49 4.80 6.75
x 1065 x 104 x 103
5.75 5.46 1.54
x 104 x 103 x 1035
In-ll3m extracted in first stage: 5.49 X ll,j X 5.2 6,43
Organic Extraction Studies
TTA Extraction of Indium113m
105
5.2 X 100
remaining after 1.54 X 163 X 5.2 8.43 X 105 X 5 . 2
3
85%.
In-113
extraction
titaees:
=
loo = o’2%b.
periodically and the put through the Usually one drop of analysis and it was after sampling and half lives later.
total volume of liquid column was noted. liquid was enough for counted immediately again some 10 to 12
Break-through of the tin occurred for the 1-gram column a t 50 ml., for the 2-gram column a t 500 ml., and for the 4-gram column a t 1330 ml. A t breakthrough, the tin activity in the eluate was 0.3 to 0.6Yo of total activity of the tin placed on the column. These data indicate a linear relationship with a slope of approximately 415 ml. of hydrochloric acid per gram of resin. Thus it is possible to determine approximately the amount of resin needed to hold the tin activity to a few tenths of a per cent for any particular flow rate. However, care must be exercised in making such a calculation because of the loss of specific activity of the indium resulting from residence time in the column if a large column were designed. For the pilot plant work that is contemplated, the 415-ml. per gram figure is low. Improvements could probably be realized using smaller particle size resins. Some enriched tin-1 12 isotope (707, enriched) has been procured and irradiated in a slow neutron flux to produce high specific activity tin-1 13 for future work.
literature Cited (1) .4,m. SOC. Testing Materials, “Symposium on Ion Exchange and Chromatography in Analytical Chemistry,” Special Tech. Publ. 195, p. 44, 1956. (2) Kinsman, Simon, Radiological Health Handbook, U. S. Government Printing Office, Cincinnati, Ohio, (1957). (3) Moeller, Therold, 2nd. Eng. Chem. Anal. Ed. 15, 270 (1943). (4) Newacheck, R. L., Beaufait, L. J., Anderson, E. E., ATucleonics 15, 122-5 (1957). (5) Sullivan, W. H., Triliner Chart of Nuclides, (2nd ed.) U. S. Government Printing Office, Washington 25, D. C. January, 1957. (6) Sunderman, D. C . , Ackerrnann, I. B., Meinke, W. W., Anal. Chem. 31, 40-4 (1959). RECEIVED for review March 30, 1960 ACCEPTED August 9, 1960
