Production of Fluorine-18 by Small Research Reactor

Journal of NUCLEAR SCIENCE and TECHNOLOGY, 4[4], p.185~189 (April 1967). 185 Production of Fluorine-18 by Small Research Reactor Yoshiaki MARUYAMA* Received October 4, 1966 High purity 18F was prepared by irradiating 1g of lithium hydroxide or otherwise 0.14g of enriched lithium carbonate containing 50% 6Li in the pneumatic tube of a TRIGA-II research reactor at a thermal neutron flux of 1.2x1012 n/cm2,sec. After cooling for 20min, the irradiated lithium hydroxide was dissolved in distilled water. The resulting 18F was separated from the irradiated target by passing the lithium hydroxide solution successively through ion exchange columns of H-, OH- and H-forms. Use was also made of the Coform ion exchange column in place of the H-form for the elimination of impurities. In the case of Li enriched lithium carbonate, the irradiated lithium carbonate was dissolved in hydrochloric acid, 6 and ion exchange columns of Ag-, OH- and H-forms were used for purifying the 18F. The product F was obtained in the form of water containing 18F. The chemical yield for the purification 18of 18F was about 80% and the final radioactive impurities were less than 1ppm. A neutron activation method for determining the isotopic abundance of 6Li was developed using the nuclear reactions 6Li(n,a)3H and 16O(t,n)18F. Aqueous solutions containing lithium were irradiated in pneumatic tube for 1hr. 18F was separated by steam distillation from the irradiated solutions and precipitated as magnesium fluoride. The chemical yield for the separation of 18F was about 80%. The sensitivity limit was estimated to be 4mg of natural lithium. I. INTRODUCTION Fluorine has in nature only one stable isotope of mass 19. The radioactive isotopes F, 18F, 20F and 21F have been produced 17 artificially, but among them only 18F has a halflife long enough to be applicable to tracer studies. The demand for 18F has been increasing in keeping with the growing importance of fluorine chemistry, and the increasing use of this isotope in medicine. Several nuclear reactions such as 18O(p,n) 8F 19F (n,2n)18f, 19F(g,n)18F, 16O(t,n)18F 1 are available for the production of 18F, but one of the most common methods is the 16O(t,n) 18F reaction, because it can be used to produce carrier-free 18F of large activity in nuclear reactor. Various workers(1)~(9) have recently prepared F in reactor using the 6Li(n,a)3H 18 and 16O (t,n)18f reactions, and compounds containing both lithium and oxygen, such as lithium carbonate, lithium nitrate, lithium oxide and lithium hydroxide, have been used as target materials. Several methods such as distillation, precipitation, ion exchange and alumina column chromatography have been reported for the purification of 18F. In this work, 18F was produced by irradiating lithium hydroxide or else 6Li enriched lithium carbonate in the TRIGA-II research reactor. A method has been devised for separating 18F from irradiated target with the use of ion exchange column. Further, a neutron activation method has been developed for determining the isotopic abundance of 6Li utilizing the 6Li(n,a)3H and 16O(ț n)18f reactions. Aqueous solutions containing lithium were irradiated in the reactor. F was separated by steam distillation 18 from irradiated solution and precipitated in the form of magnesium fluoride.. EXPERIMENTAL II 1. Target Material and Irradiation Commercial reagent grade lithium carbonate was used without further purification. Enriched lithium carbonate was obtained from the Oak Ridge National Laboratory. About 1g of lithium hydroxide, or else, 0.14g of enriched lithium carbonate containing 50%6Li was sealed in cylindrical polyethylene capsule (4mm diam., 50mm long) and irradiated in the pneumatic tube of a TRIGA-II. * Atomic Research Laboratory of Musashi Institute of Technology, Kawasaki-shi, Kanagawa-ken. 27

186 J. Nucl. Sci. Technol. research reactor for 1hr. The thermal neutron flux in the experimental hole was 1.2x 1012n/cm2,sec. 2. Purification Diaion SK No. 1-a strongly acidic cation exchange resin (100~200 mesh size) and Diaion SA No.100-strongly basic anion (100~200 mesh size) were used for the purification of 18F. After cooling for 20min, the irradiated lithium hydroxide was taken out of the polyethylene capsule and immediately dissolved in 30ml of distilled water. This solution was passed through the first -H-form cationexchange column (1.6cm diam., 6cm high) at a flow rate of 1.5ml/min. The eluate from the first column was passed through the second OH-form anion- exchange column - (1cm diam., 2cm high) underneath and connected to the first column in series; 18F was absorbed on the OH-form resin. These columns were washed with 30ml of distilled water to eliminate tritium. The first column was removed, and the third -H-form cation- exchange column (1 cm diam., 5cm high) was connected underneath the second column. The 18F absorbed on the OH-form resin of the second column was eluted with 10ml of 1N sodium hydroxide solution at a flow rate of 1ml/min. The eluate from the column was passed through the third column, and the eluate therefrom was collected in the 50ml flask. A Co-form cation exchange column (1.6cm diam., 6cm high) was also trially used as first column in onder to eliminate lithium hydroxide. In the case of 6Li enriched target, the irradiated lithium carbonate was dissolved in 5ml of 1N hydrochloric acid, and Ag-form cation exchange column (1cm diam., 5cm high) was used as the first column for eliminating lithium chloride. The eluate from the first column was passed through the same second and third columns as before. In preliminary column experiments for the determination of elution curves, the eluate was collected in a number of fractions, and the 18F activity of each fraction was counted with scintillation counter. The ion exchange columns used here are shown in Table 1. Table 1 Ion Exchange Column used for Purification 3. Activation Analysis of 6Li The relationship between 18F activity per milligram of lithium and 6Li concentration was measured by irradiating samples of varying 6Li enrichment. Samples of lithium carbonate containing 18.2, 35.3, 48.4, 72.9 and 95.6% 6Li were prepared by mixing known weights of enriched and natural lithium carbonate. About 10mg of each lithium carbonate sample was dissolved in 10ml of 0.1N nitric acid, and 1ml from each solution was sealed in a polyethylene tube (1cm diam., 2cm long). Standard lithium solutions were prepared with natural lithium carbonate. Sample and standard solutions containing lithium were irradiated together in the pneumatic tube for 1hr. After irradiation, the sample was transfer- 28

Vol. 4, No. 4 (Apr. 1967) 187 red to a round-bottomed distillation flask. Five milliliters of concentrated sulfuric acid and 2ml of 0.5N sodium fluoride solution to serve as carrier for the fluorine were added to the irradiated solution. The fluorine was separated from the irradiated solution by steam distillation. The distillate containing fluorine was collected in a 50ml beaker, and neutralized with sodium hydroxide solution, methyl orange being used as an indicator. Then the fluorine was precipitated in the form of magnesium fluoride by adding 2ml of 1N magnesium chloride solution to the distillate. The precipitate was collected on a small weighed filter paper, and washed with distilled water. The chemical yield was determined by weighing the precipitate after drying. The standard solution was treated in the same manner as before. The radioactivities of 18F were counted with a Hitachi 400 channel pulse height analyzer with 13/4" x2" NaI(T1) crystal. III. RESULTS AND DISCUSSION 1. Purity and Yield The radiochemical purity of the irradiated targets was checked by their g-ray spectra and decay curves. After cooling for 3hr, no activity other than 18F was detectable in the -ray spectra, but after the 18F activity had g decayed away, long-lived impurities could be detected. The major active impurity was Na. The active impurities, such as 24Na, and tritium which had been produced by the 6Li(n,a)3H reaction, as well as the inactive impurities constituted of lithium compounds, were eliminated with the ion exchange columns, as already described. The elution curves are shown in Figs. 1~4. Lithium hydroxide was eliminated with the first H-form cation exchange column. The active impurity 24Na was adsorbed on the H-form resin. Lithium hydroxide was also eliminated by the Co-form cation exchange column. In this case, lithium ions were adsorbed on the resin, and liberated cobalt ions were precipitated in the form of cobalt hydroxide upon reacting with hydroxyl ions. This precipitate was caught in the ion exchange column. Column: 1.5cm diam.x6cm, H-form Eluant: 0.79N LiOH solution containing 18F Flow rate: 1.5ml/min Fig. 1 Elution Curve of F Column: 1.5cm diam.x6cm, Co-form Eluant: 0.79N LiOH solution containing 18F Flow rate: 1.5ml/min Fig. 2 Elution Curve of F Column: 1.0cm diam. x2cm, OH-form Eluant : 1N NaOH solution Material : 18F Flow rate: 1ml/min Fig. 3 Elution Curve of F 29

188 J. Nucl. Sci. Technol. was later eliminated. The 18F activity induced at the end of 1hr irradiation was 0.14mCi/g LiOH. The specific activity of 18F was about 1mCi/mg F. The amount of fluorine was determined quantitatively by a photometric method utilizing the bleaching effect of fluorine ion on zirconiumalizarin lake. The results of the foregoing experiment in 18F preparation are summarized in Table 2. Table 2 Product 18F Column: 1.0cm diam.x5cm, Ag-form Eluant : 0.56N LiCI solution containing F 18 Flow rate : 0.5ml/min Fig. 4 Elution Curve of F In the case of lithium chloride, lithium ions were adsorbed on the Ag-form cation exchange resin of the first column, and liberated silver ions were precipitated in the form of silver chloride upon reacting with chlorine ions. For this reason, the eluate from the first column was obtained in the form of water containing 18F and tritium activities. The sodium hydroxide in the eluate from the second column was eliminated by the third H-form cation exchange column. The chemical yield for the purification of 8F was about 80%. In the case of Co-form 1 ion exchange column, the chemical yield was about 65%. It would appear that a part of the 18F was adsorbed on the cobalt hydroxide. The time taken for the purification of lithium hydroxide and lithium chloride solutions containing 18F were approximately 60 and 30 min respectively. The radioactive impurities in the purified solution were below 1ppm. Tritium activity was measured with a liquid scintillation counter after the other activities had decayed away, but it was not detected in the purified solution. Beg, et al(5). have purified 18F by passing the dissolved oxide through H-form cation exchange column. They report however that large tritium activity was contained in the purified solution. In the method used here, the product was obtained in the form of water containing 18F, and tritium activity 2. Sensitivity of Activation Analysis The self-shielding effect must be eliminated in order to obtain accurate results for activation analysis. The allowable concentration of lithium was determined by irradiating solutions containing natural lithium. The experimental results are plotted in Fig. 5. Fig. 5 18F Activity per milligram of Li It shows that the 18F radioactivity per milligram of lithium is independent of lithium concentration below about 3mg Li/ml. This result agrees with the value reported by Winchester, et al(10). The lithium concentration of samples was therefore held down to about 0.2mg Li/ml to prevent neutron self-shielding. Winchester, et al. have also determined the concentration of 6Li in aqueous solution by neutron activation. In their work, the 18F radioactivity has been counted with a welltype scintillation counter without chemical separation. Radiochemical separation of 18F 30

Vol. 4, No. 4 (Apr. 1967) 189 is however necessary, because the 18F activity induced in the solution of low 6Li concentration is masked by other active impurities such as 24Na and 64Cu. In this study, fluorine was separated by steam distillation from the irradiated solutions and precipitated as magnesium fluoride. The experimental results are given in Fig. 6. Fig. 6 Relationship between 18F Activities per milligram of Li and 6Li Content Figure 6 shows that the lithium solution brings about 18F radioactivity in proportion to the 6Li concentration. These experimental results demonstrate the possibility of 6Li abundance determination by neutron activation. After the separation of impurities, no activity other than 18F could be detected. The distillation curve for fluorine is shown in Fig. 7. The chemical yield for the separation of 18F was about 80% and the sensitivity limit was estimated to be about 4mg. Fig. 7 Distillation Curve of F IV. CONCLUSION 18F was prepared by irradiating lithium compounds in the TRIGA-II reactor. Separation of the 18F from irradiated targets was carried out with the use of ion exchange columns. This method, as adapted in the present experiment, would appear to be an extremely practical means of separating 6Li enriched target, and 18F was purified simply and rapidly with small ion exchange columns of Ag-, OH- and H-forms. The product 18F was obtained in the form of water containing F, probably in the form of HF. 18 A neutron activation method for the determination of isotopic abundance of 6Li was developed. This method is less accurate and sensitive than mass spectrometry, but it is simple and does not require complex apparatus. ACKNOWLEDGMENTS This research was supported in part by a grant in aid for fundamental scientific research from the Ministry of Education. REFERENCES (1) BERNSTEIN,R.B., KATZ,J.J.: Nucleonics, 11 10], 46 (1953). [ (2) BANKS,H.O., Jr.: ibid., 13 [12], 62 (1955). (3) ADAMS,R.M., SHEET,I., KATZ, J.J.: Proc. 2nd Geneva Conf., Vol. 20, p. 219 (1958), IAEA. (4) BRESESTI,M., del TURCO,A.M., OSTIDICH,A.: Radiochimica Acta, 2, 49 (1963). (5) BEG,K., BROWN,F.: Mt. J. Appl. Radiat. Isotop., 14, 137 (1963). (6) STANG,L.G., Jr.: "Production and Use of Short-lived Radioisotopes from Reactors", Vol. 1, p.3 (1963), IAEA, Vienna. (7) SHIKATA,E.: J. Nucl. Sci. Technol., 1[6], 183 (1964). (8) NAGY,G.A., BEREI,K.: J. Inorg. Nucl. Chem., 26 [4] 659 (1964). (9) THOMAS,C.C., Jr., SONDEL,J.A., KERNS,R.C.: Int. J. Appl. Radiat. Isotop., 16, 71 (1965). (10) WINCHESTER,J.W., BATE,L.C., LEDDICOTTE, RNL CF-59-7-127, (1959). G.W.: O 31