Preparation of Neptunium-239 Tracer Claude W. Sill, Health and Safety Division U. S. Atomic Energy Commission, Idaho Falls, Idaho
on the chemistry of would be facilitated convenient radioactive tracer were available. A gammaemitting nuclide is particularly desirable to permit direct counting of solids and liquids in a scintillation well counter without the time-consuming separations to eliminate absorbing materials that are necessary when beta- or alpha-emitting tracers are employed. Neptunium-239 emits principle gamma rays of 0.106, 0.228, and 0.278 m.e.v. that are relatively abundant and easily detected and is easily produced in quantity by irradiation of uranium-238 with thermal neutrons. However, neutron sources of sufficient intensity are not readily available to most laboratories, and the relatively short half life of 2.35 days necessitates repeated irradiations a t frequent intervals to keep sufficient tracer on hand t o sustain a continuing program of significant size. On the other hand, neptunium-239 is also produced by radioactive decay of alpha-emitting americium-243. The short half life of neptunium-239 becomes an advantage in this respect since one half of the equilibrium activity will grow back into the parent in only 2.35 days. Since americium-243 has a half life of 7950 years, a continuing and virtually inexhaustible supply of gammaemitting neptunium tracer is immediately available off-the-shelf by milking a solution of its americium parent. The tracer is completely carrierfree and of greater radiochemical purity than that obtained by neutron activation of natural uranium since separation from only a single nuclide is required. Americium-243 is available from Oak Ridge National Laboratory, Oak Ridge, Tenn. The present paper describes a simple and rapid procedure by which the neptunium tracer can be separated from its americium parent by extraction into long-chain amines (1). The tracer is obtained in a radiochemically pure form in a dilute perchloric acid solution in about 40 minutes, about half of which time is required for evaporation of the final solution and does not require personal attention. An alternative procedure for separating neptunium from irradiated uranium is also given for use when high-flux neutron sources are available or when particularly large quantities of tracer are required. NVESTIQATIONS
I neptunium greatly if a
EXPERIMENTAL
Separation of Neptunium-239 from Americium-243. Shake 50 ml. of 5y0 (v./v.) triisooctylamine in xylene 802
ANALYTICAL CHEMISTRY
vigorously with 5 ml. of concentrated hydrochloric acid and 1 drop of 3OY0 hydrogen peroxide in a 250-ml. separatory funnel (preferably with a Teflon stopcock) for 15 seconds to equilibrate the amine with hydrochloric acid. Discard the lower acid layer. Add 2 ml. of 72% perchloric acid to about 4 /IC. of americium-243 in a 100-ml. beaker and evaporate until only a few drops of liquid remain. Dissolve the perchloric acid in 25 ml. of concentrated hydrochloric acid with gentle warming if any turbidity persists. Cool the solution, add 2 drops of 30% hydrogen peroxide to oxidize the neptunium daughter and transfer the solution to the 250-ml. separatory funnel containing the amine with another 25 ml. of concentrated hydrochloric acid. Shake the funnel vigorously for 1 minute, allow the layers to reparate completely and draw the lower aqueous layer continuing the americium parent into a 250-ml. glassstoppered Erlenmeyer flask for storage until the next milking. Scrub the organic extract for 1 minute with 10 ml. of concentrated hydrochloric acid to remove traces of americium. Save the acid washes to recover the small quantities of americium (about 1% per wash) that are present. Periodically, evaporate the combined washes from several milkings to about 5-ml. volume and add to the main americium solution. Shake the organic extract in the separatory funnel vigorously with 50 ml. of water for 1 minute. Drain the lower aqueous layer into a 250-ml. Erlenmeyer flask, add 3 ml. each of concentrated nitric and 72% perchloric acids and evaporate to strong fumes to oxidize organic matter. If necessary, add 2 or 3 drops of a 1-to-1 mixture of concentrated nitric and perchloric acids to the boiling perchloric acid solution to facilitate oxidation of the last traces of organic matter. Evaporate the perchloric acid to a volume of about 2 ml. and cool. Add 10 ml. of water, transfer the solution to a 60-ml. bottle with a polyethylene-lined screw cap and dilute to any desired volume. The organic extractant can be reused many times. In fact, the organic layer can be left conveniently in the separatory funnel and the next milking cycle begun with the equilibration with hydrochloric acid. Separation of Neptunium-239 from Irradiated Uranium. Allow the irradiated uranium t o stand for at least 2 hours before processing t o permit most of the 23.5-minute uranium-239 t o decay to the desired neptunium-239. It will generally be convenient to let the uranium stand overnight t o permit most of the very short-lived fission products to decay. Place an appropriate sample in a 100ml. beaker and add 1 ml. of 7270 perchloric acid and 1 ml. each of lanthanum
and zirconium nitrate solutions containing 10 mg./ml. of the metal. If metallic uranium was used, add 1 ml. of concentrated hydrochloric acid and 2 drops of 30% hydrogen peroxide, cover the beaker with a watch glass and warm gently until the metal has dissolved completely. Add additional hydrogen peroxide as required to convert any dark color or precipitate of reduced uranium that tends to form to a clear yellow solution. Evaporate the solution to fumes of perchloric acid, remove the cover glass and continue the evaporation until only 1 or 2 drops of acid remain. Cool the beaker and add 1 ml. of concentrated nitric acid, 15 ml. of water and 2 drops of 30% hydrogen peroxide. Heat the solution to boiling and transfer to a 100-ml. Lusteroid centrifuge tube containing 2 ml. of 48% hydrofluoric acid. Rinse the beaker with 10 ml. of water and combine the wash with the main solution. Centrifuge a t 2000 r.p.m. for 1 minute and discard the supernate. Add 1 ml. of the zirconium nitrate solution to the centrifuge tube and swirl vigorously to suspend the precipitate as well as possible. Add 25 ml. of water containing 1 ml. of concentrated nitric acid and 2 drops of 30% hydrogen peroxide and heat the centrifuge tube in a beaker of boiling water until the precipitate dissolves completely. Add 2 ml. of 4870 hydrofluoric acid, mix thoroughly and centrifuge for 1 minute. Discard the supernate. Add 2 ml. of 2.M aluminum chloride and heat the centrifuge tube in a beaker of boiling water until the lanthanum fluoride precipitate has dissolved completely. Do not allow the solution to evaporate significantly or aluminum chloride will precipitate on subsequent addition of concentrated hydrochloric acid. Add 25 ml. of concentrated hydrochloric acid and 3 or 4 drops of 30% hydrogen peroxide and transfer the solution to a 250-ml. separatory funnel containing 50 ml. of 5% triisooctylamine in xylene that has been equilibrated with concentrated hydrochloric acid. Wash the centrifuge tube with an additional 25 ml. of concentrated hydrochloric acid and extract the neptunium as described for the americium solution. RESULTS
Solutions are counted in a 3-inch by 3-inch thallium-activated sodium iodide well crystal in volumes of up to 75 ml. contained in polystyrene counting bottles as described previously (2). I n 50 ml. of concentrated hydrochloric acid, the counting rate of an equilibrium mixture of americium-243 and its neptunium-239 daughter is approximately 2.5 X lo6 gamma c.p.m. per microcurie of americium present.
About 53% of the total gamma counting rate is due to the neptunium daughter. When the americium solution is freshly prepared as described, over 98% of the neptunium-239 is extracted by 5y0 triisooctylamine in 1 minute of vigorous shaking and 99% of the extracted tracer is stripped back into water in the same length of time. A 15% (v./v.) solution of Xmberlite LA-1 (Rohm and Haas Co., Philadelphia] Pa.) in kerosene can also be used with only slightly lower efficiency. Under the same conditions, about 93% of the neptunium is removed in a single extraction, of which 88% is stripped back into water in 1 minute and about 98Oj, in 5 minutes of T.'1'gorous shaking. Consequently, the overall yield of neptunium-239 tracer is about 1.2 x 108 gamma c.p.m. per microcurie of americium taken. If the neptunium239 is in equilibrium with its parent, the tracer from 4 pc. of americium-243 can be diluted to a volume of 50 ml. to give a solution containing l o 5 c.p.m. per milliliter. If it is desirable to milk the solution each day, the tracer should be diluted only t o 12 ml. to obtain the same concentration. Since neptunium-239 will grow back to one fourth of its equilibrium concentration in 24 hours, about 12 runs per day can be maintained indefinitely from 4 pc. of americium-243 a t the very liberal level of 105 gamma c.p.m. per run. This quantity of activity is sufficient to give a relative standard deviation of only 0.6% a t the 95% confidence level in a counting time of only 1 minute. For preliminary or less exacting work, much lower activities can be used, thus greatly extending the number of experiments that can be accommodated per day. For some unknown reason, only about 20y0 of the neptunium was extracted in 1 minute from an americium solution that had been allowed to stand for 2 or 3 months since the last milking. Most of the tracer was recovered by repeated extraction for prolonged times. The solution had a dark brownish color of organic decomposition products that had accumulated during the developmental work that is not a normal condition of the present procedure. Fuming with perchloric acid restores the original high efficiency of the extraction, and can be accomplished conveniently in the glass-stoppered Erlenmeyer flask used for storage. The problem has not recurred on the same solution after standing for 1 month. However, it would probably be prudent to increase the extraction time for solutions 1 or 2 months old and to evaporate the solution to fumes of perchloric acid if the solution is not expected to be used for much longer times. The radiochemical purity of the neptunium tracer obtained is indicated by comparing the gamma ray spectra of the neptunium and americium
ENERGY, Mev Figure 1.
Separation of neptunium-239 from americium-243 1. Equilibrium mixture of 243Amand 32ssNp 2. Final solution of 2aVNp tracer 3. Residual 243Amsolution after two extractions Gamma r a y spectra made at 2.5 k.e.v. per channel
fractions with the spectrum of an equilibrium mixture of both shown in Figure 1. No abnormalities could be detected in the spectrum of the neptunium fraction that could possibly be attributed to the americium parent. Even more significantly, the neptunium fraction was followed for 10 half lives without detecting any deviation from a 2.35-day half life that was statistically significant. After 99.9% of the neptunium-239 had decayed, no trace of americium-243 could be detected by gamma spectrometry on the remaining 0.1% of the original activity. The americium fraction was extracted one additional time to remove the last traces of neptunium and the spectrum was taken immediately so that a gamma spectrum of americium-243 itself could be obtained without abnormalities introduced by traces of the neptunium daughter. The spectra have been displaced vertically for clarity of presentation and should not be compared quantitatively.
To check the second procedure given above, 9.7 mg. of natural uranium metal was irradiated in the Materials Testing Reactor for 300 seconds in a thermal flux of 1.3 X l O I 3 neutrons per square centimeter-second. After cooling the sample overnight, 54 X 10@ gamma c.p.m. was obtained, calculated back to the end of irradiation. Such large quantities of neptunium tracer should normally be diluted only to 25 ml. and further dilutions made in 1% perchloric acid as needed. The more concentrated stock solution can then be used to replenish the working solution as the latter is depleted by the relatively rapid rate of decay. Since the quantity of neptunium-239 produced during relatively short irradiation times is proportional to the product of neutron flux, length of irradiation and quantity of uranium irradiated, or 3.8 X 1016 milligram-neutrons per square centimeter for the experiment described, the quantity obtained amounts to 1.4 X gamma c.p.m. for each milligram-neutron per square centimeter used. This empirical relationship is useful in estimating the VOL. 38, NO. 6, MAY 1966
803
quantity of tracer obtainable from other combinations of flux, time and mass. The gamma ray spectrum of the final neptunium-239 product was identical to that shown in curve 2 of Figure 1 and showed no significant activity in the region above 0.4 m.e.v. for about 2 additional decades below the abscissa shown. Since the gamma rays of many of the principle fission products present in l d a y cooled material are located in this region, the decontamination obtained in the procedure is obviously excellent. Decay of the final neptunium 239 fraction was followed for 11 half lives without detecting any deviation from a 2.35day half life that was statistically significant. Zirconium-97 was detected initially in the neptunium fraction but the quantity present was only a few hundredths percent of the neptunium-239 activity and decayed off in a few days. After 99.95% of the
neptunium-239 had decayed, zirconiumniobium-95 and cerium-141-144 were detected in the residual solution in quantities about equal to the minute quantity of neptunium-239 still remaining. Gamma spectra of all fractions discarded during the separation indicated that the overall recovery of neptunium-239 was about 957,. A run made on unirradiated uranium in the presence of 2 x lo5 c.p.m. of neptunium-239 tracer gave a recovery of 95.8’% in the final neptunium solution with about 1% in each of the 2 hydrofluoric acid supernates, the main aqueous hydrochloric acid solution, and the residual organic extractant. Similar quantities of neptunium-239 tracer can be produced with much lower, but more widely available, neutron fluxes by increasing the quantity of uranium and the duration of irradiation used. When 500 mg. of natural uranium metal was irradiated for 60 minutes in a
thermal flux of 2.5 X 1O1O neutrons per square centimeter-second, 34 x lo6 gamma c.p.m. was obtained as of the end of irradiation, corresponding to 7.6 x 10-10 gamma c.p.m. for each inilligram-neutron per square centimeter used. This rate of production is only half that obtained with the smaller quantity of uranium, possibly due either to self shielding or to differences in the effective neutron flux in the different positions. Yo additional problems were encountered due to the larger quantity of uranium irradiated for a longer time, and the final product was of nearly identical radiochemical purity. LITERATURE CITED
(1) Moore, F. L., “Liquid-Liquid Ex-
traction with High-Molecular-Weight Amines,” National Academy of Sciences, Nuclear Science Series, NAS-NS 3101 (1960). (2) Sill, C. W., Willis, C. P., ANAL.CHEM. 37, 1176 (1965).
Direct Dumas Determination of 30 to 1000 Parts per Million of Nitrogen in Oils 1. J. Oita, Research and Development Department, American Oil Co., Whiting, Ind. HE DETERMINATION of nitrogen in T p e t r o l e u m products has become increasingly important in recent years because of the development of nitrogenous additives for lubricating oils, interest in shale oil technology, and the harmful effects of nitrogen in catalytic cracking and reforming. Various modifications of the Kjeldahl method have been used for oil samples, and nitrogen content can be determined with conventional techniques down to 20 p.p.m. However, samples containing less than 20 p.p.m. nitrogen must be concentrated and the method becomes rather tedious. Also, the long digestion time is a disadvantage when the nitrogen content must be known quickly. Furthermore, if the sample contains compounds with nitrogen-nitrogen or nitrogen-oxygen linkages, low results will be obtained. The conventional Dumas method, while rapid and suitable for all types of nitrogen compounds, is limited to samples containing a t least 0.2% nitrogen. Although the combination of gas chromatography with a Dumas-type combustion system has certain advantages, none of the commercial apparatus is well suited for oil samples containing less than 100 p.p.m. nitrogen. Recently, Gustin’s (3) automatic Dumas apparatus, which is available commercially as the Coleman Nitrogen Analyzer Model 29, has been used to determine 0.2’% nitrogen in oils
804
ANALYTICAL CHEMISTRY
directly and down to 20 p.p.m. after a concentration step (2). Now Coleman has a new nitrogen analyzer, Model 29A, (1) which has larger combustion tubes, a higher temperature furnace, and a larger capacity nitrometer than the Model 29, so that it is better suited for combustion of large samples. By combining the small nitrometer of Model 29 with the combustion tube and the high temperature furnace of Model 29A, and by slightly modifying the combustion program, down to 30 p.p.m. of nitrogen can be directly determined in oils. EXPERIMENTAL
Apparatus. Modified Coleman Nitrogen Analyzer Model 29A. The 50-mi. nitrometer syringe was replaced with the 5-ml. syringe from the Model 29. The purge time was increased from 40 seconds to 100 seconds by adjusting cam number 3 of the combustion cycle control. (Cam number 1 is farthest away from the front panel.) The second half of cam 3 is rotated so that the space between the two halves is wide enough to prolong the purge time to 100 seconds. The auxiliary timer was set to control the preheat section of the cycle rather than the combustion period. This was done by rotating cam 7 so that it is activated a t 150 seconds after the start of the cycle. The space between the two halves of cam 7 should be just wide enough to accept the roller of microswitch 7. All of these changes can be
made on the equipment by the manufacturer. Copper oxide containing a small amount of platinum catalyst is used in the combustion tube. The post heater tube is filled only with metallic copper. Both reagents are available from the Coleman Instrument Co. Procedure. There are three different procedures for the three levels of nitrogen content. Materials containing more than 0.2y0 nitrogen are analyzed according to the directions in the operating manual. The sample should not be larger than 100 mg. For samples containing 0.20-0.05~0 nitrogen, weigh out about 400 mg. into a 2-inch ceramic or nickel boat, and place the boat in the tube so that it will rest 0.5 inch higher than the lower furnace. Fill the tube with copper oxide. The upper furnace should be below 400’ C. and the lower furnace should be a t 850’ C. at the start of the combustion. If more than 400 mg. is taken, keep the sample zone cold with a dry-ice-filled cooling block during the purge and preheat period. Using the auxiliary timer, extend the preheat time by 3 minutes. Apply maximum power to the upper furnace until it reaches 850’ C., and then adjust the power to an intermediate setting that will hold the temperature between 85O0-9OO0 C. Using the cycle delay switch, lower the upper furnace slowly until it reaches the lower furnace. The total lowering time depends on the sample size and volatility. A 400-mg. sample takes 12 minutes. After the upper furnace reaches the lower furnace, the rest of the procedure is done automatically.
