235
Neutron Activation Analysis for U , Especially in Limestones, by Measurement of Xe133 LARRY A. HASKIN,' HAROLD W. FEARING, and F. S. ROWLAND Department o f Chemistry, University o f Kansas, lawrence, Kan.
b The need for accurate knowledge of uranium concentrations in limestone sediments selected for geologic dating by the thermoluminescence method has prompted development of a new procedure for uranium analysis by neutron activation. The 5.27-day Xe133,formed by the thermal neutron fission of U23: is isolated from most fission product contamination by combustion of the limestone with tantalum. It is then separated from other noble gases by gas chromatography, and assayed by gas proportional counting. This techniqwe, suitable for general application to uranium in most materials, is sensitive to approximately 10-lo gram with a precision of *4%. The technique can be adapted readily for other neutron activation analyses which involve measurement of noble gas radioactivities.
A
of the low concentrations of uranium present in limestone sediments is necessary for geologic dating by the thermoluminescence method (IS), and for general interpretations of the geochemistry of uranium. Neutron activation analysis is well suited for the determination of Uu5 through isolation of particular fission products, and has also been used for U2w through isolation of both Np239 (6, 8) and U239 (6). Fissionproduced BalM was first used (6, 10) for the determination of U236,and subsequently, Ebert, Konig, and Wanke have used Xe133, purifying the xenon by adsorption on charcoal (4). All of these neutron activation techniques are sensitive to about 10-9 to 10-11 gram of uranium with a precision of about 5 to 10%. Our experiments have been designed to develop an improved technique based on Xels3. Among the general procedures described in the literature for determination of submicrogram amounts of uranium, two others are important: colorimetry (18) and fluorometry (3). Maximum sensitivity for fluorometry CCURATE MEASUREMENT
1 Present address, Department of Chemistry, University of Wisconsin, Madison,
WiS.
1298
ANALYTICAL CHEMISTRY
is in the range of to 1O-lo gram with a precision of 4 to 10%. The sensitivity of the colorimetric procedure is 0.5 pg. of uranium, with a precision of approximately 5%. Quantitative separation of the uranium from the sample prior to determination, as well as careful purification from other elements, is required for both. The neutron activation method described here offers comparable sensitivity and precision to the above-mentioned techniques, plus minimum interference by other elements, and simple, quantitative chemical procedure. I t involves fission of U235to form fission products including 5.27-day Xe133; combustion of the sample with tantalum, removing all noble gas radioactivities; when necessary, gas chromatographic separation of the rare gases, the xenon being trapped directly in a gas proportional counter; and comparison of the XeIS3activity from the sample to that from a simultaneously irradiated standard. This method, as with all those based on fission product measurement, measures the U235content of the sample, and total uranium can be determined only for the natural isotopic mixture, or when the relative isotopic abundances are otherwise known. EXPERIMENTAL
Irradiations. Finely ground 0.5- to 3-gram samples of the material to be analyzed were weighed into quartz irradiation tubes. Preliminary experiments had shown that an important fraction of noble gas formed inside the samples escapes from the powdered material during irradiation, either by the initial mass-133 fission recoil or by subsequent diffusion. The quartz tubes, 8 mm. I.D. and about 10 cm. long, were equipped with fine capillary break-off tips to facilitate their opening in vacuo, and were wrapped in aluminum foil to protect these tips during irradiation and transportation. Generally, no attempt was made to exclude air in these tubes, as activation of the Xe132in air formed much less than 0.1% of the total Xe1S8activity. Several quartz tubes were packaged together for simultaneous irradiation, and normally included appropriate standards of known uranium concentration. Care must be taken nbt to
irradiate the samples and standards in regions of rapid variation of flux with position. Self-shielding of the samples during neutron irradiation is negligible unless the bulk sample material includes elements of high neutron cross section. The irradiations were carried out a t either the Brookhaven or the Argonne National Laboratory, in neutron fluxes up to approximately l O I 3 neutrons per sq. cm. per second for 4 to 12 days. Usually Cday irradiations were carried out to maximize the Xe133 relative to possible interfering activities. Normally, a pile location with a low fast neutron/thermal neutron ratio was selected to reduce the fast neutron production of some other noble gas radioactivities (Table 11). The Xe133is not a primary fission product, but results from the beta decay of the earlier numbers of the mass-133 chain. The characteristics of this chain are summarized elsewhere (7).
Radiochemical Purification. CouWITH TANTALUM. After allowing sufficient time lapse for the mass-133 chain to decay through the IlSs to Xe133, each irradiated tube was broken open inside a vacuum line in an atmosphere of xenon carrier (approx. 5 cc. STP). If the decay of 1 1 3 3 was not complete, the samples and the standards were combusted simultaneously. The xenon carrier vias necessary to prevent subsequent loss of the radioactivity by adsorption on the surface of the combusted sampletantalum mix.ture. After a 5-minute gas mixing period, the xenon and other condensable gases were then frozen into a sidearm of the sample holder by cooling with liquid nitrogen. The sample holder was kept free from any constrictions to permit diffusion of the xenon in the presence of noncondensable gases, hence allowing rapid condensation of the entire sample. The noncondensable gases were then pumped off, and the condensable gases were expanded and recondensed into a storage bulb. The sample holder was opened to the air, removed from the vacuum line, and the solid material was transferred to a combustion tube. Samples were combusted in tubes made from Pyrex 1720 glass, which has high mechanical strength a t 700" C., and which has been used extensively in the combustion of tritium samples (9). If necessary, the combustion temperature could be increased readily by using quartz tubes. About 20 times the sample weight of tantalum metal powder was then added BUSTION
to the combustion tube, and the tube was drawn to a thick-walled capillary, attached to the vacuum line above the capillary, and pumped out to approximately 0.3 mm. of Hg pressure. The condensable gases, including the xenon carrier, were then released from the storage bulb, and recondensed inside the combustion tube. The tube was then sealed off, and the powdered contents were mixed. The samples were then heated for 8 to 20 hours a t 700" C. The primary purification from most of the radioactive impurities formed in the irradiated samples occurred in this combustion step. Little or no volatile material remains uncombined with the tantalum metal except the noble gases, and the latter are thus obtained in good radiochemical purity. When insufficient tantalum was added during the combustion, large amounts of condensable gases accompanied the noble gas radioactivity, even through the subsequent gas chromatographic separation, and interfered with the analysis. After combustion, each tube was opened in the sample holder and the xenon was condensed into an evacuated Bernstein-Ballentine type internal gas proportional counter ( 2 ) cooled with liquid nitrogen. When. separation from other noble gas radioactivities was required, the xenon carrier was condensed instead into the sample injection loop of the gas chromatography apparatus. Noble G a s Radioactivities. Neutron irradiations of limestone sediments and other materials can create a variety of noble gas radioactivities. Several possible sources of Xe133, including the neutron fission of U2a6, are summarized in Table I. Table I1 lists the most significant other noble gas radioactivities for these activation analysis experiments. When amounts of other noble gas radioactivities were sufficiently large that the Xeis3 could not be obtained by decay curve analysis, xenon was purified further by gas chromatography, which reduced the interfering radioactivities solely to isotopic xenon activities. The interference from Xe131m can be minimized, if desirable, by performing the initial separation of xenon while most of the mass-131 fission chain is still present as 8.0-day Similarly, if Xei33 from the nonfission sources listed in Table I interferes seriously, it can be eliminated completely by performing an initial tantalum combustion of the sample while the mass-133 chain is still primarily P3,discarding this xenon fraction. A second heating after a suitable time delay would then isolate XeIS3 arising solely from the fission process. In neutron activation of limestone, a very large quantity of Ar37 is formed, and the gross decay curve observed with the gas proportional counter is essentially that of the 35-day argon activity. For these samples, the gas
chromatographic separation was used routinely. G a s Chromatographic Separation of Noble Gas Radioactivities. Xenon was separated from argon readily by passage of the noble gas mixture through a 10-inch column of 13 A. Molecular Sieve a t room temperature ('/a-inch tubing, 180 ml./minute flow rate of He). The argon activity eluted 4 seconds after injection, and the xenon appeared a t about 30 seconds. Just prior to the xenon peak, the effluent gas stream was diverted into an evacuated liquid nitrogen-cooled proportional counter. After the xenon peak had completely passed, the effluent stream was returned to its original outlet and the counter, still a t liquid nitrogen temperature, was then moved
Table 1.
Tsrget
to the vacuum line; most of the helium carrier gas was pumped off. Assay of Radioactivity. The internal gas proportional counters used in these experiments have a total volume of 100 cc., and an active counting volume of 85 cc., both volumes being accurate to A l % . Propane counting gas to a total pressure of 40 cm. of Hg was added to the xenon carrier gas plus radioactivity already present in the counter from either the combustion or the gas chromatography step. The counting plateaus for all of the 20 counters used in these experiments are essentially identical, and all of the assays were made a t 34-00 volts. Previous use of these counters for tritium analysis has shown that counting of identical samples in any of the coun-
Sources of Xe13S in Neutron lrradiaiions
Approx. Cross Section, Barns
Reaction
Natural Abundance of Reacting Isotope, %.
Permissible Atom Ratio5 Element/U
0.72 26.89 99.3
...
u236
B.lax.
4
80
Xe133yield from element will be 5% of yield from Uz3b (n,f) Xe'3' in a neutron flux with a fast neutron flux 1/40 of thermal flux. (I
Table II.
Noble Gas Radioactivities That Interfere.with Xe1a3Measurements
Isotope
Decay Curve Interference, Daysa
Kfl6 Xeiaim
Half Life Fission Yield, yo FISSION PRODUCT RADIOACTIVITIES 1 0 . 3 years 0.293 12 days 0.023
55 55
OTRERRADIOACTIVITIES
rlrn Ar39 W
6
XelPQnl Xelalm Rnz29
35.0 days 265 years 1 0 . 3 years 8.0 days 1 2 . 0 days 3.825 days
Reaction ArYn, Y ) &"(fast n, a) 7)
K*O(faatn, p ) Krs4(n, Y ) Rhs'(fast n, p )
Srss(fa8tn a) XelS(n, TI Xeia(n, 7 ) Ra%
Xatural Abundance of Reacting Isotope, % 0.337 96.97 0.063 93.08 56.90 72.15 82.56 1.919 4.08 I . .
a Time after irradiation at which specific activity of this isotope would b e 10% pf the specific activity of Xe133. Assumptions: 4day irradiation, same counting efficiency, and no chemical separation.
Table 111.
Typical Analyses for Uranium in Limestones by Neutron Activation to Produce Xe133 % No. of U, P.P.M. Sample Recoil Aliquots C.P.M./Gram 1520 f 60 NBS carnotite 31 6 3 . 2 3 X 1W Limestone P E 7 4.2 3 1.10 x 104 5 . 1 f0 . 1 4.1 f0.1 Limestone PD-2 1.7 3 8.71 X 10' Limestone PC-4 2.4 3 10.0 x 10' 4.7 f0 . 4 10 2 8.63 x 103 4.1 f0 . 1 Limestone AA-11 4.5 2 5.95 x 10' 2.8 f0.1 6.5 f0.5 Limestone PD-1 3.4 2 1 . 3 8 x 104
VOL 33,
NO. 10, SEPTEMBER 1961
1299
r
\
w
l
-
a W
3\ 1O2'io
'
'
40 DAYS SINCE
50 '
' " 60
'
'
70
'
80 '
'
END OF IRRADIATION
Figure 2. Xenon from combustion of limestone No. 33A l I
DAYS
Figure 1.
l
l
50 SINCE
l
l
60
0 Observed activity X Observed activity minus long half life; short half life -5.9
l
70
days
END OF IRRADIATION
Recoil xenon from carnotite No. 1 1 R
Observed half life of 5.7 days compared to expected curve far mixture of Xe133 and Xe'31"
ters a t random gives a precision of approximately &2%. RESULTS AND DISCUSSION
A summary of the experimental results from several limestone analyses and an analysis of a National Bureau of Standards carnotite sample are shown in Table 111. In these samples, the precision of the experiments on a particular sample was estimated by altering the experimental procedure as described below. The xenon activity isolated Drior to combustion was transferred directly to a proportional counter and assayed, instead of being transferred to a storage bulb for addition to the combustion tube. This sample then gave the percentage measurements of the activity found in the gas phase by a combination of recoil and diffusion procemes, given in Table I11 as the per cent recoil. The irradiated solid material, after withdrawal of the recoil fraction, was divided into several aliquots, and each of these was assayed separately for xenon activity. The precision indicated in these experiments is that obtained among the various aliquots of irradiated solid material. The working precision of the analyses is in the range of f5% for these samples. 1300
ANALYTICAL CHEMISTRY
A separate test was made of the overall precision in the gas handling on the vacuum line, gas chromatography, and proportional counting. An irradiated pitchblende sample of high Xelsa activity was combusted, and introduced into the vacuum line. Successive aliquots of constant size were then either taken off directly into a proportional counter or into the sample injection loop of the gas chromatographic apparatus. Each of the latter aliquots was then purified by gas chromatography and diverted into a proportional counter for assay. The samples passed directly into the counters gave an average specific activity of 1714 f 45 c.p.m., and those purzed by gas chromatography gave an average specific activity of 1696 + 21 c.p.m., in excellent agreement. The larger scatter in the results for the limestones themselves also includes any natural inhomogeneity in the uranium distribution through each limestone. The precision of 5% is approximately that obtained by others in analyses of uranium in limestones. The activity observed in the NBS carnotite of Table I11 was within 3% of the calculated activity expected for Xe138, from the known conditions of neutron flux, cross section, and decay constants. However, because of the
possibilities of alterations in neutron fluxes in particular pile positions, the uranium concentrations in the irradiated limestones have always been calculated from the relative specific activities of the limestone and of an NBS standard carnotite or pitchblende included in the same irradiation package. Two typical decay curves us. time are shown in Figures 1 and 2. Figure 1 shows a decay curve for a pure xenon sample as the percentage contribution of Xelalm gradually becomes more important-the observed half life is slightly longer than that of pure Xe133. Figure 2 shows the decay curve for a xenon fraction in which not all of the activity has been separated, and the 5-day decay tails off into the longlived activity. The clean gas chromatographic separation of krypton and xenon is difficult under the rapid flow conditions and short retention times used in our arrangement, and an activity tail of K F is not unexpected. A longer column of Molecular Sieve or a lower temperature would facilitate the separation, but the catching of the broadened xenon peak in an evacuated proportional counter would be more difficult. Ultimate Sensitivity. One limit on the sensitivity of this method is, of course, the production of enough Xe138 to be determined accurately by counting. Under the conditions of analysis described in this paper, gram of uranium was equivalent to 25
c.p.m., over a background of 70 c.p.m., which could be lowered to 20 c.p.m. with a suitable anticoincidence arrangement. By using substantially higher neutron fluxes, and counters with lower backgrounds, a sensitivity to about lo-" gram of uranium should be feasible. The lower background counters would in all probability have far less volume than those used in this work, and would entail an additional transfer step from a gas chromatographic trap to the counter. Other Applications. Since the basic technique should work equally well with other rare gases by using appropriate trapping procedures for each gas, activation analysis plus tantalum combustion and 'gas chromatographic purification should be applicable to all substances capable of producing a noble gas radioactivity under irradia-
tion. For example, A r N could be activated and separated by this method for determinations of K4-Ar" for geologic dating. Similar methods have been applied recently to potassiumargon ages (1) and meteorite dating (11). LITERATURE CITED
(1) Armstrong, R., Abstracts of Geo-
chemical Society, Pittsburgh, Pa., Nov. 2-4, 1959. ( 2 ) Bernstein, W., Ballentine, R., Rev. Sa.Instr. 21, 158 (1950). (3) Bvrne. J. T.. ANAL.CHEM.29. 1408 \ - - - .3 -
(4) Ebert, K., Konig, H., Wanke, H., 2.Nafurforsch. 12a, 763 (1957). (5) Hamamchi. H., Reed, G., Turkevich,
A.. Geoihim.' et .Cosmochim. Acta
12;
337 (1957).
(6) Jervis, R.,.Mackintosh, W., Proc. 2nd United Nations International Confer-
ence on Peaceful Uses of Atomic Energy, 28,470 (1959'). (7) Katcoff, S., Nucleonics 18, No. 11,
201 (1960). (8) Mahlman, H., Leddicotte, G., ANAL. CHFM.27,823 (1965'). (9) Rowland, F. 5. Lee, J. K., White, R. M., U. S. At. hnergy C o r n . , Rept. TID-7578,p. 39 (1960). (10) Smales, A. A., Analyst 7 7 , 778 (1952). (11) Stoenner, R., Zahringer, J., Geochim. et Cosmochim. Acta 15,40 (1958 (12) Tatsumoto, M., Goldberg, lk, Zbid., 17,201 (1959). (13) Zeller, E., Wray, J., Daniels, F., Bull. Am. Assoc. Petrol. Geologists 21, 121 (1957). RECEIVED for review February 23 1961. Accepted May 1, 1961. Research s u p ported by A. E. C. Contract No. At( 11-1 )407. Division of Physical Chemistry,
136th Meeting, ACS, Atlantic q t y , N.J., September 1959. Submtted in partial fulfillment of the requirements for the Ph.D. degree by Larry A. Haskin, University of Kansas, 1960.
Radiochemical Determination of Isotopic Thorium in Uranium Process Streams HENRY G. PETROW, BERNARD SO",
and ROBERT J. ALLEN
lonics, Inc., Cambridge, Mass.
b The determination of thorium in uranium mill effluents requires an accurate sensitive method, free of interference from cationic and anionic impurities. The method developed is valuable to the determination of natural thorium and thorium-230, and can be adapted to allow for the determination of other thorium isotopes. Sensitivity and accuracy are good, and only titanium interferes sufficiently to require modification of the procedure. The method can be used for aqueous and solid samples and only conventional counting equipment is required.
I
of interest with regard to radioactive pollution by uranium mill waste streams are thorium-230, and to a lesser degree, natural thorium. There is not available currently a simple reliable method which is applicable t o thorium determination a t tracer levels. The method of Moore (4) is cumbersome when applied to a complex mixture such as uranium mill effluents. Direct extraction by thenoyltrifluoroacetone (TTA) has been recommended (2, 3), but severe inhibition of thorium extraction into TTA by phosphate, fluoride, and even sulfate occurs. SOTOPES
The method described gives satisfactory results on aqueous samples from carbonate and sulfuric acid leach liquors, as well as solubilized ore and residue samples. Excellent separation of thorium from radioactive and stable interferences is effected by a series of solvent extraction steps utilizing primary and tertiary amines. The use of amine extractants has been reviewed by Coleman et al. ( 1 ) . Solvent extraction with a tertiary amine from dilute sulfuric acid removes uranium, zirconium, and molybdenum. Solvent extraction from the same medium with a primary amine removes thorium, but not radium, lead, bismuth, protactinium, actinium, iron (11), vanadium(IV), calcium, magnesium, aluminum, alkali metals, manganese(II), or rare earths. Phosphate, fluoride, and sulfate anions do not inhibit thorium extraction, even when present in large concentrations. The concentration of nitrate, chloride, and perchlorate must be controlled, but concentrations far in excess of those normally encountered can be tolerated. Titanium, if present, follows thorium quantitatively, and special allowance must be made for titanium-bearing samples. After thorium extraction by the primary amine. thorium is re-extracted or
stripped into BM hydrochloric acid. The hydrochloric acid strip solution is scrubbed with a tertiary amine to remove traces of interferences which may have accompanied the thorium. After evaporation of hydrochloric acid, and oxidation to destroy organic material, thorium is taken up in nitric acid. Thorium-230 is then determined by radioassay and natural thorium is determined colorimetrically. APPARATUS AND REAGENTS
The recommended detection equipment consists of a 2~ proportional flow counter, capable of accepting %inch diameter planchets. coupled to a suitable scaler. Alamine 336, tricaprylyl amine, 10% by volume dissolved in benzene. The solution should be washed with an equal volume of 0.5M sulfuric acid prior to use. Alamine 336 can be purchased from the General Mills Co., Kankakee, Ill. Primene JM-T, a mixture of Cta to Czs aliphatic primary amines, 10% b y volume in benzene. This solution should be washed with an equal volume of 0.5M sulfuric acid prior to use. Primene JM-T can be purchased from R o b & Haas, Philadelphia, Pa. Alamine 336, 10% by volume in benzene. This solution is used without prior acid washing. VOL. 33, NO. 10, SEPTEMBER 1961
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