interaction parameters, although interesting, would be quite difficult to perform. It is the author’s feeling that of all the possible interaction modes in the detector, the formation of Cerenkov radiation in the tube face is the most predominate.
The effect of y-photon sensitivity is clear. It should be noted, however, that photons coincident with beta emission cannot be resolved in time by the counter system. Thus, the absolute assay of /3-y emitters is not complicated by the presence of the y component. There is a small but finite probability that gamma or X-ray photons will interact directly with the photocathode of the detector to liberate electrons. The end result of this process is identical to that observed from Cerenkov events. Thus, for photons sufficiently energetic to participate by both interaction modes, it is not possible to determine the relative efficiencies of the two routes. However, when the extremely-small density thickness of the photocathode is considered, the direct interaction probability appears to be vanishingly small. There are other possible reactions in the photomultiplier tube that may For example, complicate the response interpretation. Compton electrons formed in the face of the tube may escape into the tube envelope and be counted as a photoelectron. Energetic beta particles could also enter the tube and be similarly counted. The exact measurement of each of these
CONCLUSIONS
Most of the experimental results presented above are self explanatory and require little in the way of interpretation. It is clear that the Cerenkov counting technique can be an important adjunct to conventional liquid scintillation procedures. Among the major advantages of the method are extreme simplicity of sample preparation and the ability to count samples in a completely aqueous system without the use of organic fluors. For optimum coincident detection efficiencies, a new optical geometry appears desirable. However, any current liquid scintillation counter can be used for efficient Cerenkov measurements on a wide variety of and P-y emitters.
RECEIVED for review January 27, 1969. Accepted May 26, 1969.
Neutron Activation Analysis of Uranium in Geological Material by Measuring TelIurium- 132 A. D. Suttle, Jr., Barbara C. O’Brien, and D. W. Mueller Department of Chemistry, Texas A 6 M Uniuersity, College Station, Tex. 77843 A simple and sensitive method has been developed for determining submicrogram quantities of uranium in geological material using neutron activation. A fission product of U235, Te132, is separated radiochemically from the irradiated sample, determined by gamma spectrometry, and related to the uranium concentration of the original material. The radiochemically separated Te132emits a high intensity gamma ray at 230 keV which is free from interference from any other rad ionuclide. Under existing ir rad ia ti on conditions (3 X 10l2 n cm-2sec-1 for 10 hours), 50 nanograms of uranium can be determined with a precision of +3.5%. The proposed method has the advantages of precise determination of submicrogram and microgram quantities of uranium in geological samples with a minimum of specia Iized equipment.
NUMEROUS METHODS have been reported for determining uranium in a wide variety of natural materials ( I ) . However, when considering materials in which the concentration of uranium is in the range or parts per million to parts per billion, methods for determining uranium with high precision are few; fluorophotometry, spectrophotometry, controlledpotential coulometry, isotope dilution, and activation analysis ( I ) . Of these, activation analysis is the only method which does not require the quantitative separation of uranium from the bulk of the sample and from interfering elements prior to analysis. Naturally-occurring uranium consists of three isotopes, U234 (0.006%), UZ3j (0.72%), and U Z 3(99.274%). * Nuclear reactions of these uranium isotopes with thermal neutrons (E < 0.2 eV) and with fast neutrons (E > 0.2 MeV) are fully (1) I. M. Koltoff and Philip J. Elving, Eds., “Treatise on Analytical Chemistry,” Part 11, Vol. 9, Interscience Publishers, Inc., New York, N. Y . , 1962.
described in the literature (2). It is the reaction of the uranium-235 in geological samples with thermal neutrons resulting in fission and the production of radioactive isotopes of more than 30 elements which is utilized in the present work. Because the fission yield of each of these products is a reproducible quantity, determination of a fission product is directly related to the original concentration of U235. Furthermore, assuming the isotopic abundance of UZ3j in natural samples is constant, determination of U235is readily extended to the determination of total uranium in these materials. Essential to such an analytical procedure is that the radionuclide determined is not produced in significant yields by any other nuclear reaction. Fission products which have been used in uranium analysis of geological samples include barium-140 (3), xenon-I 33 (4), and tellurium-132 (5). The reported method using suffers from poor sensitivity (50 ppm) and involves the quantitative separation of uranium in the mineral sample prior to irradiation by coprecipitation with ferric hydroxide. An important advantage of determining uranium by activation analysis is that the uranium, or in fact any fission product, need not be quantitatively separated from the sample. Samples which are more than 50 ppm U and from which the uranium is quantitatively separated could probably be determined more conveniently by some other method. None-
’
(2) Murrey D. Goldberg, Said F. Mughabghab, Surendra N. Purohit, Benjamin A. Magurro, and Victoria N. May, “Neutron Cross Sections,” Vol. IIB, Brookhaven National Laboratory, Associated Universities, Inc., Upton, N. Y . , 1966. (3) A. A. Smales, Andyst (London),77,778 (1952). (4) Larry A. Haskin, Harold W. Fearing, and F. S. Fowland, ANAL. CHEM., 33, 1298 (1961). (5) C. Fisher and J. Beydon, BUN. SOC.C/zim. Fr., 11, C102 (1953). VOL. 41, NO. 10,AUGUST 1969
1265
Table I. Determination of Uranium in Analyzed Samples by Proposed U23sFission Method U , micrograms NBL desig. Description Taken Found Dev., 1 Phosphate rock 10.5 10.2 2.5 1 Phosphate rock 20.5 20.3 1 .o 1 Phosphate rock 12.6 12.9 2.1 5 Carnotite 10.9 11.2 3.0 5 Carnotite 46.9 48.9 4.2 75 77 77 77 77
O.O5l%U O.Ooll%U 0.Ooll~U 0.Ooll~U 0.Ooll~U
uranyl acetate uranyl acetate Q
20.5 11 .o 5.52 11.0 22.2 10.1 124
19.7 13.6 7.10 13.6 28.4 10.4 121
3.9 23.6a 28. 6a 23. 6n 27.9a 3.2 2.4 mean 2 . 7
Not included in calculation of mean.
theless, Te132appears to be the nuclide of choice. It is produced in adequate cumulative fission yield (4.3%). It has a half-life of 78 hours which allows ample time for careful radiochemical separation and, at the same time, is sufficiently radioactive to give good sensitivity without prolonged irradiations. Tellurium-132 emits a high intensity gamma ray of 230 keV (6). Gamma-counting is desirable because small variations in thickness of a solid sample do not affect the precision of the results. Finally, the radiochemical separation of Te132 from interfering nuclides and determination of its chemical yield is simple (7, 8). Aside from the advantages of its shorter half-life, tellurium-132 is superior to the fission product barium-140 ( t l 1 2= 12.8 days) which has been used extensively to determine uranium in geological materials. Because the gamma ray spectrum of Ba140is too complex for single channel gamma-counting, beta-counting is necessary. Beta-counting entails the preparation of a geometrically uniform sample and frequently corrections must be made for absorption and scattering of beta particles by the sample. The radiochemical purity of the barium sulfate precipitate must be checked by following the decay of Balao and by beta-energy measurement. Furthermore, radiochemical separation of Ba140involves many more steps than the method which will be described for separating radiochemically pure Te132. EXPERIMENTAL
Accurately weighed portions of finely ground samples are enclosed in fingers of light-weight polyethylene gloves. Sample sizes range from 0.1-1 gram depending upon the concentration of uranium. Each packet is then sealed into a small plastic vial (1.5 ml) and irradiated from 10 to 14 hours at a flux of approximately 3 x 1012 n sec-l cm-2. This flux produces 3.17 X lo4 fissions per microgram U per second. These irradiations are carried out at the Nuclear Science Center, Texas A&M University. As a further precaution, an iron wire is placed in each vial to monitor any flux varia-
tions among the samples irradiated at a given time. Following irradiation, short-lived nuclides are allowed to decay for a minimum of 20 hours before chemical separation is begun. Samples of irradiated sediment taken from the Gulf of Mexico are extremely active due to sodium-24, and these samples are allowed to decay for at least 36 hours in order to reduce the hazard to the laboratory worker. The packet consisting of the irradiated sample in the polyethylene finger is transferred to a nickel crucible containing 55 mg solid tellurium carrier, Te(OH)B (K&K Laboratories, Plainview, N. Y.),is covered with a tenfold excess of sodium peroxide and is fused at 650 “C for 30 minutes. The cooled melt is transferred with water to a 500-ml flask and dissolved completely with hydrochloric acid. The crucible is rinsed with 6N hydrochloric acid and the washings are added to the dissolved melt in the flask. The solution is adjusted to 3-4N HCl and heated almost to boiling. The tellurium in solution is reduced to tellurium metal with sulfur dioxide and hydrazine dihydrochloride (8) and collected on medium grain fritted glass. A few drops of 0.1% Aerosol OT Solution (Fisher Scientific Co., Fair Lawn, N. J.) is used as a coagulant to render the tellurium filterable. The separated tellurium is dissolved with concentrated nitric acid and the solution is evaporated almost to dryness. The residue is taken up in three successive portions of concentrated hydrobromic acid, 10, 5, and 5 ml, and evaporated after each addition according to the procedure of Glendenin (7). The residue is then dissolved, scavenged with ferric hydroxide and the filtrate is evaporated to dryness. The residue is taken up in 5 ml of 3N HC1 and the tellurium in solution is reduced to elemental tellurium as before. The precipitate is collected on a tared glass fiber filter, washed with water and finally with 95% ethanol, dried and weighed. Chemical yield is generally in the range 70-80Z. However, yields as low as 15% have given results which agree well with duplicate samples having much higher yields. The tellurium-132 is then determined by counting its 230 keV gamma ray with a single channel analyzer equipped with a 2” X 2” NaI(T1) detector. Background and each sample are counted for a preset count of 104 providing f1 % statistical accuracy. The concentration of uranium in the original sample is determined by the comparison technique. After the activities of the standard and of the unknown have been corrected for background and chemical yield, the uranium concentration of the unknown is calculated from the direct proportionality relationship of the activity and uranium concentration of the standard and the activity of the unknown. Tellurium-132 decays to iodine-132 (Iliz = 2.3 hrs). Although all the II32 gamma ray energies are higher than 230 keV (>520 keV), growth of the daughter will result in some increase in the Compton background at 230 keV. During the present work a minimum of 13 hours between the final separation and counting Telazis allowed for secular equilibrium to be established. Furthermore, activities of unknowns and standards can be corrected to a single decay time in order to take into account the decay of Te132between the times of counting the standard and the unknown. The importance of this correction is determined by the precision required. The activity of a sample of TeI32 decreases -1 % per hour. In this laboratory no decay correction was applied when the time differences were 3 hours.
310 keV cpm @ 230 + 70 keV is 1.05 i 0.05. Contamination which would not be exposed by variations in this ratio would be an ever-present radionuclide having a half-life similar to that of Te132and having a decay product with which it is in secular equilibrium. If such a nuclide is always present in exactly the same amount, it would not interfere.
In addition to the gamma spectra shown in Figure 1, complete gamma spectra have been recorded for Te132separated from irradiated uranyl acetate and from irradiated sediment, Figure 2. All of these spectra show clean separations of Te132 from other radioactive fission products. However, when naturally-occurring samples are irradiated there is a low intensity photopeak at 412 keV due to the presence of AuIg8 resulting from AuIg7(n, 7 ) . When uranyl acetate was the irradiated sample, this peak did not appear. Although the amounts of gold present in the sample analyzed thus far do not interfere with the 230 keV peak of it is conceivable that larger amounts of gold could be present in other samples in which the AuIg8 Compton contribution would increase integrated activity at 230 keV leading to erroneous conclusions concerning the concentration of uranium in the sample. Gold can be separated from Te(1V) and Te(V1) by reduction of the gold in 1.2N HC1 to the metal with hydroquinone (9). This procedure was incorporated into the chemical processing of some irradiated samples of sediment. Duplicate samples were treated in the same way but excluded the hydroquinone reduction. In each case the ratio defined above is 1.00 to 1.06 for samples of geological origin when the hydroquinone treatment is included. Their untreated counterparts give ratios of about 1.10 to 1.20. However, the uranium determination of soil samples in which this comparison has been made is not affected as long as the standard is chemically processed in the same way as the unknown. Gamma-ray peaks at 627 keV appearing in spectra shown as Figure l a and 26 have not been identified. The irradiated samples which show this unidentified peak are NBL Sample 77 and uranyl acetate. It is possible for background and electronic noise to conceal small peaks or to produce effects which could be interpreted as small peaks. However, the peaks at 627 keV do not appear to result from such extraneous causes. In fact, background has been subtracted from the spectrum of separated from irradiated uranyl acetate (Figure 2b) whereas background was not subtracted in Figure la. Sensitivity. It is difficult to evaluate the sensitivity of this method 0 - of any activation method. One can estimate the sensitivity on the basis of counting statistics alone for the irradiation time presently used and at the neutron flux presently available. All samples and background are counted for a preset count of lo4,and background is about 170 counts per minute (cprn) at the energy range 230 f 70 keV. The standard deviation, u, in the activity of the background is 1 1 . 7 cpm. One can expect good reliability in a measured activity when that activity is 6u above background. Therefore, the limit of sensitivity for this method is 50 nanograms U per gram of sample (ppb); a calculation based upon an activity of 300 cpm per microgram of uranium and a chemical yield of 7 O z . This is a valid estimate of the sensitivity of the method because the precision of the radiochemical separation is determined by the amount of carrier used rather than by its -
(9) F. E. Beamish, J. J. Russell, and J. Seath, IND.ENG.CHEM., ANAL.ED., 9, 373 (1937).
IO. 7.1'2
9 -
230
I n-
a 7-
Ad98
412
5 1
230
,
I32 670
p 2
ENERGY
*I12
773
hsV.
Figure 2. Gamma spectra of TeIa2separated from irradiated soil and uranyl acetate a, 1 gram soil; b, 124 p g U from uranyl acetate final activity. The relative standard deviation of the method is 3.5 based on reproducibility in determining uranium concentration of duplicate samples. All subsequent work conducted in this laboratory will be with samples irradiated at a flux of 1.5 X 1013 n sec-' cm-*. This increase in neutron flux for irradiation times of 10 to 14 hours should increase the sensitivity of the method. The extent of the gain in sensitivity will be dependent upon the sample. Sediment samples which are high in sodium will require decay times considerably longer than 36 hours unless the fusion and initial separation can be done remotely. Increased shielding to lower the activity of the background can be accomplished and would also serve to increase sensitivity. ACKNOWLEDGMENTS
The authors gratefully acknowledge Tin Mo for many helpful discussions and for his delayed neutron data on NBL Sample 77, Mary Custer for her assistance in experimental work, and W. M. Sackett for supplying the sample of Mississippi limestone. RECEIVED for review February 5, 1969. Accepted May 19, 1969. Investigation supported by Robert A. Welch Foundation Grant No. A-157.
VOL. 41, NO. 10,AUGUST 1969
1269
