ing properties apd evaporates faster. As a precaution, use of ethylenediamine is stipulated in the procedure. APPLICATION TO PLUTONIUM SEPARATION
The successful application of this twocycle extraction system to the recovery and determination of neptunium suggests use of the same system for the separation and determination of plutonium. K i t h a matrix described in Table I, plutonium could be reduced to
( 4 ) Moore, F. L., Ibid., 29,941 (1957). (5) Murray, B. B., U.S.Atomic Energy
+4 instead of + 3 and plutonium and neptunium extracted together. The activity contributed by the neptunium is negligible, being less than 1% of the plutonium activity.
Comm., Rept. DP-316 (1958). (6) Reid, D. L., Hanford Operations, General Electric Co., private communication, May 1959. ( 7 ) Schuman, R. P., Phillips Petroleum Co., private communication, January 1958. RECEIVED for review September 8, 1959. Accepted November 4, 1959. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960. Work performed under Contract No. AT( 10-1)205 for the U. S. Atomic Energy Commission.
LITERATURE CITED
(1) Booman,
G. L., Holbrook, W. B., ANAL.CHEJI.31, 10 (1959). (2) Maeck, W. J., Booman, G. L., Elliott, hI. C., Rein, J. E., Ibid., 30, 1902 (1958’1.
Liquid Scintillation Spectrometry for the Analysis of Zirconium-95-Niobium-95 Mixtures and Coincidence Standardization of These Isotopes J. DONALD LUDWICK General Electric Co., Hanford Atomic Products Operafion, Richland, Wash.
b
The simultaneous quantitative analysis of the isobars zirconium-95 and niobium-95 in mixtures was accomplished utilizing the liquid scintillation spectrometer. Samples with zirconium-95-niobium-95 ratios of 50 to 1 through 2 to 1 have been investigated by this procedure and were practical for rapid routine analysis. The ease of resolution of the weak beta-particle spectra of these nuclides as well as the elimination of the necessity for self-absorption correction when using the liquid scintillation method gives this technique advantages over other methods of analysis. The liquid scintillator was also used for beta detection in a coincidence arrangement for standardization of zirconium-95 and niobium-95 sources. Agreement with other standardization techniques demonstrates the utility of this method.
T
development of y-ray spectroscopy has greatly simplified many routine analyses of mixtures of gamma emitters; however, where either no y-rays are present or their energies are so similar that resolution is impractical, other methods are nechssary. Tedious chemical separations and time-consuming absorption measurements on mixtures of isotopes leave much to be desired in routine analysis. Factors favoring liquid scintillation counting for analysis of mixed beta emitters are: no self-absorption and high geometry as well as ease of sample preparation. HE
Table I.
Isotopes H3 C‘4
P33 P 32 S85 ~ 1 3 6
Ca46 Sr 90 Y90 Zr 96
Xb9s
Comparison of Some Common Beta-Emitting Isotopes
Beta Ener y hLEV 0.018 0,155 0.27 1.70 0.167
0.714 0.254 0.61 2.18 0.371 0.160
Gamma Ener y, ME%.
...
... ... ... ... .
.
I
... ...
0: 721
0.745
A short review of some common isotopes whose decay schemes lend themselves more conveniently to beta analysis is given in Table I. The final column is a relative estimate of ease of beta resolution as might be encountered in a liquid scintillator and it compares commonly occurring mixtures of beta emitters. The larger the factor the easier the resolution. This factor depends not only on the separation of the betaemitter’s energy, but also upon the absolute energy of the beta particles. Furthermore, zirconium and niobium would be a difficult pair to resolve because of their low ratio. The weak energies of their beta particles, along with complications due to y-rays, present a more difficult problem. Resolution of these two isotopes in a liquid scintillator would prove the applicability of the technique to many other systems.
Beta Energy Ratio, Ease of Resolution
t112
12.46~) 5568y 25.4d 14,30d{\ 87.ld 4 . 4 x 10’5; 152d 19.9y 61h 65d 35d j
8.6 6.3 10.2
1
3.6 2.3
The liquid scintillator has the disadvantage of a relatively high y-ray sensitivity. This must be accounted for in counting weak beta activity that is accompanied by gamma emission. I n general the liquid scintillator may be modified to accept either organic or aqueous phase materials. The amount of carrier material that can be tolerated is usually dependent only on the solubility of the scintillator for the material. Some materials, unfortunately, interfere with the scintillation process and either reduce the size of the output light pulses from the liquid or eliminate them entirely. Colored compounds and dissolved oxygen quench the scintillation pulses. Aqueous adaptable scintillators are affected by high water contents. More commonly, organic scintillators are quenched by electronegative sections of organic molecules. I n most practical cases a small amount of VOL. 32, NO. 6, MAY 1960
607
quenching is not detrimental to the ultimate purpose of the experiment. The technique of 6-y coincidence counting as an absolute method of standardization has been reviewed by several workers (1, 2, 6). The application of internal liquid scintillation counting to a 4np-7 coincidence method for absolute standardization of isotopes is the newest technique to be added to calibration systems (8). Some problems inherent in standardization with a 4np-proportional counter, such as Sample self-absorption, are eliminated in the liquid scintillator. A comparison was made of standardization results from a 2n proportional 8-7 counter and from the 4n liquid scintillation P-y coincidence counter. APPARATUS
A coincidence liquid scintillation spectrometer was used for the resolution of zirconium-95 and niobium-95. Figure 1 illustrates the detection system. The sample, which is contained in an 8-mm. quartz tube, was placed in a hole in the Lucite light pipe. This hole is' the only line of sight between the multiplier phototubes (3). The light pipe was coated with Plasite (Wisconsin Protective Co. No. 7100-B), for best reflective cover. Twg RCA 6655A multiplier phototubes were used for scintillation detection. This particular type of 2inch phototube was found to have superior resolution characteristics. The light pipe was optically coupled to the phototubes with Dow-Corning No. 200 fluid. The phototubes, operated by superstable high voltage supplies, were connected to cathode followers whose impedance was matched to the adjoining Model A-1 amplifiers. The output pulses of the amplifiers were fed to the coincidence inputs of a 256-channel analyzer. The standardization of Zr9' and NbgS reauired that one of the multidier phbtotubes in Fi ure 1 be coupled t o a 2-inch sodium iofide (thallium) crystal for y-ra detection. The crystal was moved $lush with the Lucite light pipe. The pulses from the y-ray detector were amplified with an A-1D amplifier and resolved with a differential pulse height analyzer. The liquid scintillation beta pulses were amplified with an A-1D amplifier and the discriminator output signal was delayed, using a variable delay line, to compensate for the Metal
inherent delay in the pulse height analyzer. The signals were analyzed in a coincidence analyzer. Calibration results were compared using a 2 1 proportional counter to detect the beta particles. Figure 2 illustrates the detection arrangement for coincidence calibration in this case. The electronics were essentially the same as in the liquid scintillator technique. A steel proportional counter with a thin Mylar window was used for beta detection and a 5 X 2 inch sodium iodide (thallium) crystal was used for y-ray detection. This arrangement offers the best conceivable geometry for the detector types used. PROCEDURE
Resolution of Zrg6-Nbg5. Samples of Zr9S-Nbg6 mixtures as well as separated Zrgs and Nbgs in dibutyl phosphate were introduced into the liquid scintillator. The zirconium samples were also counted after allowing build-up of the niobium daughter. The scintillator mixture found to dissolve and be the most efficient for beta detection with dibutyl phosphate was xylene, f -ter henyl (4.0 grams per liter), an 1,4-is [2-(5-phenyloxazolyl)]benzene (POPOP) (0.1 gram per liter). No quenching of the scintillator could be observed with 5% dibutyl phosphate introduction. To calibrate instruments properly, pure separated Zrg5 and Nbg5 were needed. Zirconium was extracted from an 8N hydrochloric acid mixture using 0.45M 2-thenoyltrifluoroacetone (TTA) TTA cannot be used in the scintillator because it has extraordinary quenching characteristics. Then zirconium was re-extracted from TTA with 1N hydrofluoric acid..in 8N nitric acid and the acid solution was evaporated to dryness in the presence of perchloric acid. Zirconium was finally extracted into
g
dibutyl phosphate by the procedure of Scadden and Ballou (7) after diluting to I N nitric acid. The total sample volume was about 0.4 ml. in quartz tubes. The samples were allowed to cool for about 1 hour before counting. Advantages of background reduction due to cooling as well as increased gain a t low temperatures (4)more than outweigh the expense and inconvenience of cooling. The sample was surrounded by a t least 2 inches of lead in all directions. A background counting rate of about 30 c.p.m. over the entire beta spectrum was observed under these conditions. Standardization of Zr95-Nbg5. COUNTISG. A LIQUIDSCINTILLATIOX 10-pl. spike of Zrg5 (freshly prepared) was introduced into 0.4 ml. of scintillator. In some instances it was more advantageous to calibrate aqueous solutions of the isotopes, and in this case a scintillator that was different from previous work was used. A scintillator composed of 2,5-diphenyloxazole (PPO) and POPOP in a solution of 75% dioxane, 12.5% l,2-dimethoxyethaneJ and 12.5% anisole was found compatible with 10 11. of a 1N hydrochloric acid solution of the isotopes. The scintillator was introduced into the quartz tube which was placed in the Lucite light pipe guide. Other mechanical features were similar to experimental case 1. The pulse height analyzer, discriminator, and window were adjusted to count the 0.7-m.e.v. y-ray photopeak of the isotopes. To determine the y-ray contribution to the liquid scintillator beta counting rate, a small glass capillary vial was prepared. An equal amount of calibrated solution as in the sample case was introduced into the yial. The capillary was placed inside the scintillator held in the quartz vial. The capillary walls absorbed all the beta activity while the y-rays penetrated the glass and revealed their
P
8 m m Q u a r t z Tube
-
RCA 6 6 5 5 A Multiplier Phototube
Lucite Light Pipe
Figure 1.
608
Preemplifier
Liquid scintillation coincidence light detection system
ANALYTICAL CHEMISTRY
Figure 2. Coincidence calibration detectors
background contribution to the beta counting rate. This method contrasts favorably with the tedious y-ray channel discrimination method used by other workers (8) to compensate for this interference. PROPORTIOXAL COUNTING.The isotope was placed on a thin film whose aluminum holder was positioned directly over the 5 x 2 inch sodium iodide (thallium) crystal. The 27r proportional counter was placed directly on top of the film with 1/16-inch clearance. The amount of delay needed on the beta channel was determined from coincidence counting rate data. The 7-ray contribution to the proportional counter was determined using l/8-inch aluminum absorbers between the sample and proportional counter to eliminate the beta counting rate. The entire detection system was surrounded by a t least 4 inches of lead on all sides. RESULTS AND DISCUSSION
Resolution of Zrs6-Nbg6. Figure 3 gives the beta spectra obtained from Zrs5and Kb94 as well as an approximate steady-state concentration mixture of the two nuclides. Reasonable resolution was obtained between the 0.158m.e.v. Kbe6beta particle and the mixture of beta particles from Zrg6. The energies of these ZrQ6beta particles are 4970 0.362 m.e.v., 49% 0.396 m.e.v., and 2% 0.89 m.e.v. (9). The Nb96 beta spectrum exhibits its maximum beta energy a t channel 40 on Figure 3 while the ZrQ6beta spectruni has its maximum a t about channel 94. A small tail-off from the spectrum of S b e 6is due to the gamma activity accompanying the beta decay. The tailoff from Zr95 as well as from the mixture is the result of gamma activity of ZP6Nbs6 and the small amount of 0.89m.e.v. beta particles associated with the ZrQ6decay. Cpon examination of the spectra and calibration of these two isotopes, approximately 15% of the Zrg6 beta
2400
Simultaneous Determination of Zrg5and NbRbat Time Intervals Since Zrg6 Separation
Table II.
Time,
Days
Zrg6,D.P.M.
0
32.~00 f eon 32:ijoo f 1200
1 3 5 10 30
spectrum was found to appear above the energy maximum of the NbQb beta spectrum. Through intercomparison of both these beta spectra, it was found that in the low energy region of Nbs6, 54y0 of the Zrg6 beta activity is also counted. The observed niobium counting rate in the channels 6 to 40 constitutes 25y0 of the total Nb95 activity. From this information the following formulas were derived : =
0.15
5 u'
1600
-
The background counting rate over the Nbg5 counting region is 20 c.p.m. An additional background of 6 c.p.m. 100
,
,
, ,
I
,
, ,,
'
I
l
l
1 ,
3
-
Zr-95lNb-95
001
100
1000
ratios as a function Figure 4, Zrg3-Nbe5 of time following separation
Nb-95
1400-
of zirconium and niobium. Efficiency can be increased through a proper lightreflecting system using a single phototube. The simplest system of a dualchannel analyzer will suffice. Results were obtained concerning the accuracy and precision of the method through a division of the 256 channels into just two channels covering the ZrQ6 and Nbe6 spectra. Data were taken on samples of zirconium as the niobium activity grew in. Table I1 illustrates these results and compares the values obtained and the calculated theoretical values expected. The calculated values were obtained from Figure 4, which are theoretical Zrg5-Nbs6 ratios with time, and assume no niobium concentration a t time zero. The shift in gain over a period of time was compensated for in each case through calibration of the peak position of the 60-k.e.v. -4m24I converted y-ray. The results indicate that this method may be applied in the quantitative determination of Zrgs and Xb96, to any reasonable ratio of Zre5 to Nbe6 con-
1200-
:
Q
COUNTING REGION BACKGROUND SUBTRACTED1
1000-
5 8
10 DAYS
zr-95
- 0.54(ZrQ6d.p.m.)
0.25
21-95lNb-95
-
50 3 17 0 10 3 5 3 1.9
was observed for the remaining Zrg6 counting region. A voltage increase on the multiplier phototubes will raise the counting efficiency if necessary; however, an increase in the background counting rate will also accompany this change. The specific activity of routine analysis m-ill determine the voltage and thus the efficiency needed. Coincidence counting is probably not necessary in the simultaneous counting
Kbgs d.p.m. = Nb96 c.p.m. (3 to 15 volts) or (channels 6 to 40) - background
2200
1800
Theoretical Zr/Kb Ratio ...
Zrg5 c.p.m. (>15 volts) or (>channel 40) - background
-
2000 -
... 481f28 164flO 103fO6 5 5 f 0 3 2.1 f 0.1
0
665-f 30 1,910 f 90 3,000 f 140 5,400 f 200 1 1 ;600 f 600
31,300 f 1200 31,000 f 1150 29,500f 900 24;OOO f 800
ZrQ3d.p.m.
Experimental Zr/P\'b Ratio
Sb96,D.P.M.
600 400 -
Table 111.
800
5 COUNTING REGION
Isotope "bo3
2000
0
10
20
30
4 0 50 SO 70 CHANNEL NUMBER
BO
90
100
Figure 3. Zres-Nbesspectra exhibiting separate counting regions
Zr93
Comparison of Coincidence Standardization of Zrg6 and Nbss with Liquid Scintillator and Proportional Counter
Liquid Scintillator, D.P.M.
Proportional Counter, D.P.M.
136,300f 2040 136,600 f 2300 136,800f 1370 136;OOO f 1000 138,500 i 2100 138,900 f 2200
135,200 f 2500 134,100 f 2800 134.800 f 2200 i351400 2100 135,800 f 2400 135,000 f 2200
VOL. 32, NO. 6, MAY 1960
609
centration including the radioequilibrium ratio of about 1 to 2. I n considering the utility of the liquid scintillation method, it is well to survey the methods already available for quantitative analysis of ZrQ6-?;b’J6mixtures. The close proximity of their y-ray energies, 0.74 and 0.76 m.e,v., respectively, make precise results difficult even with the finest y-ray spectrometric instrumentation. The precision of a time-consuming beta particle absorption measurement on mixtures of Zr96 and NbV6 is affected by self-absorption of the sample and by the necessary large counter corrections. iifter a consideration of these factors, the liquid scintillation method seems to be the most applicable to fast routine analysis of these isotopes. Standardization of Zre6 and Nbg5. Calculation of the absolute disintegration rate in coincidence
measurements is given by Putman (6) as :
where G y is the mean of the product of the beta and gamma detection efficiencies, while ip and zy are the means of the individual efficiencies. The ratio in brackets reduces to unity when either of the detectors is equally sensitive to all parts of the source. This is obviously true for the beta sensitivity in a liquid scintillator and equally true for the beta sensitivity of the 2a proportional counter mounted directly over a source. Results of coincidence measurements with both types of p-y counters are given in Table 111. The standard deviations of the individual measurements are also included.
LITERATURE CITED
(1) Barnothy, J., Forro, M , Reu. Sci. Instr 22,415 (1951). (2) Dunvorth, J. V., Ibid., 11, 167 (19.10). (3) Horrocks, D. L., Studier, 31. H., ANAL.CHEJI.30, 1747 (1958). ( 4 ) Kinard, F. E., Xucieonics 15, 92 f1957). ,\ - - -
(5) Putman, J. L., Atomic Energy Research Establ. Rept. I/M 26 (1953).
(6) Putman, J. L.; “Beta and Gamma Ray Spectroscopy,” K. Siegbahn, ed., Chap. 26, North Holland Publishing
co.. -19,55. -->
( 7 ) Scadden, E. hI,, Ballou, S . E., ASAL.
CHEJL25, 1602 (1953).
(8) - S t e p , J., Haasbroek, F. J., Proc. U. 1, Intern. Conf. Peaceful Cses Atomic Energy, 2nd Geneva, 21, 95
(1958), (9) Strominger, D., Hollander. J. II., Seaborg, G. T., Rem. X o d e r n Phys. 30, 585 (1958). RECEIVED for review Sovember 2 i , 1959. Accepted February 23, 1960. Work performed under Contract KO. AT(45-1)1350 for the United States Atomic Energy Commission.
Determination of the Stoichiometry of Uranium Dioxide Polarographic Determination of Uranium(V1) in Uranium Dioxide HISASHI KUBOTA Analytical Chemisfry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
The deviation of the composition of uranium dioxide from the stoichiometric oxygen-uranium ratio of 2.000 is calculated from the assay for the uranium(V1) in the oxide. The sample is dissolved in hot phosphoric acid under an inert atmosphere to preserve the oxidation states of the metal, and the uranium(V1) is determined polarographically in a phosphoric-perchloric acid medium. Because this is a direct determination of the deviation from stoichiometry, it is best applied to samples which are nearly stoichiometric, and the composition of a uranium dioxide sample which has an oxygen-uranium ratio of 2.03 or less can be determined to within 0.001 oxygen atom. m P i I u h I DIOXIDEis
widely used as a nuclear fuel in both the bulk and dispersed form. A dense, crystalline material may be prepared by the reduction of uranium trioxide monohydrate in a nonoxidizing atmosphere, such as hydrogen or argon, and high-firing at 1700’ C. Bulk UOz is formed by compressing and sintering this granular product. I t s composition approaches the theoretical 1to 2 ratio of uranium to 610
ANALYTICAL CHEMISTRY
oxygen, but the exact stoichiometry is seldom attained, because more than the stoichiometric amount of oxygen is almost invariably present. This is referred to hereafter as excess oxygen. Several approaches to the determination of this composition have been reported. They include assay methods (8, S), determination of excess oxygen (6),and determination of uranium(V1) (7). The two last-named methods give a direct measure of the deviation from stoichiometry. Among the actinide elements uranium compounds exhibit the greatest deviations from the law of definite proportions. This has been attributed to the multiplicity of stable oxidation states coupled with the small differences in energy between the states. The deviations are most pronounced with compounds of uranium and oxygen, nitrogen, sulfur, selenium, and others in which the anion-uranium bonds are partially to wholly semimetallic. As a consequence, stoichiometric LO1 has a great tendency to chemisorb oxygen and hold it in the lattice as oxide ions. The electrons that bring about the reduction of the oxygen come from U(1V) atoms which are oxidized to higher states (4). Thus, determination of the uranium in
valence states higher than 4 in EOr should serve as a measure of the excess oxygen and, subsequently, as a way to defermine the esact composition of the material. With this end in view a scheme of analysis has been developed in which the U(V1) content of dissolved KO2 is determined and directly correlated n ith the excess oxygen. A similar approach has been reported by Simmler (71, who dissolved the oxide in phosphoric acid and determined U(V1) by titration with a standard titanous solution. In the method described below, thp samples are dissolved in phosphoric acid under an argon atmosphere, and the U(V1) is determined polarographically. The uranium-phosphoric acid system has two distinct advantages for this particular determination. I t not only allows a dissolution of uranium osides ~ i t no h change in osidation states but also provides a medium in the subsequent polarographic determination of the U(V1) where a two-electron reduction of uranium can be effected, thereby providing a more sensitive procedure compared to reductions in media where one-electron reductions take place. The polarography of uranyl ions in phosphate has been under study a t this
