Analytical Applications of X=RayExcited Optical Fluorescence Direct Determinations of Rare Earth Nuclear Poisons in Uranium at the Part per Giga (1 in 10’) Level Arthur P. D’Silva and Velmer A. Fassel Institute f o r Atomic Research and Department of Chemistry, Iowa State Unioersity, Ames, Iowa
An X-ray excited optical fluorescence method for the direct quantitative determination of Gd, Sm, Dy, Eu, and Pr at the part per giga level (1 in 109) in nuclear grade uranium is described. The UOz or UaOssamples are incorporated into a quaternary oxide host system, 2 LizO.SrO UOz-2W03,prepared by heating stoichiometric proportions of Li2C03, Sr(N0J2. UOz or U308, and WO, at 825 O C for 3 hours. The internal reference element Er corrects for preparative and excitation variables. The analytical curves extend over the following concentration ranges: 0.005-0.5 ppm Gd, 0.005-0.2 ppm Dy, 0.01-0.5 ppm Pr, and 0.02-0.5 ppm for Eu and Sm.
-
THENEUTRON
ECONOMY of uranium fueled nuclear reactors may be significantly impaired by the presence of even fractional part per million levels of rare earth impurities (Gd, Sm, Dy, and Eu) possessing high capture cross sections for thermal neutrons (1). The direct determination of these rare earths at the fractional part per million level in uranium by spectrometric techniques has so far not been feasible, although attempts to d o so at higher concentration levels have met with some success (2). Usually these determinations have been effected by appropriate chemical separations and analysis of the concentrates by one of several spectrographic procedures (3, 4). These concentrates have also been analyzed by a ultraviolet excited luminescence technique
(5). X-ray excited optical fluorescence of the rare earths (6, 7) has recently emerged as a very sensitive technique for the detection and determination of the rare earths (8-11). The application of this technique to the direct determination of fractional ppm amounts of the rare earth ‘neutron poisons’ (1) C. J. Rodden, J. Chem. Educ., 36, 459 (1959). (2) R. Avni and A. Boukobza, Spectrochirn. Acta, 24B, 515 (1969). (3) C. G. Goldbeck in “Analysis of Essential Nuclear Reactor Materials,” C. J. Rodden, EX, U. S. Government Printing Office, Washington, D. C., 1964, pp 959-986. (4) “Analytical Chemistry of Uranium,” Academy of Sciences of the U.S.S.R., Vernadskii Institute of Geochemistry and Analytical Chemistry (Engl. Trans., Israel Program for Scientific Translations (I.P.S.T.),Cat. No. 2057, Jerusalem, 1963. (5) L. I. Ankina, V. V. Bagreev, T. S. Dobrolyubskaya, Yu. A. Zolotov, A. V. Karyakin, A. 2. Miklishanskii, N. G. Nikitina, P. N. Palei, and Yu. V. Yakoolev, Zh. Anal. Khim., 24, 1014 ( 1969). (6) W. Low, T. Makovsky, and S. Yatsiv in “Quantum Electronics-Paris 1963 Conference,” Vol. I, Columbia University Press, New York, N. Y., 1969. p 655. (7) V. E. Derr and J. J. Gallagher, ibid., p 817. (81 . , R. C. Linares. J. B. Schroeder. and L. A. Hurburt. Soectrochim. Acta, 21, 1915 (1965). (9) E. L. DeKalb, V. A. Fassel, T. Taniguchi, and T. R. Saranathan, ANAL.CHEM., 40, 2082 (1968). (10) R. J. Jaworowski, J. F. Cosgrove, D. J. Bracco, and R. M. Walters, Spectrochim. Acta, 23B, 751 (1968). (11) W. E. Burke and D. L. Wood in “Advances in X-Ray Analysis,” Vol. 11, J. B. Newkirk, G . R. Mallett, and H. G. Pfeiffer,Ed.. PlenumPress, New York, N. Y., 1968, pp 204-213. I
I406
in T h o ? , another fertile nuclear material has already been described (9, 12). A recent publication discussed the application of this technique to the determination of the rare earths in uranium and zirconium (13). However, in this procedure the prior separation of the rare earths from uranium and zirconium was involved. Thorium tetrafluoride was employed as a carrier for the separation and the final determinations were made in a ThOz host. Our efforts to fxtend the X-ray excited optical fluorescence technique to the direct determination of the rare earths of interest in U 0 2 or U308 have shown that these simple oxides were not suitable hosts for the rare earth activators at the fractional ppm level. However, incorporation of the oxides of uranium into ternary or quaternary oxide systems (14, 15) has led to the positive detection of the rare earths neutron poisons at the ultra-trace level. In this paper we present a more detailed account on the genesis of these host systems and their application to the determination of Gd, Sm, Dy, Eu, and Pr in uranium at the part per giga (1 in loq) level. EXPERIMENTAL FACILITIES AND PROCEDURES Apparatus. The basic instrumental facilities utilized in this study have been described (9). Some additional features have been useful in the development of this method. A Jarrell-Ash 0.25 meter monochromato: (model 8:-410) fitted with two gratings blazed for 3000 A and 6000 A and having 250-micron slits was utilized in this investigation. The rotation of the grating selector arm brought either of the two gratings into the optical path without significant wavelength displacement. The wavelength drive was fitted with a system of geared motors (Synchron No. AM-1, No. Q TM17K112 Herbach and Rademan, Inc.) which allowed both forward and backward scans of the spectrum at rates of 100, 200, or 400 A per 2.5 cm. Even with the low spectral dispersion provided by the spectrometer, 250-micrometer entrance and exit slits provided enhanced powers of detection over the narrower slits. Preparation of Calibrating Standards and Samples. In our previous communication (15) we showed that quaternary oxide phosphor systems with the general formulae aRZ1+O. bR*+O.bR4+02.cW03provided powers of detection of the relevant rare earth impurities in the part per giga (part per 109) range. The starting materials employed for the preparation of calibrating standards must therefore be highly purified. The UO? base material utilized in the preparation of our standards contained detectable G d and Dy but their concentrations were less than 10 ppg.
.
(12) T. R. Saranathan, V. A . Fassel, and E. L. DeKalb, ANAL. CHEM., 42, 325 (1970). (13) T. Nakajima, Y. Ouchi, H. Kawaguchi, and K. Takashima, Bunseki Kaguku, 19, 1183 (1970). (141 E. L. DeKalb. A. P. D’Silva. and V. A. Fassel, ANAL. CHEM., ~~
I
42, 1246 (1970). (15) A. P. D’Silva, E. L. DeKalb, and V. A . Fassel, ibid., p 1846.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
Y+:
o3
dW O ~
1
R 2 + 0 ( R = Ca,Sr, or Bo)
u4+o2
bR2+0 cU4+02 ’ dWO3
I
2~i,0
aLi20
SrO UO,. 2 ~ 0 ,
Figure 1. Phosphor genesis The quaternary oxide phosphor standards or samples were prepared by blending either UOZ or U308 with a phosphor base mixture and subjecting the blend to a heating cycle. The U 3 0 a base standards were prepared by dissolving the highly purified UOz in nitric acid to prepare a master solution. Appropriate amounts of rare earth nitrate solutions were then added to aliquots of the master solution. The resulting solutions were evaporated to dryness and ignited to lo00 “C for 3 hours to yield a graduated series of U308 standards. The host material with the empirical composition 2Li20. S r 0 . U O z . 2 W 0 3 has been shown (15) to yield superior powers of detection for rare earth impurities in the uranium base phosphor. The preparation of the quaternary oxide host system was facilitated by first blending together reagent grade LiC03 and Sr(NO3)2 and CP grade W 0 3 in the ratio 1.47/2.11/ and 4.64, respectively to yield a phosphor base mixture. The external reference element Er was incorporated into ) ~ to the preparation of the phosphor base mixthe S T ( N O ~prior ture; specifically Er in solution was added to a Sr(NO& solution in the ratio of 30 pg to 2.11 grams, respectively. The resulting solution was evaporated to dryness and the residue dehydrated at 110 “C. For the analysis of a single sample, 0.270 gram of U 0 2 or U308 was blended with 0.822 gram of the phosphor base mixture in a n agate mortar for a few minutes and ignited in a platinum crucible at 825 “C initially for a period of a n hour. The phosphor was reground and fired at the same temperature for a further period of 2 hours. It is worth noting that prior dissolution of the UOs or U308is not necessary, since the internal reference element Er is incorporated into the host through the solid-state reactions discussed above. Phosphor Genesis. The optical fluorescence of the rare earths in a wide variety of host materials under X-ray or ultraviolet excitation is well documented (6, 7, 14-16). The exceptional powers of detection achieved under X-ray excitation can be attributed in large measures to the development of a phosphor system which effectively transfers X-ray energy absorbed by the matrix (the “host”) to the fluorescing trace impurities (the “activators”) (14). Of a large number of such phosphor systems, crystalline compounds with a fluorite or scheelite structure have been found to be excellent hosts for observing rare earth fluorescence under X-ray excitation (6, 7, 14, 17). The crystal structures of CeOz, ThOz, UOz, and PUOZare known to be of the fluorite type (18). Of these only CeOp and ThOz are known to support rare earth fluorescence (17). Our efforts to prepare a UOz (or U3OS)host capable of supporting rare earth fluorescence even at the 5000-ppm level were unsuccessful. A study of (16) G. H. Dieke, “Spectra and Energy Levels of Rare Earth Ions in Crystals,” Wiley, New York, N. Y., 1968. (17) R. C Linares, J. O p f . SOC.Amer., 56, 1700 (1966). (18) E. S. Makarov, “Crystal Chemistry of Simple Compounds of U, Th, Pu and Np,” Consultants Bureau, New York, N. Y., 1959.
0.5
1.0
1.5
2.0
2.5
3.0
S r 0.U 0 2 . X W 0 3
Figure 2. Effect of variation in WO, content on rare earth fluorescent line intensities the extensive literature on lanthanide or uranium activated fluorescence in alkaline earth (19-22) or rare earth tungstates (23) indicated that these offer a more promising host system for the. observation of uranium sensitized-rare earth activated fluorescence. Host systems in which uranium sensitizes rare earth fluorescence are known (24-26), but our own observations indicate that these hosts are not analytically useful. Of the rare earth tungstates, YzOa.3W03has been reported to be a good host for observing rare earth fluorescence (23). These observations and the knowledge that tungstate phosphors are easily prepared led to the empirically developed phosphor genesis scheme based on preserving charge neutrality. This scheme is shown in Figure 1. Here it can be seen that two Y 3+ ions are replaced by a n alkaline earth and a tetravalent uranium ion pair, thus preserving charge neutrality. In a n earlier communication we reported that the phosphor SrO. U 0 2 .3 W 0 3 system provided powers of detection for Sm, Eu, Gd, and Dy in the fractional ppm range (14). When Ca is substituted for Sr in this system, a relatively intense background luminescence is developed that reduces the powers of detection of the rare earth impurities in the host. Substitution of Ba for Sr reduces the rare earth line intensities considerably and only the lighter rare earths Pr and Nd can be detected at low concentrations. ~~~~
~~
(19) F. A. Kroger, “Some Aspects of the Luminescence of Solids,” Elsevier Publishing Co., New York, N. Y . , 1948, Chap. IV. (20) H. W. Leverenz, “Luminescence of Solids,” John Wiley and Sons, New York, N. Y., 1950. (21) P. D. Johnson and “Luminescence of Inorganic Solids,” P. Goldberg, Ed., Academic Press, New York, N. Y., 1966, Chap. 5. (22) Yu. S. Leonov, Opt. Specfrosc. (USSR).,9, 145 (1960); 10, 357 (1961). (23) H. J. Borchardt, J. Chern. Phys., 39, 504 (1963). (24) Yu. I. Krasilov, Yu. A. Polyakov and Yu. P. Rudnitskii, Izu. Akud. Nauk S S S R , Neorgun. Mater., 3, 2186 (1966). (25) G. M. Gaevoi, M. E. Zhabotinskii, Yu. I. Krasilov, Yu. P. Rudnitskii, G. V. Elbert, and V. A. Kizel, Izu. Akad. Nauk SSSR, Neorgan. Muter., 5,691 (1969). (26) M. V. Hoffman, J. Elecfrochem. Soc., 117, 227 (1970).
ANALYTlCkL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
1407
I
1.0
I
x=o X* 0
0.25
0.50 0.75
x
1.00
1.25
1.50
I.
SIO.UO2 * W03
Figure 3. Effect of variation in SrO content on rare earth fluorescentline intensities
I
I
I
I
1.0
I
2.0 3.0 X . Liz00 Sr 0 ~ U O 2 ~ 2 w O B
I100
I
Figure 4. Effect of variation in LizO content on rare earth fluorescent line intensities
Fluorescent Spectra. The simple nature of the spectra of rare earth impurities in uranium present in the quaternary oxide phosphor system is evident in Figure 5 , which shows the broad background luminescence of this phosphor and the
G d line at 3118 A at a concentration of O.le ppm. The tungstate luminescence extends fcom 3400-5000 A, the uranyl ion system from -5000 to 5800 A and the uranate ion system from 6400-6800 A (29). The relative intensities of these two systems in our spectra indicate that the uranium occurs predominantly as the uranyl ion in the quaternary oxide host system. The prominent band heads of the second positive system of Nz, which arise from the X-ray excitation of air surrounding the sample are also readily seen (30). The spectra of the rare earth impurities of interest present at trace levels are shown in Figure 6 . The dotted lines indicate the background from which the relative intensities of the analytical lines are measured. It can be seen from this figure that the simple nature of the spectra gives rise to littl? spectral interference; the only exception is the Pr 6450 A line which isanot adequately resolved from a sensitive Sm line at 6441 A. If appreciable Sm occurs in a sample, the relative intensity of the Pr line can be measured above the peak of the Sm 6417 A line. The concentration of Er, the internal reference element, was arbitrarily set at 10 ppm by weight with reference to the UOz component of the h o s t o At this concentration level the intensity of the Er 5507 A is readily measured and the amount of Er added to each sample is so large that any residual Er in the base mixture at the fractional ppm level makes a negligible contribution to reference line intensity. All analytical measurements have been based o n relating recorded peak height ratios to concentration. In practice the G d spectrum was scanned first while the 3000 8, blaze grating was in position in the spectrometer. Analytical Data. The wavelengths of the analytical line pairs, the applicable concentration range, and the estimated
(27) A. Cleve, 2.Anorg. Chem., 32, 129 (1902). (28) G. Blase, J . Elecfrochem. SOC.,115, 738 (1968).
(29) H. Gobrecht and W. Weiss, Z.Physik, 140, 139 (1955). (30) B. Brochlehurst, Atomic Energy Research Establishment, Harwell, C/R 2669, 1963.
Since it is known that the molar ratio R 2 0 3 / W 0 3can vary widely in the rare earth tungstates (27), the most logical step was to determine the b, c, d ratio (Figure 1) that would give rise to optimal fluorescent line intensities. The results of these determinations, which are presented graphically in Figures 2 and 3, revealed that the optimal ratios were 1 : 1 : 2 rather than the 1 :1:3 ratio reported earlier (14). A further modification of the SrO. U 0 2 .2 w o 3 system was suggested by the reports that uranium-activated systems frequently contain lithium as a constituent (22, 28). The beneficial action of L i 2 0 additions at a molar ratio of 2 to the host material is clearly shown in Figure 4. Thus the system 2Li20.SrO. UOz.2 w 0 3 has provided the highest powers of detection of the four rare earths of interest in a uraniumcontaining host. Substitution of lithium by other alkali elements reduced the G d luminescence appreciably. The nature of the spectra obtained indicate that the 2Liz0. S r 0 . U O z . 2 W 0 3 host belongs to the scheelite class. The fact that a bright orange colored material is obtained within five minutes of heating the blend of oxides, which by themselves are not orange colored, indicates definite compound formation. The actual host has not been definitely characterized as a single-phase crystalline material. The main emphasis has been to evolve a phosphor with a high degree of reproducibility in preparation and freedom from impurity effects and with the best possible limits of detection for the rare earth neutron poisons. RESULTS AND DISCUSSION
1408
ANALYTICAL CHEMISTRY, VOL. 43,
NO. 11, SEPTEMBER 1971
Figure5 X-Ray excited optical fluorescence spectrum of Gd in 2Liz0.SrO .UOZ. 2WOa
Figure 6. X-Ray excited optical fluorescence spectrum of rare earth impurities and the Er internal reference element in 2Liz0 S r 0 . U 0 2 . 2wos +
Element Gd
Sm Eu DY Pr
Table I. Analytical Data Lowest detectable Concentration Analytical* concentration, range in line pair, A PPg UaOs,PPg Gd 3118 Er Sm Er Eu Er Dy Er Pr Er
5507 5957 5507 6153 5507 5122 5507 6450 5507
2.5
5-500
10.0
20-500
10.0
20-500
I
10.0
I
b
b
w
b-
u \
L
10
-
1.0
B
G!
I5 \
3
E
Ln
.-L
a 0
2.5 5.0
5-250 10-500
detection limits are summarized in Table I. The detection limits were determined from data obtained from ten different phosphor preparations both of the blank and the lowest standard. -From these data the concentrations that would give rise to rare earth line signals equivalent to three times the average peak-to-peak noise in the background spectrum
.*J L
0 1.0
-
- 0.1 z .-
0 ._ e
e
z
H
0.005 0.01
0.05
Concentration of Gd,Dy,Sm,Eu,Pr
0.1
.0.5
in U308(ppm)
Figure 7. Analytical curves
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
1409
Sample
Set A
1.
B C A B C
2.
A
3.
B C
Impurity Fe Ca AI
Fe
+ A1 + Ca
Table 11. Reproducibility of Phosphor Preparation Intensity Intensity ratio ratio Sm/Er cv, Gd/Er cv, 0.36 0.26 0.26 zk18.0 0.20 1.14.0 0.32 0.24 1.18 1.50 1.39 =k16.0 0.98 ~k15.0 1.@I 1.42 6.50 5.40 4z10.0 4.50 5.58 'r10.0 5.10 5.84
z
Table 111. Impurity Effects on the Determination of Rare Earths in UaOs Intensity ratio Concn in Gd/Er Sm/Er Eu/Er Ua08, ppm Dy/Er 1.44 1.17 0.54 residual 1.94 1.46 1.12 0.50 500 1.90 0.92 0.63 1.60 1.81 750 1.51 0.98 0.52 2.06 loo0 1.21 1.01 0.63 500 1.97 1.01 750 1.36 0.55 1.95 1.22 0.63 loo0 1.41 2.08 0.47 500 1.49 1.08 1.83 0.54 1.50 0.99 750 1.78 0.95 lo00 1.62 0.48 1.58 1.06 1.47 0.57 1.87 200 each 1.06 1.37 0.46 1.76 500 each 1.09 1.53 0.46 1.85 750 each
of the blank at the selected wavelengths were calculated. These concentrations are defined as the detection limit. The analytical curves are shown in Figure 7. Subtle variations in preparative procedures as well as changes in the concentration of extraneous impurities in the sample may affect the analytical line fluorescent intensities in a rather unpredicted manner (9, 20, 21). However, these intensity changes may be externally compensated by applying the well-known internal reference principle (9,12). The reproducibility of phosphor preparation was evaluated by analyzing the rare earth content of three synthetic U308 samples. Initially, three phosphor base mixtures were prepared from different lots of chemicals, each of which was utilized to prepare five different phosphor samples. Thus for each U308sample, 15 phosphor preparations were obtained. The observed coefficients of variation of the intensity ratios calculated for each sample are tabulated in Table 11, from which it is evident that the reproducibility of phosphor preparation as measured by the intensity ratios is of the order 10 to 20%.
1410
z
Intensity ratio Eu/Er 0.14 0.10 0.12 0.64 0.45 0.56 3.2 2.3 2.6
ANALYTICAL CHEMISTRY, VOL. 43,
CV, %
1.16.0 zk17.0 4z8.0
Pr/Er
1.44 1.49 1.60 1.46 1.33 1.28 1.50 1.26 1.56 1.33 1.32 1.23 1.37
The deliberate addition of Fe, Ca, and Al, which are typical of the impurities occurring in nuclear grade uranium, has been utilized to study the influence of these impurities either individually or cumulatively on the determination of the rare earths. The data for the relative intensities of each of the rare earths as well as intensity ratios are shown in Table 111. The pattern of variations observed is random and no systematic trends are evident, even though the level of impurities added is far in excess of those found in nuclear grade uranium. This significant absence of impurity effects as compared to earlier observations (9,20) may be a fortuitous characteristic of this particular host. The incorporation of the Er internal reference element, however, is highly desirable because a change in the intensity of the Er reference lines serves to monitor drastic changes in phosphor characteristics as well as instrumental variations. RECEIVED for review April 12,1971. Accepted May 19,1971. Work performed in the Ames Laboratory of the U.S. Atomic Energy Commission, Iowa State University, Ames, Iowa.
NO. 11, SEPTEMBER 1971
