demonstrated in Table 111. The values in the entire table were determined in less than half a day. All TBPO solubilities in water obtained as a function of temperature by centrifugation are plotted in Figure 2 for comparison with solubilities obtained using a constant temperature bath (6). The points fall rather well along the line drawn through the constant temperature bath values. Most values are within experimental error (=t2%); the largest deviation was +5%. Any laboratory operation involving a phase separation would appear to be accomplished most conveniently and satisfactorily by centrifugation with air cooling (open centrifuge), if the tubes are removed from the cups immediately after centrifugation. For systems whose equilibrium is disturbed by temperature changes, the air temperature must necessarily be the sxne
as the equilibrium solution temperature. However, centrifugation in :t closed centrifuge with external heat from infrared lamps can also be utilized for a quick and convenient method for determining solubilities of substances that become less soluble as the temperature rises. If centrifugation is used for any system which is temperature-sensitive, a knowledge of the final temperature after centrifugation is important. ACKNOWLEDGMENT
The authors express appreciation to
B. A. Soldano for suggesting a more detailed study on the effects of centrifugation on solution equilibrium. LITERATURE CITED
( 1 ) Alcock, IC., Grimley, S. S., Healy, T. V., Kennedy, J., &Kay, H. A. C., Trans. Faraday SOC.52,39 (1956). (2) Burger, L. I,., Forsman, R. C., U. S.
Atomic Energy Comm. HW-20936 (.\pril 2 , 1951, declassified March 2, 1957). (3) Burger, L. L., Wagner, R. M., Znd. Eng. C h e w , Chem. Eng. Data Series 3, 310 (1958).
(4) Craig, L. C Craig, D., “Extraction
and DistribuGon” in “Separation and Purification,” Vol. III, 2nd ed., pp 227, 300, “Technique of Organic Cheniistry,” A. Weissberger, ed., Interscience, New York-London, 1956. (5) Davies, W. C., Jones, W. J.. J . Chein.
SOC. 1929.33. (6) Higgins; C. E., Baldwin, ANAL. CHEY.32,233 (1960). . ( 7 ) Higgins, C. E., Baldwin, J . Org.,Chem. 21,1156 (1956). . (8) Higgns, C. E., Baldwin, Soldano. B. A,. J. Phus. Chem . 63, (1959). (9) Wunderly, H. L., Smelo, L. S., ISD. ENQ.CHEM.,ANAL.ED. 12, 754 (1940).
w. w w
RECEIVEDfor review July 6, 1959. Accepted Sovember 13, 1959. Based on work performed for the U. 6 . Atomic Energy Commission a t the Oak Ridge Xational Laboratory operated by the Union Carbide Corp.
Radioassay of Finely Divided Solids by Suspension in a Gel Scintillator SAMUEL HELF, C. G. WHITE, and R. N. SHELLEY’ Explosives and Propellanfs laborafory, Picafinny Arsenal, Dover,
b The counting of radioactive materials as suspensions in a gel scintillator is described. The radioelements included in the suspended samples are strontium90-yttrium-90, chlorine-36, sodium-22, barium-1 33, nickel-63, carbon-1 4, and hydrogen-3. Factors affecting suspension counting efficiency are discussed and a comparison is made with homogeneous solution counting for some specific cases.
T
more iibua1 methods of sample preparation for liquid scintillation counting entail making a homogeneous solution of the radioactive sample and the liquid scintillator. Because aromatic solvents, toluene in particular, are most commonly used for liquid scintillators, samples soluhle in such media present no problem. The difficulties associated \T-ith counting materials insoluble in toluene or other pure aromatic solvents can often be overcome through the use of mixed solvent systems containing polar organic liquids and/or other additives to increase solubility. Davidson and Feigelson (2) HE
1 Present address, Food & Drug Administration, Dept. of Health, Education, and Welfare, Washington, D. C.
238
ANALYTICAL CHEMISTRY
N. J.
have thoroughly reviewed and discussed many of these practical solvent systems for the homogeneous solution counting of a wide variety of radioactive materials. Homogeneous solution counting, using complex solvent systems and/or additives to increase solubility, may impart two disadvantages. First, the addition of other solvents and additives may dilute or quench the scintillation process, thereby reducing counting efficiency ( 2 ) . Second, at best, only very small amounts of tagged material can be dissolved. Thus, where lom specific activity is involved, sensitivity is limited. Moreover, it is not unlikely that for some materials, particularly inorganic salts, a suitable solvent system for homogeneous solution counting may be impossible t o obtain. To overcome the above disadvantages for very insoluble compounds, a suspension counting technique can be employed to good advantage. The radioactive material in finely divided form can be suspended directly in a simple liquid scintillator (5) or in a gel scintillator containing either aluminum stearate (3) or the thixotropic agents, Thixcin (9) and Cab-0-Si1 (8). The gel media are preferable where no settling and good reproducibility of samples are
desirable ( 7 , 9). By preparing the radioactive samples in a sufficiently finely divided state, self-absorption effects can be eliminated for all but the very low-energy beta emitters such as tritium ( 5 ) . As much as 1 gram of some carbon-14-labeled compounds can be suspended in a 20-ml. volume of a gel scintillator without appreciable effect on counting rate due to opacity (9). Previous references on suspension counting dealt primarily with tolueneinsoluble carbon-14 compounds. This paper compares the counting characteristics of a wide variety of suspended beta emitters, ranging from very low to very high emission energy, in a gel scintillator medium. A positron and a conversion electron emitter are also included among the radioactive sources. I n addition, the relative advantages of suspension over homogeneous solution counting are illustrated for certain applications. EXPERIMENTAL
Radioactive Samples. The following radioactive solids were used for this study: SrBOS04-Y%304(in equilibrium), Ni63Cz04, NaC136, Ba133C12, Na22C1, 1,3,5,7-tetranitro-l,3,5,7-tetracyclo-octane (HMX)-C1‘ (uniformly
labeled), and HMX-H3 (randomly labeled): The first four salts were prepared from appropriate sources obt.ained from the Oak Ridge National Laboratory; Na22C1 was obtained from the Xuclear Science and Engineering Corp. HhIXC14 was synthesized from formaldehydeC14 and the corresponding tribiumlabeled material was prepared by tritium irradiation of inactive HSIX. Each radioactive salt was ground as finely as possible with a mortar and p e d e . The t\vo HAZX samples could not be ground because of their potential esplosive hazard. These were precipitated as very fine powders by the rapid addition of water, with stirring, to their solutions in dimethylsulfoxide. Uenzoic-l-C14-acid and toluene-” (both from New England Suclear Corp.) w r e used as internal homogeneous rc>fvrcncestandards. Scintillator. The scintillating iiiedium used to suspend the radioLictiw samples consisted of a toluene solution containing 2,s-diphenyl(JXaZol(’ (4 grams per liter), 1,4-bis [2-(5phenyloxazolyl) ]benzene (100 mg. per litfr), and powdered Thixcin (Baker Castor Oil Co.) (25 grams per liter) as a siispt’nding agent (9). Counter. ill1 couiit’ing data were obtained ith a Packard Tri-Carb liquid scintillation counter, Model 514, consisting of two refrigerated iiiultiplier phototubes in a coincidence rircuit. Five-dram vials (Wheaton Glass Co., Millville, 5 . J.) IT-ith screw caps were used as :sample counting 11 o t tles. Estimation of Absolute Disintegration Rates. The absolute disintegration rates of the H1LIX-C14 and HMXH3 \yere determined by counting quantities of benzoic-l-Cl4 acid and toluene-H3 rctference standards, respectively, of known disintegrations per minute ktl/m) , in homogeneous solutions containing weighed amounts of each of the two compounds. To effect solution of small quantities of HlLIX for this purpose, dimethylsulfoxide in 25% by volume was added to the standard liquid scintillator solution. These determinations were made by first counting the HMX sample in the above solution and then adding the appropriate reference standard to the same counting bottle. From the counting rate of the reference standard, the counting efficiency of the particular emitt’er in this system was determined, and this value iii turn was used to icalculate the disintegration rate in d/m/mg. of the unknown sample. For the tagged inorganic salts, reference standards were not available. The absolute specific activities of these u-ere estimated by comparing the integral counting rates of a known weight of each suspended compound, in coincidence and in single channel from each multiplier phototube according to the method of Guinn (4). This method is based on the consideration that the integral counting efficiency of a sample in coincidence is the product of the individual integral counting efficiencies of each multiplier phototube
Emission Properties of Radionuclides
Table I.
Radionuclide Srw-YQo
C1“ Nie3 C’4
NaZ2
ppppP+
Ba133
e-
H3
I I
I
1
1
BW
700
900
-
9
1000 1100
,
1500
1400
.
represent “apparent” absolute specific activities.
[
RESULTS AND DISCUSSION
80
-
70
-
The important emission properties for liquid scintillation counting of the radionuclides comprising the labeled materials used for this study are listed in Table I. Except for the last two elements, all of the radionuclides are pure beta emitters and thus ideally suited for liquid scintillation counting. Sodium-22 is a positron (annihilation radiation) as well as a gamma emitter. Barium-133, another gamma emitter, is of particular interest as it also emits several monoenergetic internal conversion electrons ranging from 0.043 to 0.35 m.e.v. with the greatest number occurring in the lower energy level (1). These electrons as well as the positrons from sodium-22 should excite a liquid scintillator in the same manner as beta particles of corresponding energies. On tht> other hand, the gamma rays emitted by these two radioelements should not affect the liquid scintillator because of the low densities and small path lengths associated with the usual type of sample geometries for this counting technique. Figure 1 shows the differential spectral curres obtained by plotting counting efficiency us. high voltage for suspensions of the various emitters with the lower and upper pulse height discriminators fised at 10 and 50 volts, respectirely. Betneen 25 and 50 mg. of radioactive material wab used for each curw. These small quantities of suspended material eliminated possible opacity effects on the counting rates. Each curve is a typical frequency distribution of pulse heights produced by a beta emitter in a liquid scintillator within a finite discriminator voltage level. The peak voltage and counting efficiency is a t least qualitatively d a t e d to the maximum energy of emission from each radioelement with the esception of strontium-9Ckyttrium-90 and barium133. The relatively low efficiency value for strontium-90-yttrium-90 in a 10- to 50-volt window is undoubtedly due to the high energy yttrium-90 coinpo-
loo 90
60 5040
-
30
-
20
-
IO
-
1 4, /, yH3,
l o t 7’00
BOO BOO
7’00
900 900
1000 1100 1000 1100
1200 1300 1200 1300
1400 1400
VOLTS
Figure 2. Suspension scintillation counting of various radionuclides volts a t 10 to
(or each channel). Thus, if 2 is the absolute disintegration rate in d/m of the unknown sample, el and e2 are the counting efficiencies of channel 1 and channel 2, respectively, and A , B , and C are the observed integral counting rates less background in counts per minute of channel 1, channel 2, and in coincidence, respectively, then A
=
xe,, B = xez, and C
=
xe1e2
Substituting for el and ez in terms of A and B in the expression for C, C
or x
Ernax,
hl. E .V. 0.54-2.24 0.71 0.067 0.155 0.018 0.54 (1.26) 0.043-0.36
VOLTS
Figure 1 Suspension scintillation counting of various radionuclides in a 10- to 50-volt window
ar
(7)
I
I
I200
Emission 6-
=
=
x
x
A/x X B/x
=
A X B/x
-4 X B/C.
By use of this method with different quantities of benzoic-l-C14 acid in homogeneous solution, absolute specific activities could be determined with &2y0 accuracy if the total number of d/m in the sample exceeded 10,000 and if 1200 volts were applied to the multiplier phototubes in obtaining the integral counts. In applying the above method to suspensions of the radioactive inorganic salts, it was assumed that the particles were “infinitely thin,” as disintegrations lost through self-absorption would not be detected. Thus, the calculated disintegration rates for these materials
VOL. 32, NO. 2, FEBRUARY 1960
* 239
nent (2.24 m.e.v.) which produces pulses, the majority of which are greater than 50 volts. Thus, the resulting spectrum is predominately that of strontium-90 betas (0.54 m.e.v.). The peak efficiency for barium-133 in the 10- to 50-volt window appears to be in line with the 0.043-m.e.v. energy of the majority of its monoenergetic electrons, although the voltage a t which this peak occurs is lower than expected for this energy level as a result of the contribution of the small number of higher energy electrons. In Figure 2, curves of counting efficiency us. high voltage are shown for the same suspensions as in Figure 1 but with no upper discriminator in the circuit. This integral counting data is of more general interest because it shows the masimum detection efficiency that can be expected with these emitters and the information should apply to any liquid scintillation counter. Here, the three highest energy emitters, strontium-90yttrium-90, chlorine-36, and sodium-22, behave in a similar manner, giving welldefined plateaus starting at a relatively IOK voltage, a t efficiencies between 75 and 80%. The small differences obtained in plateau efficiency among these three emitters are not considered significant because of possible small errors in the estimation of absolute specific activities. The remaining lower energy emitters do not yield plateaus but show a definite dependence of detection efficiency with voltage. I n addition, in this lower energy range, the maximum efficiency that can be attained is directly related to the energy of the individual emitter. Again, the contribution of the higher energy monoenergetic conversion electrons to those of 0.043 m.e.v. is seen for the barium-133 curve. The factor of self-absorption is important with regard to the data depicted in Figures 1 and 2. For HiVX-C14, the absence of self-absorption was established by obtaining the same counting efficiency us. voltage relationship, for both differential and integral counting, when reference standard benzoic-l-CI4 acid was dissolved in the gel scintillator. As soluble internal standards were not available for strontium-9Ckyttrium-90, chlorine-36, sodium-22, nickel-63, and barium-133, the influence of self-absorption for these emitters could not be determined directly. However, as the values used for the absolute specific activities of these samples mere estimated on the basis of no self-absorption, the counting efficiency data for these inorganic emitters can be taken to represent those for “infinitely thin” particles. For the three highest energy emitters, it can be reasonably assumed that this condition mas actually attained. For the lover energy nickel-63 and barium-133 salts, it is possible that a certain amount of self-absorption was inherent in the
240
ANALYTICAL CHEMISTRY
100 90
I: 150
00
Figure 3. Concentration effect on pension scintillation counting
-
SUS-
suspended particles. However, as the integral counting spectra of these two samples did not differ appreciably from the one for the carbon-14-labeled suspension, this effect is probably minor for very finely divided precipitates. The tritium curves in Figures 1 and 2, on the other hand, are definitely influenced by a self-absorption factor. At 1270 volts, a reference standard of toluene-H3 in the gel scintillator gave an integral counting efficiency of 16%. As the suspended HMX-H3 sample yielded only a 10% counting efficiency under the same conditions, approximately 40% of the beta radiation is lost through self-absorption. One advantage of counting radioactive materials as suspensions rather than solutions is the absence of quenching effects (2, 6, 6) on the scintillation process. However, as the amount of the dispersed solid in a unit volume of scintillator increases, counting efficiency is reduced as a result of light scattering for colorless or white materials and also by absorption for colored particles. These opacity effects will be dependent on the optical nature of the suspended material and on the energy of the particular emitter. Colorless transparent crystals mill produce less of an effect with increasing concentration than opaque or colored crystals; the higher the energy of the emitter, the less pronounced will be the opacity effect. The influence of increasing concentration on counting efficiency is illustrated in Figure 3 for NaC136and Nis3C2O4 suspensions for concentrations up to 300 mg. in 20 ml. of scintillator. The high transparency of the sodium chloride crystals in combination with the high energy of chlorine-36 betas results in only a slight opacity effect on counting efficiency. On the other hand, this effect is pronounced for KiB3Cz04suspensions as a result of the pale green color of the crystals and the much lower energy of the emitted beta particles. For large quantities of materials such as Ni63Cz04, this opacity effect can, of course, be reduced by increasing the relative volume of scintillator to weight of suspended material within the limitations of sample bottle and chamber dimensions. For concentrations under
IOU
200
300
Figure 4. Comparison of suspension and homogeneous solution counting for HMX-C14 and HMX-H3 samples
50 mg. per 20-ml. volume, this effect is negligible. With the carbon-14- and hydrogen-3labeled H M X samples, a direct comparison of homogeneous solution us. suspension counting was possible. A1though completely insoluble in pure toluene, small amounts of this compound can be dissolved in the toluene solution scintillator containing dimethylsulfoxide in 25% by volume. In Figure 4, the counting rates for increasing quantities of HMX-Cl4 are shown in 20-ml. volumes of suspensions and homogeneous solutions. The curve for the suspension data has only a slight departure from linearity owing to opacity effects with increasing weight of suspended material, mith the homogeneous solutions, however, there is R marked decrease in counting efficiency with increasing concentration because of quenching by the nitramino compound. Figure 4 also s h o w the same comparison for suspensions and solutions of H1LIX-H3. Here the advantage of suspension over solution counting is even more marked. The weak light pulses produced by tritium betas are obviously even more strongly affected by the quenching action of H M X molecules in solution. CONCLUSIONS
By using a suspension technique for the radioactive sample, liquid scintillation counting can be extended to a wide variety of beta and electron emitters. This method is particularly useful for inorganic salts which are generally completely insoluble in the usual organic scintillator solutions. Opacity effects and self-absorption of beta particles are factors which can limit counting efficiency, but these factors only become important for the very low-energy betas such as those from
tritium. Even with these limiting factors, however, tritium compounds can be counted as suspensions with 10% detection efficiency. In the case of a carbon-14- or hydrogen-3-labeled organic compound, where a pronounced quenching occurs in homogeneous solution. the suspension technique may result in considerably higher detection efficiencies.
Holahan for the preparation of the carbon-14- and hydrogen-3-labeled HNX.
ACKNOWLEDGMENT
(1956).’ (4) Guinn, V. P., “Liquid Scintillation Counting in Industrial Research,” pp. 166-82, Proceedings of Northwestern
The authors thank T. C. Castorina for his helpful comments and F. S.
LITERATURE CITED
( I ) Crasemann, B., Pengra, J. G., Lindstrom, I. E., Phys. Rev. 108, 1500-5
(1957).
( 2 ) Davidson, J. D., Feigelson, P., Intern. J . A p p l . Radiation and Isotopes 2, 1-18 (1957). (31 Funt. B. L.. Nucleonics 14. KO.8. 83
.,
University Conference on Liquid Scintillation Counting, Pergamon Press, New York, 1958. (5) Hayes, F. X . , Rogers, B. S., Langham, W. H., h’ucleonics 14, No. 3, 48 (1956). (6) Helf. S..White. C. G.. ANAL. CHEW 29, 13’(1957). ’ ( 7 ) Nathan, K . G., Davidson, J. D., Waggoner, J. G., Berlin, N. I., J. Lab. Clin. M e d . 52, 915-17 (1958). (8) Ott, D. G., Richmond, C. R., Trujillo, T. T.. Foreman, H.. Nucleonics 17, No. 9,’106 (1959): ‘ (9) White, C. G., Helf, S., Ibid., 14, No. 10, 46 (1956). \
I
RECEIVEDfor review June 15, 1959. Accepted November 6, 1959.
Isolation of the Rare Earth Elements A Chlorina tio n-Volatiliza tion Procedure J. BERNARD ZIMMERMAN and JOHN C. INGLES Radioacfivify Division, Mines Branch, Department o f Mines and Technical Surveys, Ottawa, Canada
b A total chlorination treatment of radioactive ores and concentrates, with simultaneous volatilization a t 900” C., eliminates most of the nonrare earth elements, including thorium and scandium. A subsequent ammonia precipitation removes the alkalies and alkaline earths and isolates the rare earths, including yttrium, in a pure concentrate suitable for weighing, spectrographic examination, or colorimetric determination. With materials consisting primarily of uranium and thorium compounds, preliminary extraction from nitric acid medium with a carbon tetrachloride solution of tributyl phosphate eliminates the bulk of these elements, permitting use of a larger sample and thus extending the range of the method. Over-all recoveries of the rare earth elements are 90% or better.
T
of rare earth minerals in many ores of radioactive materials is sufficient to be of economic interest and the rare earth elements must be determined as a major constituent. In the case of uranium concentrates to be used in the production of nuclear fuels, the maximum amount of rare earth elements (particularly samarium, europium, gadolinium, and dysprosium) that can be tolerated is set a t a low level. Elements of this group are the most difficult to separate and determine, partly because of their chemical properties and partly because of the small amounts which must be determined. Until 1948 ( 1 4 ) the most difficult step HE AMOUNT
in the isolation of the group was the removal of thorium. Recently, several improved procedures have included a solvent extraction using penta-ether (dibutoxy ethylene glycol) and 8-quinolinol-chloroform extraction steps (12), but the reagent penta-ether was not readily available commercially (it is now understood that this reagent is available from the Roberts Chemical Inc., RobertsRd.,Nitro, W. Va.) (IS). A solvent extraction procedure employing tributvl phosphate-carbon tetrachloride and thenoyltrifluoroacetone extraction (22) (the thenoyltrifluoroacetone in the quantities used is expensive and requires troublesome pH adjustments), and a cellulose column method involving the use of 12.5y0 v./v. nitric acid in diethyl ether, followed by a double thenoyltrifluoroacetone extraction (6) (the use of 12.5% nitric acid in ether is hazardous) are also among those now used. LIoreover, the above methods were developed primarily for the determination of the rare earths in relatively pure thorium and uranium materials, and while they are capable of isolating microgram quantities of the rare earths, additional steps are needed to handle the many gross impurities found in ores and mill concentrates. In a survey of chlorination methods for upgrading uranium and thorium concentrates, it was noted that the chlorides of the rare earths have very low vapor pressures a t elevated temperatures, compared with the many elements which are normally difficult to separate from them. The literature data (Tables I and 11) pointed to the feasibility of
using a high-temperature chlorination procedure to volatilize most of the foreign elements. The remaining nonvolatile chlorides would apparently consist only of a few very easily separated elements. Of the reported chlorinating reagents and procedures ( 4 ) , sulfuryl chloride as a reducing and chlorinating agent in a chlorine atmosphere appeared best for the complete and rapid chlorination of the various metal oxides present. This procedure (19) has been employed for the separation of thorium and the rare earths, but it is tedious when more than milligram amounts of thorium are to be treated and results in rare earth losses (12). I n the present work, a technique has been evolved whereby the only losses are those resulting from volatilization and they are of the same order as in other procedures (6, 12, 22) (however, the solvent extraction procedures cited deal with microgram amounts of the rare earths, so that it is not strictly fair to compare the losses experienced with those found here). The method is considerably faster and requires less manipulation with the relatively impure concentrates for which it was developed than the other methods. The purity of the rare earth concentrate (particularly freedom from thorium and scandium contamination) is such that it should be possible, if increased sensitivity is desired, to complete the determination by any of the colorimetric methods now available (2, 9). If this sensitivity is not desired, the concentrate can be weighed and reported with confidence in the knowledge that it is VOL. 32, NO. 2, FEBRUARY 1960
0
241
