(11) Foulk, C. W., Bawden, A. T., J . Am. Chem. SOC. 48, 2045 (1926). (12) Franck, U. F., 2. Elektrochem. 58, 348 (1954). (13) Gauguin, R., Anal. Chim. Acta 5,200 (1951). (14) Gauguin, R., Bertin, C., BadosLambling, J., Ibid., 7, 360 (1952). (15) Gauguin, R., Charlot, G., Ibid., 7, 408 (1952). (16) Glasstone, S., Laidler, K. J., Eyrin$: H., “Theory of Rate Processes, McGraw-Hill, New York, 1941. (17) Grahame, D. C., J . Electrochem. SOC. 9 9 , 3 7 0 ~(1952). (18) Gusman, J., Rancano, A., 2. anal. Chem. 103,445 (1935). (19) Hall, L. C., Ph.D. thesis, University of Illinois, 1956. (20) Ishibashi, M., Fujinaga, T., Bull. Chem. SOC. Japan 25, 68, 238 (1952). (21) Jordan, J., ANAL. CHEM.27, 1708 (1955). (22) Kalousek, M., Collection Czechoslov. Chem. Communs. 13, 105 (1948). (23) Kambara, T., Bull. Chem. SOC. Japan 27,523,527,529 (1954). (24) Kies, H. L., Anal. Chim. Acta 6, 190 (1950).
(25) Kies, H. L., Ibid., 10, 161, 575 (1954). (26) Kolthoff, I. M., ANAL. CHEM.26, 1685 (1954). (27) Kolthoff, I. M., Eggertsen, F. T., J . Am. Chem. SOC. 61, 1036 (1939). (28) Kolthoff, I. M., Jordan, J., Ibid., 75, 1571 (1953). (29) Ibid., 76, 3843 (1954). (30) Kolthoff, I. M., Jordan, J., Heyndrickx. A.. ANAL. CHEM. 25. 884 (1953). ‘ (31) Kolthoff. I. M.. Linnane. J. .J.. “Polarography,” 2ndoed.,’ p; 420; Interscience, New York, 1952. (32) Kolthoff, I. M., Pan, Y. D., J . Am. Chem. SOC.61,3402 (1939). (33) . . Kolthoff. L. M.. Stock. J. T.. Analust 80,860 (1955). (34) Laitinen, H. A., Jennings, W. P., \ - - - - I -
\
I
(35) . . (36) (37) (38)
Parks. T. D.. IND.ENG. CHEM.. ANAL:ED. 18,’355,358 (1946). ’ Laitinen. H. A.. Kolthoff. I. M.,. J . Phis. Chem.’45, 1074 (1941). Masten, M. L., Stone, K. G., ANAL. CHEM.26, 1076 (1954). Nightingale, E. R., Ph.D. thesis, University of Minnesota, 1955. Oldham, K. B., J . Am. Chem. SOC. 77,4697 (1955).
(39) Randles, J. E. B., Somerton, K. W., Trans. Faraday SOC. 48, 937 (1952). (40) Reilley, C. N., Cooke, W. D., Furman, N. H., ANAL. CHEM. 23, 1223, 1226 (1951). (41) Riha, J., Collection Czechoslov. Chem. Communs. 16, 479 (1951). (42) Salomon, E., 2. Elektrochem. 4, i l (1897). (43) Shoemaker, K., A N A L . CHEM. 27, 553 (1955). (44) Stone, K. G., Scholten, H. G., Ibid., 24, 671 (1952). (45) Tachi, I., others, Bull. Chem. SOC. Japan 28,25,31,37 (1955). (46) Tanaka, N., Tamamuslq R., Proc. 1st Intern. Congr. Polarography, Prague, 1 , 563 (1951). (47) Willard, H. H., Fenwick, F., J . Am. Chem. SOC.44,2516 (1922). (48) Wooster, W. S., Farrington, P. S., Swift, E. H., ANAL. CHEM. 21, 1457 (1949).
RECEIVEDfor review October 20, 1956. Accepted May 25, 1957. Division of Analytical Chemistry, 130th meeting, ACS, Atlantic City, ru’. J., September 1956. Based on the Ph.D. thesis of Larry C. Hall presented at the University of Illinois, June 1956.
Quantitative Radiochemical Methods for Determination of the Sources of Natural Radioactivity JOHN N. ROSHOLT, Jr. U. S. Geological Survey, Denver, Colo.
b Study of the state of equilibrium of any natural radioactive source requires determination of several key nuclides or groups of nuclides to find their contribution to the total amount of radioactivity. Alpha activity measured by scintillation counting is used for determination of protactinium-23 1, thorium-232, thorium-230, and radium226. The chemical procedures for the separations of the specific elements are described, as well as the measurement techniques used to determine the abundances of the individual isotopes. To correct for deviations in the ore standards, an independent means of evaluating the efficiencies of the individual separations and measurements is used. The development of these methods of radiochemical analysis facilitates detailed investigation of the major sources of natural radioactivity.
T
RE radioactivity of rocks and minerals is caused primarily by the presence of uranium-238, uranium-235, thorium-232, and their disintegration products. These radioactive series are shown in Figure 1. If, during the
1398
0
ANALYTICAL CHEMISTRY
500,000 years since the minerals were deposited, no parents or their daughter products were lost and no radioactive elements were added to the mineral, equilibrium in any of the three series is established. If gains or losses occurred, the radioactive isotopes are in a state of disequilibrium, and knowledge of the isotopes not present in equilibrium abundances often provides important clues to the geochemical history of the mineral. The usefulness of a knowledge of the isotopic sources of natural radioactivity and the determination of radon-222, lead-210, and thorium-232 have been discussed (17)- The present study deals with the determination of the long-lived radionuclides and equilibrium-established short-lived daughter products needed in the investigation of radioactive disequilibrium in rocks and minerals. A study of the relationships in Figure 1 shows that the extent of disequilibria can be rigorously defined if the abundances of the following components are known: (1) uranium group, (2) thorium-230, (3) radium-226, (4) radon222 group, ( 5 ) lead-210 group, (6) protactinium-231 group, and (7) thorium-
232 series. Components 1 through 5 define the extent of equilibrium in the uranium-238 series. The equilibrium of the uranium-235 series can be defined by the amounts of uranium and protactinium-231 present. The thorium-232 series, though not involved in the equilibrium, is a major contributor of radioactivity that must be determined. Measurement of these nuclides will show that the isotopes are present in equilibrium concentrations or that disequilibrium has resulted from either recent mineral formation or gains or losses of isotopes since the mineral was formed. Other methods for the determination of some of these isotopes have been published (1, 3, 6, 7, 14, 15). Throughout this paper equivalent units are used. Uranium and thorium contents are given as actual per cent uranium or thorium. All disintegration products in the three decay series are expressed as equivalents to parent nuclides using the unit per cent equivalent, and not as the actual percentage of these daughter products. Per cent equivalent is defined as the per cent amount of primary parent, under the assumption of radioactive equilibrium,
required to support the amount of daughter actually present in the sample. This amount of parent may or may not be present in the sample. Parent and daughter decay a t the same rate under equilibrium conditions. The uranium-235 series is expressed equivalent to uranium-238 by use of the abundance and half-life ratios. Any daughter in the uranium-235 series is expressed in per cent equivalent to uranium-238, assuming the normal uranium-235-uranium-238 abundance ratio. Hence, the activity of the uranium-235 decay series product is 4.6y0 of the equivalent uranium-238 value. For example, 100 mg. of 1% uranium of normal isotope abundance in equilibrium with all of its decay products would contain the following amounts and activities of decay products:
Nuclide Content, mg. yo equivalent Alpha activity, dis./min.
Uranium238 0.9928 1.00 734.6
Probably no uranium ore exists that is in absolute radioactive equilibrium, especially with respect to the radon-222 group and the lead-210 group, because of continuous radon loss from even the most select minerals. Recent data on radium-223-radium-226 ratios (14) indicate that many uranium minerals that have been used as standards and considered to be in equilibrium deviate slightly from absolute equilibrium with respect to the longer-lived isotopes. Hence, any uranium ore standard that is to be used periodically to calibrate and check instrument operation should not necessarily be assumed to be in absolute equilibrium. An independent standardization, not influenced by the amount of radon loss from the standard source, is required for radon-222 and lead-210. The equilibrium of the ore standard can be checked by an inde-
Uranium235 0.0071 1.00 33.8
Protactinium231 3.37 X lo-' 1.00
33.8
Thorium-230 1.70 X 1.00
734.6
pendent standardization and corrected for any deviations. Calculation of the over-all efficiency of the chemical separations and subsequent measurements are used for this independent standardization. The nuclides are measured by alpha counting of chemical separates carrying the particular isotopes to be measured. A diagrammatic outline of the chemical separations is shown in Figure 2. As the amount of a nuclide is generally too small for quantitative separations, carriers are used throughout the procedure. The first separation from the sample solution is the precipitation of bismuth sulfide carrier, which carries down the bismuth and polonium isotopes. Alpha counting of this separate measures bismuth-214, bismuth212, and polonium-210, and permits the calculation of radon-222 and lead-
Radium-226 3.39 X 1.00 734.6
Radon-222 2.15 X
Lead-210 4.28 X 10-o 1.oo
1.00
734.6
734.6
U 9%
Po 91 7h go
Ac 89 Ra 80
Fr 87 Rn
86 At
a5 Po 84
61 83 Pb
82 TI 81 L
Figure 1.
___------
--J
----------
i
Classification of natural radioactivity VOL. 29, NO. 10, OCTOBER 1957
1399
210 (17). Precipitation of zirconium phosphate carrier followed by reprecipitation with praseodymium oxalate separates the isotopes of thorium, from which thorium-230 and thorium227 can be counted directly, and thorium-230 and protactinium-231 calculated; or, following additional s e p arations, radium-224 can be counted and thorium-232 calculated. I n the filtrate from the zirconium phosphate precipitation barium sulfate is precipitated as a carrier for the radium is+ topes, on which counting measurements for radium-226 and radium-223 are performed and radium-226 and protactinium-231 calculated. Repeated tests of the precipitation of each of these carriers show that the nuclides are quantitatively separated from the solution. Less than 1% of the original polonium-210 and bismuth-214, less than 27, of the original thorium isotopes, and less than 1% of the original radium isotopes remain in the solution, as indicated by measurement of the alpha activity of a second precipitation of each carrier of the isotopes. Uranium-238 and radium-226 spikes carried through the zirconium phosphate and praseodymium oxalate precipitation show less than 0.27, contamination by radium-226 measuring the subsequent build-up of radon-222 and daughter products and less than O.5y0 contamination of uranium-238 determined by fluorometric uranium analysis of the praseodymium oxalate precipitate. A much smaller amount of radium-226 combined with a uranium-238 spike shows that less than o.3y0 of the uranium-238 is carried through the precipitation of barium sulfate when the build-up of radium-226 daughter product activity is compared to the theoretical build-up of this activity. A radium-226 spike carried through the procedure allowing the build-up of radium-224 from thorium228 shows less than 0.2% contamination by radium-226 activity. APPARATUS
Polonium and bismuth isotopes in a bismuth sulfide precipitate are counted with a n alpha particle scintillation counter which includes both phosphor and photomultiplier tube in a light-tight container (17). All other measurements are made on the radioactive precipitate mixed with zinc sulfide, using a phosphor thickness of 1.5 to 20 mg. per sq. cm. The active phosphor is placed in the scintillation detector directly beneath the bare photomultiplier tube and allowed to remain in the dark for a t least 15 minutes before the high voltage is applied to the phototube and the counting is begun (4). The scintillation counter consists of a bare photomultiplier tube enclosed in B light-proof metal container, 4 inches in diameter and 10 inches high, equipped
1400
ANALYTICAL CHEMISTRY
with a felt-sealed hinged door. The photomultiplier tube, with a sensitive window surface 3 inches in diameter, is coupled through a cathode follower to a conventional scaling unit. Several of the measurements are made with a commercially available automatic sample changer modified for scintillation counting by incorporating a 5-inch photomultiplier tube in a metal housing with the entire sample changer enclosed in a light-proof box. The phototube is not exposed to light when the sample is changed. The precipitates are filtered on 50mm. Millipore filter membranes used with Millipore filter apparatus (hlillipore Filter Corp., Watertown, Mass.). The surface of the membrane is impervious to penetration by the precipitate. An aluminum ring, 50 mm. in diameter by 3 mm. high with 0.005-gage cellophane glued to its underside, is used for the zinc sulfide-precipitate mount. Plastic cement is spread on the periphery of the underside of the ring directly on the cellophane and the precipitate and membrane are mounted by adherence to this surface. A 47-mm. ring without the cellophane protector is used to mount the bismuth sulfidecarried precipitate. REAGENTS AND SOURCES
Bismuth carrier, 0.028M bismuth chloride in 5y0 hydrochloric acid solution (6 mg. of bismuth per ml.). Lanthanum “hold-back” carrier, 0.007 M , radioactively pure, lanthanum oxide solution (2 mg. of lanthanum per ml.). Zirconium carrier, 0.16M zirconium chloride solution (15 mg. of zirconium per ml.). Barium carrier, 0.146M barium chloride solution (20 mg. of barium per ml,). Praseodymium carrier, 0.142M, radioactively pure, praseodymium nitrate solution (20 mg. of praseodymium per ml.). 0.1M sodium pyrophosphate solution. Oxalic acid, crystals. 0.014M lead nitrate solution (3 mg. of lead per ml.). 0.090M ferric chloride solution ( 5 mg. of iron per ml.). Zinc sulfide, powder, phosphorescent grade, nonactivated. 0.03 millicurie lead-210-bismuth-210polonium-210 source solution. 0.05 millicurie radium-226 source solution. Enriched protactinium-231 source sample. Yational Bureau of Standards 10-9 gram radium-226 standard solution. Standard .pitchblende ore, MS-OR, 44.96’3$ uranium. National Bureau of Standards analyzed sample 2230-1, MS-VL, 3.08570 uranium. National Bureau of Standards analyzed sample 2230-2, MS-L, 6.9270 uranium. National Bureau of Standards radioactive standard, 1.0% uranium. Thorium nitrate reagent (10-year-old thorium-228 source). Thorium nitrate, equilibrium reagent (50 years old). National Bureau of Standards radioactive standard, 1.0% thorium. CHEMICAL PROCEDURE
A schematic outline of the steps in the separation procedure is shown in Figure 2.
The sample, 1 gram or less depending on total activity and ease of solution, is fused at almost red heat with 4 to 5 grams of sodium peroxide in a nickel crucible. The melt is allowed to disintegrate in water and neutralized with hydrochloric acid. The solution is diluted to 100 ml. while the acidity is adjusted to 1.0 N in hydrochloric acid. The sample should be completely in solution. If all of the silica is not taken into solution, a smaller sample should be used. Six milligrams of bismuth carrier are added, and the solution, at about 45” C., is saturated with hydrogen sulfide. The bismuth and polonium isotopes used for the radon-222 and lead-210 determination (17) are coprecipitated with the bismuth carrier and filtered on 47-mm. 1Iillipoi-e membranes. The precipitate is collected as a 35-mm. diameter circle in the center of the membrane. The filtrate containing the uranium, protactinium, thorium, actinium, radium, and lead isotopes is made free of hydrogen sulfide by evaporation on a hot plate to a volume of approximately 80 ml. Two milligrams of lanthanum “hold-back” carrier, 15 mg. of zirconium carrier, and 3 ml. of sodium pyrophosphate solution are added separately to the filtrate and the solution is boiled gently for 1 to 2 minutes while the zirconyl phosphate precipitate is forming; this precipitate carries thorium quantitatively (2, 9). The lanthanum hold-back carrier prevents the coprecipitation of actinium isotopes. The hot solution is immediately centrifuged, the supernatant liquid decanted, and the time recorded. The precipitate is retained for determination of thorium-230 or of thorium-232 where radium-224 is allowed to build up. This method for thorium-232 is used where the activity of bismuth-212 and polonium-212 is too low for accurate measurement as described (17). Measurements on zirconium phosphate precipitates from solutions containing l o + gram of radium-226 show that less than 0.2% of the radium is retained in the precipitate. Twenty milligrams of barium carrier are added to the combined supernatant liquids. Three milliliters of dilute sulfuric acid are added to the warm solution, precipitating the radium with barium sulfate (6). The precipitate is allowed to settle for 15 minutes and centrifuged, and the supernatant liquid is decanted and discarded. The precipitate is rinsed from the centrifuge tube with 30 ml. of water and 2 drops of hydrochloric acid are added. Then 150 mg. of zinc sulfide are added, the slurry is mixed well, filtered, washed, mounted, and dried, and the time is recorded. The precipitate is collected as a 35-mm. diameter circle in the center of a 50-mm. membrane. Less than a n hour should elapse between the zirconium phosphate separation and the barium sulfate filtration. When the thorium-230 measurement is desired, 1 gram of oxalic acid is added to a 50-ml. aqueous solution to dissolve the zirconium phosphate precipitate (16). One milliliter of praseodymium
Table I.
zirconium carrier and 1 ml. of sodium pyrophosphate solution are added t o the gently boiling supernatant liquid, the solution again is centrifuged, and the precipitate is washed. The supernatant liquids are combined, adjusted to p H 3 with ammonium hydroxide, cooled, and saturated with hydrogen sulfide while 150 mg. of zinc sulfide are added. An excess of sulfide ion adsorbs the lead-212, bismuth-212, and polonium-212 on the zinc sulfide completely. The latter is filtered and the time recorded. Twenty milligrams of barium carrier and 5 drops of dilute sulfuric acid are added, precipitating the radium with barium sulfate; the precipitate is allowed to settle for 10 minutes.
phosphate precipitate is combined with 50 ml. of water, 2 ml. of hydrochloric acid, and 0.3 ml. of sodium pyrophosphate solution. The solution is set aside for 3 to 20 days, depending on the amount of thorium activity expected, while radium-224 accumulates from the disintegration of its parent thorium-228. This radium-224 is to be separated and measured. The solution is heated to the boiling point; 3 mg. of zirconium carrier and 0.5 ml. of sodium pyrophosphate solution are added. The solution is centrifuged, and the supernatant liquid is decanted. The precipitate is washed, the washings are added t o the supernatant solution, and the time is recorded. Five milligrams of
carrier (20 mg. of praseodymium) diluted with 5 ml. of water is added t o the boiling oxalate solution stepwise in several increments; time is allowed for the praseodymium oxalate to crystallize after each addition. After the precipitate has settled, 300 mg. of zinc sulfide are added and mixed thoroughly. The slurry. is filtered, mounted, and dried, and the time is recorded. The precipitate is collected in a 45-mm. diameter circle in the center of a 50-mm. membrane. Less than a n hour should elapse between the zirconium phosphate separation and the praseodymium oxalate separation. If thorium-232 is t o be determined instead of thorium-230, the zirconium
Values Used for Calculating Detection Efficiences of 3-Inch Photomultiplier Tube
Ratio: PoZl0Activity RaZz6Activity in Razz6 Content in Phototube Razz6 Activity Detection Efficienc Sormalized to Po210 Normalized Ratio X by Radon PhotoZnS Source Calcd. from Razz6 (Ratio of Measurec? Activity in Tube 4 Detection Efficiency Train Analysis tube Det. by Photoof Tube 4 (10-9 G. of RaZz6)Content, Dis./Min. to Theoret. Activity) (Av. 14 ZnS Sources) Used tube, C./Min. 0.984f0.008 0.955 f 0.018 4081 f 32 0.952 f 0.012 1.885 f 0.015 1 3885 f 36 0.969 f 0.016 0.971 f 0.009 0.998 f 0 . 0 0 3 5190 =!= 24 3 5040 f 40 2.397 f O . 0 1 1 1.00 0.971f0.016 3577 f 52 0.971 3Z 0.016 4 3475 f 26 1.652 f 0 . 0 2 4 0.955 3Z0.016 0.928 f 0.022 0.928 f O . 0 1 1 5644 f 45 2.607 f 0.021 5 5239 f 45 0.960 f 0.017 0.961f0.015 0.989 f 0 . 0 0 5 2.003 f 0.020 4336 f 43 6 4168 f 52 Radium determined by Sylvia P. Furman, U. S. Geological Survey. 0
Sample Fuse with sodiu!n peroxide, leach with water, neutralize, make LON hydrochloric acid, add bismuth, saturate with hydrogen sulfide, filter I
I
Filtrate.
Predpitate. Bismuth sulfide carrier COUNT. Bismuth-214 Bismuth-212 Polonium-210
Boil out hydrogen sulfide, add zirconium, add phosphate, centrifuge
I
Precipitate. Zirconium phosphate carrier
I centrifuee
I
I
Thorium-232 determination. Accumulate short-lived radium isotopes for known interval of time centrifuge
I
Precipitate Barium sulfate carrier
Liquid. Discard
I
I
I
I
I
I
I
Thorium-230 Thorium-227 determination. Dissolve in oxalic acid, add praseodymium carrier, add zinc sulfide
I
I
Filirate. Discard
Add water add zinc sulfide, filter
~
I
I
I
1
Prkcipitate. Zinc sulfide and barium sulfate COUNT, Radium-226 Radium-223
I
L"pAdjust to pH-3 I add zinc sulfide. I saturate with ' hydrogen sulfide, filter
Filtrate. Discard
I
Preci dit ate. Zinc sulfide and praseodymium oxalate
PreciLitate. Zirconium carrier Discard
COUNT. Thorium-230 Thorium-227
1
I
I
Precipitate. Zinc sulfide carrier lead, bismuth and polonium isotopes, discard
Filtrate. Add barium, add sulfuric acid, add zinc sulfide, filter Liquid. Discard Figure 2.
I
Precipitate. Zinc sulfide, barium sulfate COUNT. Radium-224
Schematic outline of chemical procedure VOL. 29, NO.
io,
OCTOBER1957
1401
Then 150 mg. of zinc sulfide phosphor are added, the solution is stirred, and the precipitate is filtered, washed, mounted, and dried. This precipitate is also collected as a 35-mm. diameter circle in the center of the membrane. MEASUREMENTS A N D STAN DARDlZATlON
The measurements and calculations involved for each of the isotopes contained in precipitates described, followed by the over-all efficiency determination and standardization for these particular isotopes, are described below. Usually the efficiency measurements are made only initially when the method is set up, to check and correct the equilibrium of the standards and to obtain primary calibration. Thereafter, the calibrated standards are used periodically as secondary standards. To evaluate the over-all efficiency of the determination for each of the nuclides or groups, the radioactively pure carrier-free alpha emitters are adsorbed on 15 to 20 mg. per sq. cm. of zinc sulfide. The counting rate of each zinc sulfide source is determined and the alpha particle disintegration rate calculated. Each calibrated zinc sulfide source is then carried through the chemical separation and measurement procedure for the nuclide adsorbed and the counting rate is determined. The over-all efficiency of the determination of each nuclide or group is the ratio of the observed counting rate of the carrier precipitate to the alpha particle disintegration rate. As shown in the calculations, the standard counting rate (SCR) subsequently used in the analysis of ores is the product of the alpha particle disintegration rate per milligram equivalent and the over-all efficiency. The procedure for the determination of the efficiency is given below for each nuclide. All the scintillation detectors are corrected to yield total alpha particle disintegration rates for the active carrier-free zinc sulfide sources by use of a detection efficiency factor. This factor is calculated by comparison of the radium-226 activity in a radium sulfate-zinc sulfide source (barium sulfate-free) measured by the scintillation counter and the radium-226 activity calculated from the radium-226 content of the source. The radium-226 content of each source is determined by conventional radium-226 analysis, using a radon train (3). The value of detection efficiency for each tube is improved by intercomparison of several bismuth carrier-free polonium-2lCkzinc sulfide sources for each tube (Table I). Measurement of Thorium-230 and Thorium-227. It is assumed t h a t actinium-227, thorium-227, and radium-223 will be in equilibrium with
1402
ANALYTICAL CHEMISTRY
protactinium-231 in geological samples and the determination of the alpha activity of thorium-227 will be used to calculate the protactinium-231 content. The praseodymium oxalate carrier precipitate will contain thorium230 along with protactinium-231 and thorium-227 alpha emitters when products of the uranium-235 decay series are present and the thorium-232 and thorium-228 alpha emitters if any thorium-232 decay series is present. Some bismuth-211 alpha activity may be present, depending on the amount of uranium-235 series decay products in the sample and the time elapsed since the thorium separation, because lead211-bismuth-211 decaying with a 36.1minute half life is carried along with the thorium isotopes. The counting rate of the precipitate is measured a t day-to-day intervals for 6 to 10 days. Long counting-measurement periods are used for determining these rates to establish the amount of radium-223 growth from thorium-227. The thorium-230 nuclide and thorium-
0
,
I
I
227 group are resolved by use of a plot of a straight line of the form: C T = CTh"'
x fTh"'
+
CThYa
where CT is the measured counting rate, CThZz7 the thorium-227 counting rate, CThz3Othe thorium-230 counting rate, and fThZz7 the function of the growth of radium-223 and disintegration products from thorium-227. The time a t which the praseodymium oxalate precipitate was filtered is used as the zero time from which the function of thorium-227 is calculated. The function of thorium-227 with time is shown in Figure 3, plotted from the data obtained from calculation 4. The plot of the straight-line equation is shown in Figure 4, where the intercept on the ordinate of the thorium-230-thorium227 line is the counting rate of thorium-230 and the slope is the thorium227 counting rate. The thorium430 and thorium-227 determination by this method is limited to samples which do not contain quantities of the thorium-232 decay series
I
I
I
in excess of ' / b o for the ratio of per cent thorium-232-per per cent equivalent protactinium-231. Quantities of thorium232 series in excess of this amount can usually be detected in the bismuth sulfide precipitate used for radon-222 determination. The lower analytical limit for thorium-230 and thorium-227 is approximately 0.0027?0 equivalent. Efficiency Calculation and Standardization of Thorium-230, Thorium227, and Radium-223 Measurements. Thorium-230, protactinium-231, and thorium-227 are separated from 4 grams of a naturally occurring source which contains approximately 18% equivalent protactinium-231 and immediate disintegration products, 10% equivalent thorium-230, 13% equivalent radium-226, and 0.1% uranium. The sample of high-grade protactinium-231 source is digested with 20 ml. of hydrochloric acid for half a n hour and filtered, and the soluble portion is retained. Bismuth and polonium isotopes are removed from the solution and the thorium-230, protactinium-231, and thorium-227 are separated by coprecipitation with zirconium phosphate carrier. The precipitate is dissolved in oxalic acid and the solution adjusted to pH 3 with ammonium hydroxide. Two hundred milligrams of zinc sulfide and 10 drops of hydrofluoric acid are added, the slurry is stirred for 5 minutes, filtered, washed, and dried, and the time is recorded. Small fractions of this carrier-free zinc sulfide are used as sources of thorium-230, protactinium-231 , and
thorium-227. Each individual source, which should represent 300 to 1000 alpha-particle disintegrations per minute, is thinly spread on the center of a 50-mm. aluminum ring-supported paper and placed in the automatic sample changer. The alpha particle counting rate is measured a t least once a day for 10 to 15 days. The straightline equation for thorium-230 and thorium-227 is calculated and plotted from these measurements and the counting rates of the thorium-230, thorium-227, and radium-223 are determined from this plot. These counting rates are then converted to total alpha disintegration rates, using the appropriate detection efficiency factor for the phototube from Table I. The active zinc sulfide source is transferred to a nickel crucible and carried through the separation steps for the normal thorium-230-thorium-227 and radium-226-radium-223 analyses. The counting rates of the thorium-230, thorium-227, and radium-223 isotopes from this analysis are compared to the total alpha particle disintegration rates measured in the zinc sulfide source. The efficiencies of the thorium-230 and thorium-227 analyses and the standard counting rates for these nuclides are then calculated (calculation 9). The standard counting rates of thorium-230 and thorium-227 are also determined from several uranium ore samples. Table I1 shows these efficiency values and the comparison of the separately determined standard counting rates. Measurement of Radium-226, Radium-223, and Radon-222. The
barium sulfate carrier precipitate mill contain the radium-226 alpha emitter and the radium-223, radon-219, polonium-215, and bismuth-211 alpha emitters in the presence of the uranium-235 decay series, and the radium224, radon-220, and polonium-216 alpha emitters when the thorium-232 decay series is present. The counting rate of the precipitate is measured a t approximately equally spaced intervals, so that five to seven measurements are made in 2 to 4 days. Long periods of measurement are required in order t o calculate the amount of radium-223 decaying. The radium226 and radium-223 components are resolved by use of a plot of a straightline equation of the form: C T = CRaZz3
x
fRazZ8
+
CRsZz6
x
fRaZz6
or, by rearranging, the equation becomes CT/fRa2"
=
CRa'"
(fRe.z28/fRazm)
+
CRaZm
where CT is the measured counting rate, CRaZz3the radium-223 counting rate, C~,226 the radium-226 counting 3 function of the decay of rate, f ~ ~ z 2the radium-223 and decay products, and f ~ . 2 * 8 the function of the growth of radon222 and decay products. The time of the barium sulfate filtration is used as the zero time from which these functions are calculated. The function of radium-226 and the ratio of the function of radium-223 to the function of radium-226 are shon-n in Figure 3, plotted from the data ob-
Figure 4.
Plot of straight-line equations Ra*26-RaZz3 decay 0 Th*3O-ThZ2' build-up VOL. 2 9 , NO. 10, OCTOBER 1957
1403
tained from calculations 5 and 6. The plot of the straight-line equation is shown in Figure 4, where the intercept on the ordinate of the radium-226radium-223 line is the counting rate of radium-226 and the slope is the radium223 counting rate. The radium-223 content determined from this precipitate should agree rather closely with the thorium-227 content found in the thorium-230thorium-227 precipitate, as radium-223 and thorium-227 are in equilibrium in the original source. The average of these two values is then used for the protactinium-231 content. Determination of radium-226 by the scintillation method is restricted to samples that do not contain more than 20 mg. of barium plus strontium. The same restriction regarding the thorium232 series decay products exists for the radium-226radium-223 determination as for thorium-230-thorium-227. In the special case where radium-228, thorium-228, and radium-224 from the thorium-232 decay series and radium226 are present in a sample that contains thorium-228 exceeding 957, of the total initial alpha activity in the thorium precipitate, the straight-line equations, modified for the presence of radium-224, can be used to calculate the radium-226, thorium-228, and radium224 contents of the sample. I n all other limited cases radium-226 is determined by the radon train method. The lou.er analytical limit for radium226 using the scintillation counter is approximately O.OOl7, equivalent. The method for the measurement of radon-222 has been described ( I 7).
Efficiency Calculation and Standardization of Radium-226, RadiumTable II.
The counting rates of the active sources are measured a t regular intervals for a few hours. The amount of pure radium-226 activity detected is determined by extrapolation of the increasing counting rate back to the time of filtration. Several of the radium sulfate sources are used to calculate the detection efficiency of the preceding measurements. These sources are dissolved in hydrochloric acid and radium in the solution is determined by the radon train method (3). The value of the radium-226 activity determined by the photomultiplier-tube measurement is compared to the total radium-226 activity calculated from the radium-226 content determined by conventional radium analysis (calculation 8). The ratio of
Experimental Alpha-Activity Constants for Thorium-230 and Thorium-227 Determinations
Sample ZnS source
Av. and std. dev. MS-OR (44.96% U) MS-L (6.92% U ) MS-VL (3.08% U )
Efficiency, yo Th23O Than
Standard Counting Rate, C./Min./Mg. Equivalent Th23o Th22’
57.5 57.1 61.4 65.7 61.8 57 6
57.9 60.4 63.0 60.6 63.9 57.8
422 420 451 482 454 423
19.6 20.4 21.3 20.5 21.6 19.6
60.4 f 2 . 9
60.4 f 2 . 4
444 f 2 1 467 480 434 468 447 462 449 468 433 470 458 f 16
20.4f0.8 20.4 22.1 20.3 21.7 20.8 22.0 21.0 20.4 20.3 21.0 21.0 f 0 . 7
NBS (1.0% U)
Av. and std. dev. 1404
223, and Radon-222 Measurements. T h e radium-226 and radon-222 source is a 1 0 3 curie aliquot of the radium-226 solution. A 50-ml. volume containing this source is boiled under partial vacuum while being flushed with air to remove the radon. Radon decay products are removed from the solution at p H 2 to 3 by adsorption on zinc sulfide. It was determined that more than 99% of the bismuth and polonium activity is adsorbed on zinc sulfide from solutions up to 0.5N in hydrochloric acid. Two drops of dilute sulfuric acid and 150 mg. of zinc sulfide are added, and the slurry is filtered, washed, and dried. The time of precipitation is recorded. I n the presence of sulfate ion the zinc sulfide will adsorb a n appreciable amount of radium sulfate from the solution. Small fractions of this active zinc sulfide, free from barium sulfate carrier, are made into separate samples of desired activity and prepared for counting in the same manner as the thorium-230 source.
ANALYTICAL CHEMISTRY
these two values is used as the detection efficiency factor (Table I). A Kational Bureau of Standards 10- gram radium226 standard solution is used to calibrate the radon train analysis. Radon-222 is allowed to build up for 3 days or more in the active zinc sulfide sources which are to be used for over - all efficiency calculation. The counting rate of the radium-226 and radon-222 plus polonium-218 and polonium-214, which are almost in equilibrium with the radon-222 in the source, is determined (calculation 10). For a radon build-up period of 3 to 5 days, one third of the difference between the original activity of radium-226 and the latter activity of the radium226, radon-222, polonium-218, and polonium-214 is the activity of radon-222 a t the time of the latter measurement, assuming constant percentage radon loss. The activities of both the radium226 and radon-222 are corrected to yield total disintegration rates, by use of the appropriate detection efficiency factor. The active zinc sulfide is immediately transferred to a nickel crucible. The daughter products of radon-222 are separated and the counting rate of the bismuth sulfide precipitate for radon-222 analysis is determined in the usual manner. The solution is carried through the separation procedure to the isolation of the radium isotopes. The zirconium phosphate carrier io precipitated and separated. even though no thorium isotopes are present, in order to simulate the actual conditions of analysis of a sample. The counting rate of radium-226 is measured and compared to the total disintegration rate of the radium-226, thus allowing calculations of the efficiency and the standard counting rate of the radium226 analysis (calculation 9). The radium-223, allowed to build up in the carrier-free thorium-230-thorium227 precipitate, is carried through the radium isotope procedure. The efficiency and standard counting rate of radium-223 and its immediate daughters are calculated from these data (calculation 9). Uraniferous samples will contain both radium-226 and radium-223 isotopes. The counting rates of radium226 and radium-223 plus its immediate daughters in the standard ore samples are compared to the theoretical counting rates of these isotopes calculated from the over-all efficiency values of these determinations (Table 111). The over-all efficiency of the radon222 determination is calculated from the ratio of the counting rate of the polonium-214 in the bismuth sulfide precipitate and the total disintegration rate of the radon-222 computed on the radium sulfate-zinc sulfide source a t the time of fusion. The standard counting
rate is calculated from this efficiency value (calculation 10) (Table IV). The standard counting rate of radon222 has been determined, using ore samples in which the determination is independent of radon-222 loss. .
A uranium ore in a nickel crucible is placed in a refrigerator freezing compartment a t -22" C. to impound the radon. After the sample has remained in the freezer for 3 weeks, attaining approximately 98y0 of its equilibrium content due to retention of more radon, it is surrounded by liquid nitrogen for 12 to 14 days. As radon is frozen in the ore sample, greater than 99% equilibrium will be attained with the radium. The freezing point of radon is -110' C., whereas the liquid nitrogen temperature is -196" C. The sample is removed from its freezing environment, and a regular radon-222 analysis initiated immediately. The standard counting rate of radon222 is obtained and is compared to that from the efficiency calculation in Table IV. Table IV also lists the radon-222 analyses of several ore samples a t room temperature. The fraction of radon-222 in partial equilibrium with the radium-226 is calculated, showing the magnitude of the radon-222 loss a t room temperature. Measurement of Lead-210. Lead210 is separated and measured as described (I?'). The assumption is made that 138-day half-life polonium210 will be in equilibrium with lead-210 in geological samples, and the determination of the alpha activity of polonium-210 will be used to calculate the lead-210 content. Efficiency Calculation and Standardization of Lead-210 Measurement. A 50-ml. solution of radio-
actively pure lead-210, bismuth-210, and polonium-210 containing approximately 5 X curie of polonium-210 is adjusted to a pH of 2 to 3, and 150 mg. of zinc sulfide are added. Greater than 99% of the bismuth and polonium isotopes will be adsorbed on the zinc sulfide. The slurry is filtered, washed, and dried. Small fractions of this active zinc sulfide, representing approximately 3000 to 5000 alpha-particle disintegrations per minute, are prepared and counted in the same manner as the preceding nuclides. The counting rate is converted to total alpha disintegration rate by the appropriate detection efficiency factor for the phototube used. The active zinc sulfide is transferred to a nickel crucible, 4 grams of sodium peroxide are added, and the sample is carefully fused to almost red heat. Some polonium-210 will volatilize if the zinc sulfide is not continuously in contact with the sodium peroxide while being heated. The normal lead-210 determination is performed on the source, and the counting rate of the polonium-210 in the bismuth sulfide
Table 111.
Experimental Alpha-Activity Constants for Radium-226 and Radium-223 Determinations
Sample ZnS aource
Av. and std. dev. ITS-OR (44.9670 U)
Standard Counting Rate, C. /Min. /Mg . Equivalent Raz26 RaZ*3
Efficiency, ye Ra226 Razz3 89.2 89.0 88.8 89.1 88.1 90.1 90.4 90.0 89.1 88.5 8 9 . 2 zk 0 . 7
92.9 90.1 87.5 85.0 86.8 93.8 85.3 89.8 94.6 92.0 89.8
3.5
PIIS-L (6.92% U )
PITS-VL (3.08% U)
NBS (1.0% U) .4v. and std. dev.
precipitate is determined. The ratio of the counting rate of the polonium-210 in this precipitate to the total alpha disintegration rate calculated from the zinc sulfide source is the over-all efficiency value for the lead-210 determination. The standard counting rate of lead-210 is calculated from this efficiency (calculation 9). Table V shows both of these values, with lead-210 determinations on several ore samples. The ratio of the equivalent lead-210 content to the equivalent radium-226 content is calculated, indicating the magnitude of the radon-222 loss from the samples over a period of several years. Measurement of Thorium-232. The zinc sulfide-barium sulfate precipitate prepared for the thorium-232 analysis initially will contain the alpha emitters radium-224, radon220, and polonium-216. The bismuth212 and polonium-212 activity will begin to build up as the lead-212 builds up. The activity starts to increase when the zinc sulfide used to adsorb the lead-212 and bismuth212 originally present is filtered from the solution. The precipitate is measured in the same manner as the preceding nuclides. The observed counting rate is corrected to obtain the peak counting rate that radium-224 and its daughter products would have from Figure 5. The fraction of radium-224 built up fpom zero activity from thorium-228 is calculated, knowing the time elapsed between the first thorium separation and the second thorium separation isolating the radium-224 (calculation 11). The cor-
655 654 652 655 647 662 664 66 1 655 650 655 f 5 648 656 653 630 657 665 652 671 660 657 655 f 11
31 4 30 4 29 6 28 7 29 3 31 7 28 8 30 4 32 0 31 1 304zk12 32.2 32.1 30.7 30.0 30.6 32.8 28.0 29.0 31.6 30.6 30.8 f 1 . 5
Table IV. Experimental Alpha-Activity Constants for Radon-222 Determination
Efficiency Calcd. from ZnS Sources,
70
Standard Counting Rates, C/Min./Mg. Equivalent Calcd. from Frozen efficiency ores
19.7 21.6 19.3 20.2 20.4 20.0 19.6 20.7 20.4
145 159 142 149 150 147 144 152 150
148 147 148 143 144 150
20.1 f0.6
148 f 4
147 f 3
Sample MS-OR (44.96% U) GS-64 (73.46Y0 U)
PIIS-L (6.92% U ) >IS-VL (3.0870 U)
XBS (1% U)
Rn2ez Content of Pitchblende Ore Fraction Sample, of Rn22* Equiva- Equilib lent, 70 rium 42.6 43.2
0,948 0.961
68.0 70.5 66.9
0.926 0.960 0.912
6.51
0.941
2.91 2.95 0.95 0.93
0.944 0.957 0.95 0.93
VOL. 29, NO. 10, OCTOBER 1957
1405
, .
Figure 5. Build-up and decay curve of radium-224 0 Theoretical r a t i o
x
rected counting rate of the sample is the peak counting rate that radium-224 and its products have at equilibrium with the thorium-228. Correction should be made for the contribution of radium-223 and its daughter products in the uranium-235 Table V. Experimental Alpha-Activity Constants for Lead-2 10 Determination
?4
Standard Counting Rate, C/ Min./hlg. Equivalent
26.9 26.4 27.1 27.3 26.8 26.7 26.8 26.5 27.3 27.0
198 194 199 200 197 196 197 195 200 198
26.8 ==! 0 . 3
197 f 2
I
Efficiency,
Sample ZnS source
Av. and std. dev.
PbZ1O Content of Pitchblende Fraction Ore Sample, of Pbz10 Equivalent Equilib% rium MS-OR (44.96%O"U) 4 2 , 8 GS-64 (73.46% U)
MS-L (6.92% U) MS-VL (3.08% U)
NBS (170 U)
1466
42.6 42.0 70.2 69.6 68.4 6.41 2.83 2.88 2.86 2.89 0.96 0.96
ANALYTICAL CHEMISTRY
0.953 0.948 0.934 0.957 0.948 0.945 0.927 0.919 0.935 0.929 0.939 0.96 0.96
Experimental
ratio
decay series, if the sample has an appreciable uranium content, as the separation of radium-223 is identical to that of radium-224. I n the time allowed for the radium-224 build-up, no measurable radium-226 will build up from the thorium-230 accompanying the thorium-232 and thorium-228. This determination is limited to ssmples in which the uranium-thorium ratio is not in excess of approximately 5 to 1. At present the lower analytical limit where this ratio is low is approximately 1 y of thorium-232 contained in the sample. Efficiency Calculation and Standardization of Thorium-232 Determination. A thorium nitrate reagent (30 t o 50 mg.) which is a t least 1 month old is used as the source of the isotopes for this efficiency measurement. It does not have t o be in equilibrium with respect t o thorium232-radium-228-thorium-228. The reagent is dissolved in 25 ml. of water, and made up to an acidity of 0.6.V hydrochloric acid, and the solution is brought to a temperature of 45' C. Then 150 mg. of zinc sulfide are added to adsorb the bismuth-212-polonium212 isotopes quantitatively. The slurry is filtered, washed with 0.4N hydrochloric acid and with water, mounted. and dried. The precipitate is collected as a 35-mm. diameter circle in the center of the membrane. The time of filtration is recorded. The counting rate of the zinc sulfide is measured for several successive intervals and the counting rates extrapolated back to the time of filtration. This extrapolation yields the alpha counting rate for bismuth212, polonium-212, and thorium-228. Bismuth-212 and polonium-212 are allowed to build up in the solution from which the zinc sulfide was filtered.
0 X
Theoretical ratio Experimental ratio
Equilibrium of the bismuth-212 with radon-220 can be assured when all of the bismuth-212-polonium-212 activity in the precipitate has decayed. If the polonium-210 activity is in excessof O.lyo of the original bismuth-212-polonium212 activity, it is subtracted from the latter. The solution is made 1 . O N hydrochloric acid a t 60" C., and 1 ml. each of the bismuth carrier, lead nitrate, and ferric chloride solutions are added. Thorium is determined by coprecipitation of bismuth-212-polonium-212 with bismuth sulfide. The filtrate from this separation is also carried through the procedure for thorium analysis by the measurement of radium-224 and its products. The standard counting rate for bismuth-212-polonium-2 12 activity adsorbed on 150 mg. of zinc sulfide is determined by using several 20- to 30mg. portions of the 50-gear-old thorium nitrate salt, These standard values and the average are shown in Table VI. This average value of bismuth-212 expressed as counts per minute per milligram equivalent is used in calculation 11 instead of the specific activity of thorium-232, because of the difficulty involved in the efficiency determination for bismuth-212-polonium-212 adsorbed on zinc sulfide. The efficiencies and standard counting rates of the methods are calculated (calculation 11). These efficiencies, standard counting rates, the counting rates of ore samples, and a 50-year-old equilibrium thorium nitrate reagent are shown and compared in Table VI. CALCULATIONS
The decay constants used are taken from Flanagan and Senftle (8).
Calculation 1. Specific activities of uranium isotopes and equivalent amounts of each decay product (12): SDecific activity of U23s = 739.9 dis./ -min./mg. Uf38 Activitv of U23* in U of normal isotoDe abuidance = (739.9 dis./min./mg.) (0.99285) = 734.6 dis./min./mg./U Activity of U235 = (734.6 dis./min./mg. U) (0.046) = 33.8 dis./min./mg. U
Calculation 2. Specific activity of R a z z 6 and equivalent amounts of each decay product: (6.0250 X lo2$ atoms) (226.1 g.) (10-9 g.) X (8.1252 X l0-l0/rnin.) = 2165 di~./min./lO-~ K. - RaZz6 739.9 dis./min./mg. UZas = o,3417 2165 di~./min./lO-~e. RaZZ6 1 0 - 9 g. Raz26/mg. Uz3* Amount of Raz2o in U of normal isotope 734.6 dis./min./mg. U abundance = 2165 dis./min./lO-g g. RaZ26 = 0.3393 X lO-Og. RazZ6/mg.U Jq
Calculation 3. Specific activity of Th232and equivalent amounts of each decay product:
(9.481 X 10-17/min.) mg. Thzaz
=
246.1 dis./min./
Calculation 4. Function of Th2Z7: Values used for t h e svmbols in the equations are taken from Kirby (11, Tables XV, XVIII, XX, XXI). Total alpha activity/initial alpha activity
Calculation 5. Function of Razz6 ( I I , Tables XII, XXI) : Total alpha activity/initial alpha activity
=
at -
NOXR~'~'
Table VI.
Experimental Alpha-Activity Constants for Thorium-232 Determination Standard Counting Rate, Standard Efficiency, ye C./Min./Mg. ThZ3' Counting Rate RaZa4plus Razz4 plus of Bi212-Poz12 decay decay onZnS, product product C./Min./Mg. Bi212-Po212 measure- Bi*1*-PoZ1* measure- Eauivalent Sample measurement ment measurement ment Biz12 ZnS source 12.0 295 26.0 638 11.9 295 25 8 638 ... 12 0 293 25 9 634 ... 12 1 285 26.2 617 ... 11 8 284 25 5 615 ... 11.7 301 25.3 651 ... 11.9 303 25.8 656 ... ... 12 1 303 26 2 656 ... 11.8 295 25.5 638 ... 11.7 293 25.3 634 ... Av. and std. dev. 11.9 f O . 1 5 295 1 7 25 8 1 0 . 3 638 5 14 1906 thorium nitrate salt (41.4270 Th) 25 8 654 214.8 25 9 A18 215 8 . ~ 25 9 646 SiS. 3 25 9 628 214.6 25 8 638 217.2 NBS (l.Oye Th) 26.2 632 214.0 25.6 635 219.3 25.8 650 218 6 26 0 620 217 1 26 2 640 214 2 Av. and std. dev. 25.9 5 0 . 2 636 5 12 216.4 1 2 0
Efficiency
=
(activity of nuclide in carrier precipitate/detection efficiencv)j100%) (activity of nuclide in ZnS source/detection efficiency)
SCR (Th230, RaZ26,Pb210) = (efficiency) (734.6 dis./min./mg. U) = c./min./100 mg. of 1% equivalent SCR (Th227, ~ ~ 2 2 8=) (efficiency)(33.8 dis./min./me. U)= C./min./ZOO mg. - of I %' equi+al&t ' Calculation 10. ~ f i and ~ standard counting rate of Rn222 determination :
I n the study of many geochemical problems it is desirable to have a quantitative me:tsure of the amount of diseauilibrium in a radioactive ore or rock: For example, in any radioactivei age ~method ~ using ~ ~lead isotopes it is necessary to know the state of equilibrium of a given sample t o study the
Efficiency = (activity of Po214in Biz& precipitate)(detection efficiency) (100%) X (activity of RaZZ6,RnZz2,Popis, PoZ14- activity of RaZ26)in ZnS source activity of PnZz2,Poz1*, activity of Rn222 = (efficiency) (734.6 dis./min.mg. U) effect of gains or losses of some of the c./min./100 mg. of 1% equivalent elements or daughter products (18). Calculation 11. Efficiency and Comparison of the lead-210, the radonstandard counting rates of Th232 deter222, and the radium-226 content is of mination : primary importance in the evaluation of the validity of an age determination Efficiency (Bi212, Po212) = (activity of Bi212, Po212in Biz&precipitate) by the lead-210 method (10, IS). The lead-210 content in the sample is (activity in Bi212,Pozl2in ZnS source) (peak activity of Razz4,Rn220, PoZ18,Bi2l2, Poz1zin Bas04 precipitate) Efficiency (RaZz4)= (activity of Bizla, Po212 in ZnS source) X (1 - e-7.9a4(10-3)r) SCR = (efficiency) (216.4 c./min./mg. also important, in order to measure the equivalent Bi212) = c./min./100 mg. of radon loss extending back over several lye Th232 years. The comparison of the uranium, protactinium-231, thorium-230, and raDISCUSSION dium-226 abundances may be very useful to the geologist in studying the This paper is limited to the descripgeochemical processes involved in the tion of methods used for disequilibrium migration of the elements and in providanalysis and t o measurements obtained ing important clues to the geological on a few selected ores. The results of and geochemical history of deposits. the analyses of a considerable number of Tise of these measurements of nuclides samples and a proposed classification for determining the ages of recent uraof disequilibrium patterns will be prenium deposits, both high and low grade, sented in a forthcoming paper.
SCR =
Calculation 6.
Function of RaZz3:
Total alpha activity/initial alpha activity - 4.0024e-2.579(10-3)t - 2.0O1e-6.366(1O2)L1.0680e-1.152L 0.0656e-1g.25t
+
Calculation 7. Function of Razz4 (11 , Table VII) : Total alpha activity/initial alpha ac-
Calculation 8. Razz6 activity from radon train analysis: RaZza(dis./min.) = (2165 dis./min./lO-g g. Ra226)(g. of RaZ26X 10-9) Detection efficiency = Raf2eactivity by scintillation measurement RaZ26activity by radon train measurement
Calculation 9. Efficiency and standard counting rate of ThZ3O, ThZz7, RaZz6,R a Z 2 3 , and Pb21O determinations:
VOL. 29, NO. 10, OCTOBER 1957
1407
is now being studied. This method may be extremely useful in the study of Recent and Pleistocene geology, where dates can be obtained of materials containing significant amounts of uranium and compared to carbon-14 dates from the same material. I n this method an age of the uranium mineralization or assimilation is obtained when the ratios of protactinium-231uranium, thorium-230-uranium, and radium-226-uranium all yield dates in good agreement. Ages ranging up to approximately 200,000 years could be determined where this method is applicable (19). ACKNOWLEDGMENT
This work was completed as part of a program undertaken by the E. S. Geological Survey on behalf of the Division of Raw Materials, U. S. Atomic Energy Commission. The author is indebted to R. E. Stevens, J. Titcomb, F. J. Flanagan, and F. S. Grimaldi of the Geological Survey for reviewing the report and making many suggestions and to F. E. Senftle, also of the Geclogical Survey, for valuable sugges-
FIuorometric J.
tions and assistance in the course of this work and for supplying the 50year-old thorium nitrate reagent. The automatic sample changers set up by J. R. Dooley, Jr., have been of great aid in the more recent analyses. LITERATURE CITED
( 1 ) Barnes, J. W., Lang, E. J., Potratz, H. A., Los Alamos Scientific Laboratory, Rept. LA-1845 (October 1954). (2) Carney, R. J., Campbell, E. D., J . Am. Chem. SOC. 36, 1136 (1914). (3) Curtiss, L. F., Davis, F. J., J . Research Natl. Bur. Standards 31, 181-95 (1943). (4) Dalton, S. D., Golden, J., Martin, G. R., Mercer, E. R., Thomson, S. J., Geochim. et Cosmochim. Acta 3,279 (1953). (5) Doerner, H. A.. Hoskins, W. J., J . Am. Chem. SOC. 47, 662-75 (1925). (6) Eberle, A. R., Petretic, G. J., U. S.
Atomic Energy Commission, AEC Research and Development Rept., unclassified, NBL-117, 20-2
(9) Hevesy, G., Kimura, K., J . Am. Chem. SOC.47,2540 (1925). (10) Houtermans, F. G., &ad. Wiss. Heidelberg, Math. naturwiss. Kl., Sitzungsber., Abt. 11, 123-6 (1951). (11) Kirby, H. W., ANAL. CHEM. 26, 1063-71 (1964). (12) Kovarik, A. F., Adams, N. I., Jr., Phys. Rev. 98,46 (1955). (13) Kulp, J. L., Broecker, W. S., Eckelmann, W. R., Nucleonics 11, No. 8.19-21 - - -~(1953). 14) Kuroda, P. K., Ann. N . Y . Acad. Sei. 62, 177-208 (1955). 15) Nelson, L. C., Zyskowski, C. L., U. SI Atomic Energy Commission, AEC Research and Development Rept., unclassified, NBL-117, 1719 (1955). 16) Rodden, C. J., Warf, J. C., iiAnalytical Chemistry of the Manhattan Project,” National Nuclear Energy Series, Div. VIII, Vol. 1, p. 172, New York, McGraw-Hill 1950. (17) Rosholt, J. X., Jr., ANAL.CHEM.26, 1307-11 (1954). (18) Tilton, G. R., Trans. Am. Geophys. Union 37,229 (1956). (19) Urry, W. D., Am. J. Science 240, 426-36 (1942). - I
\ - - - - , -
11955).
(7) Facchini, U.! Forte, M., Malvicini, A., Rossini, T., Nucleonics 14, No. 9, 126 (1956).
(8) Flanagan, F. J., Senftle, F. E., ANAL.CHEM.26, 1595-601 (1954).
RECEIVED for review December 31, 1956. Accept,ed May 7, 1957. Publication authorized by the Director, U. S. Geological Survey.
Uranium Ana Iyzer
T. BYRNE
Rocky Flats Plant, The Dow Chemical Co., Denver, Colo.
b A fluorometric uranium analyzer has been constructed which, although simple in design, gives measurements that are virtually insensitive to line voltage variations of f 15 volts and to threefold changes in lamp intensity. An RCA 5819 photomultiplier tube detects the fluorescent light from the uranium-sodium fluoride phosphor, and a second 5 8 1 9 tube monitors the incident ultraviolet light. The output of the second photomultiplier is fed to a series regulator tube, which adjusts the voltage (and sensitivity) of both photomultipliers to compensate for variations in lamp intensity and line voltage.
A
of fluorometric uranium analyzers, based on several different designs, have been described. Some have achieved such a high degree of sensitivity and stability that many workers have concluded that further increases in precision or sensitivity must come in preparation of the fluorescent button. There would seem little justification for adding this description of a fluorometer to the abundant literature, except that it t y p s e s a design NUMBER
1408
ANALYTICAL CHEMISTRY
not previously used, which provides the required stability in a relatively simple manner. The forerunner of today’s high sensitivity direct current fluorometers was constructed by Price (14, 1 5 ) in 1944. This instrument has been modified by Pickle ( I S ) , Center (0,and Kaufman, Castillo, and Koskelo (9). Further development has led to the ORNL Q-1165 (IO), Jarrell-Ash G-M (7), and USGS transmission (6) fluorometers, which are widely used. A different approach was taken by Florida and Davey (6)and by Lynch ( I I ) , who used modulated ultraviolet light and alternating current amplification of the photocurrent. The problems of instability in ultraviolet light intensity and in the photomultiplier or phototube high voltage supply have been met in various ways. The most common approach ( 1 , 7) is to calibrate the instrument frequently against paper, plastic, glass, or uranium standards. This can limit accuracy and become time-consuming. Another approach is to provide a stabilized voltage supply. This leads into more elaborate circuitry.
Lynch’s (11) instrument measures the ratio of the fluorescent light from the unknown sample to that from a reference sample. This ratio is insensitive to changes in lamp intensity and temperature. To minimize fatigue and voltage sensitivity, Lynch uses nonmultiplying vacuum phototubes rather than photomultipliers, and amplifies phototube outputs in a two-channel selective linear amplifier. PRINCIPLES
The Rocky Flats fluorometer employs selected photomultiplier tubes rather than phototubes, because no additional amplification is required and the gain can be readily changed by changing the multiplier voltage. The photomultiplier has an inherently higher ratio of signal to noise and, over a current range of 105, sufficient linearity of response. The voltage-dependent gain can work to the operator’s disadvantage, as the voltage supply to the photomultiplier tube must be carefully regulated. This is frequently accomplished by an electronically regulated power supply. The Spectroscopy Laboratory, The Dow Chemical Co., Midland, Mich.,
