Radium Determination by Alpha Counting H. W. KIRBY .Ifourid Laboratory, Miancisbrirg, Ohio
i method is described which reduces the amount of labor and equipment involted in the assay of carrier-free radium and improves the precision and accuracy of the determination. A dilute hydrochloric acid solution of the radium sample is passed through a short column of copper powder to remove polonium. The effluent is mounted on glass disks and alpha-counted 4 to 5 hours after mounting. Twenty-four hours after mounting, the samples are counted again and the percentage increase in counts is used to determine a correction factor for the growth of radon and its daughters. A sample certified by the Rational Bureau of Standards as containing 100 =t0.7 micrograms of radium was found to contain 99.7 i 0.6 micrograms when analyzed by the method descrihed. The method is applicable to radium stock solutions and other samples of radium which are relatitelj carrier-free.
A
SI:('ESSART pre1iniin:rry to any stutlj. of the chemistry of radium is a relinble method of assay of carrier-free radium. The i n v t 4 g a t o r who is concerned with the recovery of radium from ores. from waste solutions, or even from barium concent,rutes must first be able to determine radium in a relatively carrier-free condition so t.hat the compositions of his synthetic mixtures will be accurately known. I n particular, if the recovery of radium in a carrier-free state is the objective, the accuracj- of the data c:mnot esceed the accuracy of the method used for the final assay. Seither beta counting nor g:tmina counting ean qualify as :in absolute method, since the measurements must be niade with an instrument which is regu1:rrly c:ilil,r:ited against :i standard Jample. Geometry must 1 ) ~rigidly controlled as well as shnpe and size of container and thicknws of glass if the tot:il s:miple is counted ( 1 2 ) . If :in internal (windowless) counter is used for beta counting, backscattering and particle energy become import:int. Rackscattering, in partiiwhr, n i a ~ .be as high as tj5c; of the true counting rate i l l ) . Alpha counting in a windowless chamber is the nc:trest : ~ p proach to the absolute mea?iurenient of radioactivitj- whirh is present1.1- available. The highest backscattering values for alphas do not escetd 4% of the true counting rate. \\.here samples :&remounted on glass or quartz, the backscattering contribution is only about 1% ( 2 ) . The determination of radium bj- alpha counting is complictted chiefly by the fact that radon, the first product of the r:idiuni decay chain, is a gas. The classical emanation method ( 3 ) takes advantage of the volatility of radon to determine radium iirtiirectly. The method, in its modern adaptations ($, 7 ) , is est.renicly sensitive, and, if the counter has been properly c:tlihr~tvI and maintained, provides good precision. However, the c1:iI)orate apparatus and continunl maintenance required make t'h(. method unsuitable for the routine analysis of a large number of samples. A more convenient technique involves the aeration of a radium mlut,ion a t elevated temperature to drive off radon and :illow the short-lived daughters to decay to insignificance ( 1 2 ) . This method is restricted to freahly purified samples of r:tdium since the presence of appreci:ible amounts of radium F (polonium-210) results in an erroneously high counting rate. Samples must be counted immediately after purification since no provision is made to correct' for the growth of radon and its decay products after aeration and heating have ceased. Accuracy has been sacrificed for speed in the chloride prccipitation method (1 ). Considerable self-absorption is introduced by the use of barium N R a carrier for radium, and an em-
pirical correction curve is necessary to compensate for this el lor. Here again, measurements of the counting rate must be niade immediately in order to minimize the effect of radon growth. The method provides a n excellent scpaiation of radium from Its daughters as well as from thorium and ur:iniuni and, as such, is a useful preliminary step in any radium determination where w c h contaminants are suspected.
Figure 1.
Correction for Growth of Radium Daughters
The method to be described was developed to rtxluce the rtniount of labor and equipment involved in the a m y of rehtively carrier-free radium and to improve the precision and accuracy of such a deterniin:ition. PROCEDURE
The end of a small borosilicate glass funnel is drawn to an opening of approximately 2 mm. and a small plug of glass wool is inserted. hpprosimately 0.2 gram of hydrogen-reduced copper powder ( I 50 to 200 mesh) is poured into the funnel to form a rolmnri ahout 5 mm. high and 3 to 1 mm. in crow section. The
1238
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3 flow rate should be about 1 drop per second when the stem is filled. The copper is washed with dilute (1:5) hydrochloric acid until it appears bright. The column is thoroughly rinsed with water and dried with alcohol and ether in a stream of air. The stem of the funnel is filled with a dilute hydrochloric acid solution of the radium sample to be analyzed, precautions being taken against the formation of air bubbles in the stem. The funnel is not filled above the stem. The first 2 or 3 drops of the effluent are discarded, and the remainder is collected in a small test tube or vial. The mouth of the collecting vessel is kept small to minimize evaporation. If the sample is not mounted immediately, the vessel is stoppered tightly.
Figure 2.
Regrowth of Radium E and Radium F
After passage through a column of copper powder
Within 3 days after the copper reduction, an aliquot of the polonium-free sample (up to 0.1 ml. containing from 5 X 10-5 t o 0.02 microgram of radium) is mounted on a.glass disk, the mount being spread over as large an area as possible to reduce selfabsorption. The sample is dried under an infrared lamp adjusted to a height of 2 inches from the sample. Zero time (the time when heating was begun) is noted, The dried sample is placed in a closed container, such as a pill box, and the container is kept closed except when the sample is being transferred to a counting chamber. The sample disk is counted in an air-ionization parallel-plate chamber a t 4.0, 4.5, or 5.0 hours and returned to its container until 24.0 hours after zero time, when it is counted again. The percentage increase between the two counts is computed, and the correction factor is found from Figure 1, or by direct calculation as described below. The correction factor is applied to the counts obtained at the earlier time to find the activity due t o pure radium. EXPERIMENTAL
Efficiency of The Copper Reduction Step. A radium D, E' and F equilibrium mixture (RaD-E-F) vias prepared by crushing aged radon capsules under 2 S nitric acid. A portion of the solution was converted t o the chloride by evaporating nearly to dryness with concentrated hydrochloric acid and was then diluted with distilled water to a chloride concentration of approximately 2 N . When assayed, the solution was found to contain approximately 10 microcuries of polonium per milliliter. A portion of the RaD-E-F solution was passed through the copper column, and samples of the effluent were mounted for alpha and beta counting. The samples were counted periodically for 34 days, and the amounts of radium E and radium F were calculated as percentages of their activity in the original mixture (Figure 2).
1239
The amount of radium D present was calculated by differential analysis ( 6 ) from the rate of regeneration of radium E and, separately, from the rate of regeneration of radium F. The residual radium D was found to be 94.5% of the original amount. Only 26% of the original radium E passed through the copper column while radium F was completely removed. After 5 days, less than 1yoof the original radium F had been regenerated. A4fterthe RaD-E-F elution the copper column was washed repeatedly with dilute hydrochloric acid. Beta activity appeared in the effluent (presumably from radium E) and increased to a maximum in successive washes before disappearing. The elution of radium E from the copper column was not studied quantitatively but semiquantitative data indicated that repeated use of the same copper would lead to erroneous results unless the column were first washed completely free of radium E and radium F. Accordingly, the copper was discarded after each run, and a fresh column was prepared immediately before each determination. Determination of Radon Retention. The loss of radon from a solid sample, whether by recoil or diffusion, is a function of such physical characteristics as sample thickness and crystal density. If this is so, the radon retained by the sample should be very nearly a constant percentage of the amount of radon present a t any time. To test the validity of this assumption, a group of three samples !?as counted periodically for 3 days after mounting. The radon retention values and the correction factors calculated from them at 48 and 72 hours were not significantly different from those found a t 24 hours (Table I). Because of mechanical failure or because of the intervention of a holiday or weekend, it is sometimes inconvenient to count samples 24 hours after they are mounted. The data in Table I show that it would be equally accurate to obtain the correction factor from the alpha growth after 2 or 3 days. Analysis of a Standard Sample. .Z gamma-ray standard, certified by the Sational Bureau of Standards as containing 100 f 0.7 micrograms of radium and 2 micrograms of inert barium was diluted and used as a stock solution. Dilutions of this stock solution were prepared containing 0.8 and 0.4 microgram of radium per milliliter in 2 N hydrochloric acid. Duplicates of these dilutions were prepared with RaD-E-F added to simulate an equilibrium mixture. The four dilutions were analyzed by the procedure described. I n addition to the 47 samples mounted on glass, eight samples were mounted on platinum and ignited a t red heat to determine the relative counting yields of the two types of backing material. No difference was found between the counting yield on glass and on ignited platinum, so the accepted value of 1.1148 x 1012 counts per minute per gram of radium mounted on platinum (6) was used directly.
Table I.
Calculation of Radon Retention Calculated Radon Retention, 70
Counts Per Minute at Sample
KO.
1 2 3 Av.
4.0 hours 17,903 17,900 17,831 17,878
24.0 hours 19,726 19,066 19,362 19,385
48.0 hours 21,973 20,657 20,011 20,880
Calculated Correction Factor a t Sample 1No. 1 2 3
Av.
24 0 hours 1.025 1,017 1,022 1 021
48.0 hours 1 027 1.020 1,016 1.021
72 0 hours 1.027 1.020 1.019 1 022
72.0 hours 23,512 21,942 21,477 22,310
24.0 hours 27 17 17 37 22 93 22.49
48.0 hours 29 96 20 3 0 16 12 22.13
72.0 hours 29 03 20.92 18 95 22.97
Calculated Radlurn, Counts/AIinute 24.0 hours 17,466 17,601 17,447 17,505
48 0 hours 17,432 17,549 17,550 17,510
72 0 hours 17,432 17,549 17,499 17,493
Average 17,443 17,566 17,499 17,503
The gamma-ray standard was found to have contained 99.7 & 0.6 micrograms of radium as compared with the certified value of 100 i 0.7 micrograms (Table 11). KO significant difference was found when the analysis was carried out with and without added RaD-E-F. Sensitivity of the Method. To determine whether the method could be used at low levels of activity. a dilution of the standard sample, containing 1.6 X microgram of radium per milliliter and an equivalent amount of RaD-E-F was prepared. The procedure was carried out as usual and 25-microliter samples (containing 4 X 10-6 microgram of radium) were mounted on each of two glass disks.
ANALYTICAL CHEMISTRY
1240 Because of the low counting rate, each sample was counted for an hour starting 4.0 hours after zero time and again for an hour starting 23.5 hours after zero time. The results were computed as though the two counts had been made a t 4.5 and 24.0 hours, respectively. The concentration of the dilution was found to be 1.57 X 10-3 microgram of radium per milliliter, the average deviation being 1.8%. Three additional samples were prepared and counted for two half-hour periods each, but 24hour counts were not obtained. Instead, since the average radon retention for 47 high-level samples mounted on glass was 23.6%, it was assumed that the radon retention in all c m s was about 25%. The average obtained from all five samples, computed on the basis of 25% retention, was 1.59 X microgram per milliliter. The average counting rate was 44.21 counts per minute or 2653 counts per hour. The probable error of the mean was 1.49%. Since each sample was counted for 1 hour, the total number of counts for all five samples was 13,265. It is of interest to note that the probable error inherent in measuring 13,265 random events is approximately 0.87% (square root of the total number of events). It is evident, therefore, that the precision of the method a t low levels is limited by the random nature of the radioactive decay process.
+
1.0000 a t time zero, is given by C = 4.0000 - 3.0096e-X2t 0.0006e-x8t 0.0208e-X41- 0.0118e-x6t where t is in hours, AB = 0.0075, AS = 13.633, A4 = 1.5515, XS = 2.1107. (All X values are expressed in reciprocal hours.)
+
C GROWTH OF bLPHb ACTlVITV FROM PURE RADIUM ASSUMING 100% RbDlUM RETENTION
ICALCULbTED)
s r c .wPnb
OF A RADIUM WWTION OF EWILIBRIUM, AFTER REMOVAL O F ALL PO.BI 8 Rn. ASBUMiNG l O O X R E T E l l T I O Y OF NEW R b W N ACTIVITY INITIALLY son
\,
0.C
I
7’’
DISCUSSION I
Radium decays according to the following scheme: Raz26+ RnZZz + Po218(RaAr+Pb214(RaB) + BiZl4(RaC)+ Po214(RaC’) + Pb210(RaD) + Bi21°(RaE) + Po210(RaF)+ PbZo6 Besides radium and radon, only the three polonium isotopes are significant alpha emitters. Three branch chains exist, involving alpha emitters; astatine-218 (0.04% of the decay of radium A), radium C (0.04%), and radium E (less than 10-60/~) but these may be neglected since after 1 hour their contribution to the alpha activity of the main chain is not detectable within the limits of precision of the parallel-plate counter.
lO3l
Figure 3.
Calculated -4lpha Activity of Radium
The growth and decay of radium C’, the only significant alphaemitting daughter of radium B, is given by Table 11. Analysis of Certified Sample Containing 100 =t 0.7 Micrograms of Radium Samples Counted 15 14 9 9 8‘“ 55
-
Radium Present, y/ML 0.800 0.400 0.800 0.400 0,400
Radium Found in Dilution, RaD-E-F Added, pc./Ml. of P o Cts./Min./rl. 0.0 884.4 A 6.3 0.0 445.5 A 1 . 6 0.8 888.3 A 4 . 2 0.4 448.0 A 1 . 8 0.4 444.7 A 1.8 Mean Probable error of meanb
Total Radium Found, Y
99.2 99.9 99.6 100.5 99.7 99.7 0.6
Platinum mounts. b Probable error = 0.6745
d
=
deviation from mean.
It has been shown that polonium can be quantitatively removed from a hydrochloric acid solution by spontaneous deposition on copper and that 75% of the bismuth is removed a t the same time. If, at nearly the same time, radon is completely removed from the solution by heating, the radium decay chain will be broken into two coexistent but distinct genetic lines headed, respectively, by radium and radium B. The alpha activity of each chain can be calculated from the standard equations for the growth of daughter activities (9). To simplify calculation, the following assumptions are made: ( a ) that all new radon produced after its initial expulsion from the sample is retained, together with its decay products; ( b ) that, just before its expulsion, radon and its short-lived decay products have grown to 50% of equilibrium with radium; ( c ) that the time between the copper reduction and removal of radon is negligible; and ( d ) that 100% of the bismuth, instead of 7570, is removed by copper reduction. These a m p t i o n s lead to small errors which become insignificant within 4 hours. Using the half-life values compiled by Seaborg and Perlman (IO), the relative alpha activity (C) of pure radium, taken as
B = 1.8873(e-h4l
- e-hst)
where B is the alpha activity of radium C’ relative to that of the radium parent. Several useful points calculated from these equations are given in Table I11 (C and B ) , and a plot of both equations is shown in Figure 3.
Table 111. Calculated Alpha Activity of the Radium Chain Relative to That of Pure Radium Time, Hours 4.0 4.5 5.0
A.
B. C.
D.
A B C D 1.0034 0.0034 1,0833 0.0799 1.0012 0.0012 1.0922 0.0910 1,0008 0.0008 1.1029 0.1021 1.0003 0.0003 1.1130 5.5 0.1127 1.0002 0.0002 1.1239 6.0 0.1237 1.0000 0.0000 1.3733 18.0 0.3733 1.0000 0.0000 1.4892 24.0 0.4892 1.0000 0.0000 1,9053 0.9053 48.0 1.0000 0.0000 2.2523 72.0 1.2523 Relative alpha activity of the daughters of radium growing from initially pure radium, assuming no loss of radon. Relative alpha activity of radium C’, the only significant alpha-emitting daughter of radium B, after removal of all polonium, bismuth, and radon from a radium solution which. was initially a t 50% of equilibrium with radon a n d its short-lived daughters. B 1.0). Correction factor for 100% retention of radon ( = A Correction factor for 0% retention of radon ( = B 1.0).
++ +
The method of calculating the correction factor is best explained by an example. Suppose that the samples were counted a t 4.5 and 24.0 hours and that the counting rates were 10,000 and 10,909 counts per minute, respectively. The increase in counts is 9.09%, while the increase in counts if 100% of the radon had been retained (Table 111, C) would have been
1*4892 1.0922
1 = 36.35%
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3 Therefore, the actual radon retention is 9.09 36.35
- ”%
A t 4.5 hours, the relative activity of the alpha-emitting daugho ters of radium (Table 111, A ) would have been 0.0910 if 1 0 0 ~ of the radon had been retained. But since only 25% of the radon was retained, the relative increase is only 0.0910 X 0.25 = 0.0228. In addition, if the samples were mounted immediately after the copper reduction, the radium C’ which grew from unreduced radium B will not yet have decayed to insignificance (Table I, B ) . Adding this small contribution, the total relative alpha activity 0.0012 1.0000 = 1.0240. The alpha a t 4.5 hours is 0.0228 activity directly due to radium is therefore
+
+
wo = OT66 counts per minute 1.0240 To simplify the calculation of the correction factor, a graph can be constructed relating the percentage increase in counts directly to the correction factor. For example, it has already been shown that the increase in counts (for loo’% radon retention) between 4.5 and 24.0 hours is 36.35YG,corresponding to a correction factor of 1.0922 (Table 111,C). Plotting per cent increase on the abscissa and correction factor on the ordinate, a straight line is drawn between the points having coordinates of (0.00, 1.0012) and (36.35, 1.0922). The correction factor can now be read directly if the increase in counts is known. Such a plot has been made in Figure 1 for the usual case where the second set of counts is made a t 24.0 hours, and similar graphs for other time periods can he easily constructed from the calculated values in Table 111. I t should be noted that a small error in the determination of radon retention will not significantly affect the value of the correction factor applied to the 4- or &hour counts. In the example given above, a 10% error in determining radon retention would have been reflected as a 0.270 error in the result. On the other hand, if the first set of counts had not been obtained until 24 hours after mounting, a 10% error in determining the radon retention would have caused a lvo error in the correction factor. The error which may result from neglecting the 4.0-hour counts can be illustrated by reference to Table I. Using only the 24and 48-hour counts, the average counting rate due to radium is found to be 17,079 counts per minute. From the 24.0- and i2.0hour count?, the average obtained is 16,960 counts per minute. The two values are in error hy 2.5 and 3.27,, respectively. The coi respondence betvecn the counts obtained from glass and platinum mounts is surprising a t first glance since backscattering ~ o u l dbe greater from platinum (8). In determining the specific activity of radium, Kohnian, Ames, and Sedlet ( 6 ) assumed, 011 the basis of work done with plutonium ( g ) , that the average counting yield from their platinum mounts would be 0.515111-hile for quartz and probably for glass the counting yield is approximately 27, lower ( 2 ) . However, counting yield depends upon the amount of selfabsorption as well as on the composition of the backing plate, and there is evidence to suggest that radium diffuses into a platinum surface to a significant degree: 1. Once ignited a t red heat, a platinum plate bearing radium can never again be freed of alpha activity by purely chemical means unless some of the surface platinum is dissolved. 2 Kohman et al. (e), found that a platinum sample plate which has been ignited can never again be freed of radon by redissolving and reigniting the sample. This has been confirmed by the author. 3. Radon retention, like self-absorption, is a function of sample thickness. On glass, the average radon retention was 23.6%. On unignited platinum, it was 47.1% and on platinum ignited a t red heat, the average was 70.7%.
1241
It seems likely, therefore, that there may be some absorption of backscattered and selfscattered particles as well as of primary alpha particles emitted a t low angles. Depending upon the depth of penetration, this absorption would tend to cancel out the backscattering effect, making the counting yield equal to, or even less than, that of an identical sample on glass. CONCLUSIONS
The present method has several advantages over previously published methods for the assay of carrier-free radium: No calibration of the counting equipment is necessary. The method is an absolute one and could be applied to the preparation of standard samples. If it is necessary t o assay radium in samples which are not carrier-free, such standard samples could be used for calibration of instruments and for determining selfabsorption corrections. The method requires no special training or equipment other than a parallel-plate counter and can be carried out routinely with a minimum of set-up time. Counting equipment is not immobilized for long periods of time. Duplicate samples can be counted on the same instrument. The method is accurate and precise to better than 1% when the counting rate is sufficiently high to make the counts statistically reliable. The precision of the method is satisfactory a t counting rates as low as 44 counts per minute (4 X 10-11 gram of radium). LITERATURE CITED
(1) Ames, D. P., Sedlet, J., Anderson, H. H., and Kohman, T. P., “Rapid Radiometric Assay for Radium and Application to Uranium Ore Process Solutions,” Natl. Nuclear Energy Ser.,
IV-l4B, pp. 1700-16, New York, McGraw-Hill Book Co., 1949. (2) Cunningham, B. B., Ghiorso, A . , and Hindman, J., “BackScattering of Pu239 Alpha Particles from Platinum,” Ibid., pp. 1192-6. (3) Curie, >I., Radium, 7, 65-70 (1910). (4) Fineman, P., Weissbourd, B. B., Anderson, H. H., Sedlet, J., Ames, D. P., and Kohman, T. P., “An Emanation Method for Radium dnalysis,” Natl. Sucleur Energy Ser., IV-l4B, pp. 1206-25, New York, McGraw-Hill Book Co., 1949. (5) Kirby, H. W., ASAL. CHEM.,24, 1678 (1952). (6) Kohman, T. P., Ames, D. P., and Sedlet, J., “The Specific Activity of Radium,” S a t l . Suclear Energy Ser., IV-l4B, pp. 1675-99, New York, 11cGraw-Hill Book Co., 1949. (7) Neidrach, L. TV., llitchell, A. ll.,and Rodden, C. J., “Beryllium, Magnesium! Calcium, Strontium, Barium, and Radium,’’ Natl. iVucZear Energy Ser., VIII-1, pp, 350-71, New York, McGraw-Hill Book Co., 1949. (8) Rossi, B. B., and Staub, H. H., “Ionization Chambers and Counters: Experimental Techniques,” Natl. .?TucZear Energy Ser., V-2, pp. 127-9, New York, McGraw-Hill Book Co., 1949. (9) Rutherford, E., Chadwick, J., and Ellis, C. D., “Radiations from Radioactive Substances.” p , 13, London, Cambridge University Press, 1951. (10) Seaborg, G. T., and Perlman, I., Recs. M o d Phys., 20, 585--667 (1948). (11) Seliger, H. H., Phys. Rev., 88, 408-12 (1952). (12) Tompkins, P. C., Norris, W. P., Wish, L., Finkle, R. D., and Evans, H. P., “Rlethods for the Quantification of Radium,” MDDC-699,Atomic Energy Commission, June 11, 1946. RECEIVED for review October 27, 1952. Accepted April 21, 1953. N o u n d Laboratory is operated b y Monaanto Chemical Co. under Atomic Energy Commission contract AT-33-1-GEK-53.
Gravimetric Methods for Zinc and Its Separation from Certain EIem ents-C orr ect io n In the article on “Gravimetric Alethods for Zinc and Ita Separation from Certain Elements” [Vance, J. E., and Borup, R. E., A s . 4 ~ .CHEM.,25, 611 (1953)l the legend under Figure 1 should indicate that the open circles refer to solutions to which no ammonium sulfate was added.
