a n d associated manifold pumped into t h e mixing flask, 0, by means of the Toepler pump. The tapered leads to stopcock K assured that the gas was fransferred quantitatively. A suitable amount of methane was pumped into the mixing flask from the reservoir, L, and the gases were mixed by repeated compression and expansion cycles. The mixture was then displaced into the counter system and the counters were sealed off and removed from the line for counting.
and corrected for decay to an arbitrary zero time. The pressure measurements were corrected for temperature and for change in density of mercury with temperature. The exact amount of gas in the differential volume of the counters could then be determined. This was expressed as a fraction of the total gas evolved from the known weight of sulfate used in the dilution. From these data the disintegration rate per milliliter of sulfur-35 stock solution was calculated.
COUNTING
DISCUSSION AND RESULTS
LITERATURE CITED
Two counters, differing only as to length, were used (Figure 4). The volumes of the two counter cathodes were measured by filling them with water from a calibrated buret before assembly. The sensitive volume of the counter was accurately defined by the field tubes. All dimensions were measured to A0.002 em. The long counter had a volume approximately twice that of the short counter. With 3 inches of lead for shielding, these counters had background counting rates of approsimately 2 and 1 counts per second, respectively. Counting apparatus consisted of a scaler, H.T. set, and pulse amplifier. A voltage plateau was determined for each counter. The data were corrected for dead time losses and for background. The differential plateau was derived by subtracting the results of the short counter from those of the long. The differential plateau was usually 100 volts in length with a slope of 1% per 100 volts.
A typical set of results is shown in Table 11. Four generations of S3502 were made from samples containing 5 ml. of standardized copper sulfats solution, and 3 ml. of sulfur-35 stock solution. Duplicate counts mere made on each gas sample. There is excellent agreement between both duplicate counts and between the different generations of V502. The errors involved in the measurement of low pressures of sulfur dioxide may be avoided by a suitable dilution of the gas with methane. The end effects of the counters are avoided by the differential counter technique. Wall effects were investigated by using counters of different diameters, but were not detected. This is in agreement with the results reported by Mann (6). There is little chance of isotopic fractionation in the high temperature solid state reaction used for preparing the sulfur dioxide. By suitable modification of the gashandling system, it should be possible to generate the sulfur dioxide, pump it directly into a small known volume,
(1) Bernstein, W., Ballantine, R., Rev. Sci. Znstr. 21,158-63 (1950).
CALCULATIONS
The observed counting rate was obtained from the differential plateau
measure the relatively high pressure on a mirror scale, and then mix with methane and transfer to a calibrated counter. Such a system would enable accurate, routine measurements of sulfur-35 to be performed. A study of the accuracy of the method and a comparison with other methods for the absolute assay of S35 have been carried out a t Chalk River and have been described by Merritt and others (6).
(2) Hawkings, R. C., Merritt, W. F., Atomic Energy of Canada Ltd., Chalk River, Ontario, AECL 94 (1954). (3) Johnson, R. E., Huston, J. L., J . Am. Chem. SOC.72, 1841-2 (1950). (4) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” pp. 329-43, Macmillan, New York. 1943. ~ ~ (5) Merritt, J. S., Taylor, J. G. V , Merritt, W. F., Campion, P. J., ANAL. CHEM.32,310 (1960).(6) National Academy of Sciences, National Research Council, Washington, D. C., “Measurements and Standards of Radioactivity,” Publ. 573, 102-3 (1958). ( 7 ) Slack, L., Way, X., “Radiations from Radioactive btoms,” U. S. Atomic Energy Comm., 1959. (8) Thode, H., Macnamara, J., Collins, C. B., Can. J. Research 27B, 361-73 (1949). (9) Wilkinson, D. H., “Ionization Chambers and Counters,” p. 152, Cambridge University Press, Cambridge, 1950. RECEIVED for review September 17, 1959. Accepted December 1, 1959. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, 136th Meeting, ACS, Atlantic City, N. J., September 1959. I
The Absolute Counting of Sulfur-35 JANET S. MERRITT, J. G. V. TAYLOR, W.
F. MERRITT,
and P. J. CAMPION
Atomic Energy of Canada limited, Chalk River, Ontario, Canada
b Several methods for determining the absolute disintegration rate of a stock solution of sulfur-35 are presented and compared. These include differential gas counting as sulfur dioxide, 4irb counting with suitable corrections, and a new tracer method. A technique is described for preparing sources for 47rO counting in which a minute quantity of colloidal silica is added to improve the uniformity of the deposit, The corrections associated with 474 counting are described, the largest being that due to self-absorption. In the tracer method, a P-y-emitting nuclide suitable for coincidence counting is combined with the sulfur-35 and used as a tracer to 310
ANALYTICAL CHEMISTRY
follow the efficiency for counting sulfur-35. Experimental justification for this method i s presented. The mean of these independent determinations has a standard deviation of about 1
yo.
T
published proceedings of the Easton Conference on Measurements and Standards of Radioactivity (10) suggest a general lack of agreement on the absolute standardization of sulfur-35, a radionuclide which decays by beta emission only. Because of the low beta end point energy (167 k.e.v.) self-absorption effects are large in most counting techniques (4, 6, ‘7, 9,13, 16), HE
and this may have been responsible for the discrepancies. The wide use of this nuclide makes better agreement desirable; therefore we have examined and compared several methods of standardizing a stock solution of sulfur-3.5. EXPERIMENTAL
The specific activity and radioactive concentration of the sulfur-35 stock solution were approximately 20 me. per mg. of lithium sulfate and 1 me. per ml., respectively. I n addition, the solution contained 2 p.p.m. of potassium dichromate to prevent bacterial growth. Because the results of this analysis were obtained over a period of several
months, all activity values have been corrected for decay to July 15, 1958. During the course of these measurenients the half-life value of 87.16 f 0.10 days (fd)vias confirmed. METHODA. The differential gas counting method described by Merritt and H a n kings (7') was used to obtain a value for the activity of the stock solution of 0.899 0.018 mc. per ml. This value is the average obtained from counting five separate preparations of sulfur dioxide. The standard deviation among these was 1.5%. The error of 2% shown in Table I allows for possible systematic errors ( 8 ) in the method. METHOD B. The second result shown in Table I was determined by 47$ counting samples from suitable dilutions of the stock solution, and applying corrections for absorption in the thin source mount and self-absorption.
-~ --__ Figure 1 . Effect of colloidal silica on observed 4q3 counting rate (arbitrary units)
Figure 1 shows a plot of observed 4 ~ counting p rate zs. quantity of Ludox used, in microliters of a 1 to lo4 dilution of the 15% sol. I n general, about 10 to 20 fil. of such a dilution (or about 0.2 y of silica) seems to give the best results when preparing sources which contain approximately 1 to 2 y of salt. It is felt that the colloidal silica behaves as a seeding agent, because it produces sources consisting of smaller crystals than exist in untreated sources. Although high ionic concentrations should be avoided to prevent gelation of the silica, this technique has been used successfully with solutions as acidic as 0.55. Sources having an average superficial density of 2 y per sq. cm. prepared using these two techniques were 4np counted and the results corrected for the following losses. The corrections for absorption in the gold-coated W K S source mounts and for the threshold detection energy of the counter-amplifier system have been described ( 8 ) . 1-alues for these corrections TI ere 2.1 and 0.5%, respectively. Curves of selfabsorption against maximum beta energy for both methods of source preparation have been determined ( 7 ) by
-
~-
.
-
w
2 E
0 108-
Z I-
I 1061
0
1 1041
5
I 102-
Q
s
1
3 0
*
Dilutions of the stock solution were prepared, maintaining a carrier concentration of 25 y of lithium sulfate per ml. Two source preparation techniques were empio5ed: I n the first a n aliquot was placed on the source mount and allowed to dry under an infrared lamp; in the second a small amount of colloidal silica was added. The colloidal silica used was Ludox SM, a 15% silica sol supplied by D u Pont, which has an average particle size of about 60 A. I n using colloidal silica, a small quantity of a freshly prepared dilution of the sol is placed on the source mount. An aliquot from the radioactive solution to be standardized is then delivered into this droplet. Finally the source is allowed to dry under a n infrared lamp.
_ _
1
a s LI,SO,
I
i
i
00+
i
L
0
20
40
60
SO
100 0
40
20
1 1 60
p i of i:io4 L U D O X S M .
\
30
I1 20
\
i
1
MAXIMUM
BETA
EhERGY.
Me"
Figure 2. Per cent self-absorption as a function of beta end point energy for sources of mean superficial density of 2 y/sq. cm. A.
Table 1.
Untreated sources
C. Tracer method L U P ( SO,), Co"S0, Weighted average
D. Dissolution of "thin" source Weighted average b
Colloidal silica-treated sources
Comparison of Methods for Absolute Determination of Sulfur-35 Activity of Stock Solution, Mc./Ml. ~-
Method of Assay A. Differential gas counting B. 4 4 counting Untreated sources Colloidal silica-treated sources Nbe6-S36comparison 1 y/sq. cm. 3 y/sq. cm. 60 y/sq. cm. Weighted average
a
B.
0.899 1 0 . 0 1 8 0.922 1 0 . 0 4 5 0.900*0.024 0.919 1 0 . 0 1 8 0.90410.017 0.913 dzO.068
0,910 f 0.011" 10.004b
0.918 f 0.015 0.930 =!=0.018 0.923
* 0.006b 0.0125
0.875 1 0 . 0 3 5 0.911f0.007° i 0.007*
Internal error. External error.
VOL. 32, NO. 3, MARCH 1960
31 1
4np-7 coincidence counting (1) and are
y-ray and is therefore suitable for coincidence counting. Sulfur-35 and niobium-95 sources were prepared where the sulfur-35 sources contained inactive niobium salt and lithium sulfate as carrier, and the corresponding niobium-95 sources contained inactive sulfate and niobium carrier. Three carrier concentrations were used to produce groups of sources having average superficial densities of each salt of 1, 3, and 60 y per sq. cm. The first two groups were prepared using the colloidal silica technique, whereas the thick sources were prepared on insulin-treated films ( 5 ) . It was felt that the thicker sources should minimize any differential self-absorption effects due to separation of the niobium and sulfate crystals. The average efficiency for 4 r p counting each group of niobium-95 sources as measured by the 4np-y coincidence method was used to correct the average 4 r p counting rate of the corresponding group of sulfur-35 sources. The resulting values for the activity of the sulfur-35 stock solution were reduced by 1% to allow for the difference in beta end point energy and spectrum shape of the two nuclides. No trend with source thickness was observed. The results of these measurements together with the weighted average of the five determinations of niethod B are listed in Table I.
reproduced in Figure 2. From these curves self-absorption corrections of 23 f 3% for the untreated sources and 8.3 1.4% for colloidal silica-treated sources were applied to the observed counting rates. The corrected disintegration rates are given in Table I, where the errors include the deviation among the sources and the uncertainties in the corrections. An attempt was made to follow the method recommended by Seliger, Mann, and Cavallo (14) for preparing sources of sulfur-35 for 4np counting, which these authors believed reduced selfabsorption to about 1%. Although these sources produced the highest observed 4 n counting rate for the activity of the sulfur-35 stock solution, these results were about 7% lower than the mean value determined by the other methods. This method was not included in Table I , because it was apparent that the low self-absorption reported in (16) had not been achieved. Values for the activity of the stock solution were obtained also by comparing directly a number of sulfur-35 sources with niobium-95 sources. Niobium-95 decays by emitting a beta particle with a maximum energy very close to that of sulfur-35 (160 k.e.v. compared to 167 k.e.v.) but followed by a
Lu'77( 6.8d.)
\
'
'\
Figure 3. Decay scheme for lutetium-177
\ 250
113
0
METHOD C. The analyses of method B do not satisfactorily allow for variations either in the form of the crystalline deposit or in the shape of the beta spectra. I n an attempt to overcome these objections a P-y emitting nuclide suitable for coincidence counting was combined with sulfur-35 in the same compound and used to trace the 4 r p counting efficiency of the sulfur-35. The 178-k.e.v. branch of lutetium-177 (6.8 days) was used as such a tracer in the form of lutetium sulfate. A 1 to 100 dilution of the sulfur-35 stock solution was prepared containing 50 y of lutetium sulfate per ml., more than 100 times the lithium sulfate concentration contributed from the original stock solution. This dilution also contained as lutetium sulfate a convenient amount of lutetium-177 for Coincidence counting. A series of sources covering a range of 4 n counting efficiencies was prepared by deliveringaliquots from the LuJ77(S3504)s solution into various quantities of inactive lutetium sulfate solution. The efficiency of each source for 4np counting the 178-k.e.v. branch of lutetium-177 was measured by 4rp-7 coincidence counting. The determination of this efficiency presented some difficulty, owing to the complex nature of the lutetium-177 disintegration scheme (Figure 3). The results for this branch were determined by accepting pulses in the y-channel corresponding to the peak at 321 k.e.v. only. The sources were stored in a desiccator for ten lutetium177 half lives. The remaining sulfur-35 activity in each source was 4np counted. Figure 4 shows this sulfur-35 4~ counting rate plotted against the counting efficiency of the tracer lutetium-177. The resulting straight line was estrapolated to 100% lutetium-177 counting efficiency to yield a value of 0.918 k 0.015 mc. per ml. for the absolute disintegration rate of the sulfur-35 stock solution, where the principal source of error is the uncertainty in the eytrapolation.
d
d
z *
*
1
m
75-
70
1440 k e v j
~
10c
Figure
312
ANALYTICAL CHEMISTRY
90 80 70 60 47rp COUNTING EFFICIENCY FOR TRACER (%)
5.
Evidence for validity of tracer method
A similar measurenient 11 as made USing cobalt-60 (maximum beta energy, 310 k.e.v.) as the tracer in the form of cobalt sulfate. I n this case it was impossible to allow the tracer to decay before determining the p counting rate due to sulfur-35. Therefore a high specific activity stock solution of cobalt-60 as cobalt sulfate was standardized by coincidence counting. From this cobalt-60 solution and the sulfur-35 stock solution a combined dilution was prepared, from which a series of co6OS36O4 sources was produced. For each source the 4np counting efficiency with respect to cobalt-60 was measured as before and the known absolute disintegration rate of cobalt-60 was multiplied by its efficiency to yield the 474 counting rate due to cobalt-60. This counting rate was then subtracted from the observed 470 counting rate to obtain that due to sulfur-35. A plot of sulfur35 counting rate us. cobalt-60 counting efficiency is shown in Figure 4. The resulting value for the activity of the sulfur-35 solution, 0.930 =t 0.018 me. per ml., is listed in Table I together with the weighted average of the lutetium sulfate and cobalt sulfate results. T o test this tracer method analysis, compounds \\*ereused containing pairs of p-7 emitting nuclides for which efficiencies for 47+ counting could be nieasured independently. It x a s found convenient to choose a pair in which the 4np counting efficiency for a short-lived nuclide could be measured in the presence of its longer lived partner-e.g., cerium-141 (33 dags) was combined with bromine-82 (36 hours) as cerium bromide. For each of a series of sources the efficiency for 4713 counting bromine82 (maximum beta energy, 444 k.e.v.) was determined by accepting in the gamma counter only pulses corresponding to y-ray energies greater than 300 k.e.v., thus excluding any pulses from the 145k.e.v. cerium-141 y-ray. The sources were stored in a desiccator until ten bromine-82 half lives had elapsed. Then the efficiency for 4 ~ counting 0 the 440-k.e.v. branch of the remaining cerium-141 was measured. Figure 5 shows a plot of bromine42 vs. ceriuni141 counting efficiencies. An extrapolation of the line to 100% cerium-141 efficiency leads to a bromine-82 efficiency value of 99.4 & 1.0%. Similar experiments have been made using lutetium bromide (178-k.e.v. branch of lutetium-177) and a n extreme case, sodium bromide (1400-k e.v. sodium 24). I n each case the extrapolated value for the bromine-82 efficiency deviated from 100% by no more than the experimental error. METHODD. Seliger has suggested (11) that if a source of essentially zero self-absorption could be prepared by some nonquantitative technique i t could be 4 r p counted and dissolved quanti-
tatively to prepare a standardized solution chemically identical to the unknown solution, and the solutions compared by any convenient counting method. An attempt was made to produce such a source using the electrospraying technique of Carswell and Milsted (3, 7) in which a high voltage draws a fine spray of active solution from a capillary tip to the source mount. T o estimate the success of this method in eliminating self-absorption a curve similar to those of Figure 2 showing self-absorption us. maximum beta energy for electrosprayed sources was prepared from data of Merritt, Taylor, and Campion (8). Interpolation on this curve indicated a self-absorption for the electrosprayed sulfur-35 source of 3.5%-too far short of the ideal of zero self-absorption to give the method a n y advantage over normal 4ap counting. Nevertheless, the correction was applied and, following the procedure outlined above, the activity shown in Table I, 0.875 zt 0.035 me. per ml., was obtained for the stock solution. DISCUSSION
Differential gas counting is attractive as a n absolute method because the known corrections involved are small (8). 47p counting involves much larger corrections. The correction for sourcemount absorption can be determined accurately (7, 12) and that due to the threshold detection energy of the counter amplifier system can be made small ( 7 ) . The self-absorption correction, however, is large and limits the accuracy of the method. I n this work the self-absorption has been interpolated from curves of self-absorption us. beta end point energy. The accuracy of this procedure, as mentioned above, is limited in that the shape of the beta spectrum and the form of the crystalline deposit may vary from nuclide to nuclide. However, the ease with which such a n analysis may be performed makes i t a n attractive method. The use of sources prepared by the colloidal silica technique is preferred because the correction is smaller and the precision higher than for untreated sources. The NbghS35 comparison, although equally precise, is somewhat less convenient for routine analysis. The 470 counting method suggested by Seliger (method D) is in principle a good approach. However, because the electrospraying technique failed to reduce the self-absorption to negligible proportions, it shows no advantage over straight 476 counting as in B. The tracer method of analysis has several advantages. This method is free from any uncertainties due to nonuniformity of sources, it appears to be reproducible to about 1%, and the valid-
ity of the extrapolation involved has been proved experimentally. It has been shown that the linearity of the extrapolation is to be expected on theoretical grounds by considering various models for the self-absorption process (g). Because a systematic error in the determination of the absolute efficiency for 47p counting the tracer must be considered a systematic error in the analysis, it is desirable to minimize this effect. The accuracy to Rhich the counting efficiency of a tracer may be measured is usually about 0.2%. Therefore in order to prevent this systematic error from exceeding 0.5Oj, in the disintegration rate, it is seen from Figure 5 that a tracer should be chosen having a beta end point energy of no more than twice that of the pure beta emitter . By weighting each of the four methods by the reciprocal of the square of its assigned error, a n over-all average of 0.911 i 0.007 mc. per ml. was calculated. The larger error in the means of methods B and C-Le., the internal error, was used. The agreement between the external and internal errors in the final average suggests that the methods are consistent. As the four methods are essentially independent, this consistency indicates that any systematic error in the mean greater than the standard deviation of about 1% is unlikely. LITERATURE CITED
( I ) Campion, P. J., Intern. J . Appl. Radiatzon and Isotopes 4 , 232 (1959). (2) Campion, P. J., Taylor, J. G. V., Merritt, J. S., Intern. J . Appl. Radiation and Isotopes, in press. (3) Carswell, D. J., Milsted, J., J . Nuclear Energy 4, 51 (1957). (4) Gunnink, R., Colby, L. J., Jr., Cobble, J. W., ANAL.CHEM.31,796 (1959). (5) Langer, L. M., Reo. Sci. Znstr. 20, 216 (1949). ( 6 ) LeGallic, Y., ThBnard, M., Compl. rend. 244,2909 (1957). (7) Merritt, J. S., Taylor, J. G. V., CamDion. P. J., Can. J . Chem. 37, l l 0 9 i 1959). (8) Merritt, W. F., Hawkings, R. C., ANAL.CHEM.32, 308 (1960). (9) Meyer-Schiitzmeister, L., Vincent, D. H., Z . Physik 134, 9 (1952). (10) National Academy of Sciences, NM
tional Research Council, Washington, D. C., “Measurements and Standards of Radioactivity,” Pub. 573, 34 (1958).
(11) Zbid.,p.47. (12) Pate, B. D., Yaffe, L., Can. J. Chem. 33, 929 (1955). (13) Ibid., 34, 265 (1956). (14) Seliger, H. H., Mann, W. B., Cavallo. L. M.. J. Research Natl. Bur. Stan,daids 60, 447 (1958). (15) Seliger, H. H., Schwebel, A., Xucleonics 12, No.7, 54 (1954).
RECEIVED for review September 17, 1959. Accepted December 1, 1959. Division of Analytical Chemistry, Symposium on Radiochemical Analysis, 136th Meeting, ACS, Atlantic City, N. J., September 1959. VOL. 32, NO. 3, MARCH 1960
e
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