gested ( 5 ) . These applications are now feasible as routine procedures with the present apparatus (3). LITERATURE CITED
( 1 ) Davies, P. W., Brink, F. Jr., Rev. Sci. Znstr. 13, 524 (1942). ( 2 ) Kolthoff, I. M., Lingane, J. J., “Polarography,” p. 307, Interscience Publishers, Inc., New York, 1946. ( 3 ) Lipner, H., Witherspoon, L. R.,
ChamDeaux. V. C., ANAL. CHEM.36,
204 (i964). ’ ( 4 ) Mancy, K. H., Okun, D. A,, Reilley, C. N., J . Electroanal. Chem. 4, 65 (1962). ( 5 ) Mancy, K. H., Westgarth, U’. C., J . Watsr Pollution Control Federation 34. 1037 (1962’). ( 6 ) Osaki, ‘S.,Arch. Biochem. Biophys. 100, 378 (1963). ( 7 ) Singer, T. P., Kearney, E. B., Arch. Biochem. 29, 190 (1950). (8) Umbreit, W. W., Burris, R. H.,
Stauffer, A. F., “Manometric Techniques,” 3rd ed., Burgess Publishing Company, Minneapolis, Minn., 1957. RECEIVEDfor review October 2, 1964. Accepted January 8, 1965. This investigation was supported by PHS research grant AM-01904 from NIAMED, Public Health Service, and i n part by a contract between the Division of Biology and Medicine, U. S. Atomic Energy Commission, and the Florida State University.
Decay of Cesium-1 37 Determined by Absolute Counting Methods JANET S. MERRITT and J. G. V. TAYLOR Chalk River Nuclear laboratories, Atomic Energy of Canada limited, Chalk River, Ontario, Canada Precise decay data for have been determined by absolute counting techniques aided by a rapid method for the carrier-free separation of The following values for the decay parameters were obtained: 0.003 half-life of 2.554 minutes; total internal conversion coefficient of the 662 k.e.v. transition, 0.1100 f 0.001 1 ; K-conversion coefficient, 0.0894 f 0.001 0; branching ratio for the decay of Cs137to 0.952 f 0.010;ratio of 6 6 2 k.e.v. y-ray emission to Cs137disintegra0.009. The tion rate, 0.857 f 1% obrelative uncertainty of tained for the latter ratio should make possible the absolute assay of Cs137by y-ray spectrometry to + 2% or better.
*
*
B
-
Cs13’ is a prominent longlived fission product and a very useful y-ray standard, there has been a demand for accurate measurements of its radioactivity. These measurements have been complicated by uncertainties in the decay scheme. Values reported for the branching ratio to the 662 k.e.v. level in Ba137 vary from 0.92 to 0.97. Also there has been some uncertainty in the total internal conversion coefficient for the 662 k.e.v. transition. Altogether the uncertainty in the 662 k.e.v. ?-ray abundance in the Cs137 decay has limited the accuracy with which C S ’ ~can ~ be assayed by y-ray measurements to about 5%. A direct determination of the total internal conversion coefficient, CY, of the 662 k.e.v. transition in Ba137 was made by counting simultaneously conversion electrons and y-rays from Ba13’m sources and is reported here. This determination of CY was combined with absolute p- and y-ray measurements of a Cs137 stock solution to yield a new ECAUSE
C O ~ F ~ ( Cwas N ) ~distributed throughout a column which permitted a reasonable flow rate. Cs1a7 stock solution was fed onto the column and >99% of the PI (518keV) cesium was adsorbed. The column was washed with 2 ml. of lh‘ HC1 and 13, ( l l B 0 ke\ a period was allowed for growth of the Ba13Trndaughter. Then a Ba137m milking consisted of flowing 1N HC1 through the column and collecting individual drops of the effluent. Ba13’lrn Figure Decay scheme for C S * ~ ~ was eluted promptly and sharply and a separation factor from Cs137 of >lo5 was achieved. and more precise value for the 662 Ba137rnsources suitable for 4nB counting were prepared by evaporation of k.e.v. y-ray abundance and a value for small samples of the eluate onto gold the @-branching ratio to the 662 k.e.v. coated collodion films. These films level in Ba137. These new values have could be placed very close to an infrared been incorporated in the decay scheme lamp without damage. The time reshown in Figure 1. quired for a 13a137rn milking and source preparation usually was 1 mc. (3). Response curves were obtained per mg. cesium. Part of a y-ray for ampoules in three different conspectrum of this solution taken with a tainers: 0.002-inch AI, 0.016-inch Cd, lithium-drifted germanium 7-ray specand 0.048-inch Cd lined with 0.005-inch trometer (88) is shown in Figure 2 Ta. The latter two source holders were together with a spectrum of the same designed for use when it is necessary to material with 2.0y0 C S ’ added. ~~ From absorb out x-rays accompanying electhese spectra it was concluded that tron capture or internal conversion, there was less than 0.02y0 Cs134in the but they are also useful in that they Cs’37. provide three partially independent A method was developed for the rapid response curves for checking the preseparation of carrier-free samples of cision of the method. Ba137m. This method made use of the Ba137m Counting. The 1 3 a 1 3 7 m sources adsorption of Cs137 on Co&’e(cK)~ were counted in a system consisting of a 4np-proportional counter sandwiched (10). C O ~ F ~ ( Cadsorption N)~ columns were between two 3 inch X 3 inch ?;a1 prepared as follows: about 2 ml. each y-ray detectors ( 3 ) . Essentially all of K,Fe(CN), solution, water, Cos04 ( >99.9yo) of the conversion electrons solution, and water were passed in were detected in the 4xp counter (fy). turn through small (0.03 cm.2 X 6 The y-ray emission rates of the sources cm.) columns of Dowex-1. I n this way were determined by direct comparison
c 5‘3’ (30yl
n .
VOL. 37, NO. 3, MARCH 1965
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I H)
.95
.90
85
.BO
75
70
,65
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Figure 3. Plot of 47r counting rate due to Cs137 vs. 47r counting efficiency for Bra2 for sources of Cs'37 Bra2
I
I
I
I
I
100
I
I 200 CHANNEL
Figure 2. A. B.
I
1
I
1
I 300
I
I
for and Co60 are plotted at the mean energies for the two -prays in each case. The point for Xa22 is the response for annihilation radiation after subtracting the response for the 1277 k.e.v. y-rays obtained from the Coco response. Where applicable, corrections were made for branching ratios, internal conversion, low-abundance yrays, and bremsstrahlung. The latter correction was estimated from measurements with pure &emitters. The response for 662 k.e.v. ?-rays was obtained by quadratic interpolation in the appropriate response function for each sample holder. For samples absorption in the steel wall of the well of the ionization chamber rendered the response to the Ba K-x-rays following internal conversion negligible even for the A1 holder. The bremsstrahlung correction amounted to about 0.1% in the A1 holder and less in the other two holders. From the uncertainties in the interpolations, the consistency of the results obtained with the three holders and among samples, and the accuracy of the primary standardizations, a relative error of &0.7y0was estimated for the determination of the -pray emission rate of the Cs13' stock solution. The relative standard deviation of the individual measurements was less than
I
NUMBER
y-Ray spectra of
Stock solution, using lithium-drifted germanium detector Sample with 2 7 , added
with similar sources of Cs'37 awurately mean of the ratios of electron emission weighed from the stock solution. The rate to ?-ray counting rate for the electrosprayed sources was only &0.1%. counting rates in both the 4irP and y-ray counters were followed for about 20 13a137mhalf-lives to look for any Cs'37 RESULTS A N D DISCUSSION contamination. Gamma-Ray Measurements. T h e Csl37 Counting. One met'hod used for determining the absolute total ionization chamber response curve electron emission rate of the C S ' ~ ~ for the 0.016-inch Cd source holder stock solution was the 4 ~ p - yefficiency is shown in Figure 4. Similar curves t,racing technique ( 4 , I S ) with the use of were obtained for the other two Bra2 as tracer. I n this method a series source holders. Since these curves of sources is prepared of Uraz combined were obtained with samples for which with Cs137 as CsUr and with varying the absolute disintegration rates had quantities of inactive Csl3r carrier been determined to 10.2%, their added. The efficiency for 4~ counting accuracy depends upon the reliability Ur82 is measured by the coincidence method. The contribution of the BrS2 of the y-ray abundances used. Since activity to the 4 r p counting rate for these in turn depend upon accurate each source is computed and subtracted branching ratios and internal conversion from the observed 4?rp counting rate. coefficients very few nuclides are This net counting rate is due to Cs137 available for which they are known to and is plott'ed vs. the measured lW2 0.3y0 or better. Data for the nuclides 4~ counting efficiency for each source shown in Figure 3 came from the Nuclear (Figure 3). Extrapolation by leastData Sheets (20) plus references (30) squares to 100% efficiency gives the for Ce139 and ( 1 5 ) for S'b95. The points total electron emission rat,e. A 0.15y0 correction was applied for the detection of 662 k.e.v. y-rays in the 4 r p counter (29, 35). The second method adopted for determination of the electron emission rate of the Cs137 stock solution was 4 ~ counting. p Several very thin C S ' ~ ~ sources were prepared by the (nonquantitative) electrospraying technique (6,17')from a solution of CsCl in ethanol. These sources were 4irp counted and a self-absorpt,ion correction of 0.5y0 was applied ( 1 7 ) . The y-counting rates for these sources were compared with the y-ray emission rate of the stock solution in the same manner as for the t 13a137mcounting. From these data the electron emission rat,e of the Cs'37 stock solution wa.5 calculated. The relative standard deviation for the ENERGY (MeV1
-
10.1% Internal Conversion Coefficient of 662-k.e.v. Transition. After making small cxrections for dead-time losses
Figure 4. lonization chamber curve of response vs. y-ray energy for 0.01 6-inchcadmium sample holder
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ANALYTICAL CHEMISTRY
4
10
12
14
the minute CsI3’ contamination in the Ba137” sources (< 3 X lop5 initially) was subtracted together with the backgrounds. Weighted least-squares fits were made to the decay data for both the 4ap and y-ray counters over the first 10 half-lives. The ratio of 47rp to ?-ray counting rates was taken to be the ratis of their respective intercepts from the leads-quares analyses. X correction of (1.6 f 0.3)7&was applied for the response of the 4ap counter to the 662 k.e.v. y-rays (29, 35) to get the ratio of conversion electron to y-ray counting rates. This ratio, corrected for the ?-ray counter efficiency obtained from the coniparison with accurately weighed samples of the stock solution, gave directly the total internal conversion coefficient, CY = e / y = 0.1100 i 0.0011. Allowances for possible systematic errors of f 0.6% in the y-ray calibrations and f 0.3Y0 in the conversion electron measurements were added and the sum combined in quadrature with a relative standard deviation of i 0.3y0 to obtain the quoted error of =k l.Oyo. h value of 0.813 5 0.003 for the Kltotal conversion line intensity ratio was measured for us by J. S. Geiger with the use of the Chalk River 7r& &spectrometer (5’1). Applying this measurement to our value for a , the K-conversion coefficient, c y K , was calculated to be 0.0894 f 0,0010. This and other conversion coefficient determinations (1,7-9, 21-13, 26, 19, 21, 23, 32-34, 36) are listed in Table I, where it is seen that a considerably smaller errm has been obtained with the present direct method. The theoretical K conversion coefficient for a n h14 transition was obtained from Sliv and Band (27) by numerical interpolation. The agreement between the present value and theory is satisfactory since the latter is uncertain by 1 to 3YC. A brief discussion of this result has been submitted to Nuclear Physics (31). Ba13’ Half-Life. T h e half-life of 13al3~ calculated ~ from the results of the least-squares analyses of both @- and y-counting rates for two runs was 2.554 f 0.003 minutes. The error includes an allowance of f O . l O j , for possible systematic errors, which pmbably is generous since the Ra’37“ ratio was < 3 x 1 0 - ~initially and none of the results from the two completely independent counting systems was outside this limit. C S ] 662-k.e.v. ~ ~ y-Ray Abundance. T h e total electron emission rate for ~ solution as determined the C S I ~stock by the tracer technique and 47r@ counting agreed to 0.4TC with relative standard deviations of f0.3% (Figure 3) and s = O . ~ O ~respectively. ,, The value obtained for the internal conversion coefficient was combined with the measured y-ray emission rate of the
Table I.
Values Reported for Internal Conversion Coefficients of 662-k.e.v. Transition
Reference Townsend et al. ( 3 2 ) Osoba 121) Mitchell and Peacock ( 1 9 ) Waggoner ( 3 3 ) Heath arid Bell ( 1 1 ) Dolishnyk et al. (9)
Q
ffK
0.12 0.081
0.118 0.097 0.095 0.11 0.096 0.092 0.095
Azuma ( 1 ) Wapstra ( 3 4 )
LIcGowan arid Stelson ( 1 6 ) Hicci ( 2 3 ) Yoshizawa ( 3 6 ) Hultberg and Stockendahl ( I S ) deVries et al. (8) Hultberg et al. ( 1 2 ) Daniel and Schmitt ( 7 ) This work
Theoretical value ( 2 7 )
0.114 f 0.022
0.114 f 0 . 0 3 0.1100 f 0.0011
0 92
(7)
0.0976 f 0,0055 0.093 f 0.006 0.093 f 0.005 0.095 rrt 0.004 0.093 f 0.003 0.0894 f 0,0010 0.092
ACKNOWLEDGMENT
(1L)
This work
f 0.006 f 0.008
0.935 f 0.002 0.952 f O . 0 1 0
0.95
W&&tra ( 3 4 ) Ricci (23) Yoshizawa ( 3 6 ) Daniel and Schmitt
f 0.005
0.97 0.952 rrt0 003 0.924rrt0.008
Reported value >0.95,
