olation to zero weight the intercept is the total blank, including the effect of the glass attack. This value was not significantlv higher than t'he blank referred t o abore; thus it !vas concluded that the effect the gla" att'ack "as nil. ACKNOWLEDGMENT
The author wishes to thank A. D. Kirshenbauni and D. D. Killiams for
their helpful suggestions in review of this paper. LITERATURE CITED
Goris~ p., DuffY, E., TirWY, F. H., ANAL.CHEJI.29, 1590 (195T). (2) Holt, €3. D., E.S. Atomic Energy Comm., Rept. ANL-4388 (1950). (3) Kirshenhaum, il. I)., Grosse, A. V., Ax.4~.CHEJI. 26, 1955 (1954). (4) Kirshenbaum, -1.D.: Grosse, A. \-,, d n a l . Chim. Acta 16, 225 (1957). (5) Peplrowitz, L. P., Proud, E. R., IXD. ESG.CHEX., AX'AL. ED. 2 1 , 1000 (1949).
(6) Simpson, 0 . C., Rauh, E. G., C. S. Atomic Energy Comm., Rept. CF-3748 (1946). ( i ) Tilton, G. R., Aldrich, L. T., Inghiam, M. G., Asar,. CHEX 26, 894 (1951). (8) \Yiiliams, I). D., S a v a l Research Laboratory, Memorandum Rept. 33 (1952). (9) Zaidei, A , y , , Petrov, -1,A,! Zhur. Tekh. Fir. 2 5 , 2571 (1955).
RECEIVEDfor reviev April 28, 1958. Accepted September 18, 1958, Based on work performed under the auspices of the U.S.iltomic Energy Commission.
Radiometric Determination of Krypton-85 R. B. REGIER Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
,Apparatus and procedure for the determination of krypton-85 in gas samples are described. With a single dilution, samples ranging in specific activity from about 1 X microcurie to 10 millicuries per ml. have been analyzed. The method of determining the absolute disintegration rate of krypton-85 is outlined. In routine use, the procedure i s precise to 7.1y0 a t the 95% level of confidence. The accuracy has been estimated b y comparing the analyses of high activity samples with those obtained with a mass spectrometer. In the comparison of 1 1 such samples, the average of the radiometric to mass ratios was 1 .O 1 f 0.105 a t the 95% confidence level.
lop3
R
measurements of gaseous activity are frequentlj. made by introducing the sample directly into the smsitive volume of the counter, so t'hat it is an integral part of the counter filling gas ( 2 , 3 ) . This procedure is slon- becausr the plat,eau of the counter must be measured with each filling. components of the gaseous sample not compatible with a good filling gas must be removed prior to analysis, and the maximum activity that can be tolerated in a sample is fairly lo^. A method has been developed that obviates thew disadvantages, and is both rapid and accurate over a vide range of activities. -4DIOMETRIC
APPARATUS
Figure 1 depicts the vacuum line used t o make sample dilutions and to fill the cell which contains the gas for beta counting. Pressures were measured with a Todd Universal vacuum gage, A , a NcLeod-type gage, which has three
54
ANALYTICAL CHEMISTRY
pressure scales: 0 to 0.5, 0 to 5, and 0 t o 25 nini. of mercury. The counting cell filler, B , was connected to the vacuum line with an 1 8 j i spherical joint. This filler n-as made froni a pair of standard-taper 55, '50 joints, 15-ith a three-way, 3-mm. T-bore stopcock. A female standard-taper 5,'12 joint inside thc filler supported the gascounting cell. The gas-counting cell n'as machined from brass. It was 1 inch in diameter by '/4 inch deep (inside diniensions), and its wall thickness was ' j Sinch. A Mylar nindow 1.8 mg. per sq. em. was ceinent'ed to the top of the cell n-it'h Multi Lok adhesive (Sational Starch Products, Inc.). An int'cgral part of the cell was a ':,-inch long tube Ivhich extended beneath the cell, and into which a 7320 Corning microstopcock was cemented with Apieson W v-ax. The other arm of the stopcock was attached to a male standard-taper 5,"12 joint. The over-all length of the counting cells, 6.7 to 7.0 cm., permitted use under an end-nindow proportional bet'a counter. The hole in the base of the brass cell was 0.0625 inch in diameter, small enough so that
t,he gas in the stem of the cell contributed insignificantly to the counting rate. Sample gas was adiiiit'ted to the vacuum line through stopcock 7, rvhich had a three-way T-bore plug, permitting the sample line to communicate with both t,ube D and the rest of the Tube D, connected through sto \\-ith a' Irveling bulb containing mercur!-. was used to regulate the volume of sample admitted to the systriii. Initially, D had a volume of about 125 nil., so that by appropriate manipulation of the lcveling bulb and the stopcocks, saniples could be pumped from t'he sample container into bhe vacuum line. Subsequent,ly, this \vas found unnecessary, and a 20-mm. tube about 3 inches long was substituted. Tube C was a condenser ininiersed in liquid nitrogen during portions of the sample preparation. Stopcock 5 was conncctcd u i t h plastic tubing t o a cylinder of compressed et'hylene. The IT-tube manometer, E , was filled with mercury. Rubber tubing connections )yere used b e t m e n stopcocks 6 and 7 , and hrtneen stopcock 7 and the w n p l e
Figure 1. Vacuum line for preparing krypton-85 samples
coiitaiiicr. The vacuum pump attached a t stopcock 9.
was 0
PROCEDURE
After the sample container has been connected to stopcock 7, the entire system is evacuated to a pressure of less t'han 10 microns. Stopcocks 3 and 4 should both be open to avoid rupturing the window on the counting cell. Stopcock T is turned to connect the sample line with tube D, and the sample container is opened to permit the sample to expand into the volume not occupied by mercury. Stopcock 7 is again turned to permit the sample from tube D to expand into the vacuum line. Stopcock G is then closed. After the pressure of the saiiiple has been measured Lvith the vacuuni gage, ethylene is admitted until the entire system is a t a pressure of 1 atin., :is indicated by manometer E. The gas vontained in the system is mixed by alternately condensing i t with liquid nitrogrii a t C, then rapidly vaporizing it by placing warm water around C. Four c\-clcs provide adequate mixing. Cell filler B is detached from the vacuiiiii line after stopcock 4 has been closed, and the counting cell is removed for bota counting. The cell is supported by a slotted aluminum card that fits the shelves under the proportional counter. Sufficient space remains between tlic top of the counting cell and the counter to permit insertion of a n aluniinuni absorber approximately 80 mg. per sy. cni. which is used with samples having activities too great for counting. It provides an approximately 10-fold attenuation of the counting rate. The exact. absorber attenuation was determined by counting a number of samples in which the activity was low enough to be counted properly \i-ithout t'he absorber. DETERMINATION
OF
GEOMETRY
Thallium-204 was used for calibrating the counting arrangement in terms of the absolute disintegration rate of krypt'on-85. These two nuclides are both essentially pure beta emitters with similar iiiaximuni energies [krypton-?&, 0.695 ni.e.v.; thallium-204, 0.765 m.e.17. ( I ) ] . Accordingly, it was assumed that t'hese t'wo nuclides behave in the same n.ay with respect to back-scattering and absorption in the cell nindovr- and the counbrr window. Aliquot~s of a t'hallium-204 solution that had becsn standardized by 4~ counting m r e stippled uniformly over a thin rubbcr hydrochloride film (less than 0.5 mg. per sq. cm.) cemented to a thin Lucitc. ring. This ring had a n out,side diameter of 1 inch, so the activity was distributed orer an area nearly equal to that of the ITindoIv of the counting ~ 1 1 . The foil btaring the thallium activity was placed in t,he brass counting cell and counted a t fire different, levels 16 inch apart, including t h r top and bottom planes of the brass cell.
0
0 0
TL"'
SOURCE
POSITION
Figure 2. Measurements of cell geometry
The observed counting rate was plotted against the position of the source in the cell to obtain the curve shonn in Figure 2. The ordinate, marked Geometry, n-as obtained by dividing the observed counts per minute for a given position b y the knonn disintegration rate of the source. By graphical integration the area under this curve was divided equally as shonn. Point JI defines the effective mid-plane of the counting cell, and to compute the geometry of counting it may be assumed that all the radioactivity originates in this plane. As expected, this plane n as dightly above the geometrical center of the counting cell. The gcometry factor determined by this technique was 0.227 for the counting arrangement used in this study. DISCUSSION
This procedure has been in use in the control laboratory of t'he Idaho Chemical Processing Plant' for over two years, to measure the concentration of krypton-85 in samples nhosc activity ranges from lo3 to lo7 disintegrations per minute per ml. Quality control analysis of the laboratory's precision in using this procedure shows that \Then the averagt' of duplicate analyses on a single sample is reported, there is a n uncertainty of 7.1% a t the 95% lrrel of confidence. This radiometric niethod has been direct81y comparcd 114th mass spectrometric analyses on 11 high-activity krypton-% samples (Figure 3). A half life of 10.4 years was used in converting t'he data of mass analyses to disintegration ratcs. I n the range of concent'rations compared, the a r erage ratio of the radiometric to mass analyses n-as 1.01, with a n error of 10.5% a t the 95% level of confidence. This indicates no significant' bias in the radiometric method at. these high
'Ic
ANALYSES
Figure 3. Comparison of radiometric and mass analysis
levels, and presumably there is none a t the lower concentrations found in dissolver off-gas. I n routine use approximately 50 minutes are required to make duplicate analyses of a single sample. The "memory" effect of the vacuum line for previous samples is negligible 1% hen all samples being analyzed are comparable in specific activity. Hon ex-er, v h e n samples of very high activity are introduced into the apparatus, as n a s done when the measurements shon n in Figure 3 were obtained, about 24 hours of pumping Kvith a riiechanical pump are required to remore the adsorbed activity. ACKNOWLEDGMENT
The assistance of D. G. Olson in helping to build the apparatus and derise a satisfactory operating procedure, and of G. A. Huff and J. A. Nerrill for collecting and statistically analyzing the quality control data. is gratefully acknowledgcd. LITERATURE CITED
( 1 ) Hollander,
J. &I,,Perlman, I., Sea-
borg, G. T., Rev. Modern Phys. 25, 469
(1953).
( 2 ) Reynolds, JI. B.. ?iucleonics 13. N o .
5, 54"(1955). (3) Wilson, E. J., Evans, C., Chadn-ick, J., Eakins, J., Taylor, K. T., Brit. iltomic Energy RePearch Establishment, Report, AJIRE-I/R-2216 (1955).
RECEIVED for review Julv 7, 1958. I c cepted November 3, 1958.
END OF SYMPOSIUM VOL. 31, NO. 1, JANUARY 1959
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