Some of the papers presented in the Symposium on Radiochemical Analysis, Division of Analytical Chemistry, 133rd Meeting, American Chemical Society, San Francisco, Calif., Other papers April 1958. from this symposium will be published in January
RadiochemicaI Analysis
System for Counting Tritium as Water Vapor W. F. MERRITT EnvironmentalResearch Branch, Biology and Health Physics Division, Atomic Energy o f Canada, ltd., Chalk River, Ontario, Canada
,A method i s described in which tritium is counted as water vapor in a heated proportional counter filled with methane. Introduction of the sample i s rapid and simple and convection mixing with the counting gas is obtained from the heating system. Good plateaus are obtained with small partial pressures of water vapor at 90" C. The counter i s calibrated using a standard tritiated water sample. The system i s simple and rugged and gives results reproducible within f 2y0 standard deviation.
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I . A S O F T SOLDER I A LSI M A G I N SU LATO R
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Figure 1. Tritium counting apparatus
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301'' TUNGSTEN
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HE use of tritiated water for measuring ground water movement is not \Tidespread because of the difficulties involved in counting the low energy @-particles of tritium. I n general, the sample is converted to hydrogen gas or a gaseous hydrocarbon and assayed in a Geiger or proportional counter or an ionization chamber (2, S), or is assayed directly as tritiated water by liquid scintillation counting ( 2 ) . A simpler system is possible if the tritiated r a t e r is measured into a counting system and counted as water vapor. Drever and Moljk (1) have used such a system with limited success. This paper describes an improved apparatus using a heated, circulating counter.
I I
METHOD
Details of the counter and vacuum system are shown in Figure 1. The counter was made of stainless steel 18 inches long and 1.5 inches in inside diameter, with ceramic insulators sealed in place with a synthetic resin. The valve leading to the counter was
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Figure 2. Effect of water vapor on counter performance b
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EXTERNAL S O U R C E
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VOL. 30, NO. 1 1 , NOVEMBER 1958
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a stainless steel diaphragm valve capable of Ivithstanding elevated temperatures. The side arm, 0.5 inch in outside diameter, was wrapped with a 200watt glass heating tape which supplied the heat t o the system and ensured adequate mixing of the contents of the counter by convection. The counter, valve, and sample tube rvere enclosed in a copper box which acted as both electrostatic and thermal shield. A two-stage mechanical pump was used to evacuate the system. Technical grade methane (96%) was used as the counting gas. The sample to be assayed ~ v a ~ measured into the standard-taper sample tube with a micropipet which had been treated with silicone to prevent wetting. The sample was frozen in liquid nitrogen and the tube connected to the line. The short section of line was evacuated, the stopcock closed, and the valve to the counter opened. The sample distilled into the counter in 5 minutes. Methane, which had been escaping through the mercury blowoff, was admitted slowly to the system, sm-eeping the residual water vapor into the counter. Khen the pressure of methane in the counter reached 1 atni.. the valve was closed and the counter contents were allowed t o mix for a fenminutes. The counting apparatus consisted of a fast amplifier, a H.T. supply, and a scaler and was capable of measuring counting rates up to 20.000 per second.
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2000
2
2400
0
VOLTAGE
Figure 3.
Effect of amount of water vapor
on counter performance
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RESULTS
The effects of water vapor and tritiated m t e r vapor on the performance of the counter are shown in Figure 2. The counting rates have been normalized to compare plateau shapes. The first curve shoa-s a voltage plateau ~ i t ah counter filling of methane. using a n external y-ray source. The second curve shows the effect of water vapor in the counter. The third curve was obtained when the counter \vaj filled with methane and 5 pl. of tritiated nater. The plateau of this curve is
Sample 1. 5-X standard, Oct. 28, 1957 2. 10-X standard, Oct. 28, 1957 3. 5-X 1 to 10 dilution of standard, Oct. 28, 1957
4. 10-X 1 to 10 dilution of stand-
ard, Oct. 28, 1957
5 . 5-X standard, AIarch 4, 1958 6. 10-X standard, March 4, 195s
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Figure 4.
Effect of temperature on counter performance
Table I. Calibration of System Counting Rate, C.P.11. Corr. for Corr. for 600-c.p.m. 2-,mec. Observed dead time background
x x
x x
C.P.lI.;lIi,
Standard Solution
x x
ANALYTICAL CHEMISTRY
Corr. for Decay t o Oct. 28, 1957, C.P.RI.x 10-7 4.22 4.16
4 11
x x
105 106
2.12 4.17
2.15
x
101
2.15 X lo4
2.09 x 10'
1 . 1 8 x 10:
4.18
4.37 x 104 2 . 0 0 x 105 4.03 X lo5
4 . 3 7 X 10' 2 . 0 1 x 105 1 . 0 9 x 105
4 . 3 1 X lo1 2 . 0 0 x 106 4.08 X lo5
4 . 3 1 X 10; 4.01 x 10: 4.08 x 107
4.31 4 09 4.16 A v . = 4.19 f 0.07 c.p.m./mi. 6 . 3 X 10' d.p.m./ml. Ahrolute/average = 1.62
2.11
1oj 106
"11 4.16
10s 106
4.22 4.10
1c1; 10:
Absolute disintegration rate Counter correction factor 1746
90OC 105OC
better, because dense ionization tracks from the soft tritium P-particles were originating throughout the sensitive volume of the counter. Figure 3 shows the effect of varying the amount of n-ater vapor in the counter. Good plateaus were obtained with 5 and 10 pl. of tritiated water, but the plateau had practically disappeared with 20 pl, Figure 4 shows the effect of varying the temperature. Kith increasing temperature the plateau shortened, until at 105” C. it had almost disappeared. A temperature of 90” C. was chosen for routine operation. The system was calibrated using a standard tritiated water sample (Table I). By using the known disintegration rate of the standard, it was possible to calculate an efficiency factor for the counter n-hich nas used to correct the observed counting rates. The standard deviation of the determinations was less than =t2%,
DISCUSSION
The only “memory” effect, or cross contamination between samples, was found to be due to the accumulation of tritium in the vacuum pump. For low activity samples (counting rates up to 10 times background), it is sufficient to allow the pump to flush itself with free air for a minute between determinations. For high activity samples (lo2 to lo3 times background) it is necessary t o flush the counter with one or two samples of pure water between determinations. The use of a cold trap in the pumping line is recommended for use with high activity samples. The sensitivity of the method depends on the background count. Because extreme sensitivity was not one of the design considerations, the counter was unshielded. If the very rigorous criterion is used that to be valid a
count should be equal to the background, the sensitivity of the counter is 0.0003 pc. or 0.03 p c . per ml., as a 10-pl. sample is used. By shielding the counter and relaxing the statistical requirements, the sensitivity can be stated to be about 50 times the above. The small quantity of water Tapor that the counter can tolerate places a lower limit on the specific activity of the water sample which can be measured. Xevertheless, There extreme sensitivity is not a major consideration, the system forms a fast, accurate method of tritium determination. LITERATURE CITED
(1) Drever, R. W. P., Moljk. A., Rev. Sci. Instr. 27, 650 (1956). ( 2 ) Nucleonics 16, KO.3, 62 (1958,~. (3) Ibid., p. 67.
RECEIVEDfor revien- April 28, 1958. Accepted July 21, 1958.
Low Level Plutonium-241 Analysis by Liquid Scintillation Techniques DONALD L. HORROCKS and MARTIN H. STUDIER Argonne National laboratory, lemont, 111,
b The plutonium-241 content of plutonium samples can be determined with a high degree o f confidence with the liquid scintillation spectrometer. The plutonium-24 1 P-particles are counted with a relatively high and easily reproduced efficiency of 37y0. The very low limit of detection, gram of plutonium-241, and the ease of recovery of the plutonium for further investigations give this method added advantages over other methods of analysis.
T
principal isotope of plutonium formed in reactors is plutonium239; however, isotopes of higher mass are also formed in lesser amounts by successive capture of neutrons. Because the nuclear properties of the various isotopes of plutonium are different, it is frequently desirable to know the isotopic composition of plutonium. Usually this isotopic analysis is made with EE
The term “multiplier phototube,” rather than “photomultiplier,” has been adopted b y ANALYTICAL CHEMISTRYasrecommendedb y the Institute o f Radio Engineers [IRE h o c . 45,
No. 7, 1000 ( 1 957)].
a mass spectrometer. Frequently, however, the sample and/or its plutonium241 content is so small that the plutonium-241 may be below the limit of detection of the mass spectrometer. It is very difficult to recover a sample from the mass spectrometer after the analysis has been made. For these reasons a nondestructive method of analysis for plutonium-241 with a sensitivity of 10-’6 gram has been developed. This sensitivity is not seriously affected by the presence of relatively large amounts of the other plutonium isotopes. The method consists of putting the plutonium into an organic scintillating solution and measuring the light pulses produced by the @-particles from plutonium-241 with a coincidence-type liquid scintillation spectrometer. Upon completion of determination of the plutonium-241 content, the plutonium is easily recovered for other measurements. The sensitivity of this method depends on the fact that plutonium-241 is a relatively short-lived (13-year) beta emitter. Unfortunately, the very low beta energy (18 k.e.v. maximum) makes the measurement of the plutonium-241 activity somewhat difficult. Because the range of the 0-rays is less than 1 mg. per sq. cm., the use of an
end window counter is not practical. Although plutonium-241 beta activity can be measured with an internal proportional counter, it is difficult to attain good reproducibility by this method. The problems usually encountered with the internal proportional counter method hare been eliminated with the liquid scintillator method. By dissolving the plutonium sample in the liquid scintillator (internal sample technique) the losses of the plutonium241 prays due to absorption are essentially eliminated, n hereas measurement of the 0-rays by internal proportional counters is greatly dependent upon the method of sample preparation. For internal proportional counters, the usual procedure of sample preparation has been to evaporate saniples on platinum disks and drive off volatile salts or organic matter with heat. Even very tiny amounts of extraneous material will seriously lower the amount of beta activity n hich can be measured. If the platinum is heated too hot, the sensitivity for the measurement of the 0-particles may be reduced by as much as 20%. even though no extraneous material is present. Only relatirely large amounts of extraneous materials n-ill interfere with the measurement of the light pulses produced in the liquid scinVOL. 30, NO. 1 1 , NOVEMBER 1958
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