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Low-level measurement of tritium by hydrogenation of propadiene and gas counting of propane
Low-Level Measurement of Tritium by Hydrogenation of Propadiene and Gas Counting of Propane M. WOLF,
W. RAUERT
and F. WEIGEL*
Institut fiir Radiohydromelrie der Gesellschaft fiir Strahlen- und Umweltforschung, D-8042 Neuherberg. *lnstitut ftir Anorganische Chemie der UniversitBt Miinchen, Radiochemische Abteilung D-800 Miinchen, Federal Republic of Germany (Received 7 January 1981)
A new method for low-level measurement of tritium (3H) in water is reported. In this method, a 10 or 20 ml water sample is reduced with magnesium turnings in a furnace at 570°C. The resulting hydrogen is reacted with propadiene to propane in the presence of a catalyst. The ‘H concentration in the propane is counted in a 2.6 I proportional counter at a pressure of 2 or 4 bars, respectively. With total measuring times up to 8500min per sample, an experimental detection limit down to 1 TU (A3.2 pCi ‘H/liter
H20) and ‘H standard reproducibilities of 1% were attained. After 20-fold electrolytic enrichment of ‘H in water, the experimental detection limit can be lowered to I 0.1 TU.
Introduction
Experimental Mehds
and Results
Synrhesis of propane The counting gas propane is synthetized in the high vacuum apparatus shown in Fig. 1. Up to three syn-
day are possible. The water sample to be studied (10 or 20 ml, respectively) is evaporated from a quartz flask into a furnace filled with Mg turnings (Fig. 2) in which the water vapor is reduced to hydrogen at 570°C. The furnace consists of a reaction tube made of stainless steel. length 80cm, which is charged with 250 g Mg turnings (Type LNR 6, Knapsack AG, Cologne). With one charge, a total of 120 ml H20, corresponding to 12 samples of 10 ml each, may be reduced to Hz. After recharging the Mg furnace, this is heated for at least 1 h at 63O’C under constant pumping with the diffusion pump. The hydrogen formed in the reduction is passed through a fritted filter disk followed by a cold trap cooled with liquid nitrogen and is expanded into a 601 flask containing Pd catalyst. Prior to each reduction run, 97-98% of the stoichiometric amount of propadiene (in relation to Hz) is condensed into the cold trap. After complete reduction of the water sample (H, pressure approx. 155 Torr for a 1Oml HI0 sample, approx. 310Torr for a 20 ml Hz0 sample) the stopcock leading to the bypass line is closed and the coolant is removed from the cold trap. After all propadiene has evaporated into the reaction flask, the stopcock leading to the reaction flask is closed, and the hydrogenation reaction is allowed to proceed overnight. On the following day the synthetized propane is transferred to a 0.5 1 steel cylinder (Industriewerke Karlsruhe, Augsburg AG). By means of a 0.2 I glass ampoule gas samples for mass-spectrometry or gas chromatography may be drawn. The propane yield relative to propadiene is 99 f 1%. After each run, the catalyst is heated for at
theses per
‘H concentrations in natural water samples have obtained great significance in hydrogeology (e.g. Ref. (1)) and in oceanography. Measurements may be made using /?-counting in gas counters or liquid scintillation spectrometers (e.g. Ref. (2)), or by means of mass-spectrometric determination of the ‘He formed in the decay of 3H.‘3) For the detection of very low ‘H concentrations, aside from ‘He massspectrometry, the gas counting method is generally preferred to liquid scintillation counting because of its higher sensitivity. For measurement in the gas counter the water sample, which, if necessary, is first enriched in 3H by partial electrolysis, is converted into a suitable counting gas. If one compares different counting gases (Hz, CH,, &Hz, CIH6, C3H,) with regard to high specific hydrogen content (and correspondingly high detection sensitivity), C3Hs and CzHd result as the most suitable counting gases. The demand for low ‘H content (low background) and freedom from Rn(*) as well as for high chemical purity led to propadiene (HzCaHI), which is commercially available (Deutsche Edelgas GmbH, Diisseldorf) as the starting material for propane synthesis. A detailed report on this method for low level tritium measurement (cf. also (S-7)) is given in the following paper. ANALYSES OF low
919
920 Mah vacuum Ium
I)
LJ
Cmssline
0.2 I -bfwoub
!
Mg- Furnocr 570 ‘C
0.5 I-s(ssl cylinder
.
RI -Asbaa@, catalyst
~~rampb
\-
’
IO-20 ml
HWtU
FIG;. I. Schematic drawing of high vacuum apparatus for synthesis of propane from water samples for tritium analysis.
least 3 h with infrared lamps and is evacuated with a diffusion pump. Reduction
temperature
From experiments. it was found that the reduction of water vapor starts at a temperature somewhat below 48O.C and that it proceeds readily in the temperature interval X0-640 C (m.p. of Mg approx. 650-C) with a constant HZ yield of 98 f I”,. As optimum reaction temperature. a temperature of 570-C was used routinely. because at this value temperature fluctuations were expected to have a minimum influence on the Hr yield. Comparison
oJd(ferent
Pd cutalysts
In preliminary experiments on hydrogenation of propadiene. Pt and Pd (lo”, of each metal on asbestos) showed comparable catalytic properties. Because Pt is more expensive than Pd. the following hydrogenation experiments were done using Pd only which was used in connection with different carrier materials and in different concentrations. In additional experiments lifetimes and memory effects of different types of catalysts were studied. “Lifetime” is given in terms of the number of hydrogenations which yielded useful counting gases and “memory effect” is the percentage of ‘H activity from the previous run carried over lo the run under consideration. The results have been summarized in Table 1. Freshly inserted catalysts were decontaminated by several hours heating in tritium-free Hz atmosphere. By heating after each hydrogenation run. the lifetime
of the
lo”,, Pd-asbestos catalyst was extended by a factor of 4 to 5. and the memory effect was diminished. Still. the memory effect with the 0.51, Pd-AIZOJ- and 2”, Pd-carbon catalysts was still high enough lo make these catalysts nearly unsuitable for tritium analysis. In some cases the catalyst was poisoned before attaining its average life time. In this case. the gas can be purified in such a way by either low temperature distillation or by heating with fresh catalyst that it is suitable for /3-counting. Grls chromotographic
analysis
In order lo test the purity of the gases prepared. several H 2- and C3Hs samples were assayed by the chemical laboratory of the Munich gas- and waterworks, using a gas chromatograph Perkin-Elmer 3920 with heat conductivity detector and He as carrier gas. The analytical results yielded in the case of HZ a purity of 99.9~019; with the following impurities: O2 (0.01-0.02 ~019,). NZ (0.06-0.08 ~019;). and CH, (traces). The synthesized propane had a purity of 99.8 vol:,,. the major impurity being hexane isomers (0.10-0.18 vol”,) which may have been formed by side reactions I and II in the reaction scheme shown in Fig. 3. Traces of air. CH, and CZH6 were also detected as impurities in the propane. Detection
System
The apparatus used in the detection of tritium (Fig. 4) consists of a 2.6 I gas counter (Type GPC-14. Johnston Laboratories Inc.. USA). the shielding and
921
Measwemenr of lrilium To gloss lti
,25mm
bore
FIG. 2. Longitudinal section through the Mg furnace for reduction of water samples.
TABLE
1.
Lifetime and memory effect of different types of catalysts
Type
Catalyst supplier
Total mass (g)
Pd (g)
Lifetime (runs)
Memory effect (X)
l&19 31-39 214
0.46 0.16 0.32
5% Pd on asbestos lo”/. Pd on asbestos So”/, Pd on asbestos
Fluka AC, Buchs (Switzerland)
IO 10 LO
0.5
O.S’A Pd on A&OS (pellets) 2% Pd on carbon (granules)
Kali-Chemic Engelhard. Hannover
100
0.5
211
4.60
25
0.5
217
1.54
:
922
M. WoIJet al. Mart ruwim: H,C=C =CH, -
H
“2
2
H,C=CH-CH,
CH, -CH, -CH,
(1)
Prcqodrne side rarction I: ~C=CGi,
“2
+H,C=C=CH, CQl,2-Dimethykw-
I-Makthyl2-mamybm-
CH, nC+l-CH, -CH, -CH,-CH,
,
I-Haam CH, -CH=CH-CH, 2-m
-CH, -CH,
CH, =CH -$H-CH,
"2
CH,-CH2-CH,-CH,-CH,-CH,
/’
Hexam
Pa)
-CH;
CH, 3-Memthyl-I - pantala CH,-CH=$-CH,
1,2-Dinwth$ cydobuta
CH, -CH, -FH -CH, -CH,
(2b)
C”3
-CH,
3-Mathylpamw
% 3-hlethyl-2-pantme -
CH,=F-‘?I-CH, H,C CH,
"2
CH,-CH-$Ji-CH, Hk % 2.3 - Dimathylbukna
2,3-Din~hyl-I-lwtann sida MaclbnlI.
I -MewIg-
I.3 - cwhgmk
IJ-Din&-
3-methykw-
CyCbbUkXlO
Cydobu_
cycbw I)
CH 2 =ai -CH, -$H-CH, CH, 4-Methyl -I - wlmna
-
2)
CH, -al
> 7
=a+CHHCH,
CH, -CH,
-CH,-fH-CH,
(3)
C% 2-Metrylpaflwla
t H3 4-Methyl -2-pantam
FIG. 3. Reaction scheme in the hydrogenation of propadiene.
electronics (Nuclear Enterprises Ltd, Edinburgh) and a self-constructed counter filling system. Technical data of the counter are as follows: effective volume approx. 2.2 1. maximum pressure 5 bars, Cu cathode, anode wire stainless steel, 0.025 mm dia. The counter is surrounded by a plastic scintillator (Type NE 110) to eliminate myons. Neutrons generated by interaction of myons with the shield and moderated by the plastic scintillator and by boric acid, are captured by the boron in the boric acid (cf. (8)). The detection system is set up in a room in the basement. which has 75 cm thick walls made of heavy concrete (Ilmenite). The pulses of the gas counter pass through a charge-sensitive pre-amplifier (sensitivity 0.035 pV/ion pair), and are then amplified by a factor of 12.5. A value of 5 was found to be the most favorable ratio between upper and lower threshold of the tritium
channel. Counts from the tritium channel and those counts from above the upper threshold are compared with events from the plastic scintillator in anticoincidence units. Two type 57 A.V.P. photomultipliers of Mullard Ltd are used as detectors with the plastic scintillator. The resulting anticoincidence counts and coincidence coun&. respectively. are registered and printed after preselected time intervals. Anticoincidence shielding In order lo determine the optimum anticoincidence shielding effect, the background count rate was measured under standard conditions as a function of photomultiplier voltage. It was found that the background count rate no in the tritium channel was lowered when using the highest voltage (2.2 kV) and operating the photomultipliers in coincidence. The coinci-
923
Measurement of rririum
\
I /\
I \
I.
‘Pbstic sd~IIbtce NE 110 26xe3an) (cuvlly
I
-A
VI
v2
dlanar
-Am@flt
HV
.,L_.lI,F3.ZP-l
mcoZ
dence count rate of the plastic scintillator at 2.2 kV was found to be 9OOOcpm. and is thus three times as large as the contribution caused by myons only. The influence onto the ‘H counting efficiency (due to random coincidences with pulses from the gas counter) may be neglected. High volrage factor
For the sake of reproducible results when using differential measuring technique, the gas amplification, which is a function of counter voltage, has to be adjusted to the same value for each counting gas. As reference voltage the voltage U1,,, is used, at which the coincidence counting rate, due to myons and registered by the ‘H counter, is half the value of the total coincidence counting rate. At a pressure of 2 bar in the counter, U,,2 o 4.6 kV and at 4 bar, U l/2 2 6.6 kV. The operating voltage for routine The concentration of environmental tritium is often reported in TU. 1TU (tritium unit) A 1 TR (tritium ratio)
k 3.2 pCi ‘H/l Hz0 A 0.12 Bq ‘H/l
-
HDI
measurements is obtained from U1,* by multiplication with the optimum high voltage factor F,v,O,,, (Fig. 5). In general the factor q2/n,, is used as a criterion for optimization of a low-level counting system. Even though q2/n0 has its maximum value at FHV = 1.20 (Fig. 5). a value of 1.16 was chosen for FHVVopc. because this minimizes the influence of changes in the gas amplification during long-time measurements. Corresponding measurements and considerations yielded F HV.Opc. = 1.20 for 4 bar pressure. Results and Discussho The tritium concentration c (TV)* is calculated from the measured count rate n, (cpm) by means of equation (4)
c=
l
; [‘H]/[H] = lo-” HZO.
z H
x(4 - no)
-no)
=
F (tll A
- n,)
where x = ‘H concentration
in 3H standard (TV)
(4)
924
hf. Wol/ct al.
4 - L4 - 13 -IA?
- LI
-I
-as
cf
-a6
“0 .
!tirlzz nc)[cwnJ
l.lZ
IJ4
U6
Ll6 Hi@
VdlDor
L20 fator
I22
L24
-
FM,
FIG. 5. ‘H counting efficiency q (related to the counter volume of 2.6 I). Background count rate no and optimization factor $/no (propane pressure = 2 bar inside counter) as functions of the high voltage factor FHv
A = enrichment factor n ,.,, = ‘H standard count rate @pm) n, = background count rate (cpm) F = x/(nr,, - no) = calibration factor (TU/cpm). The standard deviation u(c) is obtained from equation (4) using the law of error propagation. The relative error u’(c)/c[= 1.96u(c)/c] which is related to a 95% confidence level P (in the case of a two-sided problem) or 97.5% confidence level P (in the case of a one-sided problem) is given by equation (5)
was obtained for an initial volume of 0.4 I H@) and a’(A)/A = 0.09 for an initial volume of 1.4 l!7Q9’ The minimum tritium concentration c,in which may be distinguished with P = 97.5% from tritiumfree water samples (L no) is defined as the detection limit: Gli” = Q’(C,in)
The relative error of the tritium content in the ‘H calibration standard solution (a’(x)/x) was taken to be 0.02. For the error of the enrichment u’(A)/A = 0.06
If all the errors due to preparation and measure ment of samples are negligible compared to counting statistical (Poisson-) errors, equation (6), with
= f JCa’(nr)l’
+ Ca’hN 0 fi
f u’(n0). (6)
925
Measurement o/ iririum TABLE 2. Results of ‘H standard measurements (‘H concentration of NBS ‘H standard on Oct. 10, 1979: 263.7 TU). The count rates corrected for decay refer to
Oct. 6, 1979 ‘H
f&J (cpm)
Count rate corrected for decay Mm)
Average calibration factor F (TWpm)
4000 4am 4000
11.838 + 0.072 11.807 f 0.055 11.844 f 0.053
11.831 11.807 11.849
23.24
1167 1333 1333
22.453 f 0.211 21.987 f 0.169 22.442 f 0.171
22.629 22.208 22.615
12.24
Standard sample No.
Pressure inside counter (bar)
Measuring time (min)
Count rate
1 2 3
2 2 2
4 5 6
4 4 4
a’(no) = 1.96 fl no T, red uces to equation (7) for the smallest possible value of c,,” within
the measuring
time 7 :
Cd” = The detection limit which may be attained in practice (equation 6) has to be calculated from the experimentally determined deviation ~‘(n,,) of the count rates from a sufficiently large number of background samples. This detection limit is generally higher than the detection limit derived from counting statistics only (equation 7). Accordingly the deviation of results for ‘H-containing
samples, u’(c) has to be obtained
large number of standard samples with known ‘H content which have been prepared and measured in the same manner. Such calibration measurements have to be repeated from time to time. If only a few calibration samples are available, the standard deviation calculated in the usual way increases by multiplication with Student’s t-factor (e.g. for P = 957’ I = 4.30 for 3 equal samples, and f = 2.26 for 10 equal samples). from a sufficiently
TABLE 3.
The count rates obtained in the individual measurements are tested for statistical purity by means of Pearson’s x2-test. Typical results measured with 3H standard samples, background samples and analytical samples (partially after electrolytic enrichment) have been compiled in Tables 24.
Discussion of results The 3H standard samples measured at a filling pressure of 2 bar (corresponding to 10 ml H20) have shown a good reproducibility of 0.9% (P = 95%). For background samples experimental detection limits (P = 97.5%) were attained of 1.5 TU in 1OOOmin measuring time and 0.8 TU in 8000-8500 min measuring time. In the latter case, the detection limit as derived from counting statistics is found to be O.STU. The largest useful time of measurement, 7,,_ may be defined as that time, in which the smallest experimentally attainable fluctuations (limiting system error) correspond to the deviation due to counting statistics (cf. (11)). Since in the measurements reported here, counting statistical deviations cannot yet be neglected. one may assume that the limiting error has not
Results of background measurements (‘H concentration of background samples ~0.2 TU)
Background sample No.
Pressure inside counter (bar)
Measuring time (min)
Count rate *W0) (cpm)
Average count rate f tine) (cpm)
Average air pressure (mbar)
I
2 2
4500 4OaJ
0.468 f 0.010 0.486 & 0.011
0.477 f 0.007
965.5 959
2
2 2
4000 4000
0.486 k 0.012 0.484 + 0.012
0.485 + 0.008
z
2 2
4tmO 4167
0.481 f 0.01 I 0.495 0.01 f I
0.488 + 0.008
z
4 4 4
4000 4000 4WO
0.943 f 0.016 0.923 f 0.015 0.966 f 0.019
4 65
953 975 959
Average background count rate no f o’(no) (cpm)
0.483 f 0.024
0.944 f 0.093
M. Wol/et al.
926
TABLE4. Results of tritium analyses on groundwater and brine samples with and without preceding electrolytic enrichment. A correction of the ‘H concentration with regard to ground contaminationoO) was not carried out. The groundwater samples from Oberbachern and Munich were taken in cooperation with the Bayerisches Landesamt fur Wasserwirtschaft
Description of sample Brine samples from salt mine potash mine salt mine salt mine salt mine Oberbachern Oberbachern
Date of sampling
Mar. Dec. Mar. Mar. Mar.
12, 79 12 78 13. 79 14. 79 14. 79
Apr. 9. 79 Apr. 9. 79
Background water IAEA
Measuring time (min)
‘H concentration (TU) c * a’(c) measured Range
Enrichment factor A
-.
II
31 + 3 I8 + 2 11 +2 2+1 0.96 -+ 0.15
28-34 lb20 9-13 l-3 0.81-1.11
loo0 1333
21 23
0.14 + 0.06 0.13 + 0.05
0.084.20 0.08~.18
2333 1167
42 37
0.10 + 0.02 0.08 + 0.04
0.084.12 0.04-0.12
1000 1000 1000 4167 1167
_-
Munich, Ellwanger. deep well
Apr. 4. 79
1000
20
0.06 f 0.06
O-0.12
Munich, Paulaner Brewery, deep well 2
May 3. 79 May 3. 79 May 3. 79
1000 1167 4cmO
22 20 19
0.06 f 0.06 0.02 + 0.06 0.06 f 0.04
oJJ.12 NJ.08 0.02~.10
Munich. Metzler Rubber Co. deep well
Apr. 4. 79
1333
19
0.04 + 0.06
o-0.10
Munich, Lowenbrewery. deep well I
Apr. 4. 79
loo0
19
a T,,, of at least 3100 min results for a filling pressure of 2 bar. For a filling pressure of 4 bar (corresponding to 20 ml HaO), the reproducibility was only 4.65: for ‘H standard samples. The experimental detection limit was found to be 1.6TU for 4000min measuring time, as compared to a counting statistical detection limit of 0.5TU (this corresponds to T,,,_ of at least 400 min). The still unsatisfactory reproducibility of measuring results at 4 bar filling pressure is probably due to electronic difficulties which are caused by the high operating voltage (approx. 8 kV) and by the higher sensitivity of counting gases towards impurities. No dependence of background counting rate on atmospheric pressure was observed. As shown in Table 5. the detection limits of the )H measuring method reported here (without preceding enrichment) are comparable or lower than the detection limits attained by other authors. After preceding electrolytic enrichment, experimental detection limits of ~0.1 TU are obtained, the counting statistical detection limits being 0.05 TU after 20-fold enrichment and 0.02 TU after 50-fold enrichment. From these results it may be concluded that the ground contamination is less than 0.1 TU (cf. alao (9)). The ground contamination comprises the total possible 3H contamination of the sample due to sampling, storage, and analysis (e.g. by addition of
yet been attained. From these considerations,
-0.01
+ 0.08
O-O.07
Na20, and PbCla before or after electrolysis, respectively) and includes a well as possible original ‘H content of the water which is used as background samples (In our case, the background samples should according to hydrogeological expectation, contain no tritium). Our ground contamination is of the same order of magnitude as was found in other laboratories after electrolytic enrichment.(‘0.‘4’ The magnitude of the individual contributions to ground contamination including a possible ‘H content of background samples remain to be determined. Beside a further development of ‘H analysis by mass-spectrometric 3He measurement. the following additional possible ways may be considered to lower the ‘H detection limit in gas counter measurements: (a) Replacement of all hydrogen atoms in propane by hydrogen from the water sample to be analyzed, similar to the ethane total syntheses’*2’:
Mg2C3 + 4 Ha0 + 2 Mg(OH)2 + HC =C-CHJ HI0
+ Mg-Hz
HC=C-CH,
(8)
+ MgO
(9)
+ 2Hz +CaHa
(10)
(b) Additional ‘H enrichment (for instance 27-fold) by distillation of a major quantity of water (for
91
a/.” ”
(testphase)
2.2 etT.
I etf.
I .3 elT.
Ibid.
OESCHGER"~'
2.2 etT.
(tart phase)
and
This work
Ibid.
SC-xx
4.6 etT.
GEYH(“’
I .5 eff.
2.2 eff.
3.1 eff.
and Dc+ts~Y(~‘)
3
1.6
2.5
R~ETHERand WEISS”~’
&TLUND
SBCmmAm et ~2/.“~’
VINGGRADGV
EULITZ”
Authors
Volume of counter (1)
Cd%
CA
CH.
CH.
C2H6
Ar CH.
H2
C2H.
C2H6
Cd%
H2
CH.
CA
CA
Counting gas
1.33
4
2
3.6
I
1.87 1.39 0.35
0.94
1.0
1.4
0.48
23.2
12.2
1.3’ 0.33
I.6
I.5 0.8
1.3.
2.4’
1.8.
4mo
1000-1300 8000-8500
2600
1300
5600
1.0. 2.4’
I300
Counting time (min)
1300
5.2.
Gal. exp. (TU)
3.1. 2.5.
3.8.
3.9.
3.3.
2.6.
3.7.
cl!& count. stat. (TU)
26
0.08
0.57
36.4 72
0.55
0.73
38.5
50.4
I.0
44
2 0.7 2.84 0.22
0.25
1.28
25.9. 75
5.3
(Znom)
Background count rate
18.3’
Calibration factor F (TU/cpm)
1.33
2
2.2
Pressure inside counter (bar)
TABLE5. Characteristic data of different ‘H gas counter systems l derived or estimated from data given by the authors efi. = effective counting volume cmI..co”“l.*.l.= counting statistical detection limit (P = 97.5%) for r = 1000 min c,in.q. = experimental detection limit (P = 97.5%) for given time of measurement
C,Hs from HsC==H2
CaHs from H2C-H2
Underground laboratory Underground laboratory
C2He from C2H2
C2Hb from C2H,
C2H6 total synthesis
C,Hs from HC=C-CH,
Remarks
M. Woyet al.
928 instance 40 l).“9’ After further electrolytic gas counting
would allow theoretically
enrichment,
6. WOLFM. Diplomarbeit.
to attain de-
tection limits in the order of IO- 3 TU.
Acknowledyemenrs-The authors wish to express their thanks to Professor Dr H. MOSERfor his constant interest and support of this work. Mr M. FORSTERfor critically reading the manuscript and for enrichment of several water samples. Mrs A. KESSLERand Mrs E. STURMfor the careful laboratory work, and Mr H. RAST for his assistance in electronic problems. For useful suggestions in construction work. we thank Mr A. SCHMELLER.and for shop work Mr R. ABELTSHAUSER and Mr R. FLECK.
7.
8. 9. IO.
Radiar.
I. IAEA. fsorope Hydrolog) 197X. Vol. I and II (IAEA. Vienna. 1979). 2. CAMERONJ. F. Rudioacrire Dariny and Methods ojLowLure/ Counriny, p. 543 (IAEA. Vienna. 1967). 3. CLARKEW. B.. JENKINSW. J. and TOP Z. Inr. J. Appl. Rudiar. lsor. 27, 515 (1976). 4. EULITZG. W. Rec.. Sri. fnsrrum. 34. 1010 (1%3). 5. RAST H.. RAUERTW. and WOLF M. Low-Radioacricir! Measuremenrs and Applications, p. 141. (Slovenske pedagogicke nakladatelitvo. Bratislava 1977).
Isot. 27, 217 (1976).
II. GEYH M. A. Proc. 8th Inr. Radiocarbon Dariny Conf.. p. B 81, (Lower Hutt, New Zealand, 1972). 12. VINOGRADOVA. P.. DEVIRTS A. L. and D~BKINA E. I. Geochem. Inr. 5. 952 (1968). I ,.
References
University of Munich, unpublished (1979). EICHINGERL.. FORSTERM., RAPT H.. RAUERT W. and WOLF M. Low-Level Tritium Measuremenr. p. 43 (IAEA-TECDOC-246, Vienna. 1981). DE VR~ESHI. Nucl. Phys. 1, 477 (1956). FORSTER M. Diplomarbeit, University of Munich. unpublished (1979). WEISS W.. ROETHERW. and BADERG. Inr. 1. Appl.
SIEGENTHALERU.. OESCHGERH.. SCHOTTERERU. and H~NNI K. Inr. J. Appl. Radiur. lsor. 26. 459 (1975).
14. OSTLUNDH. G. and DORSEYH. G. Low-Radioacri&) Measurements and Applications. p. 55 (Slovenskt pedagogickt nakladatelitvo, Bratislava, 1977). 15 GROENEVELDD. J. Thesis. University of Groningen (1977). Tririum 16. ROETHER W. and WEISS W. Low-Level Measuremenr. p. 37 (IAEA-TECDOC-246, Vienna. 1981). 17. GEYH M. A. Personal communication (1980). 18. SCHOTI-ERERU. and OE~CHGERH. Law-Level Tririum Meusurement, p. 7 (IAEA-TECDOC-246, Vienna. 1981). 19. D~STROVSKYI.. AVINUR P. and NIR A. Liquid Scinrillorion Counting. p. 283 (Pergamon. Oxford, 1958).
W. RAUERT
and F. WEIGEL*
Institut fiir Radiohydromelrie der Gesellschaft fiir Strahlen- und Umweltforschung, D-8042 Neuherberg. *lnstitut ftir Anorganische Chemie der UniversitBt Miinchen, Radiochemische Abteilung D-800 Miinchen, Federal Republic of Germany (Received 7 January 1981)
A new method for low-level measurement of tritium (3H) in water is reported. In this method, a 10 or 20 ml water sample is reduced with magnesium turnings in a furnace at 570°C. The resulting hydrogen is reacted with propadiene to propane in the presence of a catalyst. The ‘H concentration in the propane is counted in a 2.6 I proportional counter at a pressure of 2 or 4 bars, respectively. With total measuring times up to 8500min per sample, an experimental detection limit down to 1 TU (A3.2 pCi ‘H/liter
H20) and ‘H standard reproducibilities of 1% were attained. After 20-fold electrolytic enrichment of ‘H in water, the experimental detection limit can be lowered to I 0.1 TU.
Introduction
Experimental Mehds
and Results
Synrhesis of propane The counting gas propane is synthetized in the high vacuum apparatus shown in Fig. 1. Up to three syn-
day are possible. The water sample to be studied (10 or 20 ml, respectively) is evaporated from a quartz flask into a furnace filled with Mg turnings (Fig. 2) in which the water vapor is reduced to hydrogen at 570°C. The furnace consists of a reaction tube made of stainless steel. length 80cm, which is charged with 250 g Mg turnings (Type LNR 6, Knapsack AG, Cologne). With one charge, a total of 120 ml H20, corresponding to 12 samples of 10 ml each, may be reduced to Hz. After recharging the Mg furnace, this is heated for at least 1 h at 63O’C under constant pumping with the diffusion pump. The hydrogen formed in the reduction is passed through a fritted filter disk followed by a cold trap cooled with liquid nitrogen and is expanded into a 601 flask containing Pd catalyst. Prior to each reduction run, 97-98% of the stoichiometric amount of propadiene (in relation to Hz) is condensed into the cold trap. After complete reduction of the water sample (H, pressure approx. 155 Torr for a 1Oml HI0 sample, approx. 310Torr for a 20 ml Hz0 sample) the stopcock leading to the bypass line is closed and the coolant is removed from the cold trap. After all propadiene has evaporated into the reaction flask, the stopcock leading to the reaction flask is closed, and the hydrogenation reaction is allowed to proceed overnight. On the following day the synthetized propane is transferred to a 0.5 1 steel cylinder (Industriewerke Karlsruhe, Augsburg AG). By means of a 0.2 I glass ampoule gas samples for mass-spectrometry or gas chromatography may be drawn. The propane yield relative to propadiene is 99 f 1%. After each run, the catalyst is heated for at
theses per
‘H concentrations in natural water samples have obtained great significance in hydrogeology (e.g. Ref. (1)) and in oceanography. Measurements may be made using /?-counting in gas counters or liquid scintillation spectrometers (e.g. Ref. (2)), or by means of mass-spectrometric determination of the ‘He formed in the decay of 3H.‘3) For the detection of very low ‘H concentrations, aside from ‘He massspectrometry, the gas counting method is generally preferred to liquid scintillation counting because of its higher sensitivity. For measurement in the gas counter the water sample, which, if necessary, is first enriched in 3H by partial electrolysis, is converted into a suitable counting gas. If one compares different counting gases (Hz, CH,, &Hz, CIH6, C3H,) with regard to high specific hydrogen content (and correspondingly high detection sensitivity), C3Hs and CzHd result as the most suitable counting gases. The demand for low ‘H content (low background) and freedom from Rn(*) as well as for high chemical purity led to propadiene (HzCaHI), which is commercially available (Deutsche Edelgas GmbH, Diisseldorf) as the starting material for propane synthesis. A detailed report on this method for low level tritium measurement (cf. also (S-7)) is given in the following paper. ANALYSES OF low
919
920 Mah vacuum Ium
I)
LJ
Cmssline
0.2 I -bfwoub
!
Mg- Furnocr 570 ‘C
0.5 I-s(ssl cylinder
.
RI -Asbaa@, catalyst
~~rampb
\-
’
IO-20 ml
HWtU
FIG;. I. Schematic drawing of high vacuum apparatus for synthesis of propane from water samples for tritium analysis.
least 3 h with infrared lamps and is evacuated with a diffusion pump. Reduction
temperature
From experiments. it was found that the reduction of water vapor starts at a temperature somewhat below 48O.C and that it proceeds readily in the temperature interval X0-640 C (m.p. of Mg approx. 650-C) with a constant HZ yield of 98 f I”,. As optimum reaction temperature. a temperature of 570-C was used routinely. because at this value temperature fluctuations were expected to have a minimum influence on the Hr yield. Comparison
oJd(ferent
Pd cutalysts
In preliminary experiments on hydrogenation of propadiene. Pt and Pd (lo”, of each metal on asbestos) showed comparable catalytic properties. Because Pt is more expensive than Pd. the following hydrogenation experiments were done using Pd only which was used in connection with different carrier materials and in different concentrations. In additional experiments lifetimes and memory effects of different types of catalysts were studied. “Lifetime” is given in terms of the number of hydrogenations which yielded useful counting gases and “memory effect” is the percentage of ‘H activity from the previous run carried over lo the run under consideration. The results have been summarized in Table 1. Freshly inserted catalysts were decontaminated by several hours heating in tritium-free Hz atmosphere. By heating after each hydrogenation run. the lifetime
of the
lo”,, Pd-asbestos catalyst was extended by a factor of 4 to 5. and the memory effect was diminished. Still. the memory effect with the 0.51, Pd-AIZOJ- and 2”, Pd-carbon catalysts was still high enough lo make these catalysts nearly unsuitable for tritium analysis. In some cases the catalyst was poisoned before attaining its average life time. In this case. the gas can be purified in such a way by either low temperature distillation or by heating with fresh catalyst that it is suitable for /3-counting. Grls chromotographic
analysis
In order lo test the purity of the gases prepared. several H 2- and C3Hs samples were assayed by the chemical laboratory of the Munich gas- and waterworks, using a gas chromatograph Perkin-Elmer 3920 with heat conductivity detector and He as carrier gas. The analytical results yielded in the case of HZ a purity of 99.9~019; with the following impurities: O2 (0.01-0.02 ~019,). NZ (0.06-0.08 ~019;). and CH, (traces). The synthesized propane had a purity of 99.8 vol:,,. the major impurity being hexane isomers (0.10-0.18 vol”,) which may have been formed by side reactions I and II in the reaction scheme shown in Fig. 3. Traces of air. CH, and CZH6 were also detected as impurities in the propane. Detection
System
The apparatus used in the detection of tritium (Fig. 4) consists of a 2.6 I gas counter (Type GPC-14. Johnston Laboratories Inc.. USA). the shielding and
921
Measwemenr of lrilium To gloss lti
,25mm
bore
FIG. 2. Longitudinal section through the Mg furnace for reduction of water samples.
TABLE
1.
Lifetime and memory effect of different types of catalysts
Type
Catalyst supplier
Total mass (g)
Pd (g)
Lifetime (runs)
Memory effect (X)
l&19 31-39 214
0.46 0.16 0.32
5% Pd on asbestos lo”/. Pd on asbestos So”/, Pd on asbestos
Fluka AC, Buchs (Switzerland)
IO 10 LO
0.5
O.S’A Pd on A&OS (pellets) 2% Pd on carbon (granules)
Kali-Chemic Engelhard. Hannover
100
0.5
211
4.60
25
0.5
217
1.54
:
922
M. WoIJet al. Mart ruwim: H,C=C =CH, -
H
“2
2
H,C=CH-CH,
CH, -CH, -CH,
(1)
Prcqodrne side rarction I: ~C=CGi,
“2
+H,C=C=CH, CQl,2-Dimethykw-
I-Makthyl2-mamybm-
CH, nC+l-CH, -CH, -CH,-CH,
,
I-Haam CH, -CH=CH-CH, 2-m
-CH, -CH,
CH, =CH -$H-CH,
"2
CH,-CH2-CH,-CH,-CH,-CH,
/’
Hexam
Pa)
-CH;
CH, 3-Memthyl-I - pantala CH,-CH=$-CH,
1,2-Dinwth$ cydobuta
CH, -CH, -FH -CH, -CH,
(2b)
C”3
-CH,
3-Mathylpamw
% 3-hlethyl-2-pantme -
CH,=F-‘?I-CH, H,C CH,
"2
CH,-CH-$Ji-CH, Hk % 2.3 - Dimathylbukna
2,3-Din~hyl-I-lwtann sida MaclbnlI.
I -MewIg-
I.3 - cwhgmk
IJ-Din&-
3-methykw-
CyCbbUkXlO
Cydobu_
cycbw I)
CH 2 =ai -CH, -$H-CH, CH, 4-Methyl -I - wlmna
-
2)
CH, -al
> 7
=a+CHHCH,
CH, -CH,
-CH,-fH-CH,
(3)
C% 2-Metrylpaflwla
t H3 4-Methyl -2-pantam
FIG. 3. Reaction scheme in the hydrogenation of propadiene.
electronics (Nuclear Enterprises Ltd, Edinburgh) and a self-constructed counter filling system. Technical data of the counter are as follows: effective volume approx. 2.2 1. maximum pressure 5 bars, Cu cathode, anode wire stainless steel, 0.025 mm dia. The counter is surrounded by a plastic scintillator (Type NE 110) to eliminate myons. Neutrons generated by interaction of myons with the shield and moderated by the plastic scintillator and by boric acid, are captured by the boron in the boric acid (cf. (8)). The detection system is set up in a room in the basement. which has 75 cm thick walls made of heavy concrete (Ilmenite). The pulses of the gas counter pass through a charge-sensitive pre-amplifier (sensitivity 0.035 pV/ion pair), and are then amplified by a factor of 12.5. A value of 5 was found to be the most favorable ratio between upper and lower threshold of the tritium
channel. Counts from the tritium channel and those counts from above the upper threshold are compared with events from the plastic scintillator in anticoincidence units. Two type 57 A.V.P. photomultipliers of Mullard Ltd are used as detectors with the plastic scintillator. The resulting anticoincidence counts and coincidence coun&. respectively. are registered and printed after preselected time intervals. Anticoincidence shielding In order lo determine the optimum anticoincidence shielding effect, the background count rate was measured under standard conditions as a function of photomultiplier voltage. It was found that the background count rate no in the tritium channel was lowered when using the highest voltage (2.2 kV) and operating the photomultipliers in coincidence. The coinci-
923
Measurement of rririum
\
I /\
I \
I.
‘Pbstic sd~IIbtce NE 110 26xe3an) (cuvlly
I
-A
VI
v2
dlanar
-Am@flt
HV
.,L_.lI,F3.ZP-l
mcoZ
dence count rate of the plastic scintillator at 2.2 kV was found to be 9OOOcpm. and is thus three times as large as the contribution caused by myons only. The influence onto the ‘H counting efficiency (due to random coincidences with pulses from the gas counter) may be neglected. High volrage factor
For the sake of reproducible results when using differential measuring technique, the gas amplification, which is a function of counter voltage, has to be adjusted to the same value for each counting gas. As reference voltage the voltage U1,,, is used, at which the coincidence counting rate, due to myons and registered by the ‘H counter, is half the value of the total coincidence counting rate. At a pressure of 2 bar in the counter, U,,2 o 4.6 kV and at 4 bar, U l/2 2 6.6 kV. The operating voltage for routine The concentration of environmental tritium is often reported in TU. 1TU (tritium unit) A 1 TR (tritium ratio)
k 3.2 pCi ‘H/l Hz0 A 0.12 Bq ‘H/l
-
HDI
measurements is obtained from U1,* by multiplication with the optimum high voltage factor F,v,O,,, (Fig. 5). In general the factor q2/n,, is used as a criterion for optimization of a low-level counting system. Even though q2/n0 has its maximum value at FHV = 1.20 (Fig. 5). a value of 1.16 was chosen for FHVVopc. because this minimizes the influence of changes in the gas amplification during long-time measurements. Corresponding measurements and considerations yielded F HV.Opc. = 1.20 for 4 bar pressure. Results and Discussho The tritium concentration c (TV)* is calculated from the measured count rate n, (cpm) by means of equation (4)
c=
l
; [‘H]/[H] = lo-” HZO.
z H
x(4 - no)
-no)
=
F (tll A
- n,)
where x = ‘H concentration
in 3H standard (TV)
(4)
924
hf. Wol/ct al.
4 - L4 - 13 -IA?
- LI
-I
-as
cf
-a6
“0 .
!tirlzz nc)[cwnJ
l.lZ
IJ4
U6
Ll6 Hi@
VdlDor
L20 fator
I22
L24
-
FM,
FIG. 5. ‘H counting efficiency q (related to the counter volume of 2.6 I). Background count rate no and optimization factor $/no (propane pressure = 2 bar inside counter) as functions of the high voltage factor FHv
A = enrichment factor n ,.,, = ‘H standard count rate @pm) n, = background count rate (cpm) F = x/(nr,, - no) = calibration factor (TU/cpm). The standard deviation u(c) is obtained from equation (4) using the law of error propagation. The relative error u’(c)/c[= 1.96u(c)/c] which is related to a 95% confidence level P (in the case of a two-sided problem) or 97.5% confidence level P (in the case of a one-sided problem) is given by equation (5)
was obtained for an initial volume of 0.4 I H@) and a’(A)/A = 0.09 for an initial volume of 1.4 l!7Q9’ The minimum tritium concentration c,in which may be distinguished with P = 97.5% from tritiumfree water samples (L no) is defined as the detection limit: Gli” = Q’(C,in)
The relative error of the tritium content in the ‘H calibration standard solution (a’(x)/x) was taken to be 0.02. For the error of the enrichment u’(A)/A = 0.06
If all the errors due to preparation and measure ment of samples are negligible compared to counting statistical (Poisson-) errors, equation (6), with
= f JCa’(nr)l’
+ Ca’hN 0 fi
f u’(n0). (6)
925
Measurement o/ iririum TABLE 2. Results of ‘H standard measurements (‘H concentration of NBS ‘H standard on Oct. 10, 1979: 263.7 TU). The count rates corrected for decay refer to
Oct. 6, 1979 ‘H
f&J (cpm)
Count rate corrected for decay Mm)
Average calibration factor F (TWpm)
4000 4am 4000
11.838 + 0.072 11.807 f 0.055 11.844 f 0.053
11.831 11.807 11.849
23.24
1167 1333 1333
22.453 f 0.211 21.987 f 0.169 22.442 f 0.171
22.629 22.208 22.615
12.24
Standard sample No.
Pressure inside counter (bar)
Measuring time (min)
Count rate
1 2 3
2 2 2
4 5 6
4 4 4
a’(no) = 1.96 fl no T, red uces to equation (7) for the smallest possible value of c,,” within
the measuring
time 7 :
Cd” = The detection limit which may be attained in practice (equation 6) has to be calculated from the experimentally determined deviation ~‘(n,,) of the count rates from a sufficiently large number of background samples. This detection limit is generally higher than the detection limit derived from counting statistics only (equation 7). Accordingly the deviation of results for ‘H-containing
samples, u’(c) has to be obtained
large number of standard samples with known ‘H content which have been prepared and measured in the same manner. Such calibration measurements have to be repeated from time to time. If only a few calibration samples are available, the standard deviation calculated in the usual way increases by multiplication with Student’s t-factor (e.g. for P = 957’ I = 4.30 for 3 equal samples, and f = 2.26 for 10 equal samples). from a sufficiently
TABLE 3.
The count rates obtained in the individual measurements are tested for statistical purity by means of Pearson’s x2-test. Typical results measured with 3H standard samples, background samples and analytical samples (partially after electrolytic enrichment) have been compiled in Tables 24.
Discussion of results The 3H standard samples measured at a filling pressure of 2 bar (corresponding to 10 ml H20) have shown a good reproducibility of 0.9% (P = 95%). For background samples experimental detection limits (P = 97.5%) were attained of 1.5 TU in 1OOOmin measuring time and 0.8 TU in 8000-8500 min measuring time. In the latter case, the detection limit as derived from counting statistics is found to be O.STU. The largest useful time of measurement, 7,,_ may be defined as that time, in which the smallest experimentally attainable fluctuations (limiting system error) correspond to the deviation due to counting statistics (cf. (11)). Since in the measurements reported here, counting statistical deviations cannot yet be neglected. one may assume that the limiting error has not
Results of background measurements (‘H concentration of background samples ~0.2 TU)
Background sample No.
Pressure inside counter (bar)
Measuring time (min)
Count rate *W0) (cpm)
Average count rate f tine) (cpm)
Average air pressure (mbar)
I
2 2
4500 4OaJ
0.468 f 0.010 0.486 & 0.011
0.477 f 0.007
965.5 959
2
2 2
4000 4000
0.486 k 0.012 0.484 + 0.012
0.485 + 0.008
z
2 2
4tmO 4167
0.481 f 0.01 I 0.495 0.01 f I
0.488 + 0.008
z
4 4 4
4000 4000 4WO
0.943 f 0.016 0.923 f 0.015 0.966 f 0.019
4 65
953 975 959
Average background count rate no f o’(no) (cpm)
0.483 f 0.024
0.944 f 0.093
M. Wol/et al.
926
TABLE4. Results of tritium analyses on groundwater and brine samples with and without preceding electrolytic enrichment. A correction of the ‘H concentration with regard to ground contaminationoO) was not carried out. The groundwater samples from Oberbachern and Munich were taken in cooperation with the Bayerisches Landesamt fur Wasserwirtschaft
Description of sample Brine samples from salt mine potash mine salt mine salt mine salt mine Oberbachern Oberbachern
Date of sampling
Mar. Dec. Mar. Mar. Mar.
12, 79 12 78 13. 79 14. 79 14. 79
Apr. 9. 79 Apr. 9. 79
Background water IAEA
Measuring time (min)
‘H concentration (TU) c * a’(c) measured Range
Enrichment factor A
-.
II
31 + 3 I8 + 2 11 +2 2+1 0.96 -+ 0.15
28-34 lb20 9-13 l-3 0.81-1.11
loo0 1333
21 23
0.14 + 0.06 0.13 + 0.05
0.084.20 0.08~.18
2333 1167
42 37
0.10 + 0.02 0.08 + 0.04
0.084.12 0.04-0.12
1000 1000 1000 4167 1167
_-
Munich, Ellwanger. deep well
Apr. 4. 79
1000
20
0.06 f 0.06
O-0.12
Munich, Paulaner Brewery, deep well 2
May 3. 79 May 3. 79 May 3. 79
1000 1167 4cmO
22 20 19
0.06 f 0.06 0.02 + 0.06 0.06 f 0.04
oJJ.12 NJ.08 0.02~.10
Munich. Metzler Rubber Co. deep well
Apr. 4. 79
1333
19
0.04 + 0.06
o-0.10
Munich, Lowenbrewery. deep well I
Apr. 4. 79
loo0
19
a T,,, of at least 3100 min results for a filling pressure of 2 bar. For a filling pressure of 4 bar (corresponding to 20 ml HaO), the reproducibility was only 4.65: for ‘H standard samples. The experimental detection limit was found to be 1.6TU for 4000min measuring time, as compared to a counting statistical detection limit of 0.5TU (this corresponds to T,,,_ of at least 400 min). The still unsatisfactory reproducibility of measuring results at 4 bar filling pressure is probably due to electronic difficulties which are caused by the high operating voltage (approx. 8 kV) and by the higher sensitivity of counting gases towards impurities. No dependence of background counting rate on atmospheric pressure was observed. As shown in Table 5. the detection limits of the )H measuring method reported here (without preceding enrichment) are comparable or lower than the detection limits attained by other authors. After preceding electrolytic enrichment, experimental detection limits of ~0.1 TU are obtained, the counting statistical detection limits being 0.05 TU after 20-fold enrichment and 0.02 TU after 50-fold enrichment. From these results it may be concluded that the ground contamination is less than 0.1 TU (cf. alao (9)). The ground contamination comprises the total possible 3H contamination of the sample due to sampling, storage, and analysis (e.g. by addition of
yet been attained. From these considerations,
-0.01
+ 0.08
O-O.07
Na20, and PbCla before or after electrolysis, respectively) and includes a well as possible original ‘H content of the water which is used as background samples (In our case, the background samples should according to hydrogeological expectation, contain no tritium). Our ground contamination is of the same order of magnitude as was found in other laboratories after electrolytic enrichment.(‘0.‘4’ The magnitude of the individual contributions to ground contamination including a possible ‘H content of background samples remain to be determined. Beside a further development of ‘H analysis by mass-spectrometric 3He measurement. the following additional possible ways may be considered to lower the ‘H detection limit in gas counter measurements: (a) Replacement of all hydrogen atoms in propane by hydrogen from the water sample to be analyzed, similar to the ethane total syntheses’*2’:
Mg2C3 + 4 Ha0 + 2 Mg(OH)2 + HC =C-CHJ HI0
+ Mg-Hz
HC=C-CH,
(8)
+ MgO
(9)
+ 2Hz +CaHa
(10)
(b) Additional ‘H enrichment (for instance 27-fold) by distillation of a major quantity of water (for
91
a/.” ”
(testphase)
2.2 etT.
I etf.
I .3 elT.
Ibid.
OESCHGER"~'
2.2 etT.
(tart phase)
and
This work
Ibid.
SC-xx
4.6 etT.
GEYH(“’
I .5 eff.
2.2 eff.
3.1 eff.
and Dc+ts~Y(~‘)
3
1.6
2.5
R~ETHERand WEISS”~’
&TLUND
SBCmmAm et ~2/.“~’
VINGGRADGV
EULITZ”
Authors
Volume of counter (1)
Cd%
CA
CH.
CH.
C2H6
Ar CH.
H2
C2H.
C2H6
Cd%
H2
CH.
CA
CA
Counting gas
1.33
4
2
3.6
I
1.87 1.39 0.35
0.94
1.0
1.4
0.48
23.2
12.2
1.3’ 0.33
I.6
I.5 0.8
1.3.
2.4’
1.8.
4mo
1000-1300 8000-8500
2600
1300
5600
1.0. 2.4’
I300
Counting time (min)
1300
5.2.
Gal. exp. (TU)
3.1. 2.5.
3.8.
3.9.
3.3.
2.6.
3.7.
cl!& count. stat. (TU)
26
0.08
0.57
36.4 72
0.55
0.73
38.5
50.4
I.0
44
2 0.7 2.84 0.22
0.25
1.28
25.9. 75
5.3
(Znom)
Background count rate
18.3’
Calibration factor F (TU/cpm)
1.33
2
2.2
Pressure inside counter (bar)
TABLE5. Characteristic data of different ‘H gas counter systems l derived or estimated from data given by the authors efi. = effective counting volume cmI..co”“l.*.l.= counting statistical detection limit (P = 97.5%) for r = 1000 min c,in.q. = experimental detection limit (P = 97.5%) for given time of measurement
C,Hs from HsC==H2
CaHs from H2C-H2
Underground laboratory Underground laboratory
C2He from C2H2
C2Hb from C2H,
C2H6 total synthesis
C,Hs from HC=C-CH,
Remarks
M. Woyet al.
928 instance 40 l).“9’ After further electrolytic gas counting
would allow theoretically
enrichment,
6. WOLFM. Diplomarbeit.
to attain de-
tection limits in the order of IO- 3 TU.
Acknowledyemenrs-The authors wish to express their thanks to Professor Dr H. MOSERfor his constant interest and support of this work. Mr M. FORSTERfor critically reading the manuscript and for enrichment of several water samples. Mrs A. KESSLERand Mrs E. STURMfor the careful laboratory work, and Mr H. RAST for his assistance in electronic problems. For useful suggestions in construction work. we thank Mr A. SCHMELLER.and for shop work Mr R. ABELTSHAUSER and Mr R. FLECK.
7.
8. 9. IO.
Radiar.
I. IAEA. fsorope Hydrolog) 197X. Vol. I and II (IAEA. Vienna. 1979). 2. CAMERONJ. F. Rudioacrire Dariny and Methods ojLowLure/ Counriny, p. 543 (IAEA. Vienna. 1967). 3. CLARKEW. B.. JENKINSW. J. and TOP Z. Inr. J. Appl. Rudiar. lsor. 27, 515 (1976). 4. EULITZG. W. Rec.. Sri. fnsrrum. 34. 1010 (1%3). 5. RAST H.. RAUERTW. and WOLF M. Low-Radioacricir! Measuremenrs and Applications, p. 141. (Slovenske pedagogicke nakladatelitvo. Bratislava 1977).
Isot. 27, 217 (1976).
II. GEYH M. A. Proc. 8th Inr. Radiocarbon Dariny Conf.. p. B 81, (Lower Hutt, New Zealand, 1972). 12. VINOGRADOVA. P.. DEVIRTS A. L. and D~BKINA E. I. Geochem. Inr. 5. 952 (1968). I ,.
References
University of Munich, unpublished (1979). EICHINGERL.. FORSTERM., RAPT H.. RAUERT W. and WOLF M. Low-Level Tritium Measuremenr. p. 43 (IAEA-TECDOC-246, Vienna. 1981). DE VR~ESHI. Nucl. Phys. 1, 477 (1956). FORSTER M. Diplomarbeit, University of Munich. unpublished (1979). WEISS W.. ROETHERW. and BADERG. Inr. 1. Appl.
SIEGENTHALERU.. OESCHGERH.. SCHOTTERERU. and H~NNI K. Inr. J. Appl. Radiur. lsor. 26. 459 (1975).
14. OSTLUNDH. G. and DORSEYH. G. Low-Radioacri&) Measurements and Applications. p. 55 (Slovenskt pedagogickt nakladatelitvo, Bratislava, 1977). 15 GROENEVELDD. J. Thesis. University of Groningen (1977). Tririum 16. ROETHER W. and WEISS W. Low-Level Measuremenr. p. 37 (IAEA-TECDOC-246, Vienna. 1981). 17. GEYH M. A. Personal communication (1980). 18. SCHOTI-ERERU. and OE~CHGERH. Law-Level Tririum Meusurement, p. 7 (IAEA-TECDOC-246, Vienna. 1981). 19. D~STROVSKYI.. AVINUR P. and NIR A. Liquid Scinrillorion Counting. p. 283 (Pergamon. Oxford, 1958).