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Low-level assay of 3H and 14C MDA limits with conventional liquid scintillation counters using the ‘LOLES’ procedure
Appl. Radar.
Pergaman PII: s096943043(96)ooo79-6
Vol. 47, No. S/IO, pp. 87%X83. 1996 Copyright 0 1996 ElsevierScience Ltd Printed in Great Britain. All rights reserved ht.
0969-8043/96
S15.00 + 0.00
Low-level Assay of 3H and 14CMDA Limits with Conventional Liquid Scintillation Counters Using the ‘LOLES’ Procedure J. M. LOS ARCOS* and L. RODRIGUEZ
BARQUERO
Metrologia de Radiaciones Ionizantes, Direction de Tecnologia, CIEMAT, Avenida Complutense 22, Madrid 28040, Spain This paper presents the results obtained with a conventional liquid scintillation counter using the procedure LOLES, developed to compensate the intrinsic and cosmic background variations. Low-level ‘H and ‘% samples of decreasing activity, down to the minimum detectable activity limits, have been assayed and their values compared to those obtained with conventional methods. The procedure enabled us to decrease the practical detection limits of a high background (90 cpm for 20 ml) general purpose counter down to 2.2 Bq L-’ for 3H and 2.9 Bq L-’ for ‘%. Copyright 0 1996 Elsevier Science Ltd
Experimental
Introduction Usual methods for low-level radioassay of ‘H and 14C by liquid scintillation counting (LSC) always require subtraction of the background count rate, which is usually estimated from an appropiate blank, similar in physicochemical properties, volume, composition and quench, to the problem sample (Peng, 1964; Mestres et al., 1993). Nevertheless, the minimum detectable activity (MDA) limits are difficult to reach without expensive and dedicated devices for background reduction. With conventional counters, the intrinsic background differences due to the glass composition of vials and the quick variations of the cosmic background during each measurement can reach up to several counts per minute (cpm) (Rodriguez Barquero et al., 1995) thus introducing significant errors when the activity being measured is equivalent to an observed net count rate of a few cpm. Very recently a new procedure, LOLES, was proposed to overcome these problems (Los Arcos and Rodriguez, 1996). In this paper, we present the application of the LOLES procedure to the radioassay of low-level ‘H and j4C samples of activity decreasing down to the MDA limits, using a conventional LSC spectrometer. Its results are compared to those obtained with the traditional procedures.
*To whom all correspondence
should be addressed.
Equipment and materials
For all the samples, we used low-potassium glass vials from Packard&, which were filled with 20 ml of commercial scintillator Insta-Gel” using a BrandR dispenser with combined uncertainty lower than 0.25% (1 SD). As quenching agent, we used CC&, which was dispensed with a Gilson” micropipette of combined uncertainty 0.35% (1 SD). The ‘H and 14Clow-activity samples were prepared from standardized tritiated water and from a standardized solution of 14C-labelled glucose in water, respectively, after dilutions to appropriate concentrations (0.3 mBq mgg ’ for ‘H and 0.8 mBq mg- ’ for ‘“C), using a pycnometer, a class A dilution flask and a 0.01 mg-sensitive MettlerTM AT210 balance for weighing the amount of radiactive material eventually added into each vial. For all measurements, we used a Wallac LKB Rackbeta 1219 Spectral” LSC spectrometer, with two photomultiplier tubes (PMTs) working in summedcoincidence mode and a ‘16Ra source for external standard quench determination. Sample preparation
The radioactive samples were prepared filling the vials with 20 ml of Insta-Gele, diluting the standardized solutions and adding aliquots to the vials, keeping always gravimetric control of each step. Several samples corresponding to activity levels of 0.1, 0.06, 0.02 and 0.01 Bq for ‘H and 0.1, 0.07, 0.04
879
J. M. Los Arcos and L. Rodriguez
880
Barquero
100
CD
Qh WC)
a
a 0
20 ml Insta-GelR
E
20 300
0
l-1024
(O-2 MeV)
A
l-520
(O-160 keV)
0
l-250
(O-20 keV)
u
FP
I
I
I
I
I
I
I
I
I
I
I
I
310
320
330
340
350
360
370
380
390
400
410
420
Quenching Fig.
I. Background
rates
Parameter
corresponding to the windows f&20 keV, O-160 keV and Insta-Gel”. for the different quenching degrees.
and 0.03 Bq for “‘C, were prepared with two quench levels without and with Ccl, enough to simulate the efficiency corresponding to 10 ml scintillator/lO ml aqueous solution, typically found in applications.
Measurements and Results
A=
NA = FA + 4 + AE
(1)
NB = FB + 4’
(2)
where A is the activity of the sample, E is the counting efficiency,
with
F, is the intrinsic background count rate of vial X (A or B), 4, d’, are the cosmic background count rates at the moment when vial A or B, respectively, was measured. Therefore, the activity A satisfies the general equation
Conventional and LOLES procedures
Two kinds of measurements have been carried out with each sample, following the conventional and the so-called ‘LOLES’ procedure. In the conventional method, two long measurements, one with the problem sample and another one with the blank vial and the same quench, were made one after the other and the background count rate directly subtracted from the sample count rate. If we call NA, NB the measured count rates of the sample and blank vials, respectively, we can express
O-2 MeV,
NA-NB
-6
E
6=
F~--&+4-4
(4)
E
For 6 to be neglected, we should ensure that both vials have the same intrinsic background and that the cosmic background was constant over the measurement time of both vials. Since differences up to 7 cpm in intrinsic background between similar vials (Rodriguez et al., 1995) and cosmic background variations up to 3 cpm in 24 h have been observed, unrealistic, unaccurate estimations can be expected at count rates below 10 cpm, with uncertainty which can reach 100% or more. The LOLES procedure (Los Arcos and Rodriguez, 1996) takes into account the different intrinsic background of sample and blank vials and the
Table I. ‘H and “C MDA limits (Bq/viai) corresponding to the highest efficiency and 24 h measurement time Nuchde ‘H ‘C
Window O-20 keV (channels l-250) 0.025
Window O-160 keV (channels I-520) 0.017
Window O-2 MeV (channels I-1024) 0.043 0.02 I
Low-level assay o f 3H a n d 14C u s i n g L O L E S
881
Table 2. Single ( + or - sign) and average (no sign) discrepancies, A(%), of 3H assays by LOLES and conventional methods at 20 ml vol. of Insta-Gel ". Associated statistical uncertainty (la) relative to exact activity, s(%), in square brackets Activity (Bq)
Window 1-1024 (0-2 MeV) Conventional
0.110
0.060
0.022
0.011
Window 1-250 (0-20 keV)
LOLES
Conventional
A(%)
is(%)]
A(%)
is(%)]
-- 13 -- 22 -- 30
[8] [14] [15]
+ 12 + 3.6 + 0.01
[111 [19] [21]
+ 49 + 22
[ll] [10]
+ 1.1 + 4.8
[15] [14]
27 37 29 28 46 47 45
[12] [24] [29] [25] [29] [21] [451
--+ ----
4.3 1.6 2.5 2.3 6 3.6 4
[16] [31] [51] [31] [38] [36] [60]
+ 25 + 19 - 3.5 + 1.8 + 43 + 37
39 + 53 + 33
[29] [63] [57]
3.3 + 7.6 -- 11
[41] [77] [74]
-- 113
[79]
+ 12
66 -- 328 - 126 + 444
[66] [111] [203] [126]
300
[147]
+ + + + + +
A(%)
LOLES
A(%)
[s(%)l
[15] [23] [13] [19] [14] [241
+ 20 + 16 + 2.5 -- 7 + 6.3 + 6
[18] [32] [16] [25] [24] [35]
21 + 20 + 0.5
[18] [33] [28]
10 + 12 -- 7
[25] [48] [39]
[119]
+ 72
[57]
+ 22
[78]
10 -- 66 + 197 + 305
[90] [175] [294] [167]
30 + 16 + 315 + 173
[39] [66] [125] [68]
14 - 4.2 + 338 + 172
[55] [95] [164] [94]
189
[212]
168
[ 86]
188
[1171
.
is(%)] .
.
.
variation of cosmic background count rates. To compensate for these effects, a first series of short
measurements, it can be shown in equations (3)-(4) that the activity in the sample can be better estimated
(15 m i n e a c h ) m e a s u r e m e n t s alternating the blank and the sample vials are carried out and then a second series is repeated after having removed the activity from the sample vial. I f w e c a l l N xr t h e a v e r a g e v a l u e o f t h e v i a l X ( A o r B ) o b s e r v e d c o u n t r a t e s , f o r s e r i e s K (1 o r 2)
by A =
Equation
(N1 -
N~) -
(4
- 4)
(5) e l i m i n a t e s t h e p r o b l e m s
(5)
of intrinsic and
Table 3. Single ( + or - sign) and average (no sign) discrepancies, A(%), of J4C assays by LOLES and conventional methods at volume 20 ml of Insta-Gel~. Associated statistical uncertainty (ltr) relative to exact activity, s(%), in square brackets Activity (Bq)
Window 1-1024 (0-2 MeV) Conventional At%)
0.144
Window 1-520 (0-160 keV)
LOLES
Conventional
A(%)
Is(%)]
A(%)
[s(%)]
A(%)
Is(%)]
5.2 6.7 4.9 6.2
[4.7] [4.3] [4.5] [4.3]
+ 3.1 -- 3.0 - 3.5 + 15.1
[6.2] [5.9] [5.9] [5.9]
-- 12.4 -- 21.3 -- 6.6 -- 13.9
[3.6] [3.6] [3.9] [3.4]
+ 6.1 -- 23.3 + 11.9 + 4.5
[5.2] [5.1] [5.1] [5.2]
-- 6.7
[4.3]
-- 2.1
[5.9]
- 21.3
[3.6]
-- 2.8
[5.3]
+ + + +
5.9
[4,3]
5.4
[6.0]
t5.1
[3.5]
0.077
-- 14.9 -- 13.1 -- 15.1 -- 6.0 -- 10.2
[8.8] [7.8} [8.5] [8.6] [8,4]
-- 15.4 -- 3.7 10.5 -- 6.6 -- 10.7
[11.5] [12.6] [11.2] [12.9] [11.9]
-- 12.7 + 1.1 -- 12.7 + 0.5 -- 11.1
[7.3] [6.9] [7.3] [6.1] [6.9]
9.4
[12.0]
7.6
0.045
+ 33.2 - 17.2 -- 17.2
[12.8] [14.5] [14.5]
-- 28.0 -- 16.7 -- 12.0
[18.3] [19.9] [20.3]
+ 11.9 -- 20.2 --20.2
[10.2] [13.0] [13.0]
- 6.2
[14.4]
-- 5.8
[21.4]
+ 3.1
- 6.2
[14.4]
- 1.1
[20.8]
+ 3.1
16.0 -- 25.9 -- 10.5 - 13.7 + 1.7 - 47.3
[14.11 [22.3] [18.6] [19.7] [21.5] [22.0]
12.7 -- 15.0 + 0.4 -- 2.8 + 12.6 - 36.4
[20.11 [32.6] [29.5] [31.2] [31.7] [32.3]
19.8
[18.6]
13.4
[31.5]
11.7
0.029
LOLES
[s(%)]
[8.4]
[6.9]
9.7 -- 11.6 + 2.2 11.7 • + 0.7 - 9.9 7.2
[5.2] [9.5] [10.3] [10.4] [9,6] [9.8] [9.9]
+ 20.7 -- 25.4 -- 11.4
[13.51 [16.1] [17.1]
[11.6]
-- 2.2
[15.8]
[11.6]
+ 11.9
[16.3]
11.7 + 2.1 + 15.6 + 16.0 + 27.5 - 31.9
[11.91 [17.7] [15.4] [17.4] [16.9] [17.8]
14;3 -- 9.4 + 4.1 + 4.5 + 16.0 - 43.4
[15.91 [24.4] [23.9] [24.8] [26.1] [26.1]
18.6
[17.0]
13.8
[25.1]
882
J.M. Los Arcos and L. Rodriguez Barquero 400
--
320 LOLES Method
240
• 1-1024 ( 0 - 2 MeV) • 1-250 (0-20 MeV) "
160
80
00|.
9
.
.
.
.
.
_~.O
_?
0
[]
[] []
+10 %
r2~-- 2
_ _ _
"d
[]
-10 % -80 D -160 Conventional Method o 1-1024 ( 0 - 2 MeV) [] 1-250 (0-20 MeV)
-240
-320
--
-400 0
D I
f
[
I
I
[
1
I
I
I
I
I
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
3H Standardized Activity (Bq) Fig. 2. Experimental discrepancies in 3H low-level assays with Insta-Gel~, using the LOLES and the conventional methods.
cosmic background making two series of short alternate measurements of blank and sample vials, instead of two long consecutive measurements. In this approach, the measurement of a single sample requires twice the time than the conventional method but with routine measurements of similar samples with the same quench, the difference N~A-- N2B needs to be evaluated only once, thus considerably simplifying the time required. Both types of procedures, the conventional [equation (3)] and the LOLES [equation (5)] have been applied to the radioassay of low-lewel 3H and ~4C samples. F o r 3H, the LOLES procedure has been applied reevaluating the difference ~ - A~Bfor each individual sample, whereas for ~4C, that difference was evaluated only once and used for different active samples, with the same quench. Results f o r 3H and 14C
The 3H and ~4C samples were measured in two ways, directly, with counting efficiency ca 45% for 3H and ca 93% for ~4C, and after quenching them with CC14, to simulate counting efficiencies typical of low-level samples, ca 22% for 3H and ca 87% ~4C.F o r each activity, two vials, A and B, were selected to contain the radioactive sample and to monitor the background, respectively. Then the triated water and the ~4C-labelled glucose solution were gravimetrically added into vial A, its quenching degree was determined and the quench of vial B was adjusted to that of vial A. In order to apply the procedure
LOLES, two series of 96 cycles of 15min measurements mentioned above were performed to obtain N~, N~,/~A, N2Baccording to equation (5). The conventional method was also applied according to equation (3) with the values N~, N~ from the first series of measurements. For 3H, the count rates of the windows 0--20 keV (channels 1-250) and 0-2 MeV (channels 1-1024) were recorded, whereas for ~4C,the count rates stored were those corresponding to the windows 0-160 keV (channels 1-520) and 0-2 MeV (channels 1-1024). The counting efficiency for 3H was directly determined by interpolation from a set of quenched standards and that of ~4C was computed by the CIEMAT/NIST method in the usual way (Grau Malonda and Garcia-Torafio, 1982; Rodriguez Barquero et al., 1991, 1992, 1994). The background count rates measured in the above-mentioned windows with 20 ml of Insta-GelTM, for the different quenching degrees used, are shown in Fig. 1 and amount to ca 28 cpm for the 3H window, ca 59 cpm for the ~4C window and ca 87 cpm for the whole window betwen 0 and 2 MeV. It can be observed that in each window the background does not change significantly with the quenching degree and, accordingly, the M D A limits are those corresponding to the highest efficiency, namely 45% for 3H and 93% for '4C and are summarized in Table 1. Tables 2 and 3 present, for 3H and ~4C respectively, the individual discrepancies between the standardized
Low-level assay of 3H and '4C using LOLES activity and the computed value obtained using both the LOLES and the conventional procedure s. Statistical uncertainties assumming Poisson distributions include covariance terms and are quoted in brackets. For 3H, the conventional subtraction of background fails to evaluate the 3H activity within 20% uncertainty. Nevertheless, the LOLES procedure is successful down to 0.022 Bq/vial, equivalent to 2.2 Bq L -~ for 20 ml samples with 50% water content, in both the 0-20 keV and 0-2 MeV, this latter having associated the smaller discrepancies, as can be seen in Fig. 2. For '4C, the procedure LOLES, although not so clearly as for 3H, provides also better results than the conventional method. Once again, the largest window, from 0 to 2 MeV, gives always smaller discrepancies than the conventional method and, in general, also smaller than those obtained in the '4C window (0-160 keV), as shown in Fig. 3. The lower performance of LOLES for ~4C against 3H, may be due to the different way in which LOLES was applied, measuring the difference N2A-- ]~B only once: small instabilities of the spectrometer (equivalent to 1 cpm or less) between the measurements series 1 and 2 could explain this effect.
Conclusions The LOLES procedure allowed to reach the MDA limits of 3H and ~4C with better accuracy than the subtraction method, using a conventional spec-
883
trometer and background count rates up to 90 cpm for 20 ml of scintillator. For 3H, a detection limit of 22 mBq/vial (2.2 Bq L - i ) , for 24 h measurements, was measured with uncertainty below 10%. For 14C, the practical detection limit found was 29 mBq/vial (2.9 Bq L -t) with uncertainty below 14%. These results indicate that the best performance of LOLES is obtained when the background characterisation of vials is done very close in time to the sample activity measurements.
References Grau Malonda A. and Garcia-Torafio E. (1982). Int. J. Appl. Radiat. Isot. 33, 249. Los Arcos J. M. and Rodriguez Barquero L. (1996) Radiocarbon. Tucson, Arizona (in press). Mestres J. M., Rajadel P. and Rauret G. (1993) Liquid Scintillation Spectrometry 1992 (Noakes E., SchonhSffer F. and Polach H. A., Eds), p. 165. Radiocarbon, Tucson, Arizona. Peng C. T. (1964). Analyt. Chem. 36, 2456. Rodrlguez Barquero L., Los Arcos J. M. and Grau A. (1991) Proc. lnt. Conf. on New Trends in Liquid Scintillation Counting and Organic Scintillators (Gatlinburg, 1989) (Ross H., Noakes H. E. and Spaulding
J. D., Eds), p. 593. Lewis Publishers, Chelsea, Mich. Rodriguez L., Los Arcos J. M. and Grau A. (1992). Nucl. Instrum. Meth. A312, 124. Rodriguez Barquero L., Los Arcos J. M., Grau Malonda A. and Garcia-Torafio E. (1994). Nucl. lnstrum. Meth. A339, 6. Rodrlguez Barquero L., Los Arcos J. M. and Jim~nez de Mingo A. (1995) C I E M A T Report 758. CIEMAT, Madrid.
Pergaman PII: s096943043(96)ooo79-6
Vol. 47, No. S/IO, pp. 87%X83. 1996 Copyright 0 1996 ElsevierScience Ltd Printed in Great Britain. All rights reserved ht.
0969-8043/96
S15.00 + 0.00
Low-level Assay of 3H and 14CMDA Limits with Conventional Liquid Scintillation Counters Using the ‘LOLES’ Procedure J. M. LOS ARCOS* and L. RODRIGUEZ
BARQUERO
Metrologia de Radiaciones Ionizantes, Direction de Tecnologia, CIEMAT, Avenida Complutense 22, Madrid 28040, Spain This paper presents the results obtained with a conventional liquid scintillation counter using the procedure LOLES, developed to compensate the intrinsic and cosmic background variations. Low-level ‘H and ‘% samples of decreasing activity, down to the minimum detectable activity limits, have been assayed and their values compared to those obtained with conventional methods. The procedure enabled us to decrease the practical detection limits of a high background (90 cpm for 20 ml) general purpose counter down to 2.2 Bq L-’ for 3H and 2.9 Bq L-’ for ‘%. Copyright 0 1996 Elsevier Science Ltd
Experimental
Introduction Usual methods for low-level radioassay of ‘H and 14C by liquid scintillation counting (LSC) always require subtraction of the background count rate, which is usually estimated from an appropiate blank, similar in physicochemical properties, volume, composition and quench, to the problem sample (Peng, 1964; Mestres et al., 1993). Nevertheless, the minimum detectable activity (MDA) limits are difficult to reach without expensive and dedicated devices for background reduction. With conventional counters, the intrinsic background differences due to the glass composition of vials and the quick variations of the cosmic background during each measurement can reach up to several counts per minute (cpm) (Rodriguez Barquero et al., 1995) thus introducing significant errors when the activity being measured is equivalent to an observed net count rate of a few cpm. Very recently a new procedure, LOLES, was proposed to overcome these problems (Los Arcos and Rodriguez, 1996). In this paper, we present the application of the LOLES procedure to the radioassay of low-level ‘H and j4C samples of activity decreasing down to the MDA limits, using a conventional LSC spectrometer. Its results are compared to those obtained with the traditional procedures.
*To whom all correspondence
should be addressed.
Equipment and materials
For all the samples, we used low-potassium glass vials from Packard&, which were filled with 20 ml of commercial scintillator Insta-Gel” using a BrandR dispenser with combined uncertainty lower than 0.25% (1 SD). As quenching agent, we used CC&, which was dispensed with a Gilson” micropipette of combined uncertainty 0.35% (1 SD). The ‘H and 14Clow-activity samples were prepared from standardized tritiated water and from a standardized solution of 14C-labelled glucose in water, respectively, after dilutions to appropriate concentrations (0.3 mBq mgg ’ for ‘H and 0.8 mBq mg- ’ for ‘“C), using a pycnometer, a class A dilution flask and a 0.01 mg-sensitive MettlerTM AT210 balance for weighing the amount of radiactive material eventually added into each vial. For all measurements, we used a Wallac LKB Rackbeta 1219 Spectral” LSC spectrometer, with two photomultiplier tubes (PMTs) working in summedcoincidence mode and a ‘16Ra source for external standard quench determination. Sample preparation
The radioactive samples were prepared filling the vials with 20 ml of Insta-Gele, diluting the standardized solutions and adding aliquots to the vials, keeping always gravimetric control of each step. Several samples corresponding to activity levels of 0.1, 0.06, 0.02 and 0.01 Bq for ‘H and 0.1, 0.07, 0.04
879
J. M. Los Arcos and L. Rodriguez
880
Barquero
100
CD
Qh WC)
a
a 0
20 ml Insta-GelR
E
20 300
0
l-1024
(O-2 MeV)
A
l-520
(O-160 keV)
0
l-250
(O-20 keV)
u
FP
I
I
I
I
I
I
I
I
I
I
I
I
310
320
330
340
350
360
370
380
390
400
410
420
Quenching Fig.
I. Background
rates
Parameter
corresponding to the windows f&20 keV, O-160 keV and Insta-Gel”. for the different quenching degrees.
and 0.03 Bq for “‘C, were prepared with two quench levels without and with Ccl, enough to simulate the efficiency corresponding to 10 ml scintillator/lO ml aqueous solution, typically found in applications.
Measurements and Results
A=
NA = FA + 4 + AE
(1)
NB = FB + 4’
(2)
where A is the activity of the sample, E is the counting efficiency,
with
F, is the intrinsic background count rate of vial X (A or B), 4, d’, are the cosmic background count rates at the moment when vial A or B, respectively, was measured. Therefore, the activity A satisfies the general equation
Conventional and LOLES procedures
Two kinds of measurements have been carried out with each sample, following the conventional and the so-called ‘LOLES’ procedure. In the conventional method, two long measurements, one with the problem sample and another one with the blank vial and the same quench, were made one after the other and the background count rate directly subtracted from the sample count rate. If we call NA, NB the measured count rates of the sample and blank vials, respectively, we can express
O-2 MeV,
NA-NB
-6
E
6=
F~--&+4-4
(4)
E
For 6 to be neglected, we should ensure that both vials have the same intrinsic background and that the cosmic background was constant over the measurement time of both vials. Since differences up to 7 cpm in intrinsic background between similar vials (Rodriguez et al., 1995) and cosmic background variations up to 3 cpm in 24 h have been observed, unrealistic, unaccurate estimations can be expected at count rates below 10 cpm, with uncertainty which can reach 100% or more. The LOLES procedure (Los Arcos and Rodriguez, 1996) takes into account the different intrinsic background of sample and blank vials and the
Table I. ‘H and “C MDA limits (Bq/viai) corresponding to the highest efficiency and 24 h measurement time Nuchde ‘H ‘C
Window O-20 keV (channels l-250) 0.025
Window O-160 keV (channels I-520) 0.017
Window O-2 MeV (channels I-1024) 0.043 0.02 I
Low-level assay o f 3H a n d 14C u s i n g L O L E S
881
Table 2. Single ( + or - sign) and average (no sign) discrepancies, A(%), of 3H assays by LOLES and conventional methods at 20 ml vol. of Insta-Gel ". Associated statistical uncertainty (la) relative to exact activity, s(%), in square brackets Activity (Bq)
Window 1-1024 (0-2 MeV) Conventional
0.110
0.060
0.022
0.011
Window 1-250 (0-20 keV)
LOLES
Conventional
A(%)
is(%)]
A(%)
is(%)]
-- 13 -- 22 -- 30
[8] [14] [15]
+ 12 + 3.6 + 0.01
[111 [19] [21]
+ 49 + 22
[ll] [10]
+ 1.1 + 4.8
[15] [14]
27 37 29 28 46 47 45
[12] [24] [29] [25] [29] [21] [451
--+ ----
4.3 1.6 2.5 2.3 6 3.6 4
[16] [31] [51] [31] [38] [36] [60]
+ 25 + 19 - 3.5 + 1.8 + 43 + 37
39 + 53 + 33
[29] [63] [57]
3.3 + 7.6 -- 11
[41] [77] [74]
-- 113
[79]
+ 12
66 -- 328 - 126 + 444
[66] [111] [203] [126]
300
[147]
+ + + + + +
A(%)
LOLES
A(%)
[s(%)l
[15] [23] [13] [19] [14] [241
+ 20 + 16 + 2.5 -- 7 + 6.3 + 6
[18] [32] [16] [25] [24] [35]
21 + 20 + 0.5
[18] [33] [28]
10 + 12 -- 7
[25] [48] [39]
[119]
+ 72
[57]
+ 22
[78]
10 -- 66 + 197 + 305
[90] [175] [294] [167]
30 + 16 + 315 + 173
[39] [66] [125] [68]
14 - 4.2 + 338 + 172
[55] [95] [164] [94]
189
[212]
168
[ 86]
188
[1171
.
is(%)] .
.
.
variation of cosmic background count rates. To compensate for these effects, a first series of short
measurements, it can be shown in equations (3)-(4) that the activity in the sample can be better estimated
(15 m i n e a c h ) m e a s u r e m e n t s alternating the blank and the sample vials are carried out and then a second series is repeated after having removed the activity from the sample vial. I f w e c a l l N xr t h e a v e r a g e v a l u e o f t h e v i a l X ( A o r B ) o b s e r v e d c o u n t r a t e s , f o r s e r i e s K (1 o r 2)
by A =
Equation
(N1 -
N~) -
(4
- 4)
(5) e l i m i n a t e s t h e p r o b l e m s
(5)
of intrinsic and
Table 3. Single ( + or - sign) and average (no sign) discrepancies, A(%), of J4C assays by LOLES and conventional methods at volume 20 ml of Insta-Gel~. Associated statistical uncertainty (ltr) relative to exact activity, s(%), in square brackets Activity (Bq)
Window 1-1024 (0-2 MeV) Conventional At%)
0.144
Window 1-520 (0-160 keV)
LOLES
Conventional
A(%)
Is(%)]
A(%)
[s(%)]
A(%)
Is(%)]
5.2 6.7 4.9 6.2
[4.7] [4.3] [4.5] [4.3]
+ 3.1 -- 3.0 - 3.5 + 15.1
[6.2] [5.9] [5.9] [5.9]
-- 12.4 -- 21.3 -- 6.6 -- 13.9
[3.6] [3.6] [3.9] [3.4]
+ 6.1 -- 23.3 + 11.9 + 4.5
[5.2] [5.1] [5.1] [5.2]
-- 6.7
[4.3]
-- 2.1
[5.9]
- 21.3
[3.6]
-- 2.8
[5.3]
+ + + +
5.9
[4,3]
5.4
[6.0]
t5.1
[3.5]
0.077
-- 14.9 -- 13.1 -- 15.1 -- 6.0 -- 10.2
[8.8] [7.8} [8.5] [8.6] [8,4]
-- 15.4 -- 3.7 10.5 -- 6.6 -- 10.7
[11.5] [12.6] [11.2] [12.9] [11.9]
-- 12.7 + 1.1 -- 12.7 + 0.5 -- 11.1
[7.3] [6.9] [7.3] [6.1] [6.9]
9.4
[12.0]
7.6
0.045
+ 33.2 - 17.2 -- 17.2
[12.8] [14.5] [14.5]
-- 28.0 -- 16.7 -- 12.0
[18.3] [19.9] [20.3]
+ 11.9 -- 20.2 --20.2
[10.2] [13.0] [13.0]
- 6.2
[14.4]
-- 5.8
[21.4]
+ 3.1
- 6.2
[14.4]
- 1.1
[20.8]
+ 3.1
16.0 -- 25.9 -- 10.5 - 13.7 + 1.7 - 47.3
[14.11 [22.3] [18.6] [19.7] [21.5] [22.0]
12.7 -- 15.0 + 0.4 -- 2.8 + 12.6 - 36.4
[20.11 [32.6] [29.5] [31.2] [31.7] [32.3]
19.8
[18.6]
13.4
[31.5]
11.7
0.029
LOLES
[s(%)]
[8.4]
[6.9]
9.7 -- 11.6 + 2.2 11.7 • + 0.7 - 9.9 7.2
[5.2] [9.5] [10.3] [10.4] [9,6] [9.8] [9.9]
+ 20.7 -- 25.4 -- 11.4
[13.51 [16.1] [17.1]
[11.6]
-- 2.2
[15.8]
[11.6]
+ 11.9
[16.3]
11.7 + 2.1 + 15.6 + 16.0 + 27.5 - 31.9
[11.91 [17.7] [15.4] [17.4] [16.9] [17.8]
14;3 -- 9.4 + 4.1 + 4.5 + 16.0 - 43.4
[15.91 [24.4] [23.9] [24.8] [26.1] [26.1]
18.6
[17.0]
13.8
[25.1]
882
J.M. Los Arcos and L. Rodriguez Barquero 400
--
320 LOLES Method
240
• 1-1024 ( 0 - 2 MeV) • 1-250 (0-20 MeV) "
160
80
00|.
9
.
.
.
.
.
_~.O
_?
0
[]
[] []
+10 %
r2~-- 2
_ _ _
"d
[]
-10 % -80 D -160 Conventional Method o 1-1024 ( 0 - 2 MeV) [] 1-250 (0-20 MeV)
-240
-320
--
-400 0
D I
f
[
I
I
[
1
I
I
I
I
I
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
3H Standardized Activity (Bq) Fig. 2. Experimental discrepancies in 3H low-level assays with Insta-Gel~, using the LOLES and the conventional methods.
cosmic background making two series of short alternate measurements of blank and sample vials, instead of two long consecutive measurements. In this approach, the measurement of a single sample requires twice the time than the conventional method but with routine measurements of similar samples with the same quench, the difference N~A-- N2B needs to be evaluated only once, thus considerably simplifying the time required. Both types of procedures, the conventional [equation (3)] and the LOLES [equation (5)] have been applied to the radioassay of low-lewel 3H and ~4C samples. F o r 3H, the LOLES procedure has been applied reevaluating the difference ~ - A~Bfor each individual sample, whereas for ~4C, that difference was evaluated only once and used for different active samples, with the same quench. Results f o r 3H and 14C
The 3H and ~4C samples were measured in two ways, directly, with counting efficiency ca 45% for 3H and ca 93% for ~4C, and after quenching them with CC14, to simulate counting efficiencies typical of low-level samples, ca 22% for 3H and ca 87% ~4C.F o r each activity, two vials, A and B, were selected to contain the radioactive sample and to monitor the background, respectively. Then the triated water and the ~4C-labelled glucose solution were gravimetrically added into vial A, its quenching degree was determined and the quench of vial B was adjusted to that of vial A. In order to apply the procedure
LOLES, two series of 96 cycles of 15min measurements mentioned above were performed to obtain N~, N~,/~A, N2Baccording to equation (5). The conventional method was also applied according to equation (3) with the values N~, N~ from the first series of measurements. For 3H, the count rates of the windows 0--20 keV (channels 1-250) and 0-2 MeV (channels 1-1024) were recorded, whereas for ~4C,the count rates stored were those corresponding to the windows 0-160 keV (channels 1-520) and 0-2 MeV (channels 1-1024). The counting efficiency for 3H was directly determined by interpolation from a set of quenched standards and that of ~4C was computed by the CIEMAT/NIST method in the usual way (Grau Malonda and Garcia-Torafio, 1982; Rodriguez Barquero et al., 1991, 1992, 1994). The background count rates measured in the above-mentioned windows with 20 ml of Insta-GelTM, for the different quenching degrees used, are shown in Fig. 1 and amount to ca 28 cpm for the 3H window, ca 59 cpm for the ~4C window and ca 87 cpm for the whole window betwen 0 and 2 MeV. It can be observed that in each window the background does not change significantly with the quenching degree and, accordingly, the M D A limits are those corresponding to the highest efficiency, namely 45% for 3H and 93% for '4C and are summarized in Table 1. Tables 2 and 3 present, for 3H and ~4C respectively, the individual discrepancies between the standardized
Low-level assay of 3H and '4C using LOLES activity and the computed value obtained using both the LOLES and the conventional procedure s. Statistical uncertainties assumming Poisson distributions include covariance terms and are quoted in brackets. For 3H, the conventional subtraction of background fails to evaluate the 3H activity within 20% uncertainty. Nevertheless, the LOLES procedure is successful down to 0.022 Bq/vial, equivalent to 2.2 Bq L -~ for 20 ml samples with 50% water content, in both the 0-20 keV and 0-2 MeV, this latter having associated the smaller discrepancies, as can be seen in Fig. 2. For '4C, the procedure LOLES, although not so clearly as for 3H, provides also better results than the conventional method. Once again, the largest window, from 0 to 2 MeV, gives always smaller discrepancies than the conventional method and, in general, also smaller than those obtained in the '4C window (0-160 keV), as shown in Fig. 3. The lower performance of LOLES for ~4C against 3H, may be due to the different way in which LOLES was applied, measuring the difference N2A-- ]~B only once: small instabilities of the spectrometer (equivalent to 1 cpm or less) between the measurements series 1 and 2 could explain this effect.
Conclusions The LOLES procedure allowed to reach the MDA limits of 3H and ~4C with better accuracy than the subtraction method, using a conventional spec-
883
trometer and background count rates up to 90 cpm for 20 ml of scintillator. For 3H, a detection limit of 22 mBq/vial (2.2 Bq L - i ) , for 24 h measurements, was measured with uncertainty below 10%. For 14C, the practical detection limit found was 29 mBq/vial (2.9 Bq L -t) with uncertainty below 14%. These results indicate that the best performance of LOLES is obtained when the background characterisation of vials is done very close in time to the sample activity measurements.
References Grau Malonda A. and Garcia-Torafio E. (1982). Int. J. Appl. Radiat. Isot. 33, 249. Los Arcos J. M. and Rodriguez Barquero L. (1996) Radiocarbon. Tucson, Arizona (in press). Mestres J. M., Rajadel P. and Rauret G. (1993) Liquid Scintillation Spectrometry 1992 (Noakes E., SchonhSffer F. and Polach H. A., Eds), p. 165. Radiocarbon, Tucson, Arizona. Peng C. T. (1964). Analyt. Chem. 36, 2456. Rodrlguez Barquero L., Los Arcos J. M. and Grau A. (1991) Proc. lnt. Conf. on New Trends in Liquid Scintillation Counting and Organic Scintillators (Gatlinburg, 1989) (Ross H., Noakes H. E. and Spaulding
J. D., Eds), p. 593. Lewis Publishers, Chelsea, Mich. Rodriguez L., Los Arcos J. M. and Grau A. (1992). Nucl. Instrum. Meth. A312, 124. Rodriguez Barquero L., Los Arcos J. M., Grau Malonda A. and Garcia-Torafio E. (1994). Nucl. lnstrum. Meth. A339, 6. Rodrlguez Barquero L., Los Arcos J. M. and Jim~nez de Mingo A. (1995) C I E M A T Report 758. CIEMAT, Madrid.