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Radioassay by an efficiency tracing technique using a liquid scintillation counter
hr. 3. ~ppl. Radiar. Isor. Vol. 35. No. 6, pp. 163--%6. Printed in Great Brirain hII rights reserved
1984 Copyright
0020-708X S-I 53.00 + 0.00 C 1984 Peqamon Press Ltd
Radioassay by an Efficiency Tracing Technique Using a Liquid Scintillation Counter HIROAKI
ISHIKAWA’,
MAKOTO
TAKIUI?
and TAMARU
ABURAI’
‘Radioisotope-Nuclear Reactor School, Japan Atomic Energy Research institute, 2-28-49, Honlcomagome, Bunkyo-ku, Tokyo. Japan and ‘Division of Radiobiology, Research Center of iMedical Science, The Jikei University School of Medicine, 3-25-8, Nishishimbashi, Minato-ku, Tokyo, Japan (Received 5 April 1983; in revisedform
17 Augusr 1983)
The various nuclides other than ‘H are radioassayed with an efficiency tracing technique. It has been clarified in this study that the liquid scintillation efficiency tracing technique possesses the following advantages; (I) sample preparation is of simplicity (2) even a nuclide different from a nuclide to be measured can be used as a reference sample, and (3) many kinds of pure B- and /?-y-emitters can be effectively radioassayed with a small error. Moreover, this technique has been systematized and
successfully computerized to be applicable for a routine radioassay.
1. Introduction A liquid scintillation counter has been developed and employed as a measuring instrument for weak p-emitters, such as 3H and “C. The remarkable advantage that radiations can be measured in the absence of self-absorption and of absorption prior to being incident to the detector (i.e. liquid scintillator) belongs to the internal-sample counting due to the liquid scintillator. The advantage is also applicable to the measurement of nuclides other than ‘H and “C, and leads to an accurate radiation measurement with a high counting efficiency.“.” The external standard channel ratio and the sample channel ratio techniques have already been established for ‘H- and “C-radioassays. These quenching correction techniques need a set of ‘H- or Y-quenched standards, which are commercially available. On the other hand, these quenching correction techniques are not very suitable for the routine radioassay of nuclides other than ‘H and “C, because it is troublesome and time-consuming to prepare individual quenched standards. An efficiency tracing technique has been utilized for the radioassay of p-emitters using a 41$-y coincidence counting,‘3.J) and can also be applied successfully to liquid scintillation measurement.(5.6) The term “efficiency tracing technique” has originated from plotting the counting rates of a sample to be measured, tracing the counting efficiencies of a reference sample used as a tracer. The sample preparation in the liquid scintillation technique is much easier than that in the 47$-g coincidence technique. The liquid scintillation efficiency tracing technique has the following merits compared to the other radiation measurement techniques: (a) Sample to be measured prepared. A.R.I. KM
can be immediately
(b) Even a nuclide different from a nuclide to be measured can be used as a reference sample. (c) This technique is eflective for many kinds of pure fi- and /?-y-emitters. The usefulness of the liquid scintillation efficiency tracing technique has not yet been systematically and practically studied. In this study, we have tried to establish the limitation of the technique and the equation for measurement error, and moreover to develop a computer process for the efficiency tracing technique.
2. Principle The liquid scintillation counting rate varies with amplifier gains, since the liquid scintillation spectrum shifts toward higher pulse height with the increase of the amplifier gains (Fig. 1). The net counting rates of the sample to be measured are plotted against the counting efficiencies of the reference sample: Letting the counting efficiencies of a reference sample be Em and the counting rates of the sample to E,,E,,..., be measured be n,, n2, . . . , n, at individual amplifier gains (G,, Gr, . . . , G,,,), the efficiency tracing curve can be obtained by plotting n vs E at the same amplifier gains (Fig. 2). In the efficiency tracing curve, the value extrapolated up to the 100% counting efficiency means the activity of the sample to be measured. The principle of this technique is based on the hypothesis that if the reference sample can be measured with a 100% counting efficiency under a given measurement condition, the sample to be measured should be necessarily measured with the counting efficiency of 100% under the same measurement condition. As illustrated in Fig. 2, it is obvious that individual efficiency tracing curves observed are, in general, represented in a
46‘3
444
HIROAKI ISHKAWA et al.
60
Hqh
200
gem
\I\
,
I 0
400
I
I
600
Dlscrlmlnator
CO
level
1000
800 I dlv
)
Fig. 1. Liquid scintillation spectra of @Co due to the change of amplifier gains. The spectrum shifts toward the higher discriminator level with the increase of amplifier gains, resulting in the increase of counting rate.
linear error.
regression
equation
a measurement
within
3. Experimental Twelve sorts of nuclides to be measured, as listed in Table 1, have been homogeneously dispersed into
15 mL of emulsion scintillator, JAERI-sol.(‘) The JAERI-sol is composed of xylene (720 mL), LiponoxNCH (280 mL), PPO (7 g/L), bis-MSB (0.5 g/L) and cont. HNO, (10 p L/L). The reference sample should be selected from the following points of view, that (I) It has long life, being accurately radioassayed. (2) It can be measured with the maximum counting efficiency of more than 90%. (3) It is a j-emitter, not giving rise to electron capture decay or isomeric transition. On the basis of the foregoing aspects, we have selected 14C as the most suitable reference sample. The reference sample (1 mL) of [‘4C]toluene [(I.027 f 0.015) x 10’ dpm] has been dissolved in Efficiency
tracing 31646
15 mL of toluene scintillator consisting of PPO (4 g/L), DMPOPOP (0.3 g/L) and toluene. A liquid scintillation counter, Packard Tricarb Model 3255, has been used in coupling with a desktop computer, Canon AX-l. When carrying out the efficiency tracing technique, the most important fact is to select the appropriate amplifier gains, with which the counting rates of the reference sample and the sample to be measured vary. The amplifier gains should be set so as to give a counting efficiency from 80 to 95% for the reference sample. In the liquid scintillation counter employed in this study, the amplifier gains of 4, 5, 6, 8, 10 and 20% under the integral counting mode of window setting (i.e. discriminator level) from 50 division to infinite seem to provide the most suitable data for the construction of the efficiency tracing curve. The differences in quenching effect, radiation energy and liquid scintillator between the reference sample and the sample to be measured do not affect the final results in practice. The slope of the efficiency tracing curve depends on the radiation energy of the sample to be measured. Besides, the slope varies with the increase of the quenching, because the pulse height is reduced by the quenching effect. In the unquenched and quenched samples containing the same activity, the different efficiency tracing curves are obtained as shown in Fig. 3. However, the extrapolated value of up to the 100% counting efficiency converges at the same point (activity). Even with having a complex decay mode, if a nuclide gives rise to the IOO’Aj-decay, its activity can be easily determined through this technique. In the case where several nuclides are present in a prepared sample, the extrapolated value obviously indicates the total activity. When measuring ‘H activity with this technique, it is apt to give a large measurement error because of very low p-ray energy. It has been proved that the efficiency tracing technique cannot be adopted to the ‘H measurement.
curve
for 32P
( dpm 1 CA 756 1 ( gain 1
Reqion
4
2
3
4
5
I
I
I
I
I
6 -
--+-+” . . . . . .. . . . . .__M
-. . ..___.__.............
+!!T?!?
32000
2
28800
,p
z
.’
. ... . . . ..
.
. ..
E
e++-++ .____.._.._...,...
__._. _. _.... ..,......
.._. ___. ___...
I
I
I
I
80
85
90
95
Efficiency Reference
nuclide
25600
.._...
22400
z 0 Z E ;
100
1%) “C
Fig. 2. Efficiency tracing curve of 3rP constructed with a desk-top computer. Based on the actually measured counting rates which are marked with six circles, the efficiency tracing curve is automatically drawn with the method of least squares. The dpm value shown above the graph indicates the activity of )?P with a standard error in parentheses.
465
Radioassay by efficiency tracing technique
IO 90
i4C
100
90
Counting
efficiency
of
reference
nuclide
(%I
Counting 14C
Fig. 3. Efficiency tracing curves for unquenched and quenched samples. Each extrapolated value up to 100% counting efficiency converges at the same point (activity), when the samples contain the same activity.
The efficiency tracing technique is not applicable to the nuclides which give rise to electron capture (EC) decay or isomeric transition. The EC nuclide is followed by the emission of x-ray or Auger electrons which makes it difficult to find the true activity with this technique (Fig. 4). When a daughter nuclide produced after j-decay has a life time of an excited level longer than the resolving time (_ 500 ns) of the counting system, the /I-ray and the subsequent y-ray will not be simultaneously counted.“) The extrapolated value is statistically determined by the method of least squares, taking into account the arithmetic weight with respect to each plot of the efficiency tracing curve. The efficiency tracing curve involves the errors due to (1) counts of the sample to be. measured and of the reference sample (2) background count, and (3) assayed value of the reference sample. The standard deviation (An(E)) of the efficiency tracing plot (ordinate) is affected by that (A,?) of the counting efficiency (abscissa), given by(*) An(E) = d.
AE.
(1)
The (dn/dE) represents a slope of the efficiency tracing curve, which is approximately (i.e. ignoring
100
90
60
efficiency
reference
of
nucltde
l%)
Fig. 4. Efficiency tracing curves for EC nuclides which are inapplicable to this study. The steeping parts of each curve are attributed to the Auger electrons and the x-rays emitted from the EC nuclides.
the arithmetic weight of each plot) derived from the method of least squares;
m~Ei(n,-n,,)-~Ei~~(nli-ns) : i
dn
I
I
dE=
2
f
\i
/
(2)
where m, n, and nb are respectively the number of plots, sample counting rate and background counting rate
Hence, the arithmetic weight (W) for each plot of the efficiency tracing curve, which is inversely proportional to the square of the total standard deviation (a), is given by
w=‘= 6’
1
An(E)‘+Anf+An;’
(3)
where An, and An,, are the counting error of the sample to be measured and that of the background, respectively. Taking into account the arithmetic weight obtained from equation (3), the activity (0) of the sample to be measured and its error (AD) can be linally calculated from the following equations using the method of least squares;
D=P+Q,
(4)
7 Wi{(n6
-nb)-(P-Ei+Q)}* (5) m -2
HIROAIUISHIKAWAet al.
466
where A and AA are the activity of the reference sample and its error, respectively. 1 w,K(n, - “t$>.
c
I x
wiEt I
p
=
Y
1 wt(n, - nb)
wiEZ.
‘T
!
12p
~WiE;.p”+WiEi)? t C
Nuclide
I -
Table I. Comparison of results obtained from thiswork with reference values
'%I %
(6)
BFe To 6'Ni '6Rb
F w~(n,- hi>
’ - F WiEl(n, -
nb,>.C WiEi I
’ =TWiE’F_i-(TWtKJ ’ (7)
P is the slope of the efficiency tracing curve and Q its intercept.
4. Results Twelve nuclides have been measured and the results obtained with the efficiency tracing technique are in good agreement with the activity of each reference nuclide commercially available (Table 1). The reference nuclides are standardized using the other measurement techniques such as the 4$-y coincidence method, y-ray spectrometry and gas-flow proportional counting. The liquid scintillation efficiency tracing technique allows the activities of various kinds of b-emitters, as shown in Table 1, to be easily determined with small measurement errors. The most attractive feature of this technique is the simplicity of the sample
l
This work (A) (xl0'dpm)
Referencevalue (B) (xlO'dpm)
I.Ol(2.19,)' 3.16(2.49') 0 4.7Y(3.1°q 7.26(1.708) 1.35(1.5"') 1.05(l.YSH) 6.66(4.59;) 5.42(0.5") 6.06(1.7!;) I.i8(1.5?0) 7.26(1.4%) 3.73(2.706)
1.01(3.0~,)* 3.08(2.1",) 4.95(2.Y" ) 7.32(2.69;) 1.35(2.73;) l.O4(0.5",) 6.9014.0%,) 5.40(3.1",) 6.00(1.5",) 1.80(1.3".) 7.32(2.5",) 3.80 (2.8”J
Differencet (",,J 0.0 2.6 -3.2 -0.3 0.0 1.0 -3.5 0.4 1.0 -1.1 -0.8 -1.9
Standarderror;tDifference:(A - B),B.
preparation and radiation measurement, mainly attributed to the advantage of the liquid scintillation measurement technique. References 1. Ishikawa H. and Takiue M. Xucl. Insrrum. Methods 112, 437 (1973). 2. Peng C. T. RCC Rmiew 17, p. 72 (The Radiochemical Centre Ltd, England, 1977). 3. Campion P. J., Taylor J. G. V. and Merritt J. S. Inr. J. Appl. Radial.
Isot. 8, 8 (1960).
4. Merritt J. S., Taylor J. G. V.. Merritt W. F. and Campion P. J. Anal. Chem. 32, 310 (1960). Yura 0. Radioisotopes 20, 493 (1971). :: Takiue M. and Ishikawa H. ?&cl. humrum. Merhodr 148, 157 (1978). 7. Fujii H., Takiue M. and Ishikawa H. Radioisotopes 30, 475 (1981).
8. Kawada Y. Inr. J. Appl. Radian. Isor. 16, 371 (1965).
1984 Copyright
0020-708X S-I 53.00 + 0.00 C 1984 Peqamon Press Ltd
Radioassay by an Efficiency Tracing Technique Using a Liquid Scintillation Counter HIROAKI
ISHIKAWA’,
MAKOTO
TAKIUI?
and TAMARU
ABURAI’
‘Radioisotope-Nuclear Reactor School, Japan Atomic Energy Research institute, 2-28-49, Honlcomagome, Bunkyo-ku, Tokyo. Japan and ‘Division of Radiobiology, Research Center of iMedical Science, The Jikei University School of Medicine, 3-25-8, Nishishimbashi, Minato-ku, Tokyo, Japan (Received 5 April 1983; in revisedform
17 Augusr 1983)
The various nuclides other than ‘H are radioassayed with an efficiency tracing technique. It has been clarified in this study that the liquid scintillation efficiency tracing technique possesses the following advantages; (I) sample preparation is of simplicity (2) even a nuclide different from a nuclide to be measured can be used as a reference sample, and (3) many kinds of pure B- and /?-y-emitters can be effectively radioassayed with a small error. Moreover, this technique has been systematized and
successfully computerized to be applicable for a routine radioassay.
1. Introduction A liquid scintillation counter has been developed and employed as a measuring instrument for weak p-emitters, such as 3H and “C. The remarkable advantage that radiations can be measured in the absence of self-absorption and of absorption prior to being incident to the detector (i.e. liquid scintillator) belongs to the internal-sample counting due to the liquid scintillator. The advantage is also applicable to the measurement of nuclides other than ‘H and “C, and leads to an accurate radiation measurement with a high counting efficiency.“.” The external standard channel ratio and the sample channel ratio techniques have already been established for ‘H- and “C-radioassays. These quenching correction techniques need a set of ‘H- or Y-quenched standards, which are commercially available. On the other hand, these quenching correction techniques are not very suitable for the routine radioassay of nuclides other than ‘H and “C, because it is troublesome and time-consuming to prepare individual quenched standards. An efficiency tracing technique has been utilized for the radioassay of p-emitters using a 41$-y coincidence counting,‘3.J) and can also be applied successfully to liquid scintillation measurement.(5.6) The term “efficiency tracing technique” has originated from plotting the counting rates of a sample to be measured, tracing the counting efficiencies of a reference sample used as a tracer. The sample preparation in the liquid scintillation technique is much easier than that in the 47$-g coincidence technique. The liquid scintillation efficiency tracing technique has the following merits compared to the other radiation measurement techniques: (a) Sample to be measured prepared. A.R.I. KM
can be immediately
(b) Even a nuclide different from a nuclide to be measured can be used as a reference sample. (c) This technique is eflective for many kinds of pure fi- and /?-y-emitters. The usefulness of the liquid scintillation efficiency tracing technique has not yet been systematically and practically studied. In this study, we have tried to establish the limitation of the technique and the equation for measurement error, and moreover to develop a computer process for the efficiency tracing technique.
2. Principle The liquid scintillation counting rate varies with amplifier gains, since the liquid scintillation spectrum shifts toward higher pulse height with the increase of the amplifier gains (Fig. 1). The net counting rates of the sample to be measured are plotted against the counting efficiencies of the reference sample: Letting the counting efficiencies of a reference sample be Em and the counting rates of the sample to E,,E,,..., be measured be n,, n2, . . . , n, at individual amplifier gains (G,, Gr, . . . , G,,,), the efficiency tracing curve can be obtained by plotting n vs E at the same amplifier gains (Fig. 2). In the efficiency tracing curve, the value extrapolated up to the 100% counting efficiency means the activity of the sample to be measured. The principle of this technique is based on the hypothesis that if the reference sample can be measured with a 100% counting efficiency under a given measurement condition, the sample to be measured should be necessarily measured with the counting efficiency of 100% under the same measurement condition. As illustrated in Fig. 2, it is obvious that individual efficiency tracing curves observed are, in general, represented in a
46‘3
444
HIROAKI ISHKAWA et al.
60
Hqh
200
gem
\I\
,
I 0
400
I
I
600
Dlscrlmlnator
CO
level
1000
800 I dlv
)
Fig. 1. Liquid scintillation spectra of @Co due to the change of amplifier gains. The spectrum shifts toward the higher discriminator level with the increase of amplifier gains, resulting in the increase of counting rate.
linear error.
regression
equation
a measurement
within
3. Experimental Twelve sorts of nuclides to be measured, as listed in Table 1, have been homogeneously dispersed into
15 mL of emulsion scintillator, JAERI-sol.(‘) The JAERI-sol is composed of xylene (720 mL), LiponoxNCH (280 mL), PPO (7 g/L), bis-MSB (0.5 g/L) and cont. HNO, (10 p L/L). The reference sample should be selected from the following points of view, that (I) It has long life, being accurately radioassayed. (2) It can be measured with the maximum counting efficiency of more than 90%. (3) It is a j-emitter, not giving rise to electron capture decay or isomeric transition. On the basis of the foregoing aspects, we have selected 14C as the most suitable reference sample. The reference sample (1 mL) of [‘4C]toluene [(I.027 f 0.015) x 10’ dpm] has been dissolved in Efficiency
tracing 31646
15 mL of toluene scintillator consisting of PPO (4 g/L), DMPOPOP (0.3 g/L) and toluene. A liquid scintillation counter, Packard Tricarb Model 3255, has been used in coupling with a desktop computer, Canon AX-l. When carrying out the efficiency tracing technique, the most important fact is to select the appropriate amplifier gains, with which the counting rates of the reference sample and the sample to be measured vary. The amplifier gains should be set so as to give a counting efficiency from 80 to 95% for the reference sample. In the liquid scintillation counter employed in this study, the amplifier gains of 4, 5, 6, 8, 10 and 20% under the integral counting mode of window setting (i.e. discriminator level) from 50 division to infinite seem to provide the most suitable data for the construction of the efficiency tracing curve. The differences in quenching effect, radiation energy and liquid scintillator between the reference sample and the sample to be measured do not affect the final results in practice. The slope of the efficiency tracing curve depends on the radiation energy of the sample to be measured. Besides, the slope varies with the increase of the quenching, because the pulse height is reduced by the quenching effect. In the unquenched and quenched samples containing the same activity, the different efficiency tracing curves are obtained as shown in Fig. 3. However, the extrapolated value of up to the 100% counting efficiency converges at the same point (activity). Even with having a complex decay mode, if a nuclide gives rise to the IOO’Aj-decay, its activity can be easily determined through this technique. In the case where several nuclides are present in a prepared sample, the extrapolated value obviously indicates the total activity. When measuring ‘H activity with this technique, it is apt to give a large measurement error because of very low p-ray energy. It has been proved that the efficiency tracing technique cannot be adopted to the ‘H measurement.
curve
for 32P
( dpm 1 CA 756 1 ( gain 1
Reqion
4
2
3
4
5
I
I
I
I
I
6 -
--+-+” . . . . . .. . . . . .__M
-. . ..___.__.............
+!!T?!?
32000
2
28800
,p
z
.’
. ... . . . ..
.
. ..
E
e++-++ .____.._.._...,...
__._. _. _.... ..,......
.._. ___. ___...
I
I
I
I
80
85
90
95
Efficiency Reference
nuclide
25600
.._...
22400
z 0 Z E ;
100
1%) “C
Fig. 2. Efficiency tracing curve of 3rP constructed with a desk-top computer. Based on the actually measured counting rates which are marked with six circles, the efficiency tracing curve is automatically drawn with the method of least squares. The dpm value shown above the graph indicates the activity of )?P with a standard error in parentheses.
465
Radioassay by efficiency tracing technique
IO 90
i4C
100
90
Counting
efficiency
of
reference
nuclide
(%I
Counting 14C
Fig. 3. Efficiency tracing curves for unquenched and quenched samples. Each extrapolated value up to 100% counting efficiency converges at the same point (activity), when the samples contain the same activity.
The efficiency tracing technique is not applicable to the nuclides which give rise to electron capture (EC) decay or isomeric transition. The EC nuclide is followed by the emission of x-ray or Auger electrons which makes it difficult to find the true activity with this technique (Fig. 4). When a daughter nuclide produced after j-decay has a life time of an excited level longer than the resolving time (_ 500 ns) of the counting system, the /I-ray and the subsequent y-ray will not be simultaneously counted.“) The extrapolated value is statistically determined by the method of least squares, taking into account the arithmetic weight with respect to each plot of the efficiency tracing curve. The efficiency tracing curve involves the errors due to (1) counts of the sample to be. measured and of the reference sample (2) background count, and (3) assayed value of the reference sample. The standard deviation (An(E)) of the efficiency tracing plot (ordinate) is affected by that (A,?) of the counting efficiency (abscissa), given by(*) An(E) = d.
AE.
(1)
The (dn/dE) represents a slope of the efficiency tracing curve, which is approximately (i.e. ignoring
100
90
60
efficiency
reference
of
nucltde
l%)
Fig. 4. Efficiency tracing curves for EC nuclides which are inapplicable to this study. The steeping parts of each curve are attributed to the Auger electrons and the x-rays emitted from the EC nuclides.
the arithmetic weight of each plot) derived from the method of least squares;
m~Ei(n,-n,,)-~Ei~~(nli-ns) : i
dn
I
I
dE=
2
f
\i
/
(2)
where m, n, and nb are respectively the number of plots, sample counting rate and background counting rate
Hence, the arithmetic weight (W) for each plot of the efficiency tracing curve, which is inversely proportional to the square of the total standard deviation (a), is given by
w=‘= 6’
1
An(E)‘+Anf+An;’
(3)
where An, and An,, are the counting error of the sample to be measured and that of the background, respectively. Taking into account the arithmetic weight obtained from equation (3), the activity (0) of the sample to be measured and its error (AD) can be linally calculated from the following equations using the method of least squares;
D=P+Q,
(4)
7 Wi{(n6
-nb)-(P-Ei+Q)}* (5) m -2
HIROAIUISHIKAWAet al.
466
where A and AA are the activity of the reference sample and its error, respectively. 1 w,K(n, - “t$>.
c
I x
wiEt I
p
=
Y
1 wt(n, - nb)
wiEZ.
‘T
!
12p
~WiE;.p”+WiEi)? t C
Nuclide
I -
Table I. Comparison of results obtained from thiswork with reference values
'%I %
(6)
BFe To 6'Ni '6Rb
F w~(n,- hi>
’ - F WiEl(n, -
nb,>.C WiEi I
’ =TWiE’F_i-(TWtKJ ’ (7)
P is the slope of the efficiency tracing curve and Q its intercept.
4. Results Twelve nuclides have been measured and the results obtained with the efficiency tracing technique are in good agreement with the activity of each reference nuclide commercially available (Table 1). The reference nuclides are standardized using the other measurement techniques such as the 4$-y coincidence method, y-ray spectrometry and gas-flow proportional counting. The liquid scintillation efficiency tracing technique allows the activities of various kinds of b-emitters, as shown in Table 1, to be easily determined with small measurement errors. The most attractive feature of this technique is the simplicity of the sample
l
This work (A) (xl0'dpm)
Referencevalue (B) (xlO'dpm)
I.Ol(2.19,)' 3.16(2.49') 0 4.7Y(3.1°q 7.26(1.708) 1.35(1.5"') 1.05(l.YSH) 6.66(4.59;) 5.42(0.5") 6.06(1.7!;) I.i8(1.5?0) 7.26(1.4%) 3.73(2.706)
1.01(3.0~,)* 3.08(2.1",) 4.95(2.Y" ) 7.32(2.69;) 1.35(2.73;) l.O4(0.5",) 6.9014.0%,) 5.40(3.1",) 6.00(1.5",) 1.80(1.3".) 7.32(2.5",) 3.80 (2.8”J
Differencet (",,J 0.0 2.6 -3.2 -0.3 0.0 1.0 -3.5 0.4 1.0 -1.1 -0.8 -1.9
Standarderror;tDifference:(A - B),B.
preparation and radiation measurement, mainly attributed to the advantage of the liquid scintillation measurement technique. References 1. Ishikawa H. and Takiue M. Xucl. Insrrum. Methods 112, 437 (1973). 2. Peng C. T. RCC Rmiew 17, p. 72 (The Radiochemical Centre Ltd, England, 1977). 3. Campion P. J., Taylor J. G. V. and Merritt J. S. Inr. J. Appl. Radial.
Isot. 8, 8 (1960).
4. Merritt J. S., Taylor J. G. V.. Merritt W. F. and Campion P. J. Anal. Chem. 32, 310 (1960). Yura 0. Radioisotopes 20, 493 (1971). :: Takiue M. and Ishikawa H. ?&cl. humrum. Merhodr 148, 157 (1978). 7. Fujii H., Takiue M. and Ishikawa H. Radioisotopes 30, 475 (1981).
8. Kawada Y. Inr. J. Appl. Radian. Isor. 16, 371 (1965).