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A system for intercomparing standard solutions of beta-particle emitting radionuclides
Nuclear Instruments and Methods in Physics Research A312 (1992) 114-120 North-Holland
NUCLEAR INSTRUMENTS 8e METHODS IN PHYSICS
RESEARCH SectionA
A system for intercomparing standard solutions of beta-particle emitting radionuclides J .M. Calhoun, J .T. Cessna and B.M. Coursey
Ionizing Radiation Dit ,isïoti, National Institute of Standards and Technology, Gaitheishurg, MD 20899, USA A system for intercomparing standard solutions of pure beta-particle emitting radionuclides is described. The CIEMAT/NIST technique cf beta-particle efficiency tracing is based on establishing a parameter in a simple calculational model, using a ; H standard with comparable quenching. To produce a ; H-in-scintillator standard which is reasonably stable over the period of the measurements, we first standardized 1 1-hexadecane solution by comparison with 3 H-water standardized by gas counting . In the second phase of the work, the ; H-hexadecane solation was used to standardize 14 C-hexadecane and l'`'Tc-tricaprylamine (TCA). Measurements were made under similar geometrical and quenching conditions for each radionuclide with a commercial scintillator and a conventional liquid-scintillation counter with two phototubes operating in coincidence . The technique was then tested at different sites in the area using a set of flame-sealed vials and state-of-the-art liquid-scintillation counters . Initial results for the "Tc-TCA and the 1981 '"Tc, Standard Reference Material (SRM) 4288 show an agreement to within 0.30% .
1. Introduction
The international reference system (SIR) for activity measurements is maintained by the Bureau International des Poids et Mesures (BIPM) [1] . The charge was given to the BIPM in the early 1900s to "establish and to maintain uniformity of the measurements in the world, to improve their accuracy where it is needed and to help developing national laboratories" . This system was formally introduced for gamma-ray emitting radionuclides in 1976. Specifications were placed on limits of activity, solution volume and ampoule type. Samples submitted to the BIPM were then assayed using the BIPM 4rrry ionization chamber . For the BIPM chamber, the current produced by gamma rays from the sample is measured and compared with the current from long-lived reference sources of aged "-'Ra . The objective of this system is to provide for a relative but precise comparison of activity measurements among different laboratories . According to Dr. G. Ratel of the BIPM, more than two dozen laboratories participate in this system, which has provided activity intercomparisons for more than 48 different radionuclides involving more than 373 independent standardizations. However, for the past several years it has been noted that the use of this system is declining . Perhaps. this can be attributed to the fact that most of the gamma-ray emitting radionuclides which have well-established standardizations have been repeatedly measured with this system . However, there are other radionuclides with important medical and industrial applications which remain outside the gamma-ray-emitting nuclide 0168-9002/92/$05 .(H)
V
SIR system . These are the pure beta-particle emitters, low-energy-photon radionuclides, and alpha-particle emitters without suitable photons . 'I he NIST has developed the system described here for intercomparing standard solutions of pure beta-particle emitting radionuclides . 2. Attributes of a SIR system for beta emitters The ideal system for intercomparing pure beta-particle emitting or other problem-radionuclide samples submitted to the BIPM would allow for: (i) specified limitations on total activity, (ii) nuclides already mixed with a fluorescent scintillator, and (iii) samples sealed in glass scintillation vials. The method of measurement for these radionuclides would be the liquid-scintillation technique . Presently, there are two basic methods which may be considered utilizing liquid-scintillation techniques. The first is the CIEMAT/NIST method of liquid-scintillation counting by efficiency tracing with 3 H [2,3]. This technique takes into account the spectrum o Y!1lat llitttl~til7W4 to {ten ~cc~arn~ rl "Hn t a.u ... of v. t .e .adionuc .. all aln v.assaycaa and tltar counting efficiency for a liquid-scintillation system with two phototubes operating in coincidence . This method has been used by the NIST for standardizing pure-beta emitters for the past 10 years [4,5] . The second is the triple-to-double coincidence ratio (TDCR) method, where a value for the activity is determined by extrapolating the ratio of the triple efficiency E-,- to the double efficiency e,) to unity [6]. This system employs three identical phototubes and the precision of the results
1992 - Elsevier Science Publisher-, 13 .V . All rights rzserved
J.M Calhoun et al. / hitercomparing standard solutions of radionuclides
Sciniillator
052 0 .51 t) 5() () 19
r~ 0 . ,l8 C
C
z 0A G 0 .-15
"
0 . .111
G
0 .4%1 0 .42 0A 1
95 100 105 70 75 80 85 90 11 »iambc-. r Fig. 1. Hydrogen-3 counting efficiency as a function of quenching for aqueous samples in four commercial liquid scintillators. ..1 )
:)0
6F)
G0
improves with energy. However, implementation of a SIR based on the TDCR method would require the construction of a new system at the BIPM, which would delay the extension of the SIR to pure-beta-particle emitting radionuclides. The present study used the liquid-scintillation efficiency tracing method using a H standard . 3
3. H-H 2 0 stability in commercial scintillators 3
In order to test the stability of H-water standards in commercial scintillators, water samples were prepared in four different commercial scintillators, identified as scintillators A--D. The aqueous loading of the scintillators for metrology applications is generally less than 2% - a useful range for most samples, for example is 15 mg to 1 g water per 10 ml scintillator . In this range of water concentration, water droplets are dispersed in the organic matrix as "reverse micelles" . In previous work at the NIST we found that the H-water samples in reverse micelles at room temperature exhibit a loss in H counting efficiency of the order of 0.15% per day . This was not observed at reduced temperature WC) . Irl 111C past couple of years a new generation of commercial scintillators has come on the market [7] . These have a higher molecular weight solvent, such as di-isopropylnapthalene, and a higher flash point than the previous scintillators based on p-xylene and pseudocumene. To test the suitability of these new materials for the proposed SIR system, four sets of samples were prepared and their H counting efficiency was followed for sufficient time to observe the trend in efficiency loss. 3
3
3
3
The H-water standard was a dilution of the NIST SRM 4927C [8] . The samples were added gravimetrically to the scintillation vials containing 10 ml scintillator in a polycone-seal glass vial (Kimble) #' . Each scintillation vial was loaded with 0-1 ml of H BO to vary the quenching in the samples . Measurements were made with a Beckman LS7800 liquid-scintillation counting system equipped with two Hamamatsu R331-05 phototubes operated in the coincidence mode, a logarithmic amplifier, and a 137CS external source for quench monitoring [9] . The samples were then counted for over 260 days to study the loss of count rate as a fu,iction of scintillator type and water loading . Fig . 1 shows the initial data set of efficiency as a function of quenching for the four scintillators. It is clear that scintillator C is less efficient than the others for the same Horrocks number (H #) [9] . Scintillator A is the pseudocumene type, B is a polyarylalkane, while C and D are based on di-isopropylnapthalene . (Efficiency is of course not the only indicator of product quality, as most commercial scintillators are formulated to incorporate large volumes of aqueous sample with various biological molecules). Figs. 2 and 3 show the loss in counting efficiency with time for scintillators B and D. The results for A and B were essentially the same. Although there were no overwhelming differences in efficiency, the highest efficiency for H and good stability were observed for Ultima Gold scintillator from Packard Instrument (The Netherlands). The counting 3
3
Mention of commercial products does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products identified are necessarily the best available for the purpose . * 1
111(b) . LIQUID SCINTILLATION
J.M Calhoun et al. / Irrtcrc"ra»r~~urirrx~ strmdrrrd salr~tians of radionurfdeti Watvr Loading 0 0 Till . 0 1 1ol . ~ 0 :' rltl . 0 ;i rnl, () ~1 tttl . 0 `'i tlll . () (i ntl. 0 7 Inl .
U . 50 () .19
r
0 A ti
r r
0 11 Il0 "I tttl . 1 0 nrl .
"
UA3 I0A 2 0 .41
L~- i-
0
L
J
M 10 .1 130 15G 111 :2 I line (dit%-, frOlll rol' . t Illl(')
52
0H
2 :3,1
260
Fig. 2. Loss in counting efficiency for ;H-water samples as a function time for scintillator B.
efficiency rate of loss was lowest for the unquenched samples (that is, 35 mg water in 10 ml scintillator), of the order of 0.03% per day in -1H efficiency. (The next generation counting system on order for the NIST Radioactivity Group will be refrigerated . Some of these measurements will be repeated to see how the scintillators behave at reduced temperature .)
4. Standardization of P-particle emitters at NIST Table 1 summarizes the sample preparation data for the four lots of samples used in this phase of the intercomparison . Three organic radiolabelled compounds were prepared that would not change with time: ; H-hexadecanc, "C-hexadccane and "Tc-tri-
Watc" r Loading 0 0 fill . 01 m1 . 0 '.? m l . 0311,1 . 0 .1 till.
O .52
0 .5 1
r-
tà
(Y50
S
A
Y
+' VLI
-> _~-
"
0 7 01, 0 11 0 9 1 0
1111. In 1 .
nil, fill . fill .
" ï.
0 .-15
0 .-1 :3 () . f_1 t 0
:2 G
:?
l fi 10 .1 1 :10 V.)G i f , :III lle (dil v~~ frorl) 1 - t4 . t III( , )
*
1' 08
Fig. 3. Loss in counting efficiency for ;H-water samples as a function time for scintillator D.
J.M. Callimin et at. / Intercomparing stantlardwhstions of radiontiefides
caprylamine . The 1 1-1-hexadecane and the "C-hexadccane were obtained from Amersharn International . The NIST 'H-water standards were compared with the -'H-hexadecane solution, thereby standardizing the -'H-hcxadecane with reference to the NIST 1978 gas counting. The "'Tc was a dilution cat' the: MIST SRM 4288 [10], a standard aqueous solution which was complexed with tricaprylaminc . In prior work, it was found that "Tc standard aqueous solutions can be added to a scintillator containing a small amount of the non-
quenching tricaprylamine, which then extracts the pertechnelate anion into the organic solvent. The radionuclide sample% (40-70 mg) were added gravimetrically to name-scaled glass vials which meet the dimcnsions of the IEC standard (11]. Each scintillation vial contains 10 ml Packard Ultima Gold scintillator. Some of the samples contain chloroform WHCI .,t ) 10-50 gl, as a quenching agent to provide a range of quenching for samples and standards . Measurements were made with a Beckrnan LS7800 liquid-scintillation counting
Table I
Samples for use in an intercomparison of pure beta-particle emitting radionuclides Nuclides
'H-hexadecane
14
C-hexadecane
"Tc-TCA
BKG
Sample no.
Sample preparation data " Sample mass [mg]
CHC1 3 [yq
13
14 15 16 17 18 19 20 21 22 23 24
53.762 55 .470 55 .523 54503 56 .899 55,399 55 .405 57 .454 51118 54 .720 55116 58380
10 10 20 30 40 50
25 27 28 29 30 31 32 33 34 35 36
54 .664 55 .589 55 .917 54.626 55151 51150 56 .012 54.376 56915 55A27 54100
10 10 20 30 40 50
37 38 40 41 42 43 44 45 46 47 48
73 .669 56.762 51176 53 .545 MAY 55 .239 54A29 55 .551 WAY 54 .235 55 .055
10 1() 20 30 40 50
activity [kBqj (rr 12(H) EST 9/3/%)
2.401 2.478 2 .480 2.434 2.541 2%4 2.475 2.566 2%2 144 2.466 2,608
50
10 ml Ultima Gold (Packard Vial : sealed glass; volume : 20 ml, max. sealed height : 63 mm ; diameter: 28 mm o.d .-, scintillator : Instrument Co.) . 111(b) . LIQUID SCINTILLATION
J.M. Gcilltraun et al. / bttercomparing stanclard soiutions of'rudiontu~lirlea
0 .52 051
>,0 .40 ca sw ~~ 0 .48 0 .47 0 31-1 H.LO
0.46
311 H20 30 pL CHC13 0 3H Hex 3H 11ex .10 /d . C1R'1s
0 .45 0 .44
0
15
30
-15
75 60 90 tinle (dilvs)
105
120
135
150
Fig . 4. Loss in counting efficiency as a function of time for 3H-water and 3H-hexadecane samples.
system as previously described . Samples were counted for over 100 days under similar geometrical and quenching conditions for each radionuclide. These measurements were also made at room temperature. Unexpectedly, the flame-sealed-in-glass 3Hhexadecane samples also exhibited a loss in counting efficiency with time. Fig . 4 compares the counting efficiency vs time for ;H-water and "H-hexadecane samples. Fig . 5 shows the change in H* with time for quenched and unquenched ;H-hexadecane and 3 H-
G4
water scintillators . A shift to higher H* for a given vial indicates some slight deterioration of the scintillators . It is not clear whether this is sufficient to explain the gradual loss in counting efficiency. Tables 2 and 3 give the results of the activity measurements by the CIEMAT/NIST liquid-scintillation method . The results for the Beckman LS7800 are based on repetitive measurements over 100 days. The technique was then tested at different sites in the area using the most contemporary liquid-scintillation coun-
~
G:2 (30
3H H,0 3H H20 30 p.L CHC13 3H Hex 3H Hex 401L CHC13
58 56 r
54 52 .50 48 -16 4-1
0
15
:30
115
00 75 90 105 120 135 150 t11r'E (davs) Fig . 5. Change in H* with time for quenched and unquenched ;H-hexadecane and 3 H-water scintillators .
J.J1f Calltcrttrt et ai. / Gttcr'ccxrrr wr~bttt stcrrtdurd scrltttwcrrts of radïcuttcclidés
Table 2 NIST results for the activity standardization of "C-hexadecane by the CIEMAT/NIST method (reference time 12(Nî FST September 3, 1990) Measurements
Average ,"
LS081190 LS081490 LS081890 LS090490 LS092090 PKI02990 LB111090 6Wa1C
54054 54!1(12 54(1(13 53979 54046 538111 54120 54122
tail/g]
Standard error 1 1- 11 0.05 0.05 0.05 0.07 11 .05 0.05 0,04 0.05
1`'" 1
0.07 -0 .03 -0 .03 -0 .07 0.05 -0 .25 0.19 0.19
Table 3 NIST results for the activity standardization of "'Tc-tricaprylamine by the CIEMAT/NIST method (reference time 1200 EST September 3, 1990)
LS081190 LS081490 LS081890 LS090490 LS092090 PKI02990 LB 111090 60001C
Average " [Bq/8] 37650 37627 37630 3763e 37657 37531 37706 37719
Standard error
0 NIST
0.03 0.02 0.02 0.02 0.02 0.02 0.03 0.03
0.03 -0 .03 -0 .03 -0 .01 0.05 -0.29 0.17 0.21
1%]
1%]
`' Average of first 5 measurements (NIST) is 37640±0.011
and for all measurements 37644±0.05% .
ters available from Beckman, LKB and Packard (see table 4). The latter results were not included in the NIST average, because we had only one opportunity to use the outside counting systems, and insufficient time to fully characterize them and understand the associated nonrandom uncertainties in the measurements. The CIEMAT/NIST general model, however, is based on a very simple model of two phototubes in coincidence, and we have assumed it would be valid for any Table 4 Liquid-scintillation counting system used in these measurements Beckman LS7800 Packard 2500TR LKB System 1410 Beckman LS6000LL
Uncertainties in the NIST activity measurements Component
.1 NIST
'' Average of first 5 measurements (NIST) is 54017±0.021, and for all measurements 54026±().05"1 .
Measurements
Table 5
NIST FDA - Rockville LKB - Rockville Microbiological Associates Rockville
Source preparation Scintillator stability Impurities Dead time Liquid-scintillation measurements (n = I I '-C) (n = 12 ""Tc) ;H reference standard Calculated efficiency Combined in quadrature
Estimate `"f uncertainty 11 ; 1 0.11) 0.10 0.01 () .03
0.112 11.111 0.1() 0.1 () 11 .20
of the commercial counting systems . The results for "C and ""'Tc in tables 2 and 3 seem to support this assumption . For the "Tc, the present value agrees to 0.3% with the value for SRM 4288 [10]. 5. Conclusion Scintillators from different manufacturers were compared to determine which new generation Scintillator would be the most suitable for use in an international intercomparison of pure-beta-particle emitters . The results indicated no significant differences, so the Scintillator exhibiting the highest counting efficiency was selected for the next phase of the study. For the proposed mini-intercomparison, samples of radiolabelled compounds were prepared on the assumption that they would be stable with time. These were -; Hhexadecane, ' 4 C-hexadecane and 9 `'Tc-tricaprylamine (TCA) . Activity measurements were made using the CIEMAT/NIST method of efficiency tracing with a Beckman LS7800 and newer instruments, with satisfactory agreement. The uncertainties in the NIST activity measurements are summarized in table 5. Each component is estimated as if it were one standard deviation, and they are added in quadrature to give a combined uncertainty of 0.20%. Samples were then distributed for a trial intercomparison to four other laboratories . Based upon the success of the trial intercomparison, a decision may be made to extend the SIR to the intercomparison of pure beta-particle emitting radionuclides. Acknowledgements
The authors gratefully acknowledge Dr. Larry L. Lucas for discussions and assistance in data acquisiI11(b) . LIQUID SCINTILLATION
120
J.M. Calhoun et al. / Interromparing xtaticlard .uwltttiotts of radionuclides
tion. We are also grateful to Robert Jones (LKB), Richard Jones (Packard) and Karen dcPianelli (Beckman) for arranging for measurements on the new LS counting systems available in the local area. References [I) A. Rytz, Int . J. Appl. Radiat. Isotopes 34 (1983) 1047 . (2) A. Grau Malonda and E. Garcia-Torapo, Int. J. Appl . Radiat . Isotopes 33 (1982) 249. [3a B.M . Coursey, W.B. Mann, A. Grau Malonda, E. Garcia-Torapo, LM . Los Arcos, J.A .B. Gibson and D. Reher, Int. J. Appl . Radiat . Isotopes 37 (1986) 403. [4] B.M. Coursey, A. Grau Malonda, E. Garcia-Torapo and J.M. Los Arcos, Trans. Amer. Nucl. Soc. 50 (1985) 13 . (S) J.M. Calhoun, B.M . Coursey, D. Gray and L. Karam, Proc. Int. Conf. on New Trends in Liquid Scintillation
Counting and Organic Scintillators, Gatlinburg, TN, 1989, eds. H. Ross, J . Noakes and J. Spaulding (Lewis, Chelsea, M1, 1991). [6] K. Pochwalski, R. Broda and T. Radoszewski, Int. J. Appl . Radiat . Isotopes 39 (1988) 165. [7] J. Thomson. Proc . Int. Conf . on New Trends in Liquid Scintillation Counting and Organic Scintillators, Gatlinburg, TN, 1989, eds . H. Ross, J. Noakes and J . Spaulding, (Lewis, Chelsea, M1, 1991). [8j M.P . Unterweger, B.M . Coursey, F.J . Schima and W.B. Mann, Int. J. Appl . Rad. Isotopes 31 (1980) 611 . [9] D.L . Horrocks, in : Advances in Scintillation Counting, eds. S.A . McQuarrie, C. Ediss and L.I . Wiebe (University of Alberta Press, 1986). [10] B.M . Coursey, J.A .B. Gibson, M. Heitzmann and J. Leak, Int. J. Appl . Rad. Isotopes 35 (1984) 1103 . [I II International Electrotechnical Committee Standard, Publication 582 (1977).
NUCLEAR INSTRUMENTS 8e METHODS IN PHYSICS
RESEARCH SectionA
A system for intercomparing standard solutions of beta-particle emitting radionuclides J .M. Calhoun, J .T. Cessna and B.M. Coursey
Ionizing Radiation Dit ,isïoti, National Institute of Standards and Technology, Gaitheishurg, MD 20899, USA A system for intercomparing standard solutions of pure beta-particle emitting radionuclides is described. The CIEMAT/NIST technique cf beta-particle efficiency tracing is based on establishing a parameter in a simple calculational model, using a ; H standard with comparable quenching. To produce a ; H-in-scintillator standard which is reasonably stable over the period of the measurements, we first standardized 1 1-hexadecane solution by comparison with 3 H-water standardized by gas counting . In the second phase of the work, the ; H-hexadecane solation was used to standardize 14 C-hexadecane and l'`'Tc-tricaprylamine (TCA). Measurements were made under similar geometrical and quenching conditions for each radionuclide with a commercial scintillator and a conventional liquid-scintillation counter with two phototubes operating in coincidence . The technique was then tested at different sites in the area using a set of flame-sealed vials and state-of-the-art liquid-scintillation counters . Initial results for the "Tc-TCA and the 1981 '"Tc, Standard Reference Material (SRM) 4288 show an agreement to within 0.30% .
1. Introduction
The international reference system (SIR) for activity measurements is maintained by the Bureau International des Poids et Mesures (BIPM) [1] . The charge was given to the BIPM in the early 1900s to "establish and to maintain uniformity of the measurements in the world, to improve their accuracy where it is needed and to help developing national laboratories" . This system was formally introduced for gamma-ray emitting radionuclides in 1976. Specifications were placed on limits of activity, solution volume and ampoule type. Samples submitted to the BIPM were then assayed using the BIPM 4rrry ionization chamber . For the BIPM chamber, the current produced by gamma rays from the sample is measured and compared with the current from long-lived reference sources of aged "-'Ra . The objective of this system is to provide for a relative but precise comparison of activity measurements among different laboratories . According to Dr. G. Ratel of the BIPM, more than two dozen laboratories participate in this system, which has provided activity intercomparisons for more than 48 different radionuclides involving more than 373 independent standardizations. However, for the past several years it has been noted that the use of this system is declining . Perhaps. this can be attributed to the fact that most of the gamma-ray emitting radionuclides which have well-established standardizations have been repeatedly measured with this system . However, there are other radionuclides with important medical and industrial applications which remain outside the gamma-ray-emitting nuclide 0168-9002/92/$05 .(H)
V
SIR system . These are the pure beta-particle emitters, low-energy-photon radionuclides, and alpha-particle emitters without suitable photons . 'I he NIST has developed the system described here for intercomparing standard solutions of pure beta-particle emitting radionuclides . 2. Attributes of a SIR system for beta emitters The ideal system for intercomparing pure beta-particle emitting or other problem-radionuclide samples submitted to the BIPM would allow for: (i) specified limitations on total activity, (ii) nuclides already mixed with a fluorescent scintillator, and (iii) samples sealed in glass scintillation vials. The method of measurement for these radionuclides would be the liquid-scintillation technique . Presently, there are two basic methods which may be considered utilizing liquid-scintillation techniques. The first is the CIEMAT/NIST method of liquid-scintillation counting by efficiency tracing with 3 H [2,3]. This technique takes into account the spectrum o Y!1lat llitttl~til7W4 to {ten ~cc~arn~ rl "Hn t a.u ... of v. t .e .adionuc .. all aln v.assaycaa and tltar counting efficiency for a liquid-scintillation system with two phototubes operating in coincidence . This method has been used by the NIST for standardizing pure-beta emitters for the past 10 years [4,5] . The second is the triple-to-double coincidence ratio (TDCR) method, where a value for the activity is determined by extrapolating the ratio of the triple efficiency E-,- to the double efficiency e,) to unity [6]. This system employs three identical phototubes and the precision of the results
1992 - Elsevier Science Publisher-, 13 .V . All rights rzserved
J.M Calhoun et al. / hitercomparing standard solutions of radionuclides
Sciniillator
052 0 .51 t) 5() () 19
r~ 0 . ,l8 C
C
z 0A G 0 .-15
"
0 . .111
G
0 .4%1 0 .42 0A 1
95 100 105 70 75 80 85 90 11 »iambc-. r Fig. 1. Hydrogen-3 counting efficiency as a function of quenching for aqueous samples in four commercial liquid scintillators. ..1 )
:)0
6F)
G0
improves with energy. However, implementation of a SIR based on the TDCR method would require the construction of a new system at the BIPM, which would delay the extension of the SIR to pure-beta-particle emitting radionuclides. The present study used the liquid-scintillation efficiency tracing method using a H standard . 3
3. H-H 2 0 stability in commercial scintillators 3
In order to test the stability of H-water standards in commercial scintillators, water samples were prepared in four different commercial scintillators, identified as scintillators A--D. The aqueous loading of the scintillators for metrology applications is generally less than 2% - a useful range for most samples, for example is 15 mg to 1 g water per 10 ml scintillator . In this range of water concentration, water droplets are dispersed in the organic matrix as "reverse micelles" . In previous work at the NIST we found that the H-water samples in reverse micelles at room temperature exhibit a loss in H counting efficiency of the order of 0.15% per day . This was not observed at reduced temperature WC) . Irl 111C past couple of years a new generation of commercial scintillators has come on the market [7] . These have a higher molecular weight solvent, such as di-isopropylnapthalene, and a higher flash point than the previous scintillators based on p-xylene and pseudocumene. To test the suitability of these new materials for the proposed SIR system, four sets of samples were prepared and their H counting efficiency was followed for sufficient time to observe the trend in efficiency loss. 3
3
3
3
The H-water standard was a dilution of the NIST SRM 4927C [8] . The samples were added gravimetrically to the scintillation vials containing 10 ml scintillator in a polycone-seal glass vial (Kimble) #' . Each scintillation vial was loaded with 0-1 ml of H BO to vary the quenching in the samples . Measurements were made with a Beckman LS7800 liquid-scintillation counting system equipped with two Hamamatsu R331-05 phototubes operated in the coincidence mode, a logarithmic amplifier, and a 137CS external source for quench monitoring [9] . The samples were then counted for over 260 days to study the loss of count rate as a fu,iction of scintillator type and water loading . Fig . 1 shows the initial data set of efficiency as a function of quenching for the four scintillators. It is clear that scintillator C is less efficient than the others for the same Horrocks number (H #) [9] . Scintillator A is the pseudocumene type, B is a polyarylalkane, while C and D are based on di-isopropylnapthalene . (Efficiency is of course not the only indicator of product quality, as most commercial scintillators are formulated to incorporate large volumes of aqueous sample with various biological molecules). Figs. 2 and 3 show the loss in counting efficiency with time for scintillators B and D. The results for A and B were essentially the same. Although there were no overwhelming differences in efficiency, the highest efficiency for H and good stability were observed for Ultima Gold scintillator from Packard Instrument (The Netherlands). The counting 3
3
Mention of commercial products does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products identified are necessarily the best available for the purpose . * 1
111(b) . LIQUID SCINTILLATION
J.M Calhoun et al. / Irrtcrc"ra»r~~urirrx~ strmdrrrd salr~tians of radionurfdeti Watvr Loading 0 0 Till . 0 1 1ol . ~ 0 :' rltl . 0 ;i rnl, () ~1 tttl . 0 `'i tlll . () (i ntl. 0 7 Inl .
U . 50 () .19
r
0 A ti
r r
0 11 Il0 "I tttl . 1 0 nrl .
"
UA3 I0A 2 0 .41
L~- i-
0
L
J
M 10 .1 130 15G 111 :2 I line (dit%-, frOlll rol' . t Illl(')
52
0H
2 :3,1
260
Fig. 2. Loss in counting efficiency for ;H-water samples as a function time for scintillator B.
efficiency rate of loss was lowest for the unquenched samples (that is, 35 mg water in 10 ml scintillator), of the order of 0.03% per day in -1H efficiency. (The next generation counting system on order for the NIST Radioactivity Group will be refrigerated . Some of these measurements will be repeated to see how the scintillators behave at reduced temperature .)
4. Standardization of P-particle emitters at NIST Table 1 summarizes the sample preparation data for the four lots of samples used in this phase of the intercomparison . Three organic radiolabelled compounds were prepared that would not change with time: ; H-hexadecanc, "C-hexadccane and "Tc-tri-
Watc" r Loading 0 0 fill . 01 m1 . 0 '.? m l . 0311,1 . 0 .1 till.
O .52
0 .5 1
r-
tà
(Y50
S
A
Y
+' VLI
-> _~-
"
0 7 01, 0 11 0 9 1 0
1111. In 1 .
nil, fill . fill .
" ï.
0 .-15
0 .-1 :3 () . f_1 t 0
:2 G
:?
l fi 10 .1 1 :10 V.)G i f , :III lle (dil v~~ frorl) 1 - t4 . t III( , )
*
1' 08
Fig. 3. Loss in counting efficiency for ;H-water samples as a function time for scintillator D.
J.M. Callimin et at. / Intercomparing stantlardwhstions of radiontiefides
caprylamine . The 1 1-1-hexadecane and the "C-hexadccane were obtained from Amersharn International . The NIST 'H-water standards were compared with the -'H-hexadecane solution, thereby standardizing the -'H-hcxadecane with reference to the NIST 1978 gas counting. The "'Tc was a dilution cat' the: MIST SRM 4288 [10], a standard aqueous solution which was complexed with tricaprylaminc . In prior work, it was found that "Tc standard aqueous solutions can be added to a scintillator containing a small amount of the non-
quenching tricaprylamine, which then extracts the pertechnelate anion into the organic solvent. The radionuclide sample% (40-70 mg) were added gravimetrically to name-scaled glass vials which meet the dimcnsions of the IEC standard (11]. Each scintillation vial contains 10 ml Packard Ultima Gold scintillator. Some of the samples contain chloroform WHCI .,t ) 10-50 gl, as a quenching agent to provide a range of quenching for samples and standards . Measurements were made with a Beckrnan LS7800 liquid-scintillation counting
Table I
Samples for use in an intercomparison of pure beta-particle emitting radionuclides Nuclides
'H-hexadecane
14
C-hexadecane
"Tc-TCA
BKG
Sample no.
Sample preparation data " Sample mass [mg]
CHC1 3 [yq
13
14 15 16 17 18 19 20 21 22 23 24
53.762 55 .470 55 .523 54503 56 .899 55,399 55 .405 57 .454 51118 54 .720 55116 58380
10 10 20 30 40 50
25 27 28 29 30 31 32 33 34 35 36
54 .664 55 .589 55 .917 54.626 55151 51150 56 .012 54.376 56915 55A27 54100
10 10 20 30 40 50
37 38 40 41 42 43 44 45 46 47 48
73 .669 56.762 51176 53 .545 MAY 55 .239 54A29 55 .551 WAY 54 .235 55 .055
10 1() 20 30 40 50
activity [kBqj (rr 12(H) EST 9/3/%)
2.401 2.478 2 .480 2.434 2.541 2%4 2.475 2.566 2%2 144 2.466 2,608
50
10 ml Ultima Gold (Packard Vial : sealed glass; volume : 20 ml, max. sealed height : 63 mm ; diameter: 28 mm o.d .-, scintillator : Instrument Co.) . 111(b) . LIQUID SCINTILLATION
J.M. Gcilltraun et al. / bttercomparing stanclard soiutions of'rudiontu~lirlea
0 .52 051
>,0 .40 ca sw ~~ 0 .48 0 .47 0 31-1 H.LO
0.46
311 H20 30 pL CHC13 0 3H Hex 3H 11ex .10 /d . C1R'1s
0 .45 0 .44
0
15
30
-15
75 60 90 tinle (dilvs)
105
120
135
150
Fig . 4. Loss in counting efficiency as a function of time for 3H-water and 3H-hexadecane samples.
system as previously described . Samples were counted for over 100 days under similar geometrical and quenching conditions for each radionuclide. These measurements were also made at room temperature. Unexpectedly, the flame-sealed-in-glass 3Hhexadecane samples also exhibited a loss in counting efficiency with time. Fig . 4 compares the counting efficiency vs time for ;H-water and "H-hexadecane samples. Fig . 5 shows the change in H* with time for quenched and unquenched ;H-hexadecane and 3 H-
G4
water scintillators . A shift to higher H* for a given vial indicates some slight deterioration of the scintillators . It is not clear whether this is sufficient to explain the gradual loss in counting efficiency. Tables 2 and 3 give the results of the activity measurements by the CIEMAT/NIST liquid-scintillation method . The results for the Beckman LS7800 are based on repetitive measurements over 100 days. The technique was then tested at different sites in the area using the most contemporary liquid-scintillation coun-
~
G:2 (30
3H H,0 3H H20 30 p.L CHC13 3H Hex 3H Hex 401L CHC13
58 56 r
54 52 .50 48 -16 4-1
0
15
:30
115
00 75 90 105 120 135 150 t11r'E (davs) Fig . 5. Change in H* with time for quenched and unquenched ;H-hexadecane and 3 H-water scintillators .
J.J1f Calltcrttrt et ai. / Gttcr'ccxrrr wr~bttt stcrrtdurd scrltttwcrrts of radïcuttcclidés
Table 2 NIST results for the activity standardization of "C-hexadecane by the CIEMAT/NIST method (reference time 12(Nî FST September 3, 1990) Measurements
Average ,"
LS081190 LS081490 LS081890 LS090490 LS092090 PKI02990 LB111090 6Wa1C
54054 54!1(12 54(1(13 53979 54046 538111 54120 54122
tail/g]
Standard error 1 1- 11 0.05 0.05 0.05 0.07 11 .05 0.05 0,04 0.05
1`'" 1
0.07 -0 .03 -0 .03 -0 .07 0.05 -0 .25 0.19 0.19
Table 3 NIST results for the activity standardization of "'Tc-tricaprylamine by the CIEMAT/NIST method (reference time 1200 EST September 3, 1990)
LS081190 LS081490 LS081890 LS090490 LS092090 PKI02990 LB 111090 60001C
Average " [Bq/8] 37650 37627 37630 3763e 37657 37531 37706 37719
Standard error
0 NIST
0.03 0.02 0.02 0.02 0.02 0.02 0.03 0.03
0.03 -0 .03 -0 .03 -0 .01 0.05 -0.29 0.17 0.21
1%]
1%]
`' Average of first 5 measurements (NIST) is 37640±0.011
and for all measurements 37644±0.05% .
ters available from Beckman, LKB and Packard (see table 4). The latter results were not included in the NIST average, because we had only one opportunity to use the outside counting systems, and insufficient time to fully characterize them and understand the associated nonrandom uncertainties in the measurements. The CIEMAT/NIST general model, however, is based on a very simple model of two phototubes in coincidence, and we have assumed it would be valid for any Table 4 Liquid-scintillation counting system used in these measurements Beckman LS7800 Packard 2500TR LKB System 1410 Beckman LS6000LL
Uncertainties in the NIST activity measurements Component
.1 NIST
'' Average of first 5 measurements (NIST) is 54017±0.021, and for all measurements 54026±().05"1 .
Measurements
Table 5
NIST FDA - Rockville LKB - Rockville Microbiological Associates Rockville
Source preparation Scintillator stability Impurities Dead time Liquid-scintillation measurements (n = I I '-C) (n = 12 ""Tc) ;H reference standard Calculated efficiency Combined in quadrature
Estimate `"f uncertainty 11 ; 1 0.11) 0.10 0.01 () .03
0.112 11.111 0.1() 0.1 () 11 .20
of the commercial counting systems . The results for "C and ""'Tc in tables 2 and 3 seem to support this assumption . For the "Tc, the present value agrees to 0.3% with the value for SRM 4288 [10]. 5. Conclusion Scintillators from different manufacturers were compared to determine which new generation Scintillator would be the most suitable for use in an international intercomparison of pure-beta-particle emitters . The results indicated no significant differences, so the Scintillator exhibiting the highest counting efficiency was selected for the next phase of the study. For the proposed mini-intercomparison, samples of radiolabelled compounds were prepared on the assumption that they would be stable with time. These were -; Hhexadecane, ' 4 C-hexadecane and 9 `'Tc-tricaprylamine (TCA) . Activity measurements were made using the CIEMAT/NIST method of efficiency tracing with a Beckman LS7800 and newer instruments, with satisfactory agreement. The uncertainties in the NIST activity measurements are summarized in table 5. Each component is estimated as if it were one standard deviation, and they are added in quadrature to give a combined uncertainty of 0.20%. Samples were then distributed for a trial intercomparison to four other laboratories . Based upon the success of the trial intercomparison, a decision may be made to extend the SIR to the intercomparison of pure beta-particle emitting radionuclides. Acknowledgements
The authors gratefully acknowledge Dr. Larry L. Lucas for discussions and assistance in data acquisiI11(b) . LIQUID SCINTILLATION
120
J.M. Calhoun et al. / Interromparing xtaticlard .uwltttiotts of radionuclides
tion. We are also grateful to Robert Jones (LKB), Richard Jones (Packard) and Karen dcPianelli (Beckman) for arranging for measurements on the new LS counting systems available in the local area. References [I) A. Rytz, Int . J. Appl. Radiat. Isotopes 34 (1983) 1047 . (2) A. Grau Malonda and E. Garcia-Torapo, Int. J. Appl . Radiat . Isotopes 33 (1982) 249. [3a B.M . Coursey, W.B. Mann, A. Grau Malonda, E. Garcia-Torapo, LM . Los Arcos, J.A .B. Gibson and D. Reher, Int. J. Appl . Radiat . Isotopes 37 (1986) 403. [4] B.M. Coursey, A. Grau Malonda, E. Garcia-Torapo and J.M. Los Arcos, Trans. Amer. Nucl. Soc. 50 (1985) 13 . (S) J.M. Calhoun, B.M . Coursey, D. Gray and L. Karam, Proc. Int. Conf. on New Trends in Liquid Scintillation
Counting and Organic Scintillators, Gatlinburg, TN, 1989, eds. H. Ross, J . Noakes and J. Spaulding (Lewis, Chelsea, M1, 1991). [6] K. Pochwalski, R. Broda and T. Radoszewski, Int. J. Appl . Radiat . Isotopes 39 (1988) 165. [7] J. Thomson. Proc . Int. Conf . on New Trends in Liquid Scintillation Counting and Organic Scintillators, Gatlinburg, TN, 1989, eds . H. Ross, J. Noakes and J . Spaulding, (Lewis, Chelsea, M1, 1991). [8j M.P . Unterweger, B.M . Coursey, F.J . Schima and W.B. Mann, Int. J. Appl . Rad. Isotopes 31 (1980) 611 . [9] D.L . Horrocks, in : Advances in Scintillation Counting, eds. S.A . McQuarrie, C. Ediss and L.I . Wiebe (University of Alberta Press, 1986). [10] B.M . Coursey, J.A .B. Gibson, M. Heitzmann and J. Leak, Int. J. Appl . Rad. Isotopes 35 (1984) 1103 . [I II International Electrotechnical Committee Standard, Publication 582 (1977).