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Čerenkov counting and Čerenkov-scintillation counting with high refractive index organic liquids using a liquid scintillation counter
International Journal of Applied Radiation and Isotopes, Vol. 29, pp. 391-394 (~) Pergamon Press Ltd. 1978. Printed in Great Britain
0020-708X/78/0601-0391 $02.00/0
Cerenkov Counting and Cerenkov-Scintillation Counting with High Refractive Index Organic Liquids Using a Liquid Scintillation Counter L E O N A R D I. W I E B E , * F. H E L U S a n d W. M A I E R - B O R S T Institut fur Nuklearmedizin, Deutsches Krebsforschungszentrum, Heidelberg, Bundesrepublik Deutschland
(Received 21 September 1977) Fluorine-18 and carbon-14 radioactivity was measured in methyl salicylate (MS), a high refractive index hybrid (~erenkov-scintillationgenerating medium, using a liquid scintillation counter. At concentrations of up to 21.4%, in MS, dimethyl sulfoxide (DMSO) quenched 14C fluorescence, and with a 10-fold excess of DMSO over MS, laF count rates were reduced below that for DMSO alone, probably as a result of concentration-independent self-quenching due to "dark-complex" formation. DMSO in lower concentrations did not reduce the counting efficiency of 18F in MS. Nitrobenzene was a concentration-dependent quencher for both 14C and taF in MS. Chlorobenzene (CB) and DMSO were both found to b~ weak (~erenkov generators with lSF. Counting efficiencies for ISF in MS, CB, and DMSO were 50.3, 7.8 and 4.370 respectively in the coincidence counting mode, and 58.1, 13.0 and 6.8% in the singles mode. 14C efficiencieswere 14.4 and 22.370 for coincidence and singles respectively, and 15.3 and 42.070 using a modern counter designed for coincidence and single photon counting. The high 14C and lSF counting efficiency in MS are discussed with respect to excitation mechanism, on the basis of quench and channels ratios changes observed. It is proposed that MS functions as an efficient (~erenkovscintillation generator for high-energy beta emitters such as aaF, and as a low-efficiency scintillator for weak beta emitting radionuclides such as ~4C.
INTRODUCTION THE GENERATIONof~erenkov radiation in liquid media has been shown to be a function of beta particle energy and refractive index of the medium,cl) Consequently, the efficiency of detection for radionuclides decaying by beta emission has been limited by the proportion of betas which have energy above the ~erenkov threshold energy, and by the proportion of the resultant ~erenkov radiation spectrum falling within the sensitivity range of the photodetectors. The use of highrefractive index solvents to lower the threshold, and the use of wave-shifting compounds to convert a portion of the ~erenkov u.v. spectrum to radiation visible to the photomultiplier have therefore been areas of endeavour to improve (~erenkov counting by liquid scintillation counter.t2, 3) An additional benefit with the use of wave shifters has been their isotropic fluorescence, allowing improved counting efficiency in the normal coincidence counter over that realized when measuring (~erenkov light which has directional characteristics34~ The wave-shifting systems have been termed "hybrid (~erenkov-scintillation"media (C-S) because the waveshifting process is analogous in part to the liquid scintillation process. ~5) A major disadvantage of the (~erenkov-scintillationmethod, in comparison with the (~erenkov method, has been its susceptibility to chemical quenching effects of the waveshifter. This,
combined with the lower counting efficiency as compared with the liquid scintillationmethod, has made the hybrid C-S procedure generally more inconvenientand unsatisfactory. Although an improved (~erenkov method with external wave-shifting has recently been published,tS) hybrid C-S systems are likely to remain in use. A unique C-S counting medium, methyl salicylate (MS), which possesses both wave-shifting properties and a high refractive index has recently been reported. ~6"7) The function of this C-S medium with chemical and colour/ chemical quench during the assay of x4C and lSF by liquid scintillation counter is now described, and comparisons are drawn to other high refractive index organic liquids. The objective was to elucidate the method of photon generation in MS by radionuclides emitting no betas with energy above the ~erenkov threshold (14C), and by radionuclides emitting positrons of which a significant proportion have energies above the (~erenkov threshold energy (1SF)" EXPERIMENTAL
Methyl salicylate (MS), (pharmaceutical grade, Merck) and dimethyl sulfoxide (DMSO) (synthesis grade, Merck) were passed through activated alumina columns prior to use. Chlorobenzene (CB) (purls p.a. > 99.5%, Fluka) and nitrobenzene (NB) (synthesis grade, Merck) were used without further purification. lSF was prepared by deuteron irradiation of 2°Ne *Permanent address: Division of Bionucleonics and Radiopharmacy, University of Alberta, Edmonton, Alberta, gas in a glass target, using the Heidelberg Cyclotron. Canada. The lSF was recovered by rinsing the target with either 391
392
~erenkov counting using a liquid scintillation counter
DMSO or dry methanol. These ~8F solutions were standardized by cross-calibration with a pre-calibrated Ge(Li) detector gamma spectrometer. High specific activity ~4C-thymidine in methanol was used for ~4C measurements. Absolute counting efficiencies for unquenched ~4C probes were estimated by internal standardization with ~4C-toluene standard (New England Nuclear). Initial, unquenched counting rates (wide window, singles mode) were 4-5 x 105 counts/ min per vial. Polypropylene vials (W. Zinsser, Frankfurt) were used for all measurements. The analyzer levels of the liquid scintillation counter (Beckman LS-150) were set so that the wide window encompassed the 3H pulse height spectrum for toluene/PPO/POPOP fluor (Beckman unquenched standard), and the narrow window covered approximately 11% of tritium counts at the lower end of that spectrum. Duplicate samples were analyzed using both the singles (one PMT) and the coincidence mode. A small number of measurements were made with a Beckman 9000 series liquid scintillation counter, to take advantage of improved photon counting instrumentation. A preset counting error of 1% (2a) was used for all experiments with the exception of a few cases where high quench and radionuclide decay in combination reduced the precision to 2% (2a). All ~8F count rates were decay corrected using a half-life value of 109.7 min3sl Sample volumes ranged from 11 ml at the beginning to 14 ml at the end of a quench sequence. All measurements were made so that the count rate without quencher added represented a relative efficiency of 100%, and the count rates upon sequential additions of quencher to that same vial represented relative efficiencies. RESULTS The lSF counting efficiency in each of the three liquid photon generating media tested (MS, DMSO and CB) using both the singles and the coincidence mode are given in Table 1. The ~4C counting efficiency in TABLEl. Estimated counting efficiencies(_ 5%) for l SF in MS, DMSO and CB using a Beckman LS-150 liquid scintillation counter in the coincidence and in the singles mode Mode Coincidence (c) Singles (s) (s - c/c) x 100
%E in liquid medium MS DMSO CB 50.3 58.1 15.5
4.3 6.8 58.1
7.8 13.0 66.7
MS (singles and coincidence) using the LS-150 and the LS-9000 spectrometers were 22.3, 14.4, 42.0 and 15.3% respectively. The addition of DMSO (chemical quenching agent) to a solution of laF in MS in quantities of up to 3 ml per 11 ml of MS (21.4% by volume) only marginally changed the counting efficiency, whereas NB (colour/ chemical quenching agent) drastically reduced the "
TABLE2. Relative counting efficiencies for 14C and 18F in MS upon addition of DMSO or NB as quenching agents 1~C
Volume quencher (/A) 0 10 30 100 300 I000 3000
18F
NB
DMSO
NB
DMSO
100 86.7 71.1 48.2 16.4 2.2 *
100 98.5 95.1 86.3 67.9 42.9 18.0
100 97.7 92.9 81.2 58.7 22.8 10.3
100 101.7 105.8 106.5 105.6 97.5 *
*Not measured. TABLE3. The influence of chemical (DMSO) and colour/ chemical (NB)quenching agents on the measurement of 14C by liquid scintillation counter using MS (11 ml) as the photon generating medium Volume quench (gl) 0 10 30 100 300 1000 3000
DMSO
NB
SCR
rel %E
SCR
rel %E
0.335 0.333 0.335 0.348 0.381 0.429 0.489
100 98.6 95.1 86.3 67.9 42.9 18.0
0.335 0.350 0.386 0.427 0.505 0.556 --
100 86.6 71.1 48.2 16.4 2.2 --
relative efficiency. A similar effect was observed for 14C in MS using NB quench, but upon addition of DMSO large reductions in count rate were also observed. Relative counting efficiencies are given in Table 2. In both cases an increase in the total number of low energy pulses increased. Data for both 14C and 18F channels ratio changes are given in Table 3. The pulse height spectrum for 14C in MS shifted to lower energies, and a decrease in counting rates occurred as a function of increasing concentrations of both NB and DMSO. Although the magnitude of the effect per unit volume was greater for NB, both the chemical and the colour/chemical quenching agents gave data which produced a common sample channels ratio (SCR) quench corrective curve. Representative data are given in Table 4. The effect of MS on ~8F ~erenkov measurement in TABLE4. Counting efficienciesfor 18F in various mixtures of DMSO and MS Volume added (111) 0 10 30 100 300 1000 3000 *Not measured.
Counting efficiencyfor 18F DMSO in MS in 11 ml MS 11 ml DMSO 50.3 51.6 53.2 53.6 53.1 49.0 *
4.3 3.2 3.1 3.0 3.2 3.8 5.4
L. I. 144ebe, F. Helus and W. Maier-Borst
TABLE5. Changes in analyzer counting rates upon addition of 300pl of either DMSO or NB to solutions of 14C or laF in MS
393
ence in counting by the coincidence or the singles modes. The 15.5~ increase in singles over coincidence count rates for MS (Table 1), as compared with a 66.7~ increase for CB, imply that a much larger Quenching Y/ochange in counting rate proportion of the light was emitted isotropically when Isotope agent Wide window Narrow window using MS. A similar (to CB) observation was made 1*C DMSO -31.3 -20.1 when using DMSO as the Cerenkov generator. NB - 83.4 - 75.5 Perhaps an equally important contributor to the taF DMSO +3.9 +30.9 relatively high lSF counting efficiency observed with NB - 40.3 + 72.1 the MS generator was the physical means of photon generation. That is, not only were photons emitted DMSO was also investigated to determine whether or isotropically as wave-shifted (~erenkov radiation, but not very large concentrations of DMSO (greater than possibly as fluorescence resulting from solvent ex21.49/o) would affect the C-S hybrid counting process. citation by the energetic positrons emitted upon decay MS was observed to act essentially as a concentration- of lSF_atoms. Evidence for the occurrence of the latter independent quencher (Table 5) upon addition of phenomenon was the generation of significant fluor10-1000~i per 11 ml DMSO (0.09-8.3~o by volume). escence in a I~C/MS system where all the betas lie No change in SCR was observed over the concentration below the theoretical (~erenkov threshold. Although the coincidence-singles difference was large (54~o in the range studied. The ~erenkov measurement of lSF in CB, a liquid LS-150), the very high counting efficiency of 42~o using with a refractive index near that of MS, was also the LS-9000 would require the refractive index for u.v. investigated briefly. The counting efficiency was slightly light at 303 nm (excitation wave length for MS ~11)) to higher than that reported for lSF in DMSO, as be at least 2.41 (a value based on the assumption that expected on the basis of refractive index differences the light collection efficiency in the LS-9000 was near between the two solvents. NB quench for lSF in CB 100%, and the known 1"C EAr of 45 keV). ts) Large was similar to that reported previously for 36C1 in singles/coincidence count rate differences have been toluene with nitromethane quench, t6) Excessive chemi- observed for 3H and for t4C (at the low energy region luminescence was observed in mixtures of CB and of the pulse spectrum) in the PPO/POPOP/toluene, DMSO when the volume of DMSO in 11 ml of CB and in this sense the 14C/MS singles/coincidence rate exceeded 100 pl and the coincidence background rate differences would be compatible with a scintillation doubled even upon addition of 10 tal. Although quan- mechanism. The existence of differing MS excitation mechanisms, titative results in coincidence and singles measurements varied, trends reported for coincidence measure- that is, photon and positron excitation by 1sF and only ments were essentially identical to those observed beta excitation by 14C, is also suggested by the differing susceptibilities to quench, both in degree and in nature during singles measurements. (Table 5). It must be recognized, however, that quenching in the case of the positron emitter was DISCUSSION complicated by the generation of a spectrum of The volume of photon-generating medium per vial secondary electrons as a result of annihilation gamma was found to influence sample count rate as reported rays interacting with the medium, the vial and the PMT previously,tl~ and was therefore kept between 11 and faces. This effect, although unimportant in unquenched 14ml. In this range, the count rate was found to systems, would undoubtedly attenuate the effectiveness fluctuate less than 3 ~ when calculated on the basis of of both colour and chemical quenching agents. The very large increase in narrow window counts may well greatest difference and lowest net counting rate. The l SF counting efficiencies in MS, DMSO and CB have arisen from this phenomenon, which has been (Table 1) indicate that although MS has a marginally shown to generate a significant number of pulses when lower refractive index (1.522) than CB (1.523),tg~ more 18F is external to the C-S hybrid medium, n~ This pulse detectable photons were produced by MS. The MS distribution would be expected to be similar to that may have unusually large refractive indices for specific observed for MS with a 137Cs external standard source wavelength regions in the low visible u.v. region which (662 keV 137Barn X-rays compared with 511 keV 7-rays would lower the theoretical (~erenkov threshold tl°) from positron annihilation). No shift to increased (calculated to be approximately 165keV by the numbers of lower level counts was previously reported method of Rosstl~), but it is not known whether or not for ~SF counting in MS containing nitromethane, but such "resonances" would be of sufficient magnitude to that agent is transparent at the excitation wave length account for the almost 4.5-fold increase observed in for MS, whereas NB has a significant adsorption. DMSO, reported to be an H-bonding quencher of singles counting. A more plausible explanation would appear to be the wave-shifting properties of MS. These salicylate fluorescence, has a u.v. cut-off in the region of properties have previously been demonstrated as a 285nm t~3~ which is well below the absorption and broadening of the pulse height spectrum, ~6'7) and have .emission maxima for MS. NB has a powerful quenching now been further substantiated by the observed differ- effect on emission fluoresence like nitromethane but,
394
ff erenkov counting using a liquid scintillation counter
because of its large absorption at 265 nm (which tails off well into the visible region and hence the yellow colour), NB also acts as a potent colour quenching agent. This strong, broad absorption band not only prevents excitation of MS, but also absorbs fluorescent radiation emitted by excited species. NB was observed to behave strictly as a concentration-dependent quenching agent, whereas studies with taF in D M S O (upon addition of small volumes of MS) indicated that D M S O quench was concentration-dependent in low concentration but concentration-independent in high concentration. The D M S O effect presumably was the result of "darkcomplex" formation3 TM A number of 1aF t~erenkov measurements were made in both CB and D M S O , the latter to observe the quench effect in D M S O - M S mixtures, and the former to compare CB and MS and (~erenkov generating media of similar refractive index. As anticipated, ~SF ~erenkov counting in CB was similar to that reported previously for 36C1 in toluene (6) in that about 50% of the (low) counts were derived not via the (~erenkov mechanism, but via fluorescence of excited solvent species. Pulse height spectra were not plotted, but on the basis of SCR data, CB and D M S O had pulse distributions similar to that observed for 32p and 36C1 in non-fluorescent media. (6' 15) The coincidence and the singles modes of measurement were applied to all experiments, and in all cases similar trends were observed. Major differences existed only in counting efficiencies, when no wave-shifting effect was evident. The singles mode was found to be somewhat undesirable, particularly for measurements where D M S O was present, because of large fluctuations in background count rate due to chemiluminescence. Except in the case of CB in D M S O , coincidence background count-rates of D M S O and D M S O mixtures did not increase by more than 100%. The chemiluminescence produced by C B - D M S O mixtures increased in intensity over a period of approximately 20min, then decayed with a half-time longer than the half-life of ~SF. Uncertainties in chemiluminescence development made it impossible to measure radionuclide activity with precision. SUMMARY The data obtained by the singles and the coincidence modes of counting of 18F and 14C as solutions in MS,
CB and D M S O using a liquid scintillation counter support the conclusion that MS is an efficient (~erenkov-scintillation medium for taF, but that it behaves as an inefficient scintillation fluor for 1"C. Additional studies are required to determine the degree of primary fluorescence in MS when counting ~SF, and to evaluate the role of Compton electrons (from ~SF annihilation gamma rays) in the observed "resistance" of MS to colour/chemical quenching of fluoresence. Acknowledgements--The authors thank Grace E. Wiebe for her assistance in preparation of the manuscript, and gratefully acknowledge the support of the Alexander yon Humboldt Foundation in the form of a Research Fellowship (L.I.W.). Beckman GmbH (Miinchen) is also thanked for providing an LS-9000 for several measurements.
REFERENCES 1. Ross H. H. Analyl. Chem. 41, 1260 (1969). 2. GELSEMA W. J., DE LIGNY C. L., LUTEN J. B. and VOSSENBERGF. G. A. Int. J. appl. Radiat. Isotopes 26, 443 (1975). 3. HEIBERGE. and MARSHALLJ. Rev. scient. Instrum. 27, 618 (1956). 4. Ross H. H. In Liquid Scintillation Science and Technology (Edited by NOUJAIMA., EDISSC. and WIESEL. I.), p. 79. Academic Press, New York (1976). 5. Ross H. H. Radiochem. radioanal. Lett. 27, 71 (1976). 6. WIEBE L. I. and EDlSS C. In Liquid Scintillation Science and Technoloqy (Edited by NOUJAIMA., EDISSC. and WIEBEL. I.), p. 93. Academic Press, New York (1976). 7. ABRAMSD. N., McQuARmE S. A., EDIss C. and WmBEL. I. In Liquid Scintillation Science and Technology (Edited by NOUJAIM A., ED1SS C, and WmBE L. 13, p. 167. Academic Press, New York (1976). 8. LEDERER C. M., HOLLANDER J. M. and PERLMAN I. (editors), Table of Isotopes, 6th edn, p. 7. John Wiley, New York (1968). 9. WEAST R. C. (editor), Handbook of Chemistry and Physics, 52nd edn, p. E201. Chemical Rubber Co., Cleveland (1971). 10. Ross H. H. (personal communication). ll. WELLER A. Z. Elektrochem 60, 1144 (1956) through PARKERC. A. Photoluminescence of Solutions, pp. 342343. Elsevier, Amsterdam (1968). 12. GILBAULT G. G. Practical Fluorescence, pp. 108-112. Marcel Dekker, New York (1973). 13. PERRY S. G., AMOS R. and BREWER P. 1. Practical Liquid Chromatography, p. 55. Plenum Press, New York (1972). 14. PESCE A. J., ROSEN C. G, and PASaY T. L. (editors), Fluorescence Spectroscopy, p. 52. Marcel Dekker, New York (1971). 15. WIEBE L. I., NOUJAIMA. A. and EDtSS C. Int. J. appl. Radiat Isotopes 22, 463 (1971).
0020-708X/78/0601-0391 $02.00/0
Cerenkov Counting and Cerenkov-Scintillation Counting with High Refractive Index Organic Liquids Using a Liquid Scintillation Counter L E O N A R D I. W I E B E , * F. H E L U S a n d W. M A I E R - B O R S T Institut fur Nuklearmedizin, Deutsches Krebsforschungszentrum, Heidelberg, Bundesrepublik Deutschland
(Received 21 September 1977) Fluorine-18 and carbon-14 radioactivity was measured in methyl salicylate (MS), a high refractive index hybrid (~erenkov-scintillationgenerating medium, using a liquid scintillation counter. At concentrations of up to 21.4%, in MS, dimethyl sulfoxide (DMSO) quenched 14C fluorescence, and with a 10-fold excess of DMSO over MS, laF count rates were reduced below that for DMSO alone, probably as a result of concentration-independent self-quenching due to "dark-complex" formation. DMSO in lower concentrations did not reduce the counting efficiency of 18F in MS. Nitrobenzene was a concentration-dependent quencher for both 14C and taF in MS. Chlorobenzene (CB) and DMSO were both found to b~ weak (~erenkov generators with lSF. Counting efficiencies for ISF in MS, CB, and DMSO were 50.3, 7.8 and 4.370 respectively in the coincidence counting mode, and 58.1, 13.0 and 6.8% in the singles mode. 14C efficiencieswere 14.4 and 22.370 for coincidence and singles respectively, and 15.3 and 42.070 using a modern counter designed for coincidence and single photon counting. The high 14C and lSF counting efficiency in MS are discussed with respect to excitation mechanism, on the basis of quench and channels ratios changes observed. It is proposed that MS functions as an efficient (~erenkovscintillation generator for high-energy beta emitters such as aaF, and as a low-efficiency scintillator for weak beta emitting radionuclides such as ~4C.
INTRODUCTION THE GENERATIONof~erenkov radiation in liquid media has been shown to be a function of beta particle energy and refractive index of the medium,cl) Consequently, the efficiency of detection for radionuclides decaying by beta emission has been limited by the proportion of betas which have energy above the ~erenkov threshold energy, and by the proportion of the resultant ~erenkov radiation spectrum falling within the sensitivity range of the photodetectors. The use of highrefractive index solvents to lower the threshold, and the use of wave-shifting compounds to convert a portion of the ~erenkov u.v. spectrum to radiation visible to the photomultiplier have therefore been areas of endeavour to improve (~erenkov counting by liquid scintillation counter.t2, 3) An additional benefit with the use of wave shifters has been their isotropic fluorescence, allowing improved counting efficiency in the normal coincidence counter over that realized when measuring (~erenkov light which has directional characteristics34~ The wave-shifting systems have been termed "hybrid (~erenkov-scintillation"media (C-S) because the waveshifting process is analogous in part to the liquid scintillation process. ~5) A major disadvantage of the (~erenkov-scintillationmethod, in comparison with the (~erenkov method, has been its susceptibility to chemical quenching effects of the waveshifter. This,
combined with the lower counting efficiency as compared with the liquid scintillationmethod, has made the hybrid C-S procedure generally more inconvenientand unsatisfactory. Although an improved (~erenkov method with external wave-shifting has recently been published,tS) hybrid C-S systems are likely to remain in use. A unique C-S counting medium, methyl salicylate (MS), which possesses both wave-shifting properties and a high refractive index has recently been reported. ~6"7) The function of this C-S medium with chemical and colour/ chemical quench during the assay of x4C and lSF by liquid scintillation counter is now described, and comparisons are drawn to other high refractive index organic liquids. The objective was to elucidate the method of photon generation in MS by radionuclides emitting no betas with energy above the ~erenkov threshold (14C), and by radionuclides emitting positrons of which a significant proportion have energies above the (~erenkov threshold energy (1SF)" EXPERIMENTAL
Methyl salicylate (MS), (pharmaceutical grade, Merck) and dimethyl sulfoxide (DMSO) (synthesis grade, Merck) were passed through activated alumina columns prior to use. Chlorobenzene (CB) (purls p.a. > 99.5%, Fluka) and nitrobenzene (NB) (synthesis grade, Merck) were used without further purification. lSF was prepared by deuteron irradiation of 2°Ne *Permanent address: Division of Bionucleonics and Radiopharmacy, University of Alberta, Edmonton, Alberta, gas in a glass target, using the Heidelberg Cyclotron. Canada. The lSF was recovered by rinsing the target with either 391
392
~erenkov counting using a liquid scintillation counter
DMSO or dry methanol. These ~8F solutions were standardized by cross-calibration with a pre-calibrated Ge(Li) detector gamma spectrometer. High specific activity ~4C-thymidine in methanol was used for ~4C measurements. Absolute counting efficiencies for unquenched ~4C probes were estimated by internal standardization with ~4C-toluene standard (New England Nuclear). Initial, unquenched counting rates (wide window, singles mode) were 4-5 x 105 counts/ min per vial. Polypropylene vials (W. Zinsser, Frankfurt) were used for all measurements. The analyzer levels of the liquid scintillation counter (Beckman LS-150) were set so that the wide window encompassed the 3H pulse height spectrum for toluene/PPO/POPOP fluor (Beckman unquenched standard), and the narrow window covered approximately 11% of tritium counts at the lower end of that spectrum. Duplicate samples were analyzed using both the singles (one PMT) and the coincidence mode. A small number of measurements were made with a Beckman 9000 series liquid scintillation counter, to take advantage of improved photon counting instrumentation. A preset counting error of 1% (2a) was used for all experiments with the exception of a few cases where high quench and radionuclide decay in combination reduced the precision to 2% (2a). All ~8F count rates were decay corrected using a half-life value of 109.7 min3sl Sample volumes ranged from 11 ml at the beginning to 14 ml at the end of a quench sequence. All measurements were made so that the count rate without quencher added represented a relative efficiency of 100%, and the count rates upon sequential additions of quencher to that same vial represented relative efficiencies. RESULTS The lSF counting efficiency in each of the three liquid photon generating media tested (MS, DMSO and CB) using both the singles and the coincidence mode are given in Table 1. The ~4C counting efficiency in TABLEl. Estimated counting efficiencies(_ 5%) for l SF in MS, DMSO and CB using a Beckman LS-150 liquid scintillation counter in the coincidence and in the singles mode Mode Coincidence (c) Singles (s) (s - c/c) x 100
%E in liquid medium MS DMSO CB 50.3 58.1 15.5
4.3 6.8 58.1
7.8 13.0 66.7
MS (singles and coincidence) using the LS-150 and the LS-9000 spectrometers were 22.3, 14.4, 42.0 and 15.3% respectively. The addition of DMSO (chemical quenching agent) to a solution of laF in MS in quantities of up to 3 ml per 11 ml of MS (21.4% by volume) only marginally changed the counting efficiency, whereas NB (colour/ chemical quenching agent) drastically reduced the "
TABLE2. Relative counting efficiencies for 14C and 18F in MS upon addition of DMSO or NB as quenching agents 1~C
Volume quencher (/A) 0 10 30 100 300 I000 3000
18F
NB
DMSO
NB
DMSO
100 86.7 71.1 48.2 16.4 2.2 *
100 98.5 95.1 86.3 67.9 42.9 18.0
100 97.7 92.9 81.2 58.7 22.8 10.3
100 101.7 105.8 106.5 105.6 97.5 *
*Not measured. TABLE3. The influence of chemical (DMSO) and colour/ chemical (NB)quenching agents on the measurement of 14C by liquid scintillation counter using MS (11 ml) as the photon generating medium Volume quench (gl) 0 10 30 100 300 1000 3000
DMSO
NB
SCR
rel %E
SCR
rel %E
0.335 0.333 0.335 0.348 0.381 0.429 0.489
100 98.6 95.1 86.3 67.9 42.9 18.0
0.335 0.350 0.386 0.427 0.505 0.556 --
100 86.6 71.1 48.2 16.4 2.2 --
relative efficiency. A similar effect was observed for 14C in MS using NB quench, but upon addition of DMSO large reductions in count rate were also observed. Relative counting efficiencies are given in Table 2. In both cases an increase in the total number of low energy pulses increased. Data for both 14C and 18F channels ratio changes are given in Table 3. The pulse height spectrum for 14C in MS shifted to lower energies, and a decrease in counting rates occurred as a function of increasing concentrations of both NB and DMSO. Although the magnitude of the effect per unit volume was greater for NB, both the chemical and the colour/chemical quenching agents gave data which produced a common sample channels ratio (SCR) quench corrective curve. Representative data are given in Table 4. The effect of MS on ~8F ~erenkov measurement in TABLE4. Counting efficienciesfor 18F in various mixtures of DMSO and MS Volume added (111) 0 10 30 100 300 1000 3000 *Not measured.
Counting efficiencyfor 18F DMSO in MS in 11 ml MS 11 ml DMSO 50.3 51.6 53.2 53.6 53.1 49.0 *
4.3 3.2 3.1 3.0 3.2 3.8 5.4
L. I. 144ebe, F. Helus and W. Maier-Borst
TABLE5. Changes in analyzer counting rates upon addition of 300pl of either DMSO or NB to solutions of 14C or laF in MS
393
ence in counting by the coincidence or the singles modes. The 15.5~ increase in singles over coincidence count rates for MS (Table 1), as compared with a 66.7~ increase for CB, imply that a much larger Quenching Y/ochange in counting rate proportion of the light was emitted isotropically when Isotope agent Wide window Narrow window using MS. A similar (to CB) observation was made 1*C DMSO -31.3 -20.1 when using DMSO as the Cerenkov generator. NB - 83.4 - 75.5 Perhaps an equally important contributor to the taF DMSO +3.9 +30.9 relatively high lSF counting efficiency observed with NB - 40.3 + 72.1 the MS generator was the physical means of photon generation. That is, not only were photons emitted DMSO was also investigated to determine whether or isotropically as wave-shifted (~erenkov radiation, but not very large concentrations of DMSO (greater than possibly as fluorescence resulting from solvent ex21.49/o) would affect the C-S hybrid counting process. citation by the energetic positrons emitted upon decay MS was observed to act essentially as a concentration- of lSF_atoms. Evidence for the occurrence of the latter independent quencher (Table 5) upon addition of phenomenon was the generation of significant fluor10-1000~i per 11 ml DMSO (0.09-8.3~o by volume). escence in a I~C/MS system where all the betas lie No change in SCR was observed over the concentration below the theoretical (~erenkov threshold. Although the coincidence-singles difference was large (54~o in the range studied. The ~erenkov measurement of lSF in CB, a liquid LS-150), the very high counting efficiency of 42~o using with a refractive index near that of MS, was also the LS-9000 would require the refractive index for u.v. investigated briefly. The counting efficiency was slightly light at 303 nm (excitation wave length for MS ~11)) to higher than that reported for lSF in DMSO, as be at least 2.41 (a value based on the assumption that expected on the basis of refractive index differences the light collection efficiency in the LS-9000 was near between the two solvents. NB quench for lSF in CB 100%, and the known 1"C EAr of 45 keV). ts) Large was similar to that reported previously for 36C1 in singles/coincidence count rate differences have been toluene with nitromethane quench, t6) Excessive chemi- observed for 3H and for t4C (at the low energy region luminescence was observed in mixtures of CB and of the pulse spectrum) in the PPO/POPOP/toluene, DMSO when the volume of DMSO in 11 ml of CB and in this sense the 14C/MS singles/coincidence rate exceeded 100 pl and the coincidence background rate differences would be compatible with a scintillation doubled even upon addition of 10 tal. Although quan- mechanism. The existence of differing MS excitation mechanisms, titative results in coincidence and singles measurements varied, trends reported for coincidence measure- that is, photon and positron excitation by 1sF and only ments were essentially identical to those observed beta excitation by 14C, is also suggested by the differing susceptibilities to quench, both in degree and in nature during singles measurements. (Table 5). It must be recognized, however, that quenching in the case of the positron emitter was DISCUSSION complicated by the generation of a spectrum of The volume of photon-generating medium per vial secondary electrons as a result of annihilation gamma was found to influence sample count rate as reported rays interacting with the medium, the vial and the PMT previously,tl~ and was therefore kept between 11 and faces. This effect, although unimportant in unquenched 14ml. In this range, the count rate was found to systems, would undoubtedly attenuate the effectiveness fluctuate less than 3 ~ when calculated on the basis of of both colour and chemical quenching agents. The very large increase in narrow window counts may well greatest difference and lowest net counting rate. The l SF counting efficiencies in MS, DMSO and CB have arisen from this phenomenon, which has been (Table 1) indicate that although MS has a marginally shown to generate a significant number of pulses when lower refractive index (1.522) than CB (1.523),tg~ more 18F is external to the C-S hybrid medium, n~ This pulse detectable photons were produced by MS. The MS distribution would be expected to be similar to that may have unusually large refractive indices for specific observed for MS with a 137Cs external standard source wavelength regions in the low visible u.v. region which (662 keV 137Barn X-rays compared with 511 keV 7-rays would lower the theoretical (~erenkov threshold tl°) from positron annihilation). No shift to increased (calculated to be approximately 165keV by the numbers of lower level counts was previously reported method of Rosstl~), but it is not known whether or not for ~SF counting in MS containing nitromethane, but such "resonances" would be of sufficient magnitude to that agent is transparent at the excitation wave length account for the almost 4.5-fold increase observed in for MS, whereas NB has a significant adsorption. DMSO, reported to be an H-bonding quencher of singles counting. A more plausible explanation would appear to be the wave-shifting properties of MS. These salicylate fluorescence, has a u.v. cut-off in the region of properties have previously been demonstrated as a 285nm t~3~ which is well below the absorption and broadening of the pulse height spectrum, ~6'7) and have .emission maxima for MS. NB has a powerful quenching now been further substantiated by the observed differ- effect on emission fluoresence like nitromethane but,
394
ff erenkov counting using a liquid scintillation counter
because of its large absorption at 265 nm (which tails off well into the visible region and hence the yellow colour), NB also acts as a potent colour quenching agent. This strong, broad absorption band not only prevents excitation of MS, but also absorbs fluorescent radiation emitted by excited species. NB was observed to behave strictly as a concentration-dependent quenching agent, whereas studies with taF in D M S O (upon addition of small volumes of MS) indicated that D M S O quench was concentration-dependent in low concentration but concentration-independent in high concentration. The D M S O effect presumably was the result of "darkcomplex" formation3 TM A number of 1aF t~erenkov measurements were made in both CB and D M S O , the latter to observe the quench effect in D M S O - M S mixtures, and the former to compare CB and MS and (~erenkov generating media of similar refractive index. As anticipated, ~SF ~erenkov counting in CB was similar to that reported previously for 36C1 in toluene (6) in that about 50% of the (low) counts were derived not via the (~erenkov mechanism, but via fluorescence of excited solvent species. Pulse height spectra were not plotted, but on the basis of SCR data, CB and D M S O had pulse distributions similar to that observed for 32p and 36C1 in non-fluorescent media. (6' 15) The coincidence and the singles modes of measurement were applied to all experiments, and in all cases similar trends were observed. Major differences existed only in counting efficiencies, when no wave-shifting effect was evident. The singles mode was found to be somewhat undesirable, particularly for measurements where D M S O was present, because of large fluctuations in background count rate due to chemiluminescence. Except in the case of CB in D M S O , coincidence background count-rates of D M S O and D M S O mixtures did not increase by more than 100%. The chemiluminescence produced by C B - D M S O mixtures increased in intensity over a period of approximately 20min, then decayed with a half-time longer than the half-life of ~SF. Uncertainties in chemiluminescence development made it impossible to measure radionuclide activity with precision. SUMMARY The data obtained by the singles and the coincidence modes of counting of 18F and 14C as solutions in MS,
CB and D M S O using a liquid scintillation counter support the conclusion that MS is an efficient (~erenkov-scintillation medium for taF, but that it behaves as an inefficient scintillation fluor for 1"C. Additional studies are required to determine the degree of primary fluorescence in MS when counting ~SF, and to evaluate the role of Compton electrons (from ~SF annihilation gamma rays) in the observed "resistance" of MS to colour/chemical quenching of fluoresence. Acknowledgements--The authors thank Grace E. Wiebe for her assistance in preparation of the manuscript, and gratefully acknowledge the support of the Alexander yon Humboldt Foundation in the form of a Research Fellowship (L.I.W.). Beckman GmbH (Miinchen) is also thanked for providing an LS-9000 for several measurements.
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