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Low-level counting by liquid scintillation—I. Tritium measurement in homogeneous systems
International Journal of Applied Radiation and Isotopes, 1969, Vol. 20, pp. 145-156. Pergamon Press. Printed in Northern Ireland
Low-Level Counting by Liquid Scintillation I. Tritium Measurement in Homogeneous Systems A. A. M O G H I S S I , H . L. K E L L E Y , J . E. R E G N I E R . a n d M . W . C A R T E R Southeastern Radiological Health Laboratory, P. O. Box 61, Montgomery, Alabama 36101
(Received 13 August 1968) The significant factors which affect low-level counting of tritium by liquid scintillation have been investigated. In a typical operating mode of the system having a background of 8-5 counts/min, one count/min above background corresponds to 380 pCi/l of water (120 T.R.). The developed system combines convenience of automatic sample counting, room temperature operation, high efficiency, high capacity, and simplicity for effective application in measuring tritium levels in environmental samples. LE G O M P T A G E 2k BAS N I V E A U PAR LA S C I N T I L L A T I O N L I Q U I D E . I. LA M E S U R E DU T R I T I U M DANS LES SYSTI~MES HOMOG]~NES On a recherch6 lea facteurs signifiants qui portent sur le comptage du tritium ~ bas niveau par la scintillation liquide. Dans une fa~on typique de fonctionnement du syst6me oh le compte de base est 8,5 pulsations/min., une pulsation/rain, au dessus de la base correspond ~k380 pCi/l, d'eau (120 T.R.). Le syst~me qu'on a mis ~t point rassemble les facilitds de compter automatiquement les &hantillons, du fonctionnement h la tempdrature ambiante, d'une efficaclt6 61evde et de la simplicit6 pour l'application effective ~ la mesure des niveaux de tritium dans les 6chantillons pris de l'environnement. H 3 M E P E H H E HH3HHX YPOBHEI~ IIOCPE~CTBOM H~H~HOCTHOlPl CI~I/IH THJIJIH I~HI/I--I. H 3 M E P E H H E VPOBHH TPHTHH B F O M O F E H H b I X CI/ICTEMAX I/Icc~e~OBaH~ BamnhIe ~aRTopH, BYt~mnIne Ha HaMepe~n~ (tIO~CqeT) Hn31~oro ypoBn~ TpHTHH Hocpe~CTBOM NNH~HOCTHOI~I CII~HHTHJIJIH~HH. B CnCTeMe ;~JIH TIIIIHqHOPO pemHMa pa6oT~, HseIollle~ ~OH B 8 , 5 IIO~CqeTOB/MIIH., O~Iltt CqeT/MIIH. Cm,i m e ~oHa COOTBeTCTByeT 380 pCi/~IIlTp BO~II~I (120 T . R . ) .
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coe)IHHHeT B eerie BLICOHHI~ HOD~HII[FIeHT HoJIe3HOPO ~e~CTBHI:I II 9(~(~eHTHBHOCTII CO BCeMH y~O6CTBaMH, HoTop~e RamT aBTOMaTHsa~HH c~eTa, pa6oTa IIpH HOMHaTH0i~TeMnepaType n
HpOCTOTa IIpHMeHeHHH. FL~SSIGSZINTILLATIONSZ~HLUNG GERINGER AKTIVIT,~TSMENGEN--I. T R I T I U M M E S S U N G IN H O M O G E N E N SYSTEMEN Die wesentlichen Faktoren, die die Fltisslgszintillatlonsz~ihlung yon geringcn Aktivit~itsmengen yon Tritium beeinflussen sind untersucht worden. Bel elner typlschen Arbeitswelse des Systems mit einer NuUrate von 8,5 Impulsen/min., entspricht ein Impuls/min. fiber die Nullrate etwa 380 pCi/1. Wasser (120 T.R.). Das entwickelte System verbindet die Bequemlichkelt der automatischen Probenz~ihlung mit hohem Wirkungsgrad, hoher Aufnahmef'~ihigkeit und Einfachhelt belm ZS,hlen von Trittium in Umgebungsproben. 145
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A. A. Moghissi, H. L. Kelley, J. E. Regnier and M. IV. Garter
SIncE the discovery of tritium in the atmosphere, (a'2) continuous efforts have been made to develop methods for its convenient determination in environmental samples. LmBY(a) used internal gas counting in conjunction with electrolytic enrichment for the determination of tritium concentrations in rain and other waters. In recent years, internal gas counters have been developed with low backgrounds and high capacities. These counters can measure environmental tritium samples without the necessity of pre-enrichment. Internal gas counters have, however, several disadvantages. Inasmuch as water vapor is an unfavorable counting gas, it is usually reacted with a metal such as magnesium or zinc, ~'4) and the resulting hydrogen is counted or reacted with acetylen0 5) to produce ethane. In both chemical reactions special care is necessary to avoid isotope effects. In addition, the method is elaborate and time consuming. No automatic sample changer has been reported for an internal gas counter and hence the method is not amenable to processing a large number of samples. Also, once a sample is counted and the gas released, a recount of the sample is no longer possible. In recent years, liquid scintillation has been utilized for the determination of tritium. Two approaches have been used for the determination of environmental levels of tritium without enrichment. KAUFMANN, et al. t°~ and BoYGE a n d CAMERON(T) used large-volume liquid scintillation to introduce more water into the sample. TAMERSIs) converted water to benzene by reacting it with calcium carbide followed by a polymerization of the resulting acetylene. Although both methods approached the sensitivity of gas counters, their application has certain of the same disadvantages of gas counting. This paper describes the development of a system for the analysis of tritium in environmental samples which has a sensitivity comparable to gas counters but utilizes conventional liquid-scintillation-counting techniques. C O M P A R I S O N OF L O W - L E V E L COUNTING SYSTEMS The present paper considers homogeneous counting systems. Kecently several suspension
counting procedures have been introduced, and a subsequent paper will discuss a system based on the suspension principle. In order to compare counting systems, "figures of merit" have been utilized. ARNOLD~9~ introduced the factor E2]B as such a number in background dominant determinations, where E is the efficiency and B is the background. K I ~ A ~ n°~ recognized the importance of the capacity of a system for the amount of the radionuclide that can be incorporated into it. BoYcE and CAMERONm considered all three factors in their figure of merit using the volume of water as the third factor. CAMERON(11) recognized the dependence of the tritium concentration of the water on the "figure of merit." In comparing counters with low and high backgrounds, he calculated the counting times for the analysis of water samples with a tritium concentration of 500 and 5000 T R (TR. is the ratio 3H:IH = l:101s), respectively, with a counting error of 5 per cent at 2a confidence level. A conceptual method for comparison of low-level counting systems is the application of the notion of minimum detectable activity (MDA). Assuming the same counting time t for the gross and the background measurements, the counting rate C which will give the same number of counts as the statistical variation of the background is calculated as follows: c. t = VB. t
(1)
where, B = background rate in counts]rain. In general, C = A. M. E (2) where, A = tritium concentration, or specific activity in dpm/ml of water; M = volume of measured sample in ml; E-----counting efficiency in cpm/dpm. T h e combination of equations 1 and 2 yields t ~/B A ~/t -- M . E (3) I f A is replaced by Y expressing the MDA for one-min counting time, Y is obtained in nCi[1 as Ve v _
M.
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(4)
Low-level counting by liquid scintillation where, K is a conversion factor and has a value of 2"22. Equation (4) is the reciprocal of BovcE and CAMERON'Sm equation. In this form, however, Y gains the physical value of the MDA at onesigma confidence level with one-minute counting time and is used subsequently as the figure of merit. EXPERIMENTAL
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Apparatus A Beckman "liquid-scintillation system" was used in this investigation. The major feature of this instrument is the application of RCA 4501 (formerly 8575) phototubes and the associated ambient temperature operation.
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Pulse-height analysis The investigation of several aspects of tritium measurement by liquid scintillation counting required a pulse-height analysis. It was recognized that the coincidence and summing amplifier of the liquid scintillation counter were not necessarily linear in response and the measurements with such a system would be applicable only on a comparative basis. The block diagram of the pulse-heightanalysis system is shown in Fig. 1. A few adjustments to the electronics were made to use the phototubes and coincidence-summingamplifier unit of the liquid-scintillation counter. To make the summing amplifier compatible with the multichannel analyzer the coincidence pulse was lengthened. It was also necessary to add an amplifier stage in the coincidence output circuit. This stage supplied the power for gating the analyzer and the negative bias to the analyzer coincidence input. It is realized that these alterations may have increased the phototube "noise" registered by the analyzer. This would have, however, a negligible effect on the measurements.
Materials Glass vials were obtained from several suppliers. They are made of low potassium glass and are henceforth designated as LKG vials. Plastic vials are made of linear polyethylene and are obtained from Nuclear Chicago Corporation (PEN) and from Packard Instrument (PEP).
FIG. 1. Block diagram of pulse-height-analysis system. 2,5-diphenyloxazole (PPO), p-bis-2-(5-phenyloxazolyl)-benzene (POPOP), p-bis-2- (4methyl-5-phenyloxazolyl)-benzene(MPOPOP), and p-b/s-(0-methylstyryl)-benzene (bis-MSB) were received from Pilot Chemicals. 2-(1-naphthyl)-5-phenyloxazole (~-NPO) was received from Packard Instrument Corporation. Naphthalene was received from Eastman Company, and Matheson Coleman and Bell Company. Spectroscopic and chromatographic grades of dioxane were received from Matheson Coleman and Bell. The tritium standard was received from the National Bureau of Standards and was used to calibrate 4 1. of tritiated water by the method of internal standard with identical system properties~a~L PROCEDURE AND R E S U L T S In liquid scintillation, as with any detection system, a number of factors influence the sensitivity obtainable with the system. The major factors influencing low-level liquidscintillation counting were studied with the object of optimizing each factor to obtain the greatest sensitivity.
Selection of counting vials It has been established that counting vials influence the counting parameters markedly. Glass seems to decrease counting efficiency for tritium counting and increase the background
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A. A. Moghissi, H. L. Kelley, J. E. Regnierand M. W. Carter
as compared to plastic (la). However, no difference in the background was observed between the two plastic vials commercially available for tritium counting. Although PEN vials were smaller and approximately 1 ml of the mixture of the scintillation solution finally selected (see Discussion) was above the neck of the vial, and partially covered by the screw cap, the counting efficiency of this vial for 25-ml samples was 3 per cent higher than that of PEP vials. This result was obtained using 3 vials each of 4 different shipments. The standard deviation of the measurement was 4-0"5 per cent at the 2a confidence level. Consequently, PEN vials were used.
Composition of the liquid scintillation solution Although a number of investigations are known dealing with the optimal composition of the liquid-scintillation mixture for low-level tritium counting, a reevaluation of the composition of the scintillation solution seemed necessary due to the fact that the optimum wavelength of the bialkali phototubes such as RCA 4501 is 3850 A. This is somewhat shorter than that of the S11 type which is the most frequently used phototube with an optimum wavelength of 4300 A. The availability of purer compounds and new primary and secondary solutes as well as room temperature operation seemed to justify such an evaluation. A common solution for water counting is a dioxane-based mixture containing 4 g of PPO, 0.05 g of P O P O P and 100 g of naphthalene/1. of dioxane. <°'a4) After its introduction, M P O P O P replaced P O P O P due to its higher solubility and longer maximum emission wavelength which was more suitable for the S11 type phototubes. BUTLER (14) used 3 m l of water and 13 ml of scintillation solution. The composition of the KAUF~ANN solution <6> was similar. Although more efficient solutions have been reported, the solubility of water in such systems is small. In a preliminary study it was found that 5 ml of water and 20 ml of the above scintillation solution had the smallest Y value. Therefore, this water concentration was utilized in subsequent studies. The optimum concentration of PPO was studied using a solution containing 100 and
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120 g of naphthalene/1, and several secondary solutes. T h e concentration of primary solute was varied from 0 to 9 g/l in 1-g increments. Figure 2 indicates that 6-8 g of PPO is the optimum concentration of primary solute. Higher concentrations did not improve the counting efficiency. The addition of the secondary solute has two objectives. One is the reemission of the absorbed light in a wavelength that is more suitable for the phototube, and in addition, it reduces the effect of quenchers. Four secondary solutes (POPOP, M P O P O P , 0~-NPO and bisMSB) were added in increasing amounts to solutions containing 4 g of PPO and 100 g of naphthalene/l, as well as 7 g of PPO and 120 g of naphthalene/1, respectively. The resulting variation in efficiency with the amount of solute is depicted in Figs. 3a, b, c and d. From the graphs in these figures, it is seen that the addition of the secondary solute improved counting efficiency in every case. Contrary to past experiences, however, P O P O P improved the counting efficiency more than M P O P O P . T h e M P O P O P emission wavelength of 4300 A is obviously too far from the optimum response of the phototubes. An example of a secondary solute which acts as a good wavelength shifter and an inferior antiquencher is 0¢-NPO with an emission wavelength of 4030 A. This is closer to the optimum response of the phototubes than any other secondary solute tested. Figure 3d shows that ~-NPO is comparable with M P O P O P , which as mentioned, due to the long wavelength,
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150
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Fxo. 5. Relationship between the water content and the counting efficiency and Y value. the composition of the liquid-scintillation solution used in this investigation contribute to the apparent discrepancy. The most likely cause for the discrepancy is, however, the application of purer compounds in the present liquidsclntillation-counting system. The presence of other quenchers decreases the optimum concentration of water which in itself is a quencher. From the present investigations, it is concluded that a liquid scintillator containing 6-7 g of PPO, 1"2-1.5 g of bis-MSB, and 120 g of naphthalene/l, of dioxane represents the optimum concentration of the liquid scintillator. It is unnecessary to weigh the components of the above scintillation solution accurately. Within the stated weight range the efficiency of such a solution is constant. Twenty ml of such a solution can incorporate 5 ml of water with a counting efficiency of 23"5 per cent at a background of 8.5 counts/min in the instrument used. Although the mixture was developed for application with ambient temperature operation, its stability down to I°C makes it of use in refrigerated systems also. Background T h e complex nature of the scintillation process requires consideration of several aspects for
a low and reproducible background. The thermal "noise" of the improved phototubes with a coincidence time of less than 50 nsec contributes less than one count/min to the background. It is essential that high-purity dioxane and scintillators be used for the preparation of the liquid scintillation solution for low-level counting of tritium to achieve high efficiencies and avoid possible chemiluminescence. Other factors affecting the background are the nature and geometry of the vial, the volume of the sample, and the counting efficiency, since the background is increased in high-efficiency solutions. Several aspects affecting the background and its reproducibility were investigated, as follows: Phosphorescence. LLOYDtln) showed that phosphorescence is particularly severe in dioxanebased liquid scintillators. In his investigations an S11 phototube was used. The application of bialkali phototubes with considerably higher quantum efficiency made, however, a reevaluation necessary. For consideration of low-level tritium counting, experiments were carried out to count the excitation of the liquid scintillation mixture after exposure to sunlight. An L K G and a PEP vial containing the scintillation mixture were placed on the roof of the laboratory for 4 h r during a summer day. Subsequently, these samples were counted at periodic intervals. A de-excitation was not achieved after 6 weeks. In order that quantitative results could be achieved, the same experiment was repeated except the irradiation time was 10 rain. The results are shown in Figs. 6a and b. Phosphorescence decay required approximately 70hr. In low-level tritium counting the solutions are seldom subject to such severe light exposures; however, in considering the long-lived component of phosphorescence, it is desirable to avoid any light which might excite the scintillation solution. In order that the contribution of vials to the phosphorescence could be determined, another set of experiments was carried out. A glass vial and its contents were irradiated for 10 rain in sunlight, and the contents were transferred into a plastic vial (which was kept in darkness) and counted periodically. In a second experiment, the plastic vial and its contents were
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irradiated, and the contents were transferred into a glass vial. T h e decay of the excitation was similar to that shown in Figs. 6. From the results of these experiments, it is evident that the phosphorescence is a property of the scintillation solution and that the contribution of the vials to the phosphorescence is, if any, small. Pure dioxane was also irradiated in sunlight under similar conditions to the scintillation solution. The decay of the phosphorescence of dioxane is shown in Fig. 6c. The decay is completed within 70 rain. It must be emphasized that the results of phosphorescence experiments do not necessarily reflect the true decay since no pulse-height analysis was attempted during these investigations. Chemiluminescence. In addition to the excitation by appropriate light, there is a possibility of production of excited states during the mixing of the components. This was investigated in the following manner. A liquid-scintillation solution was prepared as usual and 20 per cent of " d e a d " water was added. Twenty-five-ml portions of the mixture were dispensed into counting vials and subsequently counted for 150 min each. The long counting time was necessary to assure a reasonable accuracy. T h e results are presented in Fig. 7 and confirm the existence of chemiluminescence. Although a quantitative study of the decay is not possible with the results presented, they do indicate that chemiluminescence has decayed to a minimum after 500-1000 min. It was of interest to identify the contribution of the polyethylene vials to the chemiluminescence. The liquid scintillation water mixture
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Sample No. Fxo. 8. Chemiluminescence of polyethylene vlal. The lines on the right represent the statistical deviations at 2a level. was prepared in the usual manner and divided into two parts. One part was dispensed into PEP vials and the second into a volumetric flask, and both parts were stored in a dark room. After 24 hr, 95 ml were dispensed from the volumetric flask and counted. This counting was preceded by the counting of the stored PEP vial containing the same mixture. T h e minute effect made it necessary to repeat this process six times to achieve assurance and a reasonable accuracy. The results are shown in Fig. 8 and indicate the production of chemiluminescence as a result of the reaction between the polyethylene vial and the solution. T h e results indicate that the samples must be stored for a few hours, preferably overnight, to minimize the effect of chemiluminescence. Static electricity. Static electricity has been known to be of importance in low-level counting. However, its effect in liquid-scintillation counting apparently has not been investigated. Specifically, no reference to its pulse-height spectrum could be found. The application of plastic vials and the transfer of these vials in an automatic sample changer justified the evaluation of this phenomenon. Due to the complexity of the production and nature of static electricity, no quantitative evaluation of the contribution of this effect to the background nor determination of its decay was possible. I t was found, however, that when the sample was transferred in the sample changer
compartment more than five places, an increase in the background could be observed. It was presumed that this is due to static electricity produced by the conveyor system. T h e pulse height of the static electricity under normal operational conditions was simulated by rubbing a vial three times with a polyester cloth. T h e pulse-height spectrum of this sample as compared to a tritium spectrum is shown in Fig. 9. T h e figure shows that static electricity is produced and has an effect. I t also shows that it must be considered only in dealing with high-efficiency systems. I n other words, it is no surprise that this effect was not considered in the past since probably in most cases it was not measurable. T h e background of the unexcited system and a de-excited background are shown in the same figure. Ttle de-excitation was achieved by several "antistatic" compounds which are found to be equally effective. T h e counting efficiency was not affected by the antistatic spray. As a result of this investigation, all vials which are expected to move in the sample changer through a distance which might introduce static electricity are sprayed with an antistatic compound prior to counting.
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Simulated backgroundsamples. T h e composition of the background sample should be identical to that of the sample to be counted. This would require, in low-level counting, the availability of tritium-free water. Due to the large number of background samples required and because tritium-free water m a y not be available, a substitute was considered. The standard scintillation solution was quenched with dioxane containing carbon tetrachloride. Varying the amount of CC14, it was found that 24 ml of the standard scintillation solution and 1 ml of dioxane containing 13 per cent of CC14 had the same counting efficiency and similar spectrum as the water-containing sample. For these reasons, it can be used for preparation of background samples when the supply of trltium-free water is limited. No difference in the background was noted between the two types of dioxane used in this investigation.
In a high-multiplication system, such as liquid scintillation, a constant check of possible instrument drift is required. It has been the practice in this laboratory to prepare duplicate samples and count a standard sample containing a known amount of tritium activity (S-sample) between them. Originally, this was done by preparing S-samples with identical composition and volume to the low-level samples in PEN vials. It was noticed that background samples which were prepared in the same manner and contained " d e a d " water, became contaminated after a few weeks. For this reason, S-samples in sealed glass vials were considered. Since the counting efficiency of the two type vials (plastic and L K G ) was different, the spectra of both vials containing S-samples were compared. Figure I1 indicates that although the vials have different measured spectra, their difference is small on both ends of the spectrum where an instrument drift may cut a portion of the spectrum. Consequently, glass vials were used for the preparation of S-samples. No attempt was made to investigate the applicability of glass vials for other scintillation solutions or nuclides.
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Short-term stability of counting effeiency It was of special interest to establish the stability of the counting system with time. An experiment to determine the short-term stability of efficiency was carried out. A counting vial containing the optimized scintillation solution, as described in this paper, and a tritium concentration of 6.0 × 105dpm was counted immediately after the solution was placed into the vial and at subsequent intervals up to 70 rain. The changes in counting efficiency are shown in Fig. 10. A stable efficiency seems to be established an hour after injecting the solution into the vial.
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A. A. Moghissi, H. L. Kelley, J. E. Regnier and M. IV. Garter
Purification of water and urine The preparation of water samples is simple and consists of a single distillation. For low-level counting a thorough purification of urine is required. Several agents were investigated for the oxidation of organic compounds in urine. Potassium permanganate and potassium dichromate were ineffective. Barium peroxide caused strong foaming which could not be destroyed by silicon-coated glass beads. The adaptation of the OSBVmq and WERK~_AN method (le) applying potassium persulfate was found to be reasonably safe and effective. The operation was carried out in an all-glass distillation equipment. For low-level counting, a total of 30 ml of urine was required to which 1-2 g of potassium persulfate were added and kept for approximately one hour at 75-95°C. The temperature was raised and approximately 25 ml distilled. The impurities in the distillate were extracted with approximately 15 ml of toluene. The resulting solution was reasonably pure and caused no noticeable quenching. Every batch of persulfate was checked for possible tritium contamination. Samples containing high levels of activity can be counted directly. In this case the optimum concentration of tritiated water in scintillation solution is, however, different than that for pure water. In addition, the counting efficiency for each sample must be determined separately. There are several methods available for counting efficiency determination; the most convenient one is external standardization. Since the composition of urine is not constant, quenching curves for external standardization using a composite urine are not necessarily applicable to other urine samples. On the other hand, if a high degree of accuracy is not required, external standardization is adequate. For high accuracies, an internal standard with identical properties (ISISP) (12) is used with naphthalene-T as the standard activity. Optimization of background and e~ciency The optimum background and efficiency setting is, as mentioned, a function of the activity being measured. In a multichannel liquid-sclntillation counter, it is advisable to choose one of the channels for the counting of samples with very low activities.
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5 10 15 Background- CPM F]O. 12. Relationship between the counting efficiency E and the background and Y value.
In a preliminary experiment, the instrument was adjusted so that the total unquenched tritium spectrum was detected in the tritium channel. Further adjustments were made by the upper and lower discriminators. It is apparent from the tritium curve in Fig. 9 that small changes in the lower discriminator result in large changes in efficiency. The same figure indicates that the background in lower channels is small and thus the contribution of the thermal emission of the phototube to the total background is negligible. The relationship between background and efficiency caused by the variation of the upper discriminator is shown in Fig. 12. The relationship is, as expected, not linear. The optimum Y value is at a background of 6.6 counts]min and an efficiency of 21"5 per cent. For the measurement of samples with higher tritium levels, another channel of the instrument was set at the highest efficiency of 25.4 per cent having a background of 14.2 counts/min. The intermediate channel is set for a counting efficiency of 23"5 per cent and a background of 8"5 counts/min. In a large number of background measurements over a six-month period, it was established that the variation of the background was within the calculated statistical variations at 2a confidence level. DISGUSSION The present investigations indicate that the liquid-scintillation-counting system described
Low-levd counting by liquid scintillation
155
can be used for the measurement of tritium and calculated their performance. In addition, in m a n y natural waters without pre-enrichment. several gas-counting systems have been introIn addition to the previously known ease of duced recently, c1~'18~ From these data and operation of this system, the entire operation Table 1, it is apparent that the method proposed is carried out at room temperature. The in this paper combines sensitivity and conresults of experiments suggest that 20 ml of a venience. The Y values of SERHL counters dioxane solution containing 6-7 g of PPO, are favorable as compared to the reported 1.2-1.5 g of bis-MSB and 120 g of naphtha- direct water counting systems. In liquidlene/1, of dioxane can conveniently incorporate scintillation procedures only TAMER'S~8~ pro5 ml of water. The three channels of the counter cedure is more sensitive. His method requires a can be set for different levels of activity, e.g. sample preparation time of 2-3 days and is for a background of 8"5 counts/rain and counting therefore too elaborate for the processing of a efficiency of 23.5 per cent. Such low back- large number of samples. The Y values of S E R H L systems and those of grounds are achieved when phosphorescence is avoided by hindering the excitation of the gas counters are of the same order of magniscintillation solution by white light. This is tude. Although the Y values of S E R H L systems achieved in our laboratory by performing do not reach the level of the best gas-counting operations involving the scintillation solution systems (0.2 nGi/1), many gas counting systems under red light. This operation may be possible have higher values than those for the SERHL. in other laboratories with little additional effort. If higher activities are determined, the comThe scintillation solution must be kept under parison becomes even more favorable. In this total darkness when not in use. case, for backgrounds which are not too high, The present system offers the advantage of the product M . E is the determining factor the possibility of preparation of duplicate which is for many gas counters lower than those samples with little additional effort. In order for S E R H L systems. to assure the stability of a high-multiplication system such as liquid scintillation, a sample CONCLUSIONS containing a known amount of activity is Various factors which affect low-level countmeasured between the duplicates. The recoming of tritium have been investigated experimended order of counting is: background, mentally, and their effects are presented. The sample, S-sample, duplicate of sample, backcomparison of low-level counting systems is ground. When a large number of samples are discussed, and several systems are compared. counted, the counting time is reduced inasmuch The composition of the liquid-scintillation as the counting time of background samples solution was studied, and the optimum was requires only approximately one-third of the found. Procedures were investigated for a low total counting time. Table 1 is a performance summary of the and reproducible background. The comparison data given by the procedure proposed in this three channels designated as S E R H L I, II, and paper with other low-level tritium counting III, as optimized for different levels of activity. systems are favorable. CAM~.RON~11) has surveyed all low-level tritium The convenience of automatic sample changcounting systems operational through 1965 ing, the possibility of processing a large number of samples, room-temperature operation, T~a3LR 1. Performance of SERHL liquid and the minimal skill and labor requirements scintillation systems make the proposed procedure an advantageous method for monitoring environmental samples B E.M Y SERHL No. counts/rain ml nCi/1 for tritium. Mention of commercial products used in I 6.6 1.05 1.08 connection with work reported in this article II 8.5 1.18 1.11 does not constitute an endorsement by the III 14.2 1.27 1.34 Public Health Service.
156
A. A. Moghissi, H. L. Kelley, d. E. Regnier and M. W. Carter
Acknowledgement--The authors wish to express their appreciation for the technical assistance of Mrs. MAE W. WILLIAMS in performing the laboratory experiments.
REFERENCES 1. FALTING S. and HARTECK P. Z. Naturforschung.
5a, 438 (1950). 2. GRoss• A. v., JOHNSON W. M., WOLFGANGR.. L. and LmBY W. F. Science, N.Y. 113, 1 (1951). 3. KAUFMAN S. and LIBBY W. F. Phys. Rev. 93, 1337 (1954). 4. DUBBSC. A. Anal. Chem. 25, 828 (1953). 5. BAINBmDOEA. E., SANDOVALP. and SuEss H. E. Science, N.Y. 134, 552 (1961). 6. KAUFMANNW. J., Nm A., PARKS G. and HouRs R. M. Tritium in the Physical and Biological Sciences, Vol. I, p. 249 Int. Atomic Energy Agency, Vienna (1962). 7. BOYCE I. S. and CAMERON J. F. Tritium in the Physical and Biological Sciences, Vol. I, p. 231, Int. Atomic Energy Agency, Vienna (1962). 8. TAMERS M., BIBRON R. and DELIBmAS G. Tritum in the Physical and Biological Sciences. Vol. I. p. 303, Int. Atomic Energy Agency, Vienna (1962).
9. ARNOLDJ. R. NSF-NAS Publ. 573 (1958). 10. KINARD F. E. Rev. scient. Instrum. 28, 293 (1957). 11. CAMERONJ. F. A Survey of Systems for Concentration and Low-Background Counting of Tritium in Water. Radiocarbon and Tritium Dating Conference, Pullman, Washington, June (1965). 12. MooHISSI A. A. and CARTER M. W. Anal. Chem. 40, 812 (1968). 13. RAeKIN E. and PACKARDL. E. Proe. Univ. New Mexico Conf. on Organic Scintillation Detectors, p. 216, TID-7612 (1960). 14. BUTLERF. Anal. Chem. 33, 409 (1961). 15. LLOYD R. A., ELLIS S. C. and HALLOWS K. H. Tritium in the Physical and Biological Sciences. Vol. I, p. 263, Int. Atomic Energy Agency, Vienna (1962). 16. OSBURN O. L. and WERK~AN L. H. Ind. Engng Chem. Anal. Ed. 4, 421 (1932). 17. BUTTLAR H. v., FARZlNR K. and WOHLFAHRT H. D. Nud. instrum. Meth. 37, 288 (1965). 18. ALLEN R. A., SMITH D. B., OTLET R. L. and RAWSON D. S. Nucl. instrum. Meth. 45, 61 (1966).
Low-Level Counting by Liquid Scintillation I. Tritium Measurement in Homogeneous Systems A. A. M O G H I S S I , H . L. K E L L E Y , J . E. R E G N I E R . a n d M . W . C A R T E R Southeastern Radiological Health Laboratory, P. O. Box 61, Montgomery, Alabama 36101
(Received 13 August 1968) The significant factors which affect low-level counting of tritium by liquid scintillation have been investigated. In a typical operating mode of the system having a background of 8-5 counts/min, one count/min above background corresponds to 380 pCi/l of water (120 T.R.). The developed system combines convenience of automatic sample counting, room temperature operation, high efficiency, high capacity, and simplicity for effective application in measuring tritium levels in environmental samples. LE G O M P T A G E 2k BAS N I V E A U PAR LA S C I N T I L L A T I O N L I Q U I D E . I. LA M E S U R E DU T R I T I U M DANS LES SYSTI~MES HOMOG]~NES On a recherch6 lea facteurs signifiants qui portent sur le comptage du tritium ~ bas niveau par la scintillation liquide. Dans une fa~on typique de fonctionnement du syst6me oh le compte de base est 8,5 pulsations/min., une pulsation/rain, au dessus de la base correspond ~k380 pCi/l, d'eau (120 T.R.). Le syst~me qu'on a mis ~t point rassemble les facilitds de compter automatiquement les &hantillons, du fonctionnement h la tempdrature ambiante, d'une efficaclt6 61evde et de la simplicit6 pour l'application effective ~ la mesure des niveaux de tritium dans les 6chantillons pris de l'environnement. H 3 M E P E H H E HH3HHX YPOBHEI~ IIOCPE~CTBOM H~H~HOCTHOlPl CI~I/IH THJIJIH I~HI/I--I. H 3 M E P E H H E VPOBHH TPHTHH B F O M O F E H H b I X CI/ICTEMAX I/Icc~e~OBaH~ BamnhIe ~aRTopH, BYt~mnIne Ha HaMepe~n~ (tIO~CqeT) Hn31~oro ypoBn~ TpHTHH Hocpe~CTBOM NNH~HOCTHOI~I CII~HHTHJIJIH~HH. B CnCTeMe ;~JIH TIIIIHqHOPO pemHMa pa6oT~, HseIollle~ ~OH B 8 , 5 IIO~CqeTOB/MIIH., O~Iltt CqeT/MIIH. Cm,i m e ~oHa COOTBeTCTByeT 380 pCi/~IIlTp BO~II~I (120 T . R . ) .
Pa3pa6oTaHna~ CHCTeMa ~ I
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coe)IHHHeT B eerie BLICOHHI~ HOD~HII[FIeHT HoJIe3HOPO ~e~CTBHI:I II 9(~(~eHTHBHOCTII CO BCeMH y~O6CTBaMH, HoTop~e RamT aBTOMaTHsa~HH c~eTa, pa6oTa IIpH HOMHaTH0i~TeMnepaType n
HpOCTOTa IIpHMeHeHHH. FL~SSIGSZINTILLATIONSZ~HLUNG GERINGER AKTIVIT,~TSMENGEN--I. T R I T I U M M E S S U N G IN H O M O G E N E N SYSTEMEN Die wesentlichen Faktoren, die die Fltisslgszintillatlonsz~ihlung yon geringcn Aktivit~itsmengen yon Tritium beeinflussen sind untersucht worden. Bel elner typlschen Arbeitswelse des Systems mit einer NuUrate von 8,5 Impulsen/min., entspricht ein Impuls/min. fiber die Nullrate etwa 380 pCi/1. Wasser (120 T.R.). Das entwickelte System verbindet die Bequemlichkelt der automatischen Probenz~ihlung mit hohem Wirkungsgrad, hoher Aufnahmef'~ihigkeit und Einfachhelt belm ZS,hlen von Trittium in Umgebungsproben. 145
146
A. A. Moghissi, H. L. Kelley, J. E. Regnier and M. IV. Garter
SIncE the discovery of tritium in the atmosphere, (a'2) continuous efforts have been made to develop methods for its convenient determination in environmental samples. LmBY(a) used internal gas counting in conjunction with electrolytic enrichment for the determination of tritium concentrations in rain and other waters. In recent years, internal gas counters have been developed with low backgrounds and high capacities. These counters can measure environmental tritium samples without the necessity of pre-enrichment. Internal gas counters have, however, several disadvantages. Inasmuch as water vapor is an unfavorable counting gas, it is usually reacted with a metal such as magnesium or zinc, ~'4) and the resulting hydrogen is counted or reacted with acetylen0 5) to produce ethane. In both chemical reactions special care is necessary to avoid isotope effects. In addition, the method is elaborate and time consuming. No automatic sample changer has been reported for an internal gas counter and hence the method is not amenable to processing a large number of samples. Also, once a sample is counted and the gas released, a recount of the sample is no longer possible. In recent years, liquid scintillation has been utilized for the determination of tritium. Two approaches have been used for the determination of environmental levels of tritium without enrichment. KAUFMANN, et al. t°~ and BoYGE a n d CAMERON(T) used large-volume liquid scintillation to introduce more water into the sample. TAMERSIs) converted water to benzene by reacting it with calcium carbide followed by a polymerization of the resulting acetylene. Although both methods approached the sensitivity of gas counters, their application has certain of the same disadvantages of gas counting. This paper describes the development of a system for the analysis of tritium in environmental samples which has a sensitivity comparable to gas counters but utilizes conventional liquid-scintillation-counting techniques. C O M P A R I S O N OF L O W - L E V E L COUNTING SYSTEMS The present paper considers homogeneous counting systems. Kecently several suspension
counting procedures have been introduced, and a subsequent paper will discuss a system based on the suspension principle. In order to compare counting systems, "figures of merit" have been utilized. ARNOLD~9~ introduced the factor E2]B as such a number in background dominant determinations, where E is the efficiency and B is the background. K I ~ A ~ n°~ recognized the importance of the capacity of a system for the amount of the radionuclide that can be incorporated into it. BoYcE and CAMERONm considered all three factors in their figure of merit using the volume of water as the third factor. CAMERON(11) recognized the dependence of the tritium concentration of the water on the "figure of merit." In comparing counters with low and high backgrounds, he calculated the counting times for the analysis of water samples with a tritium concentration of 500 and 5000 T R (TR. is the ratio 3H:IH = l:101s), respectively, with a counting error of 5 per cent at 2a confidence level. A conceptual method for comparison of low-level counting systems is the application of the notion of minimum detectable activity (MDA). Assuming the same counting time t for the gross and the background measurements, the counting rate C which will give the same number of counts as the statistical variation of the background is calculated as follows: c. t = VB. t
(1)
where, B = background rate in counts]rain. In general, C = A. M. E (2) where, A = tritium concentration, or specific activity in dpm/ml of water; M = volume of measured sample in ml; E-----counting efficiency in cpm/dpm. T h e combination of equations 1 and 2 yields t ~/B A ~/t -- M . E (3) I f A is replaced by Y expressing the MDA for one-min counting time, Y is obtained in nCi[1 as Ve v _
M.
E
(4)
Low-level counting by liquid scintillation where, K is a conversion factor and has a value of 2"22. Equation (4) is the reciprocal of BovcE and CAMERON'Sm equation. In this form, however, Y gains the physical value of the MDA at onesigma confidence level with one-minute counting time and is used subsequently as the figure of merit. EXPERIMENTAL
147
High Voltage hototu bes
I I
Apparatus A Beckman "liquid-scintillation system" was used in this investigation. The major feature of this instrument is the application of RCA 4501 (formerly 8575) phototubes and the associated ambient temperature operation.
Amplifier Ampl.
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Pulse-height analysis The investigation of several aspects of tritium measurement by liquid scintillation counting required a pulse-height analysis. It was recognized that the coincidence and summing amplifier of the liquid scintillation counter were not necessarily linear in response and the measurements with such a system would be applicable only on a comparative basis. The block diagram of the pulse-heightanalysis system is shown in Fig. 1. A few adjustments to the electronics were made to use the phototubes and coincidence-summingamplifier unit of the liquid-scintillation counter. To make the summing amplifier compatible with the multichannel analyzer the coincidence pulse was lengthened. It was also necessary to add an amplifier stage in the coincidence output circuit. This stage supplied the power for gating the analyzer and the negative bias to the analyzer coincidence input. It is realized that these alterations may have increased the phototube "noise" registered by the analyzer. This would have, however, a negligible effect on the measurements.
Materials Glass vials were obtained from several suppliers. They are made of low potassium glass and are henceforth designated as LKG vials. Plastic vials are made of linear polyethylene and are obtained from Nuclear Chicago Corporation (PEN) and from Packard Instrument (PEP).
FIG. 1. Block diagram of pulse-height-analysis system. 2,5-diphenyloxazole (PPO), p-bis-2-(5-phenyloxazolyl)-benzene (POPOP), p-bis-2- (4methyl-5-phenyloxazolyl)-benzene(MPOPOP), and p-b/s-(0-methylstyryl)-benzene (bis-MSB) were received from Pilot Chemicals. 2-(1-naphthyl)-5-phenyloxazole (~-NPO) was received from Packard Instrument Corporation. Naphthalene was received from Eastman Company, and Matheson Coleman and Bell Company. Spectroscopic and chromatographic grades of dioxane were received from Matheson Coleman and Bell. The tritium standard was received from the National Bureau of Standards and was used to calibrate 4 1. of tritiated water by the method of internal standard with identical system properties~a~L PROCEDURE AND R E S U L T S In liquid scintillation, as with any detection system, a number of factors influence the sensitivity obtainable with the system. The major factors influencing low-level liquidscintillation counting were studied with the object of optimizing each factor to obtain the greatest sensitivity.
Selection of counting vials It has been established that counting vials influence the counting parameters markedly. Glass seems to decrease counting efficiency for tritium counting and increase the background
148
A. A. Moghissi, H. L. Kelley, J. E. Regnierand M. W. Carter
as compared to plastic (la). However, no difference in the background was observed between the two plastic vials commercially available for tritium counting. Although PEN vials were smaller and approximately 1 ml of the mixture of the scintillation solution finally selected (see Discussion) was above the neck of the vial, and partially covered by the screw cap, the counting efficiency of this vial for 25-ml samples was 3 per cent higher than that of PEP vials. This result was obtained using 3 vials each of 4 different shipments. The standard deviation of the measurement was 4-0"5 per cent at the 2a confidence level. Consequently, PEN vials were used.
Composition of the liquid scintillation solution Although a number of investigations are known dealing with the optimal composition of the liquid-scintillation mixture for low-level tritium counting, a reevaluation of the composition of the scintillation solution seemed necessary due to the fact that the optimum wavelength of the bialkali phototubes such as RCA 4501 is 3850 A. This is somewhat shorter than that of the S11 type which is the most frequently used phototube with an optimum wavelength of 4300 A. The availability of purer compounds and new primary and secondary solutes as well as room temperature operation seemed to justify such an evaluation. A common solution for water counting is a dioxane-based mixture containing 4 g of PPO, 0.05 g of P O P O P and 100 g of naphthalene/1. of dioxane. <°'a4) After its introduction, M P O P O P replaced P O P O P due to its higher solubility and longer maximum emission wavelength which was more suitable for the S11 type phototubes. BUTLER (14) used 3 m l of water and 13 ml of scintillation solution. The composition of the KAUF~ANN solution <6> was similar. Although more efficient solutions have been reported, the solubility of water in such systems is small. In a preliminary study it was found that 5 ml of water and 20 ml of the above scintillation solution had the smallest Y value. Therefore, this water concentration was utilized in subsequent studies. The optimum concentration of PPO was studied using a solution containing 100 and
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120 g of naphthalene/1, and several secondary solutes. T h e concentration of primary solute was varied from 0 to 9 g/l in 1-g increments. Figure 2 indicates that 6-8 g of PPO is the optimum concentration of primary solute. Higher concentrations did not improve the counting efficiency. The addition of the secondary solute has two objectives. One is the reemission of the absorbed light in a wavelength that is more suitable for the phototube, and in addition, it reduces the effect of quenchers. Four secondary solutes (POPOP, M P O P O P , 0~-NPO and bisMSB) were added in increasing amounts to solutions containing 4 g of PPO and 100 g of naphthalene/l, as well as 7 g of PPO and 120 g of naphthalene/1, respectively. The resulting variation in efficiency with the amount of solute is depicted in Figs. 3a, b, c and d. From the graphs in these figures, it is seen that the addition of the secondary solute improved counting efficiency in every case. Contrary to past experiences, however, P O P O P improved the counting efficiency more than M P O P O P . T h e M P O P O P emission wavelength of 4300 A is obviously too far from the optimum response of the phototubes. An example of a secondary solute which acts as a good wavelength shifter and an inferior antiquencher is 0¢-NPO with an emission wavelength of 4030 A. This is closer to the optimum response of the phototubes than any other secondary solute tested. Figure 3d shows that ~-NPO is comparable with M P O P O P , which as mentioned, due to the long wavelength,
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150
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Fxo. 5. Relationship between the water content and the counting efficiency and Y value. the composition of the liquid-scintillation solution used in this investigation contribute to the apparent discrepancy. The most likely cause for the discrepancy is, however, the application of purer compounds in the present liquidsclntillation-counting system. The presence of other quenchers decreases the optimum concentration of water which in itself is a quencher. From the present investigations, it is concluded that a liquid scintillator containing 6-7 g of PPO, 1"2-1.5 g of bis-MSB, and 120 g of naphthalene/l, of dioxane represents the optimum concentration of the liquid scintillator. It is unnecessary to weigh the components of the above scintillation solution accurately. Within the stated weight range the efficiency of such a solution is constant. Twenty ml of such a solution can incorporate 5 ml of water with a counting efficiency of 23"5 per cent at a background of 8.5 counts/min in the instrument used. Although the mixture was developed for application with ambient temperature operation, its stability down to I°C makes it of use in refrigerated systems also. Background T h e complex nature of the scintillation process requires consideration of several aspects for
a low and reproducible background. The thermal "noise" of the improved phototubes with a coincidence time of less than 50 nsec contributes less than one count/min to the background. It is essential that high-purity dioxane and scintillators be used for the preparation of the liquid scintillation solution for low-level counting of tritium to achieve high efficiencies and avoid possible chemiluminescence. Other factors affecting the background are the nature and geometry of the vial, the volume of the sample, and the counting efficiency, since the background is increased in high-efficiency solutions. Several aspects affecting the background and its reproducibility were investigated, as follows: Phosphorescence. LLOYDtln) showed that phosphorescence is particularly severe in dioxanebased liquid scintillators. In his investigations an S11 phototube was used. The application of bialkali phototubes with considerably higher quantum efficiency made, however, a reevaluation necessary. For consideration of low-level tritium counting, experiments were carried out to count the excitation of the liquid scintillation mixture after exposure to sunlight. An L K G and a PEP vial containing the scintillation mixture were placed on the roof of the laboratory for 4 h r during a summer day. Subsequently, these samples were counted at periodic intervals. A de-excitation was not achieved after 6 weeks. In order that quantitative results could be achieved, the same experiment was repeated except the irradiation time was 10 rain. The results are shown in Figs. 6a and b. Phosphorescence decay required approximately 70hr. In low-level tritium counting the solutions are seldom subject to such severe light exposures; however, in considering the long-lived component of phosphorescence, it is desirable to avoid any light which might excite the scintillation solution. In order that the contribution of vials to the phosphorescence could be determined, another set of experiments was carried out. A glass vial and its contents were irradiated for 10 rain in sunlight, and the contents were transferred into a plastic vial (which was kept in darkness) and counted periodically. In a second experiment, the plastic vial and its contents were
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irradiated, and the contents were transferred into a glass vial. T h e decay of the excitation was similar to that shown in Figs. 6. From the results of these experiments, it is evident that the phosphorescence is a property of the scintillation solution and that the contribution of the vials to the phosphorescence is, if any, small. Pure dioxane was also irradiated in sunlight under similar conditions to the scintillation solution. The decay of the phosphorescence of dioxane is shown in Fig. 6c. The decay is completed within 70 rain. It must be emphasized that the results of phosphorescence experiments do not necessarily reflect the true decay since no pulse-height analysis was attempted during these investigations. Chemiluminescence. In addition to the excitation by appropriate light, there is a possibility of production of excited states during the mixing of the components. This was investigated in the following manner. A liquid-scintillation solution was prepared as usual and 20 per cent of " d e a d " water was added. Twenty-five-ml portions of the mixture were dispensed into counting vials and subsequently counted for 150 min each. The long counting time was necessary to assure a reasonable accuracy. T h e results are presented in Fig. 7 and confirm the existence of chemiluminescence. Although a quantitative study of the decay is not possible with the results presented, they do indicate that chemiluminescence has decayed to a minimum after 500-1000 min. It was of interest to identify the contribution of the polyethylene vials to the chemiluminescence. The liquid scintillation water mixture
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Sample No. Fxo. 8. Chemiluminescence of polyethylene vlal. The lines on the right represent the statistical deviations at 2a level. was prepared in the usual manner and divided into two parts. One part was dispensed into PEP vials and the second into a volumetric flask, and both parts were stored in a dark room. After 24 hr, 95 ml were dispensed from the volumetric flask and counted. This counting was preceded by the counting of the stored PEP vial containing the same mixture. T h e minute effect made it necessary to repeat this process six times to achieve assurance and a reasonable accuracy. The results are shown in Fig. 8 and indicate the production of chemiluminescence as a result of the reaction between the polyethylene vial and the solution. T h e results indicate that the samples must be stored for a few hours, preferably overnight, to minimize the effect of chemiluminescence. Static electricity. Static electricity has been known to be of importance in low-level counting. However, its effect in liquid-scintillation counting apparently has not been investigated. Specifically, no reference to its pulse-height spectrum could be found. The application of plastic vials and the transfer of these vials in an automatic sample changer justified the evaluation of this phenomenon. Due to the complexity of the production and nature of static electricity, no quantitative evaluation of the contribution of this effect to the background nor determination of its decay was possible. I t was found, however, that when the sample was transferred in the sample changer
compartment more than five places, an increase in the background could be observed. It was presumed that this is due to static electricity produced by the conveyor system. T h e pulse height of the static electricity under normal operational conditions was simulated by rubbing a vial three times with a polyester cloth. T h e pulse-height spectrum of this sample as compared to a tritium spectrum is shown in Fig. 9. T h e figure shows that static electricity is produced and has an effect. I t also shows that it must be considered only in dealing with high-efficiency systems. I n other words, it is no surprise that this effect was not considered in the past since probably in most cases it was not measurable. T h e background of the unexcited system and a de-excited background are shown in the same figure. Ttle de-excitation was achieved by several "antistatic" compounds which are found to be equally effective. T h e counting efficiency was not affected by the antistatic spray. As a result of this investigation, all vials which are expected to move in the sample changer through a distance which might introduce static electricity are sprayed with an antistatic compound prior to counting.
• : Stotic
Electricity(SE)
Io = T r i t i u m
• : S E Removed o = Bockground
I
L
~
Channel
.
No.
FIo. 9. Comparison between the measured spectra of tritium and of static electricity.
153
Low-levelcountingby liquidscintillation Preparation of standards 1.01 l 1.00~
o g9 t0
o
o
.-° 10
I 20
o
I I 30 40 Time - Minutes
I 50
I 60
[ "70
I 8u
Fio, 10. Short-term stability of counting efficiency. R-value is defined as the ratio of counting efficiency at indicated time to the counting efficiency 5 hr after the pipetting of the scintillation solution into the vial.
Simulated backgroundsamples. T h e composition of the background sample should be identical to that of the sample to be counted. This would require, in low-level counting, the availability of tritium-free water. Due to the large number of background samples required and because tritium-free water m a y not be available, a substitute was considered. The standard scintillation solution was quenched with dioxane containing carbon tetrachloride. Varying the amount of CC14, it was found that 24 ml of the standard scintillation solution and 1 ml of dioxane containing 13 per cent of CC14 had the same counting efficiency and similar spectrum as the water-containing sample. For these reasons, it can be used for preparation of background samples when the supply of trltium-free water is limited. No difference in the background was noted between the two types of dioxane used in this investigation.
In a high-multiplication system, such as liquid scintillation, a constant check of possible instrument drift is required. It has been the practice in this laboratory to prepare duplicate samples and count a standard sample containing a known amount of tritium activity (S-sample) between them. Originally, this was done by preparing S-samples with identical composition and volume to the low-level samples in PEN vials. It was noticed that background samples which were prepared in the same manner and contained " d e a d " water, became contaminated after a few weeks. For this reason, S-samples in sealed glass vials were considered. Since the counting efficiency of the two type vials (plastic and L K G ) was different, the spectra of both vials containing S-samples were compared. Figure I1 indicates that although the vials have different measured spectra, their difference is small on both ends of the spectrum where an instrument drift may cut a portion of the spectrum. Consequently, glass vials were used for the preparation of S-samples. No attempt was made to investigate the applicability of glass vials for other scintillation solutions or nuclides.
6000"
5000"
Plas~]c 4000
2 c 3000-
Short-term stability of counting effeiency It was of special interest to establish the stability of the counting system with time. An experiment to determine the short-term stability of efficiency was carried out. A counting vial containing the optimized scintillation solution, as described in this paper, and a tritium concentration of 6.0 × 105dpm was counted immediately after the solution was placed into the vial and at subsequent intervals up to 70 rain. The changes in counting efficiency are shown in Fig. 10. A stable efficiency seems to be established an hour after injecting the solution into the vial.
2000-
1000
&o Channel Number"
1000-
FIo. 11. Comparison between the measured spectra of tritium in glass and polyethylene vials.
154
A. A. Moghissi, H. L. Kelley, J. E. Regnier and M. IV. Garter
Purification of water and urine The preparation of water samples is simple and consists of a single distillation. For low-level counting a thorough purification of urine is required. Several agents were investigated for the oxidation of organic compounds in urine. Potassium permanganate and potassium dichromate were ineffective. Barium peroxide caused strong foaming which could not be destroyed by silicon-coated glass beads. The adaptation of the OSBVmq and WERK~_AN method (le) applying potassium persulfate was found to be reasonably safe and effective. The operation was carried out in an all-glass distillation equipment. For low-level counting, a total of 30 ml of urine was required to which 1-2 g of potassium persulfate were added and kept for approximately one hour at 75-95°C. The temperature was raised and approximately 25 ml distilled. The impurities in the distillate were extracted with approximately 15 ml of toluene. The resulting solution was reasonably pure and caused no noticeable quenching. Every batch of persulfate was checked for possible tritium contamination. Samples containing high levels of activity can be counted directly. In this case the optimum concentration of tritiated water in scintillation solution is, however, different than that for pure water. In addition, the counting efficiency for each sample must be determined separately. There are several methods available for counting efficiency determination; the most convenient one is external standardization. Since the composition of urine is not constant, quenching curves for external standardization using a composite urine are not necessarily applicable to other urine samples. On the other hand, if a high degree of accuracy is not required, external standardization is adequate. For high accuracies, an internal standard with identical properties (ISISP) (12) is used with naphthalene-T as the standard activity. Optimization of background and e~ciency The optimum background and efficiency setting is, as mentioned, a function of the activity being measured. In a multichannel liquid-sclntillation counter, it is advisable to choose one of the channels for the counting of samples with very low activities.
o
30
t 30
20
20 c Y
o~1O
O0
E
___
I
ilo
_ _ _
5 10 15 Background- CPM F]O. 12. Relationship between the counting efficiency E and the background and Y value.
In a preliminary experiment, the instrument was adjusted so that the total unquenched tritium spectrum was detected in the tritium channel. Further adjustments were made by the upper and lower discriminators. It is apparent from the tritium curve in Fig. 9 that small changes in the lower discriminator result in large changes in efficiency. The same figure indicates that the background in lower channels is small and thus the contribution of the thermal emission of the phototube to the total background is negligible. The relationship between background and efficiency caused by the variation of the upper discriminator is shown in Fig. 12. The relationship is, as expected, not linear. The optimum Y value is at a background of 6.6 counts]min and an efficiency of 21"5 per cent. For the measurement of samples with higher tritium levels, another channel of the instrument was set at the highest efficiency of 25.4 per cent having a background of 14.2 counts/min. The intermediate channel is set for a counting efficiency of 23"5 per cent and a background of 8"5 counts/min. In a large number of background measurements over a six-month period, it was established that the variation of the background was within the calculated statistical variations at 2a confidence level. DISGUSSION The present investigations indicate that the liquid-scintillation-counting system described
Low-levd counting by liquid scintillation
155
can be used for the measurement of tritium and calculated their performance. In addition, in m a n y natural waters without pre-enrichment. several gas-counting systems have been introIn addition to the previously known ease of duced recently, c1~'18~ From these data and operation of this system, the entire operation Table 1, it is apparent that the method proposed is carried out at room temperature. The in this paper combines sensitivity and conresults of experiments suggest that 20 ml of a venience. The Y values of SERHL counters dioxane solution containing 6-7 g of PPO, are favorable as compared to the reported 1.2-1.5 g of bis-MSB and 120 g of naphtha- direct water counting systems. In liquidlene/1, of dioxane can conveniently incorporate scintillation procedures only TAMER'S~8~ pro5 ml of water. The three channels of the counter cedure is more sensitive. His method requires a can be set for different levels of activity, e.g. sample preparation time of 2-3 days and is for a background of 8"5 counts/rain and counting therefore too elaborate for the processing of a efficiency of 23.5 per cent. Such low back- large number of samples. The Y values of S E R H L systems and those of grounds are achieved when phosphorescence is avoided by hindering the excitation of the gas counters are of the same order of magniscintillation solution by white light. This is tude. Although the Y values of S E R H L systems achieved in our laboratory by performing do not reach the level of the best gas-counting operations involving the scintillation solution systems (0.2 nGi/1), many gas counting systems under red light. This operation may be possible have higher values than those for the SERHL. in other laboratories with little additional effort. If higher activities are determined, the comThe scintillation solution must be kept under parison becomes even more favorable. In this total darkness when not in use. case, for backgrounds which are not too high, The present system offers the advantage of the product M . E is the determining factor the possibility of preparation of duplicate which is for many gas counters lower than those samples with little additional effort. In order for S E R H L systems. to assure the stability of a high-multiplication system such as liquid scintillation, a sample CONCLUSIONS containing a known amount of activity is Various factors which affect low-level countmeasured between the duplicates. The recoming of tritium have been investigated experimended order of counting is: background, mentally, and their effects are presented. The sample, S-sample, duplicate of sample, backcomparison of low-level counting systems is ground. When a large number of samples are discussed, and several systems are compared. counted, the counting time is reduced inasmuch The composition of the liquid-scintillation as the counting time of background samples solution was studied, and the optimum was requires only approximately one-third of the found. Procedures were investigated for a low total counting time. Table 1 is a performance summary of the and reproducible background. The comparison data given by the procedure proposed in this three channels designated as S E R H L I, II, and paper with other low-level tritium counting III, as optimized for different levels of activity. systems are favorable. CAM~.RON~11) has surveyed all low-level tritium The convenience of automatic sample changcounting systems operational through 1965 ing, the possibility of processing a large number of samples, room-temperature operation, T~a3LR 1. Performance of SERHL liquid and the minimal skill and labor requirements scintillation systems make the proposed procedure an advantageous method for monitoring environmental samples B E.M Y SERHL No. counts/rain ml nCi/1 for tritium. Mention of commercial products used in I 6.6 1.05 1.08 connection with work reported in this article II 8.5 1.18 1.11 does not constitute an endorsement by the III 14.2 1.27 1.34 Public Health Service.
156
A. A. Moghissi, H. L. Kelley, d. E. Regnier and M. W. Carter
Acknowledgement--The authors wish to express their appreciation for the technical assistance of Mrs. MAE W. WILLIAMS in performing the laboratory experiments.
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5a, 438 (1950). 2. GRoss• A. v., JOHNSON W. M., WOLFGANGR.. L. and LmBY W. F. Science, N.Y. 113, 1 (1951). 3. KAUFMAN S. and LIBBY W. F. Phys. Rev. 93, 1337 (1954). 4. DUBBSC. A. Anal. Chem. 25, 828 (1953). 5. BAINBmDOEA. E., SANDOVALP. and SuEss H. E. Science, N.Y. 134, 552 (1961). 6. KAUFMANNW. J., Nm A., PARKS G. and HouRs R. M. Tritium in the Physical and Biological Sciences, Vol. I, p. 249 Int. Atomic Energy Agency, Vienna (1962). 7. BOYCE I. S. and CAMERON J. F. Tritium in the Physical and Biological Sciences, Vol. I, p. 231, Int. Atomic Energy Agency, Vienna (1962). 8. TAMERS M., BIBRON R. and DELIBmAS G. Tritum in the Physical and Biological Sciences. Vol. I. p. 303, Int. Atomic Energy Agency, Vienna (1962).
9. ARNOLDJ. R. NSF-NAS Publ. 573 (1958). 10. KINARD F. E. Rev. scient. Instrum. 28, 293 (1957). 11. CAMERONJ. F. A Survey of Systems for Concentration and Low-Background Counting of Tritium in Water. Radiocarbon and Tritium Dating Conference, Pullman, Washington, June (1965). 12. MooHISSI A. A. and CARTER M. W. Anal. Chem. 40, 812 (1968). 13. RAeKIN E. and PACKARDL. E. Proe. Univ. New Mexico Conf. on Organic Scintillation Detectors, p. 216, TID-7612 (1960). 14. BUTLERF. Anal. Chem. 33, 409 (1961). 15. LLOYD R. A., ELLIS S. C. and HALLOWS K. H. Tritium in the Physical and Biological Sciences. Vol. I, p. 263, Int. Atomic Energy Agency, Vienna (1962). 16. OSBURN O. L. and WERK~AN L. H. Ind. Engng Chem. Anal. Ed. 4, 421 (1932). 17. BUTTLAR H. v., FARZlNR K. and WOHLFAHRT H. D. Nud. instrum. Meth. 37, 288 (1965). 18. ALLEN R. A., SMITH D. B., OTLET R. L. and RAWSON D. S. Nucl. instrum. Meth. 45, 61 (1966).