Measurement of tritium in air by adsorbent

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163

(1979)

463-465,

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NORTH-HOLLAND

PUBLISHING

CO

M E A S U R E M E N T O F T R I T I U M IN A I R BY A D S O R B E N T TAKAHISA KATO Isotope Center, School of Medtcme, Mte Umverstty, Tsu, Japan Received 12 October 1978 and in revised form 20 March 1979 A scintillation method which measures tntium contamination in mr usmg hquld adsorbent is descnbed In this method trmum in t h e a t m o s p h e r e is a d s o r b e d o n t o a n a d s o r b e n t , t h e n m i x e d in a liquid s c i n t i l l a t o r , a n d m e a s u r e d b y a h q m d s c i n t i l l a t i o n

counter The adsorbent has two roles first to adsorb tritium and second to be one component of the liquid scintillator According to th~s method, the accumulated quantity of tritium is proportional to its samphng period and to the sample volume of adsorbent in which tritium is adsorbed uniformly The variation of counting rate to the mixed sample volume in a liquid scintillator as observed using a liquid scintillation counter From the results, the optimum mixed volume of sample, adsorbent for maximum sensitlwty is established, and the detectable concentration of trltmm in air found to be l0 -7 aCl/ml or less

1. Introduction Tritium is used in a variety of applications There is a continued need to measure its concentration both in tracer applications and as a contaminant The measurement of the t n t u m content, in gases and liquids, has been described by many workersl-3). However, it is very difficult to measure in air because of its low concentration and low energy beta-emission Small concentrations of tritium in air are usually measured by ionization chamber 4) into which a fixed volume of atmospheric air is allowed to flow This instrument has good accuracy and is able to measure tritium concentrations in the order o f 10-5 zzCi/ml The method has hmlted sensitivity due to volume limits of the chamber and suffers from increasing background o f the chamber on repeated use At present hquid scintillation counting 1,2) has been mainly used as one o f the most suitable detectors for low energy/J-emitters, because of high detection efficiency To measure tritium in air with a liquid scintillation counter requires trapping of the tritium as a sample This paper describes a method o f obtaining samples of very small concentration o f tritium in air by adsorption, and then measuring them with a hquld scintillation counter

2. Experimental There are many c o m p o u n d s which adsorb tritium and tritium c o m p o u n d s readily The following two prerequisites were established as necessary for such an adsorbent. It must be a powerful tritium adsorbent and be o f high purity

and transparency capable of use in a scintillator solution. Active carbon is an excellent adsorbent, however, being black and opaque reduces its efficiency as a liquid scintillator Ethylene glycol was chosen because o f its adsorbent properties and its ability to be used in liquid scintillation fluors (Bray's solution Fluors, ethylene glycol, dioxane) 5) The method IS as follows a measured a m o u n t of trltiated thymidine was dissolved in scintillator solution and its activity estimated This source was placed in a closed desiccator of known volume. A 10 ml aliquot of ethylene glycol was placed in an open vessel in the desiccator The source was allowed to volatilize thus forming the tritlated air The source was made up to its original volume and its activity again measured Thus the activity of the

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F~g 1 Adsorptive amount of tritium vs adsorbing time in hours for ethylene glycol

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air was calculated. 0.2 ml samples of glycol were periodically withdrawn, added to 9.8 ml of Bray's solution and the activity measured in the liquid scintdlatlon counter. These measurements were repeated several t~mes The lmtlal measunng period per cycle was about two months. To speed up subsequent cycles, an air pump was employed m the closed desiccator and as a result, the period was shortened to less than a day The results w~th ethylene glycol are shown in fig. 1. The curve indicates that the adsorbed amount of tritium is d~rectly proportional to its volatilized amount which is just proportional to the timeperiod for volatilization. Using these data the amount of volatilized tntmm in the desiccator, the adsorbed tritmm in the ethylene glycol and the adsorption coefficient can be estimated. Typical values are obtained as follows" the actlvlty of tritium source is 0.232/zC1. The activity of volatdized tritmm from the source is 1.46x10-2/~Ci Tntmm concentration in the desiccator was 4 76 × 10-6/zCi/ml. The adsorption coefficient is thus calculated as 2 77 × 10 -3, which is the amount of adsorbed tritium, dwided by 1 46×10 -2 giving fraction as 19 0% The author, in the first experiment as described above, has been able to trap the tritium for sampling using as adsorbent ethylene glycol Measurement was w~th a liquid scintillation counter using Bray's dioxane solution m which the adsorbent was used at a 2% concentratton In order to increase the sensitivity of th~s method, the following experiment was performed. In the sampling, if an adsorbent adsorbs tntmm w~th a fixed concentration, the activity of tritium per vial is proportional to sampled amount of the adsorbent in a vial Therefore, if the quantity of adsorbent m the scintillator is increased, the quantity of tritmm per vial is more. Hence, by increasing the counting rate of such a sample m the vial, Its sensitivity might be enhanced In the second experiment, the variation in countmg rate, accompanied by the increase of the adsorbent mixture in the scmtdlator sample per vial, is increased The adsorbent with tritium was added to Bray's scintillator, increasing ~ts quantity from 0 2 ml to 10 ml per vial. The counting rate due to activity in these samples was measured The same procedure was also used in measurement of toluene scmtillator The results are shown in fig 2 Curve I is

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Fig 2 Showing the variation m counting rate with changing volume of tntmm-loaded ethylene glycol m a wal containing a hqmd scmtdlator Curve I ts for toluene scmtdlator and curve II for dmxane

toluene scmtdlator and curve II Bray's solution A fixed concentration of tritium is added to a varying volume of ethylene glycol m the liquid scintillator It is seen that m the low concentration of the adsorbent, the curve rises linearly, i.e. the counting rate ~s proportional to concentration, but as the concentration increases, the count rate shows devlaUon from linear and becomes non-hnear Beyond saturaUon, it becomes more non-hnear Th~s saturated value of ethylene glycol occurs in a mixture of 10 ml of scmtdlator and has a proportion of about 4 ml of dloxane to 6 ml of toluene This concentration of the dloxane scmtdlator is 20 times the standard amount (0.2 ml) of Bray's. Accordingly, the quantity of tritium able to absorb in ethylene glycol is 20 times greater, but because the counting efficiency falls due to the increased quenching, the counting rate is indeed only about 7 times, therefore, sensitwlty increases by the same factor of 7 Since the sensitivity w~th 0.2 ml of the standard was of the order of 10 - 6 / . t C l / m l , It IS p o s s i b l e t o increase by the order of 10 -7/~Ci/ml in this case. As indicated in curves I and II of fig 2, the sensltwlty of toluene scintillator (curve I) can increase by a factor of 10 over that of the standard Bray's solution (curve II)

3. Discussion Ethylene glycol is a good adsorbent for tntmm m

M E A S U R E M E N T OF T R I T I U M IN AIR

air because of its homogeneity and low volatility Because of its miscibility and high efficiency it functions well as a scintillator solution. The sensitivity of the method depends upon the concentration of tritium trapped in the adsorbent, the sample volume and the efficiency of the detector. In th~s measurement the adsorbed fraction of ethylene glycol is found to be 19.0% Therefore, in practice, the product of the total amount of tritium in the room and this coefficient ~s the maximum possible amount of sample needed in order to increase the sensitivity If contaminated air with a maximum quantity of tritium IS trapped in this adsorbent and if the optimum sample volume which corresponds to the

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saturation point for maximum counting rate is measured, highest counting sensttlVlty can be obtained. I wish to thank Mr David B. Izard for his assistance with the Enghsh language

References 1) John E Noakes, M P Neary and J D Spauldmg, Nucl Instr and Meth 109 (1973) 177 2) T lwakura and Y Kaslda, Radioisotopes 24 (1975) 70 3) R A Jalbert and R D Hlebert, Nucl instr and Meth 96 (1971) 61 4) R A Jalbert, Proc 23rd Conf on Remote System Technology (1975) p 89 5) G Bray, Anal Blochem 1 (1960) 279