Application of a liquid scintillation counter to the measurement of tracing radioactivity in experiments on suspended matter uptake in aquatic environments

Radiop o ec ion - Collogues, olume 37, CI (2002) Cl-1015 Applica ion o a liquid scin illa ion coun e o he measu emen o acing adioac i i y in expe imen s on suspended ma e up ake in aqua ic en i onmen s R. El-M abe , G. Manjon1, J.M. Ab il2 and R. Ga cia-Teno io1 Depa amen o de Física A ómica, Molecula y Nuclea , Facul ad de Física, Uni e si y o Se ille, Apdo. 41060 Se illa, Spain 1 Depa amen o de Física 2, Uni e si y o Se ille, ETSA, A da. Reina Me cedes s/n, 41012 Se illa, Spain 2 Depa amen o de Física Aplicada 1, Uni e si y o Se ille, EUITA, Ca e e a de U e a km 1, 41013 Se illa, Spain Abs ac , The expe imen s on he s udy o he kine ic ans e coe icien s o conse a i e and non-conse a i e adionuclides, in aqua ic en i onmen s, need o de e mine adionuclide ac i i y in la ge numbe o samples. Di e en adioac i i y measu emen echniques can be selec ed o analyses, bu he liquid scin illa ion echnique is p e e ed due o i s easy sample ea men and i s low ime consump ion. Samples a e ini ially aced wi h a known ac i i y o a selec ed adionuclide (239Pu, MIAm o "Tc). The ans e ence o adionuclides, om he wa e o he suspended ma e , is s udied h ough he de e mina ion o he esidual ac i i y in aliquo s o wa e sample, which a e successi ely collec ed wi h ime. In his wo k, we p esen he expe imen al p ocedu es used o he 239Pu, 241Am and "Tc ac i i y measu emen by liquid scin illa ion coun ing, using a Wallac Quan ulus 1220 spec ome e . Resul s o he applica ion o his echnique o aced wa e samples a e also shown in his s udy. 1. INTRODUCTION The so p ion and elease kine ics o 239Pu, 241 Am and "Tc by eshwa e suspended pa icles we e in es iga ed o be e iden i y he biogeochemical p ocesses in ol ed and o unde es and he beha iou o hese elemen s in aqua ic en i onmen s. Time se ies da a can be ob ained om labo a o y s udies and can be analysed o s udy he ans e ence o adionuclides ac i i y om he wa e o he suspended ma e . T acing expe imen s we e pe o med du ing a la ge ime using na u al aqueous suspensions om h ee aqua ic sys ems (Ge gal ese oi , he Guadalqui i i e and he es ua y o he Tin o i e ). Scien i ic li e a u e lis s many examples o expe imen al s udies on up ake kine ics o dissol ed adionuclides by suspended loads [1, 2]. In his wo k we ca y ou a se ies o acing expe imen s wi h 239Pu, 241Am and "Tc by using en i onmen al samples o un il e ed wa e s om h ee aqua ic sys ems in he Sou h o Spain. The expe imen al p ocedu e ensu es he p ese a ion o en i onmen al condi ions. As he expe imen al me hod equi es he de e mina ion o Pu, Am and Tc ac i i ies in a la ge numbe o samples, i is impo an o selec a measu emen echnique wi h low ime consump ion. Recen ly, liquid scin illa ion coun ing has been applied o Pu, Am and Tc-iso opes. In wo ks epo ed by [3], 239+240pu we e simul aneously measu ed using a liquid scin illa ion coun e PERALS™ and wo ex ac i e scin illa o s. Liquid scin illa ion echnique is p e e ed because he edious sample ea men equi ed, e.g. o alpha spec ome y, is no necessa y. The poo esolu ion o his echnique (abou 300 keV) will be no ele an in ou measu emen s because a single alpha emi e , adio ace , will be p esen in ou samples. This echnique allows us o in es iga e newe and in e es ing aspec s o he kine ic o he elec oly ic eac ions o Pu, Am and Tc in esh wa e ecosys ems. I MATERIAL AND METHODS 21 Sampling Wa e samples we e collec ed om h ee di e en aqua ic scena ios om he Sou h o Spain. The i s s a ion was loca ed he Ge gal ese oi (NW om Se ille). The second sample was collec ed in he Guadalqui i i e , nea he own o Se ille. Wa e s we e da k g ey colou ed by he e luen s om ac o ies and u ban sewage in he su oundings. Finally a hi d sample was collec ed in he Tin o es ua y, dose o he A lan ic Ocean, du ing he low ide. A icle published by EDP Sciences and a ailable a h p://www. adiop o ec ion.o g o h p://dx.doi.o g/10.1051/ adiop o/2002118 Cl-1016 RADIOPROTECTION - COLLOQUES A each sample si e, we illed ou 25 1 plas ic bo les. Tempe a u e, pH and elec ical conduc i i y we e measu ed jus a e collec ion. Samples we e hen ca ied o ou labo a o y and s o ed in da k du ing ew hou s. The acing expe imen s gene ally begun a he ollowing mo ning. We used a nephelome e o measu e he suspended ma e con en . To con e NTU in o mg/1 we weigh ed he d y ma e ob ained a e il e ing 50 1 o wa e by using a 0.45 um NUCLEOPORE il e . This calib a ion was epea ed o each new en i onmen . In Table 1 we show he empe a u e, pH, he elec ical conduc i i y, and he suspended ma e con en o collec ed samples, co esponding wi h na u al condi ions p e ailing du ing he sampling. Table 1. Tempe a u e, pH, elec ical conduc i i y and suspended ma e con en in wa e samples measu ed jus a e collec ion. Da a can be conside ed as en i onmen al condi ions. En i onmen Tempe a u e PH Conduc i i y Suspended ma e (°C) (mS) (mg/1) Ge gal ese oi 27.1 8.35 0.46 7.37 Guadalqui i i e 20.4 7.84 1.37 12.6 T in o es ua y 19.4 7.86 66.25 7.62 He ea e he solid ac ion will be conside ed ha o med by suspended pa icles wi h a diame e la ge han 0.45 um. The dissol ed (mo e p ope ly he il e ed) ac ion is o med by he wa e , dissol ed elemen s and pa icles wi h smalle sizes. This is a p ac ical assump ion, since in mos o he expe imen al s udies his po e size is used o sepa a e bo h ac ions. 2.2 Expe imen al p ocedu e The objec i e o his wo k was o e alua e he up ake o 239Pu,241 Am and "Tc by suspended ma e in en i onmen al esh wa e s. In each expe ience, a ypical olume o 1.0 1 o wa e was ans e ed in o a glass beake . The e, he sample was spiked wi h 239Pu, 241 Am o "Tc and con inuously s i ed in o a oid any decan a ion o he suspended ma e . We ollowed he ime e olu ion o he ac i i y concen a ion in he dissol ed phase by aking aliquo s a di e en imes a e spiking. A ypical olume o 45 ml was selec ed o e e y aliquo . Suspended ma e was emo ed by cen i uga ion (20 minu es a 4000 pm). Then, 35 ml o supe na an we e ans e ed in o aplas ic con aine and acidula ed wi h 1ml o HN03 o a oid loses o plu oniumby adso p ion on o he con aine walls. Finally, 5 ml o acidula ed sample was ans e ed in o a plas ic ial and mixed wi h 15 ml o liquid scin illa ion cock ail Pha macia Op iphase 3. Nephelome ic measu emen s p o ed ha he u bidi y o he supe na an wa e s a e cen i uga ion we e simila o he measu ed ones in he il a ed ac ion. Consequen ly, cen i uga ion is an e icien p ocess o sepa a e almos he same ac ion ha we could ob ain by il a ion. Fil a ion canno be used as al e na i e echnique o sepa a e bo h ac ions om each aliquo e, since app oxima ely some 10% in he case o he Pu was e ained by he il e i sel , as p o ed by il a ing dis illa e wa e spiked wi h Pi Al hough cen i uga ion is ou bes choice, i is no a pe ec p ocess, and in some samples we go an incomple e sepa a ion, p o oking some o e -es ima ions o he speci ic ac i i y in he dissol ed ( il e ed) phase. 2.3 G anulome ic analysis and mine al composi ion s udy I is known [4, 5] ha he kine ic ans e coe icien o di ec so p ion depends on he o al a ail su ace o pa icles pe uni olume o solu ion, and on he concen a ion o ac i e si es in he sui ECORAD 2001 Cl-1017 laye o solids. This las depends on he mine al composi ion, ee edges, po es, coa ings, e c. Thus, i is necessa y o handle he pa icle size spec a and he mine al composi ion o na u al occu ing suspended loads o compa e esul s om di e en en i onmen s and o ix he condi ions o applicabili y o he de i ed coe icien s. A g anulome ic analysis o all he samples was ca ied ou by using a Ma e Size ins umen wi h 100 size bands, om 0.01 o 1000 um. A Scanning elec on mic oscope (SEM) was used o s udy he mo phology o na u al pa icles om all he samples. The SEM mic og aph o he ou samples e ealed ha he mos common shape is he sphe ical one. We can app ecia e some agg ega ion o pa icles owing o he p epa a ion p ocedu e. Sal - c ys als a e p een in mos o he samples. Finally, he mine al composi ion was de e mined by X- ay di ac ion (XRD). Some esul s a e summa ised in Table 2. Table 2.- Resul s om g anulome ic and miné alogie analysis. Diame e in (im, co esponds o he mos equen diame e , and d)2 is he diame e co esponding o pa icles wi h he highes pe cen age o olume. The speci ic su ace a ea (S.S.A.) is compu ed assuming a densi y o 2.10 g cm-3 o all he solids. En i onmen Diame e dp. Diame e <b2 S.S.A. Main mine alogical (u n) (um) (m2/g) composi ion Ge gal ese oi 0.30 19 0.28 CaC03 Calci e, Musco i e Guadalqui i i e 0.22 15 1.63 CaC03 Calci e T in o es ua y 0.18 28 0.78 CaS042H20, Bassani e 2.4 Liquid scin illa ion coun e A liquid scin illa ion spec ome e Wallac Quan ulus 1220™ was used o 239Pu, 241 Am and "Tc ac i i y measu emen s. The low backg ound o he de ec o allows he measu emen o low-le el ac i i ies. Mo eo e , alpha and be a ac i i y om he same sample can be measu ed simul aneous and sepa a ely (alpha and be a spec a) by using i s pulse shape analyse (PSA) [6]. The PSA was op imised by measu ing a 239Pu and 241 Am s anda d samples, unde he ypical measu emen condi ions, a di e en alues o he PSA h eshold le el. Resul s a e shown in Fig.l, whe e alpha in e e ence is plo ed e sus PSA le el. Taking in o accoun ha alpha in e e ence is de ined as he ac ion o alpha coun s measu ed in he be a spec um, a A PSA le el o 55 was selec ed in ou wo king condi ions, because using highe alues he alpha in e e ence canno be conside ed negligible and he coun ing e iciency dec eases. In he "Tc ac i i y measu emen s, i is no necessa y o use he PSA. PSA " e 1.- Op imisa ion o he PSA o he Wallac Quan ulus 1220™. Bes esul s do Pu and Am co esponds o PSA = 55 Cl-1018 RADIOPROTECTION - COLLOQUES When PSA is selec ed on, backg ound coun s a e acqui ed in he be a spec um, because 40K (a be a emi e ) o ial walls is he main con ibu o o backg ound o de ec o . We ha e o no e ha alpha / be a sepa a ion is no o ally e icien , and a pa o he be a emissions can be coun ed in he alpha spec um, This ac ion is known as he be a in e e ence and can be conside ed as he main con ibu ion o he alpha backg ound. Ob iously, his be a in e e ence dec eases when inc easing he PSA alue. Fo his eason he PSA alue was selec ed as he maximum alue a which he alpha in e e ence is s ill negligible. On he o he hand, we also s udied he dependence o he alpha backg ound wi h he olume o he sample. Always he dis illed wa e /scin illa o olume a io was 1:1, and he sample olume anged om 8 up o 20 ml. When he op imum PSA le el was selec ed, he alpha backg ound was simila in all cases. Fo ha eason, he sample accep ance o he scin illa o was conside ed as he main pa ame e o selec ing he sample olume. Fo example, he scin illa o used, he Wallac Op iphase Hisa e3, has a sample accep ance o 4 ml o 2M F£N03 in 10 ml o scin illa o . The samples ob ained in ou expe imen s can be conside ed as a 0.4M HN03 solu ion, bu ma e ial o iginally dissol ed in he en i onmen al wa e could be p esen also in he sample o be measu ed and could educe he sample accep ance o scin illa o . This is he case o he Guadalqui i sample, whe e i is possible o app ecia e he p esence o colloids. Fo his eason, a olume o 5 ml was selec ed o he aqueous phase, which was mixed wi h 15 ml o scin illa o . In hese condi ions, alpha backg ound and alpha e iciency we e de e mined in he di e en collec ed wa e samples. Alpha e iciency was de e mined spiking he 5 ml o he wa e samples wi h known amoun s o 239Pu, 241 Am o "Tc. Resul s a e lis ed in Table 3 o Pu measu emen s. Alpha coun ing e iciency anged om 0.67 up o 0.89 being no uni o m, p obably due o a di e en chemical quenching o he sample. Backg ound le el anged om 0.4 cpm in he ese oi wa e s, o 0.8 cpm o he Guadalqui i wa e . A highe le el o 2.4 cpm was measu ed in wa e collec ed om he Tin o es ua y. This esul can be a ibu ed o he p esence o na u al alpha emi e s in he sample [7]. Table 3. Coun ing cha ac e is ics o a Wallac Quan ulus 1220™ in he case o "'Pu, using wa e samples collec ed in di e en en i onmen al scena ios. LID is de de ec ion limi , and MDA is he minimum de ec able ac i i y; coun ing ime was 600 min in all he cases. En i onmen E iciency Backg ound MDA LID (cpm) (Bq li e 1) (cpm) Ge gal 0.67 ± 0.01 0.66 ± 0.03 0.23 0.047 Guadalqui i 0.89 ± 0.03 0.79 + 0.04 0.19 0.051 Tin o 0.67 ± 0.02 2.36 ± 0.06 0.44 0.089 3. RESULTS AND DISCUSSION In igu es 2, 3, 4, 5 and 6 we show he ime e olu ion o 239Pu, 241 Am and "Tc ac i i ies in he dissol ed phase co esponding o he ou se ies o expe imen s. Measu ed alues a e ep esen ed wi h e o ba s. F om Fig. 2, 3, 4 and 5, in loga i hmic scale, h ee di e en egions cha ac e ised by di e en slopes can be dis inguished: i s egion co esponds o a as eac ion, las ing o ew minu es, du ing which ime up o 30% o dissol ed Pu o Am is ans e ed on o suspended solids. This co esponds o a as physical-adso p ion eac ion (Pu o Am is ixed by coulombian o ces on o he ex e nal ee su ace o pa icles); in second egion, o he eac ion go e ns he Pu o Am up ake du ing 10-20 hou s ime, ans e ing up o 30% o emaining Pu in dissol ed o m. This eac ion co esponds o physical adso p ion on o he inne su ace o po es and ee-edges, and o he o ma ion o e e sible unions o a di e en na u e and/o s eng h. In hi d egion, he up ake (up o 50% o he emaining Pu o Am in solu ion) con inues du ing se e al days ( he du a ion o he expe imen s was oo sho o allow a conclusion conce ning he inal cou se o he eac ion). This eac ion may co espond o a slowe p ocess in which p e iously adso bed Pu o Am is ans e ed in o he inne ma ix o he pa icles o o ms o he non- e e sible unions. ECORAD 2001 CT-1019 2.5 2 1.5 1 0.5 0 2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 200 Fig.2 Ri e ~—• * I • 0 5 10 15 20 25 30 Time (hou s) 35 40 45 50 ; Fig.3 Es ua y -•—« I J I 0 5 10 15 20 25 30 Time (hou s) 35 40 45 50 • Fig. 4 Rese oi . I I 10 15 20 25 30 Time (hou s) 35 40 45 50 20 80 100 40 60 j. Time (hou s) '?'*'..'3'4.' Time e olu ion o 239-Pu and 241 -Am ac i i ies (in cpm and mBq) in he dissol ed phase co esponding o e ou senes o expe imen s on 239-Pu and 241-Am up ake (non-conse a i e adio ace s) in na u al aqueous suspensions. Cl-1020 RADIOPROTECTION - COLLOQUES Fig. 6 Ri e ¡ i l i I l 0 50 100 150 200 250 Time (hou s) Fig. 6: Time e olu ion o 99-Tc ac i i ies (in mBq) in he dissol ed phase co esponding o he se ie o expe imen s on conse a i e adio ace (99-Tc) up ake in na u al aqueous suspensions o un il e ed wa e s om Guadalqui i i e . F om Fig.6, he 99-Tc is conside ed as conse a i e adionuclide, he ac ion o he 99-Tc abso bed by he suspended ma e is negligible. 4. CONCLUSIONS In his wo k we applied a use ul expe imen al p ocedu e o s udy he up ake o Pu, Am and Tc by na u al aqueous suspensions, based on he use o liquid scin illa ion echnique. Acknowledgemen s This wo k has been pa ially suppo ed by I+D con ac wi h ENRESA, Spain. Re e ences 1. 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