Application Note: A Rapid Procedure for Screening Transuranium Nuclides in Urine Using Actinide Resin and Low Level
 

Application Note: A Rapid Procedure for Screening Transuranium Nuclides in Urine Using Actinide Resin and Low Level

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One of the most extensive tasks is the field of bioassay analysis is the determination of pure alpha- (and beta-) emitting radionuclides from the nuclear fuel cycle such as (234)U and (235)U, or ...

One of the most extensive tasks is the field of bioassay analysis is the determination of pure alpha- (and beta-) emitting radionuclides from the nuclear fuel cycle such as (234)U and (235)U, or anthropogenic (239)Pu and (241)Am in urine samples. However, any radiochemical method, which is applied to perform such analyses, has to be highly sensitive since even small amounts of incorporated radionuclides decaying by alpha emission may contribute to harmful doses to human organs.

Since alpha radiation has an extremely short penetration length in water and solid substances, direct counting of a salt residue of dry ashed urine is not possible. Therefore, complex radiochemical techniques have been developed for efficient separation of the transuranium elements from the bulk matrix. However, in addition to several purification steps, these methods require the production of almost weightless planar sources (e.g. via electrolytic deposition) in order to perform radioassays with proportional or surface barrier detector. In contrast to the extensive preparative techniques, fast methods using alpha/beta-LSC are of increasing interest. Due to the efficient detection of alpha emitters in a liquid scintillation cocktail, extensive radiochemical purification procedures are not necessary provided the sample is homogeneous in the liquid scintillation cocktail.

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Application Note: A Rapid Procedure for Screening Transuranium Nuclides in Urine Using Actinide Resin and Low Level Application Note: A Rapid Procedure for Screening Transuranium Nuclides in Urine Using Actinide Resin and Low Level Document Transcript

  • a p p l i c at i o n N o t e Liquid Scintillation Counting A Rapid Procedure for Screening Transuranium Authors Nuclides in Urine Using J. Eikenberg, I. Zumsteg, M. Rüthi Actinide Resin and Low S. Bajo, Paul Scherrer Institute CH-5232 Villigen (PSI) Switzerland Level a/b-LSC C. J. Passo PerkinElmer, Inc. Waltham, MA, USA M. J. Fern Eichrom Industries, Inc. Darien Illinois, USAAbstract IntroductionA fast and simple radiochemical procedure for determining One of the most extensive tasks in the field of bioassay analysisa-emitting nuclides in urine is presented. The method is is the determination of pure a- (and b-) emitting radionuclidesbased on a/b-LSC using the Tri-Carb® with alpha/beta from the nuclear fuel cycle such as 234U and 235U, or anthro-discrimination as well as on the high selectivity of Eichrom’s pogenic 239Pu and 241Am in urine samples. However, anyactinide resin for heavy isotopes with atomic numbers above radiochemical method, which is applied to perform such90 (i.e., Th, U, transuranium nuclides). Under optimized analyses, has to be highly sensitive since even small amountspulse shape discriminator settings, a very efficient a/b of incorporated radionuclides decaying by a emission maydiscrimination of only 0.1% spill-over of b into a was contribute to harmful doses to human organs.1obtained at 95% counting efficiency for a-pulses. Inaddition, using a mixture of the scintillation liquids, Gold™ AB Since a radiation has an extremely short penetration lengthand Ultima Gold F, the a-peak resolution turned out to in water and solid substances, direct counting of a salt residuebe rather high (40 keV FWHM for 239Pu on LSC scale). This of dry ashed urine is not possible. Therefore, complexallows the Tri-Carb with alpha/beta discrimination to be radiochemical techniques have been developed for efficientused as a spectrometer for screening either transuranium separation of the transuranium elements from the bulknuclides with energies exceeding 5 MeV such as 239Pu, matrix.2,3 However, in addition to several purification steps,241 Am, 244Cm or uranium isotopes between 4-5 MeV (238U, these methods require the production of almost weightless236 U, 235U, 234U) in small counting windows of 120 keV each. planar sources (e.g., via electrolytic deposition) in orderUnder these conditions, very low background count rates to perform radioassays with proportional or surface barrierof 0.05 CPM are obtained in each window, resulting in a detectors.high figure of merit (E2/B) of » 180,000 and a detection limit In contrast to the extensive preparative techniques, fastas low as 1.5 mBq/L (or 0.04 pCi/L) in a 500 minute count methods using a/b-LSC are of increasing interest.4,5 Due tointerval. the efficient detection of a emitters in a liquid scintillation cocktail, extensive radiochemical purification procedures are not necessary provided the sample is homogeneous in the liquid scintillation cocktail. Although for a counting, liquid
  • scintillation detectors are mainly used only as gross analyzers, Methods and Materials they are highly suitable for screening alpha activities in Composition of the Actinide Resin bioassay samples since low detection limits of a few mBq/L 34 U, 210Po or 226Ra from the U and Th decay Inc., Darien, Illinois, USA), is shown in Figure 1, a blank sample. In obtained. This could be shown R = 2-ethylhexyl. pre-chromatographic sup- procedure described here is based on the can be human urine, the activi- where using a simple The The separation these natural α emitters chemistryrange actinide coprecipitationhas a nominal particle size strong affinity of the resin for actinides concentration typically with port used in this study by extraordinarily 0.1 and 20 mBq/L,) mainly caused by 210Po performed using100-150 microns. Ca3(PO4 2.6 A radioassay was distribution of PerkinElmer’s (particularly in the tri-, tetra- and hexavalent oxidation states), a uptake with plant diet.9 low level model Tri-Carb with alpha/beta discrimination The Separation Procedure even from strongly acidic solutions.10 Actinide resin is com- summarizes the typical range of activities discrimination radiochemical procedure is shown liquid extractant (containing a diphosphonic acid equipped with highly efficient The schematic between a posed of a inrresponding countradiation.7,8 However, the Figure 2. The individual steps will be explained group) coated onto a chromatographic support. and b rates for α/β-LSC) for presence of anthropogenic functional in t important natural α emitting nuclides in detail below. The extractant, trade named DIPEX™ (Eichrom Industries, nuclides such as Pu, Am or Cm in urine using gross 239 241 244 nce typical blank count rates are as low asM with thecountingTri-Carb 2550TR/AB, Packard methods can be justified only oxidation of count Partial if the a net the organic matter: 0.5 LIllinois, USA), is shown in Figure 1, where R = Inc., Darien, ribution from natural components signifi- rate clearly exceeds those produced by decay of the naturally L glass beaker and The chromatographic support used in this study urine is transferred into a 1 2-ethylhexyl. xceeds reagent blank radionuclides such as 238U,mLU, 210Po or3 226Ra occurring values. This reduces 100 234 65% HNO is added. The has a nominal particle size distribution of 100-150 microns. beaker is then ting sensitivity for screening anthropogenic covered with watch glass and gently boiled for two from the In this work,decay series in a blank sample. In humans in bioassay studies. U and Th Ra and Po hours under infrared light. Subsequently, Separation Procedure The the solu-clides are urine, the activities of these natural has to cool down to room temperature. automatically eliminated using tion a emitters typicallyon chromatography without theand 20of range between 0.1 need mBq/L, mainly caused by 210Po The schematic radiochemical procedure is shown in Figure 2.purification steps. Sorption on actinide resin: 200 mg of actinide resin steps will be explained in detail below. The individual and Ra uptake with plant diet. 226 9 is added and the solution is stirred for four hours tods and Materials ensure sorption equilibrium (see Figure 3). oxidation of the organic matter: 0.5 L urine is trans- Partial Table 1 summarizes the typical range of activities (and corresponding count rates for a/b-LSC) for the mostresin from the ferred into a 1 L glass beaker and 100 mL 65% HNO3 is added.sition of the Actinide Resin Separation of the important solution: Thearation procedure described here nuclides in urine. Sinceof the resin containing the The beaker is then covered with watch glass and gently boiled natural a emitting is based on separation typical blank actinides fromaordinarilycount rates areof the resin 0.1 CPM with of the solution with strong affinity as low as for the bulk the Tri-Carb is obtained via filtration on under infrared light. Subsequently, the solution for two hourss (particularly in the tri-, tetra- and hexavalent 0.3 µm (25 mm diameter) WCN cellulose cool down to room temperature. has to nitrate n states), even from strongly acidic solu- contribution from natural Inc., Ann Arbor, Michi- alpha/beta discrimination, the membrane filters (WhatmanActinide resin is composedsignificantly exceeds reagent mounted on a 25 This glass frit membrane components of a liquid extrac- gan, USA) blank values. mm Sorption on actinide resin: 200 mg of actinide resin is taining a diphosphonic acid functional group) reduces the counting sensitivity for screening anthropogenicrate, the filtration is holder. To increase the filtration nto a chromatographic support. The extrac- performed under a vacuum using a water and the solution is stirred for four hours to ensure added pump. de named actinides in bioassay studies. In this work, Ra and Po radio- DIPEX™ (Eichrom Industries, sorption equilibrium (see Figure 3). nuclides are automatically eliminated using extraction chromatography without the need of further purification steps. Separation of the resin from the solution: The separation of the resin containing the actinides from the bulk of the solution is obtained via filtration on 0.3 μm (25 mm diameter) WCN cellulose nitrate membrane filters (Whatman Inc., Ann Arbor, Michigan, USA) mounted on a 25 mm glass frit membrane holder. To increase the filtration rate, the filtration is performed under a vacuum using a water pump. Table 1. Table 1. Typical range of the activities of natural a emitting radionuclides in human urine. of the activities of natural α emitting radionuclides in human urine. Typical range 2 2
  • After filtration, the resin on the filter will show a yellow color due to adsorption of some organic substances during thethe resin onthe filterTheshow a yellow color After After filtration, exposure process. will filtrate a filtration, the resin on the filter will show (solution)colorthen be removed. organicsome organic the yellow can adsorption adsorption of substances during due to due to of some substances during the exposure process. Thethen be removed. exposure process. The filtrate (solution) can filtrate Stripping can then be removed. (solution)of the reagent from the resin: Stripping of the reagent from the inert support (polymeric Stripping of the reagent from the resin: Stripping of the substrate) isof the reagent from the(polymeric steps Stripping performed in three consecutive reagent from the inert support resin: Stripping substrate) is using 5performed from thein each step. As in 5 mL isopropanol mL isopropanolconsecutive steps using the of the reagent in three inert support (polymeric Figure 1. Structure of the actinide resin. preceding step, the filtration three consecutive steps substrate) is performed in is performed under a in each step. As in the preceding step, the filtration is Figure 1. Structure of the actinide 1. vacuum5using a water a vacuumeach step. solutionThe organic using performed under pump. The organic As in the mL isopropanol in using a water pump. Figure resin. will be yellow will (after dissolution dissolution of a solution and be yellow and (afterof the reagent the reagent Structure of the actinide resin. preceding step, the filtration is performed under from its bed) the water pump. will beorganic solution support vacuum using a substrate willThe white in color.The from its bed) the substrate be white in color. The support canbe discarded. can then then be discarded. will be yellow and (after dissolution of the reagent from its bed) the of the solution for LSC: The organic solution is Preparation substrate will be white in color. Preparation of the solution for LSC: The organic The support can into abe discarded. transferred then 100 mL wideneck quartz glass flask and is solution is transferred into a 100 mL wideneck quartz glass flask and is taken to dryness on The organic Preparation 65% HNO3 and 1for LSC: a heating 2SO4 is then taken to dryness on a heating plate using additional infrared. Five mL of the solution mL plate using additional infrared. Fiveofwideneck quartz solution is transferred into a the reagent.65% solution is gently mL HNO3 concentrated H added for oxidation of 100 mL and 1 mL of concentrated H2SO4 is thenThea heating added for glass boiled and slowly evaporated to dryness until a flask and is taken to dryness on oxidation ofadditional infrared. solution is gently thoroughly the reagent. The plate using transparent residue is Five mL 65% HNO3is not boiled clear,of concentrated H SO drynessIfadded a and slowly evaporated to is then until for obtained. the color and 1 transparent, addition of 30% 4of H2O2 to the cooled residue mL thoroughly clear, transparent residue is obtained. If 2 oxidation of the reagent. The solution is dissolved in 2 mL is helpful. The transparent residue is then gently the color is not transparent, addition of 30% of H2O2 boiled0.5 M HCl. and slowly evaporated to dryness until a to the cooled residue is helpful. The transparent thoroughly clear, transparent residue is obtained. If residue is then dissolved in 2 mL 0.5 M HCl. Cocktail transparent, addition scintillation 2O2 the color is notpreparation and liquid of 30% of Hcounting: to theThe sample solutionis helpful. The transparent cooled residue is transferred into a 20 mL plastic Cocktailispreparation and 2liquid scintillation residue then dissolved in mL 0.5 Mof Ultima Gold AB scintillation vial containing a mixture HCl. counting: The sample solution is transferred into a 20 mL plastic scintillation(5 mL). This mixture yields optimal a/b (12 mL) and Gold F vial containing a mixture Cocktail preparation and and peakscintillation pulse shape discrimination liquid resolution of Ultima Gold AB (12 mL) and Ultima Gold Fof the a counting: The Figure yields optimal α/β pulse (5 mL). This(seesample 4). The vial istransferred into a and pulses mixture solution is shaken until aqueous 20 mLorganic phases are mixed completely and a mixture solution plastic scintillation vial containing the α shape discrimination and peak resolution ofthe cocktail pulses (see Gold AB (12 mL) about until Gold of UltimaFigure a temperatureis shaken10 °C in a refrigerator. is cooled to 4). The vial ofand Ultima aque- F ous and organic mixtureare mixed completely and (5 mL). This phases yieldscocktail must be checked for Prior to measurement, the optimal α/β pulse the cocktail separation and peak resolution of the transparent shape phase solution is(solution must be homogeneous,α discrimination cooled to a temperature of about 10(see in a refrigerator. Prior to measurement, pulsesand colorless); 4). The vialscintillation counting is performed °C Figure finally liquid is shaken until aque- the cocktail the Tri-Carb Alpha/beta discrimination and ous and organic be checked mixed completelyoption in the using must phases are for phase separation (solution must be homogeneous, transparent and of the cocktail solution is cooled to a temperature a/b-mode. colorless);°C in a refrigerator. Prior to measurement, about 10 finally liquid scintillation counting is the cocktail must be checked for phase separation performed using the Tri-Carb 2550TR/AB in the Results and Discussion α/β-mode. must be homogeneous, transparent and (solution colorless); finally liquid of various experiments to is A detailed description scintillation counting study the performed using the Tri-Carb 2550TR/AB in the sensitivity of different chemical parameters, uptake capacity Results andresin, and counting conditions can be obtained from of the Discussion α/β-mode. Eikenberg, et al. 19 A detailed description of various experiments to Results and Discussion study the sensitivity of different chemical param- eters, uptake capacity of the resin, and counting conditions can be obtained from Eikenberg, et al.19 to A detailed description of various experiments study the sensitivity of different chemical param- eters, uptake capacity of the resin, and counting Figure 2. Schematic illustration of the fast procedure for conditions can be obtained from Eikenberg, et al.19 Figure 2. separation of actinides from urine. Schematic illustration of the fast procedure for separation of actinides from urine. Figure 2. Schematic illustration of the fast procedure for separation of actinides from urine. 33 3
  • close to 100% at the crossover setting, high values of 95% were still obtained at the higher (140 ns) discriminator setting (see the “Analysis of Counting Sensitivities” section). value of 140 ns. While the counting efficiencies were close to 100%of Th, Pa, U, Pu, Am, Cm: The chemical yield or at the crossover setting, high values value of 140 ns. While the counting efficiencies were Recoveries of 95% were stillTh, Pa, U,at the higher (140 chemi- Recoveries of obtained Pu, Am, Cm: The ns) was close to 100% at theacrossover radiochemical analysis recovery following complete setting, high values discriminatoror recovery the “Analysiscomplete radio- cal yield setting (see following a of Counting of determined from the addition of radiospikes of known 95% were still obtained at the higher (140 ns) Sensitivities”analysis was determined from the addition chemical section). discriminator setting (seesample (Table 2). In particular, two activity to a blank urine the “Analysis of Counting of radiospikes of known activity to a blank urine Sensitivities”studied to check on chemical recovery: steps were section). Recoveries of Th,2). In particular, two steps chemi- sample (Table Pa, U, Pu, Am, Cm: The were stud- cal yield check on chemicalyield oncomplete radio- ied to or•recovery following a the resin The adsorption recovery: Recoveries of Th, Pa, U, Pu, Am, Cm: The chemi- chemical analysisoverall (total) chemical recovery • The was determined from the addition cal yield adsorption yield on the a complete radio- • The or recovery following to a of radiospikes of known activityresin blank urine chemical overall In particular, two from the addition analysis was determined • obtain the reproducibility of thesteps were spike sample (Table 2). (total) chemical recovery all stud- experi- ToThe results, of radiospikes chemical recovery: to a blank urine to check on of known activity ied ments were repeated at least four times for each radio- sample (Table 2). In particular, two steps were stud- iedTo obtain on chemical recovery: results, all spike to check the reproducibility of the nuclide. The sorption yield on the resin was by means of • The adsorptionwere repeated itself or via decay or ingrowth experiments yield nuclide at least four times for g-spectrometry of the on the resin Figure 3. • The daughter(total) chemical recovery on the resin of overall nuclides.The sorption yield each radionuclide. Kinetic uptake experiments: determination of sorption half- • The adsorptionof γ-spectrometry of the nuclide itself was by means yield on the resin • Thevia2decay or ingrowth ofactinide resin has an extremely Table clearly indicates that recovery all spike or overall (total) chemical the results, lives for U and Am. To obtain the reproducibility of daughter nuclides. experiments were repeated at least four times a very strong affinity for all tested actinides even from for Figure 3. each radionuclide. The sorption the results, allresin an To strong 2the reproducibilitywith yield salt content (average Table acidic urine solution that actinide the spike obtain clearly indicates of high on resin has Kinetic uptake experiments: determination of sorption half- experiments strongrepeated2at all shows actinidesfor almost salt contentwere affinity for least four times even wasextremely of γ-spectrometry of tested that there is by means 30 g/L). Table also the nuclide itself Figure 3.Figure 3. Kinetic uptake experiments: determination of sorption lives for U and Am. or via decay orstrong acidic urine yield on for high salt each radionuclide. The sorptionsolution with aresin no differenceingrowth thedaughter yields the complete from a very between of chemical nuclides.halflives for U and Am. Kinetic uptake experiments: determination of sorption half- was by means of γ-spectrometry ofThis means that additional analysis and the adsorption yield. the nuclide itself content (average salt content 30 g/L). Table 2 also lives for U and Am. or via decay or ingrowthstripping, (ii) digestion and (iii) chemical losses fromalmost daughter nuclides. an the shows that there is (i) of actinide resin has Table 2 clearly indicates thatno difference between extremely strong affinity for all tested actinides even the chemical yields for a complete analysis and transfer into the liquid scintillation vial are insignificant. Table very strong acidic urine solution withto routine This 2 clearly indicatesbe easily adopted high chemi- adsorption can hence that actinide resin has an from a methodyield. This means that additional salt contentlosses from (i) stripping, g/L). Table 2 even extremely strongsalt contentall tested actinidesalso(iii) laboratory use. affinity for 30 (ii) digestion and cal (average shows very strong acidic no difference between salt from athat there is almosturine solution with highthe chemical(averagefor acontent 30 g/L). Table 2 the content yields salt complete analysis and also adsorption yield. is almost no that additional chemi- shows that there This means difference between the cal losses from (i)for a complete analysisand (iii) chemical yields stripping, (ii) digestion and the adsorption yield. This means that additional chemi- cal losses from (i) stripping, (ii) digestion and (iii) Figure 4. Smoothed liquid scintillation spectrum of 239Pu and 244Cm obtained with Packard Tri-Carb 2550TR/AB. Figure 4.Figure 4. Smoothed liquid scintillation spectrum of 239Pu and 244Cm Smoothed liquidYield Investigations Pu and 244Cm Chemical scintillation spectrum of 239obtained with PerkinElmer Tri-Carb 2550TR/AB. obtained with Packard Tri-Carb 2550TR/AB. Figure 4. Direct spike experiments:Chemical liquid scintillation spectrumThe Pu and 244Cm Smoothed Yield Investigations of 239 counting efficien-Direct spike experiments: The counting efficienciesspiked cies were determined with radiolabeled were obtained with Packard Tri-Carb 2550TR/AB.Chemical Yieldradiolabeled spiked solutions to simulate solutions added to cocktail mixtures added todetermined with Investigations routine chemical analysis. The cocktails were mea- Table 2. Chemical recoveries obtained from a complete analyticalcocktail mixtures to simulate routine chemical analysis. The Table 2.Chemicaleach under two different discriminator set- sured Yield Investigations counting efficien-Direct spike experiments: under two different discrimi-cocktails were measured each The procedure. Chemical recoveries obtained from a complete analyticalcies werei.e., i.e., at the crossover point (125ns) and at a tings, determined with radiolabeled spikednator settings, at the crossover point (125 ns) and at a procedure.Direct spike experiments: The counting simulate efficien-solutions addedWhilecocktail mixtures to were closevalue of 140 ns. to the counting efficienciesroutine chemical analysis. The radiolabeled spikedcies were the crossover setting,cocktails were95% were determined with high values of mea-to 100% at Table 2.sured each underto cocktail mixtures to simulatesolutions added two different discriminator set- 4 Chemical recoveries obtained from a complete analyticalstill obtained at the higher (140 ns) discriminator settingtings, i.e., at the crossoverThe cocktails were mea-routine chemical analysis. point (125 ns) and at a procedure.(see the “Analysis of Counting Sensitivities” section). Table 2.sured each under two different discriminator set- Chemical recoveries obtained from a complete analyticaltings, i.e., at the crossover point (125 ns) and at a procedure. 4 4 4
  • All experiments wereliquid scintillation vial are insignifi- transfer into the performed with 200 mg resin per 0.5 cant. This method can hence be at least adopted to L sample and an extraction time of easily four hours (for routine studies see the following section entitled kinetic laboratory use. “Experiments on Uptake Kinetics”). Under these conditions All experiments were performed with 200 mg resin only about 75% Am was consistently recovered, whereas the per 0.5 L of most of thean extraction time of at least recoveries sample and investigated actinides exceeded 90%. This discrepancy is most studies see the fact that the four hours (for kinetic likely due to the following resin uptake coefficient for Am(III) more rapidlyKinetics”). section entitled “Experiments on Uptake decreases with aciditythese conditions only about 75% in thewas Under compared to those actinides present Am tetra- or hexavalent state suchwhereas the recoveries10of consistently recovered, as Th(IV), Pu(IV) or U(VI). most of the investigated actinides exceeded 90%. Recovery of Ra: In contrast to the actinides, the uptakethe This discrepancy is most likely due to the fact that of Ra on actinide resin was found to beAm(III) more rapidly resin uptake coefficient for less than 5% (Table 2). Thisdecreases with acidity compared to those actinides result is consistent with the low sorption coefficients (k-values) for the tetra- or elements (Ca2+ and Ra2+ in present inthe alkaline earthhexavalent state such )as strong acidicPu(IV) or U(VI). Th(IV), medium as obtained by Horwitz, et al.10 Even 10 in the presence of 2 M HCl solutions containing 1 M CaCl2, the Recoverythe least efficiently sorbed species Am(III) the uptake of of Ra: In contrast to the actinides, Figure 5. Bar chart showing the chemical recoveries obtained from two Figure 5. remained considerably high (k resin was foundisto be less uptake of Ra on actinide = 103). Since k lower for different resin additions. Bar chart showing the chemical recoveries obtained from Ra2+than 5% 2+ (k <1), and average urine Ca/Ra ratios are than Ca (Table 2). This result is consistent with the two different resin additions. extremely high, no additions of(k-values) for the alkaline low sorption coefficients Ca or Ba carrier are required Experiments on uptake kinetics: To obtain the times for earth elements (Ca and Ra ) in strong acidic 2+ 2+ a routine analysis. required for sorption equilibrium at a steady state, the medium as obtained by Horwitz, et al.10 Even in the uptake kinetics were studied kinetics: To obtain the Experiments on uptake for tri- and hexavalent species Recovery of Po:2Because solutions containing 1 M CaCl2, presence of M HCl oxidation of the stripped reagent using Am(III) andfor sorptionsolutions. For thesesteady times required U(VI) tracer equilibrium at a investiga- fraction uptake of the least efficiently sorbed species/ the is performed under high temperatures using HNO3 state, the uptake kinetics were studied for tri- and tions, aliquots were prepared as explained in the previous H2SO4 mixtures (boiling point of sulfuric acid = (k = 10 3). Am(III) remained considerably high 338 ˚C), the hexavalent species using Am(III) and U(VI) tracer section. This time, however, aliquots were spiked with second naturally lower forcomponent 210Po is(k <1), and Since k is occurring Ra2+ than Ca2+ efficiently solutions. For these investigations, aliquots were identical activity concentrations and the extraction was inter- average urine Ca/Ra ratios are extremely high, no eliminated since under acidic conditions at elevated prepared as explained in the previous section. This rupted at times given in Figure 3. Very rapid uptake was temperatures, Po (probably present as are required for a is additions of Ca or Ba carrier Po-oxide in the ash) time, however, aliquots were spiked with identical routine analysis. volatile. Tracer experiments with 209Po(NO3)4 spike solutions activity concentrations and the extraction was observed and in about two hours steady state conditions indeed revealed repeatedly no detectable activity in the interrupted at times given in Figure 3. Very rapid were obtained independently of the amount of added resin. liquid scintillation Po: Because oxidation of the stripped Recovery of cocktail. uptake was observed and in about two hours steady If the sorption process follows first order kinetics, the data reagent fraction is performed under high tempera- state conditions were obtained independently of the should plot on a straight line in a semi-log diagram with the Uptake studies HNOdifferentmixtures (boiling point of tures using with 3/H2SO4 resin additions: As dis- amount of added resin. remaining activity in solution plotted versus the exposure cussed above, the= 338 °C), the second naturally occur- sulfuric acid uptake coefficient of Am(III) decreases time (Figure 3). In this case the sorption exponent ksorp can rapidly with acidity. Therefore, slight neutralization ofsince ring component 210Po is efficiently eliminated the If the sorption process follows first order kinetics, be extracted from the relation: aqueous samples with NH4OH elevated the oxidation step under acidic conditions at(following temperatures, Po the data should plot on a straight line in a semi-log (probably present as Po-oxide in the ash) is volatile. with HNO3) was attempted. However, when adding NH4OH diagram with the remaining activity intsolution plot- asolution = e – ksorp • to reduce the acidity of the solution from)4 spike to 1 M Tracer experiments with 209Po(NO3 ≈ 2 M solutions ted versus the exposure time (Figure 3). In this case indeed revealed repeatedly no detectable activity in HNO3, the solutions became black and opaque. An improved (with asolution = activity in ksorp can be extracted from the the sorption exponent solution) and via regression analysis the liquid scintillation cocktail. technique to obtain a higher extraction yield is simply to relation: A more comprehensive approach is the use of of the data. the sorption half-lives (i.e., T1/2 = ln2/k). Very short half-lives a =e increase the amount of actinide resin per same sample -ksorp . t volume. The studies with different resin additions: As Uptake results for additions of 0.4 g/L and 1 g/L are solution of only eight and 20 minutes were calculated for U and Am, discussed above, the uptake coefficient of Am(III) depicted in Figure 5. Almost quantitative extraction for all respectively using this approach. the decreases rapidly withwhen taking 1 g/L actinide resin. actinides were obtained acidity. Therefore, slight neu- (with a solution = activity in solution) and via regression tralization of the aqueous samples with NH4OH analysis of the data. A more comprehensive (following the oxidation step with HNO3) was at- approach is the use of the sorption half-lives tempted. However, when adding NH4OH to reduce (i.e., T1/2 = ln2/k). Very short half-lives of only eight the acidity of the solution from ≈ 2 M to 1 M HNO3, and 20 minutes were calculated for U and Am, the solutions became black and opaque. An im- respectively using this approach. proved technique to obtain a higher extraction yield is simply to increase the amount of actinide resin per same sample volume. The results for additions of 0.4 g/L and 1 g/L are depicted in Figure 5. Almost quantitative extraction for all the actinides were obtained when taking 1 g/L actinide resin. 55
  • Table 3. Set of values used for the calculation of the LLDs for 239Pu. The set of parameters used for the calculation of the Figure 6. LLD is given in Table 3. The other parameters were α/β crossover curves as function of PDD setting obtained with 241Am and 36Cl. kept either constant (i.e., Vs = 0.5 L) or were not relevant (µ). However, it has to be noted that in contrast to procedures based on LSC, µ can only be Analysis of Counting Sensitivities omitted when almost weightless sample discs are produced. If that is not the case, absorption of α Optimum α/β discriminator settings: For gross radiation in the sample source itself has to be α/β counting systems, two parameters are essential considered seriously. to determine the sensitivity of a radioassay: back- Table 3. ground count rate (B) and counting efficiency (E), Set of values used three methods arethe LLDs for 239two proce- In Figure 7, for the calculation of compared; Pu. which can be expressed as figure of merit or E2/B.20 dures based on α/β-LSC (previous work of Eikenberg, To reduce background scatter, misclassification of β et al.)6 and a method2 developed for low level gas- Table 3. pulses counted as α has been minimized by optimiz-The set of3.parametersthe calculation(GPC). the LLDs for 239Pu. Set of values used for used for forcalculation of Although a very flow proportional counting of the LLDs for ofPu. Table Set of values used the the calculation 239 the Figure afterpulse analysis featuresLLDlow background count Tableparameters CPM was ing the pulse decay and 6. is given in Table 3. Therate of 3. of0.04LLDs for 239Pu. other only were Set of values used for the calculation the the Packard Tri-Carb 2500TR/AB. As shownkept either constant (i.e., Vs = 0.5 L) or were not ofAm and 36Cl. α/β crossover curves as function of PDD setting obtained with 241 7,8 in Figure 6, low α and β misclassification (0.6%)relevant (µ). However,used forto be noted that in The set of parameters it has the calculation of the resulted atCl. optimum pulse decay discriminatorcontrast to procedures based on LSC, µ can onlywere the Figure 6. Figure 6. a/b crossover curves as function of PDD setting obtained LLD is given in Table 3. The other parameters beAnalysis ofandcurvesof 125. Optimum E2/B values were,omitted when almost weightless 0.5 L) or were not of the kept either constant (i.e., Vs = sample calculation The set of parameters used for the discs are with 241Am and 36 (PDD) setting as function of PDD setting obtained α/β crossover Counting Sensitivities with 241Am 36 Cl. Figure 6. higher PDD settingproduced. (µ). However,the hasThebe noted thatαin were however, obtained for a slightly relevant is given in Table case, absorption of LLD If that is not it 3. to other parameters α/β crossover curves as function of PDD setting obtained Analysis of 36discriminator spill (0.1%) is extremelyradiation to procedures based on LSC, µhasL) or be with of Am α/β this value, the β settings: 241140. AtOptimum andCounting Sensitivities For gross Cl. contrast in the constant (i.e., Vs = 0.5 only were not kept either sample source itself can to beα/β counting systems, two parameters the α backgroundconsidered seriously. weightlesshas to be noted that in low (hence significantly reducing Optimum a/b discriminator settings: are essential Analysis of Counting Sensitivities For gross a/b relevant (µ). However, it sample discs are omitted when almosttocounting systems, two the loss in counting efficiency due produced. If to procedures based absorption of αonly be determine thewhile parameters radioassay: back- count rate), sensitivity of a are essential to determine contrast that is not the case, on LSC, µ canground counting some and counting is minimal.(E), (b) In Figure 7, three methods are compared; two proce-discs areAnalysis ofα/β a (B) α pulses as β efficiency rate the to count ratediscriminator settings: For Optimum Counting Sensitivities count gross sensitivity of radioassay: background radiation in when almost weightless sample be omitted the sample source itself has to α/β countingexpressed as which can be are E2/B.20whichcountingefficiency (E),figure of merit or essentialfigure produced. If that is not the case, absorption of α and can be systems, two parameters expressed as dures based on α/β-LSC (previous work of Eikenberg, considered seriously.To reduceor E2/B.20discriminatora settings: Forlimit ofet al.)6 and a method2 developed for low level gas- to be of merit background scatter, misclassification of gross to determine the To reduce background scatter, back- Comparison sensitivity of radioassay: misclas-Optimum α/β of detection limits: The lower β radiation in the sample source itself haspulses counted(LLD)countedminimized by optimiz- by flow proportional counting (GPC). Although a proce- sification of b pulses(B)been counting been are essential detection as α has the as a has efficiency (E), ground countsystems,and 95% confidence probability In Figure 7, three methods are compared; two very rate at two parameters minimizedα/β counting considered seriously.ing the pulsebecalculated from analysis ofanalysis2sampleslow background count rate of only 0.04 CPM was which can the expressed as figure analysis features 20 decay and afterpulse of merit or E /B. level was pulse decay and blank back- determine the sensitivity of a radioassay:features dures based on α/β-LSC (previous work of Eikenberg,tooptimizing background scatter,afterpulse As shown βofof thePackardequation as2500TR/AB. theusing the Tri-Carb given by Seymour, et al.:21 To reduce discrimination.7,8 Asof misclassification shown 7,8ground Tri-Carb rateα(B) and counting efficiency (E), et al.) Figuremethod methods are compared;gas- and a developed for low level 6 2 count with alpha/betaminimized by (0.6%)inpulses counted as and b β misclassification optimiz- 20 flow proportional counting (GPC). Although a very proce- Figure 6, low a and misclassification (0.6%) resulted at low α has been In 7, three two in Figure 6, expressed as figure of merit or E2/B.which can be optimum afterpulse analysis features ing the pulsepulse decay discriminator (PDD) setting of 125.lowdures based on α/β-LSC (previous work ofwasresulted at the decay andpulse decay discriminator background count rate of only 0.04 CPM Eikenberg,(PDD) setting of Tri-Carb 2500TR/AB.7,8 As were, of βTo reduce background scatter, misclassification et al.)6 and a method2 developed for low level gas- the optimum of the Packard 125. Optimum E2/B values for a slightly shownhowever, obtained forhas been minimized by(0.6%)pulses counted as α and βhowever, obtainedsetting in Figure 6, low α a slightly higher PDD optimiz- Optimum E /B values were, flow proportional counting (GPC). Although a very 2 misclassificationof 140. At at the optimum afterpulse analysis featuresing the pulse decay thestatistical valueis extremely resulted this value, and pulse(0.1%) discriminator where (K) = 1.64 = β spill decay for a confidence higher PDD setting of 140. At this value, the b spill (0.1%) low background count rate of only 0.04 CPM waslowthe Packard95%; (IsignificantlyE2/Bbackground inof (hence significantly 0reducing the α values counts (PDD) setting of 125. Optimum background were, interval of Tri-Carb=2500TR/AB.7,8the ashown is extremely low (hence ) total reducing As back-induetime t; obtained for apulses as b = chemical recovery; Figure while the andslightly ) is minimal. (0.6%)count rate),6, low αwhiletime; (Yihigher PDD setting however, (t) = counting in counting efficiency due ground count rate), loss the loss in counting efficiency β misclassification Figure 7. Evolution of the lower limit of detection for three methods Figure 7.toof 140.countingvalue, detector efficiency; (Vs) = sample (E) =at this optimum as β is minimal.resulted to counting or counting somesome a βpulse(0.1%) is extremely At the α pulses spill decay discriminator the based on GPC and LSC. Evolution of the lower limit of detection for three methods lowvolume;significantly reducing coefficient.of detection (hence and (µ) = attenuation lower values were,(PDD) setting of 125. Optimumthe2/Bbackground Comparison of detection limits: The E α limit based on GPC and LSC. countat the 95% confidence probability efficiencycalculated rate), while the lossslightly higher PDD setting in countinglower limit dueComparison of detection limits: The level was ofhowever, obtained for a (LLD)detection (LLD)value,samples usingminimal. extremelyoffrom analysissome α pulses confidence equation as given 6 to counting 140. At this blank the βas β is(0.1%) is of at the 95% spill the probabilitylevelSeymour, et al.21 from analysis of blank samples was calculatedlow (hence significantly reducing the α background byusing the equation as given limits: The lower limit of Comparison of detection by Seymour, et al.:21count rate), whileat the 95% confidence probability detection (LLD) the loss in counting efficiency dueto counting some α pulses as β isof blank samples level was calculated from analysis minimal. using the equation as given by Seymour, et al.:21Comparison of detection limits: The lower limit ofwhere (K) = 1.64 =at the 95% confidence probabilitydetection (LLD) statistical value for a confidenceinterval of calculated= totalanalysis of blank samples 95%; (I 0) from background counts inlevel was = 1.64 = statistical value for a confidence intervaltime t; the equation time; (Yi) = chemical recovery;21 of where (K) counting (t) =using (I ) = total as given by Seymour, et al.: Figure 7.(E) = counting orbackground counts in for sat;=(t) = counting Evolution of the lower limit of detection for three methods where (K) = 1.64 = statistical value time ) confidence 95%; 0 detector efficiency; (V samplevolume; and chemical 0recovery; (E) = counting counts in intervali)of 95%; attenuation coefficient. or detector based on GPC and LSC. time; (Y = (µ) = (I ) = total background time t; (t) = counting time; (Yi) = chemical recovery; efficiency; (Vs) = sample volume; and (μ) = attenuation Figure 7. (E) = counting or detector efficiency; (Vs) = sample coefficient. Evolution of the lower limit of detection for three methods volume; and (µ) = attenuation coefficient. 6 based on GPC and LSC.where (K) = 1.64 = statistical value for a confidenceinterval of 95%; (I 0) = total background counts intime t; (t) = counting time; (Yi) = chemical recovery; 6 Figure 7.(E) = counting or detector efficiency; (Vs) = sample Evolution of the lower limit of detection for three methods 6volume; and (µ) = attenuation coefficient. based on GPC and LSC.
  • Thetaken to calculate the for the using low of the GPC set of parameters used LLD calculation level LLDcounters, the new3. The other parameters were kept is given in Table procedure based on • /• -LSC yields • • either constant (i.e., Vs = LLD values. Thisrelevant is the considerably lower 0.5 L) or were not result (μ). However, it has toof the very high counting efficiencies consequence be noted that in contrast to procedures based onchemical recoveries. If,whenparticular, LSC and LSC, μ can only be omitted in almost weightless sample discs are produced. a that is not the case, absorption is carried out using If small window for analysis of a radiation in the sample source itself has tothe considered of a special group of actinides (see be “• • peak resolution and liquid scintillation quench” paragraph seriously. below), the background decreases to values about 0.05 7, three methods an extraordinary procedures In FigureCPM. This yieldsare compared; two high figure based merit (E2/B) of 180,000 of Eikenberg, et limit of of on a/b-LSC (previous work or a detection al.)6 and a method2 developed for minute counting proportional 1.5 mBq/L in a 500 low level gasflow interval. counting (GPC). Although a very low background count rate of onlyresolutionwas taken toscintillation quench: • •peak 0.04 CPM and liquid calculate the LLD using is well known counters, theresolution using LSC on It low level GPC that • •peak new procedure based is poor in comparison to • • spectrometry and hence a/b-LSC yields considerably lower LLD values. This result is the•consequence of the very high countinggross counters. •• •LSC systems are used mainly as efficiencies and - chemical recoveries.if • • particular, LSC is carried out Nevertheless, If, in pulse stretching scintillators using a used, window for analysis of a special keV can be are small FWHM values of 300-400 group of actinides (see the “a peak resolution and procedure, a near obtained.22 Since, for the current liquid scintillation quench” paragraph below), the is prepared,decreases to organic cocktail mixture background the • • peak Figure 8. Relations between Figure 8. the true emission energy and liquid resolution becomes fairly high (400 keV or 40 keV scintillation quenched a energies of actinides. Relations between the true emission energy and liquid values about 0.05 CPM. This yields an extraordinary high scintillation quenched • •energies of actinides. figure of merit (Escintillation scale).detection limit of on a liquid 2/B) of 180,000 or a This allows peak 1.5separation 500 minute counting interval. as shown in mBq/L in a between U and 238U or, 234 It is interesting to note, that for a set of a emitters with Figure 4, between transuranium nuclides such as different energies, the ratio between both energy scales is a peak resolution and • = 600 keV). Two observations Pu and 244Cm (• E electronic assignment the a given energy to the mul- to highly linear. Although of aqueous cocktail is quenched 239 liquid scintillation quench: It is well known thatinterest.resolution using LSC are symmetrically are of a peak First, the peaks is poor in comparison tichannel analyzer (MCA). Indeed stability tests a higher degree, regression analyses yielded identical slopes shaped and simple Gaussian-based fitting proce- to a spectrometry and hence a/b-LSC systems are used using 239Pu spiked cocktails revealed identical there is of exactly 10 (see Figure 8). This also implies that peak mainly as (without additional terms for peak tailing) are dures gross counters. Nevertheless, if a pulse stretching positions which scattered less assignment of a given almost no drift for the electronic then 15 keV on the scintillators are used, FWHM overlapping peaks. Second, sufficient for fitting of values of 300-400 keV can be liquid scintillation scale for samples produced within energy to the multichannel analyzer (MCA). Indeed stability obtained.22 Since, for the current procedure, a using LSC there is a significant shift of the • •energy near organic one year. This fact is helpful torevealed identical peak tests using 239Pu spiked cocktails distinguish between cocktail mixture is prepared, emission energy. with respect to the real the a peak resolution becomes two major groups of actinides which are of interest positions which scattered less then 15 keV on the liquid fairly high (400 keV or 40 keV on a liquid scintillation scale). for in vitro measurements. As shown in Figure 8, all anthropogenic transuranium nuclides are character- scintillation scale for samples produced within one year. This This allows peak phenomenon is due 234U and 238U or, as This shifting separation between to ionization quench ized is helpful to distinguish between twocompared to of fact by emission of higher • •energies major groups shown in Figure • •particles dissipate their energysuch a because the 4, between transuranium nuclides over all natural uranium isotopes (i.e., 234U, 235U, 238U). actinides which are of interest for in vitro measurements. As as 239Pu and 244Cm (DE = causing less excited niveaus in very small distance 600 keV). Two observations are the orbitals of the scintillator targets.23 However, the For in in Figure 8, all anthropogenic transuranium nuclides shown vitro screening, a distinction between these of interest. First, the peaks are symmetrically shaped and shift in energy from quenching remains constant for groups is often reasonable. For higher a energies compared are characterized by emission of instance, monitoring simple Gaussian-based fitting procedures (without additional a fixed cocktail mixture. The relation between the of employees involved in uranium mining should For in to all natural uranium isotopes (i.e., 234U, 235U, 238U). be terms for peak tailing) are sufficient for fitting of overlapping true • •emission energy and the liquid scintillation limited to uranium isotopes, whereas in nuclear vitro screening, a distinction between these groups is often peaks. Second, there is a significant shift of the a energy quenched output is demonstrated in Figure 8 for the reprocessing plants or hot laboratories handling spent reasonable. For instance, monitoring of employees involved procedure given here and an aqueous cocktail. It is using LS with respect to the real emission energy. 6 fuel elements, radiation hazards may arise predomi- in uranium mining should be limited to uranium isotopes, nantly from incorporation of 239Pu, 240Pu, 241Am and This shifting phenomenon is for to ionization quench interesting to note, that due a set of • •emitters with whereas in nuclear reprocessing plants or hot laboratories different energies, the ratio between both energy handling spent fuel instead ofradiation hazards • •over 244 Cm. Therefore, elements, counting gross may arise because the a particles dissipate their energy over a very scales is highly linear. Although the aqueous cocktail predominantly of energy, a small 239Pu, 240 of only a wide range from incorporation of windowPu, 241Am and small distance causing less excited niveaus in the orbitals of is quenched to a higher degree, regression analyses 120 keV can beinstead of counting gross a uranium 244 Cm. Therefore, taken for the group of over a wide the scintillator targets.23 However, the shift in energy from yielded identical slopes of exactly 10 (see Figure 8). isotopes energy, a small window of only 120 transura- range of (160-280 keV) as well as for the keV can be quenching remains constant for a fixed cocktail mixture. The This also implies that there is almost no drift for the nium nuclides (260-380 keV).isotopes (160-280 keV) as taken for the group of uranium relation between the true a emission energy and the liquid well as for the transuranium nuclides (260-380 keV). scintillation quenched output is demonstrated in Figure 8 for the procedure given here and an aqueous cocktail.6 77
  • Conclusions 9. Shiraishi, K., Yamamoto, M., Yoshimizu, K., Igarashi, Y.A fast and very efficient radiochemical procedure for screening and Ueno, K. (1994) Health Phys. Vol. 66, 30-35.a activities in urine was developed based on sorption using 10. Horwitz, E.P., Chiarizia, R. and Diez, M.L. React. Funct. an actinide extractive resin. A high figure of merit (180,000) Polymers (in press).is obtained by performing the radioassay with a Tri-Carbwith the Alpha/beta discrimination option. A low detection 11. Wrenn, et al., J. Rad. Nuc. Chem. Art. 156 (1992)limit of 1.5 mBq/L (0.04 pCi/L) can be obtained in about 407-412.eight hours counting time. This allows an annual throughputof about 1,000 samples for screening a activities in urine. 12. Karpas, et al., Health Phys. 71 (1996) 879-885.A complete analysis requires the use of only 0.5 L samples, 13. Dang, et al., Health Phys. 57 (1989) 393-396.additions of only 0.2 g resin and can be terminated withinone day. 14. Dahlheimer and Henrichs, Rad. Prot. Dos. 53 (1994) 207-209.References 15. Fisenne, et al., Health Phys. 53 (1987) 357-363.1. Int. Com. Radiological. Protection. ICRP Publication 54, Ann. ICRP Vol. 19 (1988). 16. ICRP Publication. 23 (1975).2. Eakins, J.D. and Gomm, P.J. (1968) Health Phys. 14, 17. Shiraishi, et al., Health Phys. 66 (1994) 30-35. 461-472. 18. Fellmann, et al., Health Phys. 57 (1989) 615-621.3. Horwitz, E., Ph Dietz, M.L., Nelson, D.M., LaRosa, J.J. and Fairman, W.D. (1990) Anal. Chim. Acta Vol. 238, 19. Eikenberg, J., Zumsteg, I., Ruethi, M., Bajo, S., Fern, M. 263-271. J. and Passo, C.J., J. Radact. Radiochem. (in press).4. Salonen, L. (1993) Sci. Tot. Environ. Vol. 130/131, 23-35. 20. Currie, L. A. (1968) Anal. Chem. 40, 586-593.5. Bickel, M., Möbius, S., Kilian, F. and Becker, H. (1992) 21. Seymour, R., Sergent, F., Knight, K. and Kyker, B. (1992) Radiochim. Acta Vol. 57, 141-151. Radioact. Radiochem. 3, 14-27.6. Eikenberg, J., Fiechtner, A., Ruethi, M. and Zumsteg, I. 22. Yu, Y.F., Salbu, B., Bjornstad, H.E. and Lien, H.J. (1990) Liquid Scintillation Spectrometry 1994, Radiocarbon Radioanal. Nucl. Chem. Lett. 145, 345-353. 1996, 283-292. 23. Horrocks, D. (1974) Academic Press. New York-London 7. Passo, C.J. and Kessler, M.J. (1992) Packard Instrument 346. Company Publication, Report PBR0012, 8.8. Passo, C.J. and Kessler, M.J. Liquid Scintillation Spectrometry 1992, Radiocarbon 1993, 51-57.PerkinElmer, Inc.940 Winter StreetWaltham, MA 02451 USA P: (800) 762-4000 or(+1) 203-925-4602www.perkinelmer.comFor a complete listing of our global offices, visit www.perkinelmer.com/ContactUsCopyright ©2011, PerkinElmer, Inc. All rights reserved. PerkinElmer® is a registered trademark of PerkinElmer, Inc. All other trademarks are the property of their respective owners. DIPEX is a trademark of Eichrom Industries, Inc.009599A_01 Printed in USA April 2011