Radiochemical separation of thorium by isotope ion exchange

Journal o f R adioanalytical and Nuclear Chemistry, Articles, Vol. 91/1 (1985) 135-148 RADIOCHEMICAL SEPARATION OF THORIUM BY ISOTOPE ION EXCHANGE* C. SEP~LVEDA MUNITA, L. T. ATALLA Radiochemistry Division, lnstitu to de Pesquisas Energ~ticas e Nucleates, Comisslio Nacional de Energia Nuclear, CaixaPosta11104 9, Silo Paulo. SP (Brazil) (Received August 1, 1984) The aim of the present work is to obtain the separation of ~3 ffiTh from the radioisotopes formed in the irradiation of Mn, U, Ba, Cs, Co and the lanthanide elements with thermal neutrons, because they may interfere in the neutron activation analysis of Th, when the activity of : 33Th is used. The experiments were performed with the resin Bio-Rad AG 50W X-4 and X-8 (100-200 mesh) in the thorium form. The separation of 23~Th from the interfering radioisotopes is based on the retention of ffis 3Th by the resin (isotope exchange) and the elution of the interfering radioisotopes with a dilute solution of Th in 0.5M HCL Batch experiments were made in order to determine the equilibrium time for the isotopic ion exchange of thorium and also the distn~oution coefficients of the interfe:ing elements between the solution and the resin. Column experiments were carried out with the purpose of establishing the conditions that allow the maximum isotope exchange of:33Th and the minimum retention of the interfering radioisotopes in the resin. With this purpose, a statistical interpretation of a four variable experimental design is presented. Introduction The determination o f thorium b y neutron activation analysis is usually carried out b y measuring the gamma.ray activity o f 23 a Pa, without chemical separation. When microamounts o f thorium are concerned, the determination requires a long irradiation time and, often, a waiting time to allow the decay o f the matrix activity. This procedure is usually employed because the direct determination o f thorium, b y measuring the 2 a 3 Th gamma-ray activity, is fimited b y the low intensity o f the total absorption peaks o f its gamma-ray spectrum. Depending on the analysis sensitivity required, the use o f NaI(TI) detectors may be indispensable. However, *From a thesis submitted by C. Sepdlveda Munita to the "Instituto de Pesquisas Energ6ticas e Nucleates (CNEN/SP)" University of S$o Paulo, in partial fulfillment of a Doctor of Science's Degree. Work supported by the Brazilian Atomic Energy Commission. Elsevier Sequoia 5. A., Lausanne Akaddmiai Kiad6, Budapest C. SEPOLVEDAMUNITA,L. T. ATALLA: RADIOCHEMICALSEPARATION as a consequence of the poor resolution of these detectors, the 86 keV main peak of the 2 a 3 Th gamma-ray Slbectrum undergoes interference from "several radioisotopes that may be formed during sample irradiation. When this happens, a fast radiochemical separation of thorium is required, in view of the short half-life of the 233 Th (22.3 min). TERA and MORRISON 1 and PETERS and DEL FIORE 2 proposed the isotopic ion exchange technique for'many radioehemieal separations and noted the reliability for short half-life radioisotope separations. As stated by TERA and MORRISON, ~ the technique presents different applications, according to the ion exchange selectivity of the elements concerned. One of them is based on the retention of radioisotope M* by an ion exchanger in the R - M form due to an isotope exchange reaction, whenever the element M has a high ion exchange selectivity. An interfering radioisotope T*, from an element T with a lower ion exchange selectivity than M, will be easily eluted from the exchanger by a solution of M. The steps pointed out b y t h e above authorst, 2 for the separation of M* from T* are Loading step: R - M + M* ~ R - M* + M R - M + T* ~- R - T* + M (isotope exchange) 0on exchange) (I) (II) (isotope exchange) (ion exchange) (III) (IV) Elution step: R - M* + M ~- R - M + M* R - T* + M ~ R - M + T* The great ion exchange selectivity of M helps the isotope exchange [Equilibrium(I)]. If the number o f gram-e.quivalents of M in the liquid and resin phases are N~ and N2, respectively, the Nl/N2 ratio must be small in order to force Equilibrium(I) to the right. In order to decrease the sorption of T by the exchanger [Equilibrium(II)], the concentration of M in the liquid phase must be high. As the operation purpose is the separation of M* from T*, the concentration of M in the liquid phase is critical. Consequently, there must be a compromise in the choice of the concentration of M in the liquid phase in order to obtain the maximum separation between the radioisotopes of interest. In order to facilitate the experiments, as far as the half-life of the tracer is concerned, all the experiments were performed with 2a4Th (24.1 d) and the best conditions for the maximum separation between 2a4Th and ~s2- ~S4Eu were established. Europium was chosen because the lanthanides have the highest ion 136 C. SEPI~ILVEDAMUNITA,L. T. AT,ALLA:RADIOCHEMICALSEPARATION exchange affinity after thorium. 3 It was proved experimentally that 2 s STh and 2 s 4Th show the same ion exchange behaviour in the conditions described. With the purpose of establishing the conditions that would allow the best separation between 2S4Th and ~s 2 - t S4Eu ' the effect of the following variables was investigated: (a) Th concentration in the liquid phase; (b) HCI concentration in the liquid phase; (c) Concentration of some interfering ions in the liquid phase; (d) Flow rate of the liquid phase through the exchanger; (e) The exchanger cross-linkage. A hydrochloric acid medium was employed, because it is suitable for the Th 4+ ion exchange separation from almost all the other cations. According to STRELOW,3 the distribution coefficient for Th 4+, when using Bio-Rad AG 50W X-8 cation exchanger, is the highest, except for ZrO 2+ ions. In order to decide which elements must be regarded as interfering, the following factors were taken into account: (a) the radioisotopes with gamma-ray energies between 50 and 130 keV; (b) the elements that, due to their nuclear characteristics, induce high activity in the matrix; (c) the affinity of the elements for the resin, according to values of the distribution coefficients, determined by STRELOW) Consequently, the behaviour of: Eu 3+, La3+, Yb 3+, UO22+,Co2§ Mn 2., ZrO 2§ Ba2+, Cs+ and Fe 3+ was studied. Experimental Reagents and tracers All reagents used were of analytical grade. Bio Rad AG 50W (X4 and X8) cation exchange resin was employed after purification with concentrated HC1 and deionized water. 2~4Th was obtained by percolating a uranyl nitrate 0.3M HF solution through aluminium oxide, as described by ABRAO.4 All the other radioactive tracers were produced by irradiating the oxides of the respective stable elements. A solution containing all the interfering elements was prepared by dissolving their carbonates or chlorides in 2M HC1. The f'mal solution (T solution) contained 200/~g/ml of each element in 1M HCI. 137 C. SEPIJLVEDAMUNITA,L. T. ATALLA: RADIOCHEMICALSEPARATION Apparatus A Nuclear Chicago single channel analyzer coupled to a 3.7 cm X 6.2 cm welltype NaI(T1) crystal was used for the measurements of the activity, when only one radioactive tracer was involved. When interfering activities were present, a TMC 400 channel analyzer coupled to a 7.5 cm X 7.5 cm well type NaI(T1) crystal was used. An automatic fraction collector "Fractomat-Buchler" was employed to collect the column effluent. The apparatus used for the radioisotope separation consisted of a glass column (5.0 cm long and 0.5 cm diameter) fitted with a small glasswool plug at the bottom. The column was joined to a glass vessel (60 ml) where the testing and washing solutions, one after the other, were placed. Resin saturation with thorium The two resin types ( X - 4 and X - 8 ) were transformed to thorium form by percolating a 50 mg/ml ThC14 solution through the column. When the saturation was reached, the resins were washed with deionized water until a negative test for thorium was obtained in the effluent. Batch experiments A portion of the resins (X--4 and X - 8 ) in the thorium form was dried at about 80 ~ for a period of about 24 hours and then 0.5 g was taken for each batch experiment. The resin and 10 ml of a 0.5N HCI solution containing a known thorium concentration and a known activity of radioactive tracer (M* or T*) were added to a 50 ml Erlenmeyer flask. The time required for reaching the equilibrium between the phases was established by shaking the flasks for periods of 5 minutes to 2 hours. It was verified that ion exchange is a slower process than isotope exchange whose equilibrium was achieved in 25 minutes. The activity of appropriate aliquots from all solutions was then measured after a shaking time of 30 minutes and the distribution coefficients were calculated by means of the following equations: IT*IR DT = IT*I-----S IM*IR and IMIR DM = [M*I----S"--IMI---~ where the subscripts R and S refer to the resin and solution, respectively. 138 C. SEPIJLVEDAMUNITA, L. T. ATALLA: RADIOCHEMICALSEPARATION In order to verify the variation of the distribution coefficients with the change of the concentration of thorium in the liquid phase, experiments with thorium solutions from 0.10 to 0.75 mg/ml were performed. The distribution of 234Th in the system was calculated, for the equilibrium conditions, from the thorium content in the phases. Although the distribution coefficient is commonly defined for the equilibrium conditions, the same nomenclature to state t h e distribution of a generic radioisotope Table 1 Values of the distribution coefficient for thorium and interfering elements as a function of the thorium concentration in the liquid phase. Liquid phase: 5 ml of 0.SM HC1; exchanger: 500 mg of R - Th (4 or 8% of DVB) Resin % of DVB Th, mg/ml 4 Distribution coefficient Th U La Eu Yb Ba Co Mn Cs 0.10 0.25 0.50 0.75 2240 900 448 298 4 2 2 2 57 38 33 32 44 30 26 24 30 22 17 16 12 lI 9 8 4 2 2 2 5 4 3 3 7 5 4 3 0.10 0.25 0.50 0.75 2180 872 436 290 3 2 2 2 14 11 10 9 14 12 10 9 12 10 9 8 10 9 8 7 3 2 2 2 2 2 2 2 2 2 2 2 Table 2 Values of the separation factors (~) for the interfering elements as a function of the thorium concentration in the liquid phase. Liquid phase: 5 ml of 0.5M HCI;exchanger: 500 mg of R - Th (4 or 8% DVB) Resin % of DVB Separation factor (c~) Th, mg/ml U La Eu Yb Ba Co Mn 0.10 0.25 0.50 0.75 560 450 224 150 39 24 14 9 51 30 17 13 75 41 26 19 187 82 50 38 560 450 225 150 448 225 150 100 0.10 0.25 0.50 0.75 727 435 218 145 156 79 44 32 156 73 44 32 182 87 48 36 218 97 54 41 727 435 218 145 Cs 320 180 113 100 1090 1090 435 435 218 218 145 145 139 C. SEPULVEDA MUNITA, L. T. ATALLA: RADIOCHEMICAL SEPARATION T*, after 30 minutes of stirring, was used, even though the equilibrium had not been reached. Table 1 presents the values of D found experimentally and Table 2 the separation factors a, calculated by means of the distribution coefficients obtained under the same experimental conditions DTh aTh/T = DT Column experiment One milliliter of the resin saturated with thorium was introduced into the column and a little glasswool plug was placed at the top. In order to maintain the equilibrium between the exchanger and the liquid phase, a ThC14 solution, with the same HC1 and Th concentrations as the sample solution, was percolated through the column. By means of this procedure, the 2a:Th daughters that may have grown during the storage were almost completely eluted, as may be seen in Fig. 1. When the eluate and the influent solution presented the same thorium concentration, it was assumed that equilibrium was reached. The concentration of thorium was measured by activation analysis through the 2 3 ~Th activity. The test solutions were prepared ~ I/'~176 t 20 mPb r 60 80 100 120 140~ Ene~U~keV Fig. I .Gamma ray spectrum of the ~32Th daughters in the thorium form resin;. 9 countin~ immediately after washing, 9 23 days after washing 140 C. SEPULVEDA MUNITA, L. T. ATALLA: RADIOCHEMICALSEPARATION by mixing a convenient volume of the T solution with the radioactive tracer . . . . (M* or T*). After drying, the residues were taken with 5 rnl of the ThCI4 solution to be used in subsequent experiments. The test solutions so obtained were percolated through the column until the t o p of the exchanger was reached. Five ml of the washing solution were immediately percolated to remove any residue that might have been left behind, The remaining 45 ml of the washing solution were percolated directly afterwards. The washing solution had the same HCI and Th concentrations as the test solution, Whenever it was necessary to increase the flow rate in the column, air pressure was applied. Two milliliter fractions of the eluate solution were collected and the activity was measured by gamma counting. The resin was quantitatively transferred into a polyethylene tube and the activity was also measured by gamma counting. Selection of the HCI, Th and interfering ion concentration in the liquid phase Europium was chosen as representative of the interfering ions. In order to obtain the best separation between 234Th and i s 2-1 s 4 Eu, the suitable HCI, Th and interfering ion concentrations were selected from previous experiments carried out with the same resin (X--4 or X - 8 ) and by keeping the liquid flow rate constant through the column (2 ml/min). Each experiment was executed twice in order to study 234Th sorption/by the exchanger and i s2-1 SaEu elution individually. The best concentration intervals established are shown in Table 3. Some experiments were carried out in order to eliminate one of the five variables. It was supposed that the effect of the flow rate of the liquid phase Table 3 Nomenclature adopted for the variables and their values Level Variables (1) Th concentration HCI c o n c e n t r a t i o n C o n c e n t r a t i o n of each interfering ion Resin cross-linking degree (2) AI = 0.25 mg/ml A2 = 0.50 mg/ml BI = 0 . 2 M B 2 = 0.5M C~ = 1.0 ~g/ml C2 = 10.0 .g/ml D~ = 4% DUB D2 = 8% DVB 141 C. SEPOLVEDA MUNITA, L. T. ATALLA: RADIOCHEMICAL SEPARATION through the column might be disregarded, considering that a fast separation is needed. Some experiments were executed at different flow rates (1.5; 2.0 and 2.5 ml/min), leaving all the other conditions constant. When the flow rate applied was 2.5 ml/min, a toss o f 234Th in the eluate solution was observed. So, a flow rate of 2.0 ml/min was chosen for all the experiments. By doing so, it was possible to make a 16-experiment design b y combining the four variables. The experiments were identified b y one letter or b y combinations o f letters; for instance, the experiment carried out in the conditions At B2 C1 D2 was indicated b y BD. Only the variables with the highest subscripts appear in the identification. The only exception made was the experiment A I B I C 1 D I which was indicated by (1)~ All the experiments were executed twice in order to obtain a good estimation o f the experimental error. Table 4 shows the values obtained for the percentages o f isotope exchange o f thorium and the ion exchange o f europium, as well as the separation factor calculated from the average for each pair o f results. Table 4 Percentage of 234 Th and t s 2 - t s 4 Eu retained by the resin and separation factors obtained in each experiment. Variables: A - thorium concentration in the solution; B - HCI concentration; C - concentration of interfering elements; D - resin cross-linking degree. Indexes 1 and 2 refer to the smallest and the greatest value, respectively Variable At Bt CI C2 B2 Ct C2 A= Bt C, C2 B2 Ct C= 142 Experimental conditions 234 Th, % t s 2 - t s 4 Eu, % Separation factor 99.7 99.9 100.3 99.8 98.1 99.9 99.4 99.5 5.4 10.1 5.7 10.7 4.9 11.7 7.4 14.7 19 9 15 8 B BD BC BCD 99.8 99.4 99.6 99.6 100.1 100.9 99.1 100.3 2.1 9.1 4.6 13.0 1.9 12.5 3.3 9.6 50 9. 25 9 Dt D= Dt Da A AD AC ACD 99.0 89.9 99.4 89.6 99.5 87.2 98.9 83.9 1.0 12.8 0.7 11.6 0.9 11.6 0.9 13.2 105 7 124 7 D~ D2 Dt D: AB ABD ABC ABCD 96.6 99.1 99.4 96.9 97.9 97.3 98.8 98.1 0.6 7.3 0.6 9.3 0.8 8.4 0.9 8.1 139 12. 132 11 Dt D2 Dt D2 (I) D C CD Dt D2 Dt D2 C. SEPOLVEDA MUNITA,L. T. ATALLA: RADIOCHEMICALSEPARATION A statistical interpretation of the results was made in order to point out the best conditions for thorium separation and also the effect of the different variables. The procedure indicated for the statistical interpretation of the results from a fourvariable experimental design, 5 in which the variable have two values and each experiment is made twice, was applied. In order to check the effect of each variable and of the combination of variables in the isotope and ion exchange, the F test at a 0.05 significance level was applied. The results for which the experimental F value was lower than the tabulated one were used to improve the estimation of the experimental error. The experimental F values are listed on Table 5. It may be observed that all the experimental conditions listed, exception made for condition AB, contribute significantly to the separation factor. Therefore, it may be concluded that the following conditions are the most favourable for the separation of ~ a4Th from interfering radioisotopes: - Thorium concentration in the solution: 0.50 mg/ml; - HC1 concentration in the solution: 0.5M; - Interfering element concentration: range: 1.0 - 10.0/zg/ml; - Percentage of DVB in the resin: 4%. After the most favourable conditions were established, the behaviour of 234Th and interfering radioisotopes, using only one radioisotope in each experiment, was studied. The retention percentages of the elements in the resin are listed in Table 6, where three results obtained with 2aaTh as tracer are also presented in order to demonstrate the uniformity of 2 a a Th and 2 S4Th behaviour. Two milliliter fractions Table 5 Experimental F values and sign of the average effect caused by the variables and their combinations in the separation factors Experimental conditions A B AB D AD BD ABD F Average effect 180 (+) 10.1 5.1 337 177 333 (+) (+) (--) (--) (--) (--) 6.2 F0.05(1.8 ) = 5.32 143 C. SEPIJLVEDA MUNITA, L. T. ATALLA: RADIOCHEMICAL SEPARATION III/ - tll ~- /It ,t j I I '!11 I /~ / Ill I / / / 20--~/ 0 A lO ,cs ~~ ,v,, . 20 9Th 30 40 SO ~ Eluate,ml Fig. 2. Elution curves of 2S~Th, 2StU, : 40 La, ! s ~ - I S4Eu ' 169yb ' I SgBa ' ~OCo' S6Mn and I s 4 Cs through the thorium satusated resin. Exchanger: 4% DVB; solution: 0.SM HC1 containing 0.50 mg Th/ml and 10 ~g/ml o f interfering ions; flow rate: 2.0 roA/min Table 6 Retention of a s 3Th, 234 Th and interfering radioisotopes b y the thorium saturated resin and value of the separation factor Tracer 234Th 23 ~Th 239 U x4eLa 1 s2-1S4Eu 169Yb s 9 Ba e e Co s 6 Mn I s 4 Cs Retention on resin, % 98.3 98.5-97.8 -99.0 0.2 1.3 0.8 0.5 0.3 <0.1 <0.1 <0.1 Separation factor* 500 75 100 200 300 >1000 > 1000 >1000 *These values represent only an of order magnitude because the contribution o f the experimental error is significant. Conditions: solution: 0.5M HCI containing 0.50 mg I'h/ml and I0/Jg/ml o f interfeting ions; flow rate: 2.0 ml/min; exchanger: 4% DVB R - Th 144 C. SEPULVEDA MUNITA, L. T. ATALLA: RADIOCHEMICALSEPARATION .2 v <~ Eluate~ml Fig. 3. Elution curves of t s 2 - 1s 4Eu obtained with 4 and 8% of DVB. Exchanger: 4 and 8% DVB; solution: 0.5M HCI containing 0.50 mg Th/ml and 10 ,g/ml of interfering ions; flow rate: 2.0 ml/min. Curve 1 - 4% DVB, curve 2 - 8% DVB o f the effluent from the exchanger were collected directly in counting tubes b y means o f an automatic sample collector. The elution curves are depicted in Fig. 2. The elution o f 1 s 2 - 1 S 4 E u through a column with a 8% DVB resin in the thorium form was studied in order to point out the difference caused b y the crosslinkage of the resin on the elution curves. In Fig. 3 the elution curves for i s 2 - 1 s 4 Eu through exchangers with 4 and 8% o f DVB are depicted. Verification of the influence o f zirconium and iron on thorium isotope exchange With this purpose, increasing masses o f zirconium or iron and 2 a 4 T h were added to the loading solutions. After the usual procedure, the 2 a 4 T h activity retained b y the exchanger was measured and compared with the known activity initially added. Tables 7 and 8 show the values for the isotope exchange o f 2 a 4 T h when ZrO 2§ and Fe 3§ respectively, are present. 1o 145 C. SEPULVEDA MUNITA, L. T. ATALLA: RADIOCHEMICAL SEPARATION Table 7 Isotope exchange of 2 ~4 Th as a function of zirconium concentration in the solution Zr, t~g/ml Retention of =34Th in the resin, % 10 20 30 40 50 60 70 99.1 98.7 98.8 98.5 98.5 96.8 96.3 Table 8 Isotope exchange of 234 Th as a function of iron eoneentratio~n in the solution Fe, mg/ml Retention of 234 Th in the resin, % 0.8 2.0 4.0 6.0 8.0 10.0 99.0 98.9 98.2 97.6 96.2 95.7 Discussion The batch experiments were made to point out the thorium concentration in the solution for which the highest separation factors and smallest loss of 2 3 a Th in the effluent were obtained. Two types o f resin (4 and 8% DVB) were used to observe the cross-linking effect on the separation factors. The results presented in Table 1 show that, when the resin with 4% DVB was used, the decontamination from the interfering ions increased rapidly until a thorium concentration of 0.25 mg/ml in the solution was achieved. This effect is less remarkable in the 8% DVB resin,: where the retention o f interfering ions is small. Consequently, it was concluded that the values of the separation factors (Table 2) are favoured by increasing the percentage o f DVB in the resin. It may be seen from Table 2 that the lanthanides are the most difficult elements to be separated froria thorium by the proposed method, in accordance with the 146 C. SEPULVEDA MUNITA, L. T. ATALLA: RADIOCHEMICAL SEPARATION distribution coefficient values determined by STRELOW.a Table 2 shows also that the most favourable range of thorium concentration in the solution in order to obtain the best decontamination is from 0.25 to 0.50 mg/ml Hydrocldoric acid concentrations of 0.2 and 0.SM were chosen, having in mind the values of the distribution coefficients calculated by STRELOWs and also because these concentrations were adopted in previous works 1,2 in which the same technique was used. It is known that the retention of the ions by the exchanger increases as the crosslinkage percentage increases. Therefore, although the batch experiments have shown that the exchanger with the highest cross-linking improves the separation factors, column experiments with both resins, 4% and 8% of DVB, were made. From the Equilibrium Reaction (II), it was presumed that the concentration of the interfering ions in the solution must be low. This condition is generally obtained when chemical separations are performed before sample irradiation. Therefore, it was assumed that, after the chemical separations, some micrograms of interfering elements still remain in the solution. Based on this assumption, the concentration of each interfering element in the loading solution was 5 or 50 ~ag/ml. Table 4 shows the values obtained in a column experiment design, where all the possible combinations with the two values for each variable were used. The statistical interpretation of the results, presented in Table 4, shows that the variation of thorium concentration in the solution (Experimental Condition A), affects both the isotope exchange and the ion exchange. The negative value of the average effect of this variable indicates that, with increasing thorium concentration in the solution, a decrease in 2S4Th and I s2-1S4Eu retention is observed. As both effects have the same sign, no indication of the effect of this variable on the separation factor may be inferred. To settle this matter, a variance analysis of the separation factor values was applied. Then, it was possible to conclude that the effect of variable A is highly significant (Table 5) and its positive sign indicates that an improvement in the separation factor is caused when the thorium concentration in the solution is increased from 0.25 to 0.50 mg/ml. Similar considerations concerning the other variables allow to conclude that the conditions which make it possible to obtain the highest separation factors are: - thorium concentration in the solution: 0.50 mg/ml; HC1 concentration in the solution: 0.SM; concentration of interfering ions: no effect between 1.0 and 10.0 #g/ml; 4% DVB resin. Once the conditions that allow the best separation between 234Th and t s a - t S4Eu were established, they were applied in determining the separation factors of the elements studied in column experiments. Table 6 shows that the lowest separation - - - 10" 147 C. SEPULVEDA MUNtTA, L. T. ATALLA: RADIOCHEMICALSEPARATION factor value was obtained for lanthanum, followed by europium and ytterbium. Among the radioisotopes produced by (n, 7) reaction from the lanthanides, 1 sa Sm is the one that interferes most strongly in the activation analysis of thorium. It may be foreseen that the separation factor for this element must be a little less than 100. It can also be observed that the differences between results obtained with 23aTh and 234Th, as tracers, are within the experimental error. Figure 3 shows that europium elution is faster when a less cross-linked resin is used. This proves the greatest readiness of interfering ion elution when they are retained in a low DVB percr resin. After determining the ideal conditions for obtaining a high separation factor, the interferences of ZrO 2+ and Fe a§ ions on 234Th isotope exchange were studied. The values presented in Table 7 show that for up to 50 #g of zirconium per milliliter (250/ag in 5 ml total), the isotope exchange of 2a4Th is virtually unaffected. As for iron, Table 8 indicates that it is tolerable up to 4.0 mg Fe/ml (20 mg total). From the study presented in this paper it may be concluded that the method proposed for radiochemical separation of 233 Th is feasible for use in thorium activation analysis through 233 Th activity. References 1. 2. 3. 4. F. TERA, G. H. MORRISON, Anal. Chem., 38 (1966) 959. J.M. PETERS, G. DEL FIORI, Radiochem. RadioanaL Lett., 16 (1974) 109 F.W. STRELOW,Anal. CherrL, 32 (1960) 1185. A. ABRAO, Chromatographic Separation and Concentration of Thorium and Rare Earths from Uranium, Using Alumina-Hydrofluoric Acid. Preparation of Carrier-Free Radio-Thorium, Institute de Energia At6mica. Publica~o 227, S~o Paulo, Brasfl, 1970. 5. L. T. ATALLA, InterpretafAo Quantitativa de Resultados Experimentais, Institute de Energia At6mica. lnformaqao 60, Sao Paulo, Brasil, 1978. 148