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Preparation of 228Ra standard solution
Applied Radiation and Isotopes 109 (2016) 222–225
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Preparation of
228
Ra standard solution
Miroslav Havelka Czech Metrology Institute, Radiová 1, 102 00 Prague, Czech Republic
H I G H L I G H T S
228Ra was isolated from 232Th salt. Two methods for Th–Ra separation. Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution. The activity of 228Ra in the standard solution was related to 232Th standard.
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2015 Accepted 24 November 2015 Available online 25 November 2015
For the preparation of a standard solution of 228Ra, 228Ra was isolated from 232Th salt. Two simple methods were developed for Th–Ra separation. Both are based on a very good solubility of thorium nitrate in organic solvents. The first one used Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution. The second method was based on solvent extraction, remaining Th in the organic phase, while Ra was concentrated in the aqueous phase. The activity of 228Ra (up to 20 kBq) in the standard solution was related to the 232Th standard by means of gamma ray spectrometry measurement. The obtained uncertainty was less than 0.7% (k ¼1). The standard solution was free of 232Th and contained the carrier in the usual concentration (1 g L 1 BaCl2, 10 g L 1 HCl). & 2015 Elsevier Ltd. All rights reserved.
Keywords: 228 Ra standard Th–Ra separation Ra–Pb co-precipitation
1. Introduction The 228Ra standard solution was proposed to be used for calibration and testing of the devices that measure naturally occurring 228 Ra. In the Czech Republic, these devices have so far been tested with 232Th standards that contain known amounts of decay products, at the same time containing also 228Ra. Besides calibrating and testing, the 228Ra solution can be used for the preparation of thoron (220Rn) emanation sources based on 228Ra. For these purposes, the solution should be free of 232Th, but it can contain Ba carrier at a reasonably low concentration. 228 Ra originates from 232Th decay (Fig. 1). Nuclear data were taken from Bé et al. (2008). The simplest way of 228Ra preparation is based on the separation of radium from “old” 232Th salt (or metallic Th) where 228Ra has been accumulated from the previous Th–Ra separation. Because of a very long 232Th half-life, the maximum specific activity of 228Ra is only 4.046 kBq/g in the case of metallic Th, or 1.596 kBq/ g in Th(NO3)4 6H2O salt, where 232Th and 228Ra are in radioactive equilibrium. Therefore, an efficient method has to be applied allowing separation of 228Ra from the large excess of Th. E-mail address: [email protected] http://dx.doi.org/10.1016/j.apradiso.2015.11.062 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
Some separation procedures include co-precipitation of Ra with Pb from Th solution (NIST, 2013) as the first (pre-concentration) step. Therefore, two procedures based on Ra–Pb co-precipitation from Th nitrate and Th-complex carbonates (aqueous) solutions were initially tested. The first one included precipitation of Pb(Ba,Ra)SO4. The second was based on Ra co-precipitation with mixed carbonates Pb(Ra,Ba)3Th(CO3)5 from Th-carbonates solution, where approximately 99% of 228Ra was separated by means of two sequential 300-mg Pb additions from 120 mL solution containing about 3.2 g Th. However, subsequently, two new more effective methods, based on a very good solubility of thorium nitrate in organic solvents, have been developed. Method I consisted in the Ra–Pb coprecipitation in the form of Pb(NO3)2 from a thorium nitrate solution in organic solvents. A disadvantage in all these Ra–Pb coprecipitation procedures was in the necessity to separate Ra from a “large” amount of Pb as one of the subsequent steps. To avoid this complication related to Ra–Pb co-precipitation, an alternative procedure, method II, based on solvent extraction, was proposed. In both methods, Th or Pb were completely separated from Ra, while other non-active chemical impurities partly passed from the original material to the final solutions, so their concentrations depend on the purity of the used salt. In both methods Ba carrier was added during the separation, which simplified the last
M. Havelka / Applied Radiation and Isotopes 109 (2016) 222–225
Fig. 1.
232
Th decay chain.
purification steps in which Ra was co-precipitated with Ba from organic solvents mixture (with the addition of mineral acids) in the form of barium chloride (which is practically insoluble in organic solvents). If needed, both methods can also be modified to prepare 228Ra solution without the addition of Ba carrier; however for long time stability of the solution sealed in glass ampoule, Ba carrier has to be added. 1.1. Principles of method I The Ra–Pb co-precipitation method utilises both the very good and the very poor solubility in organic solvents of Th(NO3)4 6H2O and Pb(NO3)2, respectively. To set up method I, the preliminary Ra–Pb co-precipitation test was made. The test proved, that three subsequent additions of 90 mg Pb to 25 mL acetic acid-HNO3 solution containing 2 g Th (5 g Th(NO3)4 6H2O, 1.4 g 65% HNO3 and 25 g acetic acid) resulted in a complete separation of 228Ra in the Pb(NO3)2 precipitates, whereas the single fractions contained 80%, 16% and 3% (in total 99%) of Ra. The Pb(NO3)2 solubility (under this condition) was estimated to be lower than 200 mg L 1. For Pb–Ra separation, new method of Ra–Ba coprecipitation from organic solvent mixture containing ammonium acetate was established. In this solution, Pb2 þ ions form a soluble complex compound with acetate anions. Because Ba carrier is added, the use of this method is limited to the cases where the Ba addition ( 1 mg for 40 mg Pb) does not interfere; otherwise a conventional method has to be applied.
223
toluene mixture saturated with water and HCl, with the addition of a small volume of HCl, whereas Ra partly passed to the aqueous phase. The Ra concentration ratio in organic and aqueous phase (distribution coefficient) corresponded to the (experimentally obtained) value of 0.015. Another possibility is to fix the aqueous phase in any adsorbent, for example in cellulose, and carry out the Th–Ra separation by solvent extraction using column chromatography arrangement. In this case, Ra was firstly retained in the column (in aqueous phase) from Th(NO3)4 solution in the mixture of acetone–toluene and HNO3, and subsequently, after washing of the column to completely eliminate Th, Ra was eluted with a diluted acid solution. In Th–Ra separation, simultaneously with Ra other elements, Ac, Pb and Bi (present as radionuclides in Th salt), passed to the aqueous phase, so that in the organic phase, mainly Th isotopes, 232 Th and 228Th (230Th), remained (220Rn and Po nuclides quickly disintegrated). These radionuclides emit alpha particles of energy o5.42 MeV, while 228Ra, 224Ra and its decay products-220Rn and 216 Po (which reach the 0.999 equilibrium in 10 min), emit alpha particles of higher energy or beta particles in range up to 3.5 MeV. This fact was used for quick monitoring of Th–Ra separation by means of LSC. The spectra of samples prepared from aliquots of the original (Th nitrate salt) solution and Th fraction in UG LLT cocktail are shown in Fig. 2. Thorium concentration in the sample prepared from Th fraction (with no detected beta emitters) was obtained from α particle peak area. Ra concentration was indicated as the activity of 228Ra, recorded in low energy channels, or as 224Ra activity. This last was calculated from the part of LSC spectra above Th peak, where alpha particles of 224Ra, 220Rn and 216Po were recorded.
2. Experimental procedure for method I The Th–Ra separation was carried out from a Th nitrate solution containing 16.56 g of Th(NO3)4 6H2O, 126.8 g acetic acid and 1.4 g 65% HNO3 ( 0.12 mol L 1 HNO3 in acetic acid). Ra was co-precipitated with Pb (as a mixed nitrate) by the addition of 45 mg Pb (0.28 mL of Pb acetate in acetic acid solution). The Pb(Ra)(NO3)2 precipitate was separated from the solution by centrifugation, washed with 3 mL of acetic acid-HNO3 mixture (21 g acetic acid, 0.28 g 65% HNO3), then dissolved in a mixture of 0.3 mL NH4C2H3O2 (ammonium acetate) -acetic acid-acetone solution (prepared from 0.77 g (NH4)2CO3, 2.1 g acetic acid and 1.75 g acetone) and 0.1 mL of acetic acid. To remove the residuals of Th, a 600
This method is based on a Th–Ra separation by liquid–liquid extraction method in HNO3, acetone–toluene system, where Th is extracted to the organic phase while Ra concentrates in the aqueous phase. Besides a very good solubility of Th(NO3)4 6H2O in organic solvent, the method also utilises the formation of Th anion complex compounds (with HNO3) extractable to acetone–toluene mixture. Considering the low Th distribution coefficient in organic and aqueous phases ( 0.35), which was determined for the chemical system used in this work, to achieve effective Th–Ra separation in batch-wise mode (to retain Th in organic phase), a low volume ratio ( 1:25) of aqueous to organic phases should be used. In order to stabilise volume ratio, the addition of HCl was applied. The aqueous phase then contained water, HCl, HNO3 and acetone, while the organic phase contained small amounts of water and HCl besides the main components (toluene, acetone and HNO3). Practically, the aqueous phase was precipitated from one-phase system – the solution of Th(NO3)4 6H2O in HNO3, acetone–
500 Count rate
1.2. Principles of method II
Th
228
Ra
400
224
Ra-220Rn-216Po
300 200 100 0
0
200
400
600
800
Ch. Fig. 2. Typical LSC spectrum of a Ra–Th sample and of a sample containing only Th. In both cases, 230Th (radionuclide impurity in Th nitrate) contributed to Th alphaparticle peak. Sources were prepared with UG LLT cocktail and were counted in TriCarb 3100TR counter.
224
M. Havelka / Applied Radiation and Isotopes 109 (2016) 222–225
second Ra co-precipitation with Pb in the form of Pb(Ra)(NO3)2 was performed by the addition of 1 mL 2 mol L 1 HNO3 in acetic acid. After washing, the precipitate was dissolved in a mixture of NH4C2H3O2-acetic acid-acetone solution (0.4 mL). Afterwards, for the Pb–Ra separation, the Ra co-precipitation with Ba was used as the first step. After the addition of acetone ( 0.57 g) to reduce the solubility of Pb acetate complex compound (closely to the limit of precipitation of the second liquid phase), two fractions of 0.8 and 0.6 mg Ba (in the form of 0.1 mol L 1 Ba(CH3COO)2 in acetic acid) were added. Furthermore, another addition of acetone ( 0.3 g) was necessary for Ba salt precipitation. After 15 min, the both precipitates were separated by centrifugation from the solution where a large part of Pb remained. Both precipitates (in total they contained 97% of Ra separated from the original Th solution, 1.4 mg Ba, less than 5 mg Pb) were dissolved (in total) in 0.05 mL 57% HI and 1.4 mL acetone– toluene mixture (1.1 g:1 g) and the solutions were combined. Further, to the sample 0.05 mL of HCl-n-butanole (1:5) a mixture was added, causing Ra co-precipitation with Ba in the form of barium chloride. After the separation of the organic phase (with the dissolved Pb), the precipitate was washed with 0.05 mL 57% HI, 1.4 mL acetone–toluene mixture and 0.05 mL HCl-n-butanole mixture. To remove the rest of HI, the barium chloride precipitate was repeatedly washed with the same organic solvent mixture as that used for barium chloride precipitation but without HI. The Ra loss for this part of Pb–Ra separation was estimated as lower than 1.5%; such result was reached mainly because of the low solubility ( 1 mg L 1) of barium chloride (radium chloride) in the selected organic solvents. The Pb–Ra separation method was similar to that described in (Havelka and Bludovsky, 2013), which was also accompanied with a very low Ra loss. A typical 228Ra yield for the co-precipitation method was about 70%, whereas 25% fraction of 228Ra remained in the treated thorium nitrate acetic acid solution, which was intended to be used again, after accumulation of 228Ra, for new preparation of 228 Ra solution. Then the presence of 25% fraction of 228Ra in thorium nitrate solution enables shortening of the time of 228Ra accumulation. 2.1. Experimental procedure for method II The solvent extraction was tested in a 20-mL plastic vial containing 1.5 g Th(NO3)4 6H2O dissolved in a mixture of 3.45 g toluene, 3.85 g acetone, 2.04 g 65% HNO3 ( 2 mol L 1 HNO3 in organic solvents). The solution was mixed with 0.25 mL 37% HCl (one phase system) and then repeatedly (5 times) with 0.2 mL 37% HCl and 0.2 mL 64% HNO3 to create aqueous phase. The phases were separated (5 times) by centrifugation. Relative activity of 228Ra in the five aqueous phase fractions was about 40, 25, 15, 7 and 3%. In view of that also about 20% of Th (in total for the five fractions) passed to aqueous phase, this procedure can be considered as a pre-concentration separation step. However, a more efficient separation was achieved in a column chromatography arrangement with the aqueous phase fixed on a layer of adsorbent. The Th–Ra separation was performed in a column containing 4 g cellulose pre-treated with water and acetone. First, 228Ra was adsorbed from a 11 g solution consisting of 1.3 g Th(NO3)4 6H2O dissolved in a mixture of 3.45 g toluene, 3.85 g acetone and 2.04 g 64% HNO3. Then, Th was completely removed from the cellulose layer with 10 mL mixture of acetone, toluene and HNO3. After that, Ra was eluted with 10 mL diluted HCl (2 mol L 1 HCl). The organic phase (Th) fraction was collected for the determination of radium loss, or for a possible reprocessing of the Th(NO3)4 6H2O salt, which could be reused. The reprocessing
of thorium nitrate-acetone-HNO3 solution is more complicated than in method I, therefore the use of this method is rather considered for the cases where the same Th material is not intended for a repeated preparation of a 228Ra solution. The 228Ra losses in Th fraction and the cellulose layer were obtained by means of gamma-ray spectrometry measurement of 228Ac (911.2 keV) photons. The Ra elution from the cellulose layer was nearly complete ( 99.9%), thus the total loss was insignificant. 2.2. Activity determination The activity of 228Ra in the prepared solution was determined by gamma-ray spectrometry using comparison measurement with a 232Th standard (in equilibrium with the decay products) by means of 228Ac (911.2 keV) photons. The standard was prepared from a known amount (mass) of metallic Th that was produced before 1960, so the 228Ra activity was 499.7% of 232Th activity. The total relative standard uncertainty, including contributions of 232 Th half-life 14.02 (6) 109 a (0.43%), the material age ( 0.2%), Th concentration in metal (0.4%) and gamma ray spectrometry comparison measurement (0.28%) was estimated to be lower than 0.7%.
3. Results and comments The preparation was repeated several times from Th(NO3)4 6H2O salt and also from Th metal. The obtained solutions of 228Ra were mainly used for testing of procedures for the preparation of solid state 220Rn (thoron) emanation sources to achieve their maximal emanation power, while a source prototype with emanation power of about 87% was produced, which was, for example, higher value in comparison with emanation power of thoron sources based on electrolytically deposited 228Th ( 41%) used for realisation of thoron reference atmosphere (Röttger et al., 2010). The newly-made 228Ra solution contained decay products in nonequilibrium concentration (228Th was separated with Th fraction), however, the solution contained some parts of 224Ra that made it possible to determine the emanation power of a newly-prepared thoron source. 228 Ra standard solutions contained 1 mg L 1 BaCl2 (carrier) and 1 10 g L HCl. They were free of Th, since both methods ensured a complete separation of 232Th (230Th). In this case, when the test of the prepared 228Ra material for the presence of 232Th was required, the chemical separation of possible Th residuals from Ra was carried out by Ra co-precipitation with Ba in the form of barium chloride from an organic solvent mixture. A comparison of these two methods showed that the loss of 228 Ra was lower for the method II. However, the method II presents more disadvantages than method I to reprocess the resulting Th solution. Considering the chemical impurities, it was expected that the original Th materials used for preparation should be sufficiently pure, because, mainly when the second method was used, the most of inactive chemical impurities passed from the original materials to the final solution. To reduce the impurity content, it is possible in some cases, depending on the chemical composition, to co-precipitate Ra with Ba in the form of barium chloride from the appropriate organic solvent mixture. This Ra Ba co-precipitate procedure in the form of barium chloride was also used for Th–Ra separation in the case of preparation of 228Th from 228Ra and 224Ra from 228Th solutions. 4. Conclusion Two methods for 228Ra separation from 232Th have been developed. 228Ra was isolated from aged Th nitrate and Th metal. The
M. Havelka / Applied Radiation and Isotopes 109 (2016) 222–225
first method was based on Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution of thorium nitrate. The second method was based on solvent extraction from a toluene–acetone–HNO3 system, where radium remained in the aqueous phase fixed on a cellulose layer in a chromatographic column. The activity (up to 20 kBq) of the obtained 228Ra standard solution was determined with standard uncertainty 0.7% (k ¼1). Besides 232Th–228Ra separation, the described Th–Ra separation procedure can also be used for other radionuclide metrology applications in 228Ra–228Th and 228Th–224Ra systems. The obtained solutions were used, among other things, for the preparation of a prototype of thoron emanation source; its emanation power was about 87%.
225
References Bé, M.M., Christé, V., Dulieu, C., Browne, E., Baglin, C., Chechev, V.P., Egorov, A., Kuzmenko, N., Sergeev, V.O., Kondev, F.G., Luca, A., Galán, M., Huang, X., Wang, B., Helmer, R.G., Schonfeld., E., Dersch, R., Vanin, V.R., Castro, R.M., Nichols, A.L., MacMahon, T.D., Pearce, A., Lee, K.B., Wu, S.C., 2008. Table of Radionuclides. BIPM, France. Havelka, M., Bludovsky, R., 2013. Activity standardisation of 226Ra by 4πα liquid scintillation counting method. Appl. Radiat. Isot. 81, 216–220. NIST, Separation and Purification of Ra-228 for SRM 4339B. 〈http://www.nist.gov/〉, 2013. Röttger, A., Honig, A., Dersch, R., Ott, O., Arnold, D., 2010. A primary standard for activity concentration of 220Rn (thoron) in air. Appl. Radiat. Isot. 68, 1292–1296.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Preparation of
228
Ra standard solution
Miroslav Havelka Czech Metrology Institute, Radiová 1, 102 00 Prague, Czech Republic
H I G H L I G H T S
228Ra was isolated from 232Th salt. Two methods for Th–Ra separation. Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution. The activity of 228Ra in the standard solution was related to 232Th standard.
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2015 Accepted 24 November 2015 Available online 25 November 2015
For the preparation of a standard solution of 228Ra, 228Ra was isolated from 232Th salt. Two simple methods were developed for Th–Ra separation. Both are based on a very good solubility of thorium nitrate in organic solvents. The first one used Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution. The second method was based on solvent extraction, remaining Th in the organic phase, while Ra was concentrated in the aqueous phase. The activity of 228Ra (up to 20 kBq) in the standard solution was related to the 232Th standard by means of gamma ray spectrometry measurement. The obtained uncertainty was less than 0.7% (k ¼1). The standard solution was free of 232Th and contained the carrier in the usual concentration (1 g L 1 BaCl2, 10 g L 1 HCl). & 2015 Elsevier Ltd. All rights reserved.
Keywords: 228 Ra standard Th–Ra separation Ra–Pb co-precipitation
1. Introduction The 228Ra standard solution was proposed to be used for calibration and testing of the devices that measure naturally occurring 228 Ra. In the Czech Republic, these devices have so far been tested with 232Th standards that contain known amounts of decay products, at the same time containing also 228Ra. Besides calibrating and testing, the 228Ra solution can be used for the preparation of thoron (220Rn) emanation sources based on 228Ra. For these purposes, the solution should be free of 232Th, but it can contain Ba carrier at a reasonably low concentration. 228 Ra originates from 232Th decay (Fig. 1). Nuclear data were taken from Bé et al. (2008). The simplest way of 228Ra preparation is based on the separation of radium from “old” 232Th salt (or metallic Th) where 228Ra has been accumulated from the previous Th–Ra separation. Because of a very long 232Th half-life, the maximum specific activity of 228Ra is only 4.046 kBq/g in the case of metallic Th, or 1.596 kBq/ g in Th(NO3)4 6H2O salt, where 232Th and 228Ra are in radioactive equilibrium. Therefore, an efficient method has to be applied allowing separation of 228Ra from the large excess of Th. E-mail address: [email protected] http://dx.doi.org/10.1016/j.apradiso.2015.11.062 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
Some separation procedures include co-precipitation of Ra with Pb from Th solution (NIST, 2013) as the first (pre-concentration) step. Therefore, two procedures based on Ra–Pb co-precipitation from Th nitrate and Th-complex carbonates (aqueous) solutions were initially tested. The first one included precipitation of Pb(Ba,Ra)SO4. The second was based on Ra co-precipitation with mixed carbonates Pb(Ra,Ba)3Th(CO3)5 from Th-carbonates solution, where approximately 99% of 228Ra was separated by means of two sequential 300-mg Pb additions from 120 mL solution containing about 3.2 g Th. However, subsequently, two new more effective methods, based on a very good solubility of thorium nitrate in organic solvents, have been developed. Method I consisted in the Ra–Pb coprecipitation in the form of Pb(NO3)2 from a thorium nitrate solution in organic solvents. A disadvantage in all these Ra–Pb coprecipitation procedures was in the necessity to separate Ra from a “large” amount of Pb as one of the subsequent steps. To avoid this complication related to Ra–Pb co-precipitation, an alternative procedure, method II, based on solvent extraction, was proposed. In both methods, Th or Pb were completely separated from Ra, while other non-active chemical impurities partly passed from the original material to the final solutions, so their concentrations depend on the purity of the used salt. In both methods Ba carrier was added during the separation, which simplified the last
M. Havelka / Applied Radiation and Isotopes 109 (2016) 222–225
Fig. 1.
232
Th decay chain.
purification steps in which Ra was co-precipitated with Ba from organic solvents mixture (with the addition of mineral acids) in the form of barium chloride (which is practically insoluble in organic solvents). If needed, both methods can also be modified to prepare 228Ra solution without the addition of Ba carrier; however for long time stability of the solution sealed in glass ampoule, Ba carrier has to be added. 1.1. Principles of method I The Ra–Pb co-precipitation method utilises both the very good and the very poor solubility in organic solvents of Th(NO3)4 6H2O and Pb(NO3)2, respectively. To set up method I, the preliminary Ra–Pb co-precipitation test was made. The test proved, that three subsequent additions of 90 mg Pb to 25 mL acetic acid-HNO3 solution containing 2 g Th (5 g Th(NO3)4 6H2O, 1.4 g 65% HNO3 and 25 g acetic acid) resulted in a complete separation of 228Ra in the Pb(NO3)2 precipitates, whereas the single fractions contained 80%, 16% and 3% (in total 99%) of Ra. The Pb(NO3)2 solubility (under this condition) was estimated to be lower than 200 mg L 1. For Pb–Ra separation, new method of Ra–Ba coprecipitation from organic solvent mixture containing ammonium acetate was established. In this solution, Pb2 þ ions form a soluble complex compound with acetate anions. Because Ba carrier is added, the use of this method is limited to the cases where the Ba addition ( 1 mg for 40 mg Pb) does not interfere; otherwise a conventional method has to be applied.
223
toluene mixture saturated with water and HCl, with the addition of a small volume of HCl, whereas Ra partly passed to the aqueous phase. The Ra concentration ratio in organic and aqueous phase (distribution coefficient) corresponded to the (experimentally obtained) value of 0.015. Another possibility is to fix the aqueous phase in any adsorbent, for example in cellulose, and carry out the Th–Ra separation by solvent extraction using column chromatography arrangement. In this case, Ra was firstly retained in the column (in aqueous phase) from Th(NO3)4 solution in the mixture of acetone–toluene and HNO3, and subsequently, after washing of the column to completely eliminate Th, Ra was eluted with a diluted acid solution. In Th–Ra separation, simultaneously with Ra other elements, Ac, Pb and Bi (present as radionuclides in Th salt), passed to the aqueous phase, so that in the organic phase, mainly Th isotopes, 232 Th and 228Th (230Th), remained (220Rn and Po nuclides quickly disintegrated). These radionuclides emit alpha particles of energy o5.42 MeV, while 228Ra, 224Ra and its decay products-220Rn and 216 Po (which reach the 0.999 equilibrium in 10 min), emit alpha particles of higher energy or beta particles in range up to 3.5 MeV. This fact was used for quick monitoring of Th–Ra separation by means of LSC. The spectra of samples prepared from aliquots of the original (Th nitrate salt) solution and Th fraction in UG LLT cocktail are shown in Fig. 2. Thorium concentration in the sample prepared from Th fraction (with no detected beta emitters) was obtained from α particle peak area. Ra concentration was indicated as the activity of 228Ra, recorded in low energy channels, or as 224Ra activity. This last was calculated from the part of LSC spectra above Th peak, where alpha particles of 224Ra, 220Rn and 216Po were recorded.
2. Experimental procedure for method I The Th–Ra separation was carried out from a Th nitrate solution containing 16.56 g of Th(NO3)4 6H2O, 126.8 g acetic acid and 1.4 g 65% HNO3 ( 0.12 mol L 1 HNO3 in acetic acid). Ra was co-precipitated with Pb (as a mixed nitrate) by the addition of 45 mg Pb (0.28 mL of Pb acetate in acetic acid solution). The Pb(Ra)(NO3)2 precipitate was separated from the solution by centrifugation, washed with 3 mL of acetic acid-HNO3 mixture (21 g acetic acid, 0.28 g 65% HNO3), then dissolved in a mixture of 0.3 mL NH4C2H3O2 (ammonium acetate) -acetic acid-acetone solution (prepared from 0.77 g (NH4)2CO3, 2.1 g acetic acid and 1.75 g acetone) and 0.1 mL of acetic acid. To remove the residuals of Th, a 600
This method is based on a Th–Ra separation by liquid–liquid extraction method in HNO3, acetone–toluene system, where Th is extracted to the organic phase while Ra concentrates in the aqueous phase. Besides a very good solubility of Th(NO3)4 6H2O in organic solvent, the method also utilises the formation of Th anion complex compounds (with HNO3) extractable to acetone–toluene mixture. Considering the low Th distribution coefficient in organic and aqueous phases ( 0.35), which was determined for the chemical system used in this work, to achieve effective Th–Ra separation in batch-wise mode (to retain Th in organic phase), a low volume ratio ( 1:25) of aqueous to organic phases should be used. In order to stabilise volume ratio, the addition of HCl was applied. The aqueous phase then contained water, HCl, HNO3 and acetone, while the organic phase contained small amounts of water and HCl besides the main components (toluene, acetone and HNO3). Practically, the aqueous phase was precipitated from one-phase system – the solution of Th(NO3)4 6H2O in HNO3, acetone–
500 Count rate
1.2. Principles of method II
Th
228
Ra
400
224
Ra-220Rn-216Po
300 200 100 0
0
200
400
600
800
Ch. Fig. 2. Typical LSC spectrum of a Ra–Th sample and of a sample containing only Th. In both cases, 230Th (radionuclide impurity in Th nitrate) contributed to Th alphaparticle peak. Sources were prepared with UG LLT cocktail and were counted in TriCarb 3100TR counter.
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M. Havelka / Applied Radiation and Isotopes 109 (2016) 222–225
second Ra co-precipitation with Pb in the form of Pb(Ra)(NO3)2 was performed by the addition of 1 mL 2 mol L 1 HNO3 in acetic acid. After washing, the precipitate was dissolved in a mixture of NH4C2H3O2-acetic acid-acetone solution (0.4 mL). Afterwards, for the Pb–Ra separation, the Ra co-precipitation with Ba was used as the first step. After the addition of acetone ( 0.57 g) to reduce the solubility of Pb acetate complex compound (closely to the limit of precipitation of the second liquid phase), two fractions of 0.8 and 0.6 mg Ba (in the form of 0.1 mol L 1 Ba(CH3COO)2 in acetic acid) were added. Furthermore, another addition of acetone ( 0.3 g) was necessary for Ba salt precipitation. After 15 min, the both precipitates were separated by centrifugation from the solution where a large part of Pb remained. Both precipitates (in total they contained 97% of Ra separated from the original Th solution, 1.4 mg Ba, less than 5 mg Pb) were dissolved (in total) in 0.05 mL 57% HI and 1.4 mL acetone– toluene mixture (1.1 g:1 g) and the solutions were combined. Further, to the sample 0.05 mL of HCl-n-butanole (1:5) a mixture was added, causing Ra co-precipitation with Ba in the form of barium chloride. After the separation of the organic phase (with the dissolved Pb), the precipitate was washed with 0.05 mL 57% HI, 1.4 mL acetone–toluene mixture and 0.05 mL HCl-n-butanole mixture. To remove the rest of HI, the barium chloride precipitate was repeatedly washed with the same organic solvent mixture as that used for barium chloride precipitation but without HI. The Ra loss for this part of Pb–Ra separation was estimated as lower than 1.5%; such result was reached mainly because of the low solubility ( 1 mg L 1) of barium chloride (radium chloride) in the selected organic solvents. The Pb–Ra separation method was similar to that described in (Havelka and Bludovsky, 2013), which was also accompanied with a very low Ra loss. A typical 228Ra yield for the co-precipitation method was about 70%, whereas 25% fraction of 228Ra remained in the treated thorium nitrate acetic acid solution, which was intended to be used again, after accumulation of 228Ra, for new preparation of 228 Ra solution. Then the presence of 25% fraction of 228Ra in thorium nitrate solution enables shortening of the time of 228Ra accumulation. 2.1. Experimental procedure for method II The solvent extraction was tested in a 20-mL plastic vial containing 1.5 g Th(NO3)4 6H2O dissolved in a mixture of 3.45 g toluene, 3.85 g acetone, 2.04 g 65% HNO3 ( 2 mol L 1 HNO3 in organic solvents). The solution was mixed with 0.25 mL 37% HCl (one phase system) and then repeatedly (5 times) with 0.2 mL 37% HCl and 0.2 mL 64% HNO3 to create aqueous phase. The phases were separated (5 times) by centrifugation. Relative activity of 228Ra in the five aqueous phase fractions was about 40, 25, 15, 7 and 3%. In view of that also about 20% of Th (in total for the five fractions) passed to aqueous phase, this procedure can be considered as a pre-concentration separation step. However, a more efficient separation was achieved in a column chromatography arrangement with the aqueous phase fixed on a layer of adsorbent. The Th–Ra separation was performed in a column containing 4 g cellulose pre-treated with water and acetone. First, 228Ra was adsorbed from a 11 g solution consisting of 1.3 g Th(NO3)4 6H2O dissolved in a mixture of 3.45 g toluene, 3.85 g acetone and 2.04 g 64% HNO3. Then, Th was completely removed from the cellulose layer with 10 mL mixture of acetone, toluene and HNO3. After that, Ra was eluted with 10 mL diluted HCl (2 mol L 1 HCl). The organic phase (Th) fraction was collected for the determination of radium loss, or for a possible reprocessing of the Th(NO3)4 6H2O salt, which could be reused. The reprocessing
of thorium nitrate-acetone-HNO3 solution is more complicated than in method I, therefore the use of this method is rather considered for the cases where the same Th material is not intended for a repeated preparation of a 228Ra solution. The 228Ra losses in Th fraction and the cellulose layer were obtained by means of gamma-ray spectrometry measurement of 228Ac (911.2 keV) photons. The Ra elution from the cellulose layer was nearly complete ( 99.9%), thus the total loss was insignificant. 2.2. Activity determination The activity of 228Ra in the prepared solution was determined by gamma-ray spectrometry using comparison measurement with a 232Th standard (in equilibrium with the decay products) by means of 228Ac (911.2 keV) photons. The standard was prepared from a known amount (mass) of metallic Th that was produced before 1960, so the 228Ra activity was 499.7% of 232Th activity. The total relative standard uncertainty, including contributions of 232 Th half-life 14.02 (6) 109 a (0.43%), the material age ( 0.2%), Th concentration in metal (0.4%) and gamma ray spectrometry comparison measurement (0.28%) was estimated to be lower than 0.7%.
3. Results and comments The preparation was repeated several times from Th(NO3)4 6H2O salt and also from Th metal. The obtained solutions of 228Ra were mainly used for testing of procedures for the preparation of solid state 220Rn (thoron) emanation sources to achieve their maximal emanation power, while a source prototype with emanation power of about 87% was produced, which was, for example, higher value in comparison with emanation power of thoron sources based on electrolytically deposited 228Th ( 41%) used for realisation of thoron reference atmosphere (Röttger et al., 2010). The newly-made 228Ra solution contained decay products in nonequilibrium concentration (228Th was separated with Th fraction), however, the solution contained some parts of 224Ra that made it possible to determine the emanation power of a newly-prepared thoron source. 228 Ra standard solutions contained 1 mg L 1 BaCl2 (carrier) and 1 10 g L HCl. They were free of Th, since both methods ensured a complete separation of 232Th (230Th). In this case, when the test of the prepared 228Ra material for the presence of 232Th was required, the chemical separation of possible Th residuals from Ra was carried out by Ra co-precipitation with Ba in the form of barium chloride from an organic solvent mixture. A comparison of these two methods showed that the loss of 228 Ra was lower for the method II. However, the method II presents more disadvantages than method I to reprocess the resulting Th solution. Considering the chemical impurities, it was expected that the original Th materials used for preparation should be sufficiently pure, because, mainly when the second method was used, the most of inactive chemical impurities passed from the original materials to the final solution. To reduce the impurity content, it is possible in some cases, depending on the chemical composition, to co-precipitate Ra with Ba in the form of barium chloride from the appropriate organic solvent mixture. This Ra Ba co-precipitate procedure in the form of barium chloride was also used for Th–Ra separation in the case of preparation of 228Th from 228Ra and 224Ra from 228Th solutions. 4. Conclusion Two methods for 228Ra separation from 232Th have been developed. 228Ra was isolated from aged Th nitrate and Th metal. The
M. Havelka / Applied Radiation and Isotopes 109 (2016) 222–225
first method was based on Ra co-precipitation with Pb in the form of Pb(NO3)2 from acetic acid solution of thorium nitrate. The second method was based on solvent extraction from a toluene–acetone–HNO3 system, where radium remained in the aqueous phase fixed on a cellulose layer in a chromatographic column. The activity (up to 20 kBq) of the obtained 228Ra standard solution was determined with standard uncertainty 0.7% (k ¼1). Besides 232Th–228Ra separation, the described Th–Ra separation procedure can also be used for other radionuclide metrology applications in 228Ra–228Th and 228Th–224Ra systems. The obtained solutions were used, among other things, for the preparation of a prototype of thoron emanation source; its emanation power was about 87%.
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