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Separation and recovery of exotic radiolanthanides from irradiated tantalum targets for half-life measurements

  • Nadine Mariel Chiera,

    Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing

    Affiliation Laboratory for Radiochemistry, Nuclear Energy and Safety Research Division, Paul Scherrer Institute, Villigen, Switzerland

  • Zeynep Talip ,

    Roles Conceptualization, Investigation, Methodology, Writing – review & editing

    zeynep.talip@psi.ch

    Affiliation Laboratory for Radiochemistry, Nuclear Energy and Safety Research Division, Paul Scherrer Institute, Villigen, Switzerland

  • Adelheid Fankhauser,

    Roles Data curation, Investigation, Writing – review & editing

    Affiliation Analytic Radioactive Materials, Paul Scherrer Institute, Villigen, Switzerland

  • Dorothea Schumann

    Roles Funding acquisition, Project administration, Supervision, Validation, Writing – review & editing

    Affiliation Laboratory for Radiochemistry, Nuclear Energy and Safety Research Division, Paul Scherrer Institute, Villigen, Switzerland

Separation and recovery of exotic radiolanthanides from irradiated tantalum targets for half-life measurements

  • Nadine Mariel Chiera, 
  • Zeynep Talip, 
  • Adelheid Fankhauser, 
  • Dorothea Schumann
PLOS
x

Abstract

The current knowledge of the half-lives (T1/2) of several radiolanthanides is either affected by a high uncertainty or is still awaiting confirmation. The scientific information deriving from this imprecise T1/2 data has a significant impact on a variety of research fields, e.g., astrophysics, fundamental nuclear sciences, and nuclear energy and safety. The main reason for these shortcomings in the nuclear databases is the limited availability of suitable sample material together with the difficulties in performing accurate activity measurements with low uncertainties. In reaction to the urgent need to improve the current nuclear databases, the long-term project “ERAWAST” (Exotic Radionuclides from Accelerator Waste for Science and Technology) was launched at Paul Scherrer Institute (PSI). In this context, we present a wet radiochemical separation procedure for the extraction and purification of dysprosium (Dy), terbium (Tb), gadolinium (Gd), and samarium (Sm) fractions from highly radioactive tantalum specimens, in order to obtain 154Dy, 157-158Tb, 148,150Gd, and 146Sm samples, needed for T1/2 determination studies. Ion-exchange chromatography was successfully applied for the separation of individual lanthanides. All separations were conducted in aqueous phase. The separation process was monitored via γ-spectrometry using suitable radioactive tracers. Both the purity and the quantification of the desired radiolanthanides were assessed by inductively coupled plasma mass spectrometry. Test experiments revealed that, prior to the Dy, Tb, Gd, and Sm separation, the removal of hafnium, lutetium, and barium from the irradiated tantalum material was necessary to minimize the overall dose rate exposure (in the mSv/h range), as well to obtain pure lanthanide fractions. With the herein proposed separation method, exotic 154Dy, 157-158Tb, 148,150Gd, and 146Sm radionuclides were obtained in sufficient amounts and purity for the preparation of samples for envisaged half-life measurements. During the separation process, fractions containing holmium, europium, and promethium radionuclides were collected and stored for further use.

Introduction

Exotic radionuclides with relatively long half-lives are of great interest in a wide range of research domains. Benchmark radionuclides for astrophysics studies are for example the nuclides 146Sm, 148,150Gd, and 154Dy [13], whose production (and destruction) pathways provide essential information on the creation and on the development of the known Universe. Long-lived radiolanthanides such as 157Tb and 158Tb, produced during irradiation of the urania-gadolinia fuel, are of extreme importance in the field of nuclear energy, since the exact knowledge on the nuclear properties of these nuclides is necessary for nuclear fuel disposal and for nuclear transmutation processes [4]. Furthermore, 157Tb was lately marked out as a relevant nuclide for the discovery of sterile neutrinos in the keV mass region [5] whereas, in a more futuristic approach, 158Tb was suggested as a valid decay-energy source in nuclear accumulators [6]. It is apparent that these studies and applications require precise nuclear data, e.g., the half-lives (T1/2), of the above-mentioned long-lived radiolanthanides. However, for 146Sm, 148,150Gd, 154Dy, and 157,158Tb, only discordant or uncertain information on their T1/2 is available. As an example, a collection of T1/2 values for 146Sm, 154Dy, and 157Tb reported in literature is given in Table 1.

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Table 1. Half-life (T1/2) values of the exotic radiolanthanides 146Sm, 154Dy, and 157Tb reported in literature.

https://doi.org/10.1371/journal.pone.0235711.t001

Discrepancies in the available T1/2 data for 150Gd are observed as well, with values differing from one another of at least 20% [10, 24, 25]. For 158Tb, former measurements showed estimated uncertainties of 30% [26] and 20% [27], with the currently adopted T1/2 = 180 ± 11 y [28] still awaiting an independent corroboration. The scientific impact of such uncertain/contradictory values is considerable. A prominent example is 146Sm, whose recent T1/2 re-evaluation might implicate a redetermination of the chronological formation of the Solar System [12]. Currently, a confirmation on this latest measured value is still pending. A similar situation holds for 148Gd, for which a more recent T1/2 = 70.9 ± 1.0 y [29] was estimated. This value, given only as a preliminary result, supports the previous measurement of Prestwood et al. [30]. However, an update on this long-term measurement is still lacking. Reasons for these shortcomings in the nuclear databases are in many cases the limited availability of the isotopes of interest, often accompanied by difficulties in the preparation of samples with a high chemical purity. With the initiative “Exotic Radionuclides from Accelerator Waste for Science and Technology–ERAWAST”, launched by Paul Scherrer Institute (PSI) [31, 32], these limitations are circumvented by using highly active accelerator waste from the PSI accelerator-facilities as a source of exotic radionuclides for scientific purposes. Detailed information on this long-term project can be found at the website “psi.ch/en/lrc/erawast”. PSI owns one of the most powerful accelerators in Europe, i.e., a ring-cyclotron with 590 MeV proton energy and 2.2 mA beam current, which feeds the SINQ neutron spallation source. Between 2000 and 2001, the second SINQ Target Irradiation Program (STIP-II) [33] was performed at PSI, with the purpose of studying radiation damage effects on construction materials induced by high-energy protons and spallation neutrons. During that project, 51 samples of tantalum were exposed to fluxes of high-energy protons and spallation neutrons in the SINQ facility. It is known from previous studies [3436] that the majority of the radionuclides produced in Ta targets irradiated with protons and spallation neutrons are lanthanides (Lns), with isotopic compositions considerably enriched in neutron-deficient nuclides such as 146Sm, 148,150Gd, 154Dy, and 157,158Tb. After a cooling period of almost 20 years, the active Ta material from the STIP-II project represents an extraordinary source of exotic Sm, Gd, Dy, and Tb radionuclides to be used within the scope of ERAWAST. It is important to mention that the method applied for extracting the desired exotic Lns from the radioactive Ta matrix is crucial, since the purity of the retrieved material strongly affects the uncertainty of the half-life determination studies. For the separation of Lns several techniques are known, summarized in [3739]. In particular, the recovery of Lns from active Ta targets was reported in [4042]. However, in those studies (mostly involving amounts of Lns in the order of milligrams), the separation efficiency for each Lns was not stated, and furthermore, the applied procedures required an extensive use of organic solvents. Recently, a method for the extraction of ppb amounts of Dy, Gd, and Sm nuclides from proton irradiated Ta targets was reported for cross section measurements [43]. The separated Dy fraction contained traces of Ho and Tb, while the Sm fraction included small amounts of Eu and Pm. However, for the purpose of that work—the determination of production cross sections—a further separation of the lanthanide fractions was not necessary and, hence, was not performed.

Here, we report a chemical separation system for the extraction of Sm, Gd, Tb, and Dy from irradiated Ta samples, in sufficient amounts and purity for future half-life measurements. The separation process was monitored using γ-spectrometry. The purity as well the isotopic composition of each Lns fraction was assessed by inductively coupled plasma mass spectrometry (ICP-MS).

Experimental

Materials

Irradiated Ta samples.

Depending on their shapes, the Ta specimens were classified as “Tensile”, “Bend bar”, and “Charpy”. For the irradiation, they were packed in layers in specimen holders, and enclosed in SS 316L tubes. Then, they were inserted in the SINQ target (Target-4), within the Pb spallation target rods, as schematized in Fig 1.

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Fig 1. Sketch (not in scale) depicting the irradiation position of each specimen rod in the lower part of the Target-4 of SINQ.

The position of the Ta samples classified as “Tensile”, “Bend bar”, and “Charpy” are indicated in green, blue, and red, respectively. The shape of the Ta samples (in scale) is illustrated as well.

https://doi.org/10.1371/journal.pone.0235711.g001

All samples were irradiated for 16 months, with a total proton charge of 10.03 Ah. Further specifications on the irradiation conditions of the Ta specimens (irradiation experiment “IB”) are given in [33]. A list of the STIP-II Ta samples retrieved and available for separation is given in Table 2. In this study, the IB18, IB19, IB22, and IB55 specimens were treated.

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Table 2. List of Ta samples irradiated during STIP-II and available for separation.

The Sample ID refers to the identification number given during the STIP-II studies. The type and the irradiation position of each sample inside the SINQ Target-4 is indicated as well. The samples used in this study are shown in bold.

https://doi.org/10.1371/journal.pone.0235711.t002

Reagents.

Ultrapure water obtained from a Milli-Q2 system water purifier (Millipore Corp., Bedford, MA) was used in all the experiments. Mineral acids were of ACS grade, procured from Sigma-Aldrich (HCl, 37%), Fischer Chemicals AG (HF, 40%), Merck (HNO3, 69%), and Alfa Aesar (H3BO3, 99.99%). Lu standard for ICP, in the form of Lu(NO3)3, was purchased from Fluka Analytical. 2-hydroxyisobutyric acid (HIBA, Alfa Aesar) solutions in the concentration range 0.09 M– 0.25 M were prepared. All the HIBA solutions were buffered at pH 4.6 by addition of liquor NH4 (25%, Merck EMSURE®). The pH of the prepared solutions was measured with a digital pH-meter (Mettler Toledo—InLab® Expert Pro-ISM sensor). During the separation process, the pH of the eluents after passing through the column was determined using pH indicator strips MColorpHastTM (pH 0–14). Cation exchange resins LN (di(2-ethylhexyl)orthophosphoric acid—HDEHP), DGA (N, N, N’, N’-tetrakis-2-ethylhexyldiglycolamide), LN3 (di-(2,4,4-trimethylpentyl) phosphinic acid) extraction resins and Sykam (surface-sulfonated styrene/divinylbenzene copolymers) were used. LN, DGA, and LN3 resins (100–150 μm particle size) were purchased from Triskem International. Macroporous cation exchange Sykam resin (12–22 μm particle size) was supplied from Sykam GmbH. LN, DGA, and LN3 resins were kept in 1 M HNO3, while the Sykam resin was conserved in MilliQ grade H2O. Before use, the Sykam resin was pre-conditioned with NH4NO3 (ACS ISO grade, Merck EMSURE®).

Radioactive tracers for γ-spectrometry.

The separation process was monitored via γ-spectrometry. The γ-emitting radionuclides contained in the irradiated Ta targets and their main γ-lines monitored during the separation are reported in Table 3.

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Table 3. List of γ-emitting radionuclides contained in the irradiated Ta targets.

For each radionuclide, the half-life (T1/2), the energy (Eγ) and intensity (Iγ) of the main γ-lines monitored during the separation are indicated. Data taken from [44].

https://doi.org/10.1371/journal.pone.0235711.t003

Additionally, γ-emitting Lns tracers were added to monitor the separation of Sm, Gd, and Dy, respectively (see Table 4).

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Table 4. List of γ-emitting radionuclides tracers added to monitor the separation of Sm, Gd, and Dy.

For each radionuclide, the half-life (T1/2), the energy (Eγ) and intensity (Iγ) of the main γ-lines monitored during the separation are indicated. Data taken from [44].

https://doi.org/10.1371/journal.pone.0235711.t004

For each separation, ca 1–5 kBq of each radiotracer was placed in a HDPE scintillation vial, brought to dryness at 70°C under N2 flow, and then re-dissolved in 0.5 mL 0.05 M HCl. γ-spectroscopic measurements were performed with a high-purity germanium (HPGe) detector. Energy calibration of the spectrometer was executed with a certified 152Eu standard source from PTB (Physikalisch-Technische Bundesanstalt, Braunschweig, Germany). γ-spectra were analyzed with the Genie 2000 Gamma Analysis software.

Separation setup.

The separation set-up consisted of the solution reservoir (i.e., CELLSTAR® polypropylene tube, capacity 15 ml), a peristaltic pump, a separation column, and the collection vessel (HDPE material, capacity 20 mL). As separation column, single fritted filtration columns (capacity 3 mL, 10–20 μm filter) filled with the resin of interest were used. Bed heights were as follows: LN resin– 3 cm; DGA resin– 1 cm; SYKAM resin– 5 cm; LN3 resin– 1 cm.

Handling of radioactive material

Separation experiments were performed in the PSI Hotlab, in a lead shielded fume hood, applying the ALARA–As Low As Reasonable Achievable–principle. In each step, the dose rate was kept as 20 uSV/h (measured at the working distance), using dose rate meters 6150AD® Automation und Messtechnik GmbH. The personnel was equipped with personal dosimeters (for the detection of gamma, beta, and neutron irradiation), operational dosimeters (electronic, with immediate dose display and with audible indication of the radiation level when thresholds for dose or dose rates are exceeded), as well extremity dosimeters (finger rings integrating a thermoluminescence-based detector).

Separation procedure

The first step in the separation procedure concerned the dissolution of the radioactive Ta specimen, followed by the removal of the Ta matrix. Then, two fractional separation methods were applied, namely Method A and Method B. Method A was conceived as a direct fractional separation of the lanthanides after their extraction from the Ta matrix. However, due to the high activity of the treated samples, an alternative separation procedure was considered (Method B). Further details are given in the following sections.

Dissolution and removal of the Ta matrix.

The procedure for the dissolution of the Ta samples and the precipitation of the Lns fluorides is described in detail in [43]. The dissolution of the “Tensile” type specimen was carried out in 4 mL 10 M HNO3 and 4 mL conc. HF. When the same mixture of acids was applied to the “Bend bar” type samples, only a partial dissolution together with fluoride precipitation was obtained. The supernatant was thus removed and transferred to another centrifuge tube. Then, the remaining part of the target was dissolved in additional HNO3 and HF. To increase the Lns separation yield, precipitation steps were repeated in supernatants by adding Lu carrier. The formation of a LuF3 precipitate helped the co-precipitation of the other lanthanides as LnF3. The obtained precipitate was separated from the supernatant, rinsed and collected. The precipitate was washed with Milli-Q water to remove any remaining Ta. The scheme for the LnF3 precipitation is shown in Fig 2.

LnF3 was dissolved in a centrifuge tube with 1 mL 0.5 M H3BO3 and 1 mL 6 M HNO3. H3BO3 was used to bind any remaining HF and to facilitate the dissolution of the aqueous insoluble lanthanide fluorides by forming tetrafluoroboric acid in solution [45]. To facilitate the dissolution the tube was placed in a water bath at 80°C. After dissolution, alternatively Method A and Method B were applied in order to sequentially isolate the desired Lns fractions from the LnF3 precipitate.

Separation Method A.

A schematic representation of the separation Method A is given in Fig 3.

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Fig 3. Separation Method A.

The separation of the lanthanides of interest from the fluoride precipitate is performed with a two-step separation procedure (SYKAM+LN3 resins).

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The pH of the dissolved LnF3 was adjusted to 1 with 3 M NH4OH, and the γ-emitting tracers were added. The solution was then loaded into the Sykam macroporous cation exchange resin. After loading, ~12 mL of 1 M NH4NO3 was used to bring the column pH to ~5. Fractional Lns separation was performed with a gradient elution technique using different HIBA concentrations. Elutions were performed at a flow rate of 0.4 mL/min. Each eluted fraction, containing 0.8 mL, was measured by γ-spectrometry. After completion of the Lns separation, the fractions containing the same element were combined. To remove the complexing agent (HIBA) from each Lns fraction, LN3 extraction resin was used as a last step. The purified lanthanides fractions were separately eluted in 1 M HNO3. The purity of the so-obtained Sm, Gd, Tb, and Dy fractions was then evaluated via ICP-MS.

Separation Method B.

A schematic representation of the separation Method B is given in Fig 4.

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Fig 4. Separation Method B.

The separation of the lanthanides of interest from the fluoride precipitate is performed with a four-step separation procedure (LN+DGA+SYKAM+LN3 resins).

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The dissolved LnF3 was loaded into the LN resin. Hf and Lu were retained in the column, while Ba and the Lns lighter than Lu were eluted with 4 M HNO3. The fractions containing Ba and the Lns were combined for subsequent separation. Then, Lu and Hf were eluted with 6 M HNO3 and 10 M HNO3, respectively. The solution containing Ba and the Lns was loaded into DGA resin. The Lns were retained in the column, while the eluted solution (4 M HNO3) contained only Ba. After changing the eluent to chloric acid, further elution with 4 M HCl allowed for the complete removal of Ba from the DGA resin. The Lns were collected using 0.05 M HCl as eluent. This solution was successively loaded into the Sykam ion exchanger resin. No pH adjustment of the loading solution was performed. The pH of the column was adjusted to 5 by passage of ca 30 mL of 0.09 M HIBA. Lns were fractionally eluted from the Sykam resin with HIBA via a gradient elution technique, as in Method A. Each Lns fraction was then transferred to nitrate form by using LN3 resin and 1 M HNO3 as eluant. The purity of the so-obtained Sm, Gd, Tb, and Dy fractions was analyzed by ICP-MS.

ICP-MS measurements

The ICP-MS measurements were performed using an Element II® Sector-Field Inductively Coupled Plasma Mass Spectrometer (SF-ICP-MS) by Thermo Fisher. The samples were diluted in 0.28M HNO3 prepared from high purity nitric acid and Milli-Q water. Dilution factors (DF) of 10, 50, or 100 were chosen after pre-measurements (see Table 5).

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Table 5. Dilution factors (DF) for the ICP-MS analysis of the Dy, Tb, Gd, and Sm fractions obtained with Method A and Method B.

https://doi.org/10.1371/journal.pone.0235711.t005

This step was necessary for having in the measurement solution a suitable amount of the main isotopes to determine their concentration, while still being able to detect other lanthanide impurities by scanning the background. As an internal standard, a certified standard solution with 10 mg/L Re was added (ESI, Elemental Scientific, Omaha, NE, USA). The concentrations of the main isotopes in the solution were determined with an external multi-element standard (TraceCERT®, Periodic Table Mix 1, 2, and 3) prepared in different concentrations (dilution series). The ICP-MS settings (gas flows, torch position, and lenses) are tuned daily by introducing an in-house multi-element solution. From this solution, B, Rh, and U (1 ppb each) are used to optimize intensity, peak shape, and U/UO value to minimize oxide formation (3–4%). The mass offset for each isotope was checked by measuring the multi-element standard solution after tuning the machine. These mass offsets in the range of -0.1 to +0.1 were imported to the operation method. The operation method was set to scan the masses between 116 and 187 several times in low- and medium-resolution. Instrument settings and data acquisition parameters are shown in Table 6.

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Table 6. Operating conditions and data acquisition parameters used for the ICP-MS measurements.

https://doi.org/10.1371/journal.pone.0235711.t006

In the measurement sequence, the samples are bracketed with the measurement of the multi-element standard dilution series. Each sample, as well each standard series, is preceded by a HNO3-blank. Each sample was measured twice independently. The detection limit of each isotope is calculated with three times the standard deviation of the Blank measurements and the dilution factor of the samples is considered. The results of the measurement are given as intensities for each mass in counts per second (cps), average of the scans. All count rates are corrected for machine background contributions by subtracting the signals previously measured on pure acid solutions. Internal normalization to cancel drift in responsiveness of the machine is done relative to 185Re (internal standard). Then, the normalized count rate versus the concentration for each mass of the external multi-element standards series (measured before and after the samples) is interpolated and applied to calculate the concentration (ppb) of each sample. The relative uncertainty of the concentrations is empirically stated as 10% (2σ). The latter includes the variations of standard measurements treated as samples, the weighing uncertainties in the preparation of the standard and sample solutions, as well the uncertainty of the concentrations in the external standard (≤ 0.8%, 2σ).

Results and discussion

Due to the long time elapsed after irradiation of the Ta samples (i.e., 17 years) short-lived reaction products were already decayed. Exemplificative γ-spectra of the LnF3 precipitate and the supernatant after fluoride precipitation are shown in Fig 5.

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Fig 5.

Exemplificative γ-spectrum of (a) the LnF3 precipitate, and (b) the supernatant after fluoride precipitation.

https://doi.org/10.1371/journal.pone.0235711.g005

As in [43], an average 98% Lns separation yield from the Ta matrix was here obtained. The detection of 133Ba and 178m2Hf radionuclides in the LnF3 precipitate (Fig 5A) is due to the sorption of Hf and Ba cations on the negatively charged surface of the LuF3 carrier, that ultimately leads to the co-precipitation of Hf and Ba in the form of fluoride complexes [46]. The LnF3 precipitate showed a contact dose rate between 0.5 and 30 mSv/h, depending on the type of Ta target initially dissolved. This dose-rate mainly derived from the decay of 172Hf (and its daughter 172Lu), 178m2Hf, 173Lu, and 133Ba.

In Method A, the dissolved LnF3 was directly loaded into the Sykam resin. Prior to the loading, the pH adjustment of the sample was performed since the distribution coefficients (Kd) of Lns in cation-exchange resins increase with lowering the acidity of the loading solution. At such conditions, the competition for the resin binding sites between the lanthanide ions and the hydroniums of the loading solution is minimized, and thus, all the Lns are retained in the resin phase. After passage of 1 M NH4NO3 and ~10 mL 0.09 M HIBA, the pH of the column was adjusted to ~5. Only at this point, the elution of the Lns was observed. This is due to the fact that at strong acidic conditions the -COOH functional group of HIBA exists predominantly in un-dissociated form, and thus the ligand shows a poor coordination strength [47]. The extraction of the Lns from the resin was performed by complexation with HIBA. An example of the separation profile with Sykam resin is presented in Fig 6.

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Fig 6. Fractional separation of Ho, Dy, Tb, Gd, Eu, Sm, and Pm fractions with Method A.

Separation of the Lns was performed via a gradient elution with HIBA at pH = 4.6 on SYKAM resin.

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The removal of the Lns from the Sykam resin was performed by applying a gradient elution technique. With increasing the HIBA concentration, the number of molecules in the mobile phase capable of chelating the Lns (III) ions is augmented. This results in a lower affinity of the Lns ions for the cation exchanger, and hence, in their elution from the separation column. Because of the lack of Nd, Pr, Ce, and La γ-emitter tracers, these elements could not be individually separated and hence were eluted together with Pm by using 0.25 M HIBA, and collected in the “Pm fraction”. Satisfactory peak separations were obtained at an elution flow rate of 0.4 mL/min. At higher flow rates, overlap of peaks due to longitudinal diffusion were observed. After performing the fractional separation, each lanthanide of interest was eluted from the Sykam resin within ~20 mL of HIBA. After the Lns separation, Ba was stripped from the column with 4 M HNO3. The concentration of each lanthanide in a reduced volume (~5 mL) of 1 M HNO3 was performed with the LN3 resin column. The upload in HIBA phase exploited the high affinity of the resin towards Lns(III) ions in low concentrated acids [48], whereas 1 M HNO3 was sufficient to elute the Lns.

In the separation Method B, the extraction of Hf, Lu, and Ba from the fluoride precipitate was prioritized in order to decrease the dose rate to which the personnel was exposed. For the removal of Hf and Lu, LN resin was chosen due to the high Kd of Hf(IV) and Lu(III) on HDEHP resin [49, 50]. An example of γ-spectra showing the qualitative separation of Hf and Lu with LN resin is shown in Fig 7.

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Fig 7.

Exemplificative γ-spectrum (a) before and (b) after the separation of Hf and Lu from the LnF3 precipitate.

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With this separation step, a reduction of ~80% of the initial emitted dose was achieved. After elution from the LN resin, the Ba and the Lns fraction (4 M HNO3) was directly loaded onto the DGA resin. It is known that Lns(III) show a high affinity for the DGA resin, regardless of the used HNO3 concentration [51]. At the same conditions, Ba(II) is instead not retained. In order to elute the Lns from the resin, a different mobile phase is thus needed. It was previously shown that Lns heavier than Nd have a log10(Kd) > 3 on DGA resin for HCl concentrations above 4 M [51]. However, when the concentration of HCl is lowered, the affinity of Lns towards the DGA resin is dramatically reduced. Thus, the elution of the Lns was pursued with 0.05 M HCl. The collected Lns were directly loaded to the Sykam column, previous addition of the γ-tracers. Due to the low acidity of the 0.05 M HCl solution, its pH adjustment was not necessary. After loading the Lns in the Sykam column, passage of ~35–40 mL of 0.09 M HIBA was in average required to adjust the pH of the column to ~5. The Lns fractional separation followed the same procedure as previously described in Method A, with the difference that no Hf and Lu elution peaks were observed (Fig 8).

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Fig 8. Fractional separation of Ho, Dy, Tb, Gd, Eu, Sm, and Pm fractions with Method B.

Separation of the Lns was performed via a gradient elution with HIBA at pH = 4.6 on SYKAM resin.

https://doi.org/10.1371/journal.pone.0235711.g008

The purity of the collected Dy, Tb, Gd, and Sm fractions was analyzed by ICP-MS. Comparative ICP-MS results are shown in Figs 912. Only signals above the limit of detection (LOD) were considered.

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Fig 9. ICP-MS spectra of the dysprosium fractions obtained with Method A and with Method B.

Dy isotopes (green bars), other lanthanides and hafnium (orange bars), isobaric interferences for Dy (green patterned bars).

https://doi.org/10.1371/journal.pone.0235711.g009

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Fig 10. ICP-MS spectra of the terbium fractions obtained with Method A and with Method B.

Tb isotopes (green bars), other lanthanides (orange bars), isobaric interferences for Tb (green patterned bars).

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Fig 11. ICP-MS spectra of the gadolinium fractions obtained with Method A and with Method B.

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Fig 12. ICP-MS spectra of the samarium fractions obtained with Method A and with Method B.

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Chemical purity of each fraction was checked by assessing the absence/presence of the nuclides reported in Table 7.

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Table 7. Masses of reference (in Dalton), corresponding to nuclides not affected by isobaric interference.

The identification of the reported mass signals in the ICP-MS spectra corresponds to the univocal presence of a specific element and thus, to the related stable and long-lived isotopes.

https://doi.org/10.1371/journal.pone.0235711.t007

These nuclides were selected as benchmarks since they are not affected by isobaric interference, i.e., each atomic mass (A) corresponds only to a single nuclide.

In all the fractions, the disruption of the natural isotopic abundance of each element, with a predominance of neutron-deficient isotopes, was observed. ICP-MS analysis showed a significant presence of Lu (~100 times the LOD) in the Dy and Tb fractions obtained with Method A. The high amounts of Lu contained in the LnF3 precipitate led to the tailing of the Lu elution peak, which affected the quality of the Dy and Tb fractions. Presence of Hf nuclides (signals at A = 178 Dalton and A = 179 Dalton) in the separated Dy was recorded as well. In the same fraction, the signal at A = 165 Dalton related to Ho was detected and, as a consequence, the presence of the long-lived isotope 163Ho (T1/2 = 4570 y [44]) cannot be excluded. Thus, isobaric interference 163Dy-163Ho at A = 163 Dalton is expected. Signal at A = 167 Dalton (4 times the LOD) was recorded as well. This indicates Er impurities in the Dy fraction. Hence, isobaric interferences 162Dy-162Er and 164Dy-164Er are presumed. In the Tb fraction, the presence of Gd (concomitant detection of signals at A = 155 Dalton and A = 156 Dalton) and Dy (signal at A = 161 Dalton) was found. It follows that the isobaric interferences 157Tb-157Gd at A = 157 Dalton and 158Tb-158Gd-158Dy at A = 158 Dalton are expected. By increasing the Sykam column bed height, a better separation resolution might be achieved when an initial Hf, Lu, and Ba separation from the LnF3 precipitate is not applied. However, this would comport a considerable increase of the time required for the separation, and hence, an unnecessary exposure of the personnel to high dose rates.

In the Sm, Gd, and Dy fractions obtained with Method B, no isobaric interferences are detected. In the Tb fraction, however, a signal 3 times above the LOD at A = 161 Dalton–i.e., 161Dy–was recorded. Due to the detection of Dy in the sample, the presence of the 158Dy isotope cannot be excluded. From the isotopic abundances in the Dy fraction, a 161Dy/158Dy ratio of 1.61 was deduced. Since the amount of 161Dy in the Tb fraction corresponds to 1.90 ppb, a 158Dy = 1.18 ppb is calculated. The signal at A = 158 Dalton in the Tb fraction corresponds to 24 ppb. Hence, the contribution of 158Dy to the 158Tb-158Dy isobaric interference is less than 5%. Overall, the mass fraction of Dy in the Tb fraction is 1%.

The total amounts of 146Sm, 148,150Gd, 154Dy, 157,158Tb contained in the Sm, Gd, Dy, and Tb samples collected with Method B and analyzed by ICP-MS are listed in Table 8, together with the minimum amount of material needed for half-life determination studies.

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Table 8. Amount (in number of atoms, Na) of 146Sm, 148,150Gd, 154Dy, 157,158Tb collected in this work by applying Method B.

The minimum amount of material (in Na) required for performing half-life measurements are indicated as well.

https://doi.org/10.1371/journal.pone.0235711.t008

The remaining irradiated Ta samples (equivalent overall to 45.29 g of Ta material) will be processed by applying the separation Method B.

Conclusions

A radiochemical separation method for retrieving ppb level of Sm, Gd, Tb, and Dy from Ta targets irradiated with proton and spallation neutrons was developed. It was proven that the removal of Ba, Hf, and Lu at the beginning of the process was necessary not only to minimize the overall dose rate exposure (in the mSv/h range), but also to obtain pure Dy and Tb fractions (i.e., devoid of Lu and Hf contaminants). ICP-MS analysis showed a disruption of the natural isotopic abundances, with a predominance of neutron-deficient nuclides such as 146Sm, 148,150Gd, 154Dy, 157,158Tb. No isobaric interference was determined in the purified Sm, Gd and Dy fractions. In the Tb fraction, 1% of Dy (mass fraction) was detected. However, the contribution of 158Dy to the 158Tb-158Dy isobaric interference was reported as less than 5%. The purified lanthanide fractions are suitable for the preparation of samples for nuclear physics experiments, i.e., for the preparation of α- or γ-sources for half-life measurements. During the separation process, fractions containing Ba, Hf, Lu, Ho, Eu, and Pm radionuclides were collected as well. Further separation of these fractions is envisaged.

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