2.1 Separation and purification of the 154Dy sample
The Dy fraction containing 154Dy was obtained from the reprocessing of 4 Ta samples from the STIP-II project. The procedure for the dissolution of the Ta samples is described in detail in [38]. Successively, a series of ion-exchange separation processes allowed us to obtain a purified Dy fraction in 1 M HNO3. During the separation process, the γ-emitter 159Dy (t1/2 = 144.4 d, Iγ = 2.29% at Eγ = 58 keV [39]) was added as an internal radio-tracer. The separation method for the retrieval of the Dy fraction is reported in detail in [40]. A scheme of the chemical separation steps is reported in Figure 1.
The homogeneous Dy fraction (in 1 M HNO3) was collected in a scintillation vial (HDPE material, capacity: 20 ml). The total mass of the collected Dy solution – from now onwards referred to as “Dy master solution” – was determined gravimetrically (averaged value of 5 consecutive weightings: 6.79404 g, see Table S1). All the gravimetric steps were performed on a certified Mettler-Toledo XP56 balance (10−6 g scale interval), in a room with controlled temperature within 20–23°C.
2.2 Mass spectrometric analysis
The concentration of 154Dy in the Dy master solution was calculated from the amount of 161Dy (deduced by SF-ICP-MS) and from the 154Dy/161Dy isotope ratio in solution (determined by MC-ICP-MS). 161Dy was chosen as reference nuclide due to the absence of isobaric interferences for mass 161. All gravimetric additions were done on a Mettler-Toledo XP56 balance.
SF-ICP-MS measurements. SF-ICP-MS analyses were conducted using a Thermo Scientific Element 2 spectrometer, applying the medium mass resolution setting in order to minimize potential effects of molecular interferences. The plasma was operated at 1350 W. All analytes were introduced into a cyclonic PFA spray chamber using an ELEMENTAL SCIENTIFIC PFA-ST nebulizer and a peripump set, with a sample consumption of ca. 130 µl∙min−1. An external linear calibration was used to establish the 161Dy concentration in the Dy master solution. In this procedure, several dilutions of a Dy standard stock solution (10 mg∙l1 in 2% HNO3, density: 1.00885 g∙ml1, concentration uncertainty: 2%, Sigma Aldrich) were repeatedly analyzed (before, in-between, and after the replicate analysis of the sample solution). In all the Dy dilutions used for the external calibration, a Re standard stock solution (10 mg∙l−1 in 2% HNO3, density: 1.00885 g∙ml−1, concentration uncertainty: 2%, ESI, Elemental Scientific, Omaha, NE, USA) was added as an internal reference. This allowed for cancelling potential temporal drift in instrumental signal response or plasma instability. The series of dilutions used for the external calibration scheme is presented in Table S2. An aliquot of the Dy master solution (averaged value of 5 consecutive weightings: 0.03000 g, see Table S3) was used for mass-spectrometry analysis. To this Dy aliquot, the same Re ESI standard stock solution used in the preparation of calibration standards were added as an internal reference. The Dy aliquot was then diluted with a 0.28 M HNO3 solution, to a total weight of 13.90972 g (averaged value of 5 consecutive weightings – see Table S3). Instrumental background signals (including potential imperfect washout between analytes) were subtracted by repeated analysis of the same acid used to prepare the external standards and the sample analytes. Each of these “blank” measurements preceded the standard and the sample analyses. The 161Dy content in the Dy master solution was obtained by correlating the background-corrected and Re-normalized 161Dy signal to the external calibration line.
MC-ICP-MS measurements. Dy isotopic ratio analysis was conducted on the Nu Instruments Plasma 3 MultiCollector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) equipped with an inductively coupled Ar-plasma ion source, 16 Faraday cups, 3 Daly detectors, and 3 secondary electron multipliers. These instrumentational characteristics allow for the simultaneous measurement of up to 22 ion beams. Analytes were introduced into the system using an Elemental Scientific Apex HF desolvating nebulizer and a self-aspiring Elemental Scientific PFA-ST Microflow at a consumption rate of ca. 50 µl∙min1. The plasma was operated at 1350 W forward power. Ion beams of masses 149 (Sm), 152 (Sm, Gd), 154-164 (Sm, Gd, Tb, Dy), 166-167 (Er), and 170 (Er, Yb) were collected simultaneously in Faraday cups connected to amplifier systems with a 1011 Ω resistors in their feedback loop. To assess potential isobaric interferences of Yb, mass 172 was monitored using a Daly ion counting detector. An aliquot of the Dy master solution was diluted by a factor of ca. 500 by addition of a pure 0.28 M HNO3 solution. To the diluted Dy aliquot, natEr was added, allowing for an empirical semi-external mass discrimination correction. Successively, six analyses of the so-prepared Dy sample were bracketed by 10 analyses of mixed Er-Dy solution standards. Each analysis consisted of 60 ten-second-long integrations of the ion beam intensities. Instrumental background signals were removed using interspersed analysis of the Dy sample and of the 0.28 M HNO3 solution used in the preparation of the analytes. Online recorded 170Er/166Er values of the admixed Er were used to determine the magnitude of instrumental mass discrimination during the analysis of the Dy sample.
2.3 Preparation of the 154Dy α-source for activity measurements
For the preparation of a thin radioactive source with the molecular plating technique, an aliquot of the Dy master solution (averaged value of 5 consecutive weightings: 2.77410 g - see Table S4) was used. The estimation of the deposition efficiency was necessary to determine the effective number of 154Dy atoms plated. The deposition yield was determined by monitoring the activity of the γ-tracer 159Dy added during the separation process (see Section 2.1). Specifically, the activity of 159Dy in the Dy aliquote before molecular plating was measured, and compared to the activity of the 159Dy plated on the deposition foil. Since isotopes of the same element behave chemically identically, the yield of deposited 159Dy is thus equal to the yield of deposited 154Dy. For a reliable deduction of the deposition yield, both 159Dy γ-activity measurements had to be performed in equal geometries. This was achieved by using a custom-made holder made of two interchangeable parts (see Figure 2), that allowed for performing γ-spectrometry measurements in two geometrically equivalent positions, namely Position A (used to quantify the activity of 159Dy before electrodeposition), and Position B (used to quantify the activity of 159Dy after electrodeposition). Technical drawings in scale of the holder are given in the Supplementary Information, Figure S1. In both positions, the distance between the radioactive source and the detector endcap was 5 cm. This distance was large enough to consider true coincidence summing effects negligible.
γ-spectroscopy measurements were performed with a BEGeTM (Broad Energy Germanium γ-detector, Mirion Technologies (Canberra), Inc.). Data acquisition and analysis were done using the GenieTM 2000 Gamma Acquisition & Analysis Software. Energy calibration was done using a 152Eu (t1/2 = 13.53 y, Iγ = 28.41% at Eγ = 121.78 keV) point-source (Physikalisch-Technische Bundesanstalt – PTB). The energy resolution FWHM (Full Width at Half Maximum) was 0.540 keV at 58.1 keV. Efficiency calibration was performed with a 133Ba (t1/2 = 10.54 y, Iγ = 32.9% at Eγ = 80.99 keV) calibrated source in HNO3 1 M (see Section 5 of the Supplementary Information for details). For Position A, a known amount of the 133Ba calibrated liquid source was put into the PEEK vial, evaporated to dryness under a N2 flow at T = 70°C, and dissolved in 400 µl of 1 M HNO3. Efficiency calibration in Position A gave a FEPE (full energy peak efficiency, φ) of (15.75 ± 0.25) % at 58 keV. For Position B, a known amount of the 133Ba calibrated liquid source was drop-deposited onto a graphite foil (thickness: 15 µm, purity: 99.8%, Flexible Graphite, GoodFellow). The liquid was evaporated by heating the graphite foil at T = 70°C. Efficiency calibration in Position B gave a φ = (15.36 ± 0.24) % at 58 keV. Details on the efficiency calibrations in Position A and B are given in Section 5 of the Supplementary Information.
γ-activity measurements before molecular plating (Position A). For the γ-spectrometry measurement in Position A, the Dy aliquot was transferred from the HDPE vial to a custom-made polyether ether ketone (PEEK) vial (internal diameter: 20 mm, thickness at the bottom: 1 mm), and evaporated to dryness at 70°C under a N2 gas flow. To ensure a complete transfer of the Dy aliquot, the HDPE vial was rinsed with 10 ml 1 M HNO3, transferring the washing solution to the PEEK vial, and evaporating the liquid to dryness. This process was repeated 5 times. Then, 400 µl of 1 M HNO3 were added in order to dissolve the dried Dy solid. The added volume corresponded to the minimum volume that would entirely cover the bottom of the PEEK vial. This step was necessary to avoid attenuation of the γ-rays of 159Dy at 58 keV due to the presence of Dy(NO3)3 crystals, as well as to ensure a specific geometry equivalent to the one of the electrodeposited radioactive source. The PEEK vial containing the Dy dissolved in 1 M HNO3 was placed in the custom-made holder. A graphite foil (thickness: 15 µm, purity: 99.8%, Flexible Graphite, GoodFellow) was inserted between the bottom of the PEEK vial and the detector endcap, as shown in Figure 2a. The γ-measurement of the 159Dy contained in the PEEK vial was performed for 5400 s.
Molecular plating. After the γ-spectroscopy measurements, the Dy solution was transferred from the PEEK vial to a HDPE vial and evaporated to dryness at 70°C under a N2 gas flow. To ensure a complete transfer of the Dy, the PEEK vial was rinsed with 5 ml 1 M HNO3, the washing solution was transferred to the HDPE vial, and the liquid was evaporated to dryness at 70°C under a N2 flow. This process was repeated 5 times. The dried Dy was then dissolved in 6 M HNO3 to promote the formation of nitrate species and again evaporated to dryness at 70°C under a N2 flow. Any organic species that might derive from the separation process described in Section 2.1 was digested by the addition of modified aqua regia, i.e., 1.5 ml 30% (w/w) H2O2 + 4.5 ml conc. HCl + 1.5 ml conc. HNO3. The solution was evaporated to dryness at 80°C under a N2 flow, and the residual solid was re-dissolved in a mixture of 2 ml conc. HNO3, 6 ml conc. HCl, and 2 ml conc. HF for the destruction and removal of any silica compound that might derive from the ion exchange resins used in the separation of the Dy fraction from the Ta matrix. The solution was then evaporated to dryness (80°C under a N2 flow), dissolved in 1 M HNO3, and re-evaporated to dryness (70°C in N2 flow). Finally, the electroplating solution was obtained by adding a 50:50 methanol (MeOH) / isopropanol (iPrOH) mixture to the dried solid residue, for a total volume of 10 ml. The liquid was then transferred to the electrodeposition cell made of polytetrafluoroethylene (PTFE). A description of the molecular plating setup can be found in [41]. Before electrodeposition, a cleaning procedure (stepwise rinsing in 1 M HNO3, MilliQ water, and iPrOH) was applied to the PTFE cell and to the spiral Pt wire (anode). The cathode, made of a copper block, was cleaned with 0.1 M citric acid, washed with MilliQ water, and rinsed with iPrOH. The graphite deposition foil (thickness: 15 µm, diameter of deposition area: 20 mm, GoodFellow Cambridge Ltd.) was cleaned with iPrOH before molecular plating. For a constant deposition temperature, the setup was implemented with a Peltier cooler at the cathode, maintaining the graphite foil at 15°C during the entire plating procedure. The distance between the two electrodes was approximately 10 mm. The electrodeposition of Dy on the graphite foil was achieved in 8 hours by applying a constant voltage of 550 V.
γ-activity measurements after molecular plating (Position B). The activity of the 159Dy contained in the Dy deposited on the graphite foil was measured by placing the foil in Position B (see Figure 2b). In between the graphite foil and the BEGeTM detector endcap, a PEEK disk (thickness = 1 mm, identical to the bottom of the PEEK vial) was inserted, as shown in Figure 2b. The γ-spectrometry measurement of the 159Dy deposited on the graphite foil was conducted for 2’160’000 seconds.
2.4 154Dy α-activity measurement
The graphite foil with the electrodeposited Dy was then transferred to an α-chamber for the measurement of the α-activity of 154Dy. α-spectrometry was performed using the Alpha Analyst Integrated Alpha Spectrometer (model A-450-21AM, Canberra) equipped with a silicon semiconductor detector (Passivated Implanted Planar Silicon – PIPS. Detector area: 450 mm2). Data acquisition and analysis were done using the GenieTM 2000 Alpha Analysis Software. Energy calibration of the detector was performed with an α-source of 148Gd (t1/2 = 74.6 y, Iα = 100% at Eα = 3.182 MeV), and a mixed 239Pu (t1/2 = 2.41 y, Iα = 70.77% at Eα = 5.157MeV), 241Am (t1/2 = 432.8 y, Iα = 84.8% at Eα = 5.486 MeV), and 244Cm (t1/2 = 18.1 y, Iα = 76.90% at Eα = 5.805 MeV) α-source. A FWHM of 24.73 keV was reached, with a sample detector distance (SDD) of 10.4 mm. The efficiency calibration of the detector was performed with a certified 241Am standard source (PTB, calibration reference n° PTB-6.11-2016-1769, A = (539 ± 11) Bq @ 01.11.2016), having the same diameter (20 mm) as the Dy deposition area on the graphite foil. Geometrical differences between the 241Am standard source and the Dy electrodeposited sample were further minimized by using holders with the same SDD for both samples. The activity of the 154Dy electrodeposited sample was measured at a defined solid angle. The α-spectrometry measurement of the electrodeposited Dy layer was conducted for 500’000 seconds.