Analytical determination of radioactive strontium and cesium by Thermal Ionization Mass Spectrometry

Journal Pre-proof Analytical determination of radioactive strontium and cesium by Thermal Ionization Mass Spectrometry M.P. Dion, Kellen W.E. Springer, Ryan I. Sumner, May-Lin P. Thomas, Gregory C. Eiden PII:

S1387-3806(19)30380-X

DOI:

https://doi.org/10.1016/j.ijms.2019.116273

Reference:

MASPEC 116273

To appear in:

International Journal of Mass Spectrometry

Received Date: 10 September 2019 Revised Date:

4 December 2019

Accepted Date: 6 December 2019

Please cite this article as: M.P. Dion, K.W.E. Springer, R.I. Sumner, M.-L.P. Thomas, G.C. Eiden, Analytical determination of radioactive strontium and cesium by Thermal Ionization Mass Spectrometry, International Journal of Mass Spectrometry (2020), doi: https://doi.org/10.1016/j.ijms.2019.116273. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Analytical Determination of Radioactive Strontium and Cesium by Thermal Ionization Mass Spectrometry M. P. Diona,1 , Kellen W. E. Springera,∗, Ryan I. Sumnera , May-Lin P. Thomasa , Gregory C. Eidena a Pacific

Northwest National Laboratory 902 Battelle Blvd., Richland, WA 99354 USA

Abstract Thermal Ionization Mass Spectrometry (TIMS) has been evaluated for the detection of the radioactive isotopes of Sr and Cs. The commercial instrument (i.e., a Thermo Scientific Triton) was investigated for the analysis of isotopic ratios of 89 Sr/90 Sr and 135 Cs/137 Cs in the presence of atomic isobars (89 Y and 90 Zr for 89,90 Sr analysis and 135,137 Ba for 135,137 Cs analysis). The decontamination achievable instrumentally was examined by isotopic ratio measurements of 89 Y/88 Sr and 90 Zr/88 Sr for Sr and 135 Ba/133 Cs and 137 Ba/133 Cs for Cs. The decontamination found was at or above 2.0E+8 for 90 Zr from 88 Sr, while the Y demonstrated a temperature dependence as it sublimed from the filament but remained better than ≈ 5E+7. The decontamination of Ba from Cs did not show any temperature dependence and remained above 5E+6 and 8E+6 for 135 Ba and 137 Ba from 133 Cs, respectively. Two standard fusion procedures one with sodium hydroxide (NaOH) plus sodium peroxide (Na2 O2 ) flux, and the second used lithium tetraborate (Li2 B4 O7 ) plus lithium metaborate (LiBO2 ) flux were evaluated for preparation of sample matrices prior to performing chemical separations. Ammonium molybdophosphate-polyacrylonitrile (AMP-PAN) and Sr-spec resin were used to isolate the Cs and Sr, respectively from a prepared background matrix (i.e., Montana Soil). A graded approach, increasing in stable background isotopes, was performed to monitor the chemical and instrumental response. The radioisotopes of Sr and Cs were produced by thermal neutron irradiation of a highly enriched uranium foil. Even though the irradiated sample was not a certified standard it does provide accurate expectation values via the published cumulative fission yield nuclear data in the Evaluated Nuclear Data Files (ENDF) [1]. The intra-element isotopic ratio results presented in this work for 89 Sr/90 Sr and 135 Cs/137 Cs agree with the published data at 1σ. Furthermore, the uncertainty of the isotopic ratio measurements with TIMS was a factor of 5 – 10 improved compared to these published values. ∗ Corresponding

author, Tel.: +1-509-372-5978 Email address: [email protected] (Kellen W. E. Springer) 1 Permanent address: Oak Ridge National Laboratory 1 Bethel Valley Rd., Oak Ridge, TN 37830 USA

Preprint submitted to Elsevier

December 14, 2019

Keywords: TIMS, environmental contamination, radiostrontium, radiocesium, fission, contamination

1. Introduction

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The presence of anthropogenic isotopes in the environment is a byproduct of global fallout from nuclear weapons testing, nuclear power production, spent fuel rod reprocessing, medical isotope production, etc. Developing methods to detect and characterize the radionuclides produced by these activities is important for estimating potential dose to inhabitants via environmental monitoring and to aid modeling efforts of the nuclear fuel cycle [2, 3]. Of particular concern is the emission of large amounts of fission product isotopes in a nuclear accident. These concerns are highlighted by the Chernobyl and Fukushima Daiichi nuclear power plant accidents which both spread vast amounts of radioactivity to the environment. Several radioisotopes released during such events are a threat to the human population because of the size of the release, the isotopes’ radioactive properties (i.e., emitted radiation and specific activity), and/or their biological and chemical behavior. These concerns have warranted several countries, including the United States, Canada and Japan, to routinely perform radiological monitoring of food sources, air and vegetation, and establish a protocol in emergency events. The monitoring program should consider radionuclides that pose an immediate threat, such as 131,132,133 I and noble gases like the radioisotopes of Xe, or other radionuclides that are transferred to the food system easily (134,137 Cs, 89,90 Sr, 238,239,240 Pu, etc.) [2, 4]. Complimentary to a monitoring program, various nuclear data such as half-lives, fission yields and cross sections are critical for modeling and calculations pertaining to the nuclear fuel cycle and associated waste management [5, 6]. The fission product isotopes of interest to this work were 89,90 Sr and 135,137 Cs. These isotopes have cumulative fission yields of 4.7% and 5.8% for 89 Sr and 90 Sr, respectively and 6.5% and 6.2% for 135 Cs and 137 Cs, respectively for thermal induced fission of 235 U. These longer lived strontium and cesium fission product isotopes pose a significant hazard because they have biological uptake pathways. These isotopes work their way into the food supply and because of the chemical similarities of strontium to calcium and cesium to potassium they can accumulate in bone and muscle, respectively [7, 8, 9]. Numerous techniques, encompassing both radiometric counting and mass spectrometry, have been studied with varying levels of complexity, detection limits and specificity for radiostrontium and radiocesium [10, 11, 12, 13, 14]. Strontium-89 and 90 can be detected by performing chemical separations and utilizing high efficiency β counting techniques; both isotopes decay by β − emission and this primary signature is the method of detection2 . However, the continuous β spectrum coupled with most low-resolution radiometric techniques are a challenge for the 2 Strontium-89 does have a 909 keV γ-ray signature but this is rarely utilized for analysis because of very low intensity (0.00956% [1]).

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individual isotopes to be completely resolved. Mass spectrometry methods, including Inductively Coupled Plasma Mass Spectrometry (ICP-MS) [15, 16, 17], Accelerator Mass Spectrometry (AMS) [18, 19, 20], Resonance Ionization Mass Spectrometry (RIMS) [21] and Thermal Ionization Mass Spectrometry (TIMS) [13, 22, 23], offer the ability to distinguish the isotopes of strontium by mass. Detection via mass spectrometry warrants investigation of the potential isobaric interferences [10] and some methods like AMS and RIMS can separate these interferences [24]. Detection of 137 Cs is commonly accomplished by gamma spectroscopy due to favorable specific activity (half-life of 30.1 years) and its γ-ray signatures. In comparison, the low specific activity of 135 Cs (half-life of 2.3 × 106 years) has historically limited the use of this isotope and impedes counting techniques. Recently however, the importance of 135 Cs pertaining to environmental monitoring has increased and mass spectrometry detection techniques have been investigated [11]. This work describes the development of chemical separations for Sr and Cs and subsequent isotopic ratio analysis by a Triton TIMS instrument. The Triton was contained in a radiological laboratory that allowed the handling of dispersible radioactive material. The intrinsic decontamination achievable within the instrument used in this work was tested in the presence of significantly larger atomic isobar interferences (i.e., Y and Zr for Sr analysis and Ba for Cs analysis). Several samples, including radioactive and stable isotopes, in various matrices were developed to evaluate the separation chemistry and the instrument sensitivity. This research measured the isotopic ratios of 89,90 Sr and 135,137 Cs as a means to reduce the steps required for detection compared to absolute quantification (i.e., measuring the atoms per gram of isotope in the irradiated sample). The results of isotopic ratio measurements of fission products of Sr and Cs using a Triton Multicollector TIMS are presented and are compared to the fission product yield ratios in the Evaluated Nuclear Data File (ENDF/B - VII.I [1]). The relative isotope ratio measurements demonstrate a precision improvement of 5 – 10 and are within (± 1σ) uncertainty of the published ENDF values. The 89,90 Sr and 135,137 Cs fission products were created by thermal fission irradiation of a highly enriched uranium (HEU) foil in a research reactor. While these samples are not a certified reference per se, the Pacific Northwest National Laboratory (PNNL) has significant experience with the production of fission products arising from thermal neutron irradiation and hence it provides an excellent means of evaluating the developed procedures. 2. Materials and Methods The fission product isotopes were produced by thermal neutron irradiation of 231 mg of HEU foil. The foil was irradiated for 200 min in the Massachusetts Institute of Technology Research (MITR) reactor operated at 90 kW on Jan 27th 2017. The foil was contained in a sealed quartz ampule and placed in vertical port 3GV; 3GV is a 3” thimble in a vertical orientation in the graphite reflector area of the reactor. This location offers a uniform, thermal flux (4 × 1012 – 1 × 1013 n · s−1 · cm−2 ) over a 24” span. Following irradiation, the foil was 3

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dissolved in 3 M nitric acid (HNO3 ) and a fraction of the total volume (40 mL) was delivered to PNNL where gamma-ray screening was performed to determine the absolute activity of the solution (i.e., total fissions per gram of solution). A 2 mL aliquot taken from the 40 mL volume was used for this research equivalent to ≈ 1E+12 fissions. The materials for chemical separations included HNO3 and hydrochloric (HCl) acids of Optima grade (Fisher Scientific); other reagents were reagent grade. Several resins were used to isolate Cs and Sr, including Dowex-50W, 8% cross-linkage, 200–400 mesh (Sigma-Aldrich), AMP-PAN 100– 600 µm (Triskem International, Chelex–100 100–200 mesh (Bio–Rad)), and Srspec resin 100–150 µm (Eichrom Technologies). These were manually packed into open columns (Environmental Express) and eluted by gravity flow. Montana Soil Standard Reference Material (SRM 2710) from the National Institute of Standards and Technology (NIST), was used as the surrogate soil matrix for this research. 2.1. Decontamination, Isobaric Interferences and Abundance Sensitivity - TIMS

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The Triton was located in a radiological laboratory area so dispersible radioactive materials could be handled and analyzed on the instrument. The Triton was evaluated for detector and instrument response, abundance sensitivity (88 Sr on 89,90 Sr) and the decontamination achievable for 135,137 Cs and 89,90 Sr in the presence of significantly larger atomic isobars (89 Y and 90 Zr for 89,90 Sr analysis and 135,137 Ba for 135,137 Cs). The Triton instrument had nine Faraday cup detectors spaced by one mass unit (Da) and one secondary electron multiplier (SEM) with a retarding potential quadrupole (RPQ) lens. Filaments used for thermal ionization of Sr were zone-refined Re (H. Cross Company 99.999% purity) in a“single” geometry (sublimation and ionization occur on a single filament). The filaments were outgassed under a vacuum of <2E-6 Torr by resistive heating of 4.5 A current for 60 min and 5.5 A current for 15 min prior to sample loading. All Sr samples were contained in ≈ 1 uL of 2 M HNO3 prior to directly loading onto the Re filaments with a pipette. The prepared solution containing the analyte was dried onto the filament by resistive heating at atmospheric conditions (typically using 1.0 A current). After this drying step, a Ta activator solution was added to the filament and heated to a very dull red (typically 2.1 A current) to enhance ionization efficiency and improve ion-beam stability [25]. To evaluate the abundance sensitivity at mass-to-charge (m/z) 89 and 90 a sample was produced of natural Sr from SRM 987 (an isotopic standard of Sr carbonate). The sample of ≈ 1E+15 atoms was loaded onto a TIMS filament. The 88 Sr ion beam was measured on the central SEM detector and the response of the two (mass) adjacent Faraday cups (into a 1011 Ω resistor) were recorded as the beam was scanned across the detectors. The data and instrument response at a filament temperature of 1700 ◦C is given in Figure 1. The SEM scan begins at the right shoulder of the 88 Sr peak and as the ion beam is scanned passed the SEM, there is signal acquired as the ion beam is transmitted through the two adjacent Faraday cups. This is not uncommon and demonstrates the sensitivity

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of the SEM to stray particles generated by the ion beam at m/z 88 interacting with the vacuum chamber. The first adjacent Faraday cup (L1) shows the response of the ion beam at m/z 88 as it is scanned across the detector array. The second Faraday cup (L2), 2 Da away from the SEM, shows the response of the ion beam at m/z 88 (measured at 90 Da on the scale in Figure 1) and the ion beam at m/z of 87 (measured at 89 Da). The abundance sensitivity of the m/z 89/88 ratio was ≈ 2E-10; only noise response of the Faraday detectors in response to the shoulder of the ion beam at m/z 88 when centered on the SEM was observed. In addition, the flat tops of the L2 response should demonstrate the natural isotopic abundance of 88 Sr (82.584 ± 0.006%) to 87 Sr (7.001 ± 0.002%) [26]; 88/87 of 11.796 ± 0.003. After analysis of the L2 data (n=7), 88/87 m/z isotopic ratio was 11.77 ± 0.02 in agreement at 1σ.

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Mass [Da] Figure 1: A mass scan showing the response of the SEM and two adjacent Faraday cups from a natural Sr standard (SRM 987) as the ion beams are scanned across the detector array. The abundance sensitivity of m/z 89/88 ratio was ≈ 2E-10. See text for further description.

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Multi-element standards were gravimetrically produced from dilutions of SRM 987 and Institute for Reference Materials and Measurements (IRMM) 635 (84 Sr in 1 M HNO3 ) for Sr analysis. Strontium, Y and Zr were loaded in approximately equal proportions onto a TIMS filament at 2.8E+15, 3.4E+15 and 1.7E+15 atoms each, respectively. The 88 Sr was measured relative to the 89 Y and 90 Zr signals. The Zr decontamination factor was consistently at or above 2.0E+8 while Y decontamination factor reaches a maximum of 2.1E+8 and reduces at filament current greater than 3000 mA to ≈ 5E+7 (>≈ 1725 ◦C) as Y begins to sublime and ionize. The results of the decontamination achievable 5

through this experiment are given in Figure 2.

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10−9 2600 2700 2800 2900 3000 3100 3200 3300 3400 Filament [mA] Figure 2: Isotope ratios of 89 Y and 90 Zr to 88 Sr measured as a function of increased filament current (temperature). The filament was mass loaded with Y, Zr and Sr stable standards. Each data point is the average of n=3 cycles of instrument measurement; each cycle consisted of an eight second integration time. The calculated m/z isotopic ratios based on the amount of elemental standard loaded on the filament for 89/88 and 90/88 were 1.19 and 0.598, respectively. An increase in the 89/88 m/z ratio was observed due to Y beginning to sublime off the filament while the 90/88 m/z ratio doesn’t show a trend with filament current.

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For investigation of the Ba decontamination from Cs, a TIMS filament was mass loaded with 2E+15 atoms of 133 Cs, 5E+14 atoms of 134 Ba, 1E+14 atoms of 135 Ba, 2E+14 atoms of 136 Ba and 2E+14 atoms of 137 Ba. Filaments used for thermal ionization of Cs were zone-refined Re (H. Cross Company) in a “double” filament geometry. The filaments were outgassed under vacuum at 4.5 A for 60 min and then 5.5 A for 15 min. In this “double” filament geometry the sample-containing filament, referred to as the evaporation filament, was heated to sublime the sample while the second filament was elevated in temperature to ionize the element of interest (i.e., ionization filament). For the Cs analysis the ionization filament was heated to approximately 850 ◦C or ≈ 1.6 A and the evaporation filament was heated to a high enough temperature to obtain a stable ion beam, typically 500–700 mA. The purified Cs sample was added to the evaporation filament directly in a solution of 1% phosphoric acid (H3 PO4 ) and produced the most stable ion beam (versus loading in HNO3 or HCl). The filament was then dried (typically at a current of ≈ 1.3 A) in an ambient atmosphere subsequent to isotopic analysis. The 133 Cs was measured on a Faraday cup in a 1011 Ω resistor relative to the 134 Ba, 135 Ba, 136 Ba and 137 Ba signals on the SEM. The results of this experiment for the isotope ratios of 135,137 Ba

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to 133 Cs (the m/z ratios 135 and 137 are of interest to this research) are given in Figure 3. The decontamination found using the average value (n=3 for each data point) of the measured isotope ratios as a function of filament temperature at m/z 134, 135, 136 and 137 were 2E+7, 1E+7, 4E+7 and 3E+7, respectively. An increased Ba interference, due to a hot ionization filament, as described in Delmore et al. was not observed [27]. A linear fit to the data in Figure 3 does return a positive slope (1E-10 for the m/z ratio of 135/133 and 3E-11 for the m/z ratio of 137/133) but the instrument response was <1 count·s−1 at m/z 135 and 137 (i.e., Ba isotopes) and therefore it is concluded that this trend is not critical in the decontamination evaluation. These results and prior published research [13, 22] suggest that accurate 89 Sr/90 Sr and 135 Cs/137 Cs ratios can be obtained even with modest chemical purification prior to TIMS analysis because of the decontamination of isobars achievable within the Triton TIMS instrument as demonstrated. Therefore, the results presented here demonstrate 135

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Evaporation Filament [mA] Figure 3: Isotope ratios of 135,137 Ba to 133 Cs as measured by increasing the filament temperature. The filament was mass loaded with Ba and Cs stable standards. The m/z ratios calculated from the amount of elemental standard used for 135/133 and 137/133 were 0.0638 and 0.109, respectively. Using these expectation values in conjunction with the average of the isotope ratios across the filament temperature leads a decontamination of Ba from Cs at m/z 135 and 137 of 1.03E-07 and 4.36E-08, respectively. 180

that the stable isotopes of the elements (i.e., the isobaric interferences) used likely impacts the sought measured ratios (i.e., the radioisotopes of Sr and Cs) by ≈ 0.01%. These results indicate that the isobaric interferences are minimized if not eliminated using the Triton TIMS for the isotopes of interest in this work.

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3. Cesium and Strontium Chemical Separations 3.1. Stable Isotopes of Cs and Sr and Limit of Detection

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The chemical separations were first evaluated with stable elemental standards supplemented with lithium tetraborate (Li2 B4 O7 66%) plus lithium metaborate (LiBO2 34%) flux material and Pb, Zn, Cu, Cs, and Sr. Mixtures were created from a modified 68 element stable standard (High Purity Standards, Charleston, SC USA - p/n ICP-MS-68A). The ICP-MS-68A standard is composed of three matrices each with a select group of elements all at 10 ppm – mainly High Purity Standards ICP-MS-68A-A (48 elements in 2% HNO3 ), High Purity Standards ICP-MS-68A-B (13 elements in 2% HNO3 + trace hydrofluoric acid (Tr HF)) and High Purity Standards ICP-MS-68A-C (7 elements in 2% HCl). The ICP-MS-68A-C was recreated with individual elemental standards minus the addition of Rh so it could be used as an internal standard for the ICP-MS analysis used to track elements through the chemical separations. Zinc (Ultra Scientific Std, Santa Clara, CA USA - 1000 ppm, CGZN1), Pb (Inorganic Ventures, Christiansburg, VA USA - 1000 ppm, CGPB1), Cu (Inorganic Ventures, Christiansburg, VA USA - 1000 ppm, CGCU1), Sr (Inorganic Ventures, Christiansburg, VA USA - 1000 ppm, CGSR1) and Cs (Inorganic Ventures, Christiansburg, VA USA - 1000 ppm, CGCS1) and the borate flux material type 66:34 (XRF Scientific, Montreal Canada) were added to mimic the proposed soil matrix and the elements of interest (e.g., Pb, Zn, Cu, Sr and Cs). The sample was 0.25 g of the borate flux material dissolved in 2M HNO3 in a 20 mL polypropylene vial. Then 0.1 mL of the 68A-A, 68A-B and the modified 68A-C were added. This mixture was supplemented with 346 µg of Pb, 435 µg of Zn, 184 µg of Cu, 6.7 µg of Cs and 20.6 µg of Sr. Chemical separations and methods were developed to isolate Cs and Sr from this sample. To retain Cs, 10 mg of ammonium molybdophosphate-polyacrylonitrile (AMP-PAN) was added to the sample solution shaken for 5 min and subsequently filtered with a 17 mm diameter polypropylene syringe filter (0.2 µm mesh). The resin was rinsed with 0.4 mL of 0.1 M HNO3 . The filtrate was collected and reserved for the Sr separation. Cesium was stripped from the resin with 8 mL of a 5 M NH4 NO3 /0.1 M HNO3 solution. This solution was heated to dryness in a quartz crucible and decomposed by heating at 350 ◦C for 1 hr. The residue was dissolved in 1 mL 0.5 M HCl and loaded onto a 0.5 mL bed volume (BV) Dowex-50W (Sigma-Aldrich USA) that was pre-conditioned with 4 BV of 6.0 M HCl followed by 4 BV of 0.50 M HCl in order to retain Cs from the sample onto the resin. The column was rinsed with 4 BV of 0.5 M HCl to primarily remove any dissolved components from the AMP-PAN resin. The Cs was finally stripped from the Dowex column with 2 BV of 6 M HCl. The Sr was present in the AMP-PAN filtrate from the Cs separation. This filtrate was acidified to 10 M HNO3 and loaded onto a 0.5 mL BV of Sr-spec resin column that was pre-conditioned with 4 BV of 0.050 M HNO3 followed by 4 BV of 10 M HNO3 . After rinsing the Sr-spec column with 4 BV of 10 M HNO3 , the Sr was eluted from the column with 8 BV of 0.05 M HNO3 . This Sr solution was transposed to 6 M HCl prior to filament loading. 8

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The instrumental response for the separated Sr sample is given in Figure 4. The filament current (proportional to temperature), the 88 Sr response as the Faraday cup voltage, and the SEM response of the ion beams at m/z 89 and 90 are plotted as a function of instrument cycle. Each cycle (n=60) was an integration time of eight seconds. The ion beam at m/z 88 is well above the detection limit and tracks with the step-like increases of the filament current (i.e., about cycle 10, 20 and 40). As the current was increased, more Sr was released from the filament and subsequently ionized, increasing the ion current on the Faraday cup which then decreases slowly until the filament current was further increased. The SEM response of the m/z 89 and 90 ion beams represent background levels at a fraction of a count·s−1 ; an indication that Sr was isolated from Zr and Y. The 3σ upper limits on the m/z ratio of 89/88 and 90/88 are 6E10 and 3E-10, respectively. Using these results and assuming 400 ppm natural Sr in an environmental soil sample, the limit of detection (LoD) for 89 Sr is 1E+9 atoms (3E+10 fissions·g−1 ) and for 90 Sr is 7E+8 atoms (1E+10 fissions·g−1 ).

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Cycle Figure 4: Triton TIMS response of the separated Sr fraction from the multi-element standard. The dashed curve is proportional to the filament temperature represented here as current. The solid black curve with data points is the 88 Sr measured on a Faraday Cup (1011 resistor) and that response tracks with the increase in filament current. The signal from the two adjacent masses (m/z 89 and 90) were measured with the SEM detector. The ion beam response from the SEM detector is noise and demonstrates removal of any potential interference at m/z 89 and 90 when measuring the radioisotopes of Sr.

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The instrumental response for the separated Cs sample is given in Figure 5. The filament current (proportional to temperature), the 133 Cs response as the Faraday cup voltage, and the SEM response of the ion beams at m/z 135 and 9

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137 are plotted as a function of instrument cycle. Each cycle (n=60) was an integration time of eight seconds. The ion beam at m/z 133 is well above the detection limit and as the filament current was slowly increased, Cs was liberated from the filament quickly due to increased volatility at a lower temperature compared to Sr. As the current was further increased, the m/z 133 response reduced non-linearly with an increased filament current starting at cycle 20 and the filament was nearly depleted of Cs by the end of the cycling. The SEM response of the m/z 135 and 137 ion beams represent background levels at a fraction of a count·s−1 during the entire experiment; an indication that Cs was isolated from Ba. The 3σ upper limits on the m/z ratio of 135/133 and 137/133 are 1E-9 and 6E-10, respectively. Using these results and assuming 2 ppm natural Cs in an environmental soil sample, the LoD for 135 Cs is 9E+6 atoms (1E+8 fissions·g−1 ) and for 137 Cs is 5E+6 atoms (9E+7 fissions·g−1 ).

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Cycle Figure 5: Triton TIMS response of the separated Cs fraction from the multi-element standard. The dashed curve is proportional to the filament temperature represented here as current. The solid black curve with data points is the 133 Cs beam measured on a Faraday Cup (1011 resistor). The signal from the two m/z of interest, 135 and 137, were measured with the SEM detector. The SEM response at m/z 135 and 137 is noise and demonstrates the chemical removal of any potential interference at these masses for the radioisotopes of Cs. 260

4. TIMS Isotopic Ratios of Radioactive Sr and Cs The radioactive isotope ratios of Sr (89,90 Sr) and Cs (135,137 Cs) were measured in a progressive approach. First, to evaluate the chemical separation meth-

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ods and instrumental techniques, the dissolved irradiated HEU sample was prepared with the addition of flux material and stable elements in a liquid matrix and Sr and Cs were isolated chemically. Finally, the dissolved HEU sample was combined with Montana Soil (SRM 2710), manually fused with sodium hydroxide (NaOH) and sodium peroxide (Na2 O2 ) flux in a 1:1 mass ratio and then processed to chemically isolate Sr and Cs. Both isolated samples of Sr and Cs were then analyzed with TIMS to quantify the intra-element ratios of 89 Sr/90 Sr and 135 Cs/137 Cs and then compared to published fission yield data in ENDF/B - VII.I [1]. An aliquot of ≈ 0.5 mL of the dissolved HEU fission product solution in 3 M HNO3 was used for initial evaluation. In order to achieve a successful detection from the Trition TIMS, 4E+11 fissions was used (4E+11 fissions is 1.9E+10, 2.3E+10, 2.6E+10 and 2.5E+10 atoms of 89 Sr, 90 Sr, 135 Cs and 137 Cs, respectively, not accounting for radioactive decay). To this sample, natural Cs and Sr (100 ng each; ≈ 5E+14 atoms of each) were added. The chemical separations were run in duplicate with a process blank. The process blank did not contain any of the fission product solution but did contain SRM 2710 and the Na flux material. To retain Cs, 5 mg of AMP-PAN was added and mixed for 5 min. This solution was drawn through a syringe filter and the filtrate, which contained Sr, was collected and reserved. The AMP-PAN was rinsed twice with 0.2 mL of 0.1 M HNO3 and the filtrate was collected (0.8 mL of 1.55 M HNO3 ). The Cs was retained on the AMP-PAN and was stripped with eight 1 mL aliquots of 5 M NH4 NO3 /0.1 M HNO3 . The strip solution was collected and heated to dryness (350 ◦C for 1 hr) in a 20 mL quartz crucible. The residue was dissolved in 1 ml of 0.5 M HCl and passed through a Dowex-50W (Sigma-Aldrich USA) column that was preconditioned. The column was washed with 4 BV of 0.5 M HCl, then rinsed with 1 BV 6 M HCl at a flow of 0.5 BV·min−1 and collected separately. The column was finally stripped with 2 BV 6 M HCl (gravity flow), collected and transferred into a perfluoroalkoxy alkane (PFA) Savillex (Eden Prairie, MN USA) vial and sent for TIMS preparation and analysis. The small volume of filtrate containing Sr was processed by adding 1.13 mL of concentrated HNO3 for a total volume of 1.93 mL in 10 M HNO3 . A preconditioned Sr spec column was used for isolation of Sr. The column was loaded with the filtrate sample and washed with 4 BV of 10 M HNO3 . Next, the column was rinsed with 1 BV of 0.05 M HNO3 ; the rinse eluate was collected separately. Finally, the column was stripped with 4 BV of 0.05 M HNO3 . The eluate was collected and heated to dryness in a 5 mL quartz crucible. The residue was dissolved with slight warming in 0.25 mL of 6 M HCl. This solution was transferred to a PFA Savillex (Eden Prairie, MN USA) vial and additionally, the crucible was rinsed twice with 0.25 mL of 6 M HCl and transferred to the vial. The vial was sent for TIMS preparation and analysis. The filaments were prepped for TIMS analysis as previously described and the results of the measurement are given in Table 1. The isotopic ratios have been time decay-corrected back to the end of neutron bombardment and are compared to the expected cumulative fission yield ratios given in ENDF/B VII.I [1]. The half-lives used for time decay-corrections were taken from NuDat 11

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2.7 [28] as 50.563 d, 28.90 y, 30.08 y and 2.3E+6 y for 89 Sr, 90 Sr, 135 Cs and 137 Cs, respectively. Agreement between the measured isotopic ratios for 89 Sr/90 Sr and the nuclear database value are within 1σ. The precision in the Sr isotopic ratio analysis has increased by a factor of approximately 5 compared to the ENDF propagated uncertainty. The two rows of 89 Sr/90 Sr highlighted by an asterisk, were samples that were prepped and loaded on a filament but allowed to decay for 80 days (1.6 half-lives of 89 Sr). This depleted the 89 Sr atoms and increased the 89 Y (stable) atoms on the filament. There was no observable change in the accuracy or precision following this atom abundance difference. Table 1: Results of decay-corrected isotopic ratios of Sr and Cs following analysis by TIMS of the liquid samples described above. These results are compared to cumulative fission yields given in ENDF/B - VII.I [1] for thermal neutron induced fission of 235 U. The standard deviations given for the ENDF values are 1σ propagated for the respective ratio. It should be noted that the uncertainty given in ENDF for the cumulative fission yield of 135 Cs is 64% due to the lack of experimental measurements, while the parent, 135 Xe is 0.7%.

Isotope Ratio

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Measured 0.8238 ± 0.0027 0.8220 ± 0.0019∗ 0.8235 ± 0.0024 0.8218 ± 0.0018∗ 0.8260 ± 0.0038 0.8228 ± 0.0033 0.8233 ± 0.0028 1.040 ± 0.013 1.030 ± 0.034 1.035 ± 0.026

ENDF

0.8185 ± 0.0116

1.057 ± 0.676

4.1. Fusion Process for Analysis As discussed in Section 3.1, the chemical separations for the stable isotope research used a lithium meta-/tetra-borate flux matrix. One complication was that the borate matrix created challenges for the proposed separations. Even though removal of the borate matrix could be accomplished, it greatly increased the time and effort of the separation processes. In addition, early experiments with fused borate glass proved that it was difficult to completely dissolve. Therefore, to allow easier solubility and eliminate the borate removal process, the fusion process for this analysis was changed to a NaOH plus Na2 O2 in a 1:1 mass ratio and performed in Zr crucibles. In addition to the flux material change, Pb was present in the previous borate dissolved solution at high concentrations. For Sr isolation, Sr-spec resin was used, and Pb preferentially retained on the column, letting the Sr pass through thus reducing the recovery efficiency. Therefore, the chemistry was modified by removal of Pb with a chelating ion exchange resin prior to the Sr-spec column. The fusion of the fission product solution and the surrogate soil (Montana Soil) had to be performed manually in a radiometric hood. The chemistry was 12

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performed in triplicate with a process blank. The process blank contained Montana Soil and the flux reagents (NaOH and Na2 O2 flux in a 1:1 mass ratio) but did not contain any of the fission product solution. The goal of this surrogate soil fusion sample was to achieve a ratio of fission products to surrogate soil of 1E+14 fissions·g−1 . Therefore, 0.01 g of Montana Soil was mixed with 0.5 g of the dissolved irradiated HEU foil. Then, 0.08 g of NaOH and 0.08 g Na2 O2 , the Montana Soil and the fission product solution were added directly to the Zr crucible and heated gently to dryness. The crucible was then placed in a muffle furnace at 900 ◦C for 4.5 min, with periodic (1.5 min interval) stirring. The fused melt had a total mass of 2.45 g and was dissolved in a final matrix of 10 mL of 2 M HNO3 . The dissolved fusion melt was shaken for 5 min with 10 mg of AMP-PAN resin and filtered with a 17 mm diameter polypropylene syringe filter (0.2 µm mesh). The resin was rinsed with 2 mL of 0.1 M HNO3 . The filtrate was collected and reserved for Sr separation. The Cs was stripped from the resin with 8 mL of a 5 M ammonium nitrate (NH4 NO3 )/0.1 M HNO3 solution. This solution was heated to dryness in a quartz crucible and decomposed by heating at 350 ◦C for 1 hr. The residue was dissolved in 2 mL 0.5 M HCl and loaded onto a 0.5 mL BV Dowex-50W pre-conditioned (as discussed previously) column to retain Cs from the sample onto the resin. The column was rinsed with 10 BV of 0.5 M HCl to remove any dissolved components from the AMP-PAN resin. The Cs was eluted from the Dowex column with 3 BV of 6 M HCl. The AMP-PAN filtrate used in the Cs separation was heated to incipient dryness in a quartz crucible, the solids were suspended in 10 mL of 2 M ammonium acetate (NH4 CH3 CO2 ) and loaded onto a 0.5 mL BV chelating ion exchange resin (Chelex 100) column that was pre-conditioned with 4 BV of 2 M NH4 CH3 CO2 (pH 5.4 ± 0.1) in-order to retain the transition metals; Pb in particular. The column was rinsed with 4 BV of 2 M NH4 CH3 CO2 (pH 5.4 ± 0.1). The column effluent was collected, heated to dryness, and decomposed by heating at 350 ◦C for 30 min. The residue was dissolved in 5 mL of 10 M HNO3 and loaded onto a 0.5 mL BV Sr-spec pre-conditioned (as discussed previously) column. The Sr-spec column was rinsed with 6 BV of 10 M HNO3 and the Sr was then eluted from the column with 8 BV of 0.05 M HNO3 that had been heated to 50 ◦C. The results of the intra-element isotopic ratio analysis for Sr and Cs following the fusion and chemical separations agree with the values reported in ENDF at 1σ as shown in Table 2. One noticeable difference is a larger uncertainty by about a factor of three increase compared to the isotopic ratios for Sr given in Table 1 and a positive experimental error. The samples in Table 2 were analyzed 181 days after irradiation and 89 Sr had decayed by almost 4 half-lives. Even if the chemical separations and fusion process were lossless, only 2E+9 atoms of 89 Sr were present in the processed 1E+14 fissions·g−1 mixture. This atom concentration is very close to the LoD of 89 Sr and this is the basis for the increased uncertainty of the 89 Sr/90 Sr isotopic ratio. The Cs ratio does not exhibit a similar increase in uncertainty (compared to Table 1) mainly because the AMP-PAN is so selective to Cs even with the addition of the Montana Soil 13

Table 2: Results of the decay-corrected isotopic ratios of radioactive Sr and Cs following analysis by the Triton TIMS after chemical separations and a fusion processing of the fission product solution. These results are compared to cumulative fission yields given in ENDF/B VII.I [1] for thermal neutron induced fission of 235 U.

Isotope Ratio 89

Sr/90 Sr x ¯

135

Cs/137 Cs x ¯

385

Measured 0.8287 ± 0.0116 0.8243 ± 0.0115 0.8265 ± 0.0115 1.049 ± 0.013 1.051 ± 0.028 1.050 ± 0.022

ENDF 0.8185 ± 0.0116

1.057 ± 0.676

matrix. Furthermore, the LoD of Cs is significantly lower compared to Sr and the Cs atom concentration in the fission product solution did not change due to radioactive decay because of the longer half-lives of the isotopes investigated. The measured Cs intra-element ratios in Table 2 have negative experimental error compared to the ENDF value but are still within 1σ standard deviation. 5. Conclusions

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A Thermo Scientific Triton TIMS, a commercial instrument, has been applied to the measurement of isotopic ratios of fission products of Sr and Cs specifically 89 Sr/90 Sr and 135 Cs/137 Cs. The sensitivity of the instrument has been evaluated for both elements in the presence of significantly larger isobaric interferences; 89 Y and 90 Zr for 89,90 Sr analysis and 135,137 Ba for 135,137 Cs. Due to the differences in volatility of the elements of interest and their isobaric interferences, there is significant decontamination achievable within the instrument. For Sr, SRM 987 was mass loaded at 1E+15 atoms and the abundance sensitivity at m/z 89 in the presence of the 88 Sr ion beam was ≈ 2E-10. Instrument decontamination for Sr was evaluated by approximately equal mass loadings of Sr, Y and Zr (2.8E+15, 3.4E+15 and 1.7E+15 atoms, respectively) onto a Re filament. The m/z 88 (Sr) ion beam was measured relative to the m/z 89 (Y) and 90 (Zr) response. The Zr decontamination was equal to or better than 2.0E+8 and flat across the filament temperature variation (1575 – 1765 ◦C). The m/z 89/88 measurement showed a temperature dependence across the filament temperature range due to sublimation of the Y off the filament; the decontamination of Y to Sr was best at the lowest filament temperature (2.1E+8) and increased to 5E+7 at filament temperatures greater than 1725 ◦C. Barium decontamination from Cs was evaluated by mass loading a filament with roughly equal amounts of natural Ba and Cs with isotopic standards; 2E+15 atoms of 133 Cs and ≈ E+14 atoms of Ba. The decontamination found for Ba from Cs at m/z 135 ranged from 5E+6 to 4E+7 and at m/z 137 ranged from 8E+6 to 7E+7 across filament temperatures of 397 – 440 ◦C; the highest decontamination was observed at lower filament temperatures.

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Fusion procedures were used to prepare the sample prior to chemical separations used to isolate Cs and Sr from the bulk matrix. As a primer, a liquid standard sample to represent a soil matrix was developed from isotopic standards and Li2 B4 O7 (66%) plus LiBO2 (34%) flux material (no fusion was performed). AMP-PAN and Sr-spec resin were used to isolate Cs and Sr, respectively. Evaluation of the separated Sr sample produced 3σ upper limits on the m/z ratio 89/88 and 90/88 of 6E-10 and 3E-10, respectively and for Cs 3σ upper limits on the m/z ratio 135/133 and 137/133 of 1E-9 and 6E-10, respectively. The LoDs assuming 400 ppm of natural Sr and 2 ppm natural Cs in a sample are 1E+9 atoms for 89 Sr, 7E+8 for 90 Sr, 9E+6 atoms for 135 Cs and 5E+6 atoms for 137 Cs. The Li flux material was difficult to dissolve completely and therefore a NaOH and Na2 O2 flux in a 1:1 mass ratio was used for subsequent experiments. Fission product isotopes were produced by thermal neutron irradiation of a 231 mg HEU foil in a research reactor with a flux of 4 × 1012 – 1 × 1013 n · s−1 · cm−2 for 200 min. A liquid sample was developed with an aliquot of the dissolved HEU foil representing 4E+11 fissions (1.9E+10, 2.3E+10, 2.6E+10 and 2.5E+10 atoms of 89 Sr, 90 Sr, 135 Cs and 137 Cs, respectively), NaOH and Na2 O2 flux in a 1:1 mass ratio and stable isotopic standards of Cs and Sr. The time decay-corrected average value (n=288) of the 89 Sr/90 Sr isotopic ratio was found to be 0.8233 ± 0.0028 compared to the ENDF value of 0.8185 ± 0.0116 and for the 135 Cs/137 Cs isotopic ratio (n=120) was found to be 1.035 ± 0.026 compared to the ENDF value of 1.057 ± 0.676 in agreement at 1σ. Finally, a sample was manually fused with a mixture of hydroxide/peroxide flux, Montana Soil and the dissolved HEU solution to achieve a ratio of 1E+14 fissions·g−1 . The results of this work were 89 Sr/90 Sr (n=96) equal to 0.8265 ± 0.0115 compared to the ENDF value of 0.8185 ± 0.0028 and 135 Cs/137 Cs (n=120) was 1.050 ± 0.022 compared to the ENDF value of 1.057 ± 0.676. Both results agree with the published nuclear data at 1σ. The uncertainty for the 89 Sr/90 Sr measured ratio of this result increased by a factor of four compared to the liquid sample due to radioactive decay of the fission product solution which depleted the 89 Sr atom abundance (4 half-lives) in the sample. Both Cs isotopes are longer lived and were not affected. The results presented in this research demonstrate the utility of TIMS for the analysis of the isotopic ratios of Sr and Cs radioisotopes in environmental samples. The uncertainty of the Cs results are more than an order of magnitude better than the ENDF published values. The uncertainty of Sr results for the liquid sample are a factor of four better than the ENDF values while the fused sample is comparable to the published data. The uncertainty given in ENDF for the fission yield of 135 Cs is very large mainly due to the long half-life and little experimental measurements of this isotope. The proposed method could provide a means to improve the nuclear data files. Acknowledgments

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The lead author would like to thank Dr. Martin Liezers (PNNL) for his insightful comments during conversations as this manuscript was developed. The 15

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research described in this paper was supported by the Nuclear Forensics Division of the Defense Threat Reduction Agency and was performed at Pacific Northwest National Laboratory, a multi-program national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RLO 1830. References

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TIMS analysis of radiostrontium and radiocesium ratios performed at environmentally relevant trace abundances.

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Y, Zr, and Ba interferences instrumentally characterized and minimized through developed chemical separations.

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The measurement uncertainties of 135Cs/137Cs and 89Sr/90Sr are significantly reduced compared to the ENDF nuclear data values.

None of the authors listed here have any conflicts of interest.