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Comparison of electrodeposition and precipitation for the preparation of samples for iodine analysis by AMS
Nuclear Inst. and Methods in Physics Research B 455 (2019) 201–203
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of electrodeposition and precipitation for the preparation of samples for iodine analysis by AMS
T
M.L. Adamic , J.E. Olson, I.R. Thomas1, M.G. Watrous ⁎
Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83402, United States
ARTICLE INFO
ABSTRACT
Keywords: Radioiodine Accelerator Mass Spectrometry AMS Electrodeposition 129 I-AMS sample preparation 129 I
Sample preparation for Accelerator Mass Spectrometry (AMS) analysis of 129I from environmental samples typically involves isolation and purification of the iodine followed by precipitation as silver iodide. The silver iodide is mixed with silver or niobium powder as a binder and medium for electrical conductivity and approximately 3 mg of the mixture pressed into a cathode for AMS analysis. Electrodeposited silver iodide on a silver clad niobium 50 µm diameter wire provides an attractive alternative to precipitation. Six inches of wire and electrodeposited silver iodide are easier to handle when pressing cathodes and minimizes possible cross contamination between samples. Although electrodeposition onto pure silver wires has been recently reported, the usage of silver clad niobium wires could offer an additional advantage. The use of niobium in the silver iodide matrix has been reported to result in higher intensity and long term stability of the iodide ion current. This paper presents a comparison of electrodeposition methods using silver and silver clad niobium wires to the traditional precipitated silver iodide mixed with silver and niobium powders. The comparison was done using materials with 129/127I ratios having nominal values of 5 × 10−10 and 5 × 10−11. For each 129/127I ratio material, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. For each ratio, the measurements of the separate days were combined for each method, yielding 168 measurements per method. An analysis of variance indicated minor statistical differences between the precipitated and electro-deposited materials, with the pooled standard error of the means being 0.2% for both ∼5 × 10−10 and ∼5 × 10−11 ratios.
1. Introduction In most 129I-AMS laboratories, iodine is chemically separated and purified from environmental samples and precipitated as silver iodide. The silver iodide is mixed with niobium or silver metal powder as a binder and medium for electrical conductivity in an AMS source and loaded as a target for the sputter ion source, where it is converted into a beam of negatively charged ions for AMS analysis. Numerous procedures have been reported to produce iodide targets. Previously our laboratory reported electrodeposition of AgI on silver wire as an alternative method to prepare samples for AMS [1]. Traditional sample preparation involves the preparation of an AgI precipitate, isolation, drying and mixing with Ag or Nb powder prior to transfer into a cathode. In comparison, electrodeposited AgI on silver or niobium clad silver wire is easily dried by dabbing on a dust free wipe, handled with tweezers and placed into a cathode. The electrodeposition sample
preparation method eliminates the need for a mortar and pestle to mix the small masses of AgI and metal powder, therefore avoiding possible cross-contamination from one sample to another. This paper describes our laboratory’s work to verify that our electrodeposition sample preparation method does not alter the 129/127I ratio and using an analysis of variance determine if the resulting samples are significantly different. 2. Experimental 2.1. Sample preparation Aliquots of ∼5 × 10−10 and ∼5 × 10−11 129/127I ratio silver iodide materials prepared at the Idaho National Laboratory (INL) [2] were dissolved in concentrated ammonium hydroxide and zinc metal [4]. Separate aliquots of the ammonium hydroxide solution were removed
Corresponding author. E-mail address: [email protected] (M.L. Adamic). URL: https://www.inl.gov (M.L. Adamic). 1 Deceased July 12, 2018. ⁎
https://doi.org/10.1016/j.nimb.2019.01.002 Received 30 January 2018; Received in revised form 25 September 2018; Accepted 4 January 2019 Available online 11 January 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 455 (2019) 201–203
M.L. Adamic et al. 127 +2
I into an offset Faraday cup located after the high energy analyzing magnet and then switching the 129I+2 into the gas filled ionization detector located after the ESA. Each AMS wheel consisted of eight samples of the original silver iodide material mixed 1:100 with silver powder (to result in current similar to the electrodeposit), eight electrodeposited on silver wire, eight electrodeposited on silver clad niobium wire, and eight re-precipitated silver iodide mixed 1:50 with niobium powder. Niobium has been reported to result in the highest intensity and long term stability of the iodide ion current [3], and was the primary reason for selecting silver clad niobium wire for the electrodeposition of AgI for AMS targets. A series of quality control (QC) samples interleaved amongst the samples are loaded on the 40 position sample wheel to correct for background and normalize the data to cross calibrate the detectors. The QC samples include matrix blanks, Woodward silver iodide mixed 1:100 with silver powder, and INL normalization materials mixed 1:100 with silver powder (129/127I = 5.62 × 10−11 ± 2.64 × 10−13 1σ calibrated using NIST SRM 3230).
Fig. 1. NEC ICAMS (figure courtesy of National Electrostatics Corp.).
3. Results and discussion
and electrodeposited on silver wire and silver clad niobium wire using the electrochemical cell previously described [1]. Eight silver iodide samples were electrodeposited over three different days on both silver wire and silver clad niobium wire. To produce an equivalent powder target from the same starting material, an aliquot of the ammonium hydroxide solution was removed and dry silver nitrate added to reprecipitate the iodide as silver iodide. The re-precipitated silver iodide was washed with Milli-Q water, dried and mixed 1:50 by mass with niobium powder. All samples were pressed into titanium cathodes for analysis by AMS.
3.1. ∼5 × 10−10 material results For the ∼5 × 10−10 129/127I ratio, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. The measurements of the separate days were combined for each method, yielding 168 measurements per method. The ∼5 × 10−10 experiment results are summarized in Table 1. At a 95% confidence the silver clad niobium wire and silver wires are not significantly different and the re-precipitated material and that diluted with silver powder are not significantly different. Standard error for means of measurements is 1.15 × 10−12 (0.2%). The range of the four methods is 2.4%. The differences noted in this data set are likely due to the sample preparation and handling and not an indication that the method of electrodeposition perturbs the 129I/127I ratio. The data for the E-11 129/127I material below further supports electrodeposition as an effective method for sample preparation for AMS.
2.2. AMS analysis All samples were analyzed at INL using the Iodine Compact Accelerator Mass Spectrometer (ICAMS) shown in Fig. 1. The ICAMS is an NEC 0.5 MV compact accelerator mass spectrometer designed to analyze iodine in the +3 or the +2 charge state. There is a low energy injection magnet followed by the accelerator and then a high energy analyzing magnet followed by an Electrostatic Analyzer (ESA). The low and high energy sides of the instrument have magnetic bouncing systems to rapidly switch the isotopes by changing the energy of the ions as they pass through the magnetic fields of the injection and analyzing magnets. Samples are loaded as solid silver iodide that has been electrodeposited on silver cladded niobium wire using the same procedure reported by our lab for electrodeposition on silver wire [1] and pressed into titanium cathodes for targets in the NEC Source of Negative Ions by Cesium Sputtering (SNICS) source. The SNICS source produces negative ions by cesium sputtering and the iodide anions are selected for analysis in the instrument. The negative ions are accelerated into a stripper cell at the center of the accelerator (0.5 MV) where they collide with the stripper gas (helium or argon) and undergo a charge reversal. The energetic process of stripping electrons from the ions eliminates most molecular interferences from the spectrum. Iodine in the +2 charge state is normally selected for the analysis resulting in a transmission of ∼40% through the accelerator using helium as a stripper gas. The isotopic measurement is performed by rapidly switching (∼7 Hz) the
3.2. ∼5 × 10−11 material results For the ∼5 × 10−11 129/127I ratio, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. The measurements of the separate days were combined for each method, yielding 168 measurements per method. The ∼5 × 10−11 experiment results are summarized in Table 2. At a 95% confidence, the samples deposited on silver wire, re-precipitated, and deposited on silver clad niobium wire, are not significantly different. The standard error for means of measurements is 1.05 × 10−13 (0.2%). Since the sample types of ∼5 × 10−10 or ∼5 × 10−11 deposited on silver wire, deposited on silver clad niobium wire and the re-precipitation came from the same solution they are the best comparison. This is an acceptable difference that can be attributed to the processing of the samples.
Table 1 ∼5 × 10−10 results summary. 5 × 10−10 ratio
1:100 w/Ag
Re-precipitated
Ag wire
Ag clad Nb wire
Average 129/127I Standard error
5.14 × 10−10 4.44 × 10−13 (0.09%)
5.11 × 10−10 4.15 × 10−13 (0.08%)
5.03 × 10−10 1.82 × 10−12 (0.36%)
5.02 × 10−10 1.27 × 10−12 (0.25%)
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Nuclear Inst. and Methods in Physics Research B 455 (2019) 201–203
M.L. Adamic et al.
Table 2 ∼5 × 10−11 results summary. 5 × 10−11 ratio 129/127
Average I Standard error
1:100 w/Ag −11
5.21 × 10 1.06 × 10−13 (0.20%)
Re-precipitated −11
5.16 × 10 8.22 × 10−14 (0.16%)
4. Conclusions
Ag wire
Ag clad Nb wire −11
5.15 × 10 1.35 × 10−13 (0.26%)
5.17 × 10−11 9.51 × 10−14 (0.18%)
or allow others to do so, for U.S. Government purposes. This information was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
Described is a comparison of electrodeposition and precipitation for the preparation of environmental iodine samples in a form appropriate for AMS analysis. Two silver iodide materials with 129/127I ratios of ∼5 × 10−10 and ∼5 × 10−11 were dissolved, electrodeposited on silver wire, electrodeposited on silver clad niobium wire, and re-precipitated as silver iodide. An aliquot of the original silver iodide material was diluted 1:100 with silver powder. AMS data obtained were compared to determine if the electrodeposition method altered the 129/ 127 I ratio. Analysis of variance indicated minor statistical differences between the precipitated and electrodeposited materials. The pooled standard error of the means was calculated to be 0.2% for both ∼5 × 10−10 and ∼5 × 10−11 ratio materials. These results also confirm that the dissolution of silver iodide did not perturb the iodine isotopic ratio. For the purpose of our laboratory’s routine measurements, these slight differences are acceptable.
References [1] M.L. Adamic, T.E. Lister, E.J. Dufek, D.D. Jenson, J.E. Olson, C. Vockenhuber, M.G. Watrous, Electrodeposition as an alternate method for preparation of environmental samples for iodide by AMS, Nucl. Instrum. Methods Phys. Res. B 361 (2015) 372–375. [2] D. Jenson, C. Vockenhuber, M.L. Adamic, J.E. Olson, M.G. Watrous, Iodine Standard Materials: Preparation and Inter-Laboratory Comparisons, AMS-13 poster CRI 11 2014. [3] Q. Liu, X. Hou, W. Zhou, Y. Fu, Accelerator mass spectrometry analysis of ultra-lowlevel I in carrier-free AgI-AgCl sputter targets, J. Am. Soc. Mass Spectrom. 26 (2015) 725–733. [4] A.C. Wahl, Fission of U235 by 14-Mev neutrons: nuclear charge distribution and yield fine structure, Phys. Rev. 99 (3) (1955) 730–739.
Acknowledgements The authors would like to thank Douglas Jenson, Ph.D. for the preparation of the INL 129/127I materials and Tedd Lister, Ph.D. for the initial electrodeposition work. This work is supported by the U.S. Department of Energy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution,
203
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of electrodeposition and precipitation for the preparation of samples for iodine analysis by AMS
T
M.L. Adamic , J.E. Olson, I.R. Thomas1, M.G. Watrous ⁎
Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83402, United States
ARTICLE INFO
ABSTRACT
Keywords: Radioiodine Accelerator Mass Spectrometry AMS Electrodeposition 129 I-AMS sample preparation 129 I
Sample preparation for Accelerator Mass Spectrometry (AMS) analysis of 129I from environmental samples typically involves isolation and purification of the iodine followed by precipitation as silver iodide. The silver iodide is mixed with silver or niobium powder as a binder and medium for electrical conductivity and approximately 3 mg of the mixture pressed into a cathode for AMS analysis. Electrodeposited silver iodide on a silver clad niobium 50 µm diameter wire provides an attractive alternative to precipitation. Six inches of wire and electrodeposited silver iodide are easier to handle when pressing cathodes and minimizes possible cross contamination between samples. Although electrodeposition onto pure silver wires has been recently reported, the usage of silver clad niobium wires could offer an additional advantage. The use of niobium in the silver iodide matrix has been reported to result in higher intensity and long term stability of the iodide ion current. This paper presents a comparison of electrodeposition methods using silver and silver clad niobium wires to the traditional precipitated silver iodide mixed with silver and niobium powders. The comparison was done using materials with 129/127I ratios having nominal values of 5 × 10−10 and 5 × 10−11. For each 129/127I ratio material, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. For each ratio, the measurements of the separate days were combined for each method, yielding 168 measurements per method. An analysis of variance indicated minor statistical differences between the precipitated and electro-deposited materials, with the pooled standard error of the means being 0.2% for both ∼5 × 10−10 and ∼5 × 10−11 ratios.
1. Introduction In most 129I-AMS laboratories, iodine is chemically separated and purified from environmental samples and precipitated as silver iodide. The silver iodide is mixed with niobium or silver metal powder as a binder and medium for electrical conductivity in an AMS source and loaded as a target for the sputter ion source, where it is converted into a beam of negatively charged ions for AMS analysis. Numerous procedures have been reported to produce iodide targets. Previously our laboratory reported electrodeposition of AgI on silver wire as an alternative method to prepare samples for AMS [1]. Traditional sample preparation involves the preparation of an AgI precipitate, isolation, drying and mixing with Ag or Nb powder prior to transfer into a cathode. In comparison, electrodeposited AgI on silver or niobium clad silver wire is easily dried by dabbing on a dust free wipe, handled with tweezers and placed into a cathode. The electrodeposition sample
preparation method eliminates the need for a mortar and pestle to mix the small masses of AgI and metal powder, therefore avoiding possible cross-contamination from one sample to another. This paper describes our laboratory’s work to verify that our electrodeposition sample preparation method does not alter the 129/127I ratio and using an analysis of variance determine if the resulting samples are significantly different. 2. Experimental 2.1. Sample preparation Aliquots of ∼5 × 10−10 and ∼5 × 10−11 129/127I ratio silver iodide materials prepared at the Idaho National Laboratory (INL) [2] were dissolved in concentrated ammonium hydroxide and zinc metal [4]. Separate aliquots of the ammonium hydroxide solution were removed
Corresponding author. E-mail address: [email protected] (M.L. Adamic). URL: https://www.inl.gov (M.L. Adamic). 1 Deceased July 12, 2018. ⁎
https://doi.org/10.1016/j.nimb.2019.01.002 Received 30 January 2018; Received in revised form 25 September 2018; Accepted 4 January 2019 Available online 11 January 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 455 (2019) 201–203
M.L. Adamic et al. 127 +2
I into an offset Faraday cup located after the high energy analyzing magnet and then switching the 129I+2 into the gas filled ionization detector located after the ESA. Each AMS wheel consisted of eight samples of the original silver iodide material mixed 1:100 with silver powder (to result in current similar to the electrodeposit), eight electrodeposited on silver wire, eight electrodeposited on silver clad niobium wire, and eight re-precipitated silver iodide mixed 1:50 with niobium powder. Niobium has been reported to result in the highest intensity and long term stability of the iodide ion current [3], and was the primary reason for selecting silver clad niobium wire for the electrodeposition of AgI for AMS targets. A series of quality control (QC) samples interleaved amongst the samples are loaded on the 40 position sample wheel to correct for background and normalize the data to cross calibrate the detectors. The QC samples include matrix blanks, Woodward silver iodide mixed 1:100 with silver powder, and INL normalization materials mixed 1:100 with silver powder (129/127I = 5.62 × 10−11 ± 2.64 × 10−13 1σ calibrated using NIST SRM 3230).
Fig. 1. NEC ICAMS (figure courtesy of National Electrostatics Corp.).
3. Results and discussion
and electrodeposited on silver wire and silver clad niobium wire using the electrochemical cell previously described [1]. Eight silver iodide samples were electrodeposited over three different days on both silver wire and silver clad niobium wire. To produce an equivalent powder target from the same starting material, an aliquot of the ammonium hydroxide solution was removed and dry silver nitrate added to reprecipitate the iodide as silver iodide. The re-precipitated silver iodide was washed with Milli-Q water, dried and mixed 1:50 by mass with niobium powder. All samples were pressed into titanium cathodes for analysis by AMS.
3.1. ∼5 × 10−10 material results For the ∼5 × 10−10 129/127I ratio, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. The measurements of the separate days were combined for each method, yielding 168 measurements per method. The ∼5 × 10−10 experiment results are summarized in Table 1. At a 95% confidence the silver clad niobium wire and silver wires are not significantly different and the re-precipitated material and that diluted with silver powder are not significantly different. Standard error for means of measurements is 1.15 × 10−12 (0.2%). The range of the four methods is 2.4%. The differences noted in this data set are likely due to the sample preparation and handling and not an indication that the method of electrodeposition perturbs the 129I/127I ratio. The data for the E-11 129/127I material below further supports electrodeposition as an effective method for sample preparation for AMS.
2.2. AMS analysis All samples were analyzed at INL using the Iodine Compact Accelerator Mass Spectrometer (ICAMS) shown in Fig. 1. The ICAMS is an NEC 0.5 MV compact accelerator mass spectrometer designed to analyze iodine in the +3 or the +2 charge state. There is a low energy injection magnet followed by the accelerator and then a high energy analyzing magnet followed by an Electrostatic Analyzer (ESA). The low and high energy sides of the instrument have magnetic bouncing systems to rapidly switch the isotopes by changing the energy of the ions as they pass through the magnetic fields of the injection and analyzing magnets. Samples are loaded as solid silver iodide that has been electrodeposited on silver cladded niobium wire using the same procedure reported by our lab for electrodeposition on silver wire [1] and pressed into titanium cathodes for targets in the NEC Source of Negative Ions by Cesium Sputtering (SNICS) source. The SNICS source produces negative ions by cesium sputtering and the iodide anions are selected for analysis in the instrument. The negative ions are accelerated into a stripper cell at the center of the accelerator (0.5 MV) where they collide with the stripper gas (helium or argon) and undergo a charge reversal. The energetic process of stripping electrons from the ions eliminates most molecular interferences from the spectrum. Iodine in the +2 charge state is normally selected for the analysis resulting in a transmission of ∼40% through the accelerator using helium as a stripper gas. The isotopic measurement is performed by rapidly switching (∼7 Hz) the
3.2. ∼5 × 10−11 material results For the ∼5 × 10−11 129/127I ratio, eight samples were analyzed for each preparation method on three days, with seven replicate AMS measurements per sample. The measurements of the separate days were combined for each method, yielding 168 measurements per method. The ∼5 × 10−11 experiment results are summarized in Table 2. At a 95% confidence, the samples deposited on silver wire, re-precipitated, and deposited on silver clad niobium wire, are not significantly different. The standard error for means of measurements is 1.05 × 10−13 (0.2%). Since the sample types of ∼5 × 10−10 or ∼5 × 10−11 deposited on silver wire, deposited on silver clad niobium wire and the re-precipitation came from the same solution they are the best comparison. This is an acceptable difference that can be attributed to the processing of the samples.
Table 1 ∼5 × 10−10 results summary. 5 × 10−10 ratio
1:100 w/Ag
Re-precipitated
Ag wire
Ag clad Nb wire
Average 129/127I Standard error
5.14 × 10−10 4.44 × 10−13 (0.09%)
5.11 × 10−10 4.15 × 10−13 (0.08%)
5.03 × 10−10 1.82 × 10−12 (0.36%)
5.02 × 10−10 1.27 × 10−12 (0.25%)
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Nuclear Inst. and Methods in Physics Research B 455 (2019) 201–203
M.L. Adamic et al.
Table 2 ∼5 × 10−11 results summary. 5 × 10−11 ratio 129/127
Average I Standard error
1:100 w/Ag −11
5.21 × 10 1.06 × 10−13 (0.20%)
Re-precipitated −11
5.16 × 10 8.22 × 10−14 (0.16%)
4. Conclusions
Ag wire
Ag clad Nb wire −11
5.15 × 10 1.35 × 10−13 (0.26%)
5.17 × 10−11 9.51 × 10−14 (0.18%)
or allow others to do so, for U.S. Government purposes. This information was prepared as an account of work sponsored by an agency of the U.S. Government. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.
Described is a comparison of electrodeposition and precipitation for the preparation of environmental iodine samples in a form appropriate for AMS analysis. Two silver iodide materials with 129/127I ratios of ∼5 × 10−10 and ∼5 × 10−11 were dissolved, electrodeposited on silver wire, electrodeposited on silver clad niobium wire, and re-precipitated as silver iodide. An aliquot of the original silver iodide material was diluted 1:100 with silver powder. AMS data obtained were compared to determine if the electrodeposition method altered the 129/ 127 I ratio. Analysis of variance indicated minor statistical differences between the precipitated and electrodeposited materials. The pooled standard error of the means was calculated to be 0.2% for both ∼5 × 10−10 and ∼5 × 10−11 ratio materials. These results also confirm that the dissolution of silver iodide did not perturb the iodine isotopic ratio. For the purpose of our laboratory’s routine measurements, these slight differences are acceptable.
References [1] M.L. Adamic, T.E. Lister, E.J. Dufek, D.D. Jenson, J.E. Olson, C. Vockenhuber, M.G. Watrous, Electrodeposition as an alternate method for preparation of environmental samples for iodide by AMS, Nucl. Instrum. Methods Phys. Res. B 361 (2015) 372–375. [2] D. Jenson, C. Vockenhuber, M.L. Adamic, J.E. Olson, M.G. Watrous, Iodine Standard Materials: Preparation and Inter-Laboratory Comparisons, AMS-13 poster CRI 11 2014. [3] Q. Liu, X. Hou, W. Zhou, Y. Fu, Accelerator mass spectrometry analysis of ultra-lowlevel I in carrier-free AgI-AgCl sputter targets, J. Am. Soc. Mass Spectrom. 26 (2015) 725–733. [4] A.C. Wahl, Fission of U235 by 14-Mev neutrons: nuclear charge distribution and yield fine structure, Phys. Rev. 99 (3) (1955) 730–739.
Acknowledgements The authors would like to thank Douglas Jenson, Ph.D. for the preparation of the INL 129/127I materials and Tedd Lister, Ph.D. for the initial electrodeposition work. This work is supported by the U.S. Department of Energy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution,
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