CW-127610-CONF-004, Rev 0 UNRESTRICTED

Journal of Analytical Atomic Spectrometry

ARTICLE

Determination of Radiogenic Silicon and its Isotopes in Neutron Irradiated Aluminum Alloys by ICP-MS

Received 00th January 20xx, a a a Accepted 00th January 20xx Y. Shi,* C. Broome , and R. Collins

DOI: 10.1039/x0xx00000x Aluminum alloy is frequently used as component material in research nuclear reactors. Thermal neutron irradiation of aluminum causes it to undergo transmutation to silicon. The production of silicon inside of aluminum alloy changes its www.rsc.org/ material and mechanical properties. Furthermore, the concentration and the isotopic composition of the radiogenic silicon provides information on the irradiation history of the material and operation history of the reactor both of which are important to nuclear forensics and nuclear archaeology. An analytical method has been developed to determine the concentration and isotopic composition of radiogenic silicon using sector field ICP-MS. Applying a mass resolution of 3000 avoided severe spectral interferences from poly-atomic and doubly charged ion species generated from the solution matrix observed using lower resolution mass spectrometers. The measured content of radiogenic silicon in aluminium alloy materials irradiated with known fluence of thermal neutrons agreed well with the theoretically predicted values.

Introduction Historically, many analytical techniques have been applied to determine silicon in aluminum based alloys. The classic With its good formability, ease of welding, and corrosion gravimetric method has been used for a long time and adapted resistance, aluminum alloy is a preferred material for some to be a standard method by ISO.7,8 This method may not suit all structural components in research nuclear reactors. Aluminum the sample types well due to potential interferences from other produces little long-lived radioactivity, however, like other impurities and alloying elements. Modern instrumental reactor materials, it undergoes changes in microstructure and methods, such as, atomic absorption spectrometry (AAS),9,10 material properties upon neutron irradiation. Low fluence fast ICP-AES,11,12 ICP-MS,13,14 and neutron activation analysis neutron irradiation causes lattice displacement which improves (NAA),15 are considered to be less hindered by interferences, ductility, however at high fluence, void generation occurs. Fast and have been successfully applied to many aluminum neutron irradiation can also produce hydrogen and helium alloyanalyses. The above mentioned methods can readily be through reactions 27Al(n, p)27Mg and 27Al(n, α)24Na that lead to applied to the determination of radiogenic silicon in irradiated swelling, blistering, and embrittlement.1 When thermal neutron materials if the un-irradiated reference material is available to irradiation dominates the exposures, radiogenic silicon is correct for the presence of non-radiogenic silicon as an alloying produced and transmuted according to the following reactions:2 or impurity element. An energy dispersive X-ray spectrometric 27Al(n,γ)28Al 28Si + β, method16 has been reported to determine the radiogenic silicon 28Si(n,γ)29Si, and content in neutron irradiated aluminum without requiring the 29Si(n,γ)30Si. un-irradiated reference material, though the detection limit was The radiogenic silicon formed precipitates inside of the as high as 200ppm. When isotopic composition of silicon is of material,2,3 which increases the material’s strength and interest, mass spectrometric techniques become the obvious decreases its ductility.4,5 Therefore, the determination of the choice. High precision Si isotopic determination by gas-source radiogenic silicon provides an assessment of fitness for service mass spectrometry17 or multi-collector ICP-MS (MC-ICP- and the likelihood of the mechanical property change. MS)18-20 have been well established for silicate rock or natural

water samples in geological applications. With the goal of From a very different perspective, the concentration and the measuring the small variation of natural Si isotopic composition isotopic composition of the radiogenic silicon can provide in geological or environmental samples, these methods require important information on the irradiation history of the materials complicated sample preparation procedures and more and the operation history of the reactor. These pieces of sophisticated and costly mass spectrometers.21-23 Since the information are of interest to nuclear forensics and nuclear isotopic composition of the radiogenic silicon formed from the archaeology.6 neutron irradiated aluminum is expected to differ significantly from its natural composition, the widely available and less a. Analytical chemistry Branch, Canadian Nuclear Laboratories, Chalk River, ON K0J costly ICP-MS meets the precision requirement. 1P0, Canada. Email: [email protected]

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(UPW) was obtained from a Millipore Direct-Q5 ultra-pure Typical quadruple mass analyzer ICP-MS (Q-ICP-MS) is not water system. Silicon standard solution was purchased from able to resolve the Si isotope peaks from many interfering ion Inorganic Venture, Christiansburg, VA, USA. This silicon species composed of polyatomic ions and doubly-charged standard was also taken as an isotopic standard for natural atomic ions generated in a typical ICP source (Table 1). With abundances. A certified reference standard of Al Alloy 6011s, collision/reaction cells, it is possible to reduce or eliminate NIST 858, was purchased from by the National Institute of some of the interfering ions; however, they may also generate Standards and Technology (NIST, Gaithersburg, MD, USA). new ions to create interferences especially when multiple neighbouring masses are to be measured simultaneously. A The sample species was obtained from different locations along sector field (SF)-ICP-MS working at a relatively low mass its length on a removed rod from the NRU reactor at Canadian resolution (M/ΔM=300) behaves the same as Q-ICP-MS, Nuclear Laboratories, Chalk River, Ontario, Canada. The however, at higher mass resolution it is more effective at material was aluminium alloy AL5052 and the irradiation resolving the analyte’s peaks from the interfering ion peaks. In period was from 1974 to 1991. The rod had received a range of this work, we used a SF-ICP-MS set at a moderate high fluencies along its length. Samples taken from the section resolution (M/ΔM=3000) and successfully analysed the outside of the reactor core were treated as the un-irradiated concentration and the isotopic composition of the radiogenic reference materials. silicon in samples of aluminum alloy material components Table 2 Instrumental conditions and measurement parameters removed from a research reactor. Plasma power 1250 W Table 1 Some poly-atomic and doubly charged ion interferences on Si isotopes Plasma gas flow rate 15 L min-1 -1 Silicon Poly- and doubly-charged atomic ions (Resolution Auxiliary gas flow rate 1.1 L min -1 isotopes required to resolve from Si isotopes) Sample gas flow rate 0.95 L min -1 12C16O+(1555), 14N14N+(957), 11B17O+(888), Sample uptake rate 200 µl min 28Si+ 10B18O+(795), 27AlH+(2250), 40Ar16O2+(16244) Acquisition mode E-scan (peak jumping) 27 28 29 30 45 13C16O+(1331),12C17O+(1280),12C16OH+(1104),15N14N+ Masses monitored Al, Si, Si, Si, and Sc 27 29Si+ (1086),11B18O+(906),14N14NH+(773), 40Ar18O2+(6775), Dwell time 0.01s for Al, 0.05s for rest 40 17 2+ 10 19 + 40 16 2+ No. of Scans (runs × passes) 8×8 Ar OH (3544), B F (832), Ar OH2 (2904) 14N16O+(1238), 12C18O+(1181), 15N15N+(1133), Mass window 125% 13C16OH+(927),12C17OH+(903), Sample per peak 20 30Si+ 12C16OH2+(815),15N14NH+(805),11B19F+(883), Search windows 125% 40 18 2+ Integration windows 60% Ar OH2 (2022) Integration type Average

Sample dissolution method Experiment The samples were cleaned by pickling with 0.1M HCl for Instrument and reagents approximately 10 minutes, weighed (~0.1g), and transferred into Teflon beakers. Ten millilitres of 6M HCl, and 1mL of 7M The ICP-MS instrument used was a sector field (SF) model HNO3 were added, and heated for 4 hours on a hotplate set at Element XR, manufactured by Thermo Scientific, Hanna- ~200oC. Some samples, especially the irradiated ones had Kunath-Str 11, d-28199 Bremen, Germany. A fumehood small whitish precipitate on the bottom of the beakers after the adaptation was attached to the instrument to enclose the sample acid digestion. Samples were centrifuged; supernatant was introduction system and the ICP ionization source for safe decanted to a separated tube and set aside. A few drops of 50% handling of radioactive samples. The sample introduction was 3 NaOH was added to the precipitate and heated for 1 hour on a through a PC compact Peltier cooled inlet system which o hotplate set at 200 C till the precipitate dissolved. The two incorporates the cyclonic spray chamber and a low flow PFA- sample fractions were diluted to 50mL each. The dissolved 50 nebulizer, all manufactured by Elemental Scientific Inc. samples were further diluted ten times for SF-ICP-MS (ESI. Omaha NE, USA). All samples, blanks, and standards determination using Sc as the internal stnadard. The results were delivered by a SC-2 DX auto-sampler with 4-channel from analysing the two fractions were mathematically standalone micro peristaltic pump for these analyses. To combined. Standard reference material NIST 858 was digested achieve the resolution required for silicon isotopic analysis, the and analyzed with the samples. The reagent blanks were medium resolution mode (R=3000) was used throughout the prepared following the same procedure. experiments. The typical optimised instrumental operating conditions and measurement parameters are given in Table 2. Results and discussion All the acids used in this study were Fisher Scientific TraceMetal grade. The sodium hydroxide was analytical grade Resolution of Si isotopic peaks from polyatomic ion peaks and its purity with respect to Si in the prepared solutions was examined to be sufficiently low for use. The ultra-pure water

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All of the three natural silicon isotope peaks suffer from 1.6 1.6

14N14NH+ 1.4 MR (R=3000) 1.4 c/s

polyatomic ion spectral interference in ICP-MS when the mass

6

analyzer’s resolution is not sufficiently high. Interference from 1.2 LR(R=300) 1.2 10

c/s

×

these polyatomic ions, mostly formed from the solution matrix 4 1.0 1.0 10 elements O, N, C, and H, is difficult to correct. The situation × 0.8 12 16 + 0.8 gets worse when analysing radiogenic silicon where the C OH 0.6 0.6

isotopic composition does not follow the natural abundance Intensity, LR requiring all isotopes to be measured. To overcome these 0.4 0.4 limitations, ICP-MS using a higher resolution, for example, the Intensity, MR 0.2 29Si+ 0.2 medium resolution setting (R=3000) in Element XR instrument, 0.0 0.0 can resolve these polyatomic ions from the corresponding 28.90 28.95 29.00 29.05 29.10 silicon isotope ions of the same mass numbers. Figures 1-a to Mass, amu Figure 1-c shows the spectra around mass 28, 29 and 30 collected at two different mass resolutions using a standard Figure 1-b ICP-MS mass scan around mass 29 at different mass resolutions -1 solution of 10 ng mL natural silicon in 2% HNO3. The complete separation of the major interference ion species from 1.0 2.0 12 16 + 14 14 +

the analyte ion species, namely, C O and N N from MR (R=3000)

28 + 12 16 + 14 14 + 29 + 14 16 + c/s

0.8 LR(R=300) 1.6 c/s

Si , C OH and N NH from Si , and N O from 4

30 + 7 10

Si using the medium resolution is clearly evident. Obviously, 10

× × at the low resolution, the overlap of these interfering masses on 0.6 1.2 the silicon isotopes was so severe that the analysis of Si 0.4 14N16O+ 0.8 isotopes was almost impossible. Furthermore, as per Table 1, 30Si+ there are only a couple potential interfering ion species, namely Intensity, LR MR Intensity, MR 0.2 0.4 40Ar16O2+ and 40Ar18O2+, which require mass resolution higher than 3000 to resolve them from the corresponding Si isotope 0.0 0.0 peaks. Experiments show that the formation of these two ion 29.90 29.95 30.00 30.05 30.10 species was not significant under the experimental conditions Mass, amu selected for the measurement. From the experiment, the Figure 1-c ICP-MS mass scan around mass 30 at different mass resolutions measured and mass bias corrected Si isotopic ratios, 29Si/28Si 30 28 and Si/ Si from 20 blank runs were found to be Accuracy and precision 0.0509±0.0036 and 0.0341±0.0036 (1s), respectively. A t-test against the standard natural silicon isotopic ratios (29Si/28Si and To evaluate the precision of Si isotopic ratio determination by 30Si/28Si) of 0.0508 and 0.0335 resulted in the t-statistic of 0.11 SF-ICP-MS, a series of Si standard solutions with natural and 0.77, respectively, which are less than the t(0.05, 19) of 2.09. isotope abundance was tested. Approximately 80 measurements Therefore, no significant difference between the measured and of 29Si/28Si and 30Si/28Si, ranging in Si concentration from 10 to the standard value was detected. In the other words, at the blank 10,000 ng mL-1, were performed. After the correction of the level of Si, interference from potential overlap ions was not mass discrimination (linear model 24), the measured values noticeable. It is also needed to point out that the Si blank level, (dots) and the standard values (lines) of the isotopic ratios were though measurable, however did not affect negatively to the plotted against the Si concentration as shown in Figure 3-a and determination of radiogenic Si concentration and its isotopic 3-b. These plots represent the precision of SF-ICP-MS in ratios in the irradiated aluminium at the concentration range. determining the Si isotopic ratios within the Si concentration range specified. It is obvious that precision deteriorates with a 1.4 1.4

decrease of the Si concentration in the test solution. In

MR (R=3000)

c/s

c/s 1.2 1.2

6 analysing real samples, the majority of the measurements were 5 LR(R=300)

-1

10 10

1.0 1.0 × performed at Si concentrations around 1000 ng mL or above, × 0.8 0.8 except for the reference specimens with only the initial natural silicon, which were analyzed at approximately ng mL-1. 0.6 14 14 + 0.6 N N Therefore, reasonably high precision was achieved in 28 +

0.4 Si 0.4 Intensity, LR determining the isotopic composition of the radiogenic Si in MR Intensity, MR 0.2 12C16O+ 0.2 aluminum alloy samples exposed to neutron irradiation. 0.0 0.0 27.90 27.95 28.00 28.05 28.10 As a quality control, NIST SRM 858 was analyzed following Mass, amu the method developed. The measure value for Si, 0.76%±0.04% (2s), was found in good agreement with the certified value of Figure 1-a ICP-MS mass scan around mass 28 at different mass resolutions 0.79%±0.01%. For the SRM, “the estimated uncertainty is based on judgment and represents an evaluation of the

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ARTICLE Journal Name combined effects of method imprecision, possible systematic 2.5%

Calculated … errors among methods, and material variability” as stated in the Measured … certificate of analysis 25. 2.0%

0.10 1.5%

0.08 1.0%

Si 28

Si/ 0.06 0.5% 29

0.04 Si Produced, Radiogenic Wt% 0.0% 0 5 10 15 Measured Measured 0.02 22 -2 Thermal Neutron Fluence, ×10 n cm

0.00 Figure 4 Measured vs. calculated radiogenic Si in irradiated aluminum alloys 1 10 100 1000 10000 100000 Silicon Concentration, ng mL-1 0.06

29 28 0.05

Figure 3-a Si/ Si measured by ICP-MS at different Si concentrations for natural Si Si29/Si28 Si30/Si28 0.04 0.07 0.03 0.06

0.02

Si 0.05 28

Si Isotopic Isotopic Si Ratios 0.01

0.04 Si/ 30 0.03 0.00 0 5 10 15 0.02 22 -2 Thermal Neutron Fluence, ×10 n cm

Measured Measured 0.01 0.00 Figure 5 Change of Si Isotopic ratio with thermal neutron fluence 1 10 100 1000 10000 100000 -1 Si Concentration, ng mL Furthermore, the change of the Si isotopic composition in the specimens exposed to different thermal neutron fluence is 30 28 Figure 3-b Si/ Si measured by ICP-MS at different Si concentrations for natural Si shown in Figure 5. Although the change of the isotopic ratios depends on both the initial Si concentration in the alloy and the Applications radiogenic Si formed due to thermal neutron irradiation, this

information could still be useful to investigate the irradiation The method developed was successfully used to determine history of the materials and the operating parameters of the silicon in samples of aluminum alloy of which some had been nuclear reactor. The fact that the radiogenic silicon are stable irradiated by neutrons in a research reactor. The concentration isotopes and alloyed with the aluminum throughout the material of radiogenic silicon in the irradiated specimens was derived by suggest that this piece of information stays there as long as the subtracting the initial concentration of silicon from the un- materials are available and accessible. The determination of the irradiated samples. As shown in Figure 4, the increase in Si concentration and the isotopic composition of radiogenic concentration is very evident. The line is the theoretically silicon could be applied to the emerging fields of nuclear calculated produced silicon concentration under different 26 forensics and nuclear archaeology. thermal neutron fluence as given in the x-axis . The dots and the uncertainty given at 95% confidence level are the measured values by the method developed. The measured radiogenic Si Conclusions concentration in these samples was found to be in a good agreement with that from the theoretical calculation. A linear Neutron irradiated aluminum alloy samples have been analysed regression between the measured and the calculated values for the concentration of radiogenic silicon and its isotopic generated a slope of 1.03±0.02, an intercept of -0.0010±0.0003, composition by sector field ICP-MS. Applying a mass and a correlation coefficient of 0.9989. The results clearly resolution of 3000, spectral interferences by the common poly- show that the concentration of the radiogenic silicon could be atomic and doubly charged ion species from the solution matrix an indicator of the neutron fluence received by the aluminium were eliminated. The accurate determination of the Si materials. These results imply that it is practical to determine concentration in aluminium alloy was demonstrated by the the thermal neutron fluence received by irradiated aluminum agreement between the measured and the certified Si materials through analysing the concentration of the radiogenic concentration in SRM NIST 858. The measured content of silicon. radiogenic silicon in the aluminium alloy materials irradiated with neutrons agreed well with the theoretically predicted calculation based on the thermal neutron fluence that they received. The radiogenic silicon isotope composition was also

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Journal Name ARTICLE determined to demonstrate the capability of the sector field 858, Aluminum Alloy 6011 (Modified), 1999, ICP-MS in determination of silicon isotopes of radiogenic Gaithersburg, MD 20899. origin.

Acknowledgements The authors are grateful to Dr. Wenjing Li and Miss Xiaoli Song for providing the test samples and the neutron fluence data. The authors would like to thank Mr. M. Stewart and J. Vienneau for their technical support on the instrumentation.

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