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Journal of Analytical Atomic Spectrometry ARTICLE 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] This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins ARTICLE Journal Name (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.
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