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Characterisation of Various Types of Alloy by K0-Neutron Activation Analysis

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Characterisation of Various Types of Alloy by K0-Neutron Activation Analysis A27 CHARACTERISATION OF VARIOUS TYPES OF ALLOY BY K0-NEUTRON ACTIVATION ANALYSIS M. WASIM, N. KHALID, M. ARIF Chemistry Division, Pakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan N.A. LODHI Isotope Production Division, Pakistan Institute of Nuclear Science and Technology, Islamabad, Pakistan Abstract Samples of certified alloys were analysed by semi-absolute, standardless k0-instrumental neutron activation analysis (k0-INAA) for compositional decoding. Irradiations were performed at Miniaturised Neutron Source Reactor (MNSR) located at Pakistan Institute of Nuclear Science and Technology, Islamabad having nominal thermal neutron fluxes of 1×1012 cm-2s-1. The experimentally optimised parameters for NAA suggested a maximum of three irradiations for the quantification of 21 elements within 5 days. The same experimental conditions produced quantitative results of 13 elements, which were not reported by the supplier of the reference materials. All reference concentrations were within 95% confidence interval of the determined concentrations. 1. INTRODUCTION Worldwide interest in the determination of elements in different materials has led to the development of many analytical techniques. The commonly used techniques include inductively coupled plasma with optical emission spectrometry (ICP-OES), X-ray fluorescence spectrometry (XRF), atomic absorption spectrometry (AAS) [1], arc/spark optical emission spectrometry, ICP with mass spectrometry and laser-induced breakdown spectroscopy [2-4]. Nuclear analytical techniques also play important role in material characterisation [5]. Among these the noticeable are particle induced X-ray emission, proton activation analysis [6,7], prompt gamma-ray neutron activation [8], fast neutron activation analysis [9,10] and thermal neutron activation analysis (NAA) [11,12]. The non-nuclear techniques apply relative standardisation, also known as classical linear calibration, whereby a calibration curve is drawn by using three or more calibration standards. Whereas, the nuclear analytical techniques have been well modelled mathematically; they require only one standard for calibration. The main disadvantage of using relative method is the requirement of a suitable calibration standard. Some techniques such as ICP and AAS require sample dissolution while XRF demands matrix-matching standards having smooth surface and very good homogenisation. Among these techniques, the one having multi-elemental, purely instrumental and non-destructive nature are highly preferred. NAA is one such technique that is multi-elemental, non-destructive, sensitive, accurate and free from contamination related problems. Its linearity range extends from ultra- trace to percentage level. The high accuracy usually reported by NAA has given this technique a good place in reference material certification [13]. NAA has its limitations as well; some elements cannot be determined by NAA because of unfavourable properties of target nuclide such as natural abundance or its cross-section for cases the properties of the product radionuclide may not be suitable such as its half-life, © International Atomic Energy Agency 1 M. Wasim et al. gamma energy or emission probability. The type of NAA, which does not involve any kind of chemical or physical separation or pre-concentration, is known as instrumental neutron activation analysis (INAA). Recently, a new standardisation has been developed in NAA known as k0-NAA, which does not require any calibration standard. The concept of k0-NAA has been adopted by a large number of laboratories worldwide. The main aim of our study is to reveal the capability of semi-absolute k0-INAA standardisation for the analysis of alloys. A typical alloy would contain Al, Co, Cr, Cu, Fe, Mg, Mn, Ni, Si, Sb, Sn, Ti, V and Zn. This study evaluates the relative merits and de-merits of k0-INAA for the analysis of alloys. A similar study has also been performed on Al-base alloys [14]. 2. METHODS AND DATA ANALYSIS The application of k0-NAA [15,16] requires the determination of thermal to epithermal neutron flux ratio (f) and epithermal neutron flux shape factor ( ) which characterise the irradiation position. Similarly, the characterisation of the counting system is required which is obtained by performing full energy peak efficiency calibration of the detector. In k0-NAA -sections in the thermal energy region. The concentration ( ) of an element a is calculated as: Asp,a 1 fQ 0,Au p,Au a (1) Asp,Au k 0,Au a f Q 0,aa p, where is the specific activity, [with is the resonance integral modified for the epithermal flux shape factor and is the thermal neutron (2200 m s-1) cross-section], is the full energy peak efficiency for the counting geometry and is the k0-factor defined for gold monitor for element “a”. In these calculations, and were determined experimentally, while and factors were taken from published data [18]. In this study Ni was quantified by 65Ni in Alloy-1, while in other samples it was measured by 58Ni(n,p)58Co reaction using the absolute method with fast neutron flux, which is calculated as: A p,ThTh Th Th sp,F Q0 F Th 1 (2) Af p,F F F F sp,Th θ is the natural abundance of target nuclide, σ is the cross-section and ϕ is the neutron flux. The subscripts neutrons” corresponding to 54Fe(n,p)54Mn reaction. Using fast flux the concentration of an element “a” is calculated as; a AMNsp/ av p F F (3) where is the atomic weight and is the Avogadro’s number. All calculations involved in this study were performed using our MATLAB based software called GammaLab ver. 1 [19], which retrieves all the related nuclear data from the NucData database [20]. 3. EXPERIMENTAL 3.1. Samples 2 M. Wasim et al. In this study five samples of certified alloys were included. These were designated as alloy-1 to alloy-5 consecutively. The list contained one sample of Ni and Cu base alloy, two samples of carbon steel, one sample of Ni-Cr-Mo steel and one sample of Cu and Sn base alloy. Composition of each alloy with manufacturer’s code is given in Table I. TABLE I: SAMPLE NAMES WITH THEIR COMPOSITION AND CODE Sample Sample Name Manufacturer’s Certified Elements Base Metal Code Code Alloy-1 Monel Alloy 400 BCR-CRM-363/1 Al, C, Co, Cr, Cu, Fe, Mn, Ni, Ni (65 %) S, Si, Ti Cu (32 %) Alloy-2 0.5% Carbon Steel BCR-CRM-159/3 C, Cr, Cu, Mn, Mo, Ni, P, S, Si Fe (98 %)) Alloy-3 1.2% Carbon Steel BCR-CRM-163/2 As, C, Cr, Cu, Mn, Mo, N, Ni, Fe (98 %) P, S, Si Alloy-4 Ni-Cr-Mo Steel BCR-CRM-219/4 Al, C, Cr, Cu, Mn, Mo, Ni, P, S, Fe (95 %)) Si, Sn Alloy-5 Gunmetal BCR-CRM-207/2 Al, As, Bi, Cu, Fe, Ni, P, Pb, Cu (87 %) Sb, Si, Sn, Zn Sn (10 %) 3.2. Instrumentation All irradiations were performed at Pakistan Research Reactor-2 (PARR-2), which is a 30 kW miniaturised neutron source reactor (MNSR). The reactor has 90% enriched uranium core (UAl4), light water moderator and Be reflector. At PARR-2, two irradiation sites, A-2 and B-2 were utilized. The f and a were determined by Al-0.1%Au wire (IRMM-530RC, EU, Geel) and ZrO2 powder (99.99%, Aldrich). Each sample weighing about 5- 100 mg was packed inside a polyethylene rabbit, which was sent for irradiation along with flux monitors. After irradiation the samples were transferred to pre-weighed polyethylene capsules for counting. The induced activity in each sample was measured by the -ray spectrometer, which consists of a p-type coaxial high purity germanium (HPGe) detector (Eurisys Mesures) with 60% relative efficiency and 1.95 FWHM at 1332 keV -ray of 60Co. The detector is connected to an Ortec-570 amplifier and Trump PCI 8k ADC/MCA card with GammaVision- 32 ver. 6 software. Full peak efficiency calibration of the detector was performed at various heights using 241Am, 133Ba, 137Cs, 60Co and 152Eu point calibration sources, which covered the energy range 59-1408 keV [16]. 4. RESULTS AND DISCUSSION The neutron flux characteristics for thermal, epithermal and fast neutrons are presented in TABLE II. The experimental conditions employed in the determination of elemental composition are presented in TABLE III, which shows irradiation, decay and counting times, radionuclides, their half-lives and gamma-energies. TABLE II: NEUTRON FLUX CHARACTERISTICS AT TWO IRRADIATION CHANNELS OF PARR-2. Irradiation f Th F channel (cm-2·s-1) (cm-2·s-1) A-2 19.10±1.07 -0.0147±0.0015 1.041012±5.131010 2.521011±5.0109 B-2 18.82±1.05 -0.0175±0.0018 1.111012±4.031010 2.311011±6.4109 3 M. Wasim et al. The experimental conditions in NAA are mostly set by the half-lives of the product radionuclides. The number of irradiations and irradiation time, therefore, depend upon the element of interest. In most of the cases, experimental conditions are optimised after a few initial runs. We started by assuming a typical composition of an alloy as: Al, Co, Cr, Cu, Fe, Mg, Mn, Ni, Si, Sb, Sn, Ti, V and Zn. TABLE III shows that irradiations were performed for three different time periods: 10 s, 1 min, and 5 h. A 10 s irradiation was required for Al, 1 min was needed for V, while the rest of the elements were determined by 5 h irradiation. All elements, except Hg were determined within 5 days of irradiation. TABLE III: EXPERIMENTAL CONDITIONS AND NUCLEAR DATA USED IN THE ANALYSIS OF ALLOYS BY K0-INAA. Irradiation Radio- Element Decay time Counting time Half-life Energy (keV) time nuclide 657.8, 884.7, 937.5, Ag 5 h 5 - 30 d 16 h 110mAg 249.76 d 1384.3 Al 10 s 5 min 30 s 28Al 2.24 min 1778.8 559.1, 657.1, 1212.9, As 5 h 2 d 1 -16 h 76As 1.09 d 1216.1 Co 5 h 5 - 30 d 16 h 60Co 1925.3 d 1173.2, 1332.5
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