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,ThTh  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 1.041012±5.131010 2.521011±5.0109 B-2 18.82±1.05 -0.0175±0.0018 1.111012±4.031010 2.311011±6.4109

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 Cr 5 h 5 - 30 d 16 h 51Cr 27.70 d 320.1 Cs 5 h 5 - 30 d 16 h 134Cs 2.0652 y 569.3, 604.7, 795.9 5 h 3 d 1 h 64Cu 12.7 h 1345.8 Cu 10 s 1 min 2 min 66Cu 5.2 min 1039.2 Fe 5 h 5 - 16 d 16 h 59Fe 44.49 d 1099.2, 1291.6 Ga 5 h 1 - 2 d 10 min-1 h 72Ga 14.1 h 630.0, 834.0, 894.2 Hg 5 h 8 d 7 h 203Hg 46.594 d 279.2 818.7, 1097.3, 1293.5, In 1 min 1 h 20 min 116m1In 54.29 min 1507.4 Mn 10 s – 1 min 5 min – 3 h 2 min - 30 min 56Mn 2.58 h 846.8, 1810.7 140.5, 181.1, 366.4, Mo 5 h 2 - 5 d 30 min – 16 h 99Mo 2.7489 d 739.5 5 h 5 - 16 d 16 h 58Co 70.86 d 810.8 Ni 10 s – 1 min 1 h – 4 h 5 min – 30 min 65Ni 2.52 h 366.3, 1115.5, 1481.8 564.2, 692.5, 1140.7, 1 min 3 h 2 min 122Sb 2.7238 d 1256.9 Sb 602.7, 645.9, 722.8, 5 h 5 - 16 d 16 h 124Sb 60.2 d 1691 121.1, 136.0, 264.7, Se 5 h 5 - 16 d 16 h 75Se 119.79 d 279.5 Sn 5 h 5 - 16 d 16 h 113Sn 115.09 d 255.1 Th 1 h 3 d – 7 d 1 h – 16 h 233Pa 26.975 d 300.2, 311.9, 340.5 V 1 min 5 min – 10 min 2 min – 10 min 52V 3.75 min 1434.1 134.2, 479.6, 134.2, W 5 h 1 - 7 d 30 min - 7 h 187W 23.72 h 618.3 Zn 5 h 5 - 16 d 24 h 65Zn 244.06 d 1115.5

4 M. Wasim et al. Two elements, Cu and Sb were quantified in both short and long irradiations. Ni was measured by (n, ) and (n,p) reactions. TABLE IV presents the results of 14 elements determined by k0-INAA with uncertainty at k = 1.

TABLE IV: DETERMINED CONCENTRATIONS IN ALLOY SAMPLES WITH CERTIFIED VALUES

Alloy-1 Alloy-2 Alloy-3 Alloy-4 Alloy-5 Elemen t Certified Determined Certified Determined Certified Determined Certified Determined Certified Determined Concentrat Concentrati Concentrati Concentrati Concentrati Concentrati Concentrati Concentrati Concentrati Concentrati ion on on on on on on on on on Al (%) 0.027 Not Detected - - - - 0.003 * Not Detected 0.013 Not Detected As (%) - - - - 0.02 0.02±0.001 - - 0.066 0.063 ±0.005 Co (%) 0.032 0.032±0.02 ------Cr (%) 0.05 * 0.05±0.003 0.16* 0.15±0.01 0.16 * 0.17±0.01 0.66 0.70±0.05 - - Cu (%) 31.9 31.0±2.6 0.15 * 0.14±0.01 0.09 * 0.09±0.01 0.088 0.087±0.009 87.35 91.88 ± 8.00 Fe (%) 1.86 1.84±0.1 98.0 a 95.6±7.9 97.8 a 99.6±8.2 94.9 a 98.9±8.4 0.029 - Mn (%) 1.26 1.08±0.09 0.77 0.78±0.05 0.3 0.3±0.02 0.81 0.73±0.05 - - Mo (%) - - 0.02 * 0.02±0.002 0.012 * 0.012±0.001 0.58 0.60±0.04 - - Ni (%) 64.7 55.6±4.6 0.12 * 0.11±0.005b 0.10 * 0.11±0.005b 2.55 2.72±0.09b 0.28 0.32 ± 0.02b Sb (%) ------0.1 0.1 ± 0.01 Si (%) 0.028 Not Detected 0.24 Not Detected 0.24 Not Detected 0.079 Not Detected 0.016 Not Detected Sn (%) ------0.011 Not Detected 9.74 9.00 ± 0.49 Ti (%) 0.03 Not Detected ------Zn (%) ------1.6 1.7 ± 0.1 *: Information values

a: Calculated as balance concentration

b: Calculated by (n,p) reaction

The uncertainty associated with the results (see TABLE IV) by k0-INAA included uncertainty in weight, peak area, efficiency, Q0, (effective resonance energy), k0 factors and replicate measurements. The measured uncertainties were mostly in the range of 5% – 10% with an average value of 7%. TABLE IV shows that most of the elements reported in the certificates of the samples were determined except Al, Si, Sn and Ti. Aluminum was certified in Alloy-1, Alloy-4 and Alloy-5 but it was not detected in any of these samples due to the presence of Mn in the first two alloys and Cu in the last alloy. Aluminum, however, was quantified in Alloy-3, which has the least quantity of Mn as compared to the other alloys. Si and Ti were not detected in any sample due to their poor limits of detection. Similarly, Sn was not detected in Alloy-4 for the same reason. Nuclear interferences can be significant in alloys. In our study, the possible nuclear interferences were: 60Ni(n,p)60Co, 65Cu(n,p)65Ni, 56Fe(n,p)56Mn, 65Cu(n,p)65Ni, 64Zn(n,p)64Cu, 63Cu(n, )60Co, 62Ni(n, )59Fe, 54Fe(n, )51Cr and 63Cu(n, )60Co reactions. However, the corrections for these interferences were less than 1% except for Alloy-2 and Alloy-5, where a correction of 2.6% for 54Fe(n, )51Cr and 4.9% for 63Cu(n, )60Co were done respectively. The performance of k0-INAA has been illustrated in FIG 1 in terms of relative deviation about the certified concentration. The average absolute relative deviation produced by k0- INAA was 4.4 %. FIG 1 shows that the results of k0-INAA are in good agreement with the

5 M. Wasim et al. certified values having a maximum relative deviation of -14.3% for Ni. Moreover, all the reference concentrations were within the 95% confidence interval of the determined concentrations.

0 As Co Cr Cu Fe Mn Mo Ni Sb Sn Zn

-5 Alloy1

Relative(%) Deviation Alloy2 Alloy3 -10 Alloy4 Element Alloy5 -15

FIG. 1: Accuracy depicted by k0-INAA in five alloys in terms of relative deviation (%).

Apart from better accuracy, NAA is also known for its higher sensitivity for many elements. The present study quantified 13 elements not certified by the alloy producer. These elements include Ag, Al, As, Co, Cs, Ga, Hg, In, Sb, Se, Th, V and W in one or more alloys as shown in TABLE V.

TABLE V: CONCENTRATION OF ELEMENTS DETERMINED IN THIS STUDY BUT NOT REPORTED IN THE CERTIFICATE OF REFERENCE MATERIALS.

-1) Element Alloy-1 Alloy-2 Alloy-3 Alloy-4 Alloy-5

Ag - - 4.035 ± 0.238 - 515±36 Al Certified - 160 ± 21 Certified Certified As 6.3 ± 0.4 275 ± 19 Certified 131 ± 10 - Co Certified 147 ± 12 106 ± 7 134 ± 12 41.1 ± 3.1 Cs 26.0 ± 0.01 35.7 ± 38.3 ± 26.9 ± - Ga - 18.4 ± 1.4 11.5 ± 0.9 16.0 ± 1.5 - Hg - - 3012 ± 205 - - In - - - - 5.8 ± 0.5 Sb 0.98 ± 0.19 39.2 ± 5.2 32.1 ± 4.6 21.5 ± 2.3 Certified Se - - 91.0 ± 4.7 - 40.0 ± 3.0 Th 14.7 ± 0.9 - 6.4 ± 0.5 - 39.2 ± 2.7 V - - 34.5 ± 3.4 - - W - 53.1 ± 3.6 101 ± 7 240 ± 16 -

6 M. Wasim et al. 5. FINAL REMARKS – CONCLUSIONS

This study reveals that by applying semi-absolute k0-INAA method using three sets of irradiations, 11 elements including As, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sb, Sn and Zn can be quantified in iron, copper and nickel base alloys. The determination of Si and Ti does not appear feasible in alloys due to the presence of Mn and Cu in most alloys. The determination of Al, Sn and V however, can be achieved for some alloys having favourable compositions. Neutron activation being multi-elemental and sensitive technique has the potential to quantify other elements usually found in alloys as impurities including Ag, Al, As, Co, Ga, In, Se, Th and W. INAA is a high performance analytical technique and now with the introduction of standardless k0-standardisation this technique has got another advantage over other elemental analysis methods.

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