J Radioanal Nucl Chem (2012) 294:409–412 DOI 10.1007/s10967-011-1467-2
Chemical characterization of graphite by instrumental neutron activation analysis Amol D. Shinde • R. Acharya • R. Verma A. V. R. Reddy
•
Received: 26 September 2011 / Published online: 19 October 2011 Ó Akade´miai Kiado´, Budapest, Hungary 2011
Abstract Nuclear and commercial grade graphite samples were analysed by instrumental neutron activation analysis (INAA) using high flux reactor neutrons. Eleven elements (Na, K, As, Sc, Fe, Cr, Co, Zn, La, Ce, and Sm) were determined in eight samples of graphite (two nuclear grade and six commercial grade) by irradiating at a neutron flux of 3 9 1013 cm-2 s-1 in CIRUS reactor and assaying the activity by high-resolution gamma ray spectrometry using 40% relative efficiency HPGe detector coupled to an MCA. Concentrations of elements were determined by relative method of INAA. Results of both types of graphites as well as detection limits achieved by INAA method are discussed in the paper. Keywords Graphite Nuclear and commercial grade INAA Relative method Trace elements
Introduction Graphite finds many applications in various technological fields owing to its high thermal and electrical conductivity, high compressive strength, inertness to many chemicals, good lubricating properties and capacity to withstand very high temperature [1, 2]. High purity graphite is used as a crucible material in various analytical instruments, A. D. Shinde R. Verma A. V. R. Reddy (&) Analytical Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra 400 085, India e-mail:
[email protected] R. Acharya Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra 400 085, India e-mail:
[email protected]
electrodes in dry batteries and for making high resistance engineering components in space and aeronautical technology. It is used as a reactant in powder metallurgy for production of hard metals, lightweight alloys and high performance carbide ceramics. Owing to its low neutron capture cross section and good moderating properties, graphite is used as a moderator or reflector in nuclear reactors. If impurities like rare earth elements and other elements which have high neutron absorption cross section are present in the graphite then its effective neutron absorption cross section becomes higher and therefore its use in nuclear reactor is not desirable due to inferior neutron economy. Additionally, impurities like Fe, Cr, Co and Zn if present in graphite produce long-lived neutron activation products like 59Fe (44.50 days), 51Cr (27.70 days), 60 Co (5.271 years) and 65Zn (244.3 days) respectively which emit high-energy gamma rays. Use of such graphite is not desirable, as it will cause high radiation dose and may deteriorate quality of other reactor materials. Thus for chemical quality control, determination of impurities in nuclear grade graphite sample by using a suitable analytical technique is important. Since chemical dissolution of graphite is cumbersome and tedious [3–5], it is always better to have techniques that can directly determine trace impurities in graphite without decomposition of the sample and matrix separation. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been established as a very efficient and sensitive technique for the direct analysis of solids. Quantification is the major limitation in LA-ICPMS of high purity materials, especially if certified reference materials (CRM) with the matching matrix composition are not available [6]. Direct determination of trace impurities in graphite is carried out by ETAAS and FAAS. However, these methods suffer from calibration
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410
Fe
59
140
60
Co
10000
51
Cr
141
La
Ce
46
Sc
153
Sm
100000
Counts
problem as well as matrix interference [7, 8]. Instrumental neutron activation analysis (INAA) has also been employed for the analysis of graphite [9]. INAA is a sensitive analytical technique used for direct (without sample dissolution) and simultaneous multielement determination of solids. In the present case, graphite being the matrix it causes low and negligible matrix effect during neutron irradiation since carbon is a low Z element with a very low (*3.5 mb) neutron absorption cross section. Though INAA is a suitable analytical technique for trace element determination, it is not much explored for the analysis of graphite. In the present work, we have analyzed two nuclear grade graphite samples. Additionally, six commercial graphite electrodes having higher impurity concentrations were analyzed in a similar way. The concentrations were determined using relative method of NAA. In the present paper, results on concentrations of impurity elements along with detection limits (DL) obtained by INAA for both types of graphite samples are discussed.
A. D. Shinde et al.
1000
100
Experimental
250
500
750
1000
1250
1500
Energy (keV)
Two nuclear grade graphite (NG-1 and NG-2) and six commercial grade graphite samples (CG-1 to CG-6) were analyzed. The electrodes were from six different makes. Samples were ground in a clean agate mortar. Samples weighing 150–250 mg were heat-sealed in clean polyethylene pouches. IAEA RMs Soil 7 and Soil 5 were sealed separately. The graphite samples and standards were irradiated in self-serve position of CIRUS reactor at a neutron flux of about 3 9 1013 cm-2 s-1 for 5–10 h. After irradiation and a cooling period of 2 days, each of the graphite samples and standards were transferred to separate clean polyethylene pouches and sealed for measuring the activity using a 40% relative efficiency HPGe detector coupled to a 8k channel analyzer (Eurisys make; resolution 2.0 keV at 1332 keV of 60Co). Figure 1 gives a typical gamma ray spectrum of one of the commercial graphite samples irradiated for 5 h. The (n, c) products namely 24Na, 42K, 76As, 46 Sc, 51Cr, 59Fe, 60Co, 65Zn, 140La, 141Ce and 153Sm were detected in the present work. For medium lived isotopes like 24Na, 42K, 76As, 10La and 153Sm cooling time and counting time were 2 days and 3,000–5,000 s respectively whereas for the rest of the long lived isotopes like 46Sc, 51 Cr, 59Fe, 60Co, 65Zn and 141Ce cooling and counting times were 7 days and 10,000–50,000 s respectively. Relevant nuclear data were taken from reference [10]. Using mass of the element in the standard (mxstd) and count rates (counts per second, cps) of standard (cpsx,std) and sample (cpsx,samp), the mass of the element present in the sample (mx, samp) was calculated by the following equation,
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Fig. 1 Gamma ray spectrum of a neutron irradiated commercial graphite sample
mx; samp ¼ mx; std
cpsx; samp Dstd cpsx; std Dsamp
ð1Þ
where D is the decay factor (exp (-ktd)), k is the decay constant and td is the decay time. The mx, samp (lg) was converted to concentration (mg kg-1) by dividing with the sample mass (g).
Results and discussion Concentrations of ten elements in nuclear grade graphite samples and 11 elements in commercial grade graphite samples were determined. Tables 1 and 2 give results obtained for the duplicate samples of graphites. The uncertainties quoted are due to propagated counting statistics. In general nuclear grade graphite samples have lower impurity concentration than the commercial grade graphites investigated in the present studies. In the case of nuclear grade graphite, it can be seen from Table 1 that concentration of the elements whose activation products have longer half-life (like Fe, Co, Zn and Cr) and medium half-life (like Na, K, La, Ce and Sm) are in the ppm to sub ppm range. The element Cr could not be determined due to its low concentration. From the results of commercial grade
Chemical characterization of graphite
411
Table 1 Concentrations and DL (mg kg-1) of elements in nuclear grade graphite samples
Table 3 Concentrations (in mg kg-1 unless % indicated) of IAEA RM Soil 5 (% unless stated)
Element
NG-1
NG-2
Element
Concentration observed
Certified value
% Error
Na
49.9 ± 0.9
53.61 ± 1.11
K
11.6 ± 0.9
1.6
Na (%)
1.81 ± 0.12
1.92 ± 0.11
-5.7
0.001
K (%)
1.84 ± 0.13
1.86 ± 0.15
-1.1
0.004
As Sc
90.4 ± 8.3 15.9 ± 1.1
93.9 ± 7.5 (15.5)
-3.7 2.6
As
DL 0.5
14.7 ± 1.0 -1
(0.36 ± 0.04) 9 10
-2
-2
(0.28 ± 0.02) 9 10
-2
Sc
(4.72 ± 0.08) 9 10
(0.31 ± 0.01) 9 10
Fe
21.8 ± 1.3
77.8 ± 1.7
Co
(3.05 ± 0.20) 9 10-2
(0.55 ± 0.02) 9 10-1
20
2.65 ± 0.09 (0.15 ± 0.01) 9 10-1
0.02
Cr
31.4 ± 3.4
28.9 ± 2.8
8.7
1.8 0.01
Fe (%)
4.63 ± 0.21
4.45 ± 0.19
4.0
Co
16.1 ± 0.8
14.8 ± 0.7
8.8
399 ± 9.4
368 ± 8.2
8.4
26.9 ± 1.8
28.1 ± 1.5
-4.3
Zn La
1.88 ± 0.09 (3.88 ± 0.06) 9 10-1
Ce
-1
(1.02 ± 0.05) 9 10
ND
0.1
Zn
Sm
(0.20 ± 0.01) 9 10-1
ND
0.01
La
ND not detected
graphite (Table 2), it was observed that CG-3 and CG-4 have higher amounts of many elements including Na, La, As and Sc as compared to the other four graphites. Except for Zn and Sm, CG-5 and CG-6 are observed to have lower concentrations of other elements. Element K was not detected in CG-5 and CG-6. Many elements present in CG1 and CG-2 graphites have comparable concentrations as in CG-3 and CG-4 except for Fe, Cr and Ce. The trace element profile of each graphite electrode of different source is specific and can be used as ‘‘fingerprint’’ elements to differentiate from one another by profile comparison. These findings will be of help in forensic investigation where such batteries are used as power source in explosive devices. The 3r detection limits (DL) of all the 11 elements in both commercial and nuclear grade graphite samples were calculated using Eq. 2 and presented in Tables 1 and 2 respectively along with the concentration values pffiffiffiffiffiffi 3 Cb DL ¼ ð2Þ LT W S
Ce
63.4 ± 2.8
59.7 ± 3.0
6.2
Sm
5.71 ± 0.11
5.42 ± 0.39
5.4
where Cb is the background counts in the sample spectrum, W is the mass of the sample in gram, S is the sensitivity in cps per microgram and LT is the live time of counting in seconds. The DL are in the range of ppm to ppb (Tables 1 and 2). The lower DL are due to high neutron flux used in the irradiation of the samples. The method was validated by analyzing IAEA RM Soil 5 as a control sample and % deviations with respect to the certified or information values [11, 12] of 11 elements were in the range of ±1–9% (Table 3). The present method shows the efficacy of INAA method using high flux reactor neutrons for non-destructive determination trace elements (in ppm to ppb range) without sample dissolution and/or chemical separations.
Conclusions The nondestructive approach of INAA was applied for the determination of 11 minor and trace elements in two
Table 2 Concentrations and DL (mg kg-1) of elements in six commercial grade graphite samples Element
CG-1
CG-2
CG-3
CG-4
Na
913 ± 19
548 ± 16
1453 ± 40
959 ± 48
K
2777 ± 110
920 ± 60
3762 ± 143
2497 ± 101
CG-5
CG-6
191 ± 3
246 ± 4
ND
ND
DL 1.1 40
As
10.4 ± 0.1
4.91 ± 0.08
20.9 ± 0.3
10.8 ± 0.1
1.06 ± 0.08
1.90 ± 0.06
0.11
Sc
2.59 ± 0.05
1.66 ± 0.06
3.66 ± 0.07
3.44 ± 0.07
0.198 ± 0.002
0.192 ± 0.003
0.03
Cr
36.2 ± 0.4
37.8 ± 0.5
16.9 ± 0.3
19.0 ± 0.3
0.14 ± 0.01
0.11 ± 0.01
Fe
6989 ± 280
6394 ± 246
5615 ± 128
5664 ± 160
1183 ± 37
1067 ± 28
0.02 41
Co
5.11 ± 0.08
2.66 ± 0.06
6.10 ± 0.08
4.45 ± 0.07
2.31 ± 0.04
2.08 ± 0.04
Zn
397 ± 6
249 ± 4
468 ± 7
58.33 ± 1.80
1120 ± 19
833 ± 13
0.06 2.1
La
8.38 ± 0.13
4.51 ± 0.08
13.39 ± 0.09
14.2 ± 0.1
1.28 ± 0.02
1.16 ± 0.02
0.3
Ce Sm
40.5 ± 0.3 3.32 ± 0.08
9.55 ± 0.17 0.86 ± 0.01
27.8 ± 0.3 2.49 ± 0.08
29.5 ± 0.3 2.55 ± 0.06
2.81 ± 0.18 38.8 ± 0.9
2.66 ± 0.16 42.0 ± 0.8
0.4 0.02
ND not detected
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different types of graphite samples. High flux neutron irradiation improved the DL and many elements in sub ppm concentration level in nuclear grade graphites could be determined. The concentrations of elements in nuclear grade graphite were found to be lower than the commercial grade graphite. The present work shows that INAA can be used as a sensitive routine method for simultaneous multielement determination at trace levels in graphite samples. Thus it is planned to prepare in-house reference material (RM) of graphite using various analytical techniques including INAA, since there is no RM of the graphite commercially available to the best of our knowledge. Acknowledgments Authors thank reactor personnel of CIRUS reactor and Mr. K. C. Jagadeesan and Dr. S. V. Thackare, RPhD, BARC for their help during the irradiation. Authors thank Dr. K.K. Swain, ACD, BARC for his help during the NAA work.
References 1. Wood AAR (1986) In: Bever MB (ed) Encyclopedia of materials science and engineering, vol I. Pergamon Press, Oxford, pp 495–500
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A. D. Shinde et al. 2. Kelly BT (1994) In: Cahn RW, Haasen P, Kramer EJ (eds) Materials science technology, vol 10A, Nuclear materials, Part 1. VCH, Weinheim, pp 365–394 3. Nadkarni RA (1984) Anal Chem 56:2233–2237 4. Buzzelli G, Mosen AW (1977) Talanta 24:383–385 5. Koshino Y, Narukawa A (1993) Analyst 118:827–830 6. Pickhardt C, Becker JS (2001) Fresenius J Anal Chem 370: 534–540 7. Koshino Y, Narukawa A (1992) Analyst 117:967–969 8. Zacharia A, Gucer S, Izgi B, Chebotarev A, Karaaslan H (2007) Talanta 72:825–830 9. Kishi T (1993) J Radioanal Nucl Chem 168:413–415 10. De Corte F, Simontis A (1989) J Radioanal Nucl Chem 133: 43–130 11. Maharia RS, Dutta RK, Acharya R, Reddy AVR (2010) J Environ Sci Health B 45(2):174–181 12. Anderson DL, Gordon GF, Lepel EA (1983) Can J Chem 61: 724–728
