Augusthy, A

Kennedy, V John

Varier, K M

IACS Institutional Repository

Indian J. Phys. 73A (2), 159-168 (1999)U P  A- an iDtcrpauonal journalPIXE facility for trace element an alyst at IGCAR, Kalpakkam, IndiaV John Kennedy*. A Augusthy*, K M Varier*, P Magudapathy,V Baskaran, S F^anchapakcsan and K G M Nair*^Department of Physics. University of Calicut, Kcrala-673 635, India Materials Science Division, Indira Gandhi Centre for Atomic Research,Kalpakkam'603 102 I mil aRec eiveJ J Apnl 199R accepted 4 August 1998Abstract : A Proton Induced X-ray Emission (PIXE) System has been designed and dc\doped with all the necessary instrumentation in one of the beam lines of the 2 MV Tandem dctelerafor at the Materials Science Division, Indira Gandhi Centre for Atomic Research (IGCARy Kalpakkam. India for measuring trace element levels in a variety of samples. The s\su tn has been calibrated using standard solutions The linearity and sensitivity were studied using 1 MeV proton beam Using the PIXE system, trace element analysis of standard water samples treun NIST (National Institute ol Standards and Technology, USA) and water samples from the riser ('hahyar has Ixen carried out This paper describes the experimental arrangement and ilctdiK alxmt the calibration and presents some of llie results.Kt'\w<»rds : PIXE accelerators, trace elemental analysis, environmental applications PACS Nos. : 2^ H) R|. 87 64 Fb. 07 85-ni1. In tro d u c tio nriiiirncierisuc X~ra> emission I’ollows the ionisation o f an atom due to the rearrangement of electrons after ejection o f a deeply hound electron. The ionisation can be induced by photons, electrons, protons, alpha particles or by other heavy ions. Proton induced X-ray emi.ssion (PIXE) is well suited because o f the very low bremsstrahlung badeground, better signal to noise ratio and high X-ray production cross section [1]. It has been successfully used by viinous groups all over the world for trace element analysis in t  variety of research lieid.s— biology, environment, archaeology, medicine and forensic science. PIXE is similai* to other lom is o f X-ray em ission techniques like X-ray fluorescence (XRfO and Electron probe m icroanalysis (i:PM A). The difference in only is the method of excitation.I orrcsponding author7^A(2b 5© 1999 lACS160 V John Kennedy el a IITic major adviuitages of P IX E  are its multielemental character, its high sensitivity (absolute cleleciion limits down to 10 g and relative detection limits down to 0.1 H g /g ), speed ol analysis (1 -1 0  mins), non-desiruclive nature and ability to analyse tiny samples (less than 1 mg). To make use of the above advantages o f the technique, appropriate experimental turangements tuid procedures have to be developed. It is therefore essential that careful attention is given to all the stages o f P IX E  experimental arrangements. Several international conferences on P IX E  and its analytical applications [2 -6 ]  have clearly  demonstrated the tlevclopmcnt and uses ol P IX E  technique. In the last few years, a few  IMXE set-ups 17-0) have brought out improved versions oT tiieir earlier experim ental arrangements( he present work describes the experimental .sc(-up designed and fabricated for routine tinahsis ol trace elements in a variety o f samples. The .set-up has been installed in one ol the beam lines o f the indigenously developed 2 M V  Tandem accelerator at the Materials Science Division. Indira Ciandhi CenU'e for A tom ic Research ( IG C A R ),  Kalpakkam. India. C'alibration ol the system with standard samples nonnally  used for Atomic absorption spectroscopy (A A S ) and N IS T  standard water samples has been discussed. The details ol the mice clement analysis o f river water samples carried out using this system are also discussed.2. Experimental arrangementThe schematic diagram o f the P IX E  facility is .shown in Figure 1. fhe P IX E  chamber has multiple ports to accommodate the S i(I.i) detector lor the detection o f X-rays and Silicon  Surlace Darner (SSB) detector for the detection of backseattcred particles, beiun viewer,VRCUUM FEED  TMROUGm SHgure I. A schcniauc dia^jram of ihc PIXE cxpcriiiienlal arrangointoiifaraday cup for beam current measurements, vacuum feedthroughs and vacuum gauges. The top flange accommodates a ladder arrangement for target mounting. The ladder can beraised o r lowered for beam alignment. A W ilson seal arrangement ha.s been provided for moving the ladder without disturbing the vacuum in the chamber. A rectangular sample bolder m ade o f aluminium with a  series o f circular holes o f diameter 10 min each in the centre, has been fabricated for mounting the samples. A 8 mm diameter. 6 |im  tliick mylar window has been provided in the 135° port inside the chamber ^ i n g  the Si(Li) detector, in order to protect the detector from the formation o f surface layers caused by sputtering and evaporation of material from the bom barded samples. The targai ladder in the top flttnge of the scattering cham ber is electrically insulated from the beam $ne and is connected to the faraday cup, so that the charge accumulated on the sample caji be collected by a current integrator. The cham ber is maintained at a pressure in the range of 10 '  -  10 * mbar. Two graphite collim ators of 4 mm diam eter each are put in the enjrance port to collimate the beam. A retractable Lithium drifted Silicon (Si(Li)) detector i j  inserted in the chamber at 135° relative to the incident ion beam for X-ray detection. The Si(Li) detector has an active area of 30 mm^ and a resolution of 160 eV at 5.9 keV (FWHM).The 2 M V tandem  accelerator at the M aterials Science D ivision, IGCAR, Kalpakkam, has been used for our PIXE studies. The details about the accelerator are described elsew here [10]. The beam emerging from the accelerator passes through a bending magnet with a field stability of 100 ppm. In order to achieve energy stability, the beam is made to pass through a slit as.sembly. The current picked up by the plates of the slit is used in a  feed back mode as an error signal, to correct the terminal voltage o f the accelerator, which decides the energy stability. The beam is collimated by two graphite collimators before it falls on the sample. The beiun then enters the target holder which has a provi.sion for mounting a number of .samples. ITie ion induced X-rays from the sample pass through a  6 pm  thick m ylar window and a 25 pm Be window before reaching the Si(Li) detector. The spectrum  is analysed and recorded in the multichannel analyser. AXIL software [11] has been used for the data analysis.3. C a lib ra tio n  o f the PIX E  systemCalibration is one o f the most important procedures in the PIXE analysis. The calibration of the PIX E set-up can be caiTied out in two ways : (a) based fully on experim ental procedures, i.e. sensitivity factors normally expressed in units o f co u n ts /p g  em '-.pC  are determ ined from the measurements performed on the standard samples which are made of pure elements or chemical compounds [12]. (b) deduced theoretically or in a semiemperical way from the calculated cross .sections o f X-ray excitation and from the absorption data with the absorber foil present between the target and the detector. We have followed the first approach using standard samples for our PIXE system as it gives a better accuracy. Thin and uniform samples are required as standard samples for the PIXE analysis.Standard samples were prepared from the standard solutions normally used for the AAS analysis. Single elem ent standard solutions (1000 ppm> of S, CJ, K, C^ a, Mn, Ni, Cu, Zn and Pd for K X-rays and Pb, Hg, Ta, W and Bi for L X-rays have been taken for our calibration study. One drop having a  volume of 200 pi of each solution was taken usingPIXE facility for trace element analysis at IGCAR, Kalpakkam, India 161162 V John Kennedy el alChe micropipet and dropped onto the 5 ^m  thick Aluminised m ylar film  and dried using;ui IH lamp.Figure 2. The linearity curve for the element K.J.I. Lmeaniv o f yield:T o test the linearity of the X-ray yield at various sample concentrations, standard samples of K, ( 'a , Pd and Pb of ctmeentrations from 100-1000 ppm were prepared. Experiments were eturied out using 1 MeV proton beam. The typical linearity curve for the element K is shown in I'igure 2. It is seen that the X-ray yields at various concentrations are linear. The above experiments were repeated several times for checking the consistency.3.2. Sensitivity determination:Experiments were cairied out using 1 MeV proton beam on the m ultielem ent standard solutions of S, C’l, K, Ca, V, ( 'r, Fe, Ni, Cu, Zn, Mo, Pd, Ta, W . Pb, Hg and Bi. The samples were prepared as de.scribed earlier. The X-ray yieldI ,  = Q a , J E p ) { N t ) ^ A Q e T ,  (1)where /. is the intensity of the element with atomic number Z, Q is the charge i.e. num ber o f protons incident on the target o, (Ep) is the X-ray production cross section with energy Ep, {Nt). is the number of atoms per unit area, M2 is the solid angle subtended by the SiG-i) detector, e is the efficiency of the detector and T is the transmission o f X-rays with a given energy through the absorbing material between the target and the detector. From eq. (1) we can write /. = K,Q(Nt\,from which K . = I . / Q( Nt ) ^ . (2 )The target sensitivity  K, is determ ined by m easuring the K  and L X -rayyields/jxg cm ‘.jtC. The sensitivity curves of K andL X-rays used in this study are shown in Figure 3.PIXE facility for trace element ana 'ysis at IGCAR. Kalpakkam, India 163If all the param eters o f eq. (1) is measured, the concentration of any elem ent can be obutined without using internal standard. Using the internal standard intensity, one canFigure 3. ScnsifiMlv tu n c  ol and X-ra> yield as a function of atomic n u iu h c r lo r  t>ur IM X I,  svs iemesiiinaic liic elem ental concentrations as follows. The mrgei sensitivity K. is actually the ptoduci of ot/.,,). .,U2. f  and 7 from  eq. (2). one can write for internal .standard= / , / Q( Nn.11 IS die respecuvc-clcrnenuil concentration to be estimated then eq. (2) becomesA', = / ,  / Qi Ni i t  ■iToin eqs. (.3) and (4). we can get the value of K, /K,  for each elementPd has been used as the internal standard for trace elem ent analysis in d illercnt siunples By taking into account the Pd yield and concentration in the spectrum, one a u iestiinate the unknown elemental concentration C  u.sing the following relation.(3)(4)C = — ^ ^  CK J ,  " (5)where T . is the internal standard concenuation, /, is the intensity of the internal standtird and f  is the intensity o f the respective elem ent. The concentrations C, and C, are proportional to (Nl)^ and (Nt%.4. T race  d e m e n t analy.si.sI'race element analysis has acquired a great significance in the estimation of pollutants in environment. PIXF. has been used for the above analysis in the last few decades (13J. Trace element analysis o f the standard reference m aterials and river water sam ples has been carried out and discussed here.164 V John Kennedy et al4.1. Analysis o f the NIST water :When validating an analytical system the complete procedure m ust be standardised using a reference material from an established agency like IAEA or NIST. It should be sim ilar to the unknown samples, in terms of analytes, matrix and physical form. We have taken the NIST SRM 1643d water reference sample for analysis. Sam ples were deposited  on aluminised mylar as described earlier. Experiments were carried out using 1 MeV proton beam from the accelerator. The typical spectrum of NIST SRM 1643d w ater sam ple is shown in Figure 4. All the elements present in the reference material are seen in theFigure 4. PK E  spectrum of .VIST standard reference material SRM 1643d showing peaks of different elements *spectrufn. Pd, used as an internal standard, is added to the sample for quantification. The experim ental PIXE spectrum has been analysed by means o f linear least square fitting procedure with the help o f AXIL softw are [111, a lter subtracting the background contribution from the blank. In the AXIL software [11], the peak overlap {e.g. between AsK« and PbL«, where the separation o f 8 eV is less than the natural line width AsK«) could be dealt with in an exact manner, by making use of the known ratios o f K j K p  and o f  the lines o f respective element. Using eq. (5), the trace elemental concentrations are estimated. The reproducibility has been checked by carrying out different runs on the sample. The values are tabulated in Table 1 along with the certified values. The typicalPIXE facility for trace element analysis at IGCAR, Kalpakkam, India 165errors are indicated in the table. From the table, it can be seen that our measurements for most o f  the elements are in close agreement with the certified values. The elements Cr. Co,Table 1 . Trace element analysis of NIST SRM 1643d water samples.ElementKCaVCrMnFcCoNiCuZnAsSeMoAgCdBaT1SrPbCertified values (ppb)2356 ± 35 31040 ± 500 35 1 ± 1.4 18.53 ± 0.20 37.66 ± 0.83 91 2 ± 3.9 25.00 ± 0.59 58.1 ± 2.7 20.5 ± 3.8 72 48 ± 0.65 56.02 ± 0.73 11.43 ± 0,17 112.9 ± 1.71.27 ± 0.057 6.47 ± 0.37506.5 ± 8.97.28 ± 0 25 294.8 ± 3.4 18 15 ± 064PDCE analysis results (ppb)2250 ± 60 30842 ± 500 32.4 ± 3.5 15 8 ± 0.2 35 ± 1.5 95 ± 4 0 23 ± 2 56 ± 2 18 ± 3 2 80 ± 2.0 48 ± 1.95 9.8 ± 1.89 105 ± 5.0 0 8  ± 0.6 5.2 ± 0.6 500 ± 22 6.5 ± 0.40 270 ± 15.0 16 ± 2Cu, Sc iuid Sr have a deviation of around 10-15%  over the certified values. The deviation mainly arises from the overlapping peaks of adjacent elements.4.2. Analysis o f river water samples :Drinking water is a significant source of essential elements of man and animals. M ost o f the water for drinking purposes is taken from the rivers. W ater samples were collected from one o f the rivers, Chaliyar, Kerala, India which is a m ajor drinking water source for the loCfil population. The river is heavily polluted by the industries situated along the river-bed, domestic wastes and agricultural wastes. Samples were prepared by dropping 200 |Xl of the water sample along with an internal standard Pd (2 ppm), onto 5 |im  aluminised mylar foils. The sam ples were irradiated by 1 MeV proton beam froni the 2 MV tandem accelerator. At the target, the beam current is kept around 5 nA to reduce dead time corrections and pile-up. The typical specu*um obtained from the river water sample is depicted in Figure 5. The K X-ray lines of P, S, CU K^Ca, Ti. Cr, Mn, Fe, Co, Cu, Zn and Pd and L X-ray lines (3f Pb are detected. The spectrum has been analysed using AXIL software, after subtracting166 V Jo h n  K en n ed y  el althe background contribution from the blank. By applying the inten.sity and concentration o f the internal standard Pd in eq. (.‘>). the elemental concentrations of all the elem ents presentin tlie river water stunples have been estimated and ttre listed in Figure 5. As can been seenFigure 5. Typical spectrum of river water showing the elemenis with conceniraiiorLsfrom the figure, major elements P, S. O . K, Ca and Fe are present in the concentration level of I(XX) ppb. the elements like Cr, C'o. ( 'u  and Pb are prc.sent in die level ol I-.*! ppb. The Zn coneentration is around 150 ppb. In general, our results arc close to the Indian Standard Specifications for Drinking water [14].4.3. Determination of minimum detection lim it:The minimum detection limit (MDL) for a given element may be defined as the m inim um  quantity o f that elem ent which m ust be present in the sam ple to yield a  significant amount of signal above the noi.se to enable its identification and measurement. If the counts in the peak N, due to trace elements exceeds three times the square root o f the baejeground Nh. taken within the FW IIM  of the peak, the peak may be ob.served. i.e. Nt > Nb. The background originating from ion interactions with matrix is independent o f the presence o f the trace elem ent [15|. The minimum measurable concentration of trace elem ent present, determined with the above criterion is .shown in Figure 6. From the figure, we can see thatPIXE facility for trace element analysis at IGCAR, Kalpakkam, India 167262116 -11 4K X-rays ^iLX-rsys•  #n----- \-----r10 20 30 40 50 e0 70 8 K > 9 0 « )Atamk:Nin«Mr(Z)F'igure 6. Calculated minimum concejitratjon of trace element*  ^ detectable t(>r proton5 on a tlun targetthe trace elem ental concentrations in the region o f 20 < Z < 30 can be estimated up to 1 pph level.5. S u m m aryThe PIXE experim ental facility has been indigenously developed with the 2 M V tandem accelerator at the M aterials Science Division. KK'AR. Kalptikkam for regular trace element analysis. The above facility has been calibrated following standard pr<x;edures. I'hc lacility is being utilised for finding out the trace elements in biological and environmental samples. O ur results obtained with the NIST standard material has shown the reliability and accuracy of the system.A cknow ledgm entThe w ork reported here has been carried out under lU C-D A EF (Indore) program m e at Kalpakkam. The authors are grateful to Dr. Kanwar Krishan and Mr. N S Thampi for their constant encouragement. W e thank C Ramesh, N Murugesan and Mr. Thiruvengadasamy of the M aterials Chemistry Divison, IGCAR for their help during the .sample preparation. (.)ne o f the authors (V J K ennedy) would like to tliank his IlIC-DAKF colleagues for their constant support and assistance.73A(2>-6168 V John Kennedy et alReferences(11 S A lohansson and J L Campbell PIXE A Novel Technique for Elemental Analysis (New York : Wiley)(1988)|2) S A loliansson (eel) Proi 1st Int. Conf on Particle Induced X-ray Emission and us Analytical Applications, Nuci Instrum Meth 142(197/)|3| S A Johansson (ed) Proc 2nd Int Con], on Particle Induced X-ray Emission and its Analytical Applications, Nud Instrum Meth 181 (1981)|4| B Marlin (ed) Proc .hd Int Conf on Particle Induced X-ray Emission and its Analytical Appltcations, Nitd InUrum Meth B3(I984)|5) R D Vis (ed) Proc 5th Int, Conf on Particle Induced X-ray Emission and its Analytical Applications, Nurl Inurum Meth B49(1990)|6I V Valkovic and G Moschini (eds) Proc 7th Int Conf on Particle Induced X-ray Emission and its Anahtical Applicatwfis, Nucl Instrum Meth. B109/110 (1996)(71 V Prolock Instrum Meth. B8S 145 (1994)|81 I Mores, M F Aldapc, R V Diaz and D Crumpton Nucl Instrum Meth B75 116 (199?)|9) A K Rath, V Vjjayan and V S Rarnamurthy Indian J Ph\s 70B 391 (1996)1101 V S 'Fliampi Indian J Pure AppL Phys 27 638 (1989)1111 AXIL X-ray Anahsis Package Users Guide (1981)112] M Ka.sahara, H Ognnara. K Yamamoio, J H Park and K Takaliashi Int J PIXE 4 155 (1994) in ]  RCecchi andG Ghermandlu Nuci Instrum Meth B15605 (1986)I M| Ifu lm  StandardSpeaficationsforDnnkmg water IS ; 10500 (1983)115] I) B Isabelle Radiat Ph\s Chem. 44 25 (1994)

PIXE Facility for Trace Element Analysis at IGCAR, Kalpakkam, India

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PIXE Facility for Trace Element Analysis at IGCAR, Kalpakkam, India

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