Mass calibration and intercalibration of Prague PIXE setup

Nuclear Instruments and Methods in Physics Research B 85 (1994) 145-149 North-Holland

Mass calibration

and intercalibration

RlOMl B

Beam Interactions with Materials&Atoms

of Prague PIXE setup

V. PotoEek * KFE FJFI &UT (Dept. of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague), V HoleZorGkka’ch2, 180 00 Praha 8, Czech Republic

The Prague PIXE system was calibrated in two steps. The first resulted in a proper description of background, absorption, and detector efficiency. Then all the parameters in the formula for quantitative thin target PIXE analysis acquired their physical sense with the exception of the detector solid angle which implicitly included the charge collection efficiency and the dead time corrections. It is stated in every analytical run through the analysis of a suitable standard. The calibration methodology was checked in some interlaboratory tests with encouraging results.

1. Introduction PIXE (particle-induced X-ray emission) as an analytical method was first reported in 1970, and about ten years after the PIXE program has also been introduced in Czechoslovakia. The technique is known as a fast, nondestructive, multielement and sensitive analytical tool that is almost ideal especially for an elemental analysis of aerosols and some types of biological samples. That is why the Prague PIXE program was influenced by needs of environmental research and why it has been strongly influenced by the programs of PIXE groups in Lund, Sweden, and Gent, Belgium. The Prague PIXE setup consists of a vertical proton beam from a 2.5 MV Van de Graaff accelerator with removable 7.5 p,rn thick Kapton exit window, vacuum target chamber with twenty position horizontal carousal for slide frames and/or 25 mm rings, Si(Li) detector PGT and Canberra signal processing electronics. Collected X-ray spectra are transported from MCA to PC, processed by suitable fitting software to remove net spectral intensities and finally either relative or absolute element concentrations are determined. The PIXE setup complete with software and methodology can be referred to as the PIXE system. At present, the Prague PIXE system is used in routine analyses and at the same time it is being continuously developed. As for acceptable target shapes, the setup is compatible with the Lund PIXE device. The level of compatibility is even higher with the Gent PIXE sys-

* Corresponding 66414818.

author, phone + 42 2 85762218, fax + 42 2

0168-583X/94/$07.00

tern because it in addition includes some software features. It facilitates interlaboratory comparative analyses and makes a direct co-operation easier even with other laboratories. To make this possible, one must calibrate and intercalibrate the system. That is why the PIXE system of the Ion Beam Laboratory will be described here with special regard to the calibration.

2. PIXE setup

The analytical equipment of the Ion Beam Group at the Department of Physical Electronics, Faculty of Nuclear Sciences and Physical Engineering of the Czech Technical University, was constructed using the proton beam from the 2.5 MV Van de Graaff accelerator of the Nuclear Center, Charles University. PIXE analyses may be conducted both in high vacuum and in an atmosphere with a pressure lower than 10’ Pa or even in open air. Proton energies from about 1.5 to about 2.4 MeV and beam intensities from some tenths to about 100 nA are used usually with a beam diameter of 6 mm in vacuum and 2 mm with exit window. The beam homogeneity is checked semi-automatically including a beam profile drawing at any time within an analytical run. A unique feature of the setup is the vertical external proton beam which enables to analyze even liquid samples. To become a real analytical device, the PIXE setup has recently been completed with appropriate software consisting of three main programs: TROJAX, TROCON and RBS. The program RBS serves both in automatic positioning of the target holder and in data

0 1994 - Elsevier Science B.V. All rights reserved

SSDI 0168-583X(93)E0444-L

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% PotoZek/Nucl. Instr. and Meth. in Phys. Res. B 8.5(1994) 145-149

transport between the multichannel analyzer and a floppy drive and/or local area computer network. Moreover, it enables to view, print and plot spectra, and to carry out some simple operations such as comparison of spectra and energy calibration statement. The spectra of characteristic X-rays are processed by the program TROJAX. It returns the intensities of separate spectral lines corrected for background, overlapping, pile-up effect and escape peaks as stated by a nonlinear least-squares fitting procedure. The program TROJAX generates for each spectrum the basic output (BAO) file and optionally also some other files for detailed information about the fitting process and the fitted spectra for graphic presentation. Some of those optional files can also be generated independently from BAO data. The BAO files from TROJAX can be directly read as input by the program TROCON. ‘Using the input data and basic physical parameters, TROCON generates amounts of separate elements either in nanograms per sample or in relative mass contributions.

3. calibration

In{1 + l/(DEF>>), the latter only for E > 1.832 keV. Published data and/or formulas were used for ionization cross sections 121, fluorescence yields [3], relative intensities [4], mass absorption coefficients [S], detector efficiencies [6], and corrections for silicon escape peaks [7]. A further multiplier was to be taken into account in Eq. (2). It represented the efficiency of charge collection from the sensitive volume and could be interpreted in terms of the peak tails. Nevertheless, that parameter seemed to be negligibly different from 1 just because of the negligible tails in the ~aIibration spectra. Another factor represented the influence of the dead time count losses in Eq. (If, but under real conditions it could also be neglected. In the case of L X-rays, Eq. (1) remained formally unchanged, but the symbol u then represented the L, X-ray production cross section. It could be determined from the ionization cross sections of Ll, L2 and L3, the Coster-Kronig transfer probabilities fZj between subshells i and j, and the fluorescence yield of the subshell L3 [8]. Ionization cross sections were determined according to Johansson [2] and the other parameters were taken either from ref. [9] or directly from its references.

3.1. Detector churacter~stics

3.3. Spectra processing software The Si(Li) detector PGT has been put into operation in May 1988. It has an active area of 30 mm2, crystal thickness 3 mm, Be window 0.5 mil, detectorwindow distance approx. 3 mm, and window axis 60” from the vertical. Its FWHM is about 160 eV/5.9 keV.

The fitting program TROJAX had been developed on the basis of the HEX-83 source file and later it was strongly influenced by AXIL. According to the latter, the background model was constructed as

3.2. Model

BACK(E)

= AL1 + AL2 * (E - EO) + AL3 *(E-EO)*

Only thin samples were used for the calibration purposes. The number of pulses in a characteristic K X-ray peak was expressed according to ref. [I] as

+ ABO * EXP[ABl +AB2*(E-EO)*

2 = (Q/e~)(~/~)~(~/4~)~, (1) where Q = the accumulated charge stated by the current integrator, e = 1.6021 x lo-l9 C, s = the beam spot area defined by the diaphragm, m = the total mass of an element in the beam spot area, A = the atomic weight of the element (atomic number divided by Avogadro’s constant), u = the K, X-ray production cross section, 0 = the solid angle subtended by the detector, T = the total parameter including both the transmission and the detector efficiency. The parameter T was expressed as

where

T= expt -/%a,

U(E) = AE”.

X (1

-

es-$

- CLsesne - F&A, -jLSi6det))

v

-

cLSisSi)fesc (2)

where separate parameters were stated using constants A, B, C, D and F as functions of energy E by the equations p(E) =AEB and f,,,(E) = 1 - C(1 - DEF

*2+

* ABS(E),

... * (E - EO)

*2+.*.j, (3)

where EO was a reference energy and ABS(E) represented the X-ray absorption [lo]. The value of EO remained constant while the values of the coefficients AL and AB were fitted in the spectra processing routine. The parameter ABS was constructed according to Eq. (2). It could be multiplied with the term of sample absorption expressed as [lo] T, = (l-exp( - U))/U,

(4)

(5)

The coefficients A and B from Eq. (51 were the only variables in ABS which could be fitted together with AL and AB. In accordance with the presumption of thin samples, the term T, was not used in the calibra-

V Poto&k/Nucl.

tion procedure but only later in the quantitative analysis. Except AL, AB, A and B, the fitted parameters in TROJAX were the calibration (both energy and FWHM), peak positions and peak widths. Optionally could also be included a limited number of absorption edges and pile-up peaks. The parameters which were not to be fitted, i.e. ABS(E) and EO, and also the input approximations of AL and AB, were expected to be constant for the given setup despite the varying conditions in separate analytical runs, and so they had to be stated carefully to remain valid for a long time. This has been done as described in the following. 3.4. Calibration

procedure

The mass calibration of the setup consisted of two main steps. The first resulted in proper input parameters for TROJAX to describe the background, absorption, and detector efficiency as well as possible. The net peak areas from the TROJAX output were then identified with 2 from Eq. (1) and the corresponding ABS(E) were identified with T. Thus all the parameters in Eq. (1) acquired their physical sense and could in principle be measured to enable determining one of them from the others. The term (0/4a) remained as the only free parameter provided the first calibration step had been done. To become the real parameter for calibration it was replaced by C-’ which implicitly included the charge collection efficiency correction and the dead time correction except of the detector solid angle itself. Thus we obtained the following equation for routine quantitative analyses of thin samples: m = (ZseA/QTu)C,

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Ins&. and Meth. in Phys. Res. B 85 (1994) 145-149

(6)

where .Z and T were to be included in the TROJAX output file, s and Q were directly measured parameters, and e, A, u were basic parameters. Eq. (6) was incorporated in the program TROCON. Determination of C became the task for the second step of the calibration procedure. The first step of the calibration, i.e. TROJAX calibration, began with the statement of the best value of EO and the input values of AL and AB in the background description (3). According to a recommendation in ref. [lo], blank spectra were analyzed by TROJAX starting with ABO = - 2, Al31 = 3, AB2 = - 0.031, AB4 = 0.001, EO = 4 keV and only one nonzero parameter AL. The parameters of the detector absorption were derived or estimated on the basis of the manufacturer’s datasheet and/or general data about Si(Li) detectors which were known from the literature. The spectra of Mylar, Nuclepore, Czech membrane filters Synpor, Slovak foil Tatrafan and occasionally other foils and filters were analyzed repeatedly by TROJAX.

Each separate parameter value including the degree of the linear polynomial (i.e. the number of nonzero coefficients AL) was changed as long as the resulting chi squared improved, and that routine was cyclically repeated for all the parameters and all the available blank spectra. Since the polynomial coefficients and EO were stated in some optimum values, the shape of the detector efficiency curve was calibrated. 35 spectra of thin MO foils evaporated onto the foil Tatrafan were taken in several well defined angles and distances between the target and the detector. The spectra were analyzed by TROJAX with unchanged background description and then the output was processed by TROCON. Separate parameters were changed (but only within physically acceptable limits) until the masses of MO stated from K, and from L, peaks were equal. The incident angle and the estimation of the detector solid angle were taken into account in the actual corrections of the input values. The transmission through the Mylar absorber was determined for its nominal thickness corrected for the incident angle by the exponential curve T,=exp(-U),

(7)

where U(E) was expressed as in Eq. (5) with A and B obtained by interpolation of published Mylar absorption data [ll]. Physical parameters of the detector and the absorber were derived from the optimum values for each MO spectrum and resulted in the following average values: Be window thickness

12.7 km

Au contact

16.0 nm

layer thickness

Si dead layer thickness

150.0 nm

Active volume thickness Mylar absorber

2.4 mm 132.0 urn

The values were then checked in all the analyses when some concentrations were known or could be known. Those tests included both old samples that had been analyzed either by PIXE or by other analytical techniques and the samples that were prepared in the laboratory. Among the old samples were the filters with aerosol, ashed needles and calibrated precipitations that had been analyzed in Lund by PIXE ten years ago, some aerosol samples that had been analyzed in the intercalibration experiment [12], the spectra of the aerosol samples that were then mineralized and analyzed by atomic absorption spectroscopy, and the samples that were analyzed by PIXE in Gent. The pure calibration samples were the residuals of KMnO, solution droplets. Experimental spectra were processed with calibrated TROJAX and then by modified TROCON (called TROCAL) which had parameters C as output instead of m. It has appeared that there was no systematic divergence in any part of the spectrum that II. PIXE/PIGE

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Instr. and Meth. in Phys. Res. B 85 (1994) 145-149

could be explained by the incorrect shape of the curve of the detector efficiency, i.e. that the only problem in routine quantitative analyses would be the correct statement of C (or, maybe, only fl) in the particular run through the analysis of a suitable standard.

4. Present state The PIXE analytical system mentioned above has been proved to be ready for analyses for the purpose of environmental and biological, including medical, research with good quality. More than 700 samples were analyzed to calibrate the system, to confirm the power of the new software and to test some types of analytical methodologies, mainly those fitted to aerosol analyses for air pollution investigation. The typical mass detection limit is about 10 ng/cm*, the reproducibility is up to lo%, typically around 3-5%. The fitting program TROJAX was compared with several other programs through analyses of some spectra from the comparative work [13] that has been sent from Gent together with the necessary input information. The spectra were taken as totally unknown (with the exception of the absorption description), and without any special tuning of the TROJAX parameters including calibration and the element/edge list. Nevertheless, the coincidence in peak areas seems to be reasonably good, as it is illustrated in Table 1.

Table 1 The ratios of some peak areas stated in standard spectra by TROJAX to average values from ref. [12] Element

Mg Al Si P s Cl K Ca Mn Fe CU Zn Y Ag Sb Ba Pb

Sample Nuclepore A

Human kidney

Orchard leaves (high statistics)

1.30 1.05 1.04 1.66 1.05 1.01 0.99 0.99 1.06 1.01 0.83 0.87

_ _

_ _

_ _ 1.01 0.96 1.18 1.01 0.87 0.87 0.98 0.87 0.92 0.81 0.85

_ _ 1.11 1.05 1.09 0.89 0.92 0.95 0.67 0.92 1.13 0.99 0.96

0.83 0.93

Table 2 Some concentrations stated in a typical sample (#99) from the sampling station Bily Kiwi, Czech Republic. Standard deviations in percent reflect accuracies of peak areas stated by fitting of spectra. Detection limits were derived from the background under the peaks in both laboratories. In Praha, they were defined as 3 * SQRtB) where B was background plus tails from other peaks in the interval from centroidFWHM to centroid + FWHM of a representative peak Ele-

Lund

ment Concentration

lWcm*l

Praha Det. limit Concentration

Det. limit

big/cm21 1ng/cm21

Wcm*l

S

539.80 (8.6%) 4.76

Cl

4.85 (61.2%) 3.48 25.57 (8.9%) 2.21 1.96 (36.5%) 1.54 _

K Ca Ti V Cr Mn Fe cu Zn As Br Pb

3.55 (9.1%) 0.67 (16.3%) 6.21 (9.2%) 0.50 (16.6%) 3.30 (10.3%) 2.48 (19.3%) 3.56 (14.3%) 2.09 (40.2%)

0.30 0.24 0.19 0.13 0.13 0.25 0.42 1.22

528.47 (3.1%) 21.92 9.86 (38.6%) 6.15 24.80 (5.9%) 2.30 1.11 (13.9%) 0.73 (13.4%) 3.49 (10.5%) 0.53 (53.9%) 6.31 (8.1%) 0.74 (12.5%) 2.94 (25.0%) 2.93 (75.0%) 4.87 (40.0%) _

0.57 0.40 0.40 0.29 0.65 0.19 0.60 0.92 1.51

Recently the first attempt in intercalibration with the Lund PIXE system was carried out. The set of samples of aerosol on Nuclepores collected in Czech sampling sites was analyzed to state elemental concentrations and compare them with those stated in Lund before. The samples were poor and almost all concentrations were close to the level of the detection limit. Despite this, no systematic errors were observed and the differences did not exceed acceptable values, as it is illustrated in Table 2. It should be noted that the Prague PIXE system is under permanent development both with respect to its setup and the software. There are only some results from intercalibration, moreover declared as preliminary. But the results of calibration and some other experience encourage us to introduce our PIXE system as prepared to reach its maturity.

References 111 K.R. Akselsson, S.A.E. Johansson and T.B. Johansson, in: X-Ray Fluorescence Analysis of Environmental Samples, ed. T.G. Dzubay (Ann Arbor Science, Collingwood, 1976) p. 175. [2] S.A.E. Johansson, Charged Particle Induced Energy Dispersive X-Ray Analysis, LUTFD2/(TFKF-3019)/163205 (1979). 131W. Bambynek, B. Crasemann, R.W. Fink, H.U. Freund, H. Mark, C.D. Swift, R.E. Price and P. Venugopalo Rao, Rev. Mod. Phys. 44 (1972) 716.

V. PotoSek /Nucl. Instr. and Meth. in Phys. Rex 3 8.5 (1994) 145-149 [4] J.H. Scofield, At. Data Nucl. Data Tables 14 (1974) 121. [5] E. Storm and H.I. Israel, Nucl. Data Tables A 7 (1970) 565. [6] S.A.E. Johansson and J.L. Campbell, PIXE - A Novel Technique for Elemental Analysis (Wiley, Chichester, 19881. [7] G. Johansson, in: Proton Induced X-Ray Emission mass calibration, computer analysis and applications to work environment aerosols LUTFD2/(TFKF-10011/l111 (1981). (81 R.S. Sokhi and D. Crumpton, Nucl. Instr. and Meth. 192 (1982) 121.

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[9] V.M. Koljada, AK Zajchenko and R.V. Dmitrenko, Rentgenospektralnyj analiz s ionnym wozbuzhdenijem (Atomizdat, Moskwa, 1978). [lo] AXIL X-Ray Analysis Package - User’s Guide (November 1981). [ll] J.W. Mayer and E. Rimini (eds.), Ion Beam Handbook for Material Analysis (Academic Press, London, 1977). 1121V. PotoEek, R. Brenner, F. Hodik and J. Voltr, J. Radioanal. Nucl. Chem. 149 (1991) 205. [13] J.L. Campbell, W. Maenhaut, E. Bombelka, E. Clayton, K. Malmqvist, J.A. Maxwell, J. Pallon and J. Vandenhaute, Nucl. Instr. and Meth. B 14 (1986) 204.

II. PIXE/PIGE