Initial inter-laboratory testing of the Rossendorf-Oxford (ROX97) secondary standard for X-ray analysis

Initial inter-laboratory testing of the Rossendorf-Oxford (ROX97) secondary standard for X-ray analysis

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B

Beam interactions with Materials 8 Atoms

ELSEVIER

Nuclear

Instruments

and Methods

in Physics

Research

B 136 13X (1998) 902 907

Initial inter-laboratory testing of the Rossendorf-Oxford secondary standard for X-ray analysis C. Neelmeijer

‘,*, M. Mader “, R. Jarjis b, T. Calligaro T. Gantz e

‘, J. Salomon

(ROX97)

‘, M. Schreiner

‘,

Abstract (ROX97 Rossendorf-Oxford. version 1997), the new paper-based secondary standard for storing, reproducing and comparing relative X-ray efficiency curves. has been distributed to different analytical laboratories for initial tests. Several standard X-ray techniques as PIXE. XRF and EDX have been involved. Quantitative inter-laboratory comparisons are presented for PIXE facilities operating in-vacuum as well as on external proton beams. In the case of XRF and EDX typical spectra taken from the standard are compared in the context of the individual experimental conditions. Regarding both the different sources of X-ray excitation and X-ray detection units the advantages and limits

of ROX97 are discussed to deduce information

on required upgrading improvements.

1. Introduction Recently, a paper-based secondary multielement standard. called ROX97 (Rossendorf-Oxford, version 1997), was developed to store, reproduce and compare the spectral efficiency curves C(E,i) of energy dispersive X-ray facilities. Here, ,I$, denotes the energy of the characteristic X-radiation emitted from a chemical element with atomic number Z,. Motivated by successful initial tests of the basic parameters [I]. using ion beam techniques, the standard paper sheets of dimension

*Corresponding

author.

E-mail: [email protected]

016%583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved PIISO168-583X(97)00871-5

0 1998 Elsevier Science B.V.

2 cm x 2 cm were distributed to different analytical groups involving not only PIXE experiments but also XRF analysis and EDX in the SEM (Scanning Electron Microscope). With respect to PIXE the aim was to gain initial quantitative data for ROX97-assisted inter-laboratory connection in practice. Therefore, the relative I:(&,) curves, stored by measuring PIXE spectra from the paper standard, are compared with corresponding calculations from the individual detection parameters, i.e. the Si(Li) detector entrance window, X-ray paths in air or He atmosphere and attenuation due to further absorbers. Whereas the detection sensitivity of PlXE does not change drastically within the energy region of

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the standard (1.5 < Exi < 15 keV) [2], the wedge structure of the XRF excitation cross section may reduce the applicability of the standard elements of low atomic number Zi. Due to the absorption effects the X-ray yield of the low-Z atoms is further reduced when working in air. The latter is of special interest when providing the standard for quick calibration of portable XRF devices [3]. For EDX the production of intense radiation background is expected from the primary electron bremsstrahlung. In addition, the low concentration of the characteristic doping elements inside the standard, i.e. in relation to that in metals or alloys, together with the porous paper fibre structure may result in low peak to background ratios. Nevertheless, the XRF and EDX test measurements serve for an initial complementary comparison to PIXE applications in order to conclude improvements to produce finally a universal secondary multielement standard for a broad scale of X-ray analytical methods.

2. Testing procedure From a sheet of ROX97 paper standard material, Ref. [I], samples of 2 cm x 2 cm were cut and measured in the several laboratories under routine conditions. Note that these samples were taken from different positions of the original size A4 for these initial tests. Sporadic quantitative fluctuations due to long-distance standard inhomogeneTable

YO3

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ities cannot be completely excluded in this regime [ 11. The individual experimental conditions are summarized in Table 1. For quantitative inter-laboratory comparison it is a necessity to guarantee identical excitation conditions. Therefore, the terminal voltages of the ion accelerators were tuned in such a way that proton beams of 2.3 MeV final energy were obtained on the paper target surface for the in-vacuum [4] and external PIXE [5,6] arrangements. As described in Table 1 the PIXE studies include macroscopic beam spots (Rossendorf and Paris) as well as measurements at the Oxford Microprobe in the 250 pm x 250 pm scanning mode. XRF experiments [7] were carried out in Vienna using the commercial device “TRACOR-Spectrace 5000” allowing measurements in-vacuum as well as in-air. From measurements at different voltages/currents (lo-50 kV/0.4-0.1 mA) the 20 kV/0.2 mA operating regime of the Rh-tube proved optimum for simultaneous emission of the 1.5 < E,, < 15 keV standard X-ray lines. For EDX analysis [S] (Dresden) the commercial “CAMSCAN-44” facility was available. The windowless Si(Li) detector was shielded by a Be absorber of 10 pm thickness to adjust reasonable counting rates by suppressing the intense K X-ray lines of oxygen and carbon from the organic paper matrix. Note that due to the metallic components inside the standard the addition of a conducting surface layer on the insulating paper material was not necessary for EDX measurements.

I

Experimental parameters of the PIXE. XRF and EDX measurements carried out by (a) R. Jarjis. University of Oxford: (b) T. Calligaro. J. Salomon. Laboratoire de Recherche des Mu&es de France, Paris: (c) C. Nrelmeijer. M. Milder. Research Centre Rossendorf:

(d) M. Schreiner.

Academy

of Fine Arts. Vienna,

Experimental Set-up

PIXE (a) In-vacuum

Excitation

2.3 MeV protons I20 pA. 30 min I pm. scanning mode, 250 pm x 250 pm 29.5 pm Be

Beam spot diameter Si(Li) entrance Absorbers

window

No absorber

and (e) T. Gantz.

Technical

University.

Dresden

PIXE (b) External. in He atmosphere 2.3 MeV protons 100 pA. I2 min I mm

PIXE Cc) External. in He atmosphere 2.3 MeV protons 200 pA. 4 min I mm

XRF Cd) In-vacuum as well as in-air Rh tube. 20 kV. 0.2 mA, 8 min 1 mm

0.25 pm BN + “funny filter” frame He: 50 mm

25.4 pm Be

I2 pm Be

He: 30.5 mm Mylar: 6.0 pm Air: 3.6 mm

No absorber or Air: 20.0 mm

EDX (e) In-vacuum 30 kV electrons. 200 nA, I3 min I pm. scanning mode. 2.5 mm x 2.0 mm windowless IO pm Be

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3. Results and discussion 3. I. PIXE Fig. 1 presents the ROX97 standard spectra as obtained at the different PIXE arrangements. Significant differences in the low energy part of the E(E,;) functions are obvious from this figure. For example in Fig. l(b), even the detection of the oxygen K-line reflects the excellent transparency of the 0.25 urn BN window of the Si( Li) detector used by the Paris group (Table 1). Note that the oxygen signal originates from both the paper matrix and the doping constituents. Hence, it would be of interest in future to test whether, from the elemental

16

(a)

12

6

W

0

2

4

6

6

10

X-ray energy / keV Fig. 1. X-ray spectra of the paper-based secondary standard ROX97 obtained at the PIXE facilities characterized in Table I: (a) Oxford Scanning Proton Microprobe (in-vacuum). (b) Laboratoire de Recherche des Mustes de France Paris (external protons in He-atmosphere), (c) Research Center Rossendorf (external protons in He-atmosphere).

stability and homogeneity point of view, the present secondary standard may also serve for the efficiency at 0.25 keV X-ray energy, i.e. the oxygen Kradiation. From the measured spectra peak areas were deduced using the GUPIX program [9]. After that the peak areas of each individual spectrum were normalised to the corresponding Zn( K,) reference peak. These normalised data sets were used to obtain inter-laboratory ratios of the experimental relative efficiency curves E(Exi)‘x’, for instance “exp. Oxford/Rossendf.“ and “exp. Paris/Rossendf.“, as given in Table 2. For comparison, Table 2 contains the corresponding theoretical ratios as deduced from the normalised c(Exi)lh values calculated on the basis of the Si(Li) entrance windows and the further absorbers (Table 1). In this context it is worthwhile to mention that even at E, = 1.49 keV, i.e. Al (K). the Paris external PIXE experiment (He flush) obtains E(E,,)‘~ = 0.84 in comparison to E(Eyi)‘h = 0.36 in the case of the Oxford in-vacuum PIXE. This is because the 29.5 urn Be-window of the Oxford Si(Li) represents a much stronger absorber than the 50 mm He-path in the Paris arrangement. Nevertheless, the precision of quantitative results depends strongly on the real experimental values c(Exi)‘“” plus their temporal stability which can easily be checked by making use of ROX97 (Table 2). The importance of the latter becomes obvious when the systematic E, dependent deviations between the measured and calculated Paris/Rossendorf ratios (Table 2) are regarded. Taking into consideration the general overall agreement of the experimental and calculated OxfordIRossendorf efficiency ratios the corresponding Paris/Rossendorf values of Table 2 reveal the presence of an additional absorber in the Paris facility. For example, the inclusion of 10% air in the He path would fairly approach the experimental and calculated Paris/Rossendorf ratios. On the other hand, the mismatch of the aluminium and titanium values in the experimental Oxford/Rossendorf curve (Table 2) probably originate from local target inhomogeneities. In this context note that the 250 urn x 250 urn scanning area, adjusted at the Oxford Microbeam is too small for calibration measurements by the aid of ROX97.

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Table 2 Ratios of relative efficiencies, normalised to E, = 8.64 keV (Zn K,): Experimental data measured by the aid of the ROX97 secondary standard in comparison to theoretical values as calculated from the detection parameters Element Z,

E,, (keV)

Oxford(“)/Rossendf.(b) Exp.

0xford(‘)/Rossendf.(2) Calc.

Paris(c)/Rossendf.‘b) Exp.

Paris’s)/Rossendf.(2J Calc.

Al Si s Cl K Ca Ti Cr Fe Ni CU Zn

I .49 1.74 2.31 2.62 3.31 3.69 4.51 5.41 6.40 7.48 8.05 8.64

1.21” 2.06 1.10 1.07 1.08 1.80” 1.10 1.07 1.00 I .04

2.85 1.93 1.34 1.22 1.10 1.08 1.04 1.02 1.01 1.oo 1.00

3.39 1.69 1.25 0.95 0.99 0.93 0.99 0.95 0.94 1.01 1.00

6.60 3.20 1.57 1.35 1.16 1.11 1.06 1.03 1.01 1.00 1.oo

1.00

1.00

1.00

1.00

1.48

a Mismatch - interpretation

see text. Statistical plus fitting errors: (a) Oxford: 4%, (b) Rossendorf: 4% (c) Paris: 6% Absorbers: (‘1Oxford: 29.5 urn Be; (2)Rossendorf: 30.5 mm He, 6 urn Mylar, 2.6 mm air, 25.4 urn Be; cl) Paris: 50 mm He, 0.25 urn BN (with frame as a “funny filter” of 20 urn B, 83”/ hole), 0.1 urn Si deadlayer.

Fe

al

In-vacuum

3.2. XRF

Shown in Fig. 2 are typical XRF spectra taken from the ROX97 paper standard in-vacuum as well as in-air (see Table 1). As expected from the given Rh X-ray tube regime, the line intensities emitted by the elements 2 > 22 dominate which preferably useful for makes the standard E, > 4.5 keV. In the case of E, < 4.5 keV both the low excitation cross sections and the high Xray absorption coefficients make it impossible to apply the present standard for in-air calibration as can be seen in Fig. 2. Possibly, the situation can be improved by making use of another tube anticathode, e.g. a Fe X-ray tube for optimum low-2 element analysis, or XRF with a 55Fe radioactive source of reasonable activity. Otherwise, regarding the higher X-ray energies 10 keV < E, < 15 keV, ROX97 proved very suitable for XRF efficiency calibration in the regime described above (Table 1). This was tested by making use of a second version of the paper standard [l], not discussed in this paper, containing e.g., the elements bromium (Br-K, = 11.92 keV) and yttrium (YK, = 14.96 keV).

2

4

6

6

10

X-ray energy I keV Fig. 2. X-ray spectra from the paper-based secondary standard ROX97 obtained at the “TRACOR-Spectrace 5000” XRF facility at the Academy of Fine Arts, Vienna (for experimental parameters see Table I ).

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To decide on the simultaneous usefulness of the standard for both PIXE and XRF also the peak to background ratios (Ypeak/Yback)i in the X-ray spectra are of interest. Comparing the in-vacuum measurements of Oxford (PIXE) and Vienna (XRF) in the X-ray energy region & < 3 keV, i.e. the region where PIXE secondary electron bremsstrahlung is emitted by the paper matrix [l], the (Ypeak/Ybac.), are quite similar for both techniques. For the higher X-ray energies the XRF ratios remain under the PIXE data by about a factor of 5. This is due to the radiation background, in the particular case, caused by scattering of the intense primary electron bremsstrahlung emitted from the X-ray tube. 3.3. EDX in the SEM Fig. 3 presents the X-ray spectrum obtained from ROX97 by 30 kV electron excitation (see Table 1). Note that a 10 urn Be absorber was positioned in front of the windowless detector to suppress the very strong carbon (E, = 0.28 keV) and oxygen (& =0.52 keV) X-ray lines from the paper matrix, hence arranging processable counting rates. Without absorber ROX97 cannot be used in a windowless Si(Li) detector regime. Consequently, the absorber function must be known if the s(Exi)exp curve, as obtained from the standard, is to be transferred for analysis in a changed routine detector regime.

I 1998) 902-907

From Fig. 3 it is obvious that despite the intense primary electron bremsstrahlung continuum the superimposed ROX97 characteristic X-ray lines are of usable intensities for fitting with reasonable precision. However, the lower peak to background ratios. relative to both PIXE and also XRF, require EDX spectra of ROX97 measured with very good statistics as given, e.g., in Fig. 3. Variation of the primary electron energy 20 keV < EC < 40 keV does not affect the statement given above.

4. Conclusions The new paper-based multielement secondary standard, code named ROX97, proves suitable and advantageous for inter-laboratory comparison and transfer of the experimental relative efficiency curves of PIXE arrangements. As known from previous proton microprobe mappings. the images of the characteristic doping elements follow the pattern of the paper fibres. Therefore, further developments should take into account material matrix compactness in order to improve the microscopic homogeneity of the dopant distribution. This is of special importance for (i) further lowering quantitative fluctuations and (ii) upgrading the standard as a useful tool also for X-ray microbeam techniques. In general. the intensities of the reference X-ray lines emitted from ROX97 are applicable for other X-ray techniques like XRF and EDX in the SEM. Therefore, the multielement standard developed on the basis of paper matrix combines the basic fundamentals of a universal secondary standard for X-ray analytical techniques. Under the latter point of view, progressive developments should include extended testing of(i) standard homogeneity by XRF and EDX plus (ii) standard stability under electron bombardment, as well as (iii) the test of doping higher concentrations of low-Z elements (13 < Zi < 20). Acknowledgements

Fig. 3. X-ray spectrum from the paper-based secondary standard ROX97 obtained at the “CAMSCAN-44” EDX device of the Technical University. Dresden (for experimental parameters see Table I ).

The present joint results have been worked out in the frame of the European Commission COST

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Action-Gl. Short-term scientific missions of M. Mader and R. Jarjis to the laboratories of Vienna and Rossendorf, respectively, formed the basis of the extensive experimental studies and fruitful inter-laboratory communications. The authors wish to express their thanks to the EC for the financial supports.

References [I] C. Neelmeijer,

J. Huller, M. Mader. B. Borchers, R. Jarjis. Nucl. Instr. and Meth. B. to be published. [2] R. Mann. C. Bauer, P. Gippner. W. Rudolph. J. Radi?ranalytical Chem. 50 (1979) 217.

[31R. Cesareo, G.E. Gigante.

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