PIXE and XRF comparison for applications to sediments analysis

Beam Interactions with Materials 8 Atoms

ELSEVIER

Nuclear Instruments and Methods in Physics Research B 132 (1997) 481488

PIXE and XRF comparison for applications to sediments analysis F. Benydich

a,*, A. Makhtari

a, L. Torrisi

b, G. Foti ’

‘I Dbpurrernent de Physique, Facultt! des Sciences, UniversitP Moulay IsmaX BP 4010. Beni M’Hamed, 50003 Meknt?. Morocco ’ Dipartimento di Fisica, Universitci di Messina. Salitu Sperone, S. Aguta Messina, ltuly ’ Dipartimento di Fisicu. Universitti di Cutania. Corso Italiu 57. 95129 Catania. Itu!,

Received 12 November 1996; revised form received 22 April 1997

Abstract The potential of particle induced X-ray emission (PIXE) and X-ray fluorescence (XRF) is compared with regard to its application to sediments analysis. The intrinsic features of each of the two techniques are addressed, and their elemental sensitivities are evaluated and compared, showing that in the case of sediments analysis, due to the complexity of their composition, the two techniques are complementary. 0 1997 Elsevier Science B.V. Keywords: PIXE; XRF; Environmental pollution;

Ion beam analysis; Sediments analysis; Spectrum fitting; Computer simutation

1. Introduction

Particle induced X-ray emission (PIXE) and X-ray fluorescence (XRF) are two useful techniques widely used in many different fields, such as medicine, biology, microelectronics, archaeology, geology and environmental sciences. Both the techniques are multi-elemental, they can be used in environmental sciences as routine methods for the determination of matrix composition of samples, or more accurately for trace elements analysis. with special regard to the detection of toxic elements dispersed in soils, atmospheric particulate and liquids [l-5].

*Corresponding :;023.

author. Tel.: 39 95 7195 111: fax: 39 38

(1168-583x/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PIISOl68-583X(97)00404-7

Elemental

composition;

Trace

element

detection;

The materials used for this study were fine sediments samples taken from the basin storage of the Barrage El-Kansera on Oued Beht River in Morocco. The choice of such a material was driven by the fact that a long term multi-disciplinary study is in course at the University of Meknes (Morocco) on monitoring the biochemical and physico-chemical quality of the waters stored in this barrage [6,7], in relation to the quality of the affluent waters and to define and monitor any source of environmental pollution resulting from urban, industrial or agricultural rejections along the river’s path upstream. Given the physico-chemica1 exchanges between water and sediments, the knowledge of the sediments composition and their trace elements content, is a necessary step in monitoring the impact of the environmental pollution on the studied site.

The composition of sediments is very complex, their matrix can contain up to ten main elements and as many trace elements. This makes the interpretation of PIXE and XRF spectra recorded for sediment samples a bit complicated. in comparison with materials of simpler composition. due to inter-elements interferences and to matrix effects. superimposed to instrumental effects such as Si-escape peaks, and pile-up peaks. Despite a!! these difficulties, in the field of rocks and sediments analysis. PIXE and XRF remain very attractive techniques due to their ability to give an almost full elemental composition of the samples under investigation in a unique measurement [S]. In this study the elementa! sensitivity of the two techniques is evaluated in the case of sediments analysis. It is shown that the two techniques are complementary in terms of qualitative and quantitative analysis. The minimum detection limit (MDL) reaches values of few atomic ppm for both PIXE and XRF.

2. Materials and methods PIXE analysis was performed using a 2 MeV proton beam issued from the Van de Graaff accelerator at the University of Catania. The beam spot was of 0.5 x 0.5 mm?. and the beam current of tens of nA. With respect to the beam direction, the sample’s normal is located at 45”. and the Si(Li) detector is at 90”. For XRF analysis the samples were excited by the 22-25 keV Ag-K X-ray lines delivered by a compact annular source, consisting of a “‘Am radioisotope source of 100 mCi activity and a Ag secondary target irradiator. With the experimental set-up used. the sample is located at most at 2 cm from the Ag ring, and at 3 cm from the Si(Li) detector. This very close packed configuration reduces the X-rays attenuation in the atmosphere. and offers a good uniformity of the sensitivity across the specimen surface. The area probed by the primary X-ray beam is about 2 cm’. For both techniques the Si( Li) detector used has a full width at half maximum (FWHM) of 180 eV at 6 keV. The acquisition times ranged between 2000 s and 4 h.

Sediments samples taken from the bottom of the basin storage of the Barrage El-Kansera on Oued Beht River in Morocco, were homogenised in an agate mortar and passed through a 200 pm nominal opening sieve, and prepared as pellets 24 and 10 mm in diameter, and 3 and 1 mm thick, respectively, for XRF and PIXE analysis. A preliminary Rutherford Backscattering Spectroscopy (RBS) analysis. using a 2 MeV He beam. has been performed on the sediment samples in order to determine their average matrix composition. This composition is used to compute the proton energy loss in the sediment matrix and to evaluate the X-ray attenuation coefficients. A computer code for fitting the experimental PIXE and XRF spectra and for their quantitative analysis has been elaborated [9]. This code after evaluating the continuous background uses a non-linear least squares fitting algorithm to adjust a mode! spectrum to the experimental one yielding thus the intensities of the X-rays lines. This fitting procedure has been featured to account for: interelements interferences; matrix effects due to self-attenuation of the emitted X-rays: Si K, escape peaks and pile-up peaks. Once the X-rays’ intensities are extracted from the experimental spectra. the quantitative analyses are performed by using the we!! known relations giving the X-rays yield

x v for protons

(

-~,p- “-

COSfli,

cos&u,

excitation,

1 &,

and

x, = C,Q&,,(!),

X

/’ ( exp

ir

/W=

---___ cos 0,”

ill,,P~

cos &“, 1

dz

(2)

for photons excitation. With Y,, being the experimental X-ray yield of the jth line of the element i. C, the atomic density of element i, Q the number of incident particles, Sz the solid angle sustained by the detector. c,, the detector efficiency for the considered line. UJ, its fluorescence yield, (T,, the ionisa-

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tion cross-section for the considered line, fl,, its branching ratio, 0,” and O,,, are, respectively, the angles of the incident beam and of the emitted X-rays directions with respect to the normal to the sample’s surface, p,, the mass attenuation coefficient in the target matrix of the emitted X-ray line, and p the target density. In Eq. (1) R, is the projected range of the incident protons, and E(z) is the protons’ energy at depth 3. In Eq. (2) /lo is the mass attenuation coefficient of the incident photon beam, t is the sample thickness. and r is the absorption jump ratio of the considered line. The depth -_in the target is taken along the protons’ trajectory in Eq. (I), and perpendicular to the sample’s surface in Eq. (3).

Energy

(keV)

2 MeV p _-3Sedlment

Fig. 1 shows typical PIXE (Fig. l(a)) and XRF (Fig. l(b)) spectra recorded for the same sediment sample. with acquisition times of 4 h. In this figure the fitted spectra and the evaluated continuous background are superimposed to the experimental data The continuous background lying under the characteristic X-ray lines is an inherent feature of the two modes of excitation. For proton excitation it originates from the secondary electrons Bremsstrahlung for X-ray energies lower than 10 keV, at higher energies it is due to the incident protons Bremsstrahlung, and Compton scattered gamma rays originating from proton-induced nuclear reactions. For photon excitation the continuous background is mainly due to the Compton scattering of X-rays in the target and in the detector. The Compton edge located at about 9 keV on the continuous background is due to the scattering and escape of the primary ‘J’Am 59 keV X-ray arriving onto the Si(Li) detector after multiple scattering in the Ag secondary target and the sample [lo]. On the PIXE spectrum (Fig. l(a)) the main peaks originate from Al, Si, K, Ca, Ti, Mn and Fe while heavier elements such as Rb, Sr, Y and Zr appear as trace elements with very low peak to background ratios. For routine acquisition times of 2000 s these elements do not appear, their corresponding peaks do not exceed the background level. tn the energy region IO-14 keV.

Samplet=4h

loo

’

200

’

400

I

’

’

600

600

Channel Energy

3. Results and discussion

15

10

5

5 /

22 keV

,

(keV) ,

X-rays

15

10 /

j

,

Sediment

Sample

,

,

<

(b)

t=4h

400

600

1

Channel Fig. I. PIXE (a) and XRF (b) spectra of a sediment sample. The contmuous line refers to the fitted spectra and the dashed line refers to the estimated background.

pile-up peaks are present. In the case of XRF analysis (Fig. l(b)), it is the heavy elements: Rb, Y, Sr, Zr and Nb that have higher peak to background ratios, compared to light elements (Si, Ca, K) whose signals are almost rubbed out. The very intense Cu signal appearing on the XRF spectrum is due to an experimental artefact. as it comes from the fluorescence of the Cu collimator of the annular X-ray source. The intensity of this peak is used as an internal clock reference when recording the XRF spectra.

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These differences in the elemental sensitivity observed between the two techniques are due to their intrinsic features: (i) the inner-shell ionisation cross-section induced either by protons or by photons; (ii) the extent of the probed depth, and matrix effects due to X-rays auto-absorption; (iii) the intensity of the continuous background underlying the characteristic X-ray lines. The experimental conditions such as the detector efficiency, the presence of any filter or absorber along the path of the detected X-rays, may also change drastically the sensitivity of both the techniques. In what follows the sensitivities of the two techniques will be evaluated andcompared with regard to the material, i.e. the sediments, under investigation in this study. Fig. 2 shows the K and L shells ionisation cross-section induced by 2 MeV protons (dashed lines) and the photo-ionisation cross-section induced by 22 keV photons (continuous line), versus the target atomic number, computed according to the semi-empiric formalisms related in the literature [ 11.13]. For proton excitation the ionisation cross-section decreases with increasing the atomic number of the target atoms. While for photon excitation the trend is inverted, the photo-ionisation cross-section increases with Z. This inverted trend of the ionisation cross-section between proton and photon excitation, explains partially the differences in elemental sensitivity of the two techniques.

Atomic

Number

Fig. 2. Variations of the lonisation cross-section versus the target’s atomic number, for 2 MeV protons (dashed lines), and the photo-ionisation cross-section for 32 keV photons (continuous lines).

The second feature that contributes to differentiate PIXE and XRF analysis, is the extent of the probed depth below the sample’s surface from which the detected X-rays originate. The extent of the probed depth depends on the penetration depth of the incident particles or photon beam and on the self-absorption of the emitted X-rays. All these effects are due to the interactions of the incident and the emitted radiations with the target matrix. and are generally termed as matrix effects. An estimation of the probed depth accounting for these matrix effects may be derived from Eqs. (1) and (2), by defining an equivalent target thickness :*, for constant ionisation cross-section and zero absorption as:

_f -XRF

- -

=

- ___

(4)

0

These relations show that for proton excitation (3) the probed depth is limited by the range of the incident particles, which is of 60 urn for 2 MeV protons in a sediment matrix, and by the self-attenuation of the emitted X-rays along their outward path; while for photon excitation (4) the limitation comes from the attenuation of the incoming X-rays and the self-absorption of the fluorescent X-rays. Fig. 3 shows the variations of the equivalent target thickness versus the atomic number of the target’s emitting element, computed according to Eqs. (3) and (4). for proton excitation (Fig. 3(a)), and photon excitation (Fig. 3(b)). The equivalent target thickness is evaluated for K lines for Z d 42 and for L lines for Z > 42. It is seen that for low energy X-rays, i.e. Z d 20 for K lines and Z d 60 for L lines, both the techniques suffer from self-attenuation of the emitted X-rays, and the equivalent target thickness is of the order of few tens of microns. At higher X-ray energies (Z 2 20 for K lines, and Z B 60 for L lines), for proton excitation the equivalent target thickness is limited by the particles’ range as already pointed out; while for photon excitation due to the reduced self-absorption effects the equivalent

F. Benvaich

rt ul. I Nucl. Instr. unn Meth. in Plr~s. Res. B 132 (1997)

2 MeV p_, o$

Ol’.“‘.““““,““l 0

20

40

Atomic

0

0

20

40

Atomic

sediment

Sample

n 60 w$

60

1

80

100

I30

100

Number

60

Number

Fig. 3. Variations of the effective target thickness versus the atomic number of the emitting target element. for (a) protons excitation and (b) photon excitation. z* is computed for K Xray lines for elements with Z < 42. and for L lines for elements aith Z > 42. target thickness increases with the target atomic number, i.e. with X-ray energy, up to values of few hundreds of urn, remaining however lower than the absorption length of the incident radiation ib = (pop)-’ = 2336 urn. A more refined comparison including both the matrix effects and the efficiency of X-ray excitation may be done by defining from Eqs. (1) and (2) and from the definition of I”* (Eqs. (3) and (4)) the elemental intrinsic sensitivity as the X-ray yield per incident particle. per unit of solid angle and per unit of atomic density of a considered element

481488

485

By taking into account the experimental parameters: the total number of incident particles, the solid angle sustained by the detector and its efficiency, one can define the effective elemental sensitivity, i.e. the X-ray yield per unit of atomic density, as Se = Q~EY. Fig. 4 shows the variations of the intrinsic and effective sensitivity ratios (&d&RF), versus the atomic number of the emitting target element; computed for K lines for Z < 42 and for L lines for Z > 42. These data show that for low energy X-ray lines PIXE is much more sensitive than XRF due to the higher ionisation cross-section of protons, which balances the strong self-absorption occurring at low energies; while at high X-ray energies XRF is more sensitive due to the combination of both high ionisation cross-section and low self-attenuation. The crossing points, at which the intrinsic sensitivity ratio equals unity, are at Z = 23 for K lines, and Z= 55 for L lines. For the effective sensitivity ratio the crossing points are shifted towards higher X-ray energies corresponding to Z= 30 and Z= 73 for, respectively. K and L lines. The positions of the crossing points of the effective sensitivities may be varied by chan-

Atomic

Number

Fig. 4. Variations of the intrinsic and effective sensitivity ratios. versus the atomic number of the emitting target element. The horizontal arrows delineate the ratio unity for each of the two scales.

ging the experimental parameters. Given that the experimental set-up is usually set to its most efficient configuration. one cannot gain too much from the detector efficiency and solid angle. The parameters that allow a certain degree of freedom are the acquisition time and the flux of the incident particles. This latter parameter favours PIXE as the beam current and diameter may easily be tuned over a large range of values. While for XRF analysis the photon flux is fixed by the activity of the radioactive source and its geometrical configuration. The conclusions drawn in the Section 2 are based on calculations derived from basic physical concepts, they must be confronted to the actual experimental measurements. For this scope the MDL achieved with each of the two techniques was extracted from PIXE and XRF spectra. The commonly accepted three-sigma criterion was used to evaluate the MDL. It says that an X-ray peak is statistically significant with a 99.87% confidence level. when its peak count Nr, satisfies the condition Nr 3 3a. where Nb is the background count [13]. Fig. 5 summarises the average MDL converted into atomic fractions. extracted from PIXE (circles) and XRF (triangles) spectra of the

0

2 YeV

Protons

_

A 22 keV Photons i

3 I

20

,

,

1

40

Atomic

,

,

I

60

,

,

1,

80

Number

Fig. 5. Variation of the minimum detection limit versus the atomic number of the emitting target element. extracted from PIXE (circles) and XRF (triangles) spectra. recorded with acquisition times of 4 h. The MDL was computed for K X-ray lines for elements with Z < 42. and for L lines for elements with z> 43.

sediment samples under investigation. The MDL is computed for K X-ray lines for elements of atomic number ranging between 13 and 41. and for L lines for elements of Z between 42 and 82. These data are in excellent agreement with the results of the preceding sensitivities calculations. They confirm the complementarity of the two techniques, that PIXE is more sensitive than XRF for low energy X-ray lines, with MDL values falling down to few atomic ppm; while at high X-ray energy, XRF is the most sensitive with MDL values ranging between 0.6 and 4 atomic ppm. Moreover, the crossing-points where the sensitivities of the two techniques intercept are perfectly reproduced by the experimental MDL measurements. The data presented on this figure refer to high statistics spectra of 4 h acquisition time. For routine acquisition times of 2000 s. the MDL values are, as one may expect. an order of magnitude higher. This figure also shows that at low X-ray energies the sensitivity of both the techniques degrades, the MDL growing up to values of lo’-lo3 atomic ppm. This is due to the vanishing detector efficiency at such low X-ray energies. For these energies XRF is also handicapped by the reduced ionisation cross-section. Table 1 summarises the average elemental concentrations. together with their relative standard deviation, extracted from PIXE and XRF spectra of sediment samples. Squares filled with the indication “ND” (for not detectable) means that the elemental concentration of the corresponding element evaluated by a given technique is below its MDL, or it has not been evaluated due to strong peak interferences. These data show that for elements for which both the techniques have given measurable values. the latter are in good agreement. Considering the statistical dispersion reported for each of the two techniques, it is seen that PIXE measurements are more precise for light elements, while for heavy elements XRF gives the most precise data. This correlates with their respective sensitivities, more the technique is sensitive more it is precise. The Ni content has not been evaluated, due to the interferences of its signal with the Ca K, sum peak on the PIXE spectra. On the XRF spectra, this is because of the low energy tail of the large

Table I Average atomic

concentrations

and their relative

standard

deviations

for sediment

samples

analysed

with PIXE and XRF

AX/ (X) (3’~)

(WXRF

AI-/(X) (‘%,I

IO-’ 10 ’ 10 1 IO J lo-” 10~ ? 10 : 10 ’ 10 5 10 i 10 a 10 2

20.48 1.83 16.12 42.11 81.64 15.12 30.74 16.4’ 39.99 60.01 25.75 19.87 ND

ND ND ND ND ND 2.03 4.32 4.15 4.73 1.80 5.09 2.00 ND

29 30 33 34

3.74 x 10 ( 2.79 x IO ( ND ND

61.95 68.68 ND ND

ND ND 1.87 x 10 ( 4.33 x 10 h

BI. Rh

35 37

ND 2.65 x 10 L

ND 97.51

3.31 x 10 h 4.31 x IO-’

Sl Y Z1Nh Pb

38 39 40 41 82

4.14 1.01 4.03 8.55 ND

73.59 84.97 58.73 167.89 ND

5.97 6.85 4.49 6.14 4.78

ND ND ND ND ND 43.44 60.09 35.28 12.34 38.06 16.79 5.71 ND ND ND 18.54 55.19 43.96 5.64 41.00 24.49 12.39 14.63 25.07

Element

(Z)

Whrxc

Al Si P S Cl K Ca Ti V Cl. MI1 FC N.

13 14 I5 16 17 19 20 21 23 24 25 26 28

6.28 x 1.40x 1.70 x 1.87 x 3.00 x 1.03 x 2.70 x 1.63 x 3.76 x 1.60 x 2.10 x 1.32 x ND

cu Zll Ai Se

x x x x

10~ 5 10 i 10-i IO h

x x x x x x x

x x x x x

IO-’ 10 7 10~’ IO-” 10 d 10 ’ IO 2

IO 5 10mh 10 q 10 ’ IO ’

ND refers to “not detected”.

Cu K, peak coming from the Cu collimator of the X-ray source, that hides the Ni signal. Zn has not bren evaluated on the XRF spectra, due to its interferences with the Cu Ka peak. Two other remarks should also be mentioned about the data of Table 1, they deal with the elemental concentrations of As, Se, Br and Pb. These elements being present at traces levels, and as PIXE sensitivity for these elements does not match that of XRF (Fig. 51, their signals do not appear on PIXE spectra. Their corresponding concentrations extracted from XRF spectra are in good agreement with those reported in the literature for soils and sedimentary rocks [14]. However, these values must be considered as indicative because of their very low levels and the strong interferences that exist between As K and Pb L lines. The trace elements content reported in this study is low. about one order of magnitude below the values reported in similar studies on European sites [15-l@.

4. Conclusions PIXE and XRF offer high analytical potentials for multi-elemental investigations in the field of environmental sciences. PIXE has a high sensitivity over a wide range of light elements. It gives relatively straightforward quantification in bulk sample. The probed depth is limited to the range of the incident particles, this makes the technique more suitable for the analysis of thin layers. XRF is more sensitive for heavy elements, it has a larger probed depth, and is suitable for large sample areas. By accurately selecting the energy of the exciting radiation, its sensitivity can be optimised for a particular elemental region of interest. Based on a radioactive source, XRF is simple and highly reliable at relatively low costs compared to the heavy instrumental set-up needed for PIXE analysis. This study has shown that in the experimental conditions adopted (i.e. 2 MeV protons for PIXE, and 22-25 keV Ag K lines for XRF) the two tech-

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ct ul. I Nucl. btstr. onrl Meth. in Phv.~. Rr.5. B 131

niques PIXE and XRF are complementary, the first being very sensitive for light elements (low energy X-ray emitters) and the second for heavier elements. Used together they represent a complete investigation tool for the analysis of environmental samples. This is particularly true for samples of complex composition like sediments, whose major constituents are light elements, and contain many heavy trace elements. The main composition of the analysed sediments is typical of silicate rocks, and their trace elements content is low, and remains at tolerable level when compared with similar studies carried out on more industrialised European sites.

Acknowledgements

References Ill PI PI [41 [51

Lb1 [71 181

[91 [1(‘1

The authors wish to acknowledge Prof. G. Pappalardo for his useful suggestions and co-operation and for putting at their disposal the facilities of the XRF laboratory at the University of Catania (Italy). This work was supported by the co-operation agreement between the Consiglio Nazionale delle Ricerche (CNR. Italy) and the Centre National pour la Coordination et la Planification de la Recherche Scientifique et Technique (CNCPRST, Morocco).

i 1997) 481488

[I II [I’1 1131 [I41 [I51 [I61 [I71 1181

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