Trace element analysis by ion induced X-ray emission spectroscopy

NUCLEAR

INSTRUMENTS

AND METHODS

142 ( ! 9 7 7 )

39-44 :

©

NORTH-HOLLAND

PUBLISHING CO.

TRACE ELEMENT ANALYSIS BY ION INDUCED X-RAY EMISSION SPECTROSCOPY* B. RAITH, M. ROTH, K. GOLLNER, B. GONSIOR, H. OSTERMANN and C. D. UHLHORN

lnstitut,/iir Experimentalphysik,

Ruhr-UniversitiJt Bochum, D-4630 Bochum, W. Germany

K and 1_ X-ray emission rates for 37 elements and continuous background radiation from a thin carbon matrix produced by bombardment with protons of 1.5, 2, 3, and 4 MeV have been investigated. K-shell ionization cross sections for 25 elements for 2 MeV proton bombardment, minimum detection limits through the periodic system in the ppm range and optimum proton energies for highest sensitivity for a given element have been extracted from these measurements. 160ions turn out to be much less sensitive than protons of the same velocity. Two examples of applications to mineralogical and environmental problems are given.

1. Introduction Particle induced X-ray emission spectroscopy (P1XE) as a m e a n s of trace element analysis enables high detection sensitivities in cases where only small sample amounts are availablet). It has been pointed out by Folkmann 2) et al. that minimum detection limits attainable in PIXE in principle are given by the production cross sections of characteristic X-rays of the trace elements in question and continuous background radiation due to the interaction of the charged particle beam with the sample. Characteristic X-ray emission for light projectiles is well understood and can be described theoretically on the basis of models and calculations for inner shell vacancy production 3) and fluorescence yields4). Continuous background radiation mainly consists of secondary electron bremsstrahlung and projectile bremsstrahlung of the bombarding particles slowed down in close collisions with the matrix nuclei. Both have been calculated in ref. 5. In case of higher bombarding energies also Compton scattering of 7-rays from nuclear reactions between projectiles and matrix nuclei contributes to the background radiationS). Especially in the case of heavy ion bombardment the advantage of enhanced inner shell vacancy production is balanced by the background enhancement due to Compton scattering. However, for analytical work knowledge of X-ray absorption, of the detector efficiency, and of the solid angle is required. Therefore it is preferable to have empirically determined X-ray yields independent of theoretical calculations and including all corrections inherent in the apparatus.

We have measured proton induced X-ray yields for 37 elements between Z -- 16 and Z = 83. Also background radiation produced in a carbon matrix has been investigated. From these measurements we extract ionization cross sections, minimum detection limits and projectile conditions for optimum detection sensitivity of a given element. Some examples of applications are described.

* Work supported by Minister ftir Wissenschaft und Forschung des Landes Nordrhein-Westfalen, D-4000 Dtisseldoff, W. Germany.

Fig. 1. K-shell ionization cross section as a function of the target atomic number for bombardment with protons of 2 MeV. The solid line represents a BEA calculation of ref. 3.

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41

TRACE ELEMENT ANALYSIS

2. Experimental 2.1. EXPERIMENTALSET-UP The measurements were performed using the 4 MV Dynamitron T a n d e m accelerator of the Ruhr-Universit~it at Bochum. The target was located at a 45 ° angle to the beam axis. The Si(Li) detector used in our experiment had a measured energy resolution of 175 eV at 5.9 keV (Mn K~). It was placed at a 90 ° angle, and the solid angle was 3.77 × l0 3 sr. A particle detector was placed at an angle between 20 ° and 90 ° depending on the projectile energy and the atomic n u m b e r of the target element. To suppress background radiation a carbon slit system was used to define the beam spot, and the walls of the target chamber were covered with a carbon foil of 0.3 m m thickness. A 200 p m Be window was used to absorb low energy X-rays. To keep dead time correction errors small the counting rate was always kept below 1000 cps. The target thicknesses varied between 3 and 20 ~ g / c m 2 evaporated on a 20/~g/cm 2 carbon foil.

get thickness and total projectile n u m b e r have been normalized to the differential cross sections of Rutherford scattering. Using these values, which include geometry, absorption and fluorescence yields, in our analytical work we are independent of theoretical cross section calculations. However, ionization cross sections can be extracted from these yield factors. In fig. 1 K-shell ionization cross sections for b o m b a r d m e n t with protons of 2 MeV are shown for 25 elements between Ca and La. The solid line represents a BEA calculation 3) without any correctioris. To determine projectile conditions for o p t i m u m sensitivity we have in addition to characteristic Xray emission measurements investigated the continuous background radiation. In these measurements 4 0 / l g / c m 2 carbon foils were bombarded for about 24 h with protons of 1.5, 2, 3, and 4 MeV and with ~60-ions of 24 MeV (1.5 MeV/amu).

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We have measured proton induced X-ray emission yields for 37 elements between Z = 16 and Z = 83 with protons of 1.5, 2, 3, and 4 MeV. Tarl

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Fig. 4. K X-ray spectra of lead produced by bombardment with protons of 5 MeV (upper spectrum) and 7 MeV. 1. T E C H N I C A L DEVELOPMENTS

42

B. RAITH et al.

Fig. 2 shows a comparison of background radiation for 2 MeV protons (upper spectrum) and 24MeV 160 projectiles (lower spectrum). Both parts of the background radiation - the secondary electron bremsstrahlung as well as the projectile bremsstrahlung - are remarkably enhanced in relation to the characteristic X-rays in case of ~60 bombardment. This is, as far as the secondary electron bremsstrahlung is concerned, in contradiction to BEA calculations, that predict the same dependence on the atomic number of projectiles for both characteristic X-rays and bremsstrahlung and therefore the same peak-to-background ratio. The fact that the high energy background is much higher than expected from theoretical calculations, is mainly due to Compton scattered 7-rays produced in nuclear reactions between projectiles and matrix nuclei. To define the minimum detection limit we have chosen as a criterion the peak-to-background ratio to be equal to or larger than 1. The sensitivity curves for protons of 1.5, 2, 3, and 4 MeV and for 160-ions of 24 MeV for spectroscopy of K X-rays are shown in fig. 3. The curves indicate that for a broad range of elements using protons of 3 MeV highest sensitivity in the ppm range is attainable. Protons of 4 MeV are less sensitive because the production of nuclear reaction 7-rays increases with increasing projectile energy more rapidly than the characteristic X-ray production does. This is demonstrated in fig. 4 where the K X-rays of lead I

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for bombardment with protons of 5 and 7 MeV measured with an intrinsic germanium detector are shown. Obviously the peak-to-background ratio decreases by a factor of two for protons of 7 MeV. In fig. 3 also a sensitivity curve for 160-ions of 24MeV (1.5 MeV/amu) is given. As mentioned above the minimum detection limit is much higher than for bombardment with protons of the same velocity, due to the considerably enhanced background radiation. In practical analytical work it is useful to know the optimum conditions for the determination of a given element, or a combination of elements of interest. Therefore in fig. 5 sensitivity versus proton energy for K X-ray detection from Ti, Fe, Zn, Mo, Cd and for L X-ray detection of Pb is shown. These elements were chosen as they are of special interest for our cur,

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Fig. 6. Proton induced X-ray spectra o f a quartz crystal. T h e lower s p e c t r u m is the higher energy part of the upper spect r u m in a linear scale.

TRACE

ELEMENT

43

ANALYSIS

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Fig. 7. X-ray spectra of carbon foils flowed off in water of the river Rhine. The samples were taken before (upper spectrum) and after the river had passed a big chemical factory.

1. T E C H N I C A L

DEVELOPMENTS

44

B. RAI'I-H et al.

rent work in biological, environmental and mineralogical fields. Lower proton energies are favourable only for low Z trace elements. Otherwise protons of 3 MeV are preferable. Higher proton energies do not promise any sensitivity improvement according to figs. 4 and 5. 3. Applications Up to now we have applied PIXE to mineralogical problems using thick targets and to environmental problems using thin targets. The investigation will be extended to biological questions using our proton microbeam of s o m e / z m diameter in the near future. In cooperation with the Institute of Mineralogy of the Ruhr-Universit~it Bochum we have investigated different quartzes looking especially for trace elements with atomic n u m b e r lower than 30. Fig. 6 shows an X-ray spectrum of a quartz of 20--30/,tm thickness bombarded with protons of 2 MeV. The upper spectrum is drawn in a logarithmic scale, the lower spectrum exhibits the high energy part of the upper one in a linear scale. However, the extraction of absolute amounts of trace elements from such thick target spectra requires to calculate X-ray absorption within the target and to consider the dependence of the ionization cross section on the proton energy during the slowing down of the projectile in the thick target; which we have not done up to now. Most of our analytical work on environmental problems was done in the area of water pollution. As a matrix we chose carbon foils of 20-40/zg/cm 2. Mainly two problems arise from this. The first problem is, that within the carbon foils there were already present nearly all elements to be analysed in the water sample. Measurements proved that they are not due to contaminations of the high purity carbon bars that were evaporated but to the bi-destilled water used for floating off the foils. A steel plate with a small hole ( 2 c m ~ ) was installed between the carbon bars and the glass slides to prevent metals and other surrounding materials, that become hot in the evaporation process, from precipitating on the slides. The second problem arises when a drop of a water sample is put on the carbon foil and is dried, the trace element contents of the water is not homogeneously distributed over the surface of the carbon foil. This is due to the fact that some of them are ions and some are floating particles.

Both problems were solved by floating off the foils in the water we wanted to analyse. Doing that all trace elements are homogeneously distributed all over the surface of the carbon foil and all analysed elements are merely due to the water sample in question. For quantitative analysis we added a certain a m o u n t of rubidium-sulphate to the water sample, to have a relationship between the concentration of the trace elements on the matrix foil (g/cm 2) as obtained by the PIXE technique and the concentration of the trace elements in the water sample (g/l). The use of rubidium-sulphate as calibration trace element does not affect the detection of neighbouring elements. Fig. 7 shows an example of such a water analysis. Two spectra of water samples from the river Rhine are depicted. The samples were taken before (upper spectrum) and after (lower spectrum) the river had passed a big chemical factory. The trace element concentration given as numbers in brackets, for the several Xray lines respectively, indicates the increasing river pollution due to the waste materials of the factory. 4. C o n c l u s i o n s Particle induced X-ray emission spectroscopy turns out to be a useful tool in trace element analysis. Thin target analysis is very easy, and sensitivities between 10 6 and 10 _7 are attainable by bombardment with protons of 1.5-4 MeV. Analyses of thick target samples are more complicated due to the slowing down of the projectiles and to X-ray absorption within the target. 160-ions appear to be less sensitive than protons of the same velocity, but they might be advantageous if lower energies are used to define a m a x i m u m penetration depth in surface layer analysis of materials.

References

1) T. B. Johansson, R. Akselsson and S.A.E. Johansson, Nucl. Instr. and Meth. 84 (1970) 141. 2) F. Folkmann, J. Borggreen and A. Kjeldgaard, Nucl. Instr.

and Meth. 119 (1974) 117. 3) j. D. Garcia, R. J. Fortner and T. M. Kavanagh, Rev. Mod. Phys. 45 (1973) 111. 4) W. Bambynek, B. Crasemann, R. W. Fink, H.-U. Freund, H. Mark, C. D. Swift, R. E. Price and P. Venugopala Rao, Rev. Mod. Phys. 44 (1972) 716. 5) F. Folkmann, C. Gaarde, T. Huus and K. Kemp, Nucl. Instr. and Meth. 116 (1974) 487.