The analysis of rare-earth bearing minerals using K and L X-ray spectra

Nuclear Instruments and Methods in Physics Research B 109/110 (1996) 606-611

B Beam Interactions with Materials & Atoms

ELSEVIER

The analysis of rare-earth bearing minerals using K and L X-ray spectra C.G. Chol

a~

, G. R e m o n d b, D.B.

Isabelle a

aCER1-CNRS, 3A, Rue de la Fdrollerie, 45071 Orldans Cedex 2, France bBRGM, DR/GGP, BP 6009, 45060 Orleans Cedex 2, France

Abstract REE bearing minerals were analysed by means of their L X-ray spectra using both PIXE and EPMA microprobes. Quantitative data were found in a good agreement for concentrations. LOD values for each REE analysis by EPMA and PIXE are estimated, In order to get rid of the multiple interferences between L lines, we used the LEGe detector to obtain well-resolved K-ray REE spectra using a 3 MeV proton beam.

1. Introduction Proton-induced X-ray emission (PIXE) and electron probe microanalysis (EPMA) have been used for spatially resolved elemental analysis of rare-earth element (REE) bearing minerals. X-ray spectra of REE containing minerals (57 < Z < 71) are usually analysed by means of their L X-ray spectra using both PIXE and EPMA equipped with energy dispersive spectrometers (EDS) or wavelength dispersive spectrometers (WDS) respectively. Thus X-ray spectra of these minerals are complex due to frequent peak interferences of the L lines of all other rare-earth elements in the 4.5-9.9 keV energy range and of the K lines of lower atomic number elements (22 < Z < 32). In order to get rid of these intereferences, most of PIXE analysis of REE were achieved by high energy proton beams in the 2085 MeV range for the high cross section of K-rays [1-5]. But in this work, we have obtained well-resolved K spectra of REE recorded with a low energy (LEGe) detector using a 3 MeV proton beam.

2. Experimental method 2.1. Materials f o r analysis

Six synthetic zircons (ZrSiO4) doped with REE (Ce, Nd, Gd, Gd + Ho, Ho and Lu) were analysed by PIXE and EPMA. The synthetic zircons doped with REE were prepared using the flux method described by Cesbron et al. [6]. Two natural monazite minerals ((La, Ce, Th) PO4) originating from Brazil and Cameroon deposits respective* Corresponding author. Tel. (33) 38 51 76 46, fax (33) 38 63 02 71, e-mail [email protected].

ly, containing many REE as major constituents, were also analysed with PIXE and EPMA.

2.2. Irradiation and measurement 2.2.1. EPMA The Cameca SX 50 type EPMA installed at the joint CNRS-BRGM laboratory was used. The EPMA is equipped with five WDS and is computer controlled for fully automated data acquisition, handling and reduction of the raw X-ray intensities. The PAP program developed by Pouchou and Pichoir [7] was used in order to derive the concentrations from the measured X-ray intensities. Glasses with Perovskite composition (REE)A10~ were used as reference material to calculate the concentrations for the synthetic zircons and the natural monazites. All synthetic zircons were measured with a 20 keV electron incident energy and a 30 nA beam current during 30 s counting time. For natural monazites, the measurements were made using a 20keV with 3 0 n A during 12s for the major elements (E Ca and REE) and during 120s for trace elements (Pb and U).

2...9.2. P1XE PIXE analyses were carried by means of the Van de Graaff accelerator at the CERI-CNRS laboratory in Or1tans, France, as already described in detail by Zine et al. [8] and Azahra [9]. All measurements were performed using a 3 MeV proton beam and with beam currents higher than 0.2 nA. The beam spots were 20 ~xm X 40 I~m in size. The specimen holder accommodates up to five specimens placed in the plane normal to the proton beam direction. A 30 mm 2 Si(Li) detector and a 100 mm 2 LEGe detector are installed respectively at +45 ° with respect to the beam axis

0168-583X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)00977-9

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and at 31 mm and 48 mm distances respectively from the specimen. The former provides a high efficiency for X-rays detection in the range of 1 to 25 keV with an energy resolution of 145 eV full width at half maximum (FWHM) at 5.9 keV and the latter offers a good efficiency in the 20 to 120keV range with an energy resolution of 153 eV FWHM at 5.9 keV and 482 eV FWHM at 122 keV. Two thick filters (104 txm A1 filter and 138 ~m Be filter) can be placed in front of the Si(Li) detector. For the detection of high energy X-ray photons (K lines of the REE), a 100 txm thick Fe filter was used in front of the LEGe detector. Energy-dispersive X-ray spectra are collected with a computerised multichannel analyser (Canberra $35+). They are transferred to a micro-processor and processed with the GUPIX program developed by Maxwell et al. [10]. The version used of the GUPIX software does not include physical parameters allowing us the data reduction of high energy K spectra (LEGe detector) according to the procedure used for the L X-ray spectra. In the absence of interferences between the high energy K spectra of the REE, the calculated statistical LOD values were applied to the peak area between two energy limits manually selected on the raw spectra.

(a)

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3.1. R E E d o p e d synthetic zircons

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Fig. l a shows a typical EPMA X-ray map, which displays the heterogeneous distribution of Gd in the Gd doped synthetic zircon. Two main areas can be distinguished by their high (A area) and low Gd concentration (B area). The quantitative profile in Fig. lb shows that within the A area, the Gd weight concentration measured with the EPMA was 1010 ppm, a value which is consistent with the value of 882ppm measured with the PIXE equipped with the A1 filter. Within the B region, the EPMA line scan in Fig. lb shows an apparent concentration of about 190 ppm which was found to be not significant when applying the usual statistical criterion for the peak (Np) and continuum (NB) intensities i.e. Np < 3 ~ / ~B. When using a Be filter, 184 ppm of Gd were detected within the B area. This result is consistent with the qualitative X-ray map in Fig. la and indicates that the Gd concentration locally varied by a factor of about 5 from area A to area B. A typical PIXE spectrum measured at the surface of the B area of the Gd doped zircon is shown in Fig. 2. This spectrum was obtained with a beam current of -0.1 nA for reducing the counting rate and the continuous background due to the tail of the Zr K peaks. As shown in Fig. 2, the PIXE spectrum revealed the presence of Cu K(x,[3 peaks which is assured to result from the excitation of the metallic copper specimen holder. Comparative quantitative results from PIXE and EPMA

11

21

31

41

51

61

71

I~1

91

point (b) Fig. 1. Heterogeneous distribution of Gd within the Gd doped zircon. (a) EPMA X-ray mapping. (b) The point analyses were performed step by step along the scanning line marked in the photo.

analyses of REE doped synthetic zircons are shown in Table 1. For the REE with atomic number greater than 64 (Gd), the deviation between EPMA and PIXE data are 1E+5 ~ i

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Fig. 2. PIXE spectrum of the Gd doped synthetic zircon. The spectrum was obtained in the B area with the 138 ~m thick Be filter.

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C.G. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 606-611

Table 1 Comparison of the results of PIXE and EPMA analysis of the REE doped synthetic zircons (two areas containing low and high concentration [wt. ppm]). (1) A1 filter 104 p,m in thickness, (2) Be filter 138 Ixm in thickness, nd: not detected Doped element (Z)

Analysed element

Ce (58) Nd (60) Gd (64)

Ce Nd Gd

Ho (67) Gd + Ho

Lu (71)

PIXE

EPMA high

low

high 292+69 (2) nd (2) 882+164 ( 11

nd nd 1010_+200

Ho Gd

nd (2) nd (2) nd (1) 184_+28 (2) 576_+70 (1) nd( 1)

2570_+220 nd

Ho

406+81 (1)

Lu

8572+97 (2)

2676_+88 ( 1) nd ( 1) 269_+37 (2) 2039+92 (1) 2422_+66 (2) 9717_+ 103 (2)

between 5% and 10% for concentrations of the analysed elements ranging between about 1000 ppm and 2000 ppm, consistent with comparisons already reported by Remond et al. [11]. As also shown in Table 1, the quantitative data derived from PIXE measurements remained consistent to each other when an AI or a Be filter was used successively. When using the A1 filter, no Gd was measured by means of PIXE for the case of the synthetic zircon doped with Ho and Gd simultaneously. However, Gd was measured at a concentration level of about 270 ppm when a Be filter was used instead of the AI filter. This result illustrates the important need of optimising the nature and the thickness of the filter as a function of both the energy of the analysed X-rays and the matrix composition. The crucial need in optimizing the instrumental factor for trace element analysis with the PIXE is also shown for the case of the analyses of the synthetic zircons doped with lighter REE. As shown in Table 1, Ce and Gd which were not detected when using the A1 filter were measured when the Be filter was used. For the case of the Nd doping REE is actually incorporated within the host zircon crystal, its concentration is lower than the LOD of PIXE. Because of the satisfactory agreement between EPMA and PIXE data for elements present at concentration levels higher than about 1000 ppm, the statistical LOD values for REE in the synthetic zircons were compared in Table 2. PIXE analyses of REE usually yield lower LOD values with a Be filter. LOD values obtained by EPMA are about 10 times higher than the ones obtained by PIXE with a Be filter. 3.2. R E E - b e a r i n g natural m o n a z i t e s

As shown in Fig. 3, the PIXE spectra measured with a 3 MeV proton energy display both L and K lines for the natural monazite originating from Brazil. Figs. 3a and 3b show the PIXE L lines recorded with a 0.1 nA beam current using a Be filter and with a 0.4 nA using an A1

2180_+120 13250

filter respectively. For the case of the spectrum using the Be filter (Fig. 3a), the 0.1 nA beam current was the lowest irradiation condition leading to a sufficient counting rate. However, these conditions did not prevent against pile-up effects with a maximum near 9 keV which is twice the mean energy of the REE photons. The use of an A1 filter with a simultaneous increase of the beam current up to 0.4 nA leads to a reduction of these pile-up effects. The multiple interferences between the REE L lines measured with a Si(li) detector are shown in Fig. 3b w i t h the expanded spectrum of the photon energy range from 4 keV to 10 keV. These interferences make difficult the qualitative and quantitative interpretation of the spectra and make poorer the LOD values compared to those which were obtained for the case of the zircon crystals containing a single REE. K line spectra in Fig. 3c using a LEGe detector exhibit less interferences than for the case of the L spectra. The analytical results of the natural monazites are presented in Table 3 in comparison with PIXE and EPMA. The results were found to be in a reasonable agreement. But in the case of a Be filter, the concentration values of Y, Sm, Eu, Gd and Dy were found to be inferior to the ones of A1 filter because of a low beam current. Concerning Pr,

Table 2 Comparison of the limits of detection (ppm) for REE in the synthetic zircons. _+: the reproducibility of LOD values derived from several analyses, nm: not measured EPMAa

Element

PIXE

(Z)

AI filter

Be filter

Ce (58) Nd (60) Gd (64) Ho (67) Lu (71)

nm nm 268_+20 128+12 nm

122_+8 100_+2 60_+ 10 68+20 49_+4

a 20 keV, 30 nA, 30 s counting time.

841 +28 670_+31 454_+ 10 390_+10 634_+ 10

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C.G. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 606-611

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Fig. 3. PIXE (EDS) spectra of the REE natural monazite (Brazil) using 3 MeV protons. (a) L lines collected with a 0.1 nA beam current and a 138 txm Be filter using a Si(Li) detector. (b) L lines recorded with a 0.4 nA beam current and a 104 p~m A1 filter using a Si(Li) detector. (c) K lines recorded with a 1.3 nA beam current and a 100 ixm Fe filter using a LEGe detector.

E u a n d Gd, the difference o f the concentration v a l u e s w a s o b s e r v e d b e t w e e n E P M A a n d P I X E o w i n g to the fact that no correction w a s m a d e for p e a k interferences o f E P M A . O n the other h a n d the g o o d residuals o f the P I X E s p e c t r u m indicate a g o o d fit to the data. T h e L O D values are c o m p a r e d in the Table 4. L O D o f L a e l e m e n t with an A1 filter w a s f o u n d 6620 p p m b e c a u s e o f the low t r a n s m i s s i o n o f a X-ray. W e o b s e r v e d also h i g h L O D v a l u e s o f R E E o w i n g to the spectral interferences o f L lines. In spite o f

the low yields o f cross section o f R E E K X-rays, L O D values o f R E E in the natural m o n a z i t e could be i m p r o v e d for the low atomic n u m b e r REE.

4. C o n c l u s i o n

T h e c o m p a r i s o n o f quantitative a n a l y s e s with the E P M A and the P I X E o f R E E present at trace levels in d o p e d

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C.G. Choi et al. / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 6 0 6 - 6 1 1

Table 3 Comparison of the PIXE and EPMA analyses of two natural monazites. (1) A1 filter 104 txm in thickness, (2) Be filter 138 ixm in thickness. nd: not detected, nm: not measured Element (Z)

Monazite (Cameroon)

Monazite (Brazil)

PIXE

EPMA

PIXE

EPMA

PK (151 a SK (16) 8

14.16+0.13 (2)

13.19+0.07

14.30+0.13 (2)

13.33-+0.08

3559-+217 (2)

nd

2727+219 (2)

nd

C1K (17) 8

910+133 (2)

nm

473_+139 (2)

nm

Ca K (20) 8 Sr K (38) 8 YK (39) 8 La L (57) a Ce L (581 a PrL (59) a Nd L (60)" Sm L (62) ~ Eu L (63) 8 Gd L (64) ~ Tb L (65) 8 Dy L (66) b Pb L (8218 Th L (90)" UL (92) b

6079 + 103 (2)

6423-+352

751 +75 (2)

96-+15 (1) nd (2) 4098+52 (1) 2999-+280 (2) 11.16-+0.66 ( 1 ) 11.11 +0.17 (2) 23.25-+0.39 (1) 22.56+0.16 (2) 2.46-+0.45 (11 2.35-+0.29 (2) 10.64+0.41 (1) 10.24-+0.31 (2) 2.38+0.25 (1) 1.83-+0.21 (2) 3668+2160 (11 nd (2) 1.32+0.16 (11 0.88-+0.17 (2) 2464+1251 (1) 3676-+ 1465 (2) 2083-+797 (1) nd (2) 1953-+45 (1) 2025 -+369 (2) 4.11 -+0.02 (1) 3.25+0.11 (2) 7752 + 104 (1) 7832+570 (2)

nm nm 10.16_+0.16 24.88+0.16 1.68-+0.10 12.46-+0.38 2.22_+0.09 6785+758 2.77-+0.09 nm nm 1610-+345 3.68+0.16 7614-+866

328_+13 (1) 247 + 114 (2) 5036+44 (11 4109_+295 (2) 12.64+0.63 (1) 13.36+0.20 (2) 27.96+0.37 (1) 28.61_+0.18 (2) 2.93+0.43 (1) 2.47_+0.33 (2) 8.07_+0.43 (1) 8.19+0.36(21 1.27_+0.23 (1) 0.83_+0.23 (2) 3444-+ 1981 (1) nd (2) 0.68_+0.14 (1) 0.29_+0.15 (2) 1931-+1010 (1) 2877_+ 1462 (2) 3040+686 (1) 1352 + 1002 (2) 99-+18 (1) nd (2) 2650_+53 (1)8 2612-+390 (2) b 54-+25 (1) nd (2)

814_+ 159 nm nm 12.20+0.12 30.69_+0.20 1.59+0.11 9.97-+0.35 1.16-+0.10 7899-+831 2.70_+0.09 nm nm nd 2517_+952 b nd

Weight %. b Weight ppm. a

synthetic zircons and o f R E E as m a j o r constituents o f natural m o n a z i t e minerals s h o w e d that: i) a relative discrepancy in the order of m a g n i t u d e o f 10% for the analysis o f the R E E p r e s e n t at concentration levels o f a few t h o u s a n d s o f p p m . ii) an i m p r o v e m e n t by a factor o f about 10 o f the limit o f detection o f PIXE with respect to E P M A . iii) for R E E concentrations r a n g i n g f r o m a few percent up to a few tens o f percent, the relative difference b e t w e e n the E P M A and P I X E data r e m a i n e d better than 10% u s i n g the L X-ray e m i s s i o n lines. T h e n u m e r o u s interferences

b e t w e e n the L lines o f R E E present s i m u l t a n e o u s l y within the minerals lead to a poorer limit of detection than that obtained for the case o f minerals containing a single REE. iv) a c o m p l e t e analysis o f the R E E is possible by c o m b i n i n g the L X-ray spectra f r o m a Si(Li) detector a n d the h i g h e n e r g y L X-ray lines f r o m a L E G e detector u s i n g a low e n e r g y proton b e a m s . T h e analysis o f e l e m e n t s with atomic n u m b e r greater than about 74 ( W ) u s i n g their K lines is still difficult due to the low X-ray production resulting f r o m low ionisation cross sections. T h e detection will be i m p r o v e d by o p t i m i s i n g the i n s t r u m e n t a l factors

C.G. Choi et al. / Nucl. h~str, attd Meth. in Phys. Res. B 109/110 (1996) 606-611

Table 4 Comparison of the limits of detection (ppm) for REE in the natural monazites. +: the reproducibility of LOD values derived from several analyses, nm: not measured Element (Z)

PIXE L Line (SiLi)

La (57) Ce (58) Pr (59) Nd (60) Sm (62) Eu (63) Gd (64) Tb (65) Dy (66)

K Line (LEGe)

A1 filter

Be filter

6620+-663 3340-+697 6083 _+248 4223 +-469 3002+93 2005_+ 269 1142 + 135 735_+200 411 -+ 163

1672+99 1157_+61 4807 _+4 l 1 5425 _+980 4787_+260 3332 + 187 2459_+ 103 1617-+30 1397_+ 19

635 802 749 3169 a 3200 a nm 4000 ~ nm nm

For Nd, Sm and Gd, the K/3 lines are used instead of K a lines in order to avoid peak interferences (see Fig. 3c).

a

s u c h as for e x a m p l e the collection solid angle o f the detector.

611

References [1] J.J.G. Durocher, N.M. Halden, F.C. Hawthorne and J.S.C. Mckee, Nucl. Instr. and Meth. B 30 (1988) 470. [2] J.S.C. Mckee, G.R. Smith, Y.J. Yeo, K. Abdul-Retha, D. Gallop, J.J.G. Durocher, W. Mulholland and C.A. Smith, Nucl. Instr. and Meth. B 40/41 (1989) 680. [3] M. Peisach and C.A. Pineda, Nucl. Instr. and Meth. B 49 (1990) 10. [4] C.A. Pineda and M. Peisach, J. Radioanal. Chem. 151 (1991) 387. [5] N.M. Halden, Nucl. Instr. and Meth. B 77 (19937 399. [6] F. Cesbron, D. Ohnenstetter, P. Blanc, O. Rouer and M.C. Sihere, C. R. Acad. Sci. Paris 316, Srrie II (19937 1231. [7] J.L. Pouchou and F. Pichoir, La Recherche Arrospatiale 3 (1984) 167. [8] E. Zine, D.B. Isabelle, G. Remond, Nucl. Instr. and Meth. 49 (1990) 446. [9] M. Azahra, Thbse Universit6 Clermont-Ferrand (1993). [10] J.A. Maxwell, J.L. Campbell and W.J. Teesdale, Nucl. Instr. and Meth. B 43 (1989) 218. [11] G. Remond, C. Gilles, D. Isabelle, C.G. Choi, M. Azahra, O. Rouer and F. Cesbron, 6th Int. Syrup, on Radiation Physics, Rabat, Morocco, 1994, Appl. Radiat. and Isotop., 46 (6/7).

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