- Email: [email protected]
High resolution PIXE set-up
Nuclear Instruments and Methods in Physics Research B 109/110 (1996) 144-147
l ll[ B Beam Int®ractlon$ with Materials & Atoms
ELSEVIER
High resolution PIXE set-up Milo~ G. Budnar J. Stefan hlstitute, Jamova 39, P.O. Box 100, SI-61111 Ljubljana, Slovenia
Abstract The resolution of the standard PIXE arrangement can be drastically improved by the use of the wavelength dispersive spectrometer (WDS). The arrangement with the plane crystal and the position sensitive detector was installed. Although the efficiency of the spectrometer is rather low due to the small solid angle (A/2/4rr = 4 x 10-7), its advantages become very pronounced inthe situation of overlapping of the adjacent characteristic lines. The spectrometer is used for the X-ray energy range from 3 to 10 keV, the soft X-rays being detected after evacuation of the crystal vacuum chamber. The resolution of the spectrometer is 15 eV at 5.9 keV. Up to now the set-up has been mainly used for the study of inner-shell atomic physics phenomena. The successful runs of the spectrometer have been performed to prove the applicability of the method for the elemental analysis.
1. Introduction Although at the PIXE analysis the energy dispersive spectrometry (EDS) is mostly used, several advantages can be achieved when the wavelength dispersive spectrometer (WDS) is employed [1,2]. Better energy resolution enables much easier separation of the closely spaced peaks and therefore enhances the signal-to-background ratio. There are also some drawbacks originating mainly from WDS's low geometrical and quantum detection efficiency, and narrow spectral range which can be detected simultaneously. Therefore the measurements are usually longer or the irradiation dose should be higher. Very often the WDS arrangements, especially the commercial ones, are more expensive than the EDS set-ups and need more space for the installation. Despite of all disadvantages mentioned, the WDS can be superior whenever high overlapping of the measured peaks is encountered with the EDS [3]. In such a case also the improved sensitivity of the trace element analysis can be expected. Facing the EDS, where only elements above Na are successfully detected, the WDS is used for the analysis of elements from Be upwards if the crystals with 2d around 10 nm are used and the spectrometer is placed in vacuum. The use of the WDS is mandatory when the studies of the secondary radiative effects is of interest [4] or when the chemical states are studied [5,6].
was used successfully also for the analysis of radiative auger effect on solid targets [8]. The spectrometer is designed as of a rhomboid type with 0.433 m hands placed outside of the vacuum chamber which encloses a plane crystal. As the position sensitive detector (PSD) the flow proportional counter with cathode strips and delay line is used. On the way to the sensitive volume of the proportional counter the X-rays from the target are attenuated in three 28 txm kapton windows, 15 mm layer of air and a 6 Ixm aluminized Mylar window. The solid angle of the detector for the particular reflected line is 4 X 10 7 sr and is estimated from the lateral resolution of the PSD which is 0.4 mm. The high-resolution spectrometer described can be employed for the measurements of X-rays with energies above 3 keV and the resolution of 15 eV at 5.9 keV can be achieved. The 7.5 cm × 3 cm plane crystals and the 12 cm x 1 cm PSD enable measurements in wide enough energy regions for successful PIXE analysis of neighbouring elements.
3. Analytical advantages of the WDS To prove the measured intensity as a peak which corresponds to the presence of the trace element in the sample the following statistical rule is normally used: Nx _> 3"~f~b.
2. Experimental set-up The arrangement used was developed for the atomic physic research [7], especially for the study of hypersatellites produced at bombardment with ions. The system
(1)
Here, N x is the net intensity above the background and ~ b b is the statistical error in the background. In the case when the trace element peak seats on the background and no lines from the elements with high concentrations are in the vicinity the estimation for the
0168-583X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved S S D I 0168-583X(95)00896-9
145
M.G. Budnar / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 144-147
minimum detection limit concentration, CMDL, can be obtained relatively simple [9] as given in Eq. (2): M'o/UL
C~¢IDL
Table 2 The influence of the high intensity line tail on the trace element CMOL as a function of the distance, (xk - xa), between the lines
3
~
f atx k - x~ (FWHM)
~/pxtrixNp (A~2141r)eT
V
(2)
°x
Ca Ks Zn Ket Ag K~x
0.50
1.50
2.50
46 194 49
6.3 26.6 6.7
0.13 0.55 0.14
-j
Here, M~o and M o are the molecular mass of the trace element and the average molecular mass of the light matrix elements, respectively, and N e is the Avogadro number. The target thickness px, the X-ray production cross-section o-x, the irradiating dose Np, the detector solid angle Ag2/ 4av, the detector efficiency e, and the transmission T are the measuring parameters. The last factor in Eq. (2) signifies the ratio between the background X-ray production do-b/dE b, summed over the range of the detector resolution (FWHM), and the characteristic line X-ray production o-x. In practice the CMDL for the particular measurement is estimated directly from the background intensity using Eq.
studied can be easily estimated. If CMDL0 means the minimum detection limit based on the background itself, the influence of the high concentration element tail can be approximated by Eq. (4): CMDL ~ CMDLo( f + 1),
(4)
where f means the ratio between the tail and the background contributions:
(1). More complex is the situation in the vicinity of the line with high intensity. Here, the background influencing the trace element CuD c comes mainly from the intense line tail. Taking the shape of the line as Gaussian the estimation of its influence on the CuD L can be clone likewise. Using similar statistical arguments as before Eq. (1) has to be corrected as shown in Eq. (3): (3)
N x --> 3~/N t + N b .
is the contribution from the high intensity line tail and depends on the energy difference between the high concentration element and the trace element line, but still more relies on the detector resolution. In Table 1 the integrated contributions from the normalised Gaussian function over one F W H M range, I = f G(x, x i, FWHM)dx, are given as a function of the distance between the adjacent lines. The results show that at peak distances bigger than 3 F W H M the influence of the high intensity line become insignificant and does not contribute to the trace element line background. For a target modelled as a low Z matrix containing only one measurable high intensity element, the influence of this element on the CMDL of the neighbor trace element Here, N t
Here, the concentration of the high intensity element is C k and Mok its molecular mass. The influence on the neighbor trace element CMDL of the high intensity element with 10% concentration in the low Z ( Z ~ 8) matrix is presented in Table 2. In the estimation the F W H M of the detector was 150 eV/5.9 keV and the proton energy 1.5 MeV. The background cross-section was obtained from the experiment [10] and o-x from the tabulations [11]. The above results clearly show that good detector resolution is mandatory for improving the CMDL if the trace element characteristic line is close to the contribution from the high concentration element. In such a case the advantage of the WDS-PIXE is fully demonstrated and despite its low efficiency it can compete with the EDS.
4. Experimental results on the metallurgical sample The advantages of WDS-PIXE system have been tested on the metallurgical samples where the situation of overlapping of the high concentration and the trace element lines is often found. The samples taken for the analysis
Table 1 The integrated contributions from the normalized Gaussian function, 1 = f G(x, x~, FWHM)dx, in the ranges from (xk -x~) - F W H M / 2 to (xk - x~) + FWHM/2 x k - x~ (FWHM) I=fG(x,x~,FWHM)dx
0.25 0.505
0.5 0.491
1.0 0.119
1.5 0.93 × 10 2
2.0 0.70×
10 -4
2.5 0.50X 10 5
3.0 0.10× 10 7
II. EXPERIMENTAL
146
M.G. Budnar / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 144-147
Table 3 Comparison of the elemental weight contributions for the high quality steel OCR 12 VM as obtained by the EDS and the WDS PIXE Z
Elements
6 13 14 15 16 22 23 24 25 26 28 29 41 42 50
C A1 Si P S Ti V Cr Mn Fe Ni Cu Nb Mo Sn
Concentration [%]
EDS-PIXE Concentration [%]
WDS-PIXE Concentration [%]
0.21 _+0.03 11.7_+0.50 1.88_+0.06 85.77_+0.30 0.15 _+0.03 0.17_+0.10
0.047_+0.007 16.5_+0.2 0.20_+0.02
0.18 0.021 0.46 O.O26 0.015 O.O09 11.7 0.42 86.44 0.33 0.168 0.017 0.20 0.017
0.40_+0.03
0.12_+0.12
were high quality steels where the amount of trace elements is very important. For the calibration of the WDS and the EDS systems, running in parallel on the same sample, thin calibration standards were used. At the analysis of the metallurgical samples the elemental intensities were obtained from the measured spectra by the AXIL code and the quantitative results on the elemental concentrations were gained by taking thick target matrix effects into account [12]. The results for the studied steel samples are given in Tables 3 and 4. The comparison with the known elemental concentrations in these samples clearly shows how high intensity lines spoil the determination of trace element concentrations in the EDS analysis. The concentrations of the trace elements determined by the WDS are in much better agreement with the known
composition, what strongly supports the parallel use of both techniques for improving the accuracy of the analysis.
5. Conclusions The strong overlapping of the characteristic lines, especially the cases of faint trace element contributions close to the ones from the high concentration elements, can be successfully solved by improving the spectrometer resolution. Here, despite its low efficiency, the WDS can be competitive with the EDS. The WDS advantages are still more expressed when detecting low Z elements. Therefore in special cases where high overlapping of the lines is
Table 4 As in Table 3 but for the steel VTOP MO1 Z
Elements
6 13 14 15 16 22 23 24 25 26 28 29 41 42 50
C A1 Si P S Ti V Cr Mn F Ni Cu Nb Mo Sn
Concentration [%]
EDS-PIXE Concentration [%]
WDS-PIXE Concentration [%]
0.36_+0.04 5.05_+0.10 1.20_+0.07 92.06_+0.40 0.04_+0.04 0.22_+0.04
0.42_+0.02 7.90_+0.12 0.40_+0.02
0.39 0.025 1.01 0.11 0.013 0.36 5.26 0.37 91.01 0.17 0.20 1.18 -
1.06_+0.31
0.19_+0.04
M.G. Budnar / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 144-147
e n c o u n t e r e d the parallel u s e o f both t e c h n i q u e s is recommended.
References [1] M. Terasawa, Int. J. PIXE 1 (3) (1991) 251. [2] M. Terasawa, I. T~ir6k and V.E Petukhov, Nucl. Instr. and Meth. B 75 (1993) 105. [3] D.H. Morse, G.S. Bench, S.RH.T. Freeman and A.E. Pontau, Nucl. Instr. and Meth. B 99 (1995) 427. [4] M. Budnar and A. Miihleisen, Nucl. Instr. and Meth. B 75 (1993) 81. [5] F. Folkman, Nucl. Instr. and Meth. B 75 (1993) 9.
147
[6] K. Yoshihara and J. Iihara, Int. J. PIXE 2 (2) (1992) 93. [7] V. Cindro, M. Budnar, M. Kregar, V. Ramgak and Z, Smit, J. Phys. B 22 (1989) 2161. [8] M. Budnar, A. Miihleisen, M. Hribar, H. Jan~ekovi~, M. Ravnikar, Z. Smit and M. Zitnik, Nucl. Instr. and Meth. B 63 (1992) 377. [9] K. Ishii, H. Orihara, Y. Iwata and K. Bessho, Int. J. PIXE 4 (1) (1994) 1. [10] K. Ishii and S. Morita, Int. J. PIXE 1 (1) (1990) 1. [11] H. Paul and J. Sacher, Atom. Data Nucl. Data Tables 42 (1989) 105. [12] Z. Smit, M. Budnar, V. Cindro, V. Ramgak and M. Ravnikar, Nucl. Instr. and Meth. B 4 (1984) 114.
II. EXPERIMENTAL
l ll[ B Beam Int®ractlon$ with Materials & Atoms
ELSEVIER
High resolution PIXE set-up Milo~ G. Budnar J. Stefan hlstitute, Jamova 39, P.O. Box 100, SI-61111 Ljubljana, Slovenia
Abstract The resolution of the standard PIXE arrangement can be drastically improved by the use of the wavelength dispersive spectrometer (WDS). The arrangement with the plane crystal and the position sensitive detector was installed. Although the efficiency of the spectrometer is rather low due to the small solid angle (A/2/4rr = 4 x 10-7), its advantages become very pronounced inthe situation of overlapping of the adjacent characteristic lines. The spectrometer is used for the X-ray energy range from 3 to 10 keV, the soft X-rays being detected after evacuation of the crystal vacuum chamber. The resolution of the spectrometer is 15 eV at 5.9 keV. Up to now the set-up has been mainly used for the study of inner-shell atomic physics phenomena. The successful runs of the spectrometer have been performed to prove the applicability of the method for the elemental analysis.
1. Introduction Although at the PIXE analysis the energy dispersive spectrometry (EDS) is mostly used, several advantages can be achieved when the wavelength dispersive spectrometer (WDS) is employed [1,2]. Better energy resolution enables much easier separation of the closely spaced peaks and therefore enhances the signal-to-background ratio. There are also some drawbacks originating mainly from WDS's low geometrical and quantum detection efficiency, and narrow spectral range which can be detected simultaneously. Therefore the measurements are usually longer or the irradiation dose should be higher. Very often the WDS arrangements, especially the commercial ones, are more expensive than the EDS set-ups and need more space for the installation. Despite of all disadvantages mentioned, the WDS can be superior whenever high overlapping of the measured peaks is encountered with the EDS [3]. In such a case also the improved sensitivity of the trace element analysis can be expected. Facing the EDS, where only elements above Na are successfully detected, the WDS is used for the analysis of elements from Be upwards if the crystals with 2d around 10 nm are used and the spectrometer is placed in vacuum. The use of the WDS is mandatory when the studies of the secondary radiative effects is of interest [4] or when the chemical states are studied [5,6].
was used successfully also for the analysis of radiative auger effect on solid targets [8]. The spectrometer is designed as of a rhomboid type with 0.433 m hands placed outside of the vacuum chamber which encloses a plane crystal. As the position sensitive detector (PSD) the flow proportional counter with cathode strips and delay line is used. On the way to the sensitive volume of the proportional counter the X-rays from the target are attenuated in three 28 txm kapton windows, 15 mm layer of air and a 6 Ixm aluminized Mylar window. The solid angle of the detector for the particular reflected line is 4 X 10 7 sr and is estimated from the lateral resolution of the PSD which is 0.4 mm. The high-resolution spectrometer described can be employed for the measurements of X-rays with energies above 3 keV and the resolution of 15 eV at 5.9 keV can be achieved. The 7.5 cm × 3 cm plane crystals and the 12 cm x 1 cm PSD enable measurements in wide enough energy regions for successful PIXE analysis of neighbouring elements.
3. Analytical advantages of the WDS To prove the measured intensity as a peak which corresponds to the presence of the trace element in the sample the following statistical rule is normally used: Nx _> 3"~f~b.
2. Experimental set-up The arrangement used was developed for the atomic physic research [7], especially for the study of hypersatellites produced at bombardment with ions. The system
(1)
Here, N x is the net intensity above the background and ~ b b is the statistical error in the background. In the case when the trace element peak seats on the background and no lines from the elements with high concentrations are in the vicinity the estimation for the
0168-583X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved S S D I 0168-583X(95)00896-9
145
M.G. Budnar / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 144-147
minimum detection limit concentration, CMDL, can be obtained relatively simple [9] as given in Eq. (2): M'o/UL
C~¢IDL
Table 2 The influence of the high intensity line tail on the trace element CMOL as a function of the distance, (xk - xa), between the lines
3
~
f atx k - x~ (FWHM)
~/pxtrixNp (A~2141r)eT
V
(2)
°x
Ca Ks Zn Ket Ag K~x
0.50
1.50
2.50
46 194 49
6.3 26.6 6.7
0.13 0.55 0.14
-j
Here, M~o and M o are the molecular mass of the trace element and the average molecular mass of the light matrix elements, respectively, and N e is the Avogadro number. The target thickness px, the X-ray production cross-section o-x, the irradiating dose Np, the detector solid angle Ag2/ 4av, the detector efficiency e, and the transmission T are the measuring parameters. The last factor in Eq. (2) signifies the ratio between the background X-ray production do-b/dE b, summed over the range of the detector resolution (FWHM), and the characteristic line X-ray production o-x. In practice the CMDL for the particular measurement is estimated directly from the background intensity using Eq.
studied can be easily estimated. If CMDL0 means the minimum detection limit based on the background itself, the influence of the high concentration element tail can be approximated by Eq. (4): CMDL ~ CMDLo( f + 1),
(4)
where f means the ratio between the tail and the background contributions:
(1). More complex is the situation in the vicinity of the line with high intensity. Here, the background influencing the trace element CuD c comes mainly from the intense line tail. Taking the shape of the line as Gaussian the estimation of its influence on the CuD L can be clone likewise. Using similar statistical arguments as before Eq. (1) has to be corrected as shown in Eq. (3): (3)
N x --> 3~/N t + N b .
is the contribution from the high intensity line tail and depends on the energy difference between the high concentration element and the trace element line, but still more relies on the detector resolution. In Table 1 the integrated contributions from the normalised Gaussian function over one F W H M range, I = f G(x, x i, FWHM)dx, are given as a function of the distance between the adjacent lines. The results show that at peak distances bigger than 3 F W H M the influence of the high intensity line become insignificant and does not contribute to the trace element line background. For a target modelled as a low Z matrix containing only one measurable high intensity element, the influence of this element on the CMDL of the neighbor trace element Here, N t
Here, the concentration of the high intensity element is C k and Mok its molecular mass. The influence on the neighbor trace element CMDL of the high intensity element with 10% concentration in the low Z ( Z ~ 8) matrix is presented in Table 2. In the estimation the F W H M of the detector was 150 eV/5.9 keV and the proton energy 1.5 MeV. The background cross-section was obtained from the experiment [10] and o-x from the tabulations [11]. The above results clearly show that good detector resolution is mandatory for improving the CMDL if the trace element characteristic line is close to the contribution from the high concentration element. In such a case the advantage of the WDS-PIXE is fully demonstrated and despite its low efficiency it can compete with the EDS.
4. Experimental results on the metallurgical sample The advantages of WDS-PIXE system have been tested on the metallurgical samples where the situation of overlapping of the high concentration and the trace element lines is often found. The samples taken for the analysis
Table 1 The integrated contributions from the normalized Gaussian function, 1 = f G(x, x~, FWHM)dx, in the ranges from (xk -x~) - F W H M / 2 to (xk - x~) + FWHM/2 x k - x~ (FWHM) I=fG(x,x~,FWHM)dx
0.25 0.505
0.5 0.491
1.0 0.119
1.5 0.93 × 10 2
2.0 0.70×
10 -4
2.5 0.50X 10 5
3.0 0.10× 10 7
II. EXPERIMENTAL
146
M.G. Budnar / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 144-147
Table 3 Comparison of the elemental weight contributions for the high quality steel OCR 12 VM as obtained by the EDS and the WDS PIXE Z
Elements
6 13 14 15 16 22 23 24 25 26 28 29 41 42 50
C A1 Si P S Ti V Cr Mn Fe Ni Cu Nb Mo Sn
Concentration [%]
EDS-PIXE Concentration [%]
WDS-PIXE Concentration [%]
0.21 _+0.03 11.7_+0.50 1.88_+0.06 85.77_+0.30 0.15 _+0.03 0.17_+0.10
0.047_+0.007 16.5_+0.2 0.20_+0.02
0.18 0.021 0.46 O.O26 0.015 O.O09 11.7 0.42 86.44 0.33 0.168 0.017 0.20 0.017
0.40_+0.03
0.12_+0.12
were high quality steels where the amount of trace elements is very important. For the calibration of the WDS and the EDS systems, running in parallel on the same sample, thin calibration standards were used. At the analysis of the metallurgical samples the elemental intensities were obtained from the measured spectra by the AXIL code and the quantitative results on the elemental concentrations were gained by taking thick target matrix effects into account [12]. The results for the studied steel samples are given in Tables 3 and 4. The comparison with the known elemental concentrations in these samples clearly shows how high intensity lines spoil the determination of trace element concentrations in the EDS analysis. The concentrations of the trace elements determined by the WDS are in much better agreement with the known
composition, what strongly supports the parallel use of both techniques for improving the accuracy of the analysis.
5. Conclusions The strong overlapping of the characteristic lines, especially the cases of faint trace element contributions close to the ones from the high concentration elements, can be successfully solved by improving the spectrometer resolution. Here, despite its low efficiency, the WDS can be competitive with the EDS. The WDS advantages are still more expressed when detecting low Z elements. Therefore in special cases where high overlapping of the lines is
Table 4 As in Table 3 but for the steel VTOP MO1 Z
Elements
6 13 14 15 16 22 23 24 25 26 28 29 41 42 50
C A1 Si P S Ti V Cr Mn F Ni Cu Nb Mo Sn
Concentration [%]
EDS-PIXE Concentration [%]
WDS-PIXE Concentration [%]
0.36_+0.04 5.05_+0.10 1.20_+0.07 92.06_+0.40 0.04_+0.04 0.22_+0.04
0.42_+0.02 7.90_+0.12 0.40_+0.02
0.39 0.025 1.01 0.11 0.013 0.36 5.26 0.37 91.01 0.17 0.20 1.18 -
1.06_+0.31
0.19_+0.04
M.G. Budnar / Nucl. Instr. and Meth. in Phys. Res. B 109/110 (1996) 144-147
e n c o u n t e r e d the parallel u s e o f both t e c h n i q u e s is recommended.
References [1] M. Terasawa, Int. J. PIXE 1 (3) (1991) 251. [2] M. Terasawa, I. T~ir6k and V.E Petukhov, Nucl. Instr. and Meth. B 75 (1993) 105. [3] D.H. Morse, G.S. Bench, S.RH.T. Freeman and A.E. Pontau, Nucl. Instr. and Meth. B 99 (1995) 427. [4] M. Budnar and A. Miihleisen, Nucl. Instr. and Meth. B 75 (1993) 81. [5] F. Folkman, Nucl. Instr. and Meth. B 75 (1993) 9.
147
[6] K. Yoshihara and J. Iihara, Int. J. PIXE 2 (2) (1992) 93. [7] V. Cindro, M. Budnar, M. Kregar, V. Ramgak and Z, Smit, J. Phys. B 22 (1989) 2161. [8] M. Budnar, A. Miihleisen, M. Hribar, H. Jan~ekovi~, M. Ravnikar, Z. Smit and M. Zitnik, Nucl. Instr. and Meth. B 63 (1992) 377. [9] K. Ishii, H. Orihara, Y. Iwata and K. Bessho, Int. J. PIXE 4 (1) (1994) 1. [10] K. Ishii and S. Morita, Int. J. PIXE 1 (1) (1990) 1. [11] H. Paul and J. Sacher, Atom. Data Nucl. Data Tables 42 (1989) 105. [12] Z. Smit, M. Budnar, V. Cindro, V. Ramgak and M. Ravnikar, Nucl. Instr. and Meth. B 4 (1984) 114.
II. EXPERIMENTAL