- Email: [email protected]
Analytical possibilities of SRXRF station at VEPP-3 SR source
Nuclear Instruments and Methods in Physics Research A 40.5 (1998) 532-536
NUCLEAR INSTRUMENTS 6 METHODS IN PHVSICS
Analytical possibilities of SRXRF station at VEPP-3 SR source V.A. Trounovaa’*,
K.Z. Zolotarevb,V.B.
Baryshevb,
M.A. Phedorinb
‘institute ofInorganic Chemistry, 63~ Novosibirsk, Russian Federation b&zdker fmsh’tuteof Nuclear Physics, 630090 Novosibirsk, Russian Federation
Abstract The purpose of this work is a general overview of metrological characteristic for SRXRF station on VEPP-3 storage ring (Budker Institute of Nuclear Physics, Novosibirsk, Russia). The materials for analysis, sample preparation and analytical techniques are described as well as some examples of researcher that has been carried out on this station.
1. Introduction The present XRF station using the synchrotron radiation beam from the VEPP-3 (2 GeV, 2 T, 100-200 mA) storage ring (Fig. 1) consists of a single-crystal vacuum monochromator (pyrolytic graphite or Si( 11 l)), a chamber for samples and an Si(Li) solid-state detector. A computer and a set of CAMAC modules provide the operation of all working parts of the station [ 1,2I]. The station was constructed for the non-destructive analysis of materials in the 3-47 keV range of excitation
/
-
Beam position control
I
I
1
Side view
lm Fig. 1. General ring.
layout of the SRXRF station on VEPP-3
storage
* Corresponding author. E-mail: [email protected] 0168~9002/98/$19.00 0 PII 50168-9002(97)00175-7
1998 Elsevier Science
B.V. All rights reserved
energy. This enables one to identify (in K-series) elements from K and Ca up to Ba, La, Ce and Nd. The sample area examined varies in the range 0.1-7 mm*, the exposure time being lo”-10’s.
2. Sampling Physical parameters of SR beam makes it possible to carry out the analysis by a “thin” layer technique. For these purposes the powdered dry material has to be tabletted using a special pressure-mould made of the chemically stable metals. The usual sizes of the tablets for SRXFA are from 5 to 12 mm in diameter and their masses are 20-50 mg. The pressure for tabletting is near 150 kg/ cm’. Several materials were prepared for the analysis. This is: (a) geological rock. rivers and ocean bottom sediments, environmental objects - soils, insects, grain and grain products, biological samples, etc. may be tabletted without filling agents; (b) the ores and technological concentrates of noble and any other metals prepared with a certain amount of polystyrene; (c) the Moon samples are 0.2-2mg fragments fixed at the surface of the thin mylar film in the special clean conditions; (d) the implanted semiconductor and high temperature su~rconductor materials are Si-based films with thickness of less than 1000 A, they may be analyzed directly. The atmospheric aerosols and water suspensions samples are Whatman-41, Nuclepore or AFAXA filters with the microparticles deposited at or near the surface of filter and may be analyzed directly the same. All the samples are fixed in the center of fluoroplastic rings between two mylar films of 2.5 ym thickness. For
V.A. Trounova
et nl. I Nucl. instr. and Meth. in @vs. Res. A JO5 (1998) 53,‘-.5_36
operations on samples preparation a chemically was used in order to avoid any contamination.
clean room
3. Techniques The SRXFA method was applied for the following kinds of analysis of elemental composition. ( 1) Quantitative multielemental analysis using an external-standard technique. The samples of international standard (NBS and other) were used as an external standard [2-S]. The mass weight of the measured samples may be from 50 mg to 0.1 mg and less. The lower limit of detection (LLD) was 0.1 ppm for element concentrations and less than I nglcm’ for surface analysis of filters 16-81. (3) Analysis of one to four elements with the use of wide-band cylindrical dispersive filter made of pyrolytic graphite for secondary monochromatization of X-ray fluorescence from the sample [9]. For example, the transmission band of such filter for detection of Ru, Rh, Pd and Ag is 20-2.3 keV and LLD value may be up to 1 ppb [IO]. (3) Analysis of metal-implanted films on Si substrata by a total-reflection technique. The LLD is 2 X 10” at/ cm’ [ 1 I I. Special reference samples were used for analysis of semiconductor films, aerosols and environmental probes, as in some cases we have no international standard samples of identical matrices. There were developed and prepared special reference samples with using admixtures of solutions of a number of elements with equal concentrations for each element. These admixtures were evaporated at the acid-cleared filters-films of Synpor type. The thickness of these Synpor filters was 2-5 mm and the diameters were 5 and IOmm liZI.
4. Rest&s and discussion The photon flux of the monochromatized SR beam generated at the station of elementary analysis amounts
Table I The ratios between the peak-Compton
areas for two different
533
4 X 10” photon/mm2 at the excitation energy 2OeV. This is two orders of magnitude higher than the photon flux of a conventional X-ray tube at the same excitation energy. The photon flux of X-ray tubes with a rotating anode (100 kW) is comparable with the SR beam. In comparison with X-ray tubes, the basic sources of the best spectral characteristics of SR are (al the possibility to choose an optimal excitation energy: (b) the natural linear polarization of the SR beam, this reduces the background below the spectral peak of the elements to be identified by one order of magnitude. As the VEPP-3 is not a dedicated source of synchrotron radiation, the SR beam generated by it can drift, and its intensity and polarization can vary in time, thereby leading to unstable conditions for measurements and deteriorating the resulting data of analysis. To get reliable analytical results, the inst~mentation includes a beam position monitor and an ionization chamber. In other words, additional monitoring and repeated measurements of the samples were conducted. In order to avoid the errors in the normalization of the peak areas of elements to the peak of Compton scattering, the ratios of these peaks for the same standard BCR-1 taken in different moments of time were compared. Good agreement of these data is seen in Table 1 (BCR- 1-I and BCR- I-2 are different spectra of the same standard). One of the metrological characteristics of the technique is its reproducibility. To illustrate, we shall examine geological standards SA-1, St-l and BIL-I (Table 2). Twenty independent measurements were made for each in order to identify five rare chemical elements Rb, Sr, Y, Zr and Nb. The different geological matrices and concentrations of these elements on the standards leads to a different value SC (relative standard deviation) for them within 4.3% to 29%. Another basic metrological characteristic of the technique is its Low Limit Detection (LLD). The LLD was defined according to the formula: C ,., ,, = 3.29Cfll(N, where
- N,)
C is the relative
spectra of the same BCR-I
content
of the element
in the
standard sample
Element
BCR-I-f
BCR-1-2
Ratio BCR-t-IIBCR-1-2
Fe Zn Rb Sr Y Zr Nb
9.049 x lo-* 3.27 X W2 2.45 x W2 2.04 x lo-’ 2.705 X lo-’ 1.748 X IO-’ 1.26 X lo-*
9.059 x lo-’ 3.31 x IO -% 2.55 x lo-’ 2.02 x 10-l 2.65 x lo-” 1.74 x lo-’ 1.32 x lo-?
0.9989 0.9858 0.9608 1xl099 1.0207 1.0046 0.9545
VI. ELEMENT
ANALYSIS
VA. Trounova et al. I Nucl. Instr. and Meth. in Phys. Res. A 405 (1998) 532-536
534
Table 2 Results of reproducibility of SRXFA Rb
Sr”
Y
Zr
Nb
SA- I Referenceb (ppm) [ZS] Average” (ppm) Stand. dev” Relative standard deviation (S,, %)
120+20 120~1.34 6 5
170240 _
3215 322 I 4.4 14
180t40 22325 13 6
1314 1420.5 2.3 16
ST-IA Reference’ (ppm) [25] Average’ (ppm) Stand. devd Relative standard deviation (S,, %)
1622 15.920.8 3.4 21
270?30 _
3427 34t0.7 3 9
130-c 10 13Okl.34 6 5
8tl 820.5 2.3 29
BIL-1 Reference” (ppm) [26] Average’ (ppm) Stand. dev” Relative standard deviation (S,, 8)
9325 95% 1.7 7.6 8
266-c-30 -
3024 30.62 1 4.7 15
156213 15321.9 8.5 5
1222 1320.5 2.5 19
a Element used as internal standard. ’ Reference: reference value of element concentration. ’ Average: average value for 20 independent measurements with standard middle arithmetic deviation d Stand. dev.: standard deviation of one measurement for 95% probability.
sample, iV,, is the full area of the peak and Ns is the background area in pulses. It has been iIiustrated by the analysis of high temperature semiconductor films (Table 3) and of the filters of atmospheric aerosols (Fig. 1) as an example. As is seen from Table 3, the LLD variation for Cu, Y and Ba takes place simultaneously with the change in the excitation energy. Fig. 2 shows that while identifying the elements from Ca to MO simultaneously, the lowest LLD is for MO 0.6 ng/mm’. This corresponds to the presence of impurities in the atmosphere at a level of 3 ppt or 3 log/g-“. It is also seen that this excitation energy is optimal for MO. And the last metrological characteristic of the technique is its accuracy. It was checked by interconve~ed dete~inations of the concentrations of chemical elements in
5 B I 0” *-
a20
,,,, I ,,,I,,, 111//1,,!,1111,11 I,,,,, 25 30 35
40
Atomic number Fig. 2. Detection limits for K-series from Ca up to MO.
Table 3 Metrological characteristics of analysis of Y-Ba-Cu films on silicon substrate (for 600 s measurement time) Element
Concentration atom/cm2
Excit. energy (kev)
Area of pea.kR (pulses)
Backgr. area (pulses)
Detection limit (atom/cm*)
CU
2.4 X 10” 2.4 x 10” 1.3 x lOI 1.3 x lOi 1.1 x IO” 1.1 x IO”
12 20 20 38 38 43
58191 10250 12276 4072 15561 46118
1782 303 472 568 419 908
5.8 x lOI 1.4 x lOI 7.8 X 1Ol3 2.9 x lOI 4.9 x lOi 2.4 x lOi4
cu Y Y Ba Ba
III8 ,,,I,
VA. Trounova
et al.
I Nucl.
Instr.
and Meth.
several international standards and the method of neutronactivation analysis [ 13,14,20]. Among many of the works carried out at the station of elemental analysis of special interest is the following ones: (I) The examination of a Moon soil specimen by Tarasov et al. [l3- 181. A bulk fragment Moon rock (2 mg) and less cannot be examined directly by none of the other available non-dest~ctive methods of analysis. It most concerns such elements as Rb, Sr. Y, Zr and Nb. It may be noted that this unique material remained for further analysis. (2) The SRXRF technique was applied in the technological cycle with simultaneous evaporation of thin HTSC films [2]. The resulting data on the concentrations of the basic components (Cu, Y. Ba) in relative units have allowed us to get information on the stoichiometry of the composition of the films of 600 A thick and less and to optimize the fabrication technology. (3) Analysis of the first 100 m core-drill sample from Lake Baikal f20] has made it possible to reveal the inhomogeneity in the distribution of chemical elements at 61-65 m level. This is a very important geochemical characteristic for future interpretations. (4) Study of the elemental composition of suspended sediments in Lake Baikal and its tributaries was made by SRXRF [S]. These samples are distinguished by the fact that they are fractions of particles, less than I mk in size, placed on Nuclepore filters and are detectable directly. (5) In analysis of atmospheric aerosols the lowest LLD
F&=24 keV
Fig. 3. Spectra of U,O,
in Phys. Res. A 43
(1998)
and KU&l
5.75
for Rb, Sr, Y, Zr and Nb was achieved and is equal to 0.6 ng/mm’. In this case. only some part of the filter-sample is examined. The whole filter is 37 mm in diameter and as matter of fact. only a 5 mm diameter section was under examination. whereas the rest can be used for other purposes. (6) The presence of uranium with a concentration of 5Oppm and higher in many geological samples introduces a substantial additional X-ray radiation within the 13.5-18 keV range where the elements from Rb to MO are identified. It is the SRXRF method that offers the possibility to vary the exciting monochromatic radiation. Owing to this. it is possible to exclude the intluence of the L-series of uranium (toriurn) and Pb. Although the energies of the fluorescence lines of the above elements are rather close, their absorption edges are far enough from each other, thereby allowing us to selectively excite the elements to be identified with high reliability. This is welt seen from Fig. 3.
5. Conclusion SRXFA is characteristic of the LLD individual for different samples, elements and measurements. We may mention the lowest limit detection 1 ppb. They can be obtained by using a cylindrical dispersion of filter for analysis of the chemical concentrations of noble metals [IO]. With the total reflection technique used. the LLD is 2 X IO” atm./cm’ 1191. For analysis of aerosols, the LLD
&,=20.3 keV “308,“-lU,3B
(0.173%)
5.~2--U5136
samples with excitation
Ea=16.6 keV
radiation
energies
24. 20.4 and 16.9 keV
VI. ELEMENT
ANALYSIS
536
EA. Trounova et al. I Nucf. Instr. and Meth. in Ph.vs. Res. A 405 (1998) 532-536
is 0.6 ng/mm’, i.e. about 3 ppt. However, in the case of a simultaneous identification of the elements from Ca to Nd, the LLD will be much different for light and heavy elements and the LLD value will always depend on the matrix of the sample under study. In matrices with a high content of Fe, Cu and Zn the LLD will be much higher than in organic or Si-based objects. Since the samples are measured in air atmosphere, the “light” elements from Ca to Fe have worse LLD values than if they were subject to exa~nation in vacuum. ?&hen stating the problem on the identification of concrete elements, one can find a solution using the above SRXFA techniques with given reproducibility and LLD.
Keferences [l] V.B. Baryshev et al., Nucl. lnstr. and Metb. A 282 (1989) 570. [2] VP. Khvostova et al., Nucl. lnstr. and Meth. A 282 (1989) 516. [3] V.B. Baryshev et al., Nucl. In&. and Meth. A 359 (1995) 305. 141 LB. Knor et al., Nucl. lnstr. and Meth. A 359 (1995) 324. [5] MS. Melgunov et al., Nucl. Instr. and Meth. A 359 (1995) 331. [6] V. Baryshev and R. Van Grieken, Abstract book of the IV Russian-French Seminar on the application of neutrons and SR for Condensed Matter Investigation June 25-July 3 (1996) 55.
[7) V.B. Baryshev et al., Nucl. lnstr. and Meth. A 359 (1995) 297. P31 L.Z. Granina et al., Nucl. lnstr. and Meth. A 359 (1995) 302. [91 A.A. Antonov et al., Nucl. lnstr. and Meth. A 308 (1991) 442. et al., Nucl. Instr. and Meth. A 282 1101 Yu.P. Kolmagorov (1989) 692. [Ill VA. Trunova et al.. Nucl. lnstr. and Meth. A 308 (1991) 321. u21 VP. Khvostova et al., Nucl. lnstr. and Meth. A 261 (1987) 295. [I31 L.S. Tarasov et al., Nucl. lnstr. and Meth. A 282 (1989) 669. iI41 L.S. Tarasov et al., Nucl. lnstr. and Meth. A 359 (1995) 3 17. [I51 L.S. Tarasov et al., Nucl. Ins&. and Meth. A 261 (1987) 263. et al., Nucl. Instr. and Meth. A 282 1161 A.F. Kudryashova (1989) 673. [I71 L.S. Tarasov et al., Nucl. Instr. and Meth. A 282 (1989) 677. [I81 L.S. Tarasov et al., Nucl. lnstr. and Meth. A 359 (1995) 3 12. Cl91 VA. Trounova et al., X-ray Spectrom. 23 (1994) 187. 1201 V.A. Trounova et al.. X-ray Spectrom. 25 ( 1996) 55. 1211 N.V Arnautov, Reference samples of chemical composition of natural mineral compounds, in: Geology and Geophysics. Novosibirsk, in Russian (1990) 49. Geostand. Newsletter. 18, Special Issue, I221 K. Govindazaju, July (1994) 53.
NUCLEAR INSTRUMENTS 6 METHODS IN PHVSICS
Analytical possibilities of SRXRF station at VEPP-3 SR source V.A. Trounovaa’*,
K.Z. Zolotarevb,V.B.
Baryshevb,
M.A. Phedorinb
‘institute ofInorganic Chemistry, 63~ Novosibirsk, Russian Federation b&zdker fmsh’tuteof Nuclear Physics, 630090 Novosibirsk, Russian Federation
Abstract The purpose of this work is a general overview of metrological characteristic for SRXRF station on VEPP-3 storage ring (Budker Institute of Nuclear Physics, Novosibirsk, Russia). The materials for analysis, sample preparation and analytical techniques are described as well as some examples of researcher that has been carried out on this station.
1. Introduction The present XRF station using the synchrotron radiation beam from the VEPP-3 (2 GeV, 2 T, 100-200 mA) storage ring (Fig. 1) consists of a single-crystal vacuum monochromator (pyrolytic graphite or Si( 11 l)), a chamber for samples and an Si(Li) solid-state detector. A computer and a set of CAMAC modules provide the operation of all working parts of the station [ 1,2I]. The station was constructed for the non-destructive analysis of materials in the 3-47 keV range of excitation
/
-
Beam position control
I
I
1
Side view
lm Fig. 1. General ring.
layout of the SRXRF station on VEPP-3
storage
* Corresponding author. E-mail: [email protected] 0168~9002/98/$19.00 0 PII 50168-9002(97)00175-7
1998 Elsevier Science
B.V. All rights reserved
energy. This enables one to identify (in K-series) elements from K and Ca up to Ba, La, Ce and Nd. The sample area examined varies in the range 0.1-7 mm*, the exposure time being lo”-10’s.
2. Sampling Physical parameters of SR beam makes it possible to carry out the analysis by a “thin” layer technique. For these purposes the powdered dry material has to be tabletted using a special pressure-mould made of the chemically stable metals. The usual sizes of the tablets for SRXFA are from 5 to 12 mm in diameter and their masses are 20-50 mg. The pressure for tabletting is near 150 kg/ cm’. Several materials were prepared for the analysis. This is: (a) geological rock. rivers and ocean bottom sediments, environmental objects - soils, insects, grain and grain products, biological samples, etc. may be tabletted without filling agents; (b) the ores and technological concentrates of noble and any other metals prepared with a certain amount of polystyrene; (c) the Moon samples are 0.2-2mg fragments fixed at the surface of the thin mylar film in the special clean conditions; (d) the implanted semiconductor and high temperature su~rconductor materials are Si-based films with thickness of less than 1000 A, they may be analyzed directly. The atmospheric aerosols and water suspensions samples are Whatman-41, Nuclepore or AFAXA filters with the microparticles deposited at or near the surface of filter and may be analyzed directly the same. All the samples are fixed in the center of fluoroplastic rings between two mylar films of 2.5 ym thickness. For
V.A. Trounova
et nl. I Nucl. instr. and Meth. in @vs. Res. A JO5 (1998) 53,‘-.5_36
operations on samples preparation a chemically was used in order to avoid any contamination.
clean room
3. Techniques The SRXFA method was applied for the following kinds of analysis of elemental composition. ( 1) Quantitative multielemental analysis using an external-standard technique. The samples of international standard (NBS and other) were used as an external standard [2-S]. The mass weight of the measured samples may be from 50 mg to 0.1 mg and less. The lower limit of detection (LLD) was 0.1 ppm for element concentrations and less than I nglcm’ for surface analysis of filters 16-81. (3) Analysis of one to four elements with the use of wide-band cylindrical dispersive filter made of pyrolytic graphite for secondary monochromatization of X-ray fluorescence from the sample [9]. For example, the transmission band of such filter for detection of Ru, Rh, Pd and Ag is 20-2.3 keV and LLD value may be up to 1 ppb [IO]. (3) Analysis of metal-implanted films on Si substrata by a total-reflection technique. The LLD is 2 X 10” at/ cm’ [ 1 I I. Special reference samples were used for analysis of semiconductor films, aerosols and environmental probes, as in some cases we have no international standard samples of identical matrices. There were developed and prepared special reference samples with using admixtures of solutions of a number of elements with equal concentrations for each element. These admixtures were evaporated at the acid-cleared filters-films of Synpor type. The thickness of these Synpor filters was 2-5 mm and the diameters were 5 and IOmm liZI.
4. Rest&s and discussion The photon flux of the monochromatized SR beam generated at the station of elementary analysis amounts
Table I The ratios between the peak-Compton
areas for two different
533
4 X 10” photon/mm2 at the excitation energy 2OeV. This is two orders of magnitude higher than the photon flux of a conventional X-ray tube at the same excitation energy. The photon flux of X-ray tubes with a rotating anode (100 kW) is comparable with the SR beam. In comparison with X-ray tubes, the basic sources of the best spectral characteristics of SR are (al the possibility to choose an optimal excitation energy: (b) the natural linear polarization of the SR beam, this reduces the background below the spectral peak of the elements to be identified by one order of magnitude. As the VEPP-3 is not a dedicated source of synchrotron radiation, the SR beam generated by it can drift, and its intensity and polarization can vary in time, thereby leading to unstable conditions for measurements and deteriorating the resulting data of analysis. To get reliable analytical results, the inst~mentation includes a beam position monitor and an ionization chamber. In other words, additional monitoring and repeated measurements of the samples were conducted. In order to avoid the errors in the normalization of the peak areas of elements to the peak of Compton scattering, the ratios of these peaks for the same standard BCR-1 taken in different moments of time were compared. Good agreement of these data is seen in Table 1 (BCR- 1-I and BCR- I-2 are different spectra of the same standard). One of the metrological characteristics of the technique is its reproducibility. To illustrate, we shall examine geological standards SA-1, St-l and BIL-I (Table 2). Twenty independent measurements were made for each in order to identify five rare chemical elements Rb, Sr, Y, Zr and Nb. The different geological matrices and concentrations of these elements on the standards leads to a different value SC (relative standard deviation) for them within 4.3% to 29%. Another basic metrological characteristic of the technique is its Low Limit Detection (LLD). The LLD was defined according to the formula: C ,., ,, = 3.29Cfll(N, where
- N,)
C is the relative
spectra of the same BCR-I
content
of the element
in the
standard sample
Element
BCR-I-f
BCR-1-2
Ratio BCR-t-IIBCR-1-2
Fe Zn Rb Sr Y Zr Nb
9.049 x lo-* 3.27 X W2 2.45 x W2 2.04 x lo-’ 2.705 X lo-’ 1.748 X IO-’ 1.26 X lo-*
9.059 x lo-’ 3.31 x IO -% 2.55 x lo-’ 2.02 x 10-l 2.65 x lo-” 1.74 x lo-’ 1.32 x lo-?
0.9989 0.9858 0.9608 1xl099 1.0207 1.0046 0.9545
VI. ELEMENT
ANALYSIS
VA. Trounova et al. I Nucl. Instr. and Meth. in Phys. Res. A 405 (1998) 532-536
534
Table 2 Results of reproducibility of SRXFA Rb
Sr”
Y
Zr
Nb
SA- I Referenceb (ppm) [ZS] Average” (ppm) Stand. dev” Relative standard deviation (S,, %)
120+20 120~1.34 6 5
170240 _
3215 322 I 4.4 14
180t40 22325 13 6
1314 1420.5 2.3 16
ST-IA Reference’ (ppm) [25] Average’ (ppm) Stand. devd Relative standard deviation (S,, %)
1622 15.920.8 3.4 21
270?30 _
3427 34t0.7 3 9
130-c 10 13Okl.34 6 5
8tl 820.5 2.3 29
BIL-1 Reference” (ppm) [26] Average’ (ppm) Stand. dev” Relative standard deviation (S,, 8)
9325 95% 1.7 7.6 8
266-c-30 -
3024 30.62 1 4.7 15
156213 15321.9 8.5 5
1222 1320.5 2.5 19
a Element used as internal standard. ’ Reference: reference value of element concentration. ’ Average: average value for 20 independent measurements with standard middle arithmetic deviation d Stand. dev.: standard deviation of one measurement for 95% probability.
sample, iV,, is the full area of the peak and Ns is the background area in pulses. It has been iIiustrated by the analysis of high temperature semiconductor films (Table 3) and of the filters of atmospheric aerosols (Fig. 1) as an example. As is seen from Table 3, the LLD variation for Cu, Y and Ba takes place simultaneously with the change in the excitation energy. Fig. 2 shows that while identifying the elements from Ca to MO simultaneously, the lowest LLD is for MO 0.6 ng/mm’. This corresponds to the presence of impurities in the atmosphere at a level of 3 ppt or 3 log/g-“. It is also seen that this excitation energy is optimal for MO. And the last metrological characteristic of the technique is its accuracy. It was checked by interconve~ed dete~inations of the concentrations of chemical elements in
5 B I 0” *-
a20
,,,, I ,,,I,,, 111//1,,!,1111,11 I,,,,, 25 30 35
40
Atomic number Fig. 2. Detection limits for K-series from Ca up to MO.
Table 3 Metrological characteristics of analysis of Y-Ba-Cu films on silicon substrate (for 600 s measurement time) Element
Concentration atom/cm2
Excit. energy (kev)
Area of pea.kR (pulses)
Backgr. area (pulses)
Detection limit (atom/cm*)
CU
2.4 X 10” 2.4 x 10” 1.3 x lOI 1.3 x lOi 1.1 x IO” 1.1 x IO”
12 20 20 38 38 43
58191 10250 12276 4072 15561 46118
1782 303 472 568 419 908
5.8 x lOI 1.4 x lOI 7.8 X 1Ol3 2.9 x lOI 4.9 x lOi 2.4 x lOi4
cu Y Y Ba Ba
III8 ,,,I,
VA. Trounova
et al.
I Nucl.
Instr.
and Meth.
several international standards and the method of neutronactivation analysis [ 13,14,20]. Among many of the works carried out at the station of elemental analysis of special interest is the following ones: (I) The examination of a Moon soil specimen by Tarasov et al. [l3- 181. A bulk fragment Moon rock (2 mg) and less cannot be examined directly by none of the other available non-dest~ctive methods of analysis. It most concerns such elements as Rb, Sr. Y, Zr and Nb. It may be noted that this unique material remained for further analysis. (2) The SRXRF technique was applied in the technological cycle with simultaneous evaporation of thin HTSC films [2]. The resulting data on the concentrations of the basic components (Cu, Y. Ba) in relative units have allowed us to get information on the stoichiometry of the composition of the films of 600 A thick and less and to optimize the fabrication technology. (3) Analysis of the first 100 m core-drill sample from Lake Baikal f20] has made it possible to reveal the inhomogeneity in the distribution of chemical elements at 61-65 m level. This is a very important geochemical characteristic for future interpretations. (4) Study of the elemental composition of suspended sediments in Lake Baikal and its tributaries was made by SRXRF [S]. These samples are distinguished by the fact that they are fractions of particles, less than I mk in size, placed on Nuclepore filters and are detectable directly. (5) In analysis of atmospheric aerosols the lowest LLD
F&=24 keV
Fig. 3. Spectra of U,O,
in Phys. Res. A 43
(1998)
and KU&l
5.75
for Rb, Sr, Y, Zr and Nb was achieved and is equal to 0.6 ng/mm’. In this case. only some part of the filter-sample is examined. The whole filter is 37 mm in diameter and as matter of fact. only a 5 mm diameter section was under examination. whereas the rest can be used for other purposes. (6) The presence of uranium with a concentration of 5Oppm and higher in many geological samples introduces a substantial additional X-ray radiation within the 13.5-18 keV range where the elements from Rb to MO are identified. It is the SRXRF method that offers the possibility to vary the exciting monochromatic radiation. Owing to this. it is possible to exclude the intluence of the L-series of uranium (toriurn) and Pb. Although the energies of the fluorescence lines of the above elements are rather close, their absorption edges are far enough from each other, thereby allowing us to selectively excite the elements to be identified with high reliability. This is welt seen from Fig. 3.
5. Conclusion SRXFA is characteristic of the LLD individual for different samples, elements and measurements. We may mention the lowest limit detection 1 ppb. They can be obtained by using a cylindrical dispersion of filter for analysis of the chemical concentrations of noble metals [IO]. With the total reflection technique used. the LLD is 2 X IO” atm./cm’ 1191. For analysis of aerosols, the LLD
&,=20.3 keV “308,“-lU,3B
(0.173%)
5.~2--U5136
samples with excitation
Ea=16.6 keV
radiation
energies
24. 20.4 and 16.9 keV
VI. ELEMENT
ANALYSIS
536
EA. Trounova et al. I Nucf. Instr. and Meth. in Ph.vs. Res. A 405 (1998) 532-536
is 0.6 ng/mm’, i.e. about 3 ppt. However, in the case of a simultaneous identification of the elements from Ca to Nd, the LLD will be much different for light and heavy elements and the LLD value will always depend on the matrix of the sample under study. In matrices with a high content of Fe, Cu and Zn the LLD will be much higher than in organic or Si-based objects. Since the samples are measured in air atmosphere, the “light” elements from Ca to Fe have worse LLD values than if they were subject to exa~nation in vacuum. ?&hen stating the problem on the identification of concrete elements, one can find a solution using the above SRXFA techniques with given reproducibility and LLD.
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