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Investigation of latent tracks in polyethyleneterephthalate and their etching
Surface Science 507–510 (2002) 911–915 www.elsevier.com/locate/susc
Investigation of latent tracks in polyethyleneterephthalate and their etching A.I. Vilensky a, D.L. Zagorski a,*, S.A. Bystrov b, S.S. Michailova b, R.V. Gainutdinov a, A.N. Nechaev a a
Shubnikov Institute of Crystallography RAS, Leninski pr., 59, 117333 Moscow, Russia b Physical-Technical Institute UrB RAS, 132 Kirov str, 426001 Izhevsk, Russia
Abstract The areas of radiation damage––latent tracks (LT) at the surface of polymer film (PET), irradiated with swift heavy ions of different types (Xe and Bi) were investigated by atomic-force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The surface of these samples was also analyzed after different types of treatment: swelling in the water and alkali chemical etching. The alkali etching process was examined at different stages and it was found, that carboxyl groups concentration at the sample surface increased, while alcohol groups concentration remains nearly constant. At the same time, the products with carboxyl groups in etching solution does not change until through pores formation. The combination of AFM, XPS methods with chemical and chromatography techniques gave information about complicated structure of LT and mechanism of etching. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion bombardment; Radiation damage; Surface defects; Etching; X-ray photoelectron spectroscopy; Atomic force microscopy
1. Introduction The polymers are known to be one of the best track detectors. On the other hand, thin polymer films, irradiated with swift heavy ions, are the source material for obtaining of so-called ‘‘nuclear filters’’ (or ‘‘track-membranes’’ (TM)). Multi-steps process––irradiation with ions (for formation of latent tracks (LT)), sensitization and chemical etching––is usually used for TM fabrication. The LT structure depends on the type of polymer, mass
*
Corresponding author. Tel.: +7-095-135-99-71; fax: +7-095135-10-11. E-mail address: [email protected] (D.L. Zagorski).
and energy of irradiated ions and determines all further processes of TM formation. The shape of defects at the surface of irradiated samples reflects the inner structure of LT, but until now they almost were not investigated in initial, virgin state. One of the ways of visualization of LT area at the surface is swelling of irradiated sample. The swelling of PET in water was investigated in [1]: but in this work structure of track was not taken into account. The appearance of atomic-force microscopy (AFM) permitted the surface of polymer materials to be studied with high resolution. AFM has been already successfully used to obtain information on the size of tracks in mica [2] and on the pore shape and size of polycarbonate [3,4]. Hillocks composed
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A.I. Vilensky et al. / Surface Science 507–510 (2002) 911–915
of polymer gel were found on irradiated PET films using AFM, chromatography and chemical etching technique [5,6]. Using X-ray photoelectron spectroscopy (XPS) with specific labels gave possibility to appreciate concentration of some functional groups at the surface. The aim of this work is to investigate tracks and their modification in poly(ethyleneterephthalate) (PET), which is the base material for production of track membranes.
2. Experiment Biaxially oriented PET films with thickness 10 mcm were used in this work. The samples were irradiated by Xe and Bi ions with energy 1–3 MeV/ nucleon and fluence 108 –109 ions/cm2 (cyclotron U-400, JINR, Dubna). AFM ‘‘Solver P47’’ (NT-MDT, Russia) working in resonance, ‘‘tapping’’ regime (silicon cantilevers (NT-MDT), frequencies 150–350 kHz and a tip curvature radius 10 nm) in air was used. For control the images in the contact and side-force modes were obtained using Si3 N4 cantilevers (Park Scientific Instruments, a resonance frequency of 120 kHz, and a tip curvature radius 10 nm). To obtain precise metric data in the xy-image plane of the surface being studied, the instrument was calibrated using atomic pattern of the highly oriented pyrographite (HOPG) surface (the accuracy was 1–3%); for those along the z-axis (vertical) the instrument was calibrated using test samples (the accuracy was 10–20%). The size of surface defects (D) was determined from the surface profiles. Two types of development were carried out: treatment in water and alkali etching. For the first treatment the irradiated films were immersed into water (20 °C) during 3 h. The alkali etching was carried out in potassium solution (KOH, 0.25 mol/l) at the temperature 75 °C. The chemical method of layer-by-layer etching was used for investigation of the chemical composition of these areas: products of polymer destruction removed from tracks into water and were periodically analyzed by chemical and chromatography methods; etching kinetic was studied at the
room-temperature in order to slow down the etching process and to investigate its different stages. The method of high-effective reverse-phase liquid chromatography (HERPLC) was applied for these products analyzing using ‘‘Milichrom 4’’ and ‘‘Milichrom A-02’’ chromatographs with columns filled by modified adsorbent ‘‘Diabond’’ and ‘‘Eurospher C 18-5’’. Water solutions of acetonitrile were used for eluation (gradient from 0% to 100% acetonitrile). X-ray photoelectron spectra were recorded with ES-2401 electron spectrometer using nonmonochromatic MgKa radiation at 200 W. The base pressure during the analysis was about 10 8 Torr. A fixed take-off angle of 45° was used. The spectrometer was calibrated using Au 4f7=2 line at 84 eV. The spectra were recorded at the pass energy 50 eV and step size of 0.1 eV. The samples were mounted onto the substrate with a double-sided adhesive tape. The spectra were charge-corrected by setting C 1s peak from the hydrocarbon contamination at the value of 285 eV. The chemical derivatization technique (CDT) [7] was used to identify functional groups from the XPS data obtained. Trifluoracetic anhydride (TFAA) was used to label alcohol groups. Trifluoroethanol was used to label carboxyl groups.
3. Results and discussion The surface tracks were detected by AFM for the Xe and Bi ions irradiated samples under investigation and these tracks surface density corresponds to the irradiation fluence. Fig. 1 demonstrates Xe-irradiated sample surface: the cavities with the diameter 7 nm were found; for Bi-irradiated samples hillocks with diameter 60–80 nm with cavity on its top (diameter 12–15 nm) were found––see Fig. 2. These holes (cavities) correspond to central area of LT––‘‘the track core’’, the area of maximal polymer destruction and carbonization. This core size agrees with the sizes obtained by other methods and calculated using adsorption energy [8]. The damaged areas were much bigger for Bi-irradiated samples, which enabled us to investigate surface defects in details.
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area: the adsorption of water during this process leads to local increasing of polymer volume, pressing out of the destruction products, which leads to disappearing of cavities at the surface. This effect has the same character for both types of irradiation; Fig. 3 illustrates this effect for Biirradiated film (where the effect was more significant). Two processes took place simultaneously during chemical treatment in alkali solution: swelling and etching. These processes are accompanied by formation of gel in area of LT. So, these areas of local swelling were detected (by AFM) as the hillocks––see Fig. 4. By further development etching prevailed and the gel was removed from the track areas (from bulk and from the surface) into the etching solution. As a result, the hillocks Fig. 1. AFM image of the surface of Xe––irradiated sample (Xe ion energy––1 MeV/nucl). Arrows indicate the cavities––ion tracks; cavity size––about 7 nm (scan size 80 80 nm2 ).
Fig. 2. 3D AFM image of single track area at the surface of Biirradiated sample (Bi ion energy––3.5 MeV/nucl) (scan size 100 100 nm2 ). Lower picture––surface profile through the hillock with cavity (crater).
Development in the water results in swelling of the Xe- and Bi-irradiated polymer in the track
Fig. 3. 3D AFM image of single track area at the surface of Biirradiated sample (Bi ion energy––3.5 MeV/nucl) after treatment in water (swelling) (scan size 150 150 nm2 ). Lower picture––surface profile through the hillock.
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Fig. 4. 3D AFM image of Xe-irradiated sample after 2 h alkali etching (with hillocks, diameter 50 nm) (scan size 800 800 nm2 ).
Fig. 6. The accumulation of PET etching products in KOH solution (S––relative peaks area): (1) accumulation of potassium terephtalate from irradiated sample, (2) accumulation of radiolysis products from irradiated PET; (3) accumulation of potassium terephtalate from control (non-irradiated) sample (chromatograph millicrome was used).
Fig. 5. 3D AFM image of Xe-irradiated sample after 5 h alkali etching (with pore entrees, diameter 7–10 nm) (scan size 800 800 nm2 ).
disappeared and the through pores (with the diameters 7 nm) were formed at their places––see Fig. 5. Then radial etching of formed pores took place. Chromatography technique was than used for etching process (of Xe-irradiated samples) and etching products in solution analyzing. Fig. 6 demonstrates these products accumulation at 20 °C. At the beginning of etching curves 1 and 3 coincide, and the etching velocity deviation is visible only after 2 h. These data show that during the first 2 h the etching velocities of irradiated and control films are equal. The potassium terephtalate concentration in the solution increases only at the
TM pore structure formation (right side of curve 1 in Fig. 6). The accumulation of radiolysis products from the destruction track area differs: after several minutes of exposure the radiolysis products appear in solution, but only after 3 h, with the TM pore structure formation, concentration of those products in the solution increased significantly. XPS was used for further investigation of the surface of Xe-irradiated PET at the different stages of etching. Fig. 7 demonstrates the results of XPS determination of alcohol (OH) and carboxyl (COOH) groups on the surface of the sample during etching process using CDT. Curve 1 demonstrates the behavior of relative carbon content in carboxyl groups at the surface layer while curve 2 demonstrates the behavior of relative carbon content in alcohol groups. It is easy to see that carboxyl groups content increased at the beginning of etching (first 2–3 h) and then decreased down to
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4. Conclusions
Fig. 7. XPS data: the dependence of relative carbon content (in atomic percents) in carboxyl (curve 1) and alcohol (curve 2) groups at the sample surface.
value close to the initial irradiated polymer. At the same time, the alcohol groups content does not change significantly. The comparison of Fig. 6 with Fig. 7 demonstrates that the etching process in track began from the first minutes of contact with alkali. This process is accompanied by concentration of carboxyl groups in tracks increasing, while the carboxyl groups concentration in etching solution remains constant. That indicates that the etching products, contains carboxyl groups, are closed in the tracks and cannot diffuse into solution. This is connected with the swelling of the all areas of tracks and formation of gel here. This gel ‘‘locks’’ the track channels and its formation can be detected by surface hillock appearance. These hillocks diameters were measured: for Xe-irradiated samples it was 50 nm, and for Bi-irradiated samples––150 nm. (This process was investigated by authors in details for Xe-irradiated PET samples––see [6].) The subsequent through-pores formation process is accompanied with gel removing due to increasing of pressure in the track. Gel, which includes products with carboxyl groups, pass into the solution. As a result of this process, the surface concentration of these groups decreased while these group concentration in etching solution increased (see Fig. 6, curve 1).
AFM and XPS methods were used for LT at the surface of PET film investigation just after irradiation and after different types and stages of chemical treatment. LT areas in polymer are found to swell both in water and in alkali solution. However, during treatment in alkali solution two process were detected––swelling and etching; in this case swelling is accompanied by surface hillocks formation and carboxyl groups in track area concentration. The gel in LT areas at the surface was detected: the process of gel formation is initiated by three-dimension macromolecular net creation during ion irradiation. This gel removed from the LT during further etching and this process leads to through pores formation. The surface hillocks and cavities appearance is connected with the processes of swelling and molecular rearrangement in the bulk of polymer in LT areas. Acknowledgements This work was supported in part by ISTC fund (project 918). References [1] T.E. Laricheva, A.A. Machula, V.K. Milinchuk, D.L. Zagorski, Colloid J. 62 (2000) 575. [2] T. Hagen, S. Grafstrom, J. Ackermann, R. Neumann, C. Trautmann, J. Vetter, N. Angert, J. Vac. Sci. Technol. B 12 (3) (1994) 1555. [3] P. Dietz, P.K. Hansma, O. Inacker, H.D. Lehmann, K.H. Lehmann, J. Membr. Sci. 65 (1–4) (1992) 101. [4] H. Kamusewitz, M. Keller, D. Paul, Thin Solid Films 264 (1–4) (1995) 184. [5] A.I. Vilensky, A.L. Tolstikhina, Russ. Chem. Bull. 48 (1999) 1100, 1104 (English translation). [6] A.I. Vilensky, O.G. Larionov, R.V. Gainutdinov, A.L. Tolstikhina, V.Y. Kabanov, D.L. Zagorski, E.V. Khataibe, A.N. Netchaev, B.V. Mchedlishvili, Radiat. Measurements 34 (1–6) (2001) 75. [7] V.I. Povstugar, S.S. Mikhailova, A.A. Shakov, Zhyrnal Analiticheskoi Khimii 55 (5) (2000) 455 (in Russian). [8] W. Enge, Radiat. Measurements 25 (1–4) (1995) 11.
Investigation of latent tracks in polyethyleneterephthalate and their etching A.I. Vilensky a, D.L. Zagorski a,*, S.A. Bystrov b, S.S. Michailova b, R.V. Gainutdinov a, A.N. Nechaev a a
Shubnikov Institute of Crystallography RAS, Leninski pr., 59, 117333 Moscow, Russia b Physical-Technical Institute UrB RAS, 132 Kirov str, 426001 Izhevsk, Russia
Abstract The areas of radiation damage––latent tracks (LT) at the surface of polymer film (PET), irradiated with swift heavy ions of different types (Xe and Bi) were investigated by atomic-force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The surface of these samples was also analyzed after different types of treatment: swelling in the water and alkali chemical etching. The alkali etching process was examined at different stages and it was found, that carboxyl groups concentration at the sample surface increased, while alcohol groups concentration remains nearly constant. At the same time, the products with carboxyl groups in etching solution does not change until through pores formation. The combination of AFM, XPS methods with chemical and chromatography techniques gave information about complicated structure of LT and mechanism of etching. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Ion bombardment; Radiation damage; Surface defects; Etching; X-ray photoelectron spectroscopy; Atomic force microscopy
1. Introduction The polymers are known to be one of the best track detectors. On the other hand, thin polymer films, irradiated with swift heavy ions, are the source material for obtaining of so-called ‘‘nuclear filters’’ (or ‘‘track-membranes’’ (TM)). Multi-steps process––irradiation with ions (for formation of latent tracks (LT)), sensitization and chemical etching––is usually used for TM fabrication. The LT structure depends on the type of polymer, mass
*
Corresponding author. Tel.: +7-095-135-99-71; fax: +7-095135-10-11. E-mail address: [email protected] (D.L. Zagorski).
and energy of irradiated ions and determines all further processes of TM formation. The shape of defects at the surface of irradiated samples reflects the inner structure of LT, but until now they almost were not investigated in initial, virgin state. One of the ways of visualization of LT area at the surface is swelling of irradiated sample. The swelling of PET in water was investigated in [1]: but in this work structure of track was not taken into account. The appearance of atomic-force microscopy (AFM) permitted the surface of polymer materials to be studied with high resolution. AFM has been already successfully used to obtain information on the size of tracks in mica [2] and on the pore shape and size of polycarbonate [3,4]. Hillocks composed
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 3 7 1 - 7
912
A.I. Vilensky et al. / Surface Science 507–510 (2002) 911–915
of polymer gel were found on irradiated PET films using AFM, chromatography and chemical etching technique [5,6]. Using X-ray photoelectron spectroscopy (XPS) with specific labels gave possibility to appreciate concentration of some functional groups at the surface. The aim of this work is to investigate tracks and their modification in poly(ethyleneterephthalate) (PET), which is the base material for production of track membranes.
2. Experiment Biaxially oriented PET films with thickness 10 mcm were used in this work. The samples were irradiated by Xe and Bi ions with energy 1–3 MeV/ nucleon and fluence 108 –109 ions/cm2 (cyclotron U-400, JINR, Dubna). AFM ‘‘Solver P47’’ (NT-MDT, Russia) working in resonance, ‘‘tapping’’ regime (silicon cantilevers (NT-MDT), frequencies 150–350 kHz and a tip curvature radius 10 nm) in air was used. For control the images in the contact and side-force modes were obtained using Si3 N4 cantilevers (Park Scientific Instruments, a resonance frequency of 120 kHz, and a tip curvature radius 10 nm). To obtain precise metric data in the xy-image plane of the surface being studied, the instrument was calibrated using atomic pattern of the highly oriented pyrographite (HOPG) surface (the accuracy was 1–3%); for those along the z-axis (vertical) the instrument was calibrated using test samples (the accuracy was 10–20%). The size of surface defects (D) was determined from the surface profiles. Two types of development were carried out: treatment in water and alkali etching. For the first treatment the irradiated films were immersed into water (20 °C) during 3 h. The alkali etching was carried out in potassium solution (KOH, 0.25 mol/l) at the temperature 75 °C. The chemical method of layer-by-layer etching was used for investigation of the chemical composition of these areas: products of polymer destruction removed from tracks into water and were periodically analyzed by chemical and chromatography methods; etching kinetic was studied at the
room-temperature in order to slow down the etching process and to investigate its different stages. The method of high-effective reverse-phase liquid chromatography (HERPLC) was applied for these products analyzing using ‘‘Milichrom 4’’ and ‘‘Milichrom A-02’’ chromatographs with columns filled by modified adsorbent ‘‘Diabond’’ and ‘‘Eurospher C 18-5’’. Water solutions of acetonitrile were used for eluation (gradient from 0% to 100% acetonitrile). X-ray photoelectron spectra were recorded with ES-2401 electron spectrometer using nonmonochromatic MgKa radiation at 200 W. The base pressure during the analysis was about 10 8 Torr. A fixed take-off angle of 45° was used. The spectrometer was calibrated using Au 4f7=2 line at 84 eV. The spectra were recorded at the pass energy 50 eV and step size of 0.1 eV. The samples were mounted onto the substrate with a double-sided adhesive tape. The spectra were charge-corrected by setting C 1s peak from the hydrocarbon contamination at the value of 285 eV. The chemical derivatization technique (CDT) [7] was used to identify functional groups from the XPS data obtained. Trifluoracetic anhydride (TFAA) was used to label alcohol groups. Trifluoroethanol was used to label carboxyl groups.
3. Results and discussion The surface tracks were detected by AFM for the Xe and Bi ions irradiated samples under investigation and these tracks surface density corresponds to the irradiation fluence. Fig. 1 demonstrates Xe-irradiated sample surface: the cavities with the diameter 7 nm were found; for Bi-irradiated samples hillocks with diameter 60–80 nm with cavity on its top (diameter 12–15 nm) were found––see Fig. 2. These holes (cavities) correspond to central area of LT––‘‘the track core’’, the area of maximal polymer destruction and carbonization. This core size agrees with the sizes obtained by other methods and calculated using adsorption energy [8]. The damaged areas were much bigger for Bi-irradiated samples, which enabled us to investigate surface defects in details.
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area: the adsorption of water during this process leads to local increasing of polymer volume, pressing out of the destruction products, which leads to disappearing of cavities at the surface. This effect has the same character for both types of irradiation; Fig. 3 illustrates this effect for Biirradiated film (where the effect was more significant). Two processes took place simultaneously during chemical treatment in alkali solution: swelling and etching. These processes are accompanied by formation of gel in area of LT. So, these areas of local swelling were detected (by AFM) as the hillocks––see Fig. 4. By further development etching prevailed and the gel was removed from the track areas (from bulk and from the surface) into the etching solution. As a result, the hillocks Fig. 1. AFM image of the surface of Xe––irradiated sample (Xe ion energy––1 MeV/nucl). Arrows indicate the cavities––ion tracks; cavity size––about 7 nm (scan size 80 80 nm2 ).
Fig. 2. 3D AFM image of single track area at the surface of Biirradiated sample (Bi ion energy––3.5 MeV/nucl) (scan size 100 100 nm2 ). Lower picture––surface profile through the hillock with cavity (crater).
Development in the water results in swelling of the Xe- and Bi-irradiated polymer in the track
Fig. 3. 3D AFM image of single track area at the surface of Biirradiated sample (Bi ion energy––3.5 MeV/nucl) after treatment in water (swelling) (scan size 150 150 nm2 ). Lower picture––surface profile through the hillock.
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Fig. 4. 3D AFM image of Xe-irradiated sample after 2 h alkali etching (with hillocks, diameter 50 nm) (scan size 800 800 nm2 ).
Fig. 6. The accumulation of PET etching products in KOH solution (S––relative peaks area): (1) accumulation of potassium terephtalate from irradiated sample, (2) accumulation of radiolysis products from irradiated PET; (3) accumulation of potassium terephtalate from control (non-irradiated) sample (chromatograph millicrome was used).
Fig. 5. 3D AFM image of Xe-irradiated sample after 5 h alkali etching (with pore entrees, diameter 7–10 nm) (scan size 800 800 nm2 ).
disappeared and the through pores (with the diameters 7 nm) were formed at their places––see Fig. 5. Then radial etching of formed pores took place. Chromatography technique was than used for etching process (of Xe-irradiated samples) and etching products in solution analyzing. Fig. 6 demonstrates these products accumulation at 20 °C. At the beginning of etching curves 1 and 3 coincide, and the etching velocity deviation is visible only after 2 h. These data show that during the first 2 h the etching velocities of irradiated and control films are equal. The potassium terephtalate concentration in the solution increases only at the
TM pore structure formation (right side of curve 1 in Fig. 6). The accumulation of radiolysis products from the destruction track area differs: after several minutes of exposure the radiolysis products appear in solution, but only after 3 h, with the TM pore structure formation, concentration of those products in the solution increased significantly. XPS was used for further investigation of the surface of Xe-irradiated PET at the different stages of etching. Fig. 7 demonstrates the results of XPS determination of alcohol (OH) and carboxyl (COOH) groups on the surface of the sample during etching process using CDT. Curve 1 demonstrates the behavior of relative carbon content in carboxyl groups at the surface layer while curve 2 demonstrates the behavior of relative carbon content in alcohol groups. It is easy to see that carboxyl groups content increased at the beginning of etching (first 2–3 h) and then decreased down to
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4. Conclusions
Fig. 7. XPS data: the dependence of relative carbon content (in atomic percents) in carboxyl (curve 1) and alcohol (curve 2) groups at the sample surface.
value close to the initial irradiated polymer. At the same time, the alcohol groups content does not change significantly. The comparison of Fig. 6 with Fig. 7 demonstrates that the etching process in track began from the first minutes of contact with alkali. This process is accompanied by concentration of carboxyl groups in tracks increasing, while the carboxyl groups concentration in etching solution remains constant. That indicates that the etching products, contains carboxyl groups, are closed in the tracks and cannot diffuse into solution. This is connected with the swelling of the all areas of tracks and formation of gel here. This gel ‘‘locks’’ the track channels and its formation can be detected by surface hillock appearance. These hillocks diameters were measured: for Xe-irradiated samples it was 50 nm, and for Bi-irradiated samples––150 nm. (This process was investigated by authors in details for Xe-irradiated PET samples––see [6].) The subsequent through-pores formation process is accompanied with gel removing due to increasing of pressure in the track. Gel, which includes products with carboxyl groups, pass into the solution. As a result of this process, the surface concentration of these groups decreased while these group concentration in etching solution increased (see Fig. 6, curve 1).
AFM and XPS methods were used for LT at the surface of PET film investigation just after irradiation and after different types and stages of chemical treatment. LT areas in polymer are found to swell both in water and in alkali solution. However, during treatment in alkali solution two process were detected––swelling and etching; in this case swelling is accompanied by surface hillocks formation and carboxyl groups in track area concentration. The gel in LT areas at the surface was detected: the process of gel formation is initiated by three-dimension macromolecular net creation during ion irradiation. This gel removed from the LT during further etching and this process leads to through pores formation. The surface hillocks and cavities appearance is connected with the processes of swelling and molecular rearrangement in the bulk of polymer in LT areas. Acknowledgements This work was supported in part by ISTC fund (project 918). References [1] T.E. Laricheva, A.A. Machula, V.K. Milinchuk, D.L. Zagorski, Colloid J. 62 (2000) 575. [2] T. Hagen, S. Grafstrom, J. Ackermann, R. Neumann, C. Trautmann, J. Vetter, N. Angert, J. Vac. Sci. Technol. B 12 (3) (1994) 1555. [3] P. Dietz, P.K. Hansma, O. Inacker, H.D. Lehmann, K.H. Lehmann, J. Membr. Sci. 65 (1–4) (1992) 101. [4] H. Kamusewitz, M. Keller, D. Paul, Thin Solid Films 264 (1–4) (1995) 184. [5] A.I. Vilensky, A.L. Tolstikhina, Russ. Chem. Bull. 48 (1999) 1100, 1104 (English translation). [6] A.I. Vilensky, O.G. Larionov, R.V. Gainutdinov, A.L. Tolstikhina, V.Y. Kabanov, D.L. Zagorski, E.V. Khataibe, A.N. Netchaev, B.V. Mchedlishvili, Radiat. Measurements 34 (1–6) (2001) 75. [7] V.I. Povstugar, S.S. Mikhailova, A.A. Shakov, Zhyrnal Analiticheskoi Khimii 55 (5) (2000) 455 (in Russian). [8] W. Enge, Radiat. Measurements 25 (1–4) (1995) 11.