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Analysis of sodium aerosol using transversely excited atmospheric CO2 laser-induced gas plasma spectroscopy
Current Applied Physics 14 (2014) 451e454
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Analysis of sodium aerosol using transversely excited atmospheric CO2 laser-induced gas plasma spectroscopy Ali Khumaeni a, Kazuyoshi Kurihara b, Zener Sukra Lie c, Kiichiro Kagawa d, Yong Inn Lee e, * a
Research Group for Laser Probing, Japan Atomic Energy Agency, Nuclear Science and Engineering Directorate, Tokai-mura, Ibaraki-ken 311-1195, Japan Department of Physics, Faculty of Education and Regional Studies, University of Fukui, Fukui 910-8507, Japan c Graduate School of Nuclear Power and Energy Safety Engineering, University of Fukui, Fukui 910-8597, Japan d Research Institute of Nuclear Engineering, University of Fukui, Fukui 910-8597, Japan e Department of Physics, Research Institute of Physics and Chemistry, Chonbuk National University, Chonju 561-756, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2013 Received in revised form 13 December 2013 Accepted 18 December 2013 Available online 8 January 2014
Taking advantages of the special characteristics of a transversely excited atmospheric (TEA) CO2 laser, the analysis of sodium aerosol has been successfully conducted by using laser-induced gas plasma spectroscopy (LIGPS) method. In this study, the sodium aerosol was deposited on a nickel metal plate; the metal plate functions as a subtarget to initiate a gas plasma. When a pulsed TEA CO2 laser was focused on the metal surface, a large-volume and high-temperature gas plasma was induced. The fine particles of sodium then entered into the gas plasma region to be dissociated and excited. By using this technique, a semi quantitative analysis of sodium aerosol was made. The detection limit of sodium was approximately 200 ppb. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Analysis of sodium aerosol Laser-induced gas plasma spectroscopy (LIGPS) Pulsed TEA CO2 laser Subtarget
1. Introduction Aerosol analysis is required in many important applications. For instance, in sodium-cooled fast reactors, radioactive sodium was found to contaminate the atmosphere around the reactors. The sodium leak is produced by vaporization or combustion of sodium, which could be released into the atmosphere from the cooling system piping or components. Because sodium aerosol is chemically very reactive, it rapidly reacts with oxygen, water, and CO2 in the atmosphere [1]. Therefore, highly sensitive method is really necessary to detect sodium leak in the atmosphere. Various kinds of sodium leak detector have been developed including sodium ionization detector and the ionization chamber, differential pressure detector, and contact leak detector [2]. However, in those techniques, a sample preparation is necessary and they are labor intensive. Laser-induced plasma has become a very popular technique for qualitative and quantitative elemental analysis in aerosol samples [3]. In this method, a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a pulsed energy of
* Corresponding author. E-mail address: [email protected] (Y.I. Lee). 1567-1739/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.12.017
approximately several tens of milli-joules is focused in aerosol under atmospheric pressure of the surrounding gas. However, the sensitivity of this method is quite poor with limit of detection (LoD) of sodium of around 55 ppm [4]. On the other hand, we found that a transversely-excited atmospheric pressure (TEA) CO2 laser is suitable for spectrochemical analysis. This is because the laser has long wavelength of 10.64 mm and long pulse duration of 200 ns. Based on our previous experiment, a high-temperature and large-volume gas plasma was induced when a TEA CO2 laser was focused on a metal surface under atmospheric pressure of the surrounding gas, while the metal itself was never ablated; this phenomenon never occurs in the case of conventional LIBS technique, in which a Nd:YAG laser is most often employed as an energy source. This plasma is very favorable for spectrochemical analysis because its plasma temperature is rather high and it has a high heat capacity [5,6]. We have demonstrated that direct and rapid analysis of powder can be realized for TEA CO2 laser-induced gas plasma method by employing a new technique, in which a powder sample was placed in a container and sent into the gas plasma region by the strong shock wave induced by the gas plasma [6]. While, for an ordinary LIBS method using YAG laser, powder samples must be prepared into a pellet prior to the analysis [7], thus the analysis is time consuming and requires additional equipments.
452
A. Khumaeni et al. / Current Applied Physics 14 (2014) 451e454
In the present paper, we have developed a sophisticated method for the analysis of sodium aerosol using the TEA CO2 laser-induced gas plasma. Namely, the sodium aerosol is deposited onto the surface of a nickel metal, which functions as a sub-target. When a pulsed TEA CO2 laser is focused onto a metal sub-target, strong gas plasma is produced. The gas plasma interacts with the fine particles of the aerosol deposited onto a sub-target. The fine particles then move into the gas plasma region to be dissociated and excited, while the metal itself never has been ablated. It should be stressed that this analytical technique is very high sensitive because this method is basically condensation method; namely, we can collect the aerosol spreading in the wide region of the atmosphere onto the metal surface by moving the metal itself or by keeping the metal surface at a fixed position for a long time so that many aerosol can be attached on the metal surface. Also by increasing the accumulation number of the spectrocehmical data in terms of repeated irradiation on the wide range of the metal surface, we can suppress the noise signal in the spectrocehmical data, thus the detection limit is significantly improved. It should be stressed again that this new analytical method has never been able to apply for ordinary LIBS, where YAG laser is used, because the metal itself is inevitably ablated, resulting in the significant reduction of the sensitivity of the elements deposited on the metal surface.
metal subtarget due to the interaction between dust and TEA CO2 laser beam. The samples used in this works are two kinds of sodium aerosol. One was sodium aerosol sample taken from marine aerosol in the Echizen Seashore (Fukui, Japan), and the other was sodium aerosol taken from the atmosphere in a room at University of Fukui, Japan. The atmospheric fine particles contain sodium as reported by Ooki et al. [8]. A nickel metal plate (30 20 0.15 mm3) was exposed in atmosphere in order to collect the atmospheric fine particles containing sodium; the metal plate functions as a sub-target. The emission spectrum was obtained using an optical multichannel analyzer (OMA) system (ATAGO Macs-320) consisting of a 0.32-m-focal-length spectrograph with a grating of 1200 groves/ mm, a 1024-channel photodiode detector array, and a microchannel plate image intensifier to detect laser plasma radiation. The spectral resolution of the OMA system is 0.2 nm. The light emission of the laser plasma was collected using an optical fiber (0.3p sr), which was fed into the OMA system. One end of the fiber was placed at a distance of 4 cm from the focusing point of the laser light and set perpendicularly to the path of the laser beam. The gate delay time and gate width of the OMA system were set at 20 and 100 ms, respectively. Each spectrum was obtained using 10 shots of laser irradiation.
2. Experimental procedure
3. Results and discussion
Fig. 1 shows the experimental setup used in this study. The pulsed TEA CO2 laser employed in this work is a Shibuya SQ-2000 laser, which was commercially developed and constructed by Shibuya Company for laser marking. The TEA CO2 laser (3 J, 10.64 mm, 200 ns) was operated at a repetition rate of 10 Hz. The laser energy was varied from 250 mJ to 1500 mJ by setting the respective apertures in the laser beam path. After passing through the aperture, the laser beam was focused by a ZnSe lens (f ¼ 20 mm) through a ZnSe window onto the target sample. The laser beam spot was 2 2 mm2 for the tight focusing condition. The samples were placed in a metal chamber with dimensions of 12 12 12 cm3. The experiment was conducted under 1 atmospheric pressure. During experiment, a nitrogen gas was flowed into the chamber, with the flow rate of 4 L per minute (Lpm). It should be mentioned that the gas flowing is required in this experiment, otherwise gas breakdown takes place in front of the
Initially, we reviewed the gas plasma generation induced by the bombardment of a pulsed TEA CO2 laser on a metal surface, which is quite different from the plasma generation induced by a Nd:YAG laser [6]. The plasma induced by a TEA CO2 laser has unique characteristics, owing to its low frequency (long wavelength of 10.64 mm) and long pulse duration (200 ns). When a TEA CO2 laser is focused onto a metal surface, electrons are released from the surface owing to a multiphoton absorption, which occurs at the focusing point of the laser light. These electrons are then accelerated to a high energy in a low-frequency electric field of laser light, which induces the cascade ionization of atoms in the gas, generating an initial gas plasma. Once this initial plasma has been produced, laser light is completely absorbed in gas plasma by inverse bremsstrahlung via freeefree transitions. This absorption is much stronger for the TEA CO2 laser than for the Nd:YAG laser because the plasma absorption coefficient is proportional to the inverse square of the frequency of laser light. Furthermore, the pulse duration of the TEA CO2 laser is relatively long (200 ns), about 20 times longer than that of the Nd:YAG laser, which means that almost all the energy from the TEA CO2 laser is absorbed by gas 7000
Na I588.9 nm Na I589.6 nm
Intensity (arb.units)
6000 5000 4000 3000 2000 1000 0 570
Fig. 1. Experimental setup used in this research.
580
590 600 Wavelength (nm)
610
620
Fig. 2. Emission spectrum of sodium (Na I 588.9 nm and 589.1 nm) taken from urban air in atmospheric room at University of Fukui Japan.
A. Khumaeni et al. / Current Applied Physics 14 (2014) 451e454
plasma. The gas plasma produced is therefore very strong and has a high temperature and a high heat capacity. It should be emphasized that no ablation on the metal surface occurred with the laser bombardment using a 750 mJ energy laser and a lens with a focal length of 100 mm. As reported by Ooki et al., both marine and urban aerosol contain particulate fine sodium [8]. In this study, at initial, we examined whether fine sodium is really deposited or not when the metal plate was placed in urban and marine atmosphere. In order to collect the particulate fine sodium from urban aerosol, we placed a nickel metal plate in atmospheric room at University of Fukui for approximately 1 h. When a pulsed TEA CO2 laser (250 mJ, 200 ns) was focused on the metal surface, a strong gas plasma was induced. It is assumed that the fine particles of the aerosol was dissociated and excited in the gas plasma region. Fig. 2 (red curve (in the web version)) shows the emission spectrum taken from the deposited aerosol. Strong emission line of Na I 588.9 nm and Na I 589.6 nm can clearly be observed with low background emission. In this experiment, 10 shots of laser irradiation were made on the metal surface. The gate delay time and gate width of OMA system were 10 ms and 100 ms, respectively. The sample was rotated with 1 rotation/minute so that the position of laser bombardment on the metal surface is always new. While the blue curve in Fig. 2, which was obtained after laser cleaning treatment, shows completely no sodium line.; in the cleaning treatment, the metal surface was irradiated by defocused laser beam without inducing the plasma, only to clean the aerosol deposited onto the metal surface. The result in Fig. 2 proved that urban aerosol contains particulate fine sodium. It should be mentioned that in this study, a pulsed Nd:YAG laser (10e80 mJ) was also employed in place of the TEA CO2 laser. However, for the case of Nd:YAG laser, the clear detection of sodium lines could not be made due to the disturbance of many strong emission lines, causing from the ablation of nickel. Further study of sodium analysis was made by using particulate fine sodium taken from marine air. To this study, we collected the sodium aerosol sample from the marine air at Echizen seashore, Fukui, Japan. The sample was taken with different times of collection (from 10 to 45 min). Fig. 3 shows the emission spectrum of sodium (Na I 588.9 nm and 589.6 nm) taken from the aerosol sample collected from the Echizen seashore for approximately 20 min. High emission intensity of sodium can clearly be detected with low background emission. The condition for data acquisition of Fig. 3 is the same with that of Fig. 2. Fig. 4 shows how the emission intensity of sodium changes with time. It can clearly be seen that with increasing time of sample
Fig. 3. Emission spectrum of sodium (Na I 588.9 nm and 589.1 nm) taken from the Echizen seashore Fukui Japan for 20 min.
453
Fig. 4. Relationship between exposure time of sample collection and sodium emission intensity (Na I 588.9 nm) taken from marine aerosol at Echizen seashore Fukui Japan.
collection, the emission intensity of sodium (Na I 588.9 nm) increases. This result certified that deposition rate of sodium aerosol is increased with increasing time. It should also be noticed that the curve is not linear. This might be because once the sodium aerosol deposits on the plate, the further sodium aerosol can easily deposit. In this study we also collected the particulate fine sodium aerosol from marine air at different distance from the seawater. Fig. 5 shows the emission spectrum of sodium taken from marine air at (a) 1 m, and (b) 50 m from the seawater. It is observed that the emission spectrum of sodium taken from shorter distance (1 m from the seawater) is much higher in emission intensity. This result
Fig. 5. Emission spectra of sodium taken from atmosphere at (a) 1 m, and (b) 50 m from the Echizen seawater.
454
A. Khumaeni et al. / Current Applied Physics 14 (2014) 451e454
Intensity (arb.units)
20000
Na I588.9 nm
Na I589.6 nm
16000 12000 8000 4000 0 570
Figure 6 580
590 600 Wavelength (nm)
610
620
Fig. 6. Emission spectrum of sodium taken from the aerosol containing 10 ppm of sodium deposited onto a metal plate.
confirmed that the concentration of sodium in marine air is different with a different distance from the seawater. Generally, we can also certify that the concentration of sodium in atmosphere is different with different places. For further exploration of the capability of this technique for quantitative analysis of sodium aerosol, we prepared sodium aerosol by using a liquid solution containing 10 ppm of Na. 1 ml Na solution was homogeneously poured onto a nickel metal plate with a dimension of 30 20 0.15 mm3. The solution was then dried by using hairdryer for 20 min so that the fine sodium was deposited onto the metal plate. Fig. 6 displays the emission spectrum taken from the aerosol containing 10 ppm of sodium deposited onto a metal plate. The typical two lines of Na I 588.8 nm and Na I 589.6 nm clearly appear with an extremely low background emission intensity. The detection limit of Na was estimated to be approximately 200 part per billion (ppb); the detection limit was derived by calculating what concentrations of the signal which yields 3 times of the noise level is, because the 3 times of the noise can be clearly identified as a signal [9]. 4. Conclusions We have demonstrated in this work that analysis of sodium aerosol can successfully be performed by using the specific characteristics of a TEA CO2 laser. In this study, sodium aerosol was deposited on a nickel metal plate, which functions as a subtarget. When a pulsed TEA CO2 laser was focused on the metal plate, a high-temperature and large-volume gas plasma was induced, without ablating the metal surface; the metal functions only to initiate gas plasma. It is assumed that the sodium aerosol was evaporated and entered into the gas plasma region to be effectively
dissociated and excited in the gas plasma region. a semi quantitative experiment has been successfully demonstrated by using sodium aerosol taken from the marine air at Echizen seashore (Fukui Japan) with different time of aerosol collection. The result certified that the emission intensity of sodium increases with increasing time of sample collection, meaning that the amount of sodium deposited on the metal surface increases with increasing time. The detection limit of Na was estimated to be 200 ppb, which derived by using only 10 shots accumulation. If we increase the number of the laser shots, the detection limit will be naturally decreased because the noise level decrease by accumulation. Thus, this present technique has high possibility to be applied to online analysis of sodium in fast reactor such as Monju reactor at Tsuruga Fukui Japan. Also it should be stressed that this technique can be applied as remote in-situ monitoring of the leaked sodium which attached on the surface as aerosol on the steel components in the reactor building; the TEA CO2 laser beam irradiated on the surface induces the gas plasma without making damage on the steel.
Acknowledgments This paper was partially supported by research funds of Chonbuk National University in 2013. Also, part of this research was supported by National Research Foundation of Korea under contract NRF-2010-0022923.
References [1] M. Murata, M. Naritomi, Y. Yoshida, M. Kokubu, Behavior of sodium aerosol in atmosphere, J. Nucl. Sci. Technol. 11 (1974) 65e71. [2] T. Aoyama, C. Ito, K. Okazaki, H. Harano, K. Watanabe, T. Iguchi, Development of sodium leak detection technology using laser resonance ionization mass spectrometry, J. Nucl. Sci. Technol. 6 (2008) 43e50. [3] W. Miziolek, V. Palleschi, I. Schechter, Laser-induced Breakdown Spectroscopy (LIBS): Fundamentals and Applications, Cambridge University Press, New York, 2006. [4] L.A. Álvarez-Trujillo, A. Ferrero, J.J. Laserna, Preliminary studies on stand-off laser induced breakdown spectroscopy detection of aerosols, J. Anal. At. Spectrom. 23 (2008) 885e888. [5] A. Khumaeni, H. Niki, Y. Deguchi, K. Kurihara, K. Kagawa, Y.I. Lee, Analysis of organic powder samples by using the metal-assisted subtarget effect in transversely-excited atmospheric (TEA) CO2 laser-induced He gas plasma at 1 atm, J. Korean Phys. Soc. 55 (2009) 2441e2446. [6] A. Khumaeni, Z.S. Lie, H. Niki, K.H. Kurniawan, E. Tjoeng, Y.I. Lee, K. Kurihara, Y. Deguchi, K. Kagawa, Direct analysis of powder samples using transversely excited atmospheric CO2 laser-induced gas plasma at 1 atm, Anal. Bioanal. Chem. 400 (2011) 3279e3287. [7] R.L. Green, M.D. Mowery, J.A. Good, J.P. Higgins, S.M. Arrivo, K. McColough, A. Materos, R.A. Reed, Comparison of near-infrared and laser-induced breakdown spectroscopy for determination of magnesium state in pharmaceutical powders and solid dosage forms, Appl. Spectrosc. 59 (2005) 340e347. [8] A. Ooki, M. Uematsu, K. Miura, S. Nakae, Source of sodium in atmospheric fine particles, Atmos. Environ. 36 (2002) 4367e4374. [9] J.D. Ingle Jr., S.R. Crouch, Spectrochemical Analysis, Prentice Hall, New Jersey, 1988.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Analysis of sodium aerosol using transversely excited atmospheric CO2 laser-induced gas plasma spectroscopy Ali Khumaeni a, Kazuyoshi Kurihara b, Zener Sukra Lie c, Kiichiro Kagawa d, Yong Inn Lee e, * a
Research Group for Laser Probing, Japan Atomic Energy Agency, Nuclear Science and Engineering Directorate, Tokai-mura, Ibaraki-ken 311-1195, Japan Department of Physics, Faculty of Education and Regional Studies, University of Fukui, Fukui 910-8507, Japan c Graduate School of Nuclear Power and Energy Safety Engineering, University of Fukui, Fukui 910-8597, Japan d Research Institute of Nuclear Engineering, University of Fukui, Fukui 910-8597, Japan e Department of Physics, Research Institute of Physics and Chemistry, Chonbuk National University, Chonju 561-756, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 August 2013 Received in revised form 13 December 2013 Accepted 18 December 2013 Available online 8 January 2014
Taking advantages of the special characteristics of a transversely excited atmospheric (TEA) CO2 laser, the analysis of sodium aerosol has been successfully conducted by using laser-induced gas plasma spectroscopy (LIGPS) method. In this study, the sodium aerosol was deposited on a nickel metal plate; the metal plate functions as a subtarget to initiate a gas plasma. When a pulsed TEA CO2 laser was focused on the metal surface, a large-volume and high-temperature gas plasma was induced. The fine particles of sodium then entered into the gas plasma region to be dissociated and excited. By using this technique, a semi quantitative analysis of sodium aerosol was made. The detection limit of sodium was approximately 200 ppb. Ó 2014 Elsevier B.V. All rights reserved.
Keywords: Analysis of sodium aerosol Laser-induced gas plasma spectroscopy (LIGPS) Pulsed TEA CO2 laser Subtarget
1. Introduction Aerosol analysis is required in many important applications. For instance, in sodium-cooled fast reactors, radioactive sodium was found to contaminate the atmosphere around the reactors. The sodium leak is produced by vaporization or combustion of sodium, which could be released into the atmosphere from the cooling system piping or components. Because sodium aerosol is chemically very reactive, it rapidly reacts with oxygen, water, and CO2 in the atmosphere [1]. Therefore, highly sensitive method is really necessary to detect sodium leak in the atmosphere. Various kinds of sodium leak detector have been developed including sodium ionization detector and the ionization chamber, differential pressure detector, and contact leak detector [2]. However, in those techniques, a sample preparation is necessary and they are labor intensive. Laser-induced plasma has become a very popular technique for qualitative and quantitative elemental analysis in aerosol samples [3]. In this method, a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser with a pulsed energy of
* Corresponding author. E-mail address: [email protected] (Y.I. Lee). 1567-1739/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.12.017
approximately several tens of milli-joules is focused in aerosol under atmospheric pressure of the surrounding gas. However, the sensitivity of this method is quite poor with limit of detection (LoD) of sodium of around 55 ppm [4]. On the other hand, we found that a transversely-excited atmospheric pressure (TEA) CO2 laser is suitable for spectrochemical analysis. This is because the laser has long wavelength of 10.64 mm and long pulse duration of 200 ns. Based on our previous experiment, a high-temperature and large-volume gas plasma was induced when a TEA CO2 laser was focused on a metal surface under atmospheric pressure of the surrounding gas, while the metal itself was never ablated; this phenomenon never occurs in the case of conventional LIBS technique, in which a Nd:YAG laser is most often employed as an energy source. This plasma is very favorable for spectrochemical analysis because its plasma temperature is rather high and it has a high heat capacity [5,6]. We have demonstrated that direct and rapid analysis of powder can be realized for TEA CO2 laser-induced gas plasma method by employing a new technique, in which a powder sample was placed in a container and sent into the gas plasma region by the strong shock wave induced by the gas plasma [6]. While, for an ordinary LIBS method using YAG laser, powder samples must be prepared into a pellet prior to the analysis [7], thus the analysis is time consuming and requires additional equipments.
452
A. Khumaeni et al. / Current Applied Physics 14 (2014) 451e454
In the present paper, we have developed a sophisticated method for the analysis of sodium aerosol using the TEA CO2 laser-induced gas plasma. Namely, the sodium aerosol is deposited onto the surface of a nickel metal, which functions as a sub-target. When a pulsed TEA CO2 laser is focused onto a metal sub-target, strong gas plasma is produced. The gas plasma interacts with the fine particles of the aerosol deposited onto a sub-target. The fine particles then move into the gas plasma region to be dissociated and excited, while the metal itself never has been ablated. It should be stressed that this analytical technique is very high sensitive because this method is basically condensation method; namely, we can collect the aerosol spreading in the wide region of the atmosphere onto the metal surface by moving the metal itself or by keeping the metal surface at a fixed position for a long time so that many aerosol can be attached on the metal surface. Also by increasing the accumulation number of the spectrocehmical data in terms of repeated irradiation on the wide range of the metal surface, we can suppress the noise signal in the spectrocehmical data, thus the detection limit is significantly improved. It should be stressed again that this new analytical method has never been able to apply for ordinary LIBS, where YAG laser is used, because the metal itself is inevitably ablated, resulting in the significant reduction of the sensitivity of the elements deposited on the metal surface.
metal subtarget due to the interaction between dust and TEA CO2 laser beam. The samples used in this works are two kinds of sodium aerosol. One was sodium aerosol sample taken from marine aerosol in the Echizen Seashore (Fukui, Japan), and the other was sodium aerosol taken from the atmosphere in a room at University of Fukui, Japan. The atmospheric fine particles contain sodium as reported by Ooki et al. [8]. A nickel metal plate (30 20 0.15 mm3) was exposed in atmosphere in order to collect the atmospheric fine particles containing sodium; the metal plate functions as a sub-target. The emission spectrum was obtained using an optical multichannel analyzer (OMA) system (ATAGO Macs-320) consisting of a 0.32-m-focal-length spectrograph with a grating of 1200 groves/ mm, a 1024-channel photodiode detector array, and a microchannel plate image intensifier to detect laser plasma radiation. The spectral resolution of the OMA system is 0.2 nm. The light emission of the laser plasma was collected using an optical fiber (0.3p sr), which was fed into the OMA system. One end of the fiber was placed at a distance of 4 cm from the focusing point of the laser light and set perpendicularly to the path of the laser beam. The gate delay time and gate width of the OMA system were set at 20 and 100 ms, respectively. Each spectrum was obtained using 10 shots of laser irradiation.
2. Experimental procedure
3. Results and discussion
Fig. 1 shows the experimental setup used in this study. The pulsed TEA CO2 laser employed in this work is a Shibuya SQ-2000 laser, which was commercially developed and constructed by Shibuya Company for laser marking. The TEA CO2 laser (3 J, 10.64 mm, 200 ns) was operated at a repetition rate of 10 Hz. The laser energy was varied from 250 mJ to 1500 mJ by setting the respective apertures in the laser beam path. After passing through the aperture, the laser beam was focused by a ZnSe lens (f ¼ 20 mm) through a ZnSe window onto the target sample. The laser beam spot was 2 2 mm2 for the tight focusing condition. The samples were placed in a metal chamber with dimensions of 12 12 12 cm3. The experiment was conducted under 1 atmospheric pressure. During experiment, a nitrogen gas was flowed into the chamber, with the flow rate of 4 L per minute (Lpm). It should be mentioned that the gas flowing is required in this experiment, otherwise gas breakdown takes place in front of the
Initially, we reviewed the gas plasma generation induced by the bombardment of a pulsed TEA CO2 laser on a metal surface, which is quite different from the plasma generation induced by a Nd:YAG laser [6]. The plasma induced by a TEA CO2 laser has unique characteristics, owing to its low frequency (long wavelength of 10.64 mm) and long pulse duration (200 ns). When a TEA CO2 laser is focused onto a metal surface, electrons are released from the surface owing to a multiphoton absorption, which occurs at the focusing point of the laser light. These electrons are then accelerated to a high energy in a low-frequency electric field of laser light, which induces the cascade ionization of atoms in the gas, generating an initial gas plasma. Once this initial plasma has been produced, laser light is completely absorbed in gas plasma by inverse bremsstrahlung via freeefree transitions. This absorption is much stronger for the TEA CO2 laser than for the Nd:YAG laser because the plasma absorption coefficient is proportional to the inverse square of the frequency of laser light. Furthermore, the pulse duration of the TEA CO2 laser is relatively long (200 ns), about 20 times longer than that of the Nd:YAG laser, which means that almost all the energy from the TEA CO2 laser is absorbed by gas 7000
Na I588.9 nm Na I589.6 nm
Intensity (arb.units)
6000 5000 4000 3000 2000 1000 0 570
Fig. 1. Experimental setup used in this research.
580
590 600 Wavelength (nm)
610
620
Fig. 2. Emission spectrum of sodium (Na I 588.9 nm and 589.1 nm) taken from urban air in atmospheric room at University of Fukui Japan.
A. Khumaeni et al. / Current Applied Physics 14 (2014) 451e454
plasma. The gas plasma produced is therefore very strong and has a high temperature and a high heat capacity. It should be emphasized that no ablation on the metal surface occurred with the laser bombardment using a 750 mJ energy laser and a lens with a focal length of 100 mm. As reported by Ooki et al., both marine and urban aerosol contain particulate fine sodium [8]. In this study, at initial, we examined whether fine sodium is really deposited or not when the metal plate was placed in urban and marine atmosphere. In order to collect the particulate fine sodium from urban aerosol, we placed a nickel metal plate in atmospheric room at University of Fukui for approximately 1 h. When a pulsed TEA CO2 laser (250 mJ, 200 ns) was focused on the metal surface, a strong gas plasma was induced. It is assumed that the fine particles of the aerosol was dissociated and excited in the gas plasma region. Fig. 2 (red curve (in the web version)) shows the emission spectrum taken from the deposited aerosol. Strong emission line of Na I 588.9 nm and Na I 589.6 nm can clearly be observed with low background emission. In this experiment, 10 shots of laser irradiation were made on the metal surface. The gate delay time and gate width of OMA system were 10 ms and 100 ms, respectively. The sample was rotated with 1 rotation/minute so that the position of laser bombardment on the metal surface is always new. While the blue curve in Fig. 2, which was obtained after laser cleaning treatment, shows completely no sodium line.; in the cleaning treatment, the metal surface was irradiated by defocused laser beam without inducing the plasma, only to clean the aerosol deposited onto the metal surface. The result in Fig. 2 proved that urban aerosol contains particulate fine sodium. It should be mentioned that in this study, a pulsed Nd:YAG laser (10e80 mJ) was also employed in place of the TEA CO2 laser. However, for the case of Nd:YAG laser, the clear detection of sodium lines could not be made due to the disturbance of many strong emission lines, causing from the ablation of nickel. Further study of sodium analysis was made by using particulate fine sodium taken from marine air. To this study, we collected the sodium aerosol sample from the marine air at Echizen seashore, Fukui, Japan. The sample was taken with different times of collection (from 10 to 45 min). Fig. 3 shows the emission spectrum of sodium (Na I 588.9 nm and 589.6 nm) taken from the aerosol sample collected from the Echizen seashore for approximately 20 min. High emission intensity of sodium can clearly be detected with low background emission. The condition for data acquisition of Fig. 3 is the same with that of Fig. 2. Fig. 4 shows how the emission intensity of sodium changes with time. It can clearly be seen that with increasing time of sample
Fig. 3. Emission spectrum of sodium (Na I 588.9 nm and 589.1 nm) taken from the Echizen seashore Fukui Japan for 20 min.
453
Fig. 4. Relationship between exposure time of sample collection and sodium emission intensity (Na I 588.9 nm) taken from marine aerosol at Echizen seashore Fukui Japan.
collection, the emission intensity of sodium (Na I 588.9 nm) increases. This result certified that deposition rate of sodium aerosol is increased with increasing time. It should also be noticed that the curve is not linear. This might be because once the sodium aerosol deposits on the plate, the further sodium aerosol can easily deposit. In this study we also collected the particulate fine sodium aerosol from marine air at different distance from the seawater. Fig. 5 shows the emission spectrum of sodium taken from marine air at (a) 1 m, and (b) 50 m from the seawater. It is observed that the emission spectrum of sodium taken from shorter distance (1 m from the seawater) is much higher in emission intensity. This result
Fig. 5. Emission spectra of sodium taken from atmosphere at (a) 1 m, and (b) 50 m from the Echizen seawater.
454
A. Khumaeni et al. / Current Applied Physics 14 (2014) 451e454
Intensity (arb.units)
20000
Na I588.9 nm
Na I589.6 nm
16000 12000 8000 4000 0 570
Figure 6 580
590 600 Wavelength (nm)
610
620
Fig. 6. Emission spectrum of sodium taken from the aerosol containing 10 ppm of sodium deposited onto a metal plate.
confirmed that the concentration of sodium in marine air is different with a different distance from the seawater. Generally, we can also certify that the concentration of sodium in atmosphere is different with different places. For further exploration of the capability of this technique for quantitative analysis of sodium aerosol, we prepared sodium aerosol by using a liquid solution containing 10 ppm of Na. 1 ml Na solution was homogeneously poured onto a nickel metal plate with a dimension of 30 20 0.15 mm3. The solution was then dried by using hairdryer for 20 min so that the fine sodium was deposited onto the metal plate. Fig. 6 displays the emission spectrum taken from the aerosol containing 10 ppm of sodium deposited onto a metal plate. The typical two lines of Na I 588.8 nm and Na I 589.6 nm clearly appear with an extremely low background emission intensity. The detection limit of Na was estimated to be approximately 200 part per billion (ppb); the detection limit was derived by calculating what concentrations of the signal which yields 3 times of the noise level is, because the 3 times of the noise can be clearly identified as a signal [9]. 4. Conclusions We have demonstrated in this work that analysis of sodium aerosol can successfully be performed by using the specific characteristics of a TEA CO2 laser. In this study, sodium aerosol was deposited on a nickel metal plate, which functions as a subtarget. When a pulsed TEA CO2 laser was focused on the metal plate, a high-temperature and large-volume gas plasma was induced, without ablating the metal surface; the metal functions only to initiate gas plasma. It is assumed that the sodium aerosol was evaporated and entered into the gas plasma region to be effectively
dissociated and excited in the gas plasma region. a semi quantitative experiment has been successfully demonstrated by using sodium aerosol taken from the marine air at Echizen seashore (Fukui Japan) with different time of aerosol collection. The result certified that the emission intensity of sodium increases with increasing time of sample collection, meaning that the amount of sodium deposited on the metal surface increases with increasing time. The detection limit of Na was estimated to be 200 ppb, which derived by using only 10 shots accumulation. If we increase the number of the laser shots, the detection limit will be naturally decreased because the noise level decrease by accumulation. Thus, this present technique has high possibility to be applied to online analysis of sodium in fast reactor such as Monju reactor at Tsuruga Fukui Japan. Also it should be stressed that this technique can be applied as remote in-situ monitoring of the leaked sodium which attached on the surface as aerosol on the steel components in the reactor building; the TEA CO2 laser beam irradiated on the surface induces the gas plasma without making damage on the steel.
Acknowledgments This paper was partially supported by research funds of Chonbuk National University in 2013. Also, part of this research was supported by National Research Foundation of Korea under contract NRF-2010-0022923.
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