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Intensified microwave coupled by a loop antenna (diameter of 3 mm) has been employed to enhance the laser-induced breakdown spectroscopy (LIBS) emission. In this method, a laser plasma was induced on Gd2O3 sample at a reduced pressure by focusing a pulsed Nd:YAG laser (532 nm, 10 ns, 5 mJ) at a local point, at which electromagnetic field was produced by introducing microwave radiation using loop antenna. The plasma emission was significantly enhanced by absorbing the microwave radiation, resulting in high-temperature plasma and long-lifetime plasma emission. By using this method, the enhancement of Gd lines was up to 32 times, depending upon the emission lines observed. A linear calibration curve of Ca contained in the Gd2O3 sample was made. The detection limit of Ca was approximately 2 mg/kg. This present method is very useful for identification of trace elements in nuclear fuel and radioactive materials.
© 2013 Optical Society of America
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) has become an increasingly popular technique for qualitative and quantitative analysis of elements in many kinds of samples in industries and research laboratories [1–4]; the popularity of this method is indicated by a large number of published papers in many journals (over 1500 papers in the past 5 years). This is because the technique enables one to carry out rapid and direct remote analysis without time consuming.
In conventional LIBS method, a pulsed Nd:YAG laser is usually focused onto a sample target to induce a small luminous plasma. Elemental composition of the target can be obtained by analyzing the atomic emission from the plasma [5–8]. However, one of the drawbacks of LIBS is its low sensitivity to perform the analysis of trace elements in the materials in various fields and the analysis of elements, which have complex spectral lines, such as nuclear fuel elements.
Several methods have been developed to improve the sensitivity by enhancing the LIBS emission intensity. Recently, double pulse LIBS (DP LIBS) has been widely studied [9]. Uebbing et al. and Sattmann et al. examined collinear and orthogonal reheating multipulse LIBS of solid in air [10,11]. Killinger et al. used a pulsed CO2 laser to enhance the Nd:YAG LIBS signal emission of a remote target [12]. The latest study about DP LIBS has been performed by X. Liu et al. by examining the effect of laser pulse energy on double femtosecond LIBS [13]. R. Sanginés et al. studied the emission enhancement mechanisms in orthogonal DP LIBS [14]. Some researchers combined LIBS with laser-induced fluorescence (LIBS-LIF) to improve the detection limit [15]. However, both double pulse LIBS and LIBS-LIF methods are costly because the other laser system is required.
The other method has been used to enhance the emission intensity of spectral lines without the use of an additional laser. Envimetrics, Inc. and Liu et al. applied microwave to enhance the plasma emission of the metal samples [16,17]. Kearton et al. and Y. Liu et al. reported intensity enhancements by using laser-assisted microwave plasma spectroscopy (LAMPS) [18,19]. Ikeda et al. used microwave to extend the lifetime of plasma emission to millisecond [20,21]. However, in the methods, waveguides and microwave cavity were required. Meir and Jerby performed material identification by using breakdown spectroscopy induced by localized microwave [22].
For the convenience application of microwave, we offer a new technique of microwave-assisted LIBS (MA-LIBS) coupled by a loop antenna for the enhancement of LIBS emission. In this technique, a small plasma is induced by a low energy of pulsed laser at a local point, at which electromagnetic field was produced by intensified microwave radiation. A microwave (MW) is applied through a loop antenna via coaxial cable to enhance the plasma emission. In order to locally intensify the MW electromagnetic field in space for the effective plasma enhancement, the antenna was formed as a loop with a diameter of 3 mm. This present method is very potential to be used for the enhancement of emission intensity of elements, which enables us to improve the sensitivity of analytical results, and the selection of high resolution spectrometer, of which the sensitivity is generally lower than ordinary spectrometer. The use of such spectrometer is highly desirable because the emission spectrum of nuclear fuel material is highly complicated.
2. Experimental procedure
The basic experimental setup used in this study is shown in Fig. 1. The second-order harmonic of Q-switched Nd:YAG laser (Quanta-Ray, Spectra Physics, 532 nm, 10 ns) was directly focused onto a sample surface through a quartz window using quartz lens with a focal length of 200 mm to induce a luminous plasma on a sample target. During the experiment, the laser energy was fixed at 5 mJ by using a polarizer. It should be mentioned that our new method does not require a high-power laser because the power used for the plasma expansion was supplied by the microwave. The plasma emission was enhanced by intensified microwave induced through a loop antenna; the length of antenna is 30 mm and the antenna was made a loop shaped with a diameter of 3 mm as shown in the insert of Fig. 1. It should be mentioned that during experiment, the antenna was not ablated by the microwave-enhanced laser-induced plasma and hence no contamination of the antenna elements in the emission spectrum. Microwave (MW) was generated by using a magnetron at a frequency of 2.45 GHz (MUEGGE MG0500D-215TC, 400 W, 0-1 ms). The MW has a maximum power at 400 W. The MW impedance can experimentally be adjusted for plasma generation by a three-stab tuner. The Nd:YAG laser and MW were operated in the synchronization mode with a delay time of 10 μs for the Nd:YAG laser bombardment relative to the microwave generation.
The diameter of loop antenna used in this study was 3 mm. Based on our experiment using antenna with a diameter of 3 mm and 9 mm, the optimum enhancement of emission intensity was obtained when the diameter of 3 mm was used. The mechanism of laser plasma enhancement by using antenna coupled-MW is assumed as follows: a luminous plasma with high electron density is generated by a pulsed laser on a sample target near the center of the antenna, at which locally intensified electromagnetic field is produced. After several μs of plasma generation, the electron density of laser plasma decreases and after achieving the cutoff density of 2.45 GHz MW, the electrons absorb the microwave radiation. The electrons are then accelerated to provide kinetic energy to excite the atoms and ions by multiple collisions among electrons-atoms and electrons-ions. The plasma emission can be sustained long lifetime because the MW duration is long lifetime, from several hundreds of μs to ms. As a result, the plasma emission intensity is enhanced due to larger integration time. We assumed that in the case of small diameter of 3 mm, the electromagnetic field induced in the central axis of the loop antenna is much higher than that of the large diameter of 9 mm, allowing optimum collisions among electrons-atoms and electrons-ions. Thus, in this study, an antenna with a small diameter of 3 mm was used throughout the experiments.
The sample used in this experiment was gadolinium oxide (Gd2O3) pellet as a simulated nuclear fuel pellet. Gd has complex spectra as well as uranium. It is difficult to choose strong and entirely separated spectral lines for evaluation in Gd due to the complex spectra. Therefore, relatively strong and identifiable lines of the neutral atomic line 422.56 nm (Gd I) and ionic line 342.25 nm (Gd II) are used in this study. For quantitative analysis, the Gd2O3 sample containing various concentrations of Ca (60, 125, 250, and 500 mg/kg) were employed.
During the experiment, the sample was placed in a metal chamber equipped by windows, on which the fine gold mesh (lattice constant of 100 μm and wire diameter of 30 μm) was attached. The chamber functioned to block the microwave radiation. The experiment was made at a reduced pressure of air surrounding gas.
The emission spectrum was obtained by using an ICCD camera (Andor, iStar) through a high-resolution echelle spectrometer (ARYELLE) with the resolution of 50 pm at the wavelength of 360-460 nm. The light emission of the laser plasma was collected by using an optical fiber, which fed into the spectrometer. The plasma radiation at 2-5 mm from the sample surface was imaged in a ratio of 1:1 onto one end of the fiber by using a quartz lens (f = 100 mm).
3. Results and discussion
3.1 Influence of ambient pressure
In order to obtain an optimum enhancement intensity of LIBS emission of Gd2O3 material target using intensified MW, Initially the influence of ambient pressure on Gd emission signal intensities was investigated. The pressure dependence of the Gd I 422.56 nm and Gd II 342.25 nm emissions of a Gd2O3 sample taken by using conventional LIBS and MA-LIBS is shown in Fig. 2. In this study, the laser energy was 5 mJ and the MW power and duration were 400 W and 800 μs, respectively. For data acquisition both in conventional LIBS and MA-LIBS, the gate delay time was 1 μs and the measurement time duration was 800 μs. Each point plotted in the graph was averaged on 100 laser shots and 3 spectra. It is seen that in the conventional LIBS case, the emission intensity of Gd I 422.56 nm slowly increases from 0.27 kPa to 6.67 kPa and finally decreases slowly. The emission intensity of Gd II 342.25 nm rises slowly up to 1.33 kPa and finally becoming a monotonous decrease. Similar phenomenon at reduced pressure has also been obtained by Knight et al. from nanosecond laser plasmas [23], namely at reduced pressure, the emission intensity increases at certain pressure and further decreases with decreased pressure. This behavior can be explained by pressure-dependent changes in the mass of material vaporized and the frequency of collision between species in the plasma. The increased intensity is due to the decreased plasma shielding effect and hence increased mass ablation of material target. At a reduced pressure, a nanosecond laser plasma is expanding in a less dense atmosphere, resulting in a less dense shock wave. The reduced density in the shock wave reduces the plasma shielding, allowing more photons to reach the target. Increment of the number of photon interacting with the sample surface increases the sample ablation, resulting in more intense spectrum. The decreased emission intensity at reduced pressure is because the shock wave plasma freely expands and hence the decrease of frequency of collision between species in the plasma, decreasing the intensity of emission spectrum.
Fig. 2 Pressure dependence of the Gd I 422.56 nm and Gd II 342.25 nm emissions of a Gd2O3 sample taken by using LIBS with and without microwave.
In the MA-LIBS case, from 6.67 kPa to 66.67 kPa, the intensity profile of Gd I and Gd II lines is almost the same with the case of LIBS; the difference is only in emission intensity, namely the MA-LIBS intensity is slightly higher than LIBS intensity. However, significant increment of both Gd I and Gd II emission intensities was observed below 6.67 kPa, with an intensity peak at 1.33 kPa. The effect of pressure on the enhancement may give influence to the signal increment as reported by authors [24]. The Significant enhancement at a reduced pressure is explained as follows: as the air pressure is decreased, the density of air surrounding gas decreased, allowing the decrease of the number of electron-air molecules collisions in the plasma region. Thus, the collision probability among electrons-atoms and electrons-ions in the plasma is higher at reduced pressure, resulting in significant increment at a reduced pressure. This result certified that optimum pressure to realize maximum emission intensity was around several hundreds of Pa.
From Fig. 2, we can obtain the intensity enhancement of the Gd as a function of ambient pressure. The enhancement was defined as the ratio between the MA-LIBS intensity and LIBS intensity after subtracting background emission. Figure 3 shows the enhancement of the Gd I 422.56 nm and Gd II 342.25 nm intensity dependent on pressure of air surrounding gas when the intensified MW was on. Varying enhancement factors of Gd emission have been obtained at different ambient pressures. For both Gd I and Gd II, it is seen that week enhancement was observed from near atmospheric pressure to 6.67 kPa. The significant enhancement was achieved below 6.67 kPa. The intensity peak for Gd I and Gd II was obtained at 1.33 kPa and 0.27 kPa, with the intensity enhancement of 10 and 32 times, respectively. The microwave plays an important role on this increment. When the microwave radiation is introduced into the laser plasma, the electrons are accelerated, increasing the kinetic energy. Along with the increment of kinetic energy, the number of collisions between electrons and constituents increases, which affects the plasma temperature and the densities of electrons, ions, and neutral atoms in the plasma, allowing to the improvement of emission intensity. A detailed study on the mechanism of enhanced emission process will be made in the near future. In this present study the ambient pressure was set at 0.27 kPa in order to achieve an optimum enhancement of Gd emission intensity.
Fig. 3 Enhancement dependence of the Gd I 422.56 nm and Gd II 342.25 nm emissions on ambient pressure by using MA-LIBS method.
3.2 Emission spectrum from Gd2O3 sample
The emission spectrum of Gd2O3 taken by using MA-LIBS and conventional LIBS from 300 nm to 450 nm are shown in Fig. 4. Red curve shows the emission spectrum taken by using MA-LIBS method, while blue curve (dashed line) is for conventional LIBS. The spectrum was taken at 0.27 kPa of ambient pressure and the laser energy was 5 mJ. The spectra were accumulated for 100 pulses for both microwaves on and off, with each laser shot hits at a fresh point of the moving sample surface to avoid shot-to-shot fluctuation. The measurement was started after a delay time of 1 μs from laser bombardment and the measurement time duration was 800 μs.
In order to make a comparison of Gd2O3 spectrum between MA-LIBS and conventional LIBS methods, the experimental conditions were set the same including laser power, detection system, and ambient pressure. As shown in Fig. 4, the emission intensity of Gd for conventional LIBS is thoroughly very week; the LIBS intensity is shown in the right-y axis. The intensity is extremely enhanced once the intensified MW (400 W, 1 ms) was introduced. Total emission intensity increased by approximately 40 times. No OH and N2 cluster emissions were detected in LIBS, but these are very clear OH and N2 cluster spectra when using MA-LIBS. It can clearly be seen that many Gd lines were observed in the spectrum. Many lines of Gd are also overlapped with OH and N2 clusters. Gd has complex spectral lines as well as nuclear fuel material such as uranium. Therefore, it is difficult to choose strong and entirely separated spectral lines for evaluation in Gd due to the complex spectra. In this study, strong and identifiable lines of the neutral atomic line Gd I 422.56 nm and ionic line Gd II 342.25 nm are used. For the plasma temperature measurements, several ionic lines of Gd were depicted from Fig. 4. These lines along with their spectroscopic parameters, taken from the Kurucz atomic database [25], are shown in Table 1.
Table 1. Wavelength, upper level energy, upper level degeneracy, and transition probability for the ionic Gd emission lines used in this study.
Figure 5 and Fig. 6 show the zoomed windows of Fig. 4 in the wavelength ranges of 340-345 nm and 418-423 nm, respectively. It can clearly be seen that enhancement of emission intensity of Gd lines including Gd II 342.25 nm and Gd I 422.56 nm as shown in Fig. 5 and Fig. 6 occurs when the MW was introduced. The enhancement factors for Gd II and Gd I are 32 and 8 times, respectively. Unidentified molecular band also appears in Fig. 6. Even though the intensity enhancement increases with the MW power increased, our MW power was limited to 400 W.
3.3 Boltzmann plot method for determination of plasma temperature
In order to determine the plasma temperature, Boltzmann plot method was applied. The Boltzmann plot is an established method to determine the plasma temperature from relative line intensities. For plasma in local thermodynamic equilibrium (LTE) [26,27], based on the emission coefficient relationship, the Boltzmann plot can be reordered as follows
Where I is the integrated intensity of spectral lines occurring between the upper energy level k and lower energy level i, λ is the transition line wavelength, gk is the degeneracy of the upper energy level, Ak is the transition probability, kB, T, and Ek are Boltzmann constant, the plasma temperature, and energy of the upper energy level, respectively, C is a constant for a given atomic species. The plasma temperature T can be determined from slope of a linear fit according to Eq. (1) as shown in Eq. (2),Figure 7 shows typical Boltzmann plot based on measurements of spectral lines list in Table 1 by using MA-LIBS and conventional LIBS methods. The slope of the curve yields a plasma temperature of approximately 7500 K in MA-LIBS and approximately 5700 K in conventional LIBS. The plasma temperature in the MA-LIBS case was effectively enhanced compared to the case of conventional LIBS. It is assumed that the increased plasma temperature in the MA-LIBS case is attributed to the effect of reheating by microwave radiation as in the case of double pulse LIBS (DP-LIBS) [28]. In any case, the plasma temperature in LIBS is driven by electron collisions. When the microwave was introduced, the electrons in the laser plasma accelerate. Thus multiple collisions of electrons increase, resulting in the increase of plasma temperature. The estimated values of plasma temperature at several delay time in MA-LIBS are displayed in Fig. 8. Time duration of each point plotted in the graph was 5 μs. It is seen in the figure that the plasma temperature is almost stable with the delay time. It should be noticed that the plasma temperature is quiet high of 7500 K in average and keeps long lifetime. This elongated plasma temperature is attributed to the microwave duration. Multiple collisions of electrons in the microwave-enhanced laser plasma continuously happen as long as the microwave radiation was introduced in the laser plasma region. For the plasma to be in LTE, atomic and ionic should be populated and depopulated predominantly by collisions rather than by radiation. This requires that the electron density has to be high enough to ensure a high collision rate. The corresponding lower limit of electron density is given by the McWhirter criterion [29]
Where (eV) is the largest energy transition for which the condition holds and Te (K) the plasma temperature. We have estimated the electron density in this present microwave-enhanced laser plasma from studying the stark broadening of H I 656.28 nm line observed in the emission spectrum at long wavelength. The electron density was estimated to be in the order of 1018 cm−3. By using the plasma temperature obtained in the present MA-LIBS method, the McWhirter criterion was found to be fulfilled, certifying that the plasma was in the LTE condition.Fig. 7 Boltzmann plot made from the analysis of twenty two Gd II lines, considering the intensities at 2.67 kPa of air surrounding gas. The continuous line represents the result of a linear best fit. I and λ are the intensity and the wavelength of a transition from upper level k of energy Ek and statistical weight g to lower level i with A as the corresponding transition probability. The slope gives the temperature as approximately 7500 K and 5700 K in MA-LIBS and standard LIBS methods, respectively.
3.4 Lifetime of plasma emission
The lifetime of emission intensity of Gd in the MA-LIBS and LIBS methods was measured with a delay time.
Figure 9 shows how the emission intensities of Gd I 422.25 nm and Gd II 342.25 nm taken by using MA-LIBS and conventional LIBS changes with delay time. Time duration of each point plotted in the graph was 5 μs for both in LIBS and MA-LIBS cases. At initial, the emission intensities of Gd I and Gd II in the case of conventional LIBS and MA-LIBS methods are almost the same. However, the lifetimes of Gd I and Gd II in the conventional LIBS are very short of around 5 μs with suddenly decrease in intensities. Even though both emission intensities of Gd I and Gd II in LIBS can be increased by increasing the laser energy, no significant effect was found in the plasma lifetime. While in the MA-LIBS case, the emission lines of both Gd I and Gd II last long lifetime up to approximately 800 μs as long as microwave duration. Also, both emission intensities keep almost stable in emission intensity until 500 μs before becoming slow decrease. As a consequence of the prolonged plasma lifetime, the emission intensity of the Gd is enhanced due to larger integration time. These results approved that the MW has an important role not only to enhance the plasma temperature, but also to extend the lifetime of plasma emission. Therefore, our develop method is very potential to be used as a high intense optical emission source in very high resolution spectrometer for identifying the elements, which have complex spectral lines such as nuclear fuel elements.
Fig. 9 Time profiles of emission intensity of Gd I 422.56 nm and Gd II 342.25 nm using LIBS with and without microwave.
3.5 Quantitative analysis of impurity in Gd2O3 sample
The method presented in this paper was adopted for a study on a quantitative analysis of the Gd2O3 sample containing trace elements of Ca in different concentrations. For this study, Ca ionic line at 393.37 nm was used. Figure 10 shows the emission spectra of Ca II 393.37 nm taken from the Gd2O3 sample containing 125 mg/kg, 250 mg/kg, and 500 mg/kg of Ca by using the MA-LIBS technique. The emission spectra were taken using 50 laser shots with an average of five measurements. The laser energy was set at 5 mJ and the MW power was 400 W. Figure 11 Shows the emission spectra of Ca from the same sample as used in Fig. 10 using the standard LIBS method, without microwave. To obtain each spectrum, 100 shots of laser irradiation with an average of five measurements were employed. By comparing the spectra in Figs. 10 and 11, it is seen that in the case of MA-LIBS method, total emission intensity of the spectra was enhanced and many analytical lines of Gd can clearly be observed. The enhancement factors for Ca II 393.37 line is almost 100 times. The microwave radiation in MA-LIBS method plays an important role on the enhancement of analytical spectra.
Fig. 11 Emission spectra of Ca II 393.37 nm taken from the Gd2O3 sample by using standard LIBS method.
Finally, in order to examine the effectiveness of our present technique in terms of quantitative analysis, calibration curves were obtained using the Gd2O3 sample containing different concentrations of Ca. Figure 12 shows the calibration curve obtained by using the MA-LIBS method (dashed red line) and conventional LIBS method (solid blue line). Each point plotted on the calibration curves of Fig. 12 was an average of 5 spectra. In order to compensate for the uncontrolled fluctuation of the laser plasma intensity produced on the sample, internal standardization was carried out using the Gd II 391.65 nm line. The Gd II 391.65 nm line was used as an internal standard because no interference line was found and its upper energy level is very close to that of the analyzed lines as well, thus reducing the possibility of temperature ñuctuations affecting the ratios; the upper energy levels of Gd II 391.65 nm and Ca II 393.37 nm lines are 3.8 eV and 3.2 eV from the ground states of the Gd1+ and Ca1+ ions, respectively.
It is clearly seen in the figure that the linearity of calibration curve for the MA-LIBS method is much better than that of the conventional LIBS. The detection limit of Ca in the Gd2O3 using the MA-LIBS method was approximately 2 mg/kg, and for the case of conventional LIBS method, the detection limit were approximately 48 mg/kg. The detection limit was derived by calculating the signal concentration which yielded 3 times the noise level because this was clearly identified as a signal that could be distinguished from the noise [30]. These results clearly indicate that the analysis by MA-LIBS method is significantly superior in sensitivity compared to the case of conventional LIBS method.
4. Conclusions
For the first time, a new method of microwave-assisted laser-induced breakdown spectroscopy (MA-LIBS) coupled by a loop antenna was developed and applied to enhancement of LIBS signal emission. In this study, microwave was injected through the antenna to expand the LIBS plasma. The results certified that compared to that of LIBS method, the luminous plasma induced in MA-LIBS has higher temperature of approximately 7500 K and longer lifetime of approximately 800 μs. The enhancement factor on several lines of Gd up to 32 times was achieved for MA-LIBS method. Identification of trace elements in the Gd2O3 sample can successfully be made. The calibration curves have successfully been obtained from the Gd2O3 sample containing different concentrations of Ca by using the MA-LIBS method and conventional LIBS. The detection limit of Ca in the Gd2O3 sample using the MA-LIBS method is approximately 2 mg/kg, while for the standard LIBS case are approximately 48 mg/kg. The results indicates that enhancement of the sensitivity can effectively be realized by using the LIBS coupled by microwave (MA-LIBS). In the present study, the experiment was made at a reduced pressure because our microwave has limited power of 400 W. Based on our experimental result, the emission intensity of Gd by using present MA-LIBS in air at near atmospheric pressure was slightly increased compared to that of conventional LIBS case. Therefore, for the next future, we plan to improve our technique by increasing a microwave power in order to realize significant intensity enhancement on elemental analysis at normal pressure. The final goal of our present method is for practical application of nuclear fuel and radioactive material analysis. In the application, the material target is placed in a metal chamber. It is known that in the analysis of nuclear fuel and radioactive materials, very complex spectrum was obtained. Thus, to clearly detect the analytical lines, the high-resolution spectrometer is usually used. Unfortunately, the spectrometer is dark enough and requires bright plasma as an emission source. Our present microwave-enhance laser plasma induced at low pressure using low laser energy for material ablation is suitable to be used as an emission source because in order to induce the plasma, we do not need to use waveguides and microwave cavity, which is inconvenient in the practical application.
Acknowledgments
Present study includes the result of “Development of laser remote analysis for next generation nuclear fuel and applied study by MOX sample” entrusted to Japan Atomic Energy Agency by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).
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