Detection of uranium in solids by using laser-inducedbreakdown spectroscopy combined with

laser-induced fluorescence

X. K. Shen and Y. F. Lu*Department of Electrical Engineering, University of Nebraska—Lincoln, Lincoln, Nebraska 68588-0511, USA

*Corresponding author: ylu2@unl.edu

Received 20 December 2007; revised 8 March 2008; accepted 11 March 2008;posted 12 March 2008 (Doc. ID 91017); published 4 April 2008

Detection of uranium in solids by using laser-induced breakdown spectroscopy has been investigated incombination with laser-induced fluorescence. An optical parametric oscillator wavelength-tunable laserwas used to resonantly excite the uranium atoms and ions within the plasma plumes generated by aQ-switched Nd:YAG laser. Both atomic and ionic lines can be selected to detect their fluorescence lines.A uranium concentration of 462ppm in a glass sample can be detected by using this technique at anexcitation wavelength of 385:96nm for resonant excitation of U II and a fluorescence line wavelengthof 409:0nm from U II. © 2008 Optical Society of America

OCIS codes: 140.3440, 300.2140, 300.2530, 300.6170, 300.6365.

1. Introduction

The technique of laser-induced breakdown spectro-scopy (LIBS) offers a real-time and in situ elementalanalysis with little or no sample preparation. When apowerful laser beam is focused onto a sample target,a hot luminous plasma will be generated. By spec-trally analyzing the atomic emissions from the lumi-nous plasma, the elemental composition of the targetcan be deduced. In recent years, LIBS has been suc-cessfully used in a number of applications, such asaerosol detection [1], culture heritage analysis [2],biomedical applications [3], and industrial applica-tions [4]. LIBS is also a suitable real-time in situ ana-lysis technique for monitoring nuclear material [5].LIBS can be used to detect trace uranium in solid

and liquid samples, which has applications in pro-cess control in nuclear fuel reprocessing facilitiesand nonproliferation and environmental monitoring.However, the spectra of atomic and single-ionized ur-anium are extremely complex. Thousands of emis-sion lines arise from transitions among energy

states belonging to numerous electron configurationswhose terms show considerable mixing [6]. In addi-tion, most uranium atomic lines have strong interfer-ence from other elements such as cobalt, nickel, andiron. So far, only Wachter and Cremers have reporteda limit of detection of 0:1 g=L for pure uranium solu-tion by using LIBS [7]. Therefore, efforts to improvethe sensitivity of uranium detection by LIBS arerequired. LIBS combined with laser-induced fluores-cence (LIF, also termed laser-excited atomic fluores-cence spectroscopy, LEAF) has the potential to solvethe problem. LIBS combined with LIF (LIBS-LIF)was first proposed by Measures and Kwong [8,9].In this technique, a pulse from a wavelength-tunablelaser, which is tuned to a specific wavelength match-ing an electronic transition in an element of interest,is applied to the plasma plume that is produced bythe first laser pulse. The fluorescence emissionsare then evaluated for element analysis. LIBS-LIFhas high selectivity and sensitivity. It has been suc-cessfully used in the analysis of cadmium and thal-lium in solid samples [10]. Telle et al. studied LIBScombined with LIF in metallic samples and the po-tential of real-time and on-line remote analysis usingLIBS-LIF [11]. On-line monitoring of the process of

0003-6935/08/111810-06$15.00/0© 2008 Optical Society of America

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laser cleaning of limestone by using LIBS and LIFhas been described by Gobernado-Mitre et al. [12]The optimization of experimental conditions forthe detection of uranium by using LIBS-LIF, in-cluding the selection of appropriate spectral lines,is necessary because of the density of atomic andionic lines and the strong interference from otherelements.In this work, detection of uranium in solids was

investigated by using LIBS-LIF in open air. Bothatomic and ionic lines were selected for fluorescenceanalysis, and uranium ionic lines were found to havebetter sensitivity and selectivity than atomic linesfor detection of uranium. The time-delay dependenceof LIF signals for a uranium ionic line has been alsoinvestigated to optimize the sensitivity.

2. Experimental Setup

The schematic diagram of the experimental setup forLIBS-LIF is shown in Fig. 1. Both a Q-switched Nd:YAG laser operating at 532nm (Continuum, Power-lite Precision II 8000, pulse duration of 6ns) and anoptical parametric oscillator (OPO) wavelength-tunable laser (Continuum, Panther EX, wavelengthtunable from 215nm to 2:55 μm, pulse duration of6ns) were used in the experiments. Both lasers wereoperated in the external trigger mode and weretriggered by a digital delay generator (Stanford Re-search System DG535, 5ps delay resolution) for syn-chronization. The repetition rate was 10Hz. The Nd:YAG laser beam was focused onto a sample at nor-mal incidence by a convex lens (15 cm focal length).To avoid overablation, the sample was mounted ona programmable translation stage. The laser was fo-cused to a spot size of around 1mm in diameter. Theexperiments were performed in open air. The OPOlaser beam was focused by a UV-grade lens with a

focal length of 20 cm and was horizontally propa-gated through the center of the plasma plume pro-duced by the Nd:YAG laser. The laser fluences forthe Nd:YAG laser and the OPO tunable laser were20 and 0:5 J=cm2, respectively. The size of the lumi-nous plasmas was about 4mm in diameter. The spotsize of the OPO tunable laser was around 1mmin diameter. The distance of the OPO laser abovethe sample surface was optimized to about 2mm.The time delay between the OPO tunable laserand the Nd:YAG laser was adjusted by the DG535delay generator. The light emissions from the laser-induced plasma plumes were coupled to the entranceslit of a spectrometer (Andor Technology, Shamrock303 i) by a pair of UV-grade quartz lenses (L1,5 cm focal length; L2, 20 cm focal length). The opticalemissions at the center of the plasmas where theOPO laser beam propagated were studied. Thespectrometer has three gratings with 150, 600, and2400 lines=mm. The spectral resolution for the2400 line grating is 0:1nm. A 512 × 512 pixel inten-sified CCD, Andor Technology, iStar DH-712) was at-tached to the exit focal plane of the spectrograph andwas used to detect the spectrally resolved lines. Theintensified CCD detector was operated in the gatedmode. Data acquisition and analysis were performedwith a personal computer. Two uranium glasssamples were used in the experiments: one fromthe National Institute of Standards and Technology(NIST) with a uranium concentration of 462ppm(parts in 106), the other Vaseline glass having auranium concentration of around 1%.

3. Results and Discussion

First, aluminum and calcium species in the uraniumglass from NIST were investigated by using bothLIBS and LIBS-LIF to show the high selectivity of

Fig. 1. (Color online) Schematic diagrams of the experimental setup for LIBS-LIF.

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LIBS-LIF. The concentrations of aluminum and cal-cium in the samples were around 1% and 8%, respec-tively. Figures 2(a) and 2(b) show the typical LIBSand LIBS-LIF signals for the uranium glass fromNIST and the partial Grotrian diagrams for alumi-num atoms and calcium ions, illustrating the opticaltransitions that have been used in our investigation.The OPO tunable laser was operated at a time delayof 5 μs. In Fig. 2(a), the OPO tunable laser was oper-ated at 309:28nm, which resonantly excited an alu-minum atomic transition line (Al I at 309:28nm). InFig. 2(b) the OPO laser was operated at 393:37nm,which resonantly excited a calcium ionic transitionline (Ca II at 393:37nm). The dashed curves repre-sent the LIBS spectra obtained by using only theNd:YAG laser, whereas the solid curves representthe LIBS-LIF spectra obtained by using both lasers.The spectra were recorded with a time delay of5 μs after the plasma ignition and a gate width of10ns. All the spectra were averaged over 600 shotsto improve the signal-to-noise ratio. As is shown inFig. 2(a), the emission intensities from the aluminum

atomic lines (Al I at 394.4 and 396:2nm) were signif-icantly enhanced. The signal enhancement ratio forthe aluminum atomic lines is around 30. The emis-sion intensities from the calcium ionic lines (Ca IIat 393.4 and 396:8nm) were nearly unchanged.Therefore, the LIBS-LIF approach has high selectiv-ity to minimize the matrix effect. Similarly, as canbe seen in Fig. 2(b), the emission intensity for thecalcium ionic line (Ca II at 396:8nm) was obviouslyenhanced, while the emission intensity for the alu-minum atomic line (Al I at 396:2nm) was nearlyunchanged. The signal enhancement ratio is around8. It is shown that the LIBS-LIF can selectivelyenhance both atomic and ionic lines.

In order to find the optimal transition lines inLIBS-LIF for detection of uranium, a Vaseline glasswith a uranium concentration of around 1%was usedin the experiments. Figures 3(a) and 3(b) show theLIBS and LIBS-LIF signals for the Vaseline glassand simplified Grotrian diagrams for uranium atomsand ions, illustrating the optical transitions used inour investigation. In Fig. 3(a), the OPO tunable laser

Fig. 2. (Color online) LIBS-LIF (solid curves) and LIBS (dashed curves) spectra of a uranium glass from NIST and the partial Grotriandiagrams for aluminum atoms and calcium ions. (a) The OPO tunable laser was operated at 309:28nm to resonantly excite Al I;(b) the OPOtunable laser was operated at 393:37nm to resonantly excite Ca II.

1812 APPLIED OPTICS / Vol. 47, No. 11 / 10 April 2008

was operated at 348:94nm, which resonantly exciteda uranium atomic transition line (U I at 348:94nm).In Fig. 3(b), the OPO tunable laser was operated at385:96nm to resonantly excite a uranium ionic tran-sition line (U II at 385:96nm). The time delay be-tween the OPO laser and Nd:YAG laser was 4 μs.The laser fluences were the same as the previous ex-periments. The spectra were recorded with a time de-lay of 4 μs after the plasma ignition and a gate widthof 10ns. As can be seen from Figs. 3(a) and 3(b), bothatomic (356:7nm) and ionic uranium (409:0nm) lineswere significantly enhanced. The signal enhance-ment ratio for uranium atomic transitions is around10, while that for uranium ionic transitions is around25. This shows that both uranium atomic and ionictransitions are suitable for LIBS-LIFmeasurements.Since ionic species in plasma plumes usually have

much shorter lifetimes than atoms, the time-delaydependence of LIF signals for a uranium ionicline (409:0nm) has been investigated by adjustingthe time delay between the two lasers by usingthe DG535 delay generator. Figure 4 shows theLIBS-LIF and LIBS signals for U II at 409:0nm as

Fig. 3. (Color online) LIBS-LIF (solid curves) and LIBS (dashed curves) spectra for a Vaseline uranium glass and the partial Grotriandiagrams for uranium atoms and ions. (a) The OPO tunable laser was operated at 348:94nm to resonantly excite U I; (b) the OPOwavelength-tunable laser was operated at 385:96nm to resonantly excite U II.

Fig. 4. (Color online) LIF (filled squares, solid curve) and LIBS(filled circles, dashed curve) signals for a uranium ionic transitionline (U II at 409:0nm) as a function of the time delay between thetwo lasers.

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a function of the time delayΔtLIBS-LIF. As can be seenfrom Fig. 4, the LIBS signal for U II at 409:0nm de-creased rapidly as the time delay increased. After atime delay of 4 μs, the uranium ionic line almost dis-appeared, and no distinct signals can be obtained.However, in the case of LIBS-LIF, the uranium ionicline was still obviously observed as ΔtLIBS-LIF in-creased from 4 to 18 μs. This indicates that uraniumions in a lower-energy state existed in the plasmaplume for a long time, around 20 μs. This resultmay possibly be attributed to the relatively low ioni-zation potential of uranium (6:19 eV) [13]. In a plas-ma, recombination between electrons and ions willoccur. It is easy for the uranium atoms to be strippedof orbital electrons, since the ionization potential ofuranium atoms is low. Therefore, there were stillsome uranium ions in the plasma after a time delayof around 20 μs. Wachter et al. also reported that ur-anium ionic lines were significantly stronger thanatomic lines [7].In the NIST glass sample, the concentration of ur-

aniumwas as low as 462ppm. In addition, there werea total of 61 elements in the glass sample, includingcobalt, nickel, and iron, which have strong interfer-ence with the uranium atomic lines and hence makeit much more difficult to detect the uranium. For ex-ample, a cobalt atomic line (Co I at 348:94nm) is veryclose to a uranium atomic line (U I at 348:94nm), anda nickel atomic line (Ni I at 356:64nm) is also veryclose to a uranium atomic line (U I at 356:66nm).Unlike uranium atomic lines, uranium ionic linesare free from interference from most elements.Therefore, uranium ionic line transitions are moresuitable for LIBS-LIF detection of trace uraniumconcentrations. Figure 5 shows the LIBS-LIF andLIBS signals for the uranium glass when the OPOtunable laser was operated at 385:96nm. The timedelay between the two lasers was 3:5 μs, which pro-vided the best signal enhancement ratio for U II at

409:0nm. As can be seen from Fig. 5, the presence ofuranium in the sample could be determined. To ob-tain a better limit of detection for uranium in solidsamples by using LIBS-LIF, further investigationof the calibration curve for uranium concentrationare needed.

4. Conclusions

In summary, detection of uranium in solids by usingLIBS-LIF has been investigated in open air. It hasbeen shown that LIBS-LIF has high selectivityand sensitivity and is suitable for real-time and insitu analysis of trace uranium in solid samples.Although atomic and ionic lines can be selected to de-tect their fluorescence lines, uranium ionic lines aremore suitable for LIBS-LIF measurements, since thestrong uranium atomic lines have interference fromother elements. The optimal time delay between thetwo lasers was found to be around 3:5 μs. With thistechnique, a uranium concentration of 462ppm ina glass sample can be detected.

This research was financially supported by theU.S. Department of Energy (Nuclear and Radiologi-cal National Security Program, National NuclearSecurity Administration) through the cooperativeagreement DE-FC52-04NA25688. This work alsoreceived support, in part, from the U.S. Office ofNaval Research (ONR) through the Multidisciplin-ary University Research Initiative (MURI) program.

References1. D. W. Hahn and M. M. Lunden, “Detection and analysis of

aerosol particles by laser-induced breakdown spectroscopy,”Aerosol Sci. Technol. 33, 30–48 (2000).

2. D. Anglos, K. Melesanaki, V. Zafiropulos, M. J. Gresalfi, andJ. C. Miller, “Laser-induced breakdown spectroscopy for theanalysis of 150-year-old daguerreotypes,” Appl. Spectrosc.56, 423–432 (2002).

3. O. Samek, D. C. S. Beddows, H. H. Telle, G. W. Morris, M.Liska, and J. Kaiser, “Quantitative analysis of trace metal ac-cumulation in teeth using laser-induced breakdown spectro-scopy,” Appl. Phys. A 69, S179–S182 (1999).

4. R. Noll, H. Bette, A. Brysch, M. Kraushaar, I. Monch, L. Peter,and V. Sturm, “Laser-induced breakdown spectrometry—applications for production control and quality assurance inthe steel industry, ” Spectrochim. Acta Part B 56, 637–649(2001).

5. J. P. Singh, F. Y. Yueh, H. Zhang, and K. P. Karney, “A preli-minary study of the determination of uranium, plutonium,and neptunium by laser induced breakdown spectroscopy,”Recent Res. Devel. Appl. Spectrosc. 2, 59–67 (1999).

6. L. R. Carlson, J. A. Paisner, E. F. Worden, S. A. Johnson, C. A.May, and R. W. Solarz, “Radiative lifetimes, absorption crosssections, and the observation of new high-lying odd levels of238U using multistep laser phonoionization,” J. Opt. Soc.Am. 66, 846–853 (1976).

7. J. R. Wachter and D. A. Cremers, “Determination of uraniumin solution using laser-induced breakdown spectroscopy,”Appl. Spectrosc. 41, 1042–1048 (1987).

8. R. M. Measures and H. S. Kwong, “TABLASER: trace (ele-ment) analyzer based on laser ablation and selectively excitedradiation,” Appl. Opt. 18, 281–286 (1979).

Fig. 5. (Color online) LIBS-LIF (solid curve) and LIBS (dashedcurve) spectra for the uranium glass from NIST. The OPO wave-length-tunable laser was operated at 385:96nm to resonantlyexcite U II.

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9. H. S. Kwong and R. M. Measures, “Trace element laser micro-analyzer with freedom from chemical matrix effect,” Anal.Chem. 51, 428–432 (1979).

10. F. Hilbk-Kortenbruck, R. Noll, P. Wintjens, H. Falk, and C.Becker, “Analysis of heavy metals in soils using laser-inducedbreakdown spectrometry combined with laser-induced fluor-escence,” Spectrochim. Acta Part B 56, 933–945 (2001).

11. H. H. Telle, D. C. S Beddows, G. W. Morris, and O. Samek,“Sensitive and selective spectrochemical analysis of metallicsamples: the combination of laser-induced breakdown spectro-

scopy and laser-induced fluorescence spectroscopy,” Spectro-chim. Acta Part B 56, 947–960 (2001).

12. I. Gobernado-Mitre, A. C. Prieto, V. Zafiropulos, Y. Spetsidou,and C. Fotakis, “On-line monitoring of laser cleaning of lime-stone by laser-induced breakdown spectroscopy and laser-induced fluorescence,” Appl. Spectrosc. 51, 1125–1129 (1997).

13. A. Coste, R. Avril, P. Blancard, J. Chatelet, D. Lambert, J.Legree, S. Liberman, and J. Pinard, “New spectroscopic dataon high-lying excited levels of atomic uranium,” J. Opt. Soc.Am. 72, 103–109 (1982).

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