Spectrochimica Acta Part B 96 (2014) 12–20

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Spectrochimica Acta Part B

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Evaluation of laser-induced breakdown spectroscopy analysis potentialfor addressing radiological threats from a distance☆

I. Gaona, J. Serrano, J. Moros, J.J. Laserna ⁎Universidad de Málaga, Departamento de Química Analítica, E-29071 Málaga, Spain

☆ Selected paper from the 7th Euro-MediterraneanBreakdown Spectroscopy (EMSLIBS 2013), Bari, Italy, 16–⁎ Corresponding author. Tel.: +34 952 13 1881; fax: +

E-mail address: laserna@uma.es (J.J. Laserna).

http://dx.doi.org/10.1016/j.sab.2014.04.0030584-8547/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 November 2013Accepted 7 April 2014Available online 16 April 2014

Keywords:LIBSRadiological threatDirty bombStandoff sensing

Although radioactivematerials are nowadays valuable tools in nearly all fields ofmodern science and technology,the dangers stemming from the uncontrolled use of ionizing radiation aremore than evident. Since preparednessis a key issue to face the risks of a radiation dispersal event, development of rapid and efficient monitoringtechnologies to control the contamination caused by radioactive materials is of crucial interest. Laser-inducedbreakdown spectroscopy (LIBS) exhibits appealing features for this application. This research focuses on theassessment of LIBS potential for the in-situ fingerprinting and identification of radioactive material surrogatesfrom a safe distance. LIBS selectivity and sensitivity to detect a variety of radioactive surrogates, namely 59Co,88Sr, 130Ba, 133Cs, 193Ir and 238U, on the surface of common urban materials at a distance of 30 m have beenevaluated. The performance of the technique for nuclear forensics has been also studied on different modelscenarios. Findings have revealed the difficulties to detect and to identify the analytes depending on the surfacebeing interrogated. However, as demonstrated, LIBS shows potential enough for prompt and accurate gatheringof essential evidence at a number of sites after the release, either accidental or intentional, of radioactivematerial.The capability of standoff analysis confers to LIBS unique advantages in terms of fast and safe inspection offorensic scenarios. The identity of the radioactive surrogates is easily assigned from a distance and the sensitivityto their detection is in the range of a few hundreds of ng per square centimeter.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Radioactivity refers to the emission of ionizing (high-energy) radia-tion, namely alpha (α), beta (β), X or gamma (γ), from the spontaneousand random transformation (radioactive decay) of an unstable atomicnucleus, resulting in new elements or a lower energy state of theatoms [1]. Most of the ionizing radiation we are exposed to (about 82%of the total) consists of natural background radiation from radon gasand other natural terrestrial sources (radioactive elements in rocks,soil, water, and plants), whereas the remaining 18% is from man-madesources. Since the discovery of X-rays in 1895, man-made radiationhas been becoming a very common tool that lets us do many thingsthat would be impossible without it [2]. Thousands of new practicaland beneficial uses of ionizing radiation have been devised till itscomplex structure within the present-day life [3]. Ionizing radiation isubiquitous and indispensable for medical techniques and practices ofdiagnosing and treating – e. g. for identifying broken bones and healingof tumors [4] – as well as on nuclear power plants for sustaining

Symposium on Laser Induced20 September 2013.34 952 13 2000.

exothermic processes to generate heat and electricity [5], to mentionthe most common. Its use also extends to metabolic studies, geneticengineering and environmental protection studies. The ionizingradiation from radioisotopes is also used in agriculture to breed newseed varieties and to preserve different varieties of food [6]. Similarly,it is applied in industry, manufacturing and engineering for improvingthe quality of manufactured goods [7].

Although the benefits of radiation are more than apparent, livingand working with it is not only hazardous but also may entail risksthat cannot be overlooked. With a view to putting in perspective themenaces from ionizing radiation, all of them can cause to any form oflife, whether it be human, plants, or animals many adverse conse-quences not just at a cellular level but on a genetic level as well. Theamount of damage caused by ionizing radiation depends on its half-life and on both the dose and the period of exposition.

Radiological dispersal events from nuclear and radioactive sourcesmay arise from an unfortunate natural disaster. Just to cite the mostrecent example, the incidents at Fukushima nuclear reactors becauseof the earthquake and the subsequent tsunami of March, 2011, demon-strate the extreme dangers associated to these events. Furthermore,malicious actions by those who seek to unleash a radiological mayhemare todaymore than a real threat [8]. In these cases, themost commonlyused civilian radiation sources are malevolently exploited to prepare aradiological dispersive device (RDD), also known as “dirty bomb”,

13I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

using conventional explosives to pulverize and disperse the radioactivematerial.

Preparedness to establish the release or presence of an injuriousagent in a given location is a fundamental interest for any successfulresponse policy against these threats. Sensing technologies are neededat the three stages of the dispersal incident: I) before, allowing acontinuousmonitoring either to prevent the incident (detect-to-protect)or for early warning in the case that the incident occurs (detect-to-treat); II) during, allowing first responders to identify the precise nature(detect-to-identify) and the extent (detect-to-quantify) of the releaseand to organize the response accordingly; and III) after, allowingconfirmation either on the persistence of the threat or on a completearea decontamination (detect-to-confirm). Analytical detection thuscontributes to at least four of the core objectives of civil protection,i.e. prevention, protection, response and recovery.

A variety of field deployable and hand-held portable instrumentsare available for detecting and measuring radioactive materials [9].Hand-held survey meters are able to search, locate, and detect alpha(α), beta (β), gamma (γ), and neutron radiations. However, releasingthe radionuclide identity is available only for some particular emittingsources. For instance, while radiation from 90Sr – a pure beta emitter –can be easily detected and measured with a Geiger–Mueller surveymeter connected to a pancake probe, it will not be detected by a sodiumiodide instrument or other types of gamma identification devices. Inaddition, some instruments saturate and provide low or no readings ina very high radiation field [10].

Thus, since one of the most critical steps in first responding to aradiological disaster is the identification of the emitter present, it is inthis direction that laser-induced breakdown spectroscopy (LIBS) mayprovide a fruitful and meaningful contribution. Technology based onLIBS has appealing features for radiological analysis including thecapacity for fast and simultaneous detection of almost all elements, insitu operation using field-deployable designs, and contactless inspec-tion with nanogram sensitivity. From these properties, LIBS-basedsensors may be able to make a quick determination of the elementresponsible for the radiation, and the distribution profile of the elementin the contaminated zone. It is not a coincidence that LIBS trackinginstruments have become the most complete sensing systems to applyto chemical, biological, radiological, and nuclear (CBRN) threats.

The potential of LIBS to detect and discriminate chemical andbiological warfare surrogate agents on a variety of substrates and inthe presence of interferents has been explored [11,12]. Detection ofaerosolized CBW agents has been also studied [13]. A number of studieshave evidenced the extensive experience of LIBS on recognition ofexplosive residues [14–17]. The use of LIBS for the design of a potentialrapid in situ field portable unit for nuclear safeguard inspection, envi-ronmental surveillance and detecting weapons of mass destruction[18], as well as in-field forensic applications [19] is currently beingexplored. Interests have been mostly focused on open air detection ofuranium in solids [20–22], as a residue on aluminum, plastic and ceramicsurfaces [18], and as a surrogate of highly radioactive glass waste [23].Detection of other surrogates for nuclear sensitive materials such asstrontium [24,25] and cesium [25] has been also discussed.

The present investigation focuses on the assessment of the LIBSpotential in benefit of designing a field deployable sensor to tackle theradiological sensing from a distance. The measurement range in theseexperiments has been established in 30 m (the minimum operatingdistance for our sensor). A number of tasks including identification ofsurrogate radiological residues and establishing their detectableamounts have been faced. Selectivity and sensitivity of LIBS to confirmdifferent elements, namely 59Co, 88Sr, 130Ba, 133Cs, 193Ir and 238U, oversurfaces linked to common urban materials have been verified. Solidsupports have been used to emulate the sensing procedure at suspectedlocations as most of the airborne radioactive dust has settled to theground in about 10 min. The LIBS power for forensic analysis has beenalso tested.

2. Experimental

2.1. LIBS sensors

The standoff LIBS sensor used in the present investigation has beendescribed in detail in previousworks [26,27], so only an outline summa-rizing the key features is quoted here. A scheme of the instrument ispresented in Fig. 1. The distance from the instrument to the samplesinspected is 30 m. A Q-switched 1064 nm Nd:YAG twin laser system(10 Hz, 850 mJ pulse−1, 5.5 ns pulse width) is used as an excitationsource. Both delivered laser beams are spatially overlapped, but with atemporal delay (600 ns) inherent to the electronics. A flexible deliveryof laser beams towards a telescope is reached through a waveguidingarticulated arm that integrates 5 laser mirrors. At the exit of the opticalarm, a fused-silica dichroic mirror directs the laser beam towards aclassical Cassegrain telescope (400 mm aperture, open truss design). Itshould be noted that this dichroic mirror system reduces the transmis-sion performance in the UV and IR regions. Particularly, these dips atboth edges of the spectrum coincide with the third harmonic and theown fundamental Nd:YAG radiation for which the reflectance of thismirror is designed to be a maximum. This effect is inherent to reflectivedielectric coatings and cannot be avoided [28]. The telescope ismounted on a fork that allows it to move up-and-down (altitude) andside-to-side (azimuth) and fitted with an autofocus system, thereforeallowing the scan of large areas at changing distances. The telescopemeets a dual role, first focusing the laser pulses on the distant target,and then, gathering the light from the produced plasma plume. Theplasma light collected by the telescope is imaged into the tip of a600 mm optical fiber (four-furcated cable, 4 × 600 mm fibers, all legsSMA terminated, total 2 m long, splitting point in the middle) thatguides the light to the entrance of a four-channel miniature Czerny–Turner spectrograph (75 mm focal length). The spectrometers are eachfitted to a CCD detector. With this device, an effective spectral windowranging from 350 nm to 950 nm is available. Default timing parametersof 1.28 μs for delay time and 1.1 ms for integration time are synchro-nized to the shooting of the laser pulse for LIBS data acquisition.

For performance comparison, a similar laboratory setup has been alsotested. The excitation source is a Q-switched 1064 nm Nd:YAG laser(20 Hz, 54 mJ pulse−1, 8 ns pulse width). The laser beam is deliveredto the target through an optical train involving a 1064 nm plane mirrorand a 75-mm focal length plane-convex quartz lens. Light from thecreated plasma is collected using a collimating lens securely attachedto the tip of the optical fiber. The emission response is gathered usingthe same detection device as in the standoff configuration.

In both approaches, laser spot size and laser energy at thetarget have been matched to get identical irradiance at the sample(3 GW·cm−2) for plasma plume production.

2.2. Samples

In this attempt to elemental detection and quantification of potentialradioactive sources, surrogate, non-radioactive materials have beenoperated. Hydrated salts including Co(NO3)2·6H2O, SrCl2·6H2O,UO2(CH3COO)2·2H2O, and IrBr3·xH2O, have been used as Co, Sr, U andIr suppliers, respectively. Other inorganic compounds including BaSO4

and Cs2CO3 have been utilized as convenient sources of Ba and Cs,correspondingly.

Starting from the idea that a LIBS sensor is aimed to assess an areacontaminated with radioactive material, several common items easilyfound in urban areas have been examined: aluminum from trafficsigns, glass from bus shelters, building bricks, concrete and pavement(sidewalk). These objects have been used as supports for locating theresidues. For identification and characterization of the surrogates ofinterest, the compounds have been spread onto the raw surface (with-out any physical treatment such as a buffing to smooth) of each supportin milligram quantities. In parallel, surface concentrations of elements

Optical fiber

Czerny–Turner spectrographs

Articulated optical arm

Nd:YAG lasers (@1064 nm)

Cassegraintelescope

PC

Target

Fig. 1. Schematic diagram of the assembling of the key elements of the LIBS apparatus for standoff surveillance.

14 I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

at few (0.5–2.0) mg per square centimeter level have been disposed toascertain the LIBS detection capability at a distance.

3. Results and discussion

Implicit in risk assessment during the inspection of a contaminatedzone is the identification and quantification of the radiation sourceaccording to the presence or absence in the operational scenario of aparticular analyte. In the successive sections, a discussion on the capa-bilities of LIBS for standoff detection and quantification of radioactivesurrogates over distinct surfaces is presented. The ability for a paralleldetection of the radioactivity source and the corresponding explosivepropellant after the possible detonation of a “dirty bomb” has beenalso appraised.

3.1. Detection specificity

First, identifiable spectral features of radioactive surrogates on thesurface of tested objects were established. The identification capabilityof a residue may be affected by the substrate composition, since partof the support is usually ablated together with the material of interest.Accordingly, elements composing the substrate contribute to theplasmaplume and, therefore, are also involved in the emission observed thusreducing the probabilities of a positive finding. Fig. 2 shows the standoffemission spectra of the tested supports. As observed, the LIBS spectrumof aluminum is quite simple. Sensitive atomic lines at 394.40 nm and396.15 nm together with the emission sequence of gaseous aluminumoxide (AlO) are easily identifiable [29]. In contrast, the spectrum ofglass exhibits virtually no spectral features. This fact is due to the lowoptical absorption of glass at 1064 nm that hampers the heating andsubsequent vaporization of the surface to result in plasma formation.The spectrum of glass scarcely shows emission signals from its mainconstituents (SiO2, Na2O, CaO, MgO and K2O) [30]. On this substrate,emissions from the analytes are free from spectral interferences and abroad selection of lines can be used for identification purposes. Anentirely opposed situation occurs with the spectral profiles of mortarand clay. As shown in Fig. 2, the spectra of these materials are full of

spectral features as a result of their multielemental composition —

Al2O3·2SiO2·H2O and varying amounts of oxides from Ca, K, Mg andNa. Strong molecular emissions of CaO and CaOH in the spectral region(540 nm–660 nm) are also observed [31].

Beyond the greater or lesser richness of emission features within thesupport signal, effectiveness in identifying the presence of an analytedepends on the potential masking between spectral features of both,residue and substrate [32]. In this connection, Table 1 lists the mostprominent atomic and ionic emission lines that identify each element.As noted, the analytical use of these lines depends on the spectralcharacteristics of the substrate where the residue is located. Whileover glass, all the LIBS features are interference-free, in the rest ofsupports, only specific lines are suitable to prove the presence of thespecies in the target.

As an example of the LIBS detection capability, Fig. 3 depicts theemission spectra of plasmas from the different radioactive surrogateswhen located on aluminum.As shown, regardless of the signal intensity,at least one spectral feature within the spectrum is free from interfer-ence, thus allowing identification of each surrogate element. In thespectra, a number of oxide bands are evidenced. In the case of Sr,emission bands from strontium dioxide radicals appear in the spectralregion covering from 590 nm to 690 nm [24], whereas the lines of AlOare absent. Similarly, AlO is absent in the spectrum of Ba, which exhibitslines of barium oxide, a weak emitter in the yellow and green regions ofthe spectrum (495 nm–590nm) [33]. The absence of AlO in thesemate-rials is due to the thermodynamically favored reaction of dissociatedatmospheric oxygen with Sr and Ba atoms evaporated from the sampleas compared to the reaction of oxygen with aluminum. By contrast,cobalt and iridium do not exhibit emissions from their correspondingoxides [34,35]. Although these elements have also the capability toform oxygen clusters, emissions from these compounds are not observedin the spectrum.

As a result of their similar affinity for oxygen, Cs and U oxide bandsare accompanied by emissions of AlO. Emissions of CsnOm clustersappear in the blue-green region of the spectrum near 550 nm, close tothose of AlO. However, emissions from UO2, UO3, and U3O8 [36] occurin the same region than those from AlO – spanning from 475 nm to

Wavelength (nm)350 450 550 650 750 850 950

Al(I)

Na(I)

Ca(II)

CaO

Ca(I)Mg(I)

K(I)

Mg(I)

AlO

CaO

Ca(I)

Mg(I)

Ca(II)

Al(I)

Si(I)

Fe(I)

Fe(I)Sr(I)

380 390 400 410Wavelength (nm)

Ti(I)

Al(I) 2nd order

Hα

O(I)

N(I)

Ca(I)

Ca(I)Ca(I)

Ca(I)

Ca(I)

Fig. 2. Standoff LIBS spectra of the tested blank supports.

15I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

600 nm –, thereby causing a superposition of both spectral systems.The dominance of AlO in the spectrum is due to the minute amount ofuranyl residue deposited on the aluminum support.

Despite themany factors contributing to the spectral behavior of theconcerned elements, it can be argued that their oxygen affinity is wellmatched with their electronegativity (χ − using the Pauling scale) aswell as with their electronic structures. For instance, while Ba (χ =0.89) and Sr (χ= 0.95) are alkaline earthmetal elements, a relationshipexists between Co (χ = 1.88) and Ir (χ = 2.20), which are Group 9elements. The alkaline-earth metals have electronic configurationswith a filled s subshell but easily lose electrons to become positiveions (cations). As a result, these elements having a lower electronegativ-ity as compared to that of Al (χ= 1.61) exert a stronger attractive forceover the oxygen. Due to the high plasma temperature and the excessoxygen, Ba and Sr are able to form oxides (O2−), as well as peroxides(O2

2−). Contrarily, the transition metals are characterized by a partially-filled d subshell in their free elements and cations. Consequently, theirrelatively high electronegativity implies a lower or no flexibility informing oxides in the case that the Al or other elements having lesselectronegativity are present. In any case, although the formation ofall these oxides leads to a general alteration, mainly a decrease onintensity, of the atomic and ionic spectral signals of the elements dueto the depletion of such species within the plasma plume, their occur-rence within the emission spectrum – clearly confirmed in Fig. 3(A) –,may also act as an exclusive marker indicating the presence of aparticular element.

3.2. Potential for detection

The sensitivity of a sensor is indicated by the minimum amount ofa particular analyte that must be present in the analysis scenario in

order to warn with guarantees about its presence. In order to checkthe detection potential of the standoff LIBS method, the limit ofdetection (LOD) of the considered elements on different surfaces hasbeen studied. For this purpose, microvolumes of aqueous solutionsfrom concerned salts – at thousand ppm level – were homogeneouslydispersed over each support. Once thewater was evaporated, ten singlelaser shots were directed on refreshed locations within the area. Fromthe average of these ten emission responses, LODs have been computedby using the following equation:

LOD ¼ 0:01 � 3 � RSDbckgrnd � CSnet

Sbckgrnd

where RSDbckgrnd is the relative standard deviation (expressed inpercent) of the spectral background, Snet is the net emission intensityof the line considered, Sbckgrnd is the intensity of the background signal,and C (expressed inmass per unit area) is the calculated surface amountof analyte exposed to the laser shot (depending on the laser spot sizethat ablates the targetedmaterial) [37]. Except in self-reversed or inter-fered lines, LODs have been calculated from themost sensitive emissionline for each particular analyte. Table 2 lists the standoff detection limitsfor the radionuclide surrogates in terms of mass of the element per unitarea. For comparison purposes, the corresponding detection limitsachieved in close-contact LIBS are also reported.

Standoff LODs vary from a few up to several hundred microgramsper square centimeter. Apart from the inherent spectral properties ofthe elements, some differences in the LODs on a same support mayarise in connection with the capability for oxide formation. The LOD ofan element may change since oxide formation alters the population ofits atomic and ionic states. Clear examples are the cases of Sr and Ba

Table 1Level of selectivity for the most relevant emission signals from the radioactive surrogates depending on the surface where located.

Surrogate radionuclide Emission wavelength (nm)Supports

Aluminum Glass Mortar Clay

59Co 350.23 (I)

352.98 (I)

356.93 (I)

389.41 (I)

399.53 (I)

411.88 (I)

412.13 (I)

88Sr 407.77 (II)

421.55 (II)

460.73 (I)

679.10 (I)

687.83 (I)

707.01 (I)

868.89 (II)

130Ba 455.40 (II)

493.41 (II)

553.55 (I)

614.17 (II)

649.69 (II)

728.03 (I)

133Cs 455.53 (I)

459.32 (I)

672.44 (II)

697.33 (I)

852.11 (I)

894.35 (I)

193Ir 357.37 (I)

362.87 (I)

380.01 (I)

397.63 (I)

439.95 (I)

238U 385.96 (II)

393.20 (II)

409.01 (II)

502.74 (I)

591.54 (I)

682.69 (I)

16 I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

on aluminum. Despite the low LODs reported for these elements, thecapability for their detection is underestimated. Their tendency toform emitting oxides (as shown in Fig. 3(A)) leads to less intense linesfrom atoms and ions. That is why standoff LODs substantially lower(better) than 17 μg cm−2 and 2 μg cm−2 should be expected for Srand Ba, respectively.

Differences in the LOD of a particular analyte in different surfacesresult also from the chemical and physical properties of the complete

matrix [38]. Although the dependence of the LODs with the substrateis not tied to any fixed pattern, it is possible to extract some detailedinfluences. Despite the low ablation rate induced in pure glass, efficientdetection of elements is ensured at this support. This circumstanceis due to the presence of the compound itself, which leads to a largeroptical absorption. Furthermore, the scarce contribution of thesupport to the final emission signal leads to a larger signal-to-noiseratio.

Wavelength (nm)350 450 550 650 750 850

Wavelength (nm)350 450 550 650 750 850 950

(A) (B) (C)

Wavelength (nm)350 450 550 650 750 850 950

Wavelength (nm)350 450 550 650 750 850 950

Wavelength (nm)350 450 550 650 750 850 950

Wavelength (nm)350 450 550 650 750 850 950

Co(I)412.13 nm

Sr(I) 707.01 nm

Ba(II) 649.69 nm Ir(I)

439.95 nm

Cs(I) 697.33 nm

U(I)591.94 nm

950

Fig. 3. Standoff LIBS spectra for different residues of radioactive surrogates on aluminum. From left to right: (A) Sr and Ba, (B) Co and Ir, and (C) U and Cs. The spectrumof the blank supportis also displayed for comparative purposes. More details in the body of the text.

17I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

On the other side, although intense plasma plumes are produced inaluminum, mortar and clay surfaces, significant differences in detectionsensitivity are evidenced. The variations in the LOD for a same elementon these different matrices is elucidated through the properties of theplasmas formed. In order to verify this extreme, the electron tempera-ture (Te) and electron number density (Ne) of the plasmas of bariumresidues on these supports have been calculated. Electron temperatureswere calculated using the following equation:

lnλmnImn

Amngm

� �¼ − Em

KTeþ ln hcNð Þ

where λmn is thewavelength of the transition lines, Imn is the integratedline intensity of the transition involving an excited level (m) and alower level (n), Amn (s−1) is the transition probability, gm (s−1) is thestatistical weight of the excited level (m), Em (eV) is the energy of theexcited level, K (eV K−1) is the Boltzmann constant, h (J s) is the Plank'sconstant, c (m s−1) is the speed of light, and N(T) (m−3) is the totalnumber density [39]. By plotting the left hand side term vs. Em, theplasma temperature is obtained from the slope of the straight line.Electron number densities were computed from the line profiles of theisolated barium line (Ba II) at 649.69 nm [40]. Although Ne is associatedwith a global contribution of different broadening mechanisms, it wascomputed from the measured full-width at half maximum (FWHM) of

Table 2LIBS LODs of radioactive surrogates when encountered on surfaces of different common urban

Surrogate isotope Emission line (nm) Aluminum Glass

Close-contact Standoff Close-con

59Co 412.13 (I) 25 25 10588Sr 707.01 (I) 13 17 75130Ba 649.69 (II) 2 2 25133Cs 697.33 (I) 15 7 190193Ir 439.95 (I) 20 11 50238U 591.54 (I) 30 21 80

a Limits of detection have been calculated from depositions of iridium salt in solid form due

the Stark broadened line profile. The instrumental broadening contribu-tion was subtracted. The following relation was used:

Δλ12¼ 2ω

Ne

1016

� �

where ω is the electron impact width parameter.Fig. 4(A) shows the Boltzmann plots obtained in the three supports

considered. The slope of the curves yields Te values of 14,000 K, 6500K and 3700 K in aluminum, clay and mortar, respectively. In the samevein, Fig. 4(B) plots the correspondence between the conditions for Baresidue plasmas and the associated LOD at each support (Table 2). Asobserved, large values of temperature and density of the plasmaplume lead to the best detection. Consequently, the LOD calculated inthe aluminum surface is better than that in mortar and clay.

Regarding the sensitivity of standoff LIBS when compared to theclose-contact counterpart, no significant differences in the LODs of Co,Sr, Ba and U are observed. Contrarily, the standoff LODs of Cs and Ir areeven better than those calculated in close-contact. Such results areunforeseen, to some extent, since, if all operating conditions were iden-tical (in terms of laser energy and spot size), the standoff LIBS signalshould be smaller than the proximity measurement due to the inversesquare law drop of the signal with the range [41]. Hence, LODs shouldbe predictably larger for standoff mode as compared to close-contactframework. However, in the standoff measurements the area sampled

materials.

LOD (μg cm−2)

Mortar Clay

tact Standoff Close-contact Standoff Close-contact Standoff

133 155 127 300 23086 125 153 140 19312 23 27 8 948 1050 307 750 44020 280 a 107 a 225 a 120 a

69 150 167 320 250

to the matrix induced effects when the aqueous solutions were used.

AluminumClay

A

B

Mortar

Em(eV)

mnImn

Amngm

Ln

2.8 2.9 3.0 3.1 3.2 3.3-26

-25

-24

-23

-22

-21

Mortar

Aluminum

Clay

2000

4000

6000

8000

10000

12000

14000

16000

Te

(K)

LOD (µg cm-2)

Ne(

cm-3 )

x1016

20

18

16

14

1.6

1.8

2.0

0 5 10 15 20 25 30

λ

Fig. 4. (A) Saha–Boltzmann plots obtained from the spectral emissivity of selected lines ofBa for plasma temperature calculation on aluminum (black), clay (blue) andmortar (red)supports and (B) dual correspondence between the electron temperature (●) and theelectron density (♦) characterizing the plasmas of Ba residues on the concerned supportsand the associated sensitivity.

18 I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

by the laser beam is by far larger than that excited in the close-contactapproach. Consequently, the standoff spot size of 1500 μm in diameteris much more crucial for sampling than that of 450 μm for close-contact approach; a circumstance that also mitigates the lack ofuniformity on the distributions of the residues. Nonetheless, differencesin LODs between approaches are much more pronounced whenabsolute amount is computed. For instance, the 280 μg cm−2 and the107 μg cm−2 of Ir, that are detected in mortar, turn into quantitiesof ca. 0.5 μg and 2.0 μg, for close-contact and standoff configurations,respectively. In any case, whatever the approach, the technique exhibitsa large sensitivity for the considered elements.

3.3. Identification of forensic evidences [42]

In the present section, the suitability of the LIBS portable device forfingerprinting radioactive surrogates in potential scenarios of nuclearforensics has been investigated. Two modeled scenes on the post-detonation of a dirty bomb and the accidental release of radioactivematerial have been considered.

3.3.1. Premeditated radiological dispersionThe investigation of a dirty bomb scene mainly seeks identifiable

chemical evidences at witness debris materials close to the “actual” tar-get to determine any type of intention. Nonetheless, these compellingevidences are very limited. As the afterburning products resultingfrom the detonation of an organic explosive are mainly water vapor

and gases like CO,H2 andN2, the expected post-blast residues are limitedto solid carbon and traces of the non-reacted explosive accompanied byseveral interfering impurities, in amounts that are orders of magnitudehigher [43]. In this context, two LIBS sensing tests over supposed debrisresulting from detonations by trinitrotoluene (TNT) of Cs- and Co-baseddirty bombs have been performed. Aluminumplates (15× 15 cm2) havebeen used as debris, emulating fragments of the sealedmetal containersthat store the radioactive source. Particles from TNT-surrogate solidmixtures at variable proportions (from 0%–100% w/w up to 100%–0%w/w) have been dispersed between well-defined areas (2 × 10 cm2

each) over the aluminum surfaces. The CN molecular signal and theintense atomic emissions of Cs at 697.33 nm and Co at 412.13 nmhave been used asmarkers of the explosivematerial and the radioactivesurrogates, respectively.

Fig. 5 depicts the spatial mappings on the composition and distribu-tion of the dispersed evidence. As shown, in the case of TNT:Csmixtures(left panels), the chemical screening for both the explosive and theradioactive surrogate is accurately described regardless of the concen-tration ratio studied. For the TNT:Co mixtures (right panels), someevidences pass unnoticed. As seen, TNT is virtually not detected unlessit is at a ratio of 75% to the radioactive surrogate (upper map). Likewise,cobalt (map below) is not detected even when it accounts for a onequarter of the mixture. In contrast, in the hypothetical case of a Cs-based dirty bomb, both the element and the explosive are detectedwhatever the concentration in the mixture is. Thus, from the point ofview of a positive finding, the interrogation of this scenario revealsevidence of a detonation. The simultaneous detection of TNT and Cs ata proportion of 25:75 fits with a minute amount of explosive, whichhas survived the blast, within a relatively large amount of radioactivematerial.

3.3.2. Accidental radiological releaseFor the study of the accidental release of radioactive material to the

environment, a well-delimited area (24 × 24 cm2) within the surface ofa rustic terracotta flooring tile has been contaminated with residues ofIr, Sr, Co, and Cs. Identification tests have been organized across thesurface which was divided into four quadrants (12 × 12 cm2 each),one for each element. The inspected target has been located at 30 mfrom the sensor and scanned with a lateral resolution of 1 cm. Fig. 6shows the photograph of the prepared scenario together with theresulting chemicalmaps associated to the overall 24 cm×24 cm sectionwhen theparticular emission of each analyte ismonitored. The emissionlines used were 439.95 nm for Ir, 707.01 nm for Sr, 412.13 nm for Co,and 697.33 nm for Cs. As observed, standoff LIBS analysis along thenormal direction to the interrogated surface fits neatly with the spatialdistribution profiles of the original depositions for the concerned ele-ments. The particular identity of each surrogate is accurately monitoredwithout interferences from other more outstanding emissions withinthe spectral signal.

4. Conclusions

The present work deals with assessing the potential of LIBS as a fieldsensor for monitoring of radiological threats from a safe distance. Fromthemeasurement of a variety of radioactive surrogates – Co, Sr, Ba, Cs, IrandU –, LIBS has displayed a high selectivity to detect the presence of allthese elements on the surface of different common urban materials.This study also confirms that the sensitivity of LIBS is satisfactory forthe distant detection of these materials. Standoff LIBS is an effectiveoption to complete the suite of available radiation detection technolo-gies on gathering chemical evidence at a disaster area.

Acknowledgments

This work was supported by the Project CTQ11-24433 of theMinisterio de Economia y Competitividad, Secretaría de Estado de

CN

(38

8.38

nm

)

Cs

(I)

(697

.33

nm)

Co

(I)

(412

.13

nm)

CN

(38

8.38

nm

)

100:0 75:25 50:50 25:75 0:100 (% w/w) 100:0 75:25 50:50 25:75 0:100 (% w/w)

(A) (B)

Fig. 5.Mapping of chemical distribution of evidence for hypothetical aluminumdebris corresponding to simulated post-detonations of two dirty bombs: Cs-TNT (left panels) and Co-TNT(right panels). Intensitymaps, by laser-assisted standoff emission spectroscopy, of evidencesmixtures atfive variable levels – vertical stripes – for the explosive (CN band at 388.83 nm) aswell as the radioactive surrogates, Cs (697.33 nm) and Co (412.13 nm), respectively.

Ir

Co

Sr

Cs

Fig. 6.Mapping of residuals of different radioactive surrogates over a terracottaflooring tile. Intensitymaps, by laser-assisted standoff emission spectroscopyof evidence for Ir (439.95nm),Sr (707.01 nm), Co (412.13 nm), and Cs (697.33 nm), respectively.

19I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

20 I. Gaona et al. / Spectrochimica Acta Part B 96 (2014) 12–20

Investigación, Desarrollo e Innovación of Spain and from the Consejeriade Innovación, Ciencia y Empresa de la Junta de Andalucía (Project P07-FQM-03308). I. Gaona also thanks the concession of her FPI researchcontract, through the CTQ2007-60348 Project, to the SpanishMinisteriode Economía y Competitividad, Secretaría de Estado de Investigación,Desarrollo e Innovación.

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