2762 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 5, OCTOBER 2010

Matrix Characterization of the Sea Floorin the Threat Material Detection Processes

Jasmina Obhodas, Davorin Sudac, and Vladivoj Valkovic

Abstract—Results obtained by using the fast neutron activationanalysis (FNAA) with the associated alpha particle for under-water analysis (salinity of 38 g/L) of Ca, Mg-rocks, and Si-rocks,which are the main types of rocks found in the Adriatic Sea, havebeen presented. Geochemical data of coastal sea sediments in theAdriatic Sea acquired by the energy dispersive X-ray fluorescence(EDXRF) analysis are also presented. The data obtained can beused to evaluate the background for neutron sensor underwaterapplications used in threat material analysis.

Index Terms—Adriatic Sea, coastal sea sediments, energy disper-sive X-ray fluorescence (EDXRF), fast neutron activation analysis(FNAA).

I. INTRODUCTION

M ATRIX characterization of sea sediments and rocks is akey for the application of the neutron systems in under-

water threat materials inspections. As used specifically herein,threat material refers primarily to military explosives in objectssuch as mines, grenades, torpedoes, different types of ammuni-tion, and chemical warfare disposed on the sea floor. Ammuni-tion dumpsites, beingmostly unprotected and neglected, presenta serious threat to human security (underwater explosions) andenvironmental security (corrosive action and leakage of chem-icals) and could be objects of possible misuse (terrorist activi-ties). In addition to explosives, a possible presence of chemicalwarfare agents (CWAs) in the suspicious objects imposes evenmore serious threat.Fast neutron activation analysis (FNAA) with the associated

alpha particle using the sealed tube neutron generator [1]is used for noninvasive assessment of the composition or con-tent of objects that are closed or otherwise inaccessible. In the

reaction, a 14-MeV neutron and an alpha particle that areemitted nearly back to back are produced. The portable neu-tron generator is capable to produce fast (14 MeV) neutron irra-diation with a maximum output up to 10 n/s by . Irradiationof an object by fast neutrons induces reactions in which gammarays are produced. Detection of gamma rays enables chemicalcomposition evaluation of the activated material. These gammarays are detected in coincidence with the associated alpha par-

Manuscript received June 24, 2009; revised October 27, 2009 and January 02,2010; accepted January 08, 2010. Date of current version October 15, 2010. Thiswork was supported in part by the EU-FP7 Project No. 218148 UNCOSS—Un-derwater Coastal Sea Surveyor and by the Croatian Ministry of Science, Edu-cation and Sport.J. Obhodas and D. Sudac are with the Department of Experimental Physics,

Institute Ruder Boskovic, Zagreb 10000, Croatia (e-mail: jobhodas@irb.hr;dsudac@ irb.hr).V. Valkovic is with the A.C.T.d.o.o., Zagreb 10000, Croatia (e-mail:

valkovic@ irb.hr).Digital Object Identifier 10.1109/TNS.2010.2040629

ticles, allowing determination of neutrons direction and time offlight. In such a way, the origin of the gamma rays can be deter-mined, allowing chemical composition evaluation along depthprofiles of the inspected object in only one measurement.Recently, it has been shown that the FNAA with the associ-

ated alpha particle can be used in underwater applications [2].The particular advantages of this technique in threat material un-derwater inspection are fast interrogation, reliability, high pen-etration, the possibility of incorporation in mobile underwatervehicles, and the possibility of obtaining three-dimensional in-formation about the composition of an object under inspection.Since the inspected military objects disposed on the sea floorare usually large and dense (the presented neutron probe is notsensitive for objects under 1 kg) and should not be moved if thestate of the detonator is not known, the FNAA with the associ-ated alpha particle is the most suitable method for the task.The coastal sea is the area where human activities are very in-

tensive, and therefore the security risk arising from the presenceof threat material has to be minimized. Most of the backgroundin the gamma spectra measured when investigating suspiciousobject on the bottom of the sea is generated by the surroundingsea sediments and rocks. Therefore, the chemical compositionof sea floor sediments and rocks has to be known. Since theAdriatic Sea is of a special interest to us, we have producedgeochemical maps of coastal sea sediments based on samplingpoints distributed approximately every 8 km of the eastern Adri-atic coast in order to examine possible interferences of sedi-ment chemical composition with the threat material inspection.By producing geochemical maps, we can localize areas of in-creased concentrations in sediments, present either because ofanthropogenic influence or natural variations in the geologicalbackground, where interferences with the threat material anal-ysis can be expected.In addition, it has to be noted that after years and decades of

lying on the bottom of the sea, dumped ammunition could vi-sually change to an unrecognizable appearance, especially foruntrained eyes. By using visual inspection only, the military ob-jects can be easily mistaken for rocks, and vice versa.The geochemical maps have been produced based on the

results obtained by the energy dispersive X-ray fluorescence(EDXRF) analysis. The EDXRF is the most often used tech-nique for sediment analysis. It is a mature technique, verysuitable for bulk samples and very fast, and requires a smallamount of a sample, 2 g or less. It cannot measure lightelements that constitute explosives (C, O, H, and N), but itcan provide information on additional elements in sediments

, which present background and therefore caninterfere with the signal from the threat material, especially inchemical warfare inspection.

0018-9499/$26.00 © 2010 IEEE

OBHODAS et al.: MATRIX CHARACTERIZATION OF THE SEA FLOOR IN THE THREAT MATERIAL DETECTION PROCESSES 2763

Fig. 1. Prototype of the underwater system “Surveyor” positioned in the labo-ratory test pool for underwater measurements.

II. EXPERIMENTAL SETUP

A. Neutron ProbeThe experimental work regarding the underwater qualitative

chemical composition analysis by using a neutron probe basedon FNAA with the associated alpha particle has been conductedin the Laboratory for Nuclear Analytical Methods of the RuderBoskovic Institute (Zagreb, Croatia). The prototype of the un-derwater system “Surveyor” (Fig. 1) containing a neutron gen-erator, shielding, and gamma ray detector (NaI scintilator) hasbeen used [1].The neutron generator used in the “Surveyor” can be rotated

by two stepmotors so that different volumes of elements, chosenby the relative position of the neutron generator and gamma raydetector, can be inspected. In such a way, a chemical composi-tion profile could be measured. The gamma detector is placedinside an aluminum holder of thickness 5 mm. The detector isshielded on one side by a 5-cm-thick lead.The submarine body is made of polyester and Kevlar, with

a wall thickness of 1.5 cm. Below the detector, there is an alu-minum window of dimensions 46 26 0.5 cm .

B. EDXRF ProbeCroatia has 5835 km of coastline (1777 km of mainland and

4058 km of islands). Over 700 sediment samples have beencollected and analyzed so far (Fig. 2) for the concentrations of16 chemical elements (K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga,As, Br, Rb, Sr, Y, Pb) by tube-excited EDXRF.Measurements were performed with a W anode (60 kV,

60 mA), and Mo secondary target in orthogonal geometry,with measurement parameters of 35 kV and 35 mA. The ir-radiation time was 1000 s. The X-ray spectra were collectedwith a Si(Li) detector ( eV at 5.9 keV) andanalyzed by using the IAEA’s QXAS program package “Directcomparison of count rate” method. As a standard referencematerial (SRM), the IAEA SL-1 has been used. The energiesof principal emission lines were used for the quantizationof all measured elements except for Pb, for which Pb wasused. The SRM SL-1 has been measured in regular intervals,after every 10 samples.

Fig. 2. Coastal sea sediment sampling points in seven Adriatic Sea counties:1) Istarska; 2) Primorsko-goranska; 3) Licko-senjska; 4) Zadarska; 5) Sibensko-kninska; 6) Splitsko-dalmatinska; 7) Dubrovacko-neretvanska.

Fig. 3. Test with grenade and rock.

Sediment samples were grouped in seven categories: bays,beaches, villages, ports, marinas—pier area, marina—serviceareas, and others (sea mud, river inflows, and similar). Thedata obtained were used to produce contour maps of concen-trations [3]–[6] indicating the locations of “hot spots” whereconcentration values were significantly higher in relation to thebackground.

III. EXPERIMENTAL RESULTS

A. Rocks Gamma Spectra

Fig. 3 shows the grenade as a target and rock as background,prepared for the analysis by the ”Surveyor” system. The anal-ysis was performed with the target in and the target out, and theresulted spectra are shown in Fig. 4. In such a way, the capabilityof the system to distinguish the threat material (explosive insidegrenade casing) from the background (rock) has been tested.In another measurement, rocks collected from the coastal sea

floor (depth 10 m) were placed into a tank filled with water

2764 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 5, OCTOBER 2010

Fig. 4. Gamma-ray spectra from 60-mm grenade (top spectrum) and back-ground (grenade removed, bottom spectrum). Total number of neutrons was

and measurement time was 3975 s.

(salinity of 38 g/L), and the resulting gamma-ray spectra weremeasured.In this way, two main types of rocks found on the Adriatic sea

floor have been analyzed, prevailed limestones and dolomites(Ca, Mg-rocks) and magmatic rocks (Si-rocks), which can befound in the vicinity of the most distant islands in the easternAdriatic (Jabuka, Svetac). The inspection of spectra obtainedfor limestones and dolomites revealed that peaks attributable tooxygen, magnesium, and calcium were clearly apparent, whilecarbon, silicon, and iron were significantly weaker. In magmaticrocks, the peaks of oxygen, silicon, and iron were clearly visible,while carbon and calciumwere less distinguished. The nitrogen,which is a major component of explosives, was not detected inany type of rocks. Compared to measurements in the air, un-derwater measurements suppress the peaks of carbon and cal-cium. The chlorine from sea water interferes with some peaksof silicon and iron. The peaks corresponding to the chemical el-ements in the measured spectra have been detected (see Fig. 4)at 4.44 MeV for carbon; 6.13 MeV including escape peaks at5.11 and 5.62 MeV for oxygen; 1.37 MeV for magnesium; 1.78and 2.84 MeV for silicon; 1.22, 1.29, 1.73, 1.76, and 2.13 MeVfor chlorine; 0.755 MeV for calcium; and 0.85 and 1.24 MeVfor iron.The gamma spectra of rocks measured in the air and under

the water by the ”Surveyor” system are shown in Fig. 5.

B. Sediment Concentration Values

We also present geochemical data for coastal sea sediments inthe eastern Adriatic Sea. These data can be used for evaluatingthe background for neutron sensor underwater applications forthreat material inspection.For example, interferences from background can be expected

for Fe, Ni, Cu, and Zn, which are often found in casing ofthreat material; for lead, which is frequently found in explo-sive primers; and for arsenic and bromine, which are main com-pounds of many CW agents.The sediments can contain increased concentrations of the

aforementioned elements because of the natural geological

background or anthropogenic pollution. Therefore, in additionto the use of chemical analysis of explosives, the CWA and their“containers” by means of the determination of the presence orabsence of particular elements of interest or the determinationof their ratios, the matrix that produces background has to bewell studied because the effects from compounds in the matrixcan result in signal enhancement or suppression. In this way,the matrix effect can affect the reproducibility and accuracy ofthe method increasing the false signal occurrence.Tables I and II show the results of descriptive statistics for

the analyzed samples distinguished as: 1) bays and beaches; and2) ports and marinas. Concentration values for bays and beachesrepresent background or unpolluted values and can be used forthe definition of increased concentrations which are most oftenthe result of anthropogenic pollution. We have chosen to definepolluted values as higher than the mean background value 3standard deviations.Fig. 6 shows the statistical distribution of polluted sediment

samples in seven coastal counties regarding concentrations ofFe, Ni, Cu, Zn, As, Br, and Pb. The percentage of the total pol-luted area amounts from 2% for Br to 17% for Cu. Approxi-mately 50% of the polluted locations are found in northern Is-tria and Primorsko-goranska counties, with respect to all listedelements (from 45% to 68%, for Cu and Fe, respectively) ex-cept Br. The increased concentrations of Br are found mostly inthe south (Dubrovacko-neretvanska and Splitsko-dalmatinskacounties) and the central eastern Adriatic (Zadarska county).Themost polluted locations relate to increased concentrations

of Cu and Zn (17% and 13%, respectively), which is the resultof their usage as biocide elements in antifouling paints.

IV. DISCUSSIONWe have demonstrated that a sealed-tube neutron generator

can be used under the water to distinguish between stones andobjects containing explosives. Although the experiments doneso far have been performed in laboratory conditions, they mightserve as a “proof of principle.”The sea floor sediments are the source of background in “in

situ” threat material inspection of objects laying on the sea floor.Therefore, the chemical characterization of the sea sediments isof paramount importance. Since the first field tests are plannedin the Croatian coastal sea [7], special attention has been paid tothe investigation of concentration levels in the eastern Adriaticcoastal sea sediments.In previous studies of different parts of the Adriatic Sea,

authors have shown that sea sediments have characteristicfingerprints related to the C content and particularly the C Nratio [8]–[10]. According to Fagneli [11], the western and thesoutheastern parts of the Adriatic Sea are affected by localriverine inputs, primarily by the river Po, whose sediments aretransported to the southwest by the Western Adriatic CoastalCurrent [12]. These sediments are characterized by a lowC N atomic ratio and high organic carbon and nitrogen con-centrations when compared to marine and terrestrial sediments.Average values in riverine sediments reported by Giani [13]are 8.1 and 1.0 mg g for C and N, respectively. Unlikethe western Adriatic, in the eastern Adriatic, the C content ispredominantly marine in origin (algal sapropelic) with a minor

OBHODAS et al.: MATRIX CHARACTERIZATION OF THE SEA FLOOR IN THE THREAT MATERIAL DETECTION PROCESSES 2765

Fig. 5. Gamma-ray spectra from two types of stones obtained in air and water.

TABLE IDESCRIPTIVE STATISTICS FOR BAYS AND BEACHES

TABLE IIDESCRIPTIVE STATISTICS FOR PORTS AND MARINAS

input of terrestrial components [13]. The other contributorsto the C content in sea sediments are micropollutants suchas polycyclic aromatic hydrocarbons (PAHs), polychlorinatedbiphenyls (PCBs), and chlorinated pesticides (DDT). The con-centrations of these organic micropollutants in the Adriatic Seaare measured in ng g [9], [14], hence their presence cannotinterfere with the FNAA and the associated alpha particlemethod, which is sensitive to much higher concentrations andbest suited for the inspection of large objects. To conclude, theC and N content and its variations in the Adriatic Sea sedimentsare predominantly controlled by geochemical environment andindirectly by anthropogenic contribution of C and N by

the river sediments. The C and N concentration values in theAdriatic Sea sediments are in the range that cannot interferewith underwater threat material inspection performed by the“Surveyor” system.The evaluation of the results obtained by the EDXRF analysis

presented in this paper has located numerous areas with chem-ical concentrations of elements increased by an order of mag-nitude when compared to the natural background concentra-tions (obtained from unpolluted sediments in bays and beaches).Many other authors investigated the elemental concentrationlevels in polluted [15]–[17] and unpolluted [9], [15], [18] sedi-ments in the Adriatic Sea, but none of these studies are so com-

2766 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 5, OCTOBER 2010

Fig. 6. Distribution of polluted sediment samples in seven coastal counties: I—Istarska; PG—Primorsko-goranska; LS—Licko-senjska; Z—Zadarska;SK—Sibensko-kninska; SD—Splitsko-dalmatinska; DN—Dubrovacko-neretvanska.

prehensive regarding the number of samples and the variety oflocations covering different geochemical environments and dif-ferent levels of human impact on marine ecosystems. Unfortu-nately, some elements found in increased concentrations in sedi-ments can also be components of chemical warfare agents [5] orcasing of ammunition. These high values can interfere with theunderwater determination of the threat material presence in theinvestigated objects. Typically, increased values of Zn, Cu, As,and Pb in sediments can be found in ports and harbors. Those arethe locations where human activities are most intensive, and anysuspicious object needs to be analyzed for the presence of threatmaterials in order to prevent accidents that can be potentiallydangerous for the security of local population and infrastructure.The problem of interferences can be solved by simultaneous

use of neutron and x-ray sensors. The work on such a system is

under progress [19]. Further improvements of the system willinclude experiments with different geometries, utilization of adetector with a higher sensitivity and integration of the systemin an automated submarine [6].

ACKNOWLEDGMENTThe authors appreciate the help of K. Nad during the sample

collection campaign. The authors are grateful to the reviewersof the IEEETRANSACTIONS ONNUCLEAR SCIENCE, whose com-ments and suggestions greatly improved the quality of the paper.

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