Neutron radiation system for explosives detection in cargo containers

A.A. Ananiev, S.G. Belichenko, E.P. Bogolyubov, O.V. Bochkarev, E.V. Petrov, Yu.G. PolkanovAll-Russian Research Institute of Automatics named after N.L. Dukhov

(VNIIA) Sushchevskaya 22, 127055 Moscow, Russian Federation(Dated: May 5, 2008)

A measuring inspection system created in VNIIA and intended for active interrogation of cargocontainers to detect explosives is presented. The system is based on (a) the 14 MeV “tagged” neutronmethod in combination with nanosecond neutron analysis, and (b) the detection of characteristicsecondary gamma-radiation from the reaction of inelastic neutron scattering on sample objects.Experimental results for the detection of an explosive simulator – melamine (C3H6N6) are presentedfor different container loadings (wood, iron) and for different arrangements of the explosive simulator.

I. INTRODUCTION

Active gamma-neutron method – Associated ParticleImaging (API) – is used for the inspection of large con-tainers, to detect concealed explosives [1, 2, 3]. Anothername for this method is Method of Tagged Neutrons(MTN) and Nanosecond Neutron Analysis (NNA).

Standard active gamma-neutron method is based onusing the inelastic scattering of fast neutrons, accompa-nied by characteristic gamma-radiation. The identifica-tion of separate nuclei inside an object is based on reg-istering energy spectra of emitted gamma-quanta. Thisradiation provides information concerning the presenceof nitrogen, oxygen and carbon, entering explosive com-position, in a cargo. Gamma-radiation energy of inelas-tic neutron scattering on carbon, nitrogen and oxygennuclei exceeds sufficiently the energy of gamma-quanta,emitted during inelastic neutron scattering on nuclei ofother elements, thus enabling one to select them withhigh efficiency. MTN additionally enables event local-ization (regions where characteristic gamma-quanta areemitted) using information concerning the direction ofprobe neutron escape and the place of event along theneutron trajectory. So the complex separation of use-ful and background events is conducted using amplitude,spatial and time discrimination. Efficiency of backgrounddiscrimination by the NNA-MTN method may come toseveral orders [4, 5].

As a result key chemical elements entering explosivecomposition – carbon, nitrogen, oxygen – are deter-mined. This allows one to improve sufficiently the detec-tion probability and decrease false alarm probability, ascompared to methods aimed at determination of nitrogenonly (for example, Thermal Neutron Analysis method).

When MTN is used, the place of event is fixed in somediscrete volume (voxel), and the whole space under in-spection can be presented as a summation of voxels. Thenumber of “useful” events, accumulated in voxels, is pro-portional to the number of nuclei of chemical elementsto be detected. This allows one to first reconstruct a3D image of the elemental composition of an object andthen evaluate the relative nuclear density of carbon, ni-trogen and oxygen, as well as other chemical elements ofinterest.

FIG. 1: General view of measuring module.

II. MEASURING MODULE

The measuring module of the inspection system forexplosive detection and identification in cargo containersrepresents an assembly of: 12 gamma-detectors (BGO)with dimensions of 63 mm diameter ×63 mm; ING-27neutron generator with built-in 9-pixel semiconductoralpha-detector (30×30) mm2 [6]; unit of gamma-detectorradiation shielding from generator neutrons, made of bo-rated polyethylene + iron, and a supporter, where all theabove units are arranged (Fig. 1). Overall dimensions ofthe module: length – 140 cm, width – 80 cm, height –(110 ö180) cm. Height can be varied within specifiedlimits via supporter hydraulic drive. The whole measur-ing module can be moved manually on the floor to get arequired working position, i.e., near the wall of the cargocontainer under inspection. Total mass of the module isabout 300 kg.

Assembly of 12 gamma-detectors is arranged alongthe square sides, 3 pieces on each. The square size(45×45 cm2) is dictated by solid angle of alpha-detector(linear opening angle – 26.8◦) and the distance fromsquare center to generator target center (about 80 cm).So gamma-detectors are at the edge of the beam of

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“tagged” neutrons and are not preventing their free pass-ing through the square plane.

III. EXPERIMENTAL RESULTS

Experimental investigations were conducted for twomaterials loaded into container: wood and iron. Aver-age density of filling material was equal to 0.4 g/cm3.Melamine powder (C3H6N6) was used as explosive sim-ulator. Melamine was packaged in paper bags of 25 kgmass and 60×30×13 cm3 dimensions. Melamine packeddensity in a bag was about 1.07 g/cm3. So with men-tioned real full density, the partial density of nitrogen inmelamine sample we used was about 0.71 g/cm3. Thisvalue corresponds to nitrogen content in explosive, suchas octogen (0.72 g/cm3). As a rule we used at one timetwo melamine bags with total mass of 50 kg, wrappedwith polyethylene and arranged in the container alongthe axis of the central “tagged” neutron beam.

In all measurements melamine was arranged along theaxis of the central “tagged” neutron beam. All mea-surements were conducted at neutron generator inten-sity (6.0 − 7.0) × 107 n/s. Each measurement time wasequal to 30 minutes. Threshold values for each gamma-detector were selected so, that pulses with amplitudeabove 1 MeV only were registered. Gamma-detectorswere calibrated according to 4.44 MeV and 3.93 MeVcarbon peaks, 6.13 MeV and 5.62 MeV oxygen peaks.

For each energy distribution we firstly plotted a dia-gram of the time distribution of the difference betweensignal arrival from alpha- and gamma-detectors. Thenwe analyzed this distribution to get the energy distribu-tion with account of all 12 detectors. A time interval ofcoincidences of 2 ns duration, where signal/backgroundratio is the maximum, was selected for each detector. Itshould be noted that signal/background ratio decreaseswith increase of duration of coincidence time interval,but statistics grows at that. Therefore we can get more

FIG. 2: View of experiment with wood assembly.

FIG. 3: Total amplitude distribution (energy spectrum ofgamma-quanta) for 12 gamma-detectors for melamine ar-ranged at the distance of 30 cm from the wall of containerloaded with wood. Background curve corresponds to mea-surement, when melamine is absent and its place is filled withwood.

statistics during less measurement time. This is impor-tant, if deterioration of signal/background ratio is per-missible, when making decision about explosive presenceor absence in container.

A. Measurements in container loaded with wood

Wood was used as a loading material with relativelylow Z value. We loaded the container with pine boardsof room-dry humidity (12%). Measurements in con-tainer loaded with wood were conducted for three casesof melamine position relative to container wall:

– melamine was placed closely behind container wall;– melamine was placed at the distance of 30 cm from

container wall;– melamine was placed at the distance of 60 cm from

container wall.

Dimensions of parallelepiped filled with wood and withmelamine inside it were as follows: 100 cm width, 110 cmdepth and 60 cm height (Fig. 2).

Figure 3 shows clear 5.1 and 2.3 MeV nitrogen peaks,much exceeding the background distribution. This is oneof sure signs of explosive presence.

The signal/background ratio decreases as the distancefrom melamine to container wall grows. Along with mea-surement geometry this is conditioned by the sufficientattenuation of the neutron beam by wood due to the ef-fective neutron scattering by nuclei of light elements: hy-drogen, carbon and oxygen, as well as by the similarityof energy gamma-spectra from melamine and wood.

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FIG. 4: Steel tube assembly simulating container loading withiron with density of 0.4 g/cm3. Melamine is arranged closelyto container wall. Top layer of tubes is taken away.

B. Measurements in container loaded with iron

As loading material with relatively high Z value, weused iron in the form of steel tubes 3.2 cm in diameterand 2 − 3 mm thick. Measurements in the containerloaded with iron (Fig. 4) were conducted for three casesof melamine position relative to the container wall:

– melamine was placed closely behind container wall;– melamine was placed at the distance of 30 cm from

container wall;– melamine was placed at the distance of 60 cm from

container wall.

Dimensions of parallelepiped filled with steel tubes andwith melamine inside it were as follows: 95 cm width,120 cm depth and 60 cm height. Tubes were spaced uni-formly over parallelepiped, so that the average densitywas equal to 0.4 g/cm3. Measurement results are pre-sented on Fig. 5.

As in the case with wood loading, the sig-nal/background ratio decreases with increasing distancefrom melamine to container wall and filling of vacantspace with iron. But unlike wood, iron doesn’t containlight elements, contributing to the same energy peakson amplitude distribution as main explosive elements.Therefore in the case of iron loading one can see dis-tinct peaks characterizing melamine: carbon (4.44 and3.93 MeV), nitrogen (5.1 and 2.3 MeV).

The number of melamine counts over the backgroundfor the range (range width 250 keV) of 4.4 MeV carbonand 5.1 MeV nitrogen peaks, expressed in σ standarddeviations for two cases of container loading (wood andiron) was deduced in the analysis process.

In the case of iron loading, C/N ratio for melamineis much higher than C/N ratio for the background (thesame spectrum regions were taken into account), and inthe case of wood loading the corresponding C/N ratio is

FIG. 5: Total amplitude distribution (energy spectrum ofgamma-quanta) for 12 gamma-detectors for melamine ar-ranged at the distance of 30 cm from the wall of containerloaded with iron with density of 0.4 g/cm3. Background curvecorresponds to measurement, when melamine is absent and itsplace is filled with steel tubes.

much lower than the background one. Moreover, withincrease of the distance from melamine to container wallC/N ratio begins to change. At the same time the ex-cess of the number of nitrogen and carbon counts overthe background in selected regions is always more than3σ. This testifies to the presence of some other material,different from the given loading. The exception is mea-surement in wood with melamine placed at the distanceof 60 cm.

Reference C/N ratios, obtained in experiments, can betaken as a first approximation for the algorithm of mak-ing decisions about the presence or absence of melaminein a container. These values will be compared with cor-responding ratios obtained in the course of inspection ofnew objects.

The algorithm of explosive identification based on themethod of neural networks (NN) calling for preliminarytraining was considered [7]. Training consists in the selec-tion of network parameters (when working with referencespectra), enabling NN to scope with task in the best way– when output value of NN for the reference spectrumof explosive simulator is more than 0.8. Output valuesof neurons, corresponding to the rest reference spectra(wood and iron) of NN training, shall be less than 0.2(Fig. 6). Spectra of characteristic γ radiation resultedfrom the irradiation of the examined sample by “tagged”neutron beams and corresponding different time inter-vals are compared with melamine reference spectra, usingNN trained for melamine. The melamine reference spec-trum was collected in real experimental conditions in theabsence of any shielding materials, arranged around themelamine reference sample.

NN method is able to identify melamine both in ironand wood in a time interval of 1 to 5 minutes, de-pending on the depth of melamine location. An excep-

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FIG. 6: NN output value (probability) in the case of melamine placed at the distance of 30 cm from container wall loaded withwood at different depths: 10 cm (wood in front of melamine) and 40 cm (melamine).

tion is melamine arrangement in wood at a distance of60 cm from the container wall. In this case NN doesn’tmake a distinction between energy spectra of wood andmelamine.

IV. CONCLUSION

The measuring module for explosive detection andidentification in cargo containers was tested. Analysisof obtained data shows that explosive location near con-tainer front wall is the most favorable case from the view-point of detection efficiency. As the distance from con-tainer wall to explosive grows, the signal/background ra-tio decreases. This is conditioned by the measurement ge-ometry, as well as by the attenuation of the probe neutronbeam and produced inelastic scattering gamma-quantaby material loaded in container. Nevertheless whenmelamine is placed at the distance of 60 cm from con-tainer wall loaded with iron, the signal/background ratiois more than 3σ for 4.4 MeV carbon and 5.1 MeV nitro-gen peaks and 5 minutes measurement time. In the case

of container loaded with wood, the signal/background ra-tio is less than 2σ for melamine placed at the distance of60 cm from the container wall. Along with attenuationand scattering of probe neutron radiation, this is condi-tioned by the presence of light nuclei (carbon, oxygen)in wood, and results in the similarity between the energygamma-spectra of melamine and wood.

As for further improvement of measuring modulecharacteristics, efforts should be made to increase sig-nal/background ratio by upgrading gamma-detectors andelectronics. It would be well to analyze appropriatenessof additional mechanical collimation of gamma-detectors.

Acknowledgments

Authors express their thanks to A.M. Polishchuk andA.Yu. Udaltsov for assistance in preparation and per-formance of experiments, as well as to our colleaguesfrom JINR (Dubna) V.M. Bystritsky, N.I. Zamyatin,Yu.N. Rogov, M.G. Sapozhnikov and V.M. Slepnev forfabrication of alpha-detector, electronics and processingof experimental results using neural networks.

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