ENTE PER LE NUOVE TECNOLOGIE,L'ENERGIA E L'AMBIENTE

Associazione EURATOM-ENEA sulla Fusione

A PORTABLE NEUTRON SOURCE FORLANDMINES DETECTION

MASSIMO RAPISARDA, MAURIZIO SAMUELLI

ENEA- Divisione FusioneCentro Ricerche Frascati, Roma

Presented at the 6th Intern. Conf. on Application of Nuclear Techniques “Nuclear Technologyfor Safety, Security and Industrial Development”

Crete, Greece, June 20-26, 1999

RT/ERG/FUS/2001/9

This report has been prepared and distributed by: Servizio Edizioni Scientifiche - ENEACentro Ricerche Frascati, C.P. 65 - 00044 Frascati, Rome, Italy

The technical and scientific contents of these reports express the opinion of the authors but notnecessarily those of ENEA.

Abstract

In the framework of a joint development of applications of the Plasma Focus machines, by ENEA,University of Ferrara and University of Bologna, a neutron source is studied, suitable for thedetection of explosives, either landmines or devices hidden in air luggage, exploiting the emissionfor radiative capture of the 10.8 MeV gamma ray from the 14N contained in the majority ofexplosives. The device is based on a plasma focus neutron source of portable dimensions emitting109 neutrons per second (patent pending). Here we report the Monte Carlo simulations performedwith the MCNP code to design the device structure for use in landmine detection. The original 2.1MeV neutrons are focused and moderated by layers of lead and polyethylene toward the ground,and absorbed by layers of borated polyethylene and cadmium toward the air. The shape and thethickness of the shields are optimised to produce the highest thermal neutron flux at variousdepths (0-15 cm) in an average ground with different contents of water (10% to 30%). Resultsshow that it is possible to generate a flux of at least 10-5/cm2 thermalised neutrons (between 0.01and 0.1 eV) per source neutron in an area of about 1 m2 around the axis of the source, highenough to generate a an adequate number of 10.8 MeV gamma rays in the buried explosive.

Keywords: Plasma Focus, neutron generator, landmines, explosives, demining, MCNP

Riassunto

Nell’ambito di una collaborazione fra ENEA, Università di Ferrara e Università di Bologna sullosviluppo congiunto di applicazioni delle macchine Plasma Focus viene studiato un generatore dineutroni da impiegare per la rivelazione di mine nel terreno o di bombe nascoste nel bagaglioaereo. I neutroni termici, inducono nell’Azoto 14 contenuto negli esplosivi l’emissione, in seguitoa cattura radiativa, di caratteristici raggi gamma a 10.8 MeV. Il dispositivo è basato su un Plasma

Focus trasportabile capace di emettere 109 neutroni al secondo (brevettato). Qui riportiamo lesimulazioni Monte Carlo fatte col codice MCNP per progettare ed ottimizzare il dispositivo per laricerca di mine nel terreno. I neutroni da 2.1 MeV generati dal Plasma Focus sono focalizzati emoderati da strati di piombo e polietilene verso il suolo e schermati da strati di polietilene borato ecadmio verso l’aria. La forma e gli spessori degli schermi sono ottimizzati per produrre ilmassimo flusso termico di neutroni nel primi 15 cm di suolo, tenendo conto di diverseconcentrazioni di umidità (da 10% a 30% di acqua). I risultati indicano che è possibile produrre un

flusso di almeno 10-5/cm2 neutroni termalizzati (tra 0.01 e 0.1 eV), per neutrone di sorgente, nelmetro quadrato di suolo più vicino al dispositivo, un livello sufficiente a generare un numeroadeguato di raggi gamma da 10.8 MeV dall’esplosivo sepolto.

INDEX

INTRODUCTION...........................................................................................................................................................7

THE DEVICE..................................................................................................................................................................7

RESULTS OF THE SIMULATION ..............................................................................................................................9

CONCLUSIONS ...........................................................................................................................................................13

REFERENCES ..............................................................................................................................................................14

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A PORTABLE NEUTRON SOURCE FOR LANDMINESDETECTION

INTRODUCTION

The vast majority of explosives used in landmines are based on chemical compounds, whichcontain Nitrogen in a percentage of about 20%. Nitrogen 14, which provides the 99.6% ofnatural Nitrogen, has the probability of about 12-15% to emit a 10.8 MeV gamma ray afterthe capture of a thermal neutron. Commonly, in the soil composition there are no elementsable to emit gamma rays so energetic, following neutron capture, apart from 30Si, that has atransition at 10.6 MeV, about 150 times less probable than that of N, per thermal neutron [1].This suggests that the presence of a 10.8 MeV gamma ray from a neutron irradiated sample isa valuable marker of a suspect explosive.

Explosive detectors based on this reaction have been already proposed [1] and are normallybased on fast neutrons emitted by a strong passive source (Cf, AmB, AmBe) moderated by ashield of low Z material. In this paper we suggest the use of an active source to generate theneutrons, in order to overcome two unpleasant side effects of the passive one: the continuoushigh level of emission that renders uneasy its manipulation and transportation, and thepossibility that an explosion spreads radioactive material in the surroundings. An activesource, with no radioactive material among its components, emitting neutrons only whenoperated by personnel behind a shield has evident handling advantages.

THE DEVICE

A joint program has been started with INFN (The EXPLODET project) [2]. Our work regardsthe study of the characteristics of an active neutron source. The device has been designedkeeping in mind these considerations:

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a) It should be transportable, to be taken to the minefields, and should be operated at a safedistance from the ground. Its weight should not exceed a few hundreds of kg and itsdimension the cubic metre.

b) Its cost should be contained. For practical purposes several of these devices could beoperated in various regions, and the possibility of loss, due to an explosion, should becontemplated. So it should be made of materials readily available and not expensive.

Fig. 1 - Vertical section of the model lying on the ground. In particular (A) is the Plasma Focusvacuum chamber, (B) the lead shield around it, (C) the absorbing shield of borated polyethylene, (D)the labyrinth duct for the vacuum pumps, (E) the layer of polyethylene to slow the neutrons towardsthe ground, (F) the ground, divided in three layers of 5 cm, each one segmented in cylindrical shells of200 cm3 with radii of 10, 20, 30, 40 and 50 cm

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In our device neutrons are generated by a plasma focus of small dimensions (the cylindricalvacuum chamber is about 10 cm wide and 20 cm long) capable to produce up to 109 n/s in D-D with a transportable set of capacitors. Although the neutron emission energy average is 2.5MeV, its spectral characteristic are not isotropic: along the anode/cathode axis the spectrum ispeaked at 2.9 MeV in the “forward”(anode → cathode) direction and at 2.1 MeV in the“reverse” (cathode → anode) direction. Therefore a clever position the chamber allows topoint the softer part towards the ground. The plasma focus is inserted in a material structuredesigned to shield the neutron flux towards the air, while maximising its effect on a possiblelandmine buried in the ground. This second aim is achieved by surrounding the source with alayer of lead that, besides shielding the gamma rays towards the outside, has the effect ofcreating another 20% neutrons through the (n,2n) reaction and scattering the particles as ifthey were produced by a more extended source. A further layer of polyethylene moderates theneutrons towards the ground.

Cousins et al. [1] claim that a 106 n/s 252Cf source is sufficient to detect 1 kg of N, countingwith a 2”x 2” NaI(Tl) for 8 hours. A 109 n/s source should then allow the detection of 100 g ofNitrogen, counting for 5 minutes. With this threshold in mind we designed the structure of ourdevice to optimise its performances by means of the MCNP (Monte Carlo Neutron andPhoton) [3] transport code.

RESULTS OF THE SIMULATION

The present work is focused on the simulation of the neutron fluxes in the ground, notworrying for the detection of the gamma rays, which has been treated elsewhere moreextensively by INFN [2]. On the other hand a given neutron flux and spectrum will generate apredictable number of gamma rays from a specific amount of Nitrogen.

In principle we do not know where the mine is buried, so the obvious aim was to achieve auniform neutron illumination, as thermal and intense as possible, in a rather large and deepportion of the ground. And since the (n,γ) capture cross section in Nitrogen is rather smooth inproximity of thermal energies, we decided to neglect the detailed tuning of the spectrum infavour of a simpler maximisation of the “thermalised” neutron flux (that we define in therange 0.01 - 0.1 eV) in the ground.

The first step has been the optimisation of the moderator layer between the source and theground. The ground itself has moderating power, but the possibility that the mine could be

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superficial requires that the neutrons must be already thermalised when they leave the device.So we tried different moderator thicknesses and studied the transport of the neutron flux at

Fig. 2 - Flux of thermalised neutrons (0.01- 0.1 eV) in neutrons/cm2 per neutron emitted by thePlasma Focus on axis, at three depths in the ground for different thicknesses of the polyethylenemoderator

various energies in a disc of soil 1 m of diameter and 15 cm deep. Here we report thedependence of the flux at various energies in an exemplary case.

The first and the second columns of fig. 2 show respectively the total neutron flux and thethermal neutron flux along the source axis at three depths in the soil, with the bare PlasmaFocus source at 30 cm from the ground, taking into account only geometrical effects. Thethird column shows the effects of the upper structure without the lower polyethylenemoderator on the thermal flux. The thermal flux greatly improves with the insertion of thelower moderator (columns 4 - 10). Eventually the best thickness of the polyethylenemoderator results to be around 5 cm.

Off axis these findings are confirmed at various depths in the ground. The dependence of thethermalised flux in the first 5 cm of soil is shown in fig. 3 in function of the thickness of themoderator, stating that 5 cm of polyethylene are the best compromise and will be used in the

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remainder of this study. In the simulations of fig. 3 the device is laying on the ground and itcan be noticed how the presence of the absorbing shield (borated polyethylene) around thesource limits the cone of emission causing the rapid drop in the underground flux at about 30cm from the axis.

This drop vanishes if we rise the device on the ground, widening the interception of the cone.Increasing the distance of the base of the apparatus from 0 to 50 cm on the level of theground, as shown in fig. 4, we observe that the distribution of the thermalised neutronsspreads over an area of about a square metre, in which the flux is rather uniform and alwaysgreater than 10-6/cm2 per fast source neutron (that we take as a lower limit for the operation ofthe device). In fact with the device suspended at 30 cm from the ground (corresponding to adistance of 60 cm from the source to the soil) which represents a practical solution to avoidthe risk of explosions, the flux is still close to 10-5/cm2 in a reasonable area around the axis.

Fig. 3 – Flux of thermalised neutrons (0.01 – 0.1 eV) in the first 5 cm of soil in function of thedistance from the axis and of the thickness of the moderator (n/cm2 per source neutron)

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Fig. 4 – Flux of thermalised neutrons (0.01 – 0.1 eV) in the first 5 cm of soil in function of thedistance from the axis and of the height of the device on the ground (n/cm2 per source neutron,device with 5 cm of polyethylene)

All the calculations shown above have been made in the case of dry soil1. On the other hand,water is often present in the ground and its moderating power must be taken into account, sowe considered the practical case of a device suspended at 30 cm from the surface and repeatedthe simulations with different water contents in the soil composition. Fig. 5 shows that watercontent of 30% leads to an increase of almost a factor 2 in the flux of thermalised neutronsand that also lesser contents extend the range of operation of the device. Considering thatmany European soils are normally humid and that in many cases water can be spread on thefloor before irradiation it follows that the operation of the device could be facilitated inseveral cases, not to mention the search of mines in shallow damps.

A final remark: the high concentration of Silicon present in all soils increases the backgroundlevel with 10.6 MeV gamma rays proportionally to the irradiated area, while the number of

1 Atomic composition: O 62.7%, Si 21.2%, Al 6.5%, Fe 1.9%, Ca 1.9%, Na 2.6%, K 1.4%, Mg 1.8%.

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Fig. 5. Flux of thermalised neutrons (0.01 -0.1 eV) in the first 5 cm ofsoil, in function of thedistance from the axis and of the watercontent of thesoil (n/cm2 per source neutron, 5 cm

moderator, device suspended at 30 cmabove the ground)

1.00E-06

1.00E-05

1.00E-04

0 cm 10 cm 20cm 30 cm 40cm 50cm

Distance from the axis

dry soil10%water20%water30%water

Fig. 5 – Flux of thermalised neutrons (0.01-0.1 eV) in the first 5 cm of soil, in function of thedistance from the axis and of the water content of the soil (n/cm2 per source neutron, 5 cmmoderator, device suspended at 30 cm above the ground)

10.8 MeV depends only on the quantity of Nitrogen contained in the mine. Roughly speaking,about 15 g of 30Si are present in every cubic decimetre of superficial soil, i.e. ≈ 5 kg in thefirst 30 cm of every square meter of surface, with a gamma production (>10 MeV)comparable to that of 30 g of Nitrogen. This means that, to detect small quantities ofexplosive, it is not useful to increase too much the irradiated area, unless one is not ready todiscriminate 2% in energy at 10 MeV.

CONCLUSIONS

Monte Carlo calculations show that a Plasma Focus of limited dimensions with an emittingcapability of 109 n/s can be opportunely shielded to become a neutron source suitable foractivating the 14Ni contained in landmines buried in the ground. The device can be safelyoperated by the personnel and transported without problems, being an active source notcontaining radioactive material among its components. The shielding and moderatingmaterials are chosen to allow for a flux of 104 thermalised neutrons/s (0.01 - 0.1 eV) in anarea of about 1 m2 for a depth of about 10 cm near the source, which we consider the lower

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limit of operativity. The shape of the various shields can be improved to achieve a morehomogeneous distribution of the neutron flux over the ground surface, but still at this level ofdetail the device offers an adequate performance. Further studies will be dedicated to thedesign and optimisation of the detector assembly, aiming to the spatial identification of thesignal.

REFERENCES

[1] T. Cousins et al. J. Rad. Nucl. Chemistry, 235, 1-2, 1998, 53-58.

[2] The EXPLODET project; M. Lunardon and G. Viesti editors, Int. Rep. DFPD 99/NP/05(1999) Padua, Italy.

[2] A General Purpose Monte Carlo Code for Neutron and Photon Transport, LA-7336-M,Los Alamos Nat. Lab. (1981).