The Use of Neutron Generators for the The Use of Neutron Generators for the Detection of Hidden Explosives and Illicit Detection of Hidden Explosives and Illicit

MaterialsMaterials

Giancarlo Nebbia

Istituto Nazionale di Fisica Nucleare and Dip. di Fisica, Universita’ di Padova, Italy

IAEA HQ 13 June 2005

The detection of The detection of hiddenhidden explosivesexplosives:: a a shiftshift in in the the politicalpolitical agenda after 9/11 ?agenda after 9/11 ?

a) a) Detection of Detection of landmineslandmines buriedburied byby armiesarmies and/or and/or irregularirregular groupsgroups duringduringrecentrecent conflictsconflicts

b) b) Detection of Detection of unexplodedunexploded ordnanceordnance fromfrom recentrecent asas wellwell asas old old conflictsconflicts (i.e. World War II)(i.e. World War II)

c) c) Detection of Detection of explosiveexplosive devicesdevices in the in the protectionprotection of of sensiblesensible sitessites

d) d) Detection of Detection of illicitillicit traffickingtrafficking of fissile of fissile and and otherother threatthreat materialsmaterials (WMD) in the (WMD) in the passengerspassengers and and freightfreight transportationtransportationsystemsystem

DifferentDifferent toolstools forfor differentdifferent needsneeds

For manual humanitarian demining one needs low-cost, handheld, operator-friendly equipment to substitute/compliment the use of metal detectors.

For military demining more sophisticated and expensive tools could be implemented.

Detection of Detection of explosivesexplosives in the in the tradetrade system :system :countermeasurescountermeasures againstagainst illicitillicit traffickingtrafficking

The size of the container industry isenormous : in FY 2002 the world’s total movement in containersamounted to about 72 M TEU (“Twenty-foot Equivalent Unit) thatare transported by ships and deposited inside the harboursCustoms areas.

There is an increasing riskthat sizeable amounts of “threat materials”, includingexplosives, be hidden in cargo and transported bymeans of the standard commercial network.

Present inspectionsystems at ports are based on x-ray or γ-rayradiography.

Courtesy of SAIC, San Diego, CA, USA

Although the picturesare rather detailed and 3D imaging is possible, the number of suspectunidentified areas is stillhigh.

ChemicalChemical compositioncomposition of of differentdifferent materialsmaterials((CourtesyCourtesy of of A.BufflerA.Buffler))

NeutronNeutron inducedinduced reactionsreactions

Thermal neutron capture

Inelastic scattering

Backscattering

γγ--rayray transitionstransitions of of relevantrelevant elementselements((MeVMeV))

Element capture inelastic

H 2.2 none

C 4.42

N 10.82 1.6 - 2.3 - 5.1

O 3.8 – 6.1

TNIS : TNIS : TaggedTagged NeutronsNeutrons InspectionInspectionSystem System basedbased on the “on the “AssociatedAssociated ParticleParticle

TechniqueTechnique””

In the d + t reaction a neutron with energy of 14 MeV and an alpha particlewith energy of 3.5 MeVare emitted “back-to-back” in the COM.

WhyWhy the the associatedassociated particleparticle techniquetechnique ??

Energy (MeV)2 3 4 5 6 7

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Energy (MeV)2 3 4 5 6 7

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Gamma ray spectrumof a graphite samplewithout “neutrontagging” with the associated particle

Gamma ray spectrum of the same graphite samplewith “neutron tagging” bythe associated particle

VdGVdG beamlinebeamline withwith γγ--rayray detector detector arrayarray and and soilsoilboxbox

The The AssociatedAssociated Particle Detector (Phase I)Particle Detector (Phase I)

Channel1200 1220 1240 1260 1280 1300 1320 1340

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TDC

I II III

Alpha-Gamma time-of-flight and gamma energy signals are recordedand used to recognize the elementalcomposition of a well definedirradiated area.

Different elementsare detected in different volume cells (“voxels”) using the taggedneutron beams.

The Associated Particle Detector (Phase II)The Associated Particle Detector (Phase II)

YAP:Ce (YAlOYAP:Ce (YAlO33 Cerium loaded)Cerium loaded)Φ = 40 mm, t = 0.5 mm coated with a layer of 1 mg/cm2 of metallic silver

Stainless steel CF63 flange

UV-extended sapphire window Φ = 48 mm, t = 3 mm

PMT HamamatsuR1450

YAP:Ce

This detector is fully compatible with installation in a sealed neutron generator

Performances of the YAP(Ce) crystal in lab tests

Energy resolution of YAP(Ce) for Eα = 5.4 MeVand Eγ = 59 KeV

Timing resolution of twoYAP(Ce) detectors for twocoincident 511 KeV γ - rays

The The experimentalexperimental setupsetup at the at the InstituteInstitute RuderRuderBoskovicBoskovic in in ZagrebZagreb ((CroatiaCroatia))

Beamlines dedicated to inspection of suitcases and containers forairport and harbour security

Irradiation of a 10x10x10 cm. graphite sample hidden inside the suitcase

Right : alpha-gamma timing spectrum

Left : gamma energy gated on the “graphite” in the timing spectrum

Inserting two differentlandmines (PROM-1 = 400 gr. of TNT and TMA-1A = 5 Kg. of TNT).Effect of the explosive on the gamma energy spectrum(gated on the alpha-gammatiming ).

A A sealedsealed neutronneutron generatorgenerator withwith the the associatedassociatedparticleparticle detector detector

First First resultsresults withwith the the sealedsealed tubetube

Time (ns)10 20 30 40 50 60 70 80 90

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Energy (MeV)1 2 3 4 5 6

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6000NaI(Tl) spectrum

(b)

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YAP:Ce spectrum(c)

Time (ch)400 600 800 1000

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Graphite sample irradiation spectra obtained from an NaI(Tl) detector without “nanosecond coincidence” with the α-particle

Graphite sample irradiation spectra obtained from an NaI(Tl) detector with a 3 nanosecond coincidence with the α-particle

Layout of an inspection station

There is need for 3D tagging capability !

Airport Luggage

Sea Container

The Associated Particle Detector (Phase III)The Associated Particle Detector (Phase III)

YAP:Ce detector (Φ = 40 mm, t = 0.5 mm)read by 3 PMT's Hamamatsu R4141 (Φ = 13.5 mm)

Study of position sensitivityStudy of position sensitivity

the light collection efficiency depends on the relative position of the alpha particles hitting the detector surface with respect to the PMT center (dsource-PMT)

ΦPMT < ΦYAP:Ce =>=>

Assumption: Gaussian pulse height distributions dependent only on dsource-PMT

Parameterization of the amplitude and width values as a function of dsource-PMT

source-PMT centerd0 2 4 6 8 10 12 14 16 180

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Sigma

Energy (ch)200 400 600 800 1000 1200 1400

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220=1253 ch0X

FWHM = 144 ch = 962 ch0X

FWHM = 182 ch = 524 ch0X

FWHM = 215 ch

Channels

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Reconstruction algorithmReconstruction algorithm

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛ −=

3

1

2exp

),(),(),(

icalci

calcii

yxyxAAyxf

σ

Picalc : (Ai1

calc, Ai2calc, Ai3

calc)

(σi1calc, σi2

calc, σi3calc)

P exp : (A1exp, A2

exp, A3exp)

Looking for the coordinates thatminimize the function f(x,y)

=>

(xrec,yrec)

P(x,y)

PMT 1

PMT 2 PMT 3

X

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d2d3

x (mm)

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P (x,y)Entries 2338Mean x 0.09359Mean y -1.997RMS x 0.8258RMS y 0.7255

P (x,y)Entries 2338Mean x 0.09359Mean y -1.997RMS x 0.8258RMS y 0.7255

Test of the Test of the reconstructionreconstruction algorithmalgorithm

x (mm)-8 -6 -4 -2 0 2 4 6 8

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m)

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PMT1

PMT2 PMT3

Φholes = 1 mm

Reconstructedposition

The Associated Particle Detector (Phase IV)The Associated Particle Detector (Phase IV)

YAP:Ce detector (Φ = 40 mm, h = 0.5 mm)

read by a 2x2 multi-anodePMT Hamamatsu R5900U-00-M4 (18x18 mm2)

x (mm)-10 -5 0 5 10

x (mm)-10 -5 0 5 10

y (m

m)

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PMT1

PMT2PMT3

PMT4x (mm) -20246810

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Test of the Test of the reconstructionreconstruction algorithmalgorithm

A

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RMSyRMSxPosition

Test Test withwith a cross a cross collimatorcollimator

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Collimator with two crossed 7×1 mm2 slits placed in front of the YAP:Ce detector

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Test Test withwith 2 2 graphitegraphite samplessamples

Neutron beam

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BA

Results from the “Center of Gravity” methodResults from the “Center of Gravity” method

Pros:•Use of a single, large YAP(Ce) crystal

•Position resolution of the order of 2 mm (may improve to 1 mm)

Cons:•Analysis of amplitude signals -> position resolution depends on precise spectroscopy of the alpha signal

•Data acquisition rate of spectroscopic signal could be rather high (up to 107

counts/second)

Is it possible to achieve a suitable position resolution by simple “threshold” discrimination on the fast PMT signals ?

The Associated Particle Detector (Phase V)The Associated Particle Detector (Phase V)

64-elements 6x6 mm YAP(Ce) crystal matrix

64-pixels 6x6 mm H8500 Hamamatsu PMT

The H8500 PMT was coupled to the matrix YAP(Ce) crystals by a 4 mm thick quartz window. The system was irradiated with an alpha source through a 2 mm pinhole collimator

The four crystals indicated by the red square have been irradiated simultaneously through a square collimator.The red arrows show the position of the “direct alpha hit” signal in the amplitude spectrum for each crystal.

Setting a low threshold on signal #36 (pink area) results in cutting the “direct alpha hit” signal in #27 which is the farthest away.

Setting a high threshold on signal #36 results in cutting the “direct alpha hit” signals in #28 and #35 which are adjacent to #36.

In the response amplitude spectra of all crystals (pixels) the signal from “direct alpha hit” and from hits on adjacent crystals are totally decoupled.In this configuration one can use the “threshold” to determine position

For applications that require more performing position

resolution new solutions are needed for the “alpha tracking

system”

What next ?

+HV (2000V)G-10

Copper

Needle: Anode Copper:Cathode

Pre Amp

+HV

Pre Amp

The microstructure

needle-pad detector

-Vdrift

Detector holder

120x120 mm2

Electronics

Active Area

2D detector

X-ray tube

Working principle

+ ++ +

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Photon

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Position read-out

X1

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X1 – X2Px= X1 + X2Y1 – Y2PY= Y1 + Y2

X1

X2

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Y2

2D matrix detector

Image of a half razor

blade

The position The position resolution is of the resolution is of the

order of 150 µm order of 150 µm FWHMFWHM

The Needle-Pad detector works in any multiplication gas ( the residual gas inside a sealed neutron generator would be a good option) but the pressure needed to provide the electron-ion track is too high for applications inside a neutron generator.

One needs a first stage electron converter and amplifier.

Widely used electron multipliers

Microchannelplate detector

Microsphere detector

These electron multipliers have an average gain of 105 – 107. Biased at less than nominal voltage they can yield amplifications largely sufficient to work as a first stage converter and it has been demonstrated that they can work in much worse vacuum conditions (around 10-3 torr)

Main problem: good vacuum required for operation (10-6 torr)

Conclusions:Conclusions:

• The use of the “associated particle technique” to tag 14 MeV neutrons for inspection of large items improves the quality of the γ-ray spectra largely reducing the background (experimental results on sealed generators show a factor 50 improvement on S/N )

• Inspection of large items with miscellaneous loads (like a suitcase in an airport or a maritime container) requires the identification of a suitable size “voxel” to be irradiated

• The use of YAP(Ce) scintillators as alpha particle detectors for the tagging system is fully compatible with a sealed neutron generator and allows some “pixelation”, but other options are being considered to obtain better performances

• It is possible to reach a “voxel” size of few cm (2-3) in 2D in a close-in inspection system for suitcases and of about 30x30x30 cm3 in any location inside a container using different position sensitive PMTs

•Reaching sub-millimeter position resolution would allow to apply APT to other inspection methods (i.e. fast neutron radiography) that require real “imaging” performances

First test on the detection of fissile First test on the detection of fissile materialmaterial

TOF TOF spectraspectra forfor PbPb and DUand DU

Time of Flight spectra with Time of Flight spectra with γγ−−γγ coincidencescoincidences

Three different samples with the same weight have been irradiated for few minutes.Alpha-gamma-gamma triple coincidences have been recorded. In the Iron and Lead case one sees a very small of coincidences due to the detection of gammas in cascade.In the DU case one can notice the increase of coincidences due to the high multiplicity of fission fragment gamma decay.