giancarlo nebbia - iaea
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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
Co
un
ts
0
500
1000
1500
Energy (MeV)2 3 4 5 6 7
Co
un
ts
0
100
200
300
400
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
Co
un
ts
0
100
200
300
400
500
600
700
800
900
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
Coun
ts
0
500
1000
1500 Time distribution(a)
Energy (MeV)1 2 3 4 5 6
Coun
ts
0
2000
4000
6000NaI(Tl) spectrum
(b)
Energy (ch)0 100 200 300 400 500
Coun
ts
0
2000
4000
YAP:Ce spectrum(c)
Time (ch)400 600 800 1000
Coun
ts
0
500
1000
1500 (a)
Energy (ch)0 100 200 300 400 500
Coun
ts
0
200
400
(c)
Energy (MeV)1 2 3 4 5 6
Coun
ts
0
200
400
600 (b)
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
0.2
0.4
0.6
0.8
1
Amplitude
Sigma
Energy (ch)200 400 600 800 1000 1200 1400
Co
un
ts
0
20
40
60
80
100
120
140
160
180
200
220=1253 ch0X
FWHM = 144 ch = 962 ch0X
FWHM = 182 ch = 524 ch0X
FWHM = 215 ch
Channels
Coun
ts
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
Y
d1
d2d3
x (mm)
-4-2024y (mm)
-6 -4 -2 0 2 4
0
50
100
150
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
x (mm)-8 -6 -4 -2 0 2 4 6 8
y (m
m)
-10
-8
-6
-4
-2
0
2
4
6
8
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)
-10
-5
0
5
10
PMT1
PMT2PMT3
PMT4x (mm) -20246810
y (mm)
-20246810
0
200
400
600
800
1000
1200
1400
Test of the Test of the reconstructionreconstruction algorithmalgorithm
A
C
D
B
1.01.0D0.90.9C0.70.5B0.70.7A
RMSyRMSxPosition
Test Test withwith a cross a cross collimatorcollimator
x (mm)
-6-4
-20
24
6y (mm)
-6-4
-20
24
6
0
200
400
600
800
Collimator with two crossed 7×1 mm2 slits placed in front of the YAP:Ce detector
5.2
-2.0
xrec (mm)
-3.3
4.0
yrec (mm)
-4
4
yA (cm)
4B
-4A
xA (cm)Sample
x (mm)-15 -10 -5 0 5 10 15
y (m
m)
-15
-10
-5
0
5
10
15
0
10
20
30
40
50
60
x (mm)
-15-10
-50
510
15
y (mm)-15-10-5051015
010203040506070
Test Test withwith 2 2 graphitegraphite samplessamples
Neutron beam
x
y
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
+ ++ +
++ +
++ +
+ ++ +
+
+
Photon
+ + ++ + +
+ + ++ + +
+
+
Position read-out
X1
Y1 X2
Y2
X1 – X2Px= X1 + X2Y1 – Y2PY= Y1 + Y2
X1
X2
Y1
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.
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