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Using Fast Neutrons to Detect Explosives and Illicit Materials
Andy BufflerDepartment of Physics
University of Cape Town, South Africa
International Symposium on Utilization of Accelerators, Dubrovnik, Croatia, 5-9 June 2005(Paper IAEA-CN-115/59)
International Workshop onFast Neutron Detectors
and ApplicationsUniversity of Cape Town, Cape Town, South Africa
3 - 6 April 2006
Jointly organized by:University of Cape Town, Cape Town, South Africa
Physikalisch-Technische Bundesanstalt, Braunschweig, GermanyiThemba Laboratory for Accelerator Based Sciences, Faure, South Africa
www.fnda2006.deAlso incorporating courses on unfolding methods in spectrometry and MCNPX
Post 11 September 2001:
Increased awareness to protect the global supply chain from acts of terrorism and smuggling of contraband.
The problem: find the contraband!
Contraband = nuclear materialsillicit drugsmoneybiological materialsexplosives
G8 Action Plan on Transport Security
Cooperative actions needed for improved security in the areas of:
• people movements• container security• aviation security• maritime security• land transportation
The (United States) Container Security Initiative
... Establish security criteria to identify high-risk containers
... Pre-screen containers using approved technology before they depart from the country of export
detectors
detectorsinterrogatingradiation
object
Analysis and
Decisionexitingradiation
What is in here? What signature?
Do you want ... an image (regular or tomographic), ... or elemental characterization, ... or both (elemental image) ?
... the context of the problem is everything.
Analysis of bulk materials using fast neutrons
Transmittedneutrons
n
Capture γ-rays andinelastic scattering
γ-rays
γ
Elastically and inelastically
scattered neutrons
fast neutrons
n, n’
false negatives tolerated
difficulty of problem
bombs in aircraft luggage
buriedlandmines
illicit materials
(non-threat) in cargo
analysis of a “known”
object
The context is everything ... What is the real problem that needs to be solved?
nuclear materials in cargo
WoolWoodWaterSugar
SoybeanSilk
RayonPVC
PolyurethanePolyethylene
PolyesterPaperOrlonNylon
NeopreneMethanolMelamine
Lucite (Perspex)EthanolDacronCottonBarley
Ammonium acetateAcetamide
PCPMorphineMandrax
LSDHeroin
CocaineTetrylTNTRDX
Picric AcidPETN
Octogen (HMX)NitroglycereneNitrocellulose
EGDNDynamite
Composition 4 (C-4)Composition B
Ammonium Nitrate
0 20 40 60 80 100
Miscellaneoussubstances
Illicit drugs
Explosives
Atom fraction (%)
H C N O
Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV)
Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV)
Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV)
Rice Cocaine C4 explosive Glass Aluminium Oxygen
Acetone Apples Sarin Leather Silicon Nitrogen
Polyethylene Coffee Water Plastic Iron Carbon
Fast Neutron Analysis gamma ray signatures
Pulsed Fast Neutron Analysis(PFNA)
Compact sealed tube neutron generators provide cheap fast neutrons
2He(d,n)3He 2.5 MeV neutrons
or
3He(d,n)4He 14.1 MeV neutrons
MF Physics A-325: 109 14 MeV neutrons s-1, µs-pulsed
~ 1 m
Pulsed Elemental Analysis with Neutrons (PELAN) [Vourvopoulos]
Portable system based around a 14 MeV sealed tube neutron generator and BGO detector to detect de-excitation γ-rays from neutron inelastic scattering interactions.
Fast neutron inelastic scattering
object
Inelastic scattering γ-rays
mono-energetic fast neutrons γ
Neutronflux
Time10-15 µs 80-100 µs
Scattering Capture ActivationInteraction: (n, n’ γ) (n, γ) (n, α), (n, p)Gammas: Prompt Prompt DelayedElements: C, O H, N, S, Cl, Fe, ... O, Al, Si, ...
Fast neutrons during pulsesThermal neutrons between pulses
µs-pulsed 14 MeV sealed tube neutron generator
tritium target
α-particledetector
interrogated object
γ-ray spectrum for each volume element
γ-ray detector
electronics
n
neutron
α
dAPSTNG
Tαxα
yα
Tγ Eγ
Associated particlesealed tube neutron generatorutilising the T(d,n)α reaction.
The 14.1 MeV neutron is tagged in time and direction by detecting the associated α-particle released in the opposite direction.
... allows ns-timed measurements to be made.
γ
0 5 1 0 1 50
2
4
6
8
1 0
E N D F /B -V I
1 H
1 2 C
1 4 N
1 6 O
Sca
tterin
g cr
oss
sect
ion
(b)
I n c id e n t n e u tro n e n e rg y (M e V )0 5 10 15Incident neutron energy (MeV)
10
8
6
4
2
0
Scat
terin
g cr
oss
sect
ion
(b)
Total scattering cross section(ENDF/B-VI)
1H12C14N16O
Pulsed Fast Neutron Transmission Spectroscopy spectrum
... ns-pulsed broad energy neutron beam is attenuated in the sample according to total scattering cross section for each element in the sample.
Unfolding analyses of the transmitted neutron spectra allows the elemental content of the sample to be determined [Overley].
transmittedneutrons
ns-pulsed broad energy neutron beam
n
0.0 50.0 100.01/v (ns m-1)
102
105
104
103
106
Cou
nts
unattenuatedbeam
semtexexplosivein beam
( )( )
0
0
ln
lnn nn
g g g
I IR
I Iµµ
= =
Fast Neutron and Gamma Radiography
transmittedneutrons
and γ-rays
14 MeV STNG neutrons and 60Co γ-rays n, γ
neutrons and γ-rays
γ-rays only
The energy and intensity distributions of the scattered neutron field are functions of the:
• incident neutron energy• angle of scattering• mass of the scattering nuclide
Neutrons in, neutrons out
Fast Neutron Scattering Analysis (FNSA)Andy Buffler and Frank Brooks, et al.
Department of Physics, University of Cape Town, South Africa
Elastically and inelastically
scattered neutrons
fast monoenergetic
neutrons n, n’
Fast Neutron Scattering Analysis (FNSA)
monoenergetic neutron beam
(two multiplexed energies)
scatteringobject
Forward neutron detector
Backward neutrondetector
shielding
... development work undertaken at UCT and iThemba LABS ...
A scattering signatures obtained from selections of the raw scattering data which are assembled serially, into a single, spectrum-like, distribution.
Scattering signatures for pure elements exhibit distinct individual characteristics for each element.
The scattering signature
En = 6.8 MeV
En = 7.5 MeV
For example: one set based on pulse heightonly (does not require a pulsed neutron beam)
0
400
800
H
0
400
θlab: 150o 45o
Nor
mal
ized
cou
nts
per c
hann
el N
C
0
400
0
400O
0
400
S
Al
0
400
0
400Fe
0 150 300 450 6000
400Pb
Channel number
H
C
N
O
Pb
Al
Fe
S
FNSA
Scattering signatures measured for unknown samples are unfolded to determine the elemental composition of the sample.
The measured atom fractions uniquely characterize the elemental composition of the scattering sample.
H C N O Al S FePb0.0
0.2
0.4
0.6
0.8Methanol Ammonium Heroin nitrate simulant
atom
frac
tion
H C N O Al S FePb
H C N O Al S FePb
The identification of specific materials from measured atom fractions can be facilitated by introducing a χ2-based screening procedure to compare the atom fractions measured for an “unknown” sample with the known atom fractions of a large set of candidate materials.
... so is there a real future for neutrons inside an airport terminal ?
It all depends on ...
... the perceived threat
... political pressure
... financial concerns
... radiation safety compliance
... the physics
The hardest problem to solve (by far) is the detection of explosives in airline luggage.
Combined detection probabilities for a multi-stage system
Passenger profiling
X-ray scanning
OR
pd= 0.50pfa=0.05
pd = 0.90pfa= 0.03
pd= 0.95pfa=0.08
Stage 1
neutron system
ANDpd = 0.94pfa < 0.001
pd = 0.99pfa= 0.01
Overall system
Stage 2
The ultimate neutron-based system for the detection of explosives and illicit drugs in airline luggage.
backwardneutron and γ-ray detectors
forward neutron and γ-ray detectors
fast neutron (and γ-ray)source
luggageconveyor
fan beam
collimatorshielding
backwardneutron and γ-ray detectors
neutron beam... system based on the
fusion of signatures of:
• transmitted and scattered fast neutrons
• backscattered thermal neutrons (from H)
• γ-rays from inelastic scattering and thermal neutron capture
… but will this solve the problem?
Fast neutron menu
Mono-energetic beam (single or multiplexed)Neutron generator ... 2.5 MeV or 14.1 MeV ?
... µs - pulsed ?
... associated particle ?LinacVan de Graaff or small cyclotron ... ns-pulsed?
White (broad energy) beamRadioisotopic (252Cf, Am-Be, Pu-Be ...)Van de Graaff or small cyclotron ... ns-pulsed?
… but unless you are not worried about size, cost, time, … For contrast, you need two (or more) flavours of interrogating radiation … and a signature with at least two components.
Closing thoughts
Massive research and development since Lockerbie has notresulted in a significant case for fast neutrons to be the solution to all problems.Explosive detection with neutrons has been the “flavour of the decade” with very little practical accomplishment.The most successful developments have occurred when there has been total collaboration between the laboratory scientists and the end users.Fast neutrons are likely to be useful for specific, targeted applications: the detection of threat (nuclear, …) materials in air and sea cargo, and for the analysis of materials of “known” composition (possibly landmine detection).