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Contraband detection with fast neutrons
Andy BufflerDepartment of Physics
University of Cape Town
9th International Symposium on Radiation Physics, Cape Town, 26-31 October 2003(Tuesday 28 October, 9:15-10:00)
Bombing of Pan Am Flight 103 (Boeing 747-121) at Lockerbie, Scotland on 21 December 1988
Terror attacks on the World Trade Centre and the Pentagon on 11 September 2001
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
... relatively easy
... not directly life threatening
... not life threatening
... a new problem to consider
... extremely hard!
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 technology before they depart the country of export
What signature?What is in here?
object
detectors
detectors
Analysis and
Decisionexitingradiation
interrogatingradiation
Do you need ... an image (regular or tomographic), ... or elemental characterization, ... or both (elemental image) ?
... the context of the problem is everything.
X-ray imaging
... well established and accepted in public places
... object identification based mainly on visual imaging.
X-ray imaging
Finding explosives visually is hard ...
... since most X-ray systems in use are not sensitive to the light elements H, C, N and O.
... either improve X-ray technologies (may not be good enough) [ Geoff Harding’s talk on Thursday:
Explosives detection with X-ray scatter tomography]
... or need elemental characterization (using neutrons?)
Gamma ray imaging
Transmittedγ-rays
γ γobject
137Csor 60Co
VACIS: Vehicle and Container Inspection Systems
Mobile VACIS ... used with permission from
VACIS
Elemental analysis using fast neutrons
Neutrons: PenetratingSensitive to both light and heavy elementsUndergo specific interactions with different nuclidesEasily produced and detected
Transmittedneutrons
fast neutrons
n
Capture γ-rays
thermal neutrons γ
object
Inelastic scattering γ-rays
fast neutrons γ
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
Thermal neutron capture
Capture γ-rays
thermal neutrons γ
14N + thermal neutron → 15N + γ (10.83 MeV)1H + thermal neutron → 2H + γ (2.223 MeV)
Thermal Neutron Analysis(TNA) systems have undergone extensive testing in airports ... in combination with X-ray systems.
... TNA now marketed as a small parcel screening system.
TNA spectrum for a bag with and without a small explosive
0 2 4 6 8 10 12 14
Energy (MeV)
100 000
10
100
1
10 000
N (in air andcosmic rays)
N (explosive)
H peakbackgroundammonium nitrate
Cou
nts
1 000
... TNA systems have also been developed for vehicle screening.
vehicle being screened
neutron source and γ-ray detectors
Thermal neutron capturealso used to characterize unexploded ordnance
CW agent
neutron
252Cf source
HPGedetector
Gamma ray
1.63
2+
0+
3−
2−
1+ 0+
0+
3−
4.44 MeV5.112.31
2.80
0.72
6.13
16O12C 14N
1+
Jπ
Fast neutron inelastic scatteringobject
Inelastic scattering γ-rays
mono-energetic fast neutrons γ
Neutron Inelastic scattering used in online analysis and control in the minerals industry
(e.g. fast neutron coal gauge [Sowerby])
252Cf source
shielding
conveyor
lump coke
γ-ray detector
Fast Neutron Inelastic Scattering to detect explosives in luggage
Energy
γ-ray detectorssuitcase
accelerator
shielding
suitcase in
suitcase out
net suitcase
C4.44 MeV
O6.13 MeV
FNA gamma ray signatures
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
Pulsed Fast Neutron Analysis(PFNA)
Pulsed Fast Neutron Analysis (PFNA)
Accelerator-based system using mono-energetic 8 MeV neutrons and arrays of NaIdetectors to detect de-excitation γ-rays from neutron inelastic scattering interactions.
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
~ 1 m
MF Physics A-325: 109 14 MeV neutrons s-1, µs-pulsed
Pulsed Elemental Analysis with Neutrons (PELAN) [Vourvopoulos]
object
Inelastic scattering γ-rays
mono-energetic fast neutrons γ
Fast neutron inelastic scattering
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.
µs-pulsed 14 MeV sealed tube neutron generator
Fast neutrons during pulsesThermal neutrons between pulses
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, ...
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
Associated Particle Imaging
... see ISRP9 poster by Vlado Valković
Inspection of shipping containers for threat materials
(... describes using 14 MeV “tagged” neutrons to scan shipping containers for nuclear materials, explosives, etc.)
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
transmittedneutrons
... 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].
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
PFNTS luggage screening system proposed
by Tensor Technology
ns-pulsed accelerator provides neutrons having broad energy spectrum
2D neutron detector array
luggage on conveyor
Panel on the assessment of the practicality of Pulse Fast Neutron Transmission Spectroscopy for Aviation Safety
Report: 1999Washington D.C.
Recommendations:1. Do not consider accelerator-based technologies to have promise for deployment as a primary screening procedure for checked baggage inspection. Any screening procedure relying on an accelerator cannot compete with available technologies on either cost or practicality bases2. Do not fund any large accelerator-based hardware development projects. Combinations of experimental work with existing laboratory equipment, mathematical modeling and simulation can better define the potential of nuclear technologies without the expense or time required to design and build new hardware.
Fast neutron resonance radiography
... see ISRP9 poster by Volker Dangendorf et al.
Detectors for neutron resonance radiography with time-of-flight methods
(... describes results from tests using three different neutron imaging systems)
Neutrons in, neutrons out
Elastically and inelastically
scattered neutrons
fast monoenergetic
neutrons n, n’
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
Fast Neutron Scattering Analysis (FNSA)Andy Buffler and Frank Brooks, et al.
Department of Physics, University of Cape Town, South Africa
Fast Neutron Scattering Analysis (FNSA)
scatteringobject
Backward neutrondetector
shielding
monoenergetic neutron beam
(two multiplexed energies)
Forward neutron detector
... 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
The hardest problem to solve (by far) is the detection of explosives in airline luggage.
... so is there a future for neutrons in airports ?
It all depends on ...
... the perceived threat
... political pressure
... financial concerns
... radiation safety compliance
...
Detection of explosives in airline baggage by fast neutrons
Focus on screening of outgoing baggage – requirements include high throughput and low false alarm rate.
Signal: Appropriate HCNO atom fractionHigh density of HCNO (1.6 – 2.0 g cm-3)
Effective explosive detection systems must offer:(a) Rapid operation (< 20 s per bag)(b) Reliable detection
Zero false negative rate (must detect all explosives > 200 g) Small false positive rate (few false alarms)
(c) Safe use(d) Ease of use
It is not clear whether or not the new generation of X-ray systems will fully meet all these requirements.
Stage 1First (rapid) screening using X-ray method (10 seconds)
Bag “suspect” ?
Yes (~3% of bags)
Bag moved to Stage 2 station
Stage 2Thorough neutron-based screening
(1 – 2 minutes)Bag still “suspect” ?
Stage 3Visual inspection
No
Yes( < 0.1% of bags)
No
A fast neutron system could be used as the second stage of a multi-stage screening system, in order to provide corroborative evidence via an independent and more sensitive analysis.
Multi-stage interrogation protocol for explosives in outgoing 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
Explosives detection signal(e.g. nitrogen density)
Freq
uenc
y di
strib
utio
n
distribution of nitrogen density in
bags with noexplosives
Threshold(adjustable)
distribution of nitrogen density
in bags withexplosives
probability of signal being caused by
either a false positive or a false negative
moving the detection threshold his way increases the risk of missing a real explosive
moving the detection threshold his way increases false alarms
Detection of illicit drugs in airline baggage by a neutron based system
Focus on the screening of incoming baggage
Signal: Appropriate HC atom fraction High H concentration
Requirements less stringent and less precise than for explosives detection.
(Detecting > 0.5 kg of illicit drugs with 90% efficiency is useful.)
Which is the best neutron system to use?
Which signal / signatures for explosives detection?
... gamma rays from thermal neutron capture ?
... gamma rays from inelastic neutron scattering
... neutrons transmitted through the sample ?
... neutrons scattered out of the beam ?
... ???
Development of the optimum neutron-based system has been hampered by commercial and intellectual property concerns.
Proposed neutron-based system for the detection of explosives and illicit drugs in airline luggage.
forward neutron and γ-ray detectors
backwardneutron and γ-ray detectors
fast neutron 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
... using a sealed tube neutron generator or RFQ linac in a controlled area, inaccessible to passengers.
Which neutron source?
Mono-energetic beam (single or multiplexed)Sealed tube neutron generator ... 2.5 MeV or 14.1 MeV ?
... µs - pulsed ?
... associated particle ?Radiofrequency quadrapole 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?
Conclusion
Elemental analysis and imaging technologies based on fast neutrons have been developed for a wide range of contexts.
Fast neutrons can be used to analyze the important elements for contraband detection (H, C, N and O) with high sensitivity.
Although it is unlikely that fast neutrons will replace X-rays as the primary probes in screening technologies at airports, neutron systems may find use as part of a confirmatory stage in a multi-stage protocol.
Proper comparative testing of the different neutron methods under development for explosives detection in luggage is still lacking.
It is therefore more likely that neutron-based systems will find increasing use in the detection of nuclear materials, the screening of small packages, the detection of illicit materials such as narcotics, and the search for buried landmines.
Thank you
