contraband detection with fast neutrons - faculty of … · deployment as a primary screening...

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