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).