Using Fast Neutrons to Detect Explosives and Illicit Materials

Transcription

1 Using Fast Neutrons to Detect Explosives and Illicit Materials Andy Buffler Department 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)

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3 International Workshop on Fast Neutron Detectors and Applications University 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, Germany ithemba Laboratory for Accelerator Based Sciences, Faure, South Africa Also incorporating courses on unfolding methods in spectrometry and MCNPX

4 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 materials illicit drugs money biological materials explosives

5 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

6 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

7 What is in here? What signature? interrogating radiation object exiting radiation detectors Analysis and Decision detectors Do you want... an image (regular or tomographic),... or elemental characterization,... or both (elemental image)?... the context of the problem is everything.

8 Analysis of bulk materials using fast neutrons n, n Elastically and inelastically scattered neutrons fast neutrons n Transmitted neutrons γ Capture γ-rays and inelastic scattering γ-rays

9 The context is everything... What is the real problem that needs to be solved? false negatives tolerated analysis of a known object illicit materials (non-threat) in cargo buried landmines nuclear materials in cargo bombs in aircraft luggage difficulty of problem

10 Ammonium Nitrate Composition B Composition 4 (C-4) Dynamite EGDN Nitrocellulose Nitroglycerene Octogen (HMX) PETN Picric Acid RDX TNT Tetryl Cocaine Heroin LSD Mandrax Morphine PCP Acetamide Ammonium acetate Barley Cotton Dacron Ethanol Lucite (Perspex) Melamine Methanol Neoprene Nylon Orlon Paper Polyester Polyethylene Polyurethane PVC Rayon Silk Soybean Sugar Water Wood Wool H C N O Atom fraction (%) Explosives Illicit drugs Miscellaneous substances

11 Fast Neutron Analysis gamma ray signatures Rice Cocaine C4 explosive Glass Aluminium Oxygen Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Acetone Apples Sarin Leather Silicon Nitrogen Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Polyethylene Coffee Water Plastic Iron Carbon Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV) Energy (MeV)

12 Pulsed Fast Neutron Analysis (PFNA)

13 Compact sealed tube neutron generators provide cheap fast neutrons 2 He(d,n) 3 He 2.5 MeV neutrons or 3 He(d,n) 4 He 14.1 MeV neutrons ~ 1 m MF Physics A-325: MeV neutrons s -1, µs-pulsed

14 Fast neutron inelastic scattering mono-energetic fast neutrons object γ Inelastic scattering γ-rays 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. Pulsed Elemental Analysis with Neutrons (PELAN) [Vourvopoulos]

15 µs-pulsed 14 MeV sealed tube neutron generator Neutron flux Fast neutrons during pulses Thermal neutrons between pulses µs µs Time Scattering Capture Activation Interaction: (n, n γ) (n, γ) (n, α), (n, p) Gammas: Prompt Prompt Delayed Elements: C, O H, N, S, Cl, Fe,... O, Al, Si,...

16 APSTNG Associated particle sealed tube neutron generator utilising the T(d,n)α reaction. α-particle detector α d n neutron 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. T α x α y α tritium target T γ E γ γ γ-ray detector interrogated object γ-ray spectrum for each volume element electronics

17 Scattering Scattering cross cross section (b) (b) ENDF/B-VI Total scattering cross section 1 H 12 C 14 N 16 O (ENDF/B-VI) 1 H 12 C 14 N 16 O Incident neutron energy (MeV) Incident neutron energy (MeV)

18 Pulsed Fast Neutron Transmission Spectroscopy spectrum ns-pulsed broad energy neutron beam n transmitted neutrons... 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]. Counts semtex explosive in beam unattenuated beam /v (ns m -1 )

19 Fast Neutron and Gamma Radiography 14 MeV STNG neutrons and 60 Co γ-rays n, γ transmitted neutrons and γ-rays R µ µ n = = ln ln ( 0 I ) n In ( 0 I I ) g g g

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21 γ-rays only neutrons and γ-rays

22 Neutrons in, neutrons out fast monoenergetic neutrons n, n Elastically and inelastically scattered neutrons 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

23 Fast Neutron Scattering Analysis (FNSA) monoenergetic neutron beam (two multiplexed energies) shielding scattering object Backward neutron detector Forward neutron detector... development work undertaken at UCT and ithemba LABS...

24 FNSA θ lab : 150 o H H 45 o The scattering signature C C 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. For example: one set based on pulse height only (does not require a pulsed neutron beam) E n = 6.8 MeV E n = 7.5 MeV Normalized counts per channel N N O O Al Al S S Fe Fe Pb Pb Channel number

25 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. atom fraction Methanol Ammonium Heroin nitrate simulant HCNOAlSFePb HCNOAlSFePb HCNOAlSFePb The identification of specific materials from measured atom fractions can be facilitated by introducing a χ 2 -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.

26 The hardest problem to solve (by far) is the detection of explosives in airline luggage.... 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

27 Combined detection probabilities for a multi-stage system p d = 0.50 p fa =0.05 Passenger profiling X-ray scanning p d = 0.90 p fa = 0.03 OR Stage 1 p d = 0.95 p fa =0.08 AND Overall system p d = 0.94 p fa < Stage 2 neutron system p d = 0.99 p fa = 0.01

28 The ultimate neutron-based system for the detection of explosives and illicit drugs in airline luggage. forward neutron and γ-ray detectors... 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 backward neutron and γ-ray detectors but will this solve the problem? fan beam collimator neutron beam luggage conveyor fast neutron (and γ-ray) source backward neutron and γ-ray detectors shielding

29 Fast neutron menu Mono-energetic beam (single or multiplexed) Neutron generator MeV or 14.1 MeV?... µs - pulsed?... associated particle? Linac Van de Graaff or small cyclotron... ns-pulsed? White (broad energy) beam Radioisotopic ( 252 Cf, 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.

30 Closing thoughts Massive research and development since Lockerbie has not resulted 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).

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