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) 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 www.fnda2006.de Also 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 materials illicit drugs money biological materials explosives 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 What is in here? What signature? object detectors interrogating radiation Analysis and exiting Decision radiation detectors 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 Elastically and inelastically scattered neutrons n, n’ fast n Transmitted neutrons neutrons γ Capture γ-rays and inelastic scattering γ-rays The context is everything ... What is the real problem that needs to be solved? analysis of false a “known” object illicit negatives materials tolerated (non-threat) in cargo buried landmines nuclear bombs in materials aircraft in cargo luggage difficulty of problem H C N O Ammonium Nitrate Composition B Composition 4 (C-4) Dynamite EGDN Nitrocellulose Nitroglycerene Octogen (HMX) Explosives PETN Picric Acid RDX TNT Tetryl Cocaine Heroin LSD Mandrax Morphine Illicit drugs PCP Acetamide Ammonium acetate Barley Cotton Dacron Ethanol Lucite (Perspex) Melamine Methanol Miscellaneous Neoprene Nylon Orlon substances Paper Polyester Polyethylene Polyurethane PVC Rayon Silk Soybean Sugar Water Wood Wool 0 20 40 60 80 100 Atom fraction (%) 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) 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 ~1m MF Physics A-325: 109 14 MeV neutrons s-1, µs-pulsed object Fast neutron mono-energetic inelastic scattering fast neutrons 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] µs-pulsed 14 MeV sealed tube neutron generator Fast neutrons during pulses Thermal neutrons between pulses Neutron flux 10-15 µs 80-100 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, ... APSTNG α-particle d detector Associated particle n α sealed tube neutron generator neutron utilising the T(d,n)α reaction. tritium target The 14.1 MeV neutron is γ interrogated tagged in time and direction object by detecting the associated α- particle released in the Tα xα opposite direction. γ-ray γ-ray spectrum yα for each volume detector element ... allows ns-timed measurements to be made. Tγ Eγ electronics 10 10 E N D F /B -V I Total scattering cross section (ENDF/B-VI) H 1 12 8 1H C 8 N 14 Scattering cross section (b) 12C 16 O 14N Scattering cross section (b) 16O 6 6 4 4 2 2 0 0 0 5 10 15 0 5 e n t n e u t r o n e n e r g y ( 10 V ) I n c id Me 15 Incident neutron energy (MeV) Pulsed Fast Neutron Transmission Spectroscopy spectrum ns-pulsed broad n transmitted energy neutron beam neutrons 106 ... ns-pulsed broad energy neutron beam is 105 attenuated in the sample according to total unattenuated scattering cross section beam Counts for each element in the 104 sample. semtex explosive Unfolding analyses of 103 in beam the transmitted neutron spectra allows the elemental content of 102 the sample to be 0.0 50.0 100.0 determined [Overley]. 1/v (ns m-1) Fast Neutron and Gamma Radiography 14 MeV STNG neutrons transmitted and 60Co γ-rays n, γ neutrons and γ-rays µn ln ( I n I n ) 0 R= = µ g ln ( I g I g ) 0 γ-rays only neutrons and γ-rays 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 Fast Neutron Scattering Analysis (FNSA) shielding scattering object monoenergetic neutron beam (two multiplexed energies) Backward neutron Forward detector neutron detector ... development work undertaken at UCT and iThemba LABS ... o o θlab: 150 45 FNSA 800 H 400 H 0 C The scattering signature 400 C 0 N Normalized counts per channel A scattering signatures obtained from 400 N selections of the raw scattering data 0 O which are assembled serially, into a 400 O single, spectrum-like, distribution. 0 Al 400 Al Scattering signatures for pure 0 elements exhibit distinct individual 400 S S characteristics for each element. 0 Fe 400 Fe For example: one set based on pulse height En = 6.8 MeV 0 Pb Pb only (does not require a 400 En = 7.5 MeV pulsed neutron beam) 0 0 150 300 450 600 Channel number Scattering signatures 0.8 measured for unknown Methanol Ammonium Heroin samples are unfolded to nitrate simulant determine the elemental 0.6 composition of the sample. atom fraction 0.4 The measured atom fractions uniquely characterize the 0.2 elemental composition of the scattering sample. 0.0 H C N O Al S FePb 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. 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 Combined detection probabilities for a multi-stage system Stage 1 pd= 0.50 pfa=0.05 Passenger profiling pd= 0.95 pfa=0.08 Overall OR system AND X-ray scanning pd = 0.94 pd = 0.90 pfa < 0.001 pfa= 0.03 pd = 0.99 neutron system Stage 2 pfa= 0.01 The ultimate neutron-based forward system for the detection of neutron and explosives and illicit drugs in γ-ray detectors airline luggage. neutron ... system based on the beam backward fusion of signatures of: neutron and luggage γ-ray detectors conveyor • transmitted and scattered fast neutrons backward • backscattered thermal neutron and neutrons (from H) fan γ-ray detectors • γ-rays from inelastic scattering and beam collimator fast thermal neutron capture shielding neutron (and γ-ray) … but will this solve the problem? source Fast neutron menu Mono-energetic beam (single or multiplexed) Neutron generator ... 2.5 MeV or 14.1 MeV ? ... µs - pulsed ? ... associated particle ? Linac Van de Graaff or small cyclotron ... ns-pulsed? White (broad energy) beam Radioisotopic (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 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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