Dubrovnik 2005 by sdsdfqw21

VIEWS: 12 PAGES: 31

									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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