Logo of the Physikalisch-Technische Bundesanstalt

Energy-Selective Neutron Imaging for Detection of Contraband in Luggage and Cargo

11.12.2009

Most established X- and γ-ray methods for contraband and explosives detection in luggage and cargo depend critically on shape recognition and therefore on human operator skill. Furthermore, photon radiography only permits very limited differentiation among elements in the low-Z range and the latter can be rendered quasi undetectable by high-Z element shielding. In contrast, Fast-Neutron Resonance Radiography (FNRR) is one of the most promising methods for fully automatic detection and identification of contraband concealed in luggage and cargo. This is related to the fact that neutrons probe the nuclear properties of the absorber and exhibit highly characteristic structure in their various interaction cross sections with different isotopes and at diverse neutron energies.

Furthermore, neutron transmission depends only weakly on absorber Z and neutrons readily penetrate high-Z materials. For example, figure 1 shows the calculated transmission spectra of MeV neutrons through 10 cm material of water, polyethylene and Tri-Acetone Tri-Peroxide (TATP). TATP is an improvised explosive made of standard household chemicals, that has been employed by terrorists in Israel, the U.K. and a recently failed attempt in Germany. The various dips and bumps in each spectrum are characteristic of its elemental composition, and specifically of its relative abundance of carbon, nitrogen and oxygen.

Figure 1 : Calculated neutron transmission through 10 cm thick TATP, water and polyethylene, as function of neutron energy

A pre-requisite for FNRR is the availability of various neutron transmission images at precisely determined neutron energy. This is achieved by using a broad energy pulsed neutron beam and measuring neutron Time-of-Flight (TOF) for energy selection. This method, known as Pulsed Fast-Neutron Transmission Spectroscopy (PFNTS) requires a high resolution fast-neutron imaging system with TOF-spectrometry capability. In recent years we have investigated several approaches, including gas-detectors and scintillator-based optical detectors operating using various optical readout techniques [1 - 5].

Here we report about the most recent development of a "Time-Resolved Integrative Optical Neutron" (TRION) detector which allows for energy-selective neutron imaging at very high neutron fluxes and is the favourite concept for a possible commercial cargo and luggage inspection system. A comprehensive overview of this concept can be found in [6, 7].

In the basic concept of the TRION detector a neutron image is converted in a scintillator screen to an optical image. This optical image is viewed through a dedicated electro-optical chain capable of providing a neutron transmission image for a well defined, preselected neutron energy. This energy window is cut out from the available broad-energy neutron spectrum using a few-ns wide TOF window where neutrons are allowed to expose a camera image. With this first TRION version images at different neutron energies require successive measurements, since only one single energy frame is available at a time. This is not very economical because out of the broad neutron spectrum only the neutrons in a narrow energy window are employed, the others delivering an excessive radiation dose to the sample, to no useful effect. Therefore, we are developing methods for imaging simultaneously multiple pre-selectable energy (or TOF) windows. Recently we have made operational a 4-energy system, sketched out in figures 2 and 3, which underwent a 1st beam test earlier in 2008. Here the single frame electro-optical read out scheme is replaced by a large-area visible-light amplifier (OPA for OPtical Preamplifier) and 4 independently gated high-speed intensified cameras. Compared to the previous system we can now measure images at 4 different energies simultaneously. The new high speed gating system with shortest exposure time at 4 ns allows also for shorter neutron flight distances or, alternatively, provides better energy resolution compared to the previous system. More details on the new system and the results of the recent tests can be found in [5, 6]. With this system we have made measurements earlier in 2008 performing tomography on simple objects and evaluating its spatial resolution, contrast sensitivity and signal/noise performance. Based on this experiments the system is now further improved - in particular the gain stability of the 4 ICCD channels, where tomography is very demanding. For this a stabilised LED-based reference light source was developed in collaboration with the PTB optics department (WG 4.15) which provides a gain correction matrix for all cameras and projections during a tomographic measurement. A rigorous evaluation of the new system is planned for December 2008 at the accelerator facility of PTB (Department 6.4).

Figure 2 : Engineering drawing of a multiple-energy TRION detector, whose readout is based on an optical preamplifier (OPA) and individually gated intensified CCD cameras (ICCD).

Figure 3 : Photography of the new TRION, here equipped with only 2 ICCDs cameras. Present status is with 4 ICCDs.

Reference

  1. V. Dangendorf, C. Kersten, G. Laczko, D. Vartsky, I. Mor, M. B. Goldberg, G. Feldman, O. Jagutzki, U. Spillman, A. Breskin, R. Chechik:
    Detectors for energy resolved fast neutron imaging,
    Nucl. Instr. and Meth. A 535, p. 93, 2004
  2. V. Dangendorf, R. Lauck, F. Kaufmann, J. Barnstedt, A. Breskin, O. Jagutzki, M. Kraemer, D. Vartsky:
    Time-Resolved Fast-Neutron Imaging with a Pulse-Counting Image Intensifier,
    Proc. of International Workshop on Fast Neutron Detectors and Applications, PoS(FNDA2006)008, 2006, available: pos.sissa.it//archive/conferences/025/008/FNDA2006_008.pdf
  3. D. Vartsky, I. Mor, M. B. Goldberg, I. Mardor, G. Feldman, D. Bar, A. Shor, V. Dangendorf, G. Laczko, A. Breskin, R. Chechik:
    Time Resolved Fast Neutron Imaging: Simulation of Detector Performance,
    Nucl. Instr. Meth. A 542, p. 197, 2005
  4. I. Mor, D. Vartsky, I. Mardor, M. B. Goldberg, D. Bar, G. Feldman, V. Dangendorf:
    Monte-Carlo simulations of time-resolved, optical readout detector for fast-neutron transmission spectroscopy,
    Proc. Int. Workshop on Fast Neutron Detectors and Applications, POS(FNDA2006)064, 2006, available: pos.sissa.it//archive/conferences/025/008/FNDA2006_064.pdf
  5. I. Mor:
    Energy resolved fast neutron imaging via time resolved optical readout,
    MScThesis, 2006, available: jinst.sissa.it/theses/2006_JINST_TH_002.jsp
  6. V. Dangendorf et al:
    Multi-Frame Energy-Selective Imaging System for Fast-Neutron Radiograph,
    submitted to IEEE Transactions of Nuclear Science, Jul. 2008
  7. I. Mor et al:
    A Time-Resolved Integrative Optical Neutron Imaging System for Fast-Neutron Resonance Imaging, in preparation,
    to be published in JINST (http://www.iop.org/EJ/journal/jinst)