Detection of explosive substances by tomographic inspection using neutron and gamma-ray spectroscopy

In recent years the detection and identification of hazardous materials has become increasingly important. This work discusses research and development of a technique which is capable of detecting and imaging hidden explosives. It is proposed to utilise neutron interrogation of the substances under investigation facilitating the detection of emitted gamma radiation and scattered neutrons. Pulsed fast neutron techniques are attractive because they can be used to determine the concentrations of the light elements (hydrogen, carbon, nitrogen, and oxygen) which can be the primary components of explosive materials. Using segmented High Purity Ge (HPGe) detectors and digital pulse processing [R.J. Cooper, G. Turk, A.J. Boston, H.C. Boston, J.R. Cresswell, A.R. Mather, P.J. Nolan, C.J. Hall, I. Lazarus, J. Simpson, A. Berry, T. Beveridge, J. Gillam, R.A. Lewis, in: Proceedings of the 7th International Conference on Position Sensitive Detectors, Nuclear Instruments and Methods A, in press; I. Lazarus, D.E. Appelbe, A. J. Boston, P.J. Coleman-Smith, J.R. Cresswell, M. Descovich, S.A.A. Gros, M. Lauer, J. Norman, C.J. Pearson, V.F.E. Pucknell, J.A. Sampson, G. Turk, J.J. Valiente-Dobon, IEEE Trans. Nucl. Sci., 51 (2004) 1353; R.J. Cooper, A.J. Boston, H.C. Boston, J.R. Cresswell, A.N. Grint, A.R. Mather, P.J. Nolan, D.P. Scraggs, G. Turk, C.J. Hall, I. Lazarus, A. Berry, T. Beveridge, J. Gillam, R.A. Lewis, in: Proceedings of the 11th International Symposium on Radiation Measurements and Application, 2006. ] the scatter path of incident photons can be reconstructed to determine the origin of the gamma-rays without the need for mechanical collimation by applying the Compton camera principle [V. Schonfelder, A. Hirner, K. Schneider, Nucl. Instr. and Meth. 107 (1973) 385; R.W. Todd, J.M. Nightingale, D.B. Everett, Nature 251 (1974) 132. ]. In addition, it is proposed to utilise the scattered neutrons which recoil from the materials being assayed, detecting them with a fast neutron detector providing data for inversion to tomographic images. In this paper, we present our approach to the design and implementation of a system for the efficient screening of goods in luggage and cargo containers. The simulation in a Monte Carlo framework using GEANT4 has been carried out for the imaging of gamma-ray events using the Compton camera design which will be discussed. The results of Compton camera measurements using HPGe detectors and the subsequent reconstructed images will also be presented