Nuclear Instruments and Methods in Physics Research A 582 (2007) 638–643 Photon attenuation and neutron moderation correction factors for the inspection of cargo containers with tagged neutrons C. Carasco a , B. Perot a,Ã , G. Viesti b , V. Valkovic c , D. Sudac c , S. Bernard a , A. Mariani a , J.-L. Szabo d , G. Sannie d , M. Lunardon b , C. Bottosso b , S. Moretto b , S. Pesente b , P. Peerani e , V. Sequeira e , M. Salvato e a Commissariat a` l’Energie Atomique, 13108 St Paul-lez-Durance, France b INFN and Universita` di Padova, Via Marzolo 8, I-35131 Padova, Italy c Institute Ruder Boskovic, 54 Bijenicka c. 10000 Zagreb, Croatia d Commissariat a` l’Energie Atomique, 91191 Gif-Sur-Yvette, France e European Commission, Joint Research Centre, I-21020 Ispra, Italy Received 31 July 2007; received in revised form 3 September 2007; accepted 3 September 2007 Available online 7 September 2007 Abstract The EURopean Illicit TRAfficking Countermeasures Kit (EURITRACK) inspection system uses 14 MeV neutrons produced by the D(T,n a) reaction to detect explosives in cargo containers. Fast-neutron-induced reactions inside the container produce gamma rays, which are detected in coincidence with the associated alpha particle. The definition of the neutron path and the time-of-flight measurement allow positioning the source of the gamma ray inside the container, while the chemical composition of the target material is correlated with the energy spectrum of the coincident gamma rays. However, in case of dense cargo, neutron moderation and photon attenuation inside the container make difficult the reconstruction of the material composition from the measured gamma-ray energy spectrum. An analytical method has been developed and validated against experimental data, which allows obtaining the chemical carbon-to-oxygen and carbon-to-nitrogen ratios of the inspected items from the gamma-ray energy spectra. The principle of the method is presented along with validation tests. r 2007 Elsevier B.V. All rights reserved. PACS: 28.20.Gd; 29.30.Kv; 25.20.Dc; 89.20.Bb Keywords: EURITRACK; Associated particle technique; Fast-neutron inspection; Explosive detection; Photon attenuation; Neutron moderation 1. Introduction The development of non-intrusive devices for the detection of explosives hidden in cargo containers is playing a major place in Homeland Security R&D programs. Controls based on X- or gamma-ray scanners provide only information on the shape and density of the transported goods and can be greatly improved by the use of fast neutrons, which give information about elemental composition of the irradiated goods [1]. Prompt gamma rays following fast-neutron-induced reactions are specific to a given nucleus and can be used to identify chemical elements. Consequently, the yield of specific gamma rays in the recorded energy spectrum can be related to the amount of the corresponding element. In particular, explosive materials can be in principle identified by means of fast- neutron interrogation and discriminated against common goods, due to their specific carbon-to-oxygen (C/O) and carbon-to-nitrogen (C/N) chemical ratios. However, the detected gamma-ray yield can be strongly affected by photon attenuation and neutron moderation effects in the cargo container material. Consequently, in order to avoid misidentification, the C/O and C/N ratios inferred from the measured gamma-ray spectrum have to be corrected for the above effects. ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.09.002 Ã Corresponding author. Tel.: +33 442254048; fax: +33 442252367. E-mail address: bertrand.perot@cea.fr (B. Perot).