The limitations of associated alpha particle technique for contraband container inspections Davorin Sudac * , Sasa Blagus, Vladivoj Valkovic Rudjer Boskovic Institute, P.O. Box 180, 10002 Zagreb, Croatia Available online 19 April 2007 Abstract Inspection of a shipping container for the presence of the threat materials has been investigated in the laboratory by using a 14 MeV neutron beam, a BaF 2 gamma detector and the associated alpha particle technique. The associated alpha particle technique is proposed as a part of a two sensor system for contraband container inspections. This method is effective in the reduction of background radiation with the possibility of collimating electronically the neutron beam. The intrinsic time resolution has been experimentally estimated to be 1.3 ns (FWHM), which allows inspection of a minimum voxel having 7 cm depth along the neutron flight path. The neutron beam intensity plays a crucial role as a limiting factor for the acquisition time reduction. Single counting rates of the gamma and alpha detector were investigated as a function of the neutron intensity, distance between the gamma detector and the neutron source and the type of shielding. The time and the energy spectra for different neutron intensities were evaluated. Ó 2007 Elsevier B.V. All rights reserved. PACS: 25.40.Fq; 29.40.M; 82.80.Ej Keywords: The associated alpha particle technique; Contraband container inspections; Tagged neutrons 1. Introduction In the past few years many efforts have been put into developing a new system for contraband container inspec- tions. An overview of the different nuclear methods which were used or will be used to build such a system can be found, for example, in [1]. One of the possible nuclear tech- niques is the associated alpha particle technique which will be discussed here. In the 3 H(d,n) 4 He reaction neutrons are emitted in all directions and they interact not only with the interrogated object but also with all surrounding materials. The reduc- tion of the background radiation is a key point in practical use of the fast neutrons. As a neutral particles neutrons are not easy to collimate, however the ‘‘collimations’’ can be done electronically by detection of the associated alpha particles emitted at 180° in centre of mass from the neutron direction. The dimension of the tagged neutrons cone is defined by the alpha particle detector solid angle. The inspected volume is defined by the size of neutron beam cone and the time of flight measurements. The neutrons ‘‘tagged’’ in this way interact with interrogated objects and produce gamma rays by inelastic A(n,n 0 c)A processes. The implementation of the associated alpha particle technique in a system for contraband containers inspec- tions, for suitcase inspections and buried explosives detec- tion is shown in [2–5] and references therein. Yet, there are some limitations that complicate the design of such a sys- tem and make a possible limit on inspection time and dimensions of the voxel under the investigation. Some of these limits have already been discussed in [4] where 7.5 kg of TNT in soil background was successfully identi- fied inside the shipping container. 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.04.108 * Corresponding author. Tel.: +385 1 4561 161/1531; fax: +385 1 4680 239. E-mail address: dsudac@irb.hr (D. Sudac). www.elsevier.com/locate/nimb Nuclear Instruments and Methods in Physics Research B 263 (2007) 123–126 NIM B Beam Interactions with Materials & Atoms