Nuclear Instruments and Methods in Physics Research A 820 (2016) 141–145

Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A

journal homepage: www.elsevier.com/locate/nima

Spectroscopic neutron for a scanning system

Jill Rahon a, Areg Danagoulian a,n, Thomas D. MacDonald a, Zachary S. Hartwig b, Richard C. Lanza a a Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, United States b Plasma Science and Fusion Center, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, United States article info abstract

Article history: Detection of cross-border smuggling of illicit materials and contraband is a challenge that requires rapid, Received 14 January 2016 low-dose, and efficient radiographic technology. The work we describe here is derived from a technique Received in revised form which uses monoenergetic gamma rays from low energy nuclear reactions, such as 11B(d,nγ)12C, to 24 February 2016 perform radiographic analysis of shipping containers. Transmission ratios of multiple monoenergetic Accepted 9 March 2016 gamma lines resulting from several gamma producing nuclear reactions can be employed to detect Available online 10 March 2016 materials of high atomic number (Z), the details of which will be described in a separate paper. Inherent Keywords: in this particular nuclear reaction is the production of fast neutrons which could enable neutron Active interrogation radiography and further characterization of the effective-Z of the cargo, especially within the range of Nuclear security lower Z. Previous research efforts focused on the use of total neutron counts in combination with X-ray Radiography radiography to characterize the hydrogenous content of the cargo. We present a technique of performing Transmission transmitted neutron spectral analysis to reconstruct the effective Z and potentially the density of the cargo. This is made possible by the large differences in the energy dependence of neutron scattering cross-sections between hydrogenous materials and those of higher Z. These dependencies result in harder transmission spectra for hydrogenous cargoes than those of non-hydrogenous cargoes. Such observed differences can then be used to classify the cargo based on its hydrogenous content. The studies presented in this paper demonstrate that such techniques are feasible and can provide a contribution to cargo security, especially when used in concert with gamma radiography. & 2016 Published by Elsevier B.V.

1. Introduction The attenuation ratios of multiple monoenergetic gamma lines resulting from the 11B(d,nγ)12C nuclear reaction have been suc- The risk of smuggling special nuclear materials (SNM), such as cessfully used to generate initial radiographs indicating material of and , through US ports presents a significant a high atomic number (high-Z) shielded within low and medium-Z security challenge. From 1993 to 2013 there were 16 recorded cargo [2]. This multiparticle reaction also generates neutrons fi attempts to traffic SNM through points of entry, a difficult threat to whose kinetic energies are de ned by the nuclear kinematics of detect as over 40,000 cargo containers are moved through US the reaction, including the reaction Q value, incident deuteron kinetic energy, and final state of the residual 12C nucleus. These ports daily [1]. In addition to the challenge of detecting SNM neutrons can be employed to complete the radiographic assess- hidden within the flow of commerce, general contraband and ment of cargo by providing excellent low-Z materials discrimina- conventional high explosive trafficking presents an additional tion. This research introduces monoenergetic neutron radiography measure of complexity for customs and law enforcement. The as a component in a comprehensive radiographic system and employment of comprehensive radiographic techniques that discusses preliminary findings in the characterization of low-Z improve atomic number (Z) reconstruction at the lower values of cargo. Another technique, Neutron Resonance Radiography (NRR) effective Z will improve the overall picture of cargo security. A has previously demonstrated an ability to distinguish between novel radiographic approach is being developed that will permit various low Z materials when applied to screening of aircraft cargo active detection of illicit cargoes with the benefit of high materials containers [3,4]. NRR generates monoenergetic fast neutrons by discrimination sensitivity. selecting neutron production angles and as such it requires com- plex apparatus for neutron energy selection, which can be difficult to deploy in the field. The technique presented in this work is n Corresponding author. advantageous due to the simplicity of the beam source: both E-mail address: [email protected] (A. Danagoulian). nuclear reactions and continuous energy sources generating quasi- http://dx.doi.org/10.1016/j.nima.2016.03.025 0168-9002/& 2016 Published by Elsevier B.V. 142 J. Rahon et al. / Nuclear Instruments and Methods in Physics Research A 820 (2016) 141–145 monochromatic neutrons can be used as part of the described methodology. It should be clarified that the purpose of this paper is not to present a particular configuration of a radiographic system, but rather to present a new concept for the reconstruction of cargo information based on the spectral analysis of transmitted neutron beams. While prior research leveraging neutron transmission exists [5–8], most of it focuses on measurements of total neutron counts, without exploiting the spectral information. The spectro- scopic technique described in this work can reveal additional information about the cargo type. It can be adapted to various radiographic configurations using a neutron source, assuming the source's neutron energies span the 1–15 MeV range. While the presented data was acquired with a system employing a 3 MeV RFQ accelerator, an excellent tool for research, future field- deployed systems by no means would be limited to this parti- cular configuration. Rather, alternate, more compact and simpler sources can be used, such as:

DT sources producing 14 MeV neutrons, with neutron energies Fig. 1. Energy-dependent total neutron interaction cross-sections for high density broadened using (n,2n) reactions, or small quantities of mod- polyethylene (HDPE), aluminum, and lead (natural isotopic abundance). The HPDE erating material total cross-section is the result of a concentration-weighted sum of carbon and combined DT and DD sources hydrogen cross-sections per the reduced polyethylene molecule, CH2 (i.e.: σHDPE ¼σCþ2σH). This technique is especially well suited for future gamma radiographic systems which leverage gammas from nuclear reac- radiography, neutron radiography employs a similar principle: the 11 12 σ tions, e.g. B(d,nγ) C. While these reactions produce gammas to dependence of the neutron scattering cross-section, n(E), on be used in radiographic applications, they also produce neutrons, incident neutron energy varies greatly for different elements. In allowing for the neutron spectroscopic techniques to be applied analogy with gamma radiography, this circumstance can be with the goal of inferring additional information about the exploited to independently reconstruct the effective Z of the cargo σ cargo type. by using the variability of n(E), which is strongest for lower Z This work presents initial proof of concept measurements, materials. Hydrogenous materials, such as polyethylene, have a σ demonstrating the feasibility of the neutron spectroscopic analysis total n(E) which decreases comparatively faster with increasing approach to the challenge of cargo classification. As described in energy to that of metals and higher Z materials. The energy the conclusion, future work needs to be performed to determine dependence in this interaction, visible in Fig. 1, can be used the limitations and capabilities of this methodology and determine towards Z-sensitive radiographic inspection. the technological development which is necessary to transform Though materials of higher-Z display cross-section resonances this basic concept into a commercial application. in this energy range, they experience a lower relative change in cross-section from low-MeV energies to their plateau, occurring at approximately 10 MeV neutron energy, as shown in Fig. 1.Itis 2. Concept possible to exploit this relative strength in attenuation from low to high incident neutron energies across materials of different Z to The U.S. Customs Service's Container Security Initiative seeks to distinguish low-Z cargo content. prescreen 80% of cargo prior to inbound shipment, relying upon manifest-assisted automated targeting and rapid scanning of high- 2.2. Fast neutron production risk cargoes [9]. Low density, high hydrogen content goods, including furniture, manufacturing articles, and textiles, account The high positive Q-value of the 11B(d,nγ)12C source reaction for 63% of U.S. imports via twenty foot equivalent ISO containers (þ13.73 MeV) contributes to prolific generation of 4.4 and [10]. A scanning system with the sensitivity to distinguish both 15.1 MeV γ rays, provided the incident deuteron energy is above high and very low-Z from medium-Z materials provides a more the 1.63 MeV threshold (ET) [11]. These two monochromatic complete solution in nondestructive interrogation for the detec- gamma energies are the comparison points for the radiographic tion of illicit substances, including weapons materials and general technique described above. The reaction also generates neutrons contraband. in discrete energies, which are primarily determined by the kinetic energy of the deuteron and the energy level of the 12C nucleus in 2.1. Principles of fast neutron radiography the final state [12]. These discrete neutron energies, ranging from 1.6 to 16.7 MeV (in the case of a 3.0 MeV incident deuteron), In gamma radiography, the relative strength of the mass provide an opportunity analogous to gamma radiography to con- attenuation coefficient of a material can be measured through duct transmission analysis of the cargo. Table 1 contains a partial attenuated gamma fluxes at different known interrogation beam listing of neutron energies emitted by the reaction of a 3.0 MeV energies. At high gamma energies, higher Z materials have an deuteron on 11B [11,12]. increased pair production contribution to the attenuation coeffi- The neutron elastic scattering cross-section on hydrogen has cient, relative to Compton scattering. Given an areal density broad variations throughout the energy range of the neutrons measurement, the ratio of attenuated flux at an energy favoring produced by the 11B(d,nγ)12C reaction, from 4 barns at 1 MeV Compton scattering to an energy favoring pair production is used incident neutron energy to 0.2 barns at 16 MeV. for atomic number discrimination of high and medium-Z materials The intensity of the beam of transmitted neutrons, I, through a (aluminum, Z¼13, and greater) [2]. As in dual energy gamma target of thickness x and density ρ can be calculated by the Download English Version: https://daneshyari.com/en/article/8170575

Download Persian Version:

https://daneshyari.com/article/8170575

Daneshyari.com