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Cargo inspection with fast neutrons and gamma radiation


In the past few years, different concepts for spatially high-resolving detectors for neutron radiography and spectrometry in pulsed 1 - 10 MeV neutron fields have been developed at PTB. The aim of this was to detect explosives in small and medium-sized cargo units, in air cargo and in vehicles. Due to the fact that there is an increasing interest in detecting also smuggled fissionable material (U, Pu) in the international transport of goods (this interest was triggered in particular by the current U.S. legislation which prescribes that the complete freight destined for the U.S. has to be inspected already at the port of dispatch) - a program has been launched in our department which allows also high-Z materials in cargo to be identified. In this program, methods as similar as possible to those used for the detection of explosives by neutron radiography shall be used for the (accelerator-supported) beam generation and detection technique.

Radiography with 2 discrete gamma energies - the so-called "Dual Discrete Energy Gamma Radiografie, DDEGR" - represents a suitable complementary technique for the identification of heavy elements. This method was proposed a few years ago by Dr. Goldberg, who is currently staying at PTB for a sabbatical.

Figure 1 : Spectral neutron fluence from the 11B(d,n+γ)12C nuclear reaction

In the year under report, the spectral neutron yield in the range up to 20 MeV and the excitation curve for the different gamma lines - especially those at 4.4 MeV and 15.1 MeV - were investigated in the working group for the nuclear reaction 11B(d,n+γ)12C recommended for beam generation. Figure 1 shows the spectral neutron fluence for 3 different deuteron energies, and Figure 2 shows the excitation curve for the relevant 4.4 MeV and 15.1 MeV gamma lines. The result confirms that suitable neutron and gamma spectra and yields can be achieved with this reaction already at moderate deuteron energies (e.g. 6.5 MeV).

Recently, a more exact experiment has been performed in order to study in detail the gamma spectrum and the yield of the gamma lines at 4.4 MeV and 15.1 MeV which depends on the deuteron energy - in particular to deduce also yields with thin 11B targets.

Figure 2 : Excitation function for the 4.4 MeV and 15.1 MeV gamma lines of the 11B(d,n+γ)12C nuclear reaction

Parallel to the experimental investigations, a research project for the development of the combined neutron and gamma screening of cargo was proposed to the German and Israeli Ministries of Science and Technology within the scope of the initiative "German-Israeli Cooperation in Civil Security Research". Six German and three Israeli teams from research institutions, authorities and enterprises are participating in this project proposal. These groups shall investigate all aspects - from fundamental scientific-technical questions of the radiation source and the radiation detection to safety (as, for example, radiation protection) up to legal, commercial and social matters - for a potential later marketing and use at airports. The project proposal has successfully passed the first selection round and is, at present, in the final assessment process.

* M.B. Goldberg, Method & System for Detecting Substances such as Special Nuclear Materials, U.S. Patent No. 7,381,962 , 2008