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Explosives detection using neutrons

Abstract:

The physical properties of neutrons made it possible that immediately after their discovery, in the year 1932, technologies by activation and detection of gamma-emission began to be developed and applied as a useful process for identifying and characterizing materials as early as the 1940s. However, it has been from the last twenty years when neutrons have become a powerful tool for non-invasive studies in various branches of science and engineering, such as physics, geotechnical exploration, biology, medicine, materials engineering, and interest in applying these technologies in border, port and airport security equipment has increased, complementing existing systems based on radiographic images that use X-rays and gamma radiation. This impulse has been largely motivated by the development of neutron sources based on compact generators, which helped to expand the fields of application, by increasing the available neutron fluxes in a very wide energy range. Likewise, advances in the design of solid-state and scintillation gamma ray detectors have significantly improved detection sensitivity, and in combination with the development of increasingly precise algorithms and decision techniques, it has enhanced the obtaining of statistically valid results, and the unambiguous identification of the target materials and samples. The fields of application of neutron techniques are very diverse, although the greatest developments have occurred in the identification of terrorist threats using CBRNE materials (chemical, biological, radiological, nuclear and explosive) that pose a clear danger to the health of individuals. As a consequence, prevention of these types of incidents is among the top priorities of all countries and organizations. From the advance of these technologies, the field of application has been extended to other areas such as the detection of mines and unexploded ordnance, the applications of geoterrestrial and seabed reconnaissance, and finally the application to space exploration, incorporating neutron equipment into Rover exploration vehicles for planets and asteroids. The main objective of this work is to select the key components that would make up a thermal neutron analysis system (ANT), so that it is a portable device that can be integrated into systems or remotely operated vehicles, versatile, in the sense being able to use in different fields of application, without using isotopes and any radioactive components, so that when the system is off it does not require any precautions related to radiological safety and protection. The elementary process of applying neutronic techniques consists of: 1) irradiating the material to be characterized with neutrons; 2) detect and measure the response of that material to irradiation; 3) analyse and process the information to determine what material it is. Monte Carlo (MC) methods have been used to design an explosives detection system (EDS) to identify these materials and other threats as drugs or buried mines. Using several versions of MCNP code, different types of EDS were modelled, and neutron transport, prompt y-rays production and gamma-rays detection were simulated in order to optimizing the system. The performance of neutron sources, moderation materials, gamma radiation detectors, and system configurations have been compared. Once the key elements of the EDS have been optimized through simulations, a significant amount of experimental validation measurements have been carried out using 241Am/Be9 isotopic neutron sources, gamma and neutron detectors or Boner Spheres Spectrometer (BSS), among others. The results achieved at the laboratory show great similarity and coherence with the simulations. The works were carried out in the neutronics hall of the Neutron Measurement Laboratory of the Energy Engineering Department at the Polytechnic University of Madrid (NML-EED-UPM).