Caracterización teórico-experimental de nuevos sistemas de detección de neutrones para el control de mercancías en fronteras y puntos críticos

Abstract

In homeland security, one of the main lines of defense against nuclear terrorism is the control at borders of goods and persons entering and leaving customs (land, sea, air) by means of Radiation Portal Monitors, RPM. These devices are part of the "Second Line of Defense, SLD", of the program of the U.S. Department of Defense, which are installed in different parts of the World too. In Spain, through the initiative Megaports, they are implemented in the ports of Algeciras, Valencia and Barcelona. RPMs were designed to detect traces of radiation emitted from an object passing through them with the purpose of intercepting illicit traffic of nuclear or radioactive materials. RPMs usually consist on a set of gamma-ray and neutron detectors. Neutron detectors are used for Special Nuclear Materials (SNM) detection, like 239 Pu, with large size 3 He proportional counters widely used, that are placed inside of a polyethylene box of high density, HDPE. 3 He detectors are characterized by having high efficiency for neutron detection. However, since late the last decade, 3 He shortage worldwide has encouraged searching for alternative neutron detectors. The aim of this thesis is to study the features of alternative new neutron detectors of 10 B+ZnS(Ag), that are based on a scintillator detector design to detect particles , ZnS(Ag) mixed with highly enriched 10 B. Neutrons experiment a reaction with the 10 B of the detector, 10 B(n,) 7 Li, emitting an  particle and 7 Li, which are detected by the ZnS(Ag). Using Monte Carlo methods, with MCNP5, MCNPX and MCNP6 codes, the responses of different detectors with variable geometries have been estimated for 29 monoenergetic neutron sources with values from 10-9 to 20Mev, also, for two isotopic neutron sources of 241 AmBe and 252 Cf. The response in counts per second per nanogram of sensitive material, cps/ng from a moderated neutron source of 252 Cf at a distance of more iv than 200 cm, has been also calculated through the reactions that occur in the 10 B. Different models were developed based in distinct cases, indoors, outdoors, and different geometries. These detectors are studied to achieve at least the efficiency established by the ANSI 42.35 Standard, so that be able to detect a neutron source of 252 Cf of 20.000 n/s passing through the detector at more than 200 cm distance, which requires an efficiency higher than 2,5 cps/ng 252 Cf at 200 cm, according to studies carried out in Pacific Northwest National Laboratory. With the MCNP6 code, the case of different SNM (239 Pu, "Highly Enriched Uranium" HEU at 70% and 94%, 252 Cf) is evaluated inside a customs transport vehicle passing in front of four different RPMs with 10 B+ZnS(Ag) detectors. With a set of models and measurements we verified that these neutron detectors of 10 B+ZnS(Ag) are an interesting and valid alternative to replace 3 He detectors installed in the RPMs at borders.