Patent Number: 053735382
Section: summary

DESCRIPTION The present invention relates to a system for the detection of substances and in particular explosives by the neutron irradiation thereof. It more particularly applies to the checking of luggage, especially in airports. Systems for the detection of explosives involving the irradiation of the latter by neutrons are already known and reference can be made to FR-A-2201765 and EP-A-0227497. It is pointed out that there are three processes for producing gamma photons by the interaction of neutrons with nuclei: the capture of a neutron by a nucleus producing an isotope, which is deexcited by the emission of a gamma radiation and which is referred to by capture gamma radiation, said interaction essentially taking place with thermal neutrons; the inelastic scattering of a neutron on a nucleus, which is deexcited by the emission of prompt gamma radiation, said interaction only existing with fast neutrons having an adequate energy (i.e. an energy at least equal to that of the prompt gamma radiation); and the activation of a nucleus by a thermal or fast neutron, which creates a radioactive nucleus having a certain life and which disintegrates on emitting activation gamma radiation. In addition, standard explosives have carbon, hydrogen, oxygen and nitrogen atoms and the most interesting elements for the detection of the presence of an explosive are nitrogen and oxygen, particularly due to the separability of the gamma lines or peaks, created by the inelastic scattering of fast neutrons, the energy of the gamma photons being between 2.3129 and 6.1304 MeV. The use of capture gamma radiation for nitrogen at 10.83 MeV is also of interest due to the good signal-to-noise (S/N) ratio in the energy range close to 10 MeV. The aforementioned, known detection systems suffer from the disadvantage of giving rise to an excessive probability of a lack of detection (approximately 10.sup.-2 or more) in the case of checks carried out at high speeds and for small explosive quantities. The present invention aims at obviating this disadvantage by proposing a system for the detection of substances and in particular explosives, which makes it possible to reduce this lack of detection probability and to even reach very low values for the same (10.sup.-4, 10.sup.-10 or even less than 10.sup.-10) using high resolution detection means for detecting the gamma radiation. More specifically, the present invention relates to a system for the detection of a substance which may be contained in an object, characterized in that it comprises: means for irradiating the object by thermal and/or fast neutrons, means for detecting the gamma radiation which may then be emitted by the object and electronic means for processing the signals supplied by the detection means, said electronic processing means being provided for counting the gamma photons corresponding to each line of a plurality of characteristic lines of at least one chemical element of the substance, determining, for each line i, a false detection probability for the chemical element associated with said line, i.e. the probability PFi of the detected signal corresponding to said line i being due to a background noise, determining the product of these false detection probabilities, comparing this product with a threshold fixed by the system users and notifying these users if the product is below the threshold fixed by them, the object then being assumed to contain the substance. Therefore the important elements of the invention making it possible to reduce the lack of detection probability compared with known systems are: the choice of one or a plurality of chemical elements contained in the substance which it is wished to detect, e.g. nitrogen alone or nitrogen and oxygen or nitrogen, oxygen and carbon in the case of an explosive, the choice of a plurality of lines characteristic of the element or elements chosen and whereof the corresponding gamma photons are counted, e.g. in the case of an explosive, the lines at 5.106 MeV and at 10.83 MeV of nitrogen or one and/or the other of the preceding lines, together with one or more characteristic lines of oxygen and optionally, in addition, respectively characteristic lines of elements such as carbon, hydrogen and chlorine (chloro explosive) and evaluating the false detection probability relative to each of these lines (or an information quantity linked with this probability). Each false detection probability PFi can be determined by the formula: ##EQU1## in which Ni represents the count, during a time Dt, which corresponds to the line i and which is due to nuclear reactions induced by neutrons and also to a background noise M'2i relative to the line i during the time Dt. Each background noise M'2i can be determined by forming the sum of a physical background noise M2i, relative to the line i, and the upper limit Ci of the counts relative to the said line i, during the time Dt, on objects liable to contain the substance, but which are known not to contain it. According to a first embodiment of the system according to the invention, the electronic processing means are provided for: determining, for each line i, an information quantity Ii relative to the said line i and defined by the formula: ##EQU2## (after determining Ni and then PFi), determining the sum of these information quantities, comparing said sum with a threshold fixed by the system users and notifying said users if said sum exceeds the threshold with which it is compared, the object then being assumed to contain the substance. According to a second embodiment, allowing a shorter calculation time and therefore a shorter object checking time, the electronic processing means are provided for: determining, for each line i, an information quantity Ii relative to the said line i and defined by the formula: ##EQU3## (after determining Ni, but without having to determine PFi), in which K and a are stored functions of a background noise M'2i relative to the line i and N1i is the integral part of the difference Ni-M'2i, Ni representing the count, during a time Dt, which corresponds to the line i and which is due to nuclear reactions induced by neutrons and also to the background noise M'2i relative to the line i, during the time Dt, determining the sum of these information quantities, comparing said sum with a threshold fixed by the system users and notifying these users if said sum exceeds the threshold with which it is compared, the object then being assumed to contain the substance. The system according to the invention is applicable both to a "closed" configuration and to an "open" configuration. The irradiation means can comprise a fast neutron source and an enclosure for thermalizing said fast neutrons, in which is located the source and which serves to receive the object, or can comprise a fast neutron source for irradiating the object, i.e. directly and without a thermalization enclosure. In order to be able to check the entire object, the detection means can comprise a plurality of gamma radiation detectors, the electronic processing means having a plurality of detection chains, respectively associated with the detectors and the configuration of the detectors can be of a matrix type. According to a preferred embodiment of the system according to the invention, which makes it possible to greatly reduce the lack of detection probability compared with known systems, the gamma radiation detection means are high resolution detection means. To this end, it is e.g. possible to use one or a plurality of high purity germanium detectors. Preferably, the irradiation means incorporate a neutron source, the detection means being protected from direct radiation from the source and are collimated towards the object. In a preferred embodiment of the invention, the irradiation means incorporate a pulsed neutron source for supplying neutron bursts and the electronic processing means cooperate with the detection means for carrying out measurements in the time intervals where it is certain that there is only a detection of one of the categories of gamma photons produced during the irradiation of the object by neutrons. This pulsed neutron source is e.g. used for a synchronous detection of prompt gamma photons and a detection of capture gamma photons between neutron bursts. Thus, for each type of detection, it is possible to obtain a S/N ratio well above that obtained with a continuous neutron source. In the present invention, the irradiation means preferably have a 14 MeV neutron source produced by fusion reactions. Thus, a neutron source with such an energy makes it possible to check most chemical elements. Finally, in a special embodiment of the invention appropriate for the detection of explosives, the electronic processing means count the gamma photons of a plurality of lines, which respectively characterize nitrogen, oxygen and possibly carbon, in the case of nitro explosives, or other elements in the case of other explosives.