Induced nuclear reactions: processes – systems – and elements – Nuclear transmutation – By neutron bombardment
Patent
1991-06-20
1994-12-13
Walsh, Donald P.
Induced nuclear reactions: processes, systems, and elements
Nuclear transmutation
By neutron bombardment
376161, 376166, G21G 106
Patent
active
053735382
DESCRIPTION:
BRIEF 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
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Grenier Gerard
Rambaut Michel
Commissariate a l'Energie Atomique
Voss Frederick H.
Walsh Donald P.
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