engleski

Matrix characterization of sea floor in threat material detection processes

Matrix characterization is a key for the neutron prototype systems used for underwater threat materials inspections. As used specifically herein, threat material refers primarily to military explosives in objects such as mines, grenades, torpedoes and ammunition disposed on the sea floor. Ammunition dump sites being mostly unprotected and neglected present a serious threat to human security, environmental security and could be possible objects of misuse. In addition to explosive, the possible presence of chemical warfare (CW) agents in the suspicious objects imposes even more serious threat. Fast Neutron Activation Analysis (FNAA) with associated alpha particle using the sealed tube d+t neutron generator is used for noninvasive assessment of the composition or contents of the object which is closed or otherwise inaccessible. Recently, it was shown that FNAA with associated alpha particle can be used in underwater applications . The particular advantages of this technique in threat material underwater inspection are fast interrogation, reliability, high penetration, possibility of incorporation in mobile underwater vehicle and possibility of obtaining three-dimensional information about the composition of the object under inspection. Since the inspected military objects disposed on the sea floor are usually large and dense and should not be moved if the state of the detonator is not known, the FNAA with associated alpha particle is the best candidate for the task. Most of the background in the gamma spectra measured when investigating suspicious object on the bottom of the sea is generated by surrounding sea sediment and rocks. Therefore, the chemical composition of sea floor sediments and rocks has to be known. In addition, after years and decades of lying at the bottom of the sea, dumped ammunition could change to unrecognizing appearance, especially for untrained eye. By using visual inspection only, the military objects can be easily mistaken for rocks and vice versa. Here we present results of underwater analysis (salinity of 38 g/l) of the main type of rocks found in the Adriatic Sea by using FNAA with associated alpha particle. The chemical composition of two main types of rocks found in the Adriatic sea floor have been evaluated, prevailing limestones and dolomites (Ca, Mg– rocks) and magmatic rocks (Si-rocks) which can be found in vicinity of the most distant islands in Eastern Adriatic (Jabuka, Svetac). Inspection of spectrum obtained for limestones and dolomites revealed that peaks attributable to oxygen, magnesium and calcium were clearly apparent while carbon, silicon and iron were significantly weaker. In magmatic rocks the peaks of oxygen, silicon and iron were clearly visible while carbon and calcium were less distinguished. The nitrogen, which is a major component of explosives, was not detected in any type of rocks. Compared to measurements in the air, underwater measurements suppress the peaks of carbon and calcium, but enhance peaks of oxygen and magnesium. In addition, the chlorine from the sea water interferes with some peaks of silicon and iron. The peaks corresponding to listed chemical elements in the obtained FNAA spectra have been detected approximately at 4.44 MeV for carbon, 6.13 MeV including escape peaks at 5.11 MeV and 5.62 MeV for oxygen, 1.37 MeV for magnesium, 1.78 MeV and 2.84 MeV for silicon, 1.22 MeV, 1.29 MeV, 1.73 MeV, 1.76 MeV, 2.13 MeV for chlorine, 0.755 MeV for calcium and 0.85 MeV and 1.24 MeV for iron. We also present geochemical maps of coastal sea sediments in selected areas of the Adriatic Sea. These geochemical maps can be used for evaluating background for neutron sensor applications. For example, mercury, lead or other heavy metals are frequently found in explosive primers, thus the presence of such atomic species may be indicative of the presence of military explosive. Similarly, phosphorous, sulfur, chlorine, fluorine, arsenic, bromine etc. that are found in CW agents can indicate the presence of a CW dump sites. Anyhow, the sediments can contain increased concentrations of above mentioned elements because of natural geological background or anthropogenic pollution of other source types which can interfere with the threat material inspection. Therefore, in addition to the use of chemical analysis of explosives, CW and their “ containers” by means of determination of presence or absence of particular elements of interest or determination of their ratios, the matrix that produces background has to be well studied in order to not be misinterpreted as threat material. Besides, the effects from compounds in the matrix can result in signal enhancement or suppression. In this way, matrix effect can affect the reproducibility and accuracy of the method.

Matrix characterization; threat material

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