Patent Number: 
Section: description

During the inspection of suspect installations samples of different kinds are taken, such as smear samples from surfaces or dust samples from air filters. They are sealed on the spot between sheets of organic material such as polycarbonate, which are sufficiently transparent for allowing later-on the size measurement by means of a microscope and which are altered by the fission products of actinides contained in the sample. The sample quantity should preferably be in the range of 1 to 10 mg. It might therefore be useful to collect a large number of samples on successive portions of a common carrier tape which after sealing sample-by-sample can be enrolled on a reel similar to that of a movie film. The different individual xe2x80x9cpicturesxe2x80x9d can be associated to a data track similar to the sound track on a movie film tape. This track stores data concerning the identification and the place of origin of the respective sample. As soon as the entire spool has been charged with samples it can be transferred to a neutron irradiation facility. The neutron fluence and the irradiation time are adjusted conveniently in order to obtain a number of tracks or xe2x80x9cstarsxe2x80x9d of tracks on the sheets which can be easily analyzed. After the neutron irradiation process the entire tape is etched in an appropriate etching medium such as NaOH. This etching enhances the tracks. FIG. 2 shows an example of tracks from one particle. The next step is a first visual screening in order to eliminate samples from further consideration which apparently have not produced any tracks or only tracks which can be attributed to natural uranium. These samples are considered free of any actinides. The remaining samples are then analyzed under an optical or electron microscope in order to determine the size of particles which have given rise to tracks. These tracks show in general a starlike configuration in the organic sheet (see FIG. 2). The density of tracks around the center of the particle depends on the size of the particle, on the enrichment in fissile actinides (e.g. U-235), on the irradiation time and the neutron fluence. The step revealing the enrichment of the respective particle is constituted by a comparison of the track density around the center of the particle with a standard chart on which a large number of stars is represented. An example of such a chart for uranium is schematically shown in FIG. 3. The stars have been obtained by the same fission track process applied to standard particles of well defined stepped sizes and enrichment ratios. FIG. 1 shows a micrograph of some standard particles of about 4 xcexcm diameter which can be used to establish this chart. Preferably the stars of this chart are arranged in lines and columns, one ordinate being associated to size and the other to the enrichment ratio. The steps relating to size should be adapted to the sizes of U-235 particles which are expected to be found in the samples. The steps relating to the enrichment ratio are selected according to the mission to be accomplished: It might be useful to provide small steps around the natural isotope ratio and coarse steps for high enrichment rations as those particles will in any case be further studied by mass spectrometry. The star whose track density corresponds best to that of the (analyzed particle in the selected size ordinate allows to determine the enrichment ratio U-235 to U-238. In particular there might be marked on the chart a zone of images corresponding to natural uranium (having an enrichment ratio of about 0.7 to 99.3). Stars not corresponding to this zone will be isolated and analyzed more closely by mass spectrometry, such as Secondary Ion Mass Spectrometry (SIMS). A particular feature of the method according to the invention is that unnecessary expensive analytical work such as SIMS is avoided and only reserved to a small number of suspect samples with abnormal enrichment rations. Another advantage is that the samples are collected and sandwiched between organic sheets on the spot and do not need a clean laboratory which otherwise would be necessary when using other analytical techniques. Finally the method according to the invention does not destroy the samples. After neutron irradiation and etching, samples which have not been perused for the SIMS process can be stored in archives and can be verified afterwards. Although the invention has been conceived for the analysis of environmental samples revealing non-declared nuclear activities, it should be understood that it can also be applied to routine safeguard and installation monitoring purposes. In such applications other radionuclides than U-235 can be of interest, such as plutonium. Of course, the chart must be adapted to fit for other nuclides. For example, a specific standard fission track chart should be prepared for plutonium with images of the typical alpha tracks produced by plutonium particles. The tracks of alpha particles can be separated from those of fission products . To achieve this, one organic sheet of the sandwich structure is covered on the inner side with a thin foil of a material stopping the fission products. Thus, a layer of gold having a thickness of a 8 xcexcm and evaporated onto one of the organic sheets, will stop all fission fragments whereas alpha particles pass there-through and can still produce tracks in the underlying sheet. As a result, these tracks correspond to alpha particles only whereas the opposite sheet of the sandwich structure shows tracks due both to these alpha particles and to fission products. The visual analysis of the tracks (size measurement and comparison with the chart) can be replaced by an automatic system of optical evaluation. In this case the standard chart is replaced by a corresponding set of data in the form of a computer fitting program concerning the characteristics of the different stars. Similar programs exist already for fitting X-ray diffraction, mass spectrometry and others. This would speed up the evaluation of the large number of samples on a spool and increase the reliability of the method.