[automatically translated] One of the most complex problems in the measurement with spectrometric range of air particulate samples collected on filter is the determination, for the geometry of measurement used, the photoelectric efficiency curve (Full-Energy_Peak Efficiency-FEPE). The measuring geometry is taken into account, called "packet-sample", size 6 cm × 6 cm × 0.7 cm, is realized starting from of large cellulose filter (45 cm x 45 cm) used for the sampling that, after aspiration, it is sprayed, cut, compound and pressed up to reach the dimensions indicated for the spectrometric measurement. The main difficulties are related to the filter size, the shape of the final sample, the density and other physical characteristics not perfectly reproducible. These factors make it virtually impossible to identify a calibration standard among those commercially available. For the evaluation of the FEPE were prepared different standards, including: - Filters on the surface of which has been distributed a known amount of KCl; - Filters of which was distributed uniformly a modest amount of ThO2, sealed to ensure the radioactive equilibrium with the decay products; The determination of the activity of the used radioactive products were carried out for comparison with measurements of a calibrated source of 152Eu to 25 cm. The efficiency curve is obtained in the hypothesis of having a uniform distribution of the particulate, realized by the alternate positioning of the filter stripes between the center and the periphery of the same filter, and the repeatability of the shape and density of the sample, which, moreover, has been verified by measurement of the dust collection amount with respect to the weight of the white filter. Two types of detectors were finally considered: for the measurement of low energy a planar detector ORTEC ™ GLP photons while for those of higher energy, two HPGe detectors, a Streamline ™ ORTEC detector GEM18180 (18% relative efficiency) and a detector ORTEC GEM ™ 50195 of low background type (60% efficiency). The curve of efficiency determined for each detector, for a total range of energy of a few keV to 2600 keV, has allowed to derive values of activity not dissimilar from those of previous determinations (eg. 137Cs present in the filters taken following the Chernobyl accident ).
|Number of pages||17|
|Publication status||Published - 2011|