Abstract. One of the most important operations, which have to be performed at the decommissioning of a nuclear installation, is the radioactive characterization of the dismantled components. This is done usually, in situ, by using a portable measuring device. Such a portable gamma spectrometry device was set up at the Institute for Nuclear Research (SCN) and was adapted to measure surface contamination. This was required for the preparedness activities of SCN's multizonal reactor (RMZ) decommissioning. By economic reasons one renounced to start using this reactor to test CANDU fuel performances and nowadays there is considered its dismantling. Since the reactor was never operated, we assumed that its components are only superficially contaminated due to the positioning of RMZ pool nearby the TRIGA-SSR and post irradiation examining cells. Is being expected for a series of RMZ 's components to be contaminated over the exemption levels and therefore, during the decommissioning activities, there is required a permanent control of surface contamination. The radioactive characterization of the dismantled components will serve furthermore in the selection of contaminated materials to reduce the radioactive wastes. The collimated gamma-ray spectrometry device used was calibrated by using a Monte Carlo based computer program. Generally, the issues related with the Monte Carlo simulation of radiation transport are solved by using algorithms, which are based on an imitative approach of processes occurring at the emission, transport and interaction of radiation with materials of source and detector. These algorithms are time consumers even when variance-reducing procedures are used. A semiempirical method can be used to calibrate a gamma detector, especially when this one has a collimator to reduce the range of incidence angle. One can demonstrate that detection efficiency for a particular geometry can be evaluated like the product between the efficiency in a standard detection geometry (usually a point source one) and the ratio between the effective solid angle in the given geometry and the effective solid angle in standard geometry.

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