Measurement of low-activity uranium contamination by gamma-ray spectrometry for nuclear decommissioning

Abstract Nuclear decommissioning takes places after dismantling, and consists of studying the possible presence of any residual contamination. To do so, every surface inside the facility must be radiologically characterized, and every contamination removed. In the case of an alpha contamination linked with a low gamma activity, using a High-Purity Germanium detector (HPGe) combined with hypothesis tests on the count statistics can be helpful to assess said contamination. Gamma-ray activity enables to determine the total contamination activity provided both are related, like in the case of an uranium-based contamination. Hypothesis tests determine the statistical power of the measurement, allowing decision making when dealing with countings below the detection limit. A representation of the tests results using Receiver Operating Characteristics (ROC) curves allow the user to select an appropriate time of measurement, true detection and false alarm rates, in accordance to the required specifications. This paper presents the feasibility study of such a method, applied to the detection of a low-activity gamma surface contamination of uranium on concrete, with varying enrichment levels.