Activity estimation of radioactive sources is one of the basic concepts in radiation safety. In laboratory conditions this is a well-established technique. However, in field applictions the measurement is much more complex because of an unknown measurement geometry, including shielding between the source and the detector. The source could be heavily masked, the signal being small, but the actual activity might still be very large. To draw correct conclusions from the spectrometric measurements, new analysis methods have to be developed.
The spectrum baseline contains information about the shielding of the source. The analysis of the continuum provides an interesting approach to provide crucial information for the activity estimation. The present research consortium aims to demonstrate the problem and to provide initiatives for better analysis of unknown gamma emitters which are either sealed or behind complex obstacles, such as a concrete wall.
Reference measurements will be performed with high-activity sources (> 1 GBq), such as Cs-137 and Co-60 in different environments and shielding around the sources. The measurement geometry must be realistic from the point of view of field operations, including accidents. The source-detector distances shall be more than 5 m, preferably > 10 m. Gamma spectrometric measurements are then performed with different shielding materials between the source and the detector (1 – 100 mm Pb, Fe or water).
The spectrum analysis is initially performed in three ways: (1) traditional peak analysis resolving individual peaks and then determining their peak area ratios for the attenuation calculation, (2) step analysis underneath each peak and (3) continuum shape analysis far away from the peaks. From these analyses an algorithm is proposed which can be used to conclude the activity and its uncertainty of an unknown shielded source.