Basic tasks in all monitoring projects involving gamma spectroscopy are design optimization and calibration of the Ge gamma spectrometer system which in general consists of one or more Ge detectors, the corresponding collimators, and the object with radioactive material (container, drum, wall, material on a conveyor etc.). A simplified approach to these problems has been developed by RWE NUKEM GmbH, leading to the software tool “ISO-Mod” which provides a straightforward comprehensive approach for the assessment and analysis of the most common cases without any need for other resources. The detector response is measured with calibration sources as a function of gamma energy, distance and polar angle. The effect of collimators is evaluated for rotational symmetry and for rectangular design based on polygon cross sections, with the possibility to have one or more layers of specific materials. The object model allows considering cubic (e. g. box, material on conveyor) or cylindrical (e. g. drum) absorption matrix and activity distributions with additional optional absorption layers. For considering the absorption in the object matrix and in the collimator material, mass attenuation coefficients are available for a wide range of energy. A number of selected gamma energies and matrix densities can be evaluated in a single evaluation run using Point Kernel and Monte Carlo simulation methods. For selected energies and densities, ISO-Mod provides the probability distribution of the calibration data. This allows investigating the sensitivity to potentially non-homogeneous activity distributions, e. g. to assess the improvement achieved when using more than one detector since up to 4 detectors can be considered simultaneously. All design data and results are — hierarchically structured — stored in a particular database. ISO-Mod is currently being used in different monitoring projects involving conveyor belts, containers and drums with radioactive material. Good agreement was found between the calculations and the results of measurements using calibrated point and homogeneous extended volume sources. Particular procedures are applied for locating hot spots in waste packages with multi-detector systems.

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