Collimation technique for HPGe-detector gamma spectrometry in intense radiation fields

Sammanfattning: The aim of this master thesis was to develop a rotation-based collimation system for large coaxial high-purity germanium detectors for use in intense radiation fields. A collimator was constructedformed as a cylinder surrounding the detector, with a 90° slit opening to the detector. The rotational asymmetry caused by the slit was used to trace the direction of the incident gamma rays as well as reducing dead time problems at high-count rates. High-purity germanium detectors have an energy resolution superior to scintillation detectorssuch as sodium iodide and are therefore more suited for radionuclide identification. Germanium detectors are an important complement to the more effective sodium iodide detectors, which often are used in mobile in situ gamma spectrometry. A problem with large high purity germanium detectors is often that the dead time rises and becomes too high for accurate measurements even at as low gamma dose rates as a fewmicrosieverts per hour. Without any modifications of the detector geometry measurements could be impossible in many situations, for instance after a reactor failure scenario with release of radioactive material. A mobile system with the collimator and detector mounted in an aluminum rack was built. The system has been tested for localization of radioactive sources. The collimator attenuates atmost 89 ± 1 percent of the photons at 1332 keV, which reduces the dead time significantly. By rotating the collimator the reduction of photon fluence rate could be varied from 3 ± 3 percent to 89 ± 1 percent.