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- Khaplanov, Anton, 1980-
(författare)
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Applications of Pulse Shape Analysis Techniques for Segmented Planar Germanium Detectors
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The application of pulse shape analysis (PSA) and γ-ray tracking techniques has attracted a great deal of interest in the recent years in fields ranging from nuclear structure studies to medical imaging. These new data analysis methods add position sensitivity as well as directional information for the detected γ-rays to the excellent energy resolution of germanium detectors. This thesis focuses on the application of PSA on planar segmented germanium detectors, divided into three separate studies. The pulse shape analysis technique known as the matrix method was chosen due to its ability to treat events with arbitrary number and combinations of interactions within a single detector. It has been applied in two experiments with the 25-fold segmented planar pixel detector -- imaging and polarization measurements -- as well as in a simulation of upcoming detectors for DESPEC at NuSTAR/FAIR. In the first experiment, a point source of 137Cs was imaged. Events where the 662 keV γ-rays scattered once and were then absorbed in a different segment were treated by the PSA algorithm in order to find the locations of these interactions. The Compton scattering formula was then used to determine the direction to the source. The experiment has provided a robust test of the performance of the PSA algorithm on multiple interaction events, in particular those with interactions in adjacent segments, as well as allowed to estimate the realistically attainable position resolution. In the second experiment, the response of the detector to polarized photons of 288 keV was studied. The polarization of photons can be measured through the observation of the angular distribution of Compton-scattered photons, Hence the ability to resolve the interaction locations had once again proven useful. The third study is focused on the performance of the proposed planar germanium detectors for the DESPEC array. As these detectors have not yet been manufactured at the time of this writing, a set of data simulated in GEANT4 was used. The detector response was calculated for two of the possible segmentation patterns -- that with a single pixelated contact and one where both contacts are segmented into mutually orthogonal strips. In both cases, PSA was applied in order to reconstruct the interaction locations from this response. It was found that the double-sided strip detector can achieve an over-all better position resolution with a given number of readout channels. However, this comes at the expense of a small number of complex events where the reconstruction fails. These results have also been compared to the performance of the 25-fold pixelated detector.
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| 2. |
- Khaplanov, Anton, 1980-
(författare)
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Position-sensitive germanium detectors for gamma-ray tracking, imaging and polarimetry
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern germanium detectors are often manufactured with two-dimensionally segmented electrical contacts. Signals induced in each segment are read out simultaneously when a photon is detected. Detailed pulse shape analysis (PSA) of these signals allows to resolve positions of individual γ-ray interactions with a precision of at least a few mm. The track of a photon can then be reconstructed using γ-ray tracking. Using these techniques, highly efficient large-volume germanium detectors can replace detector systems where previously highly granulated detector arrays were required, and/or large fractions of photons had to be rejected. The ability to reconstruct the direction of an incoming photon and its scattering path makes it possible to use segmented detectors for γ-ray imaging and polarimetry. Doppler correction of photon energies in experiments where γ rays are emitted from fast ion beams can be greatly improved due to improved determination of the emission angle with respect to the beam. Furthermore, arrays of many detectors can be built without the need for conventional anticoincidence detectors for escape suppression. Instead, photons escaping a detector crystal can be tracked through neighbouring ones.In this work position reconstruction accuracy was evaluated for segmented detectors in a number of geometries in realistic applications. Particular emphasis has been put on the reconstruction of data sets containing events of arbitrary complexity in terms of the number of hit segments and interactions per segment. The imaging and polarization sensitivities of a single planar germanium pixel detector have been evaluated experimentally. In these measurements, photons absorbed in two, often adjacent, segments were reconstructed. Simulated interactions of γ-rays with the detectors of the proposed DESPEC germanium array were analysed yielding the position resolution obtainable in realistic experimental situations, as well as its dependence on photon energy, event complexity, noise and other sources of error.
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