Spatial characterisation of a 36-fold segmented AGATA detector via a novel scanning system.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2011)
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|Item Type:||Ph.D. Thesis|
|Title:||Spatial characterisation of a 36-fold segmented AGATA detector via a novel scanning system|
The core principle of segmented gamma-tracking germanium detector arrays like AGATA and GRETA, that will be used in future for 4pi gamma detection, is the application of pulse shape analysis (PSA). The 3D position sensitivity of the HPGe detectors is based on differences in the shape of the charge pulses associated with different interaction points inside the whole volume. It is therefore necessary for this analysis to have a data base containing pulses for all the interaction points inside the detector volume. A full 3D scanning of the detectors, which experimentally determines pulse shapes for each position inside the active volume, is therefore needed. In this thesis, a novel scanning system is presented to determine the HPGe detector pulse shapes as a function of the gamma-ray interaction position inside the detector. The device is based on a pulse shape comparison scan (PSCS) and the positron annihilation correlation (PAC) method which makes it about 100 times faster than the conventional coincidence based scanners. The performance and efficiency of the system is superior because of using a position sensitive detector (PSD)/gamma camera. It consists of a LYSO scintillating crystal optically coupled to a position sensitive photomultiplier tube. The individual multianode readout (IMAR) approach is used to achieve a spatial resolution of ~ 1 mm and to optimize its field of view to ~ 28 cm^2. A Compton scattering imaging technique is implemented to perform an accurate position calibration of the gamma camera. The employment of PSD yields an added advantage of imaging capability which allows to study e.g. the details of the inner structure of HPGe detectors and electric field anisotropy effects. The position response of a planar HPGe detector is obtained using the apparatus and the risetime distribution plots are compared with those obtained via a conventional scanning system. However, to validate the aforementioned scanning principle, an AGATA symmetric detector is tested. The risetime values are measured as a function of the interaction position in both the coaxial and quasi-planar region of the detector. Furthermore, the Multi Geometry Simulation (MGS) code is used to generate theoretical distribution plots for comparison. The transition in charge carrier transport behaviour as a function of the depth is studied for the region of the complex electric field. Systematic deviations between simulation and measurement are observed for the critical front part of the AGATA detector. They are interpolated as due to a non-linear impurity concentration profile of the germanium crystal, asking for rigorous scanning of all detectors in the future.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 530 Physik|
|Divisions:||05 Department of Physics > Institute of Nuclear Physics|
|Date Deposited:||17 Jun 2011 08:29|
|Last Modified:||07 Dec 2012 12:00|
|Referees:||Pietralla, Prof. Dr. Norbert and Aumann, Prof. Dr. Thomas and Durante, Prof. Dr. Marco and Roth, Prof. Dr. Robert|
|Refereed:||6 June 2011|