Position-sensitive germanium detectors for gamma-ray tracking, imaging and polarimetry

University dissertation from Stockholm : KTH

Author: Anton Khaplanov; Kth.; [2010]


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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