Development of 3D Imaging Detectors for High Energy Astronomy Instrumentation

Observations within medium-energy X- and gamma-ray astronomy are characterized by the technical challenges associated with instrument requirements. Current observatories suffer from poor sensitivity in this energy range compared to its neighboring bands, resulting in the mediumenergy X- and gamma-ray domain remaining largely unexplored. Meaningful advances in this field depend on the next generation of space observatories, where state-of-the-art detector technology plays an important role.

This thesis focuses on the development of the 3D CZT drift strip detector, a promising candidate for future space missions within medium-energy X- and gamma-ray astronomy. This detector offers excellent intrinsic spatial and spectral resolution but requires further development to achieve a higher Technology Readiness Level. The thesis work is divided into three main components: an investigation into enhancing the 3D CZT drift strip detector model by mapping electron mobility and lifetime of the detector in three dimensions, a comprehensive characterization study of 10 3D CZT drift strip detector modules, and finally a simulation study of the detector operating in a small Compton camera configuration in orbit to assess its feasibility for use on small platforms like CubeSats.

In the first study, three-dimensional maps of electron mobility and lifetime in the 3D CZT drift strip detector were presented. Notably, these maps revealed material non-uniformity, which resulted in a substantial improvement in the 3D CZT drift strip detector model performance. The study underlined that knowledge of the detector material is essential for thorough understanding and characterization of the detector and for modeling its response. Areas of severe charge trapping or poor charge transport can be identified and included in the models as well. Furthermore, a reliable model depicting the pulse shape formation in the detector can be used to generate training data for future artificial neural network models, which can be a crucial for data size reduction.

The second study presented a full characterization work of 10 newly fabricated 3D CZT drift strip detector modules. A key finding of this study, was that the leakage current and overall performance of the modules were within the desired range, even though the modules were fabricated with a simpler electrode deposition process, compared to previous detector versions. This allows for a simpler manufacturing process of future modules. The study elevated the 3D CZT drift
strip detector technology to a more modular design with a simpler electrode geometry, while maintaining sub-millimeter intrinsic spatial resolution, moderate spectral resolution at 122 keV, and good spectral resolution at 661.6 keV. Advances in the overall setup and dedicated readout electronics can reduce the dominant electronic noise and help further improve the spectral resolution of the detector. Additionally, it was highlighted that the 3D CZT drift strip detector
technology might not only benefit the future of space exploration, but a technology transfer can make it valuable on Earth, for example, in future breast cancer diagnostic tools.

Lastly, the simulation study indicated that it may be possible to operate the detector on a small Compton camera payload, with some limited science goals. In terms of technology demonstration purposes, a small payload like this could be valuable.

This thesis brings the 3D CZT drift strip detector technology closer to a space ready module. It summarizes the findings and developments made throughout the project, and wraps up with an outlook on the future work for the 3D CZT drift strip detector, to hopefully, one day operate it
in space.