Advanced Radiation Detectors for Particle Physics and Medical Imaging

Xiaopeng Wu

Research output: ThesisDoctoral ThesisCollection of Articles


The edgeless or active edge pixel detector has gained increasingly attention due to its superiority in defect-free edges and the capability to minimize the insensitive area at edges of the detector. The main focus of this thesis is design, fabrication and characterization of edgeless radiation detectors aiming for large area detection of high energy physics and medical imaging applications. Edgeless detectors with various polarities are fabricated on Float Zone silicon and Magnetic Czochralski silicon (MCz-Si). Characterization of edgeless detectors is performed with the electrical and isotope measurement apparatus built by the author at VTT, the 3-D spatial scanning system at IEAP-CTU in Prague, and the proton beam test setup at the Van de Graaff accelerator laboratory. The leakage currents are 1-100 nA/cm2 at full depletion voltages for detectors with different thicknesses. Charge collection efficiency of edge pixels in a detector is influenced by electric field distortion that is mainly determined by detector thicknesses and edge-to-pixel distances. Spectroscopic measurements show good energy resolutions of the edge pixels, indicating an excellent charge collection at the edge of the detector. Apart from edgeless detectors, the author investigates radiation-hard AC-coupled stripe detectors for high-luminosity radiation environments. The detectors are fabricated on MCz-Si wafers with an optimized process to avoid generation of thermal donors. The detectors have full depletion voltages in the range of 120-150 V and leakage currents about 55 nA/cm2 at 200 V bias voltage. The test beam result shows a signal-to-noise ratio greater than 40 achieved by the detector under study. The result demonstrates that MCz-Si detectors can reliably be manufactured in the industrial scale semiconductor process. As a part of the thesis work, a reactor based on the Chloride Vapor Phase Epitaxy technique is built to grow high-purity gallium arsenide (GaAs) epitaxial material. Diodes and pixel detectors are fabricated on an epitaxial GaAs substrate. The full depletion voltage of the GaAs detector with 110 μm epi-layer thickness is in the range of 8–15 V and the leakage current density is about 10 nA/cm2. Good spectroscopic results are obtained from both a GaAs diode and a hybridized Medipix GaAs detector.
Translated title of the contributionAdvanced Radiation Detectors for Particle Physics and Medical Imaging
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Savin, Hele, Supervising Professor
  • Kalliopuska, Juha, Thesis Advisor, External person
  • Härkönen, Jaakko, Thesis Advisor, External person
Print ISBNs978-952-64-0992-4
Electronic ISBNs978-952-64-0993-1
Publication statusPublished - 2022
MoE publication typeG5 Doctoral dissertation (article)


  • edgeless detector
  • pixel detector
  • MCz-Si
  • GaAs


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