Si, CdTe and CdZnTe radiation detectors for imaging applications

The structure and operation of CdTe, CdZnTe and Si pixel detectors based on crystalline semiconductors, bump bonding and CMOS technology and developed mainly at Oy Simage Ltd. And Oy Ajat Ltd., Finland for X- and gamma ray imaging are presented. This detector technology evolved from the development of Si strip detectors at the Finnish Research Institute for High Energy Physics (SEFT) which later merged with other physics research units to form the Helsinki Institute of Physics (HIP). General issues of X-ray imaging such as the benefits of the method of direct conversion of X-rays to signal charge in comparison to the indirect method and the pros and cons of photon counting vs. charge integration are discussed. A novel design of Si and CdTe pixel detectors and the analysis of their imaging performance in terms of SNR, MTF, DQE and dynamic range are presented in detail. The analysis shows that directly converting crystalline semiconductor pixel detectors operated in the charge integration mode can be used in X-ray imaging very close to the theoretical performance limits in terms of efficiency and resolution. Examples of the application of the developed imaging technology to dental intra oral and panoramic and to real time X-ray imaging are given. A CdTe photon counting gamma imager is introduced. A physical model to calculate the photo peak efficiency of photon counting CdTe pixel detectors is developed and described in detail. Simulation results indicates that the charge sharing phenomenon due to diffusion of signal charge carriers limits the pixel size of photon counting detectors to about 250 μm. Radiation hardness issues related to gamma and X-ray imaging detectors are discussed.

Työssä esitellään Suomessa kehitettyjä uudentyyppisiä digitaalisia röntgen- ja gammakuvausantureita, jotka perustuvat pii- ja CdTe-puolijohdeteknologiaan. Uuden tekniikan anturit osoitetaan toimivan hyvin lähellä teoreettista optimia. Erinomainen röntgenkuvan laatu saavutetaan pienellä säteilyannoksella. Antureiden rakenne ja toiminta selostetaan seikkaperäisesti ja niiden soveltamisesta hammaskuvaukseen ja reaaliaikaiseen videoröntgenkuvantamiseen annetaan esimerkkejä. Työssä esitellään myös fotonilaskentaan perustuvan CdTe gammakuvausanturin toiminta simuloinnein ja mittauksin sekä käsitellään antureiden säteilynkestoon liittyviä kysymyksiä.