Development of (Cd,Zn)Te X-ray and gamma ray radiation detectors for medical and security applications

Full text: There is a growing need for large area X-and Gamma radiation detectors for penetrating radiations in various fields of application e.g. astronomy, detectors for nuclear medicine, biosensor materials, security, non-proliferation of hazardous materials, and environmental applications etc. Direct X-rays conversion into electric charges in a semiconductor is envisaged with better spectroscopic characteristics to improve contrast and quantitative measurements compared to indirect detection using scintillators. The family of II-VI semiconductor materials combine a range of excellent properties such as their high sensitivity due to the high mobility-lifetime products, their high energy resolution as a consequence of the electron-hole pair formation energy, their reasonable maturity in terms of microelectronic technologies required for commercial detector fabrication, wide range of stopping power and band-gaps available. In particular, CdTe and CdxZn1-xTe (CZT) with Zn=0.1 offer a favorable combination of physical and chemical properties that makes it attractive as a room temperature X-ray detector material of choice for many applications involving photon energies up to several hundreds of keV. From the scientific experience accumulated in the past years, the detector properties are strongly dependent on a series of parameters which must be strictly controlled during crystal growth, such as the homogeneity, stoichiometry and the related intrinsic defects which appear during the material growth, a high mobility-lifetime for electron and holes is mandatory etc. Production of detector-grade CdTe and CdZnTe on industrial scale is still a challenge and optimal growth methods and growth conditions have been under intensive investigation. Progress in crystal growth and characterization achieved in a project of Institute partnership between Charles University in Prague and University of Freiburg, Germany which was sponsored by Alexander von Humboldt Foundation, will be presented. A complex approach to analyze distribution of defects and defect-related physical properties is as grown crystals based on comparison of various mapping techniques (photoluminescence, photoconductivity, contactless resistivity, mobility-lifetime product, infrared microscopy) will be presented. The obtained maps reflect the combined effect of crystal growth and cooling conditions. Their analysis can serve as information source for simulations of growth process. Special attention will be paid to presence of point, line and volume defects (inclusions and precipitates) generated in as grown and annealed crystals and to their influence to charge collection efficiency. We show in this contribution results of developments of CdTe based sensors for several applications, like an innovative energy resolved X-ray measuring station which was build up for investigations on small animals, low contrast objects and for non-destructive material analysis, where a photon counting semiconductor Medipix detector is being used as X-ray detector. Devices suitable for radiation monitoring will be also presented. (authors)