X-ray detectors for NDE applications

A tremendous development in the field of imaging radiation detectors has taken place in the last decade. Conventional X-ray film has been replaced by digital X-ray imaging systems in a no. of ways. Such systems mainly consist of silicon charge coupled devices (CCDs) where incident photons create electron-hole pairs in the thin silicon absorption layer near the surface. In contrast to visible light, which is absorbed within a 2 micro m layer of silicon, the penetration of X-ray is much deeper due to higher photon energy. This disadvantage is often circumvented by the use of a scintillator absorption layer. Due to scattering of the low energy fluorescence photons, resoln. and contrast of the X-ray images decrease. In order to eliminate these disadvantages, hybrid detectors consisting of direct converting semiconductors and readout electronics parts are fabricated. For this configuration, it is advantageous that both parts can be optimized sep. and different materials can be used. Because of the well developed technol., the readout chip is fabricated out of silicon. As absorbing material, silicon is less suitable. In a silicon substrate of 500 micro m thickness, only 15% of a 30 keV radiation is absorbed and converted into charges. In order to increase the absorption, materials with a higher at. mass have to be used. Several compd. semiconductors can be used for this purpose. One of them is GaAs, which is available as high quality semi-insulating wafer material. For detector optimization, GaAs wafers from several manufacturers with different properties were investigated. Test structures with Schottky and PIN diodes were fabricated. The I/V curves of the diodes, the spectral response from 5 up to 150 keV, the carrier concn., and the carrier mobility were measured and compared. A survey of the results and the criteria for material selection resulting from these measurements will be provided in the paper.