  The performance of hybrid GaAs pixel detectors as X-ray imaging sensors were
investigated at room temperature. These hybrids consist of 300 mu-m thick GaAs
pixel detectors, flip-chip bonded to a CMOS Single Photon Counting Chip (PCC).
This chip consists of a matrix of 64 x 64 identical square pixels (170 mu-m x
170 mu-m) and covers a total area of 1.2 cm**2. The electronics in each cell
comprises a preamplifier, a discriminator with a 3-bit threshold adjust and a
15-bit counter. The detector is realized by an array of Schottky diodes
processed on semi-insulating LEC-GaAs bulk material. An IV-charcteristic and a
detector bias voltage scan showed that the detector can be operated with
voltages around 200 V. Images of various objects were taken by using a standard
X-ray tube for dental diagnostics. The signal to noise ratio (SNR) was also
determined. The applications of these imaging systems range from medical
applications like digital mammography or dental X-ray diagnostics to non
destructive material testing (NDT). Because of the separation of detector and
readout chip, different materials can be investigated and compared.
