AN EXPERIMENTAL PHOTODIODE-ARRAY UNIT FOR ULTRAHIGH-RESOLUTION RECORDING OF X-RAY IMAGES

The geometrical resolution of modern solid-state X-ray imagers (photod iode arrays, PDA, and charge-coupled devices, CCD) is defined by their sensitive-pixel sizes and ranges from 10 up to 20 mu m, i.e. these im agers yield considerably to photoemulsions in the resolution (0.1-1 mu m of emulsions). In the present paper, an attempt to create a recordi ng unit with a spatial resolution comparable to that of photoemulsions is described. First results of testing of the unit is described. The last was fabricated on the basis of a PDA containing 1024 sensitive pi xels which are 150 mu m high and are positioned with a 25 mu m step. T he feature of the given PDA consists of the presence of an individual charge-sensitive amplifier in each pixel. This allowed us to realize a n algorithm of the digital double correlated sampling and to remove KT C noise which, under ordinary conditions, limits the PDA sensitivity. To diminish the noise level, the unit is provided with a double-correl ated-sampling analog highway, which removes 1/f noises of the on-chip read preamplifiers and possesses the LF filter qualities, and with coo ling of the PDA by a two-stage battery of Peltier cells. The unit was tested in the mode of the integrating X-ray imager. It is expected, by corresponding computer processing of the separate X-ray photon absorp tion events, that one will succeed in determining each event coordinat e while allowing for the signal charge distribution between pixels due to charge diffusion in the PDA substratum.