An experimental photodiode array unit for ultrahigh-resolution recording of X-ray images

Abstract

The geometrical resolution of modern solid-state X-ray imagers (photodiode arrays, PDA, and charge-coupled devices, CCD) is defined by their sensitive-pixel sizes and ranges from 10 up to 20 μm, i.e. these imagers yield considerably to photoemulsions in the resolution (0.1–1 μm of emulsions). In the present paper, an attempt to create a recording 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 pixels which are 150 μm high and are positioned with a 25 μm step. The feature of the given PDA consists of the presence of an individual charge-sensitive amplifier in each pixel. This allowed us to realize an algorithm of the digital double correlated sampling and to remove KTC noise which, under ordinary conditions, limits the PDA sensitivity. To diminish the noise level, the unit is provided with a double-correlated-sampling analog highway, which removes 1 f noises of the on-chip read preamplifiers and possesses the LF filter qualities, and with cooling 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 absorption events, that one will succeed in determining each event coordinate while allowing for the signal charge distribution between pixels due to charge diffusion in the PDA substratum.