Bismuth tri-iodide polycrystalline films for digital X-ray radiography applications

Bismuth tri-iodide polycrystalline films were grown by the physical vapor deposition method. Glass 1´´×1´´ in size was used as the substrate. Palladium was deposited previously onto the substrates as the rear contact. For growth, bismuth tri-iodide 99.999% was heated at 130-170°C under high vacuum atmosphere (5×10^-5 mmHg) or under Ar pressure for 20 hours. Film thickness was measured by the transmission of 59.5 keV ²⁴¹Am emission, giving values ranging from 90 to 130 μm (5%). Film grain size was measured by scanning electron microscopy, and it gave an average of (50±20) μm. Detectors were made with the films by depositing palladium as the front contact (contact area 4 mm²) and then performing acrylic encapsulation. Resistivities of 6×10¹² Ω.cm and current densities of 240 pA/cm² at 20 V were obtained for these detectors. The electron mobility and lifetime and the electron mobility-lifetime product were measured by the transient charge technique, which gave values of 4.4 cm²/V.s, 3.3×10^-7s and 1.4×10^-6 cm²/V respectively. X-ray film response was checked by irradiating the films with a ²⁴¹Am source and with an X-ray beam, for different beam energies and intensities and for several bias voltages applied to the detector. A linear response with exposure rate was obtained. Finally, the results were compared with previous ones for monocrystals of bismuth tri-iodide and polycrystalline films of alternative materials like lead iodide and mercuric iodide.