Bismuth triiodide polycrystalline films as beta direct and digital imaging detectors for medical applications

We report here the use of bismuth triiodide polycrystalline films as beta direct and digital detectors for medical imaging applications. First, films were grown by the physical vapor deposition method on glass substrates 2"×2" in size, with gold previously deposited as rear contact. For the growth, bismuth triiodide 99.99% was heated at 280°C and 400 mmHg of Ar initial pressure, during 20-60 hours (for different film thickness). Film thickness gave values between 20 and 50 μm (±5%). Film grain size was 10 μm (±10%). Detectors were made by depositing gold onto the films as front contact (areas 7-100 mm²) and then performing acrylic encapsulation. Resistivities up to 3 ×10¹⁵ Ωcm were obtained for these detectors. The film response to beta radiation was checked by irradiating the films with beta emitters of different energies and doses and for several bias voltages applied to the detectors. Response correlates with the expected one, considering film thickness and the stopping power of the material, and results to be linear with the exposure rate. The ability of distinguish beta radiation in the presence of a gamma background was also checked. A phantom with a linear beta source arrangement was used to check the films feasibility of imaging separate tumors.