Improvements on Bismuth Tri-iodide Platelets For Room Temperature X-ray Detection

Bismuth tri-iodide synthesized from bismuth carbonate and potassium iodide and then treated with hydriodic acid was used as starting material. Platelets were grown by physical vapor deposition (PVD), with a source temperature of 320 degC, during 1 day. Growth temperature was about 250 degC. Platelets are up to 30 times 20 mm² in size, and from 50 to 80 mum in thickness. Platelet surface exhibits extreme brightness and higher uniformity than the ones previously reported, confirmed by optical and scanning electron microscopy. Several reactants were used for platelets´ surface etching; any of them did not improve the crystal surface. Detectors were made with the as-grown platelets by Pd thermal deposition (contact areas of 0.071 cm²), with and without guard-ring, Pd wire attachment with aquadag and acrylic encapsulation. Detector X-ray response was measured by irradiation with a ²⁴¹Am source of 3.5 mR/h. Resistivities up to 1.7 times 10¹³ Omega-cm were obtained, the best reported for detectors made with monocrystals of this material, which correlates with the fact that these detectors show the best signal to dark relation as response to an X-ray moderate radiation exposure: 11.0, with a bias of 60 V. This new generation of platelets give better surfaces, higher resistivities, lower dark currents and better signal to dark relation than previous ones.