Lead iodide platelets: correlation between surface, optical, and electrical properties with X- and γ-ray spectrometric performance

Lead iodide platelets grown by physical-vapor deposition were characterized by several properties related to spectrometric performance. Platelets were grown from both synthesized lead iodide without purification, and purified by repeated evaporation. The growth was performed by heating the starting material under vacuum (10^-5 mm Hg) or in high-purity Ar atmosphere (500 mm Hg) at temperatures from 390 to 440°C, for 10 to 25 days. Platelets grew at 200-250°C and perpendicular to the ampoule wall. Platelet size is up to 13×8 mm², and their thickness is from 50 to 100 μm. The crystals were characterized by optical microscopy, atomic force microscopy, scanning electron microscopy, and low temperature photoluminescence. These characterizations showed that platelets have an exceptional transparence, that they grew from hexagonal grains, and that their surface has a mean roughness in the range from 1.1 to 4.7 nm. Low-temperature photoluminescence peak position and broadness confirmed the high purity of the starting material, and bands confirmed strong components from free and bonded excitons and low superficial imperfections. Electrical properties were measured by making radiation detectors with front palladium thermal deposition contacts and acrylic encapsulation. Resistivity values up to 4×10¹³ Ω.cm and current density values as low as 288 pA/cm² (20 V) were obtained. X-ray spectrometry was performed with lead iodide detectors, and an energy resolution of 1.8 keV was achieved for the ²⁴¹Am 14 keV radiation. Correlation between optical and surface properties, electrical parameters, and detector performance are presented. Also, the different results were compared with previous ones for lead iodide crystals grown by other methods and with alternative materials.