Purification of bismuth tri-iodide as material for radiation detector purposes

First attempts to purify bismuth tri-iodide for radiation detector applications are presented. Bismuth tri-iodide synthesized from bismuth subcarbonate and potassium iodide was used as starting material. Sample composition was confirmed by powder X-ray diffraction. A first set of samples (at 400 mmHg of Ar) was purified by zone refining in a vertical zone refining furnace at 460/spl deg/C and at a speed of about 3 cm/hr down the length of the ingot, 100 passes. One of these samples was then purified by sublimation. A second set of samples was purified by repeated sublimation under Ar atmosphere (400 mmHg) with the source sample at 370/spl deg/C and the cold extreme of the ampoule at room temperature. The starting and purified products' stoichiometry was determined. Also the starting and purified materials purity was determined by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). Zone refining appears as an appropriate method for purification of several impurities, while sublimation, for others. On the other hand, zone refining clearly deteriorates the material stoichiometry (from BiI/sub 2.82/ to BiI/sub 2.75/), while a combination of the two methods, that is, a first zone refining process of about 100 passes, followed by, at least, one sublimation, results to be the most appropriate purification method, in order of obtaining a pure but also stoichiometric material (BiI/sub 3.08/). This process has a yield of 20%. The purification way proposed here turns over to be the same what was found for lead iodide purification, although easier, because of the lower tendency for bismuth tri-iodide to decompose.