Zone refining of TlBr detector material

In my experience, the material purity should be exceed more than 6N to be a detector. Because commercial TlBr has a purity of 4N, it is necessary to improve the purity of TlBr before makeing the radiation detector. The zone purification method which uses the difference in solubility of impurities in liquid and solid, is the most common method for increasing the purity of a material to the 6N grade. After 50 times zone refining, TlBr detector showed the gamma-ray response on the Am-241.