Cesium Iodide is a material with high ?-ray stopping power due to its relative high density and atomic number. For scintillation counting, it is used either in its undoped form or doped with sodium or thallium. CsI is resistant to thermal and mechanical shock. Compared to NaI(Tl), it is relatively soft and plastic, and does not cleave. Because it has no cleavage plane, it is quite rugged. So it is well suited for well logging, space research or other applications where severe shock conditions are encountered.