The most important scintillation material NaI(Tl) is hygroscopic and is only used in hermetically sealed metal containers to preserve its properties. All water soluble scintillation materials should be packaged in such a way that they are not attacked by moisture. Some scintillation crystals may easily crack or cleave under mechanical pressure whereas others, like CsI, are plastic and only will deform. In table 3.1 below, the most important aspects of commonly used scintillation materials are listed. The list is not exhaustive and new materials are developed regularly