Sodium Iodide crystals doped with Thallium (or NaI(Tl) crystals) have a very high scintillation or luminescence efficiency and a good energy resolution, NaI(Tl) scintillators has the largest light-out and an emission range well matched to photomultipliers with bialkali photocathodes at a high efficiency, and combination with its easy availability of variety size and geometries and low cost, NaI(Tl) scintillators has become most widely used scintillators. NaI(Tl) crystals is a kind of highly hygroscopic materials, it must be put in a hermetical sealing housing in using.

The well type NaI(Tl) scintillator is a gamma ray detector with a cylindrical part (or a well) that is fabricated concentrically in a cylindrical NaI(Tl) crystal, then the well-type NaI(Tl) crystals is encapsulated in an aluminium housing. For measuring, specimen could be put in the well part and emitted scintilating light would be obtained from the optical window on the top. This well shaped design is especially useful for detection of low-level radiation.  Shanghai North Optics offers the customized well type NaI(Tl) scintillation crystals upon customer's request, and the Photomultiplier Tube (PMT) and electronics could be integrated if required .