Photograph of a CsI Crystal Being Wrapped.

The crystals used in the CsI electromagnetic calorimeter all have a unique outer wrapping designed to give the crystal a uniform response along its length. To first order this is accomplished by using dark wrapping on the end of the crystal which is read out with a photomultiplier tube and reflective wrapping on the end farthest from the tube. The relative amounts of dark and reflective wrapping as well as the boundary between the two is different for every crystal.