  To study an alternative to BaF2, as the crystal choice for the Mu2e
calorimeter, thirteen pure CsI crystals from Opto Materials and ISMA producers
have been characterized by determining their light yield (LY) and longitudinal
response uniformity (LRU), when read with a UV extended PMT. The crystals show
a LY of ~ 100 p.e./MeV (~ 150 p.e./MeV) when wrapped with Tyvek and coupled to
the PMT without (with) optical grease. The LRU is well represented by a linear
slope that is on average around -0.6 %/cm. The timing performances of the Opto
Materials crystal, read with a UV extended MPPC, have been evaluated with
minimum ionizing particles. A timing resolution of ~ 330 ps (~ 440 ps) is
achieved when connecting the photosensor to the MPPC with (without) optical
grease. The crystal radiation hardness to a ionization dose has also been
studied for one pure CsI crystal from SICCAS. After exposing it to a dose of
900 Gy, a decrease of 33% in the LY is observed while the LRU remains
unchanged.
