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In Table 
we show the results of a
GEANT simulation of the effects of material in front of the barrel
crystal calorimeter for CLEO II and for various CLEO III configurations.
For this simulation the 
's were generated uniformly
in |cos
(
between 45 and 135
) with a momentum
of 250 MeV/c. Both photons were required to be detected in the good barrel
region. The material of the particle identification systems was distributed
as might occur in an actual implementation. The radiation lengths are
representative and might differ slightly in a constructed detector.
The results of this simulation indicate a linear loss of
efficiency even for small amounts of material and an indication of loss of
mass resolution when more than 0.2 
is traversed. In
Fig. we show the mass resolution attained in the CLEO II
detector when one photon is constrained to the good barrel region
and the other passes through varying material radiation thicknesses.
A striking degradation of the mass resolution
from 5 to 12 MeV/c
can be observed. This work indicates that in the central
region the total amount of material in front of the calorimeter should be kept
below 20% in order to maintain the calorimeter performance relative to
CLEO II. It also indicates a need to reduce the amount of material in
the barrel/endcap transition region and in front of the endcaps.
