  We describe the results of detailed 3-D Monte Carlo simulations of a "CZT
calorimeter" that can be used to detect photons in the keV to several MeV
range. Several astrophysics applications require the detection of photons in
the energy range of keV up to several MeV with good position and energy
resolution. For certain applications Cadmium Zinc Telluride (CZT) detectors
might be the detector option of choice. Up to now, CZT detectors have mainly
been used in the energy range between a few keV to ~1 MeV, we describe the
results of detailed 3-D Monte Carlo simulations of a "CZT calorimeter" that can
be used to detect photons in the keV to several MeV range. The main objective
of these studies is to evaluate the feasibility of CZT calorimeters, to study
their performance and detect and understand performance limiting factors. Such
a calorimeter consists of many layers of closely packed pixellated CZT detector
units. Our simulations of single detector units reproduce experimental results,
indicating that our simulations capture the main factors that limit the
performance of a detector unit. Overall the conclusion of our simulation study
is that between 1 cm and 1.5 cm thick detector units can be used to build a
calorimeter with good performance over the energy range from ~20 keV to ~10
MeV.
