  We are currently developing Cadmium Zinc Telluride (CZT) detectors for a
next-generation space-borne hard X-ray telescope which can follow up on the
highly successful NuSTAR (Nuclear Spectroscopic Telescope Array) mission. Since
the launch of NuSTAR in 2012, there have been major advances in the area of
X-ray mirrors, and state-of-the-art X-ray mirrors can improve on NuSTAR's
angular resolution of ~1 arcmin Half Power Diameter (HPD) to 15" or even 5"
HPD. Consequently, the size of the detector pixels must be reduced to match
this resolution. This paper presents detailed simulations of relatively thin (1
mm thick) CZT detectors with hexagonal pixels at a next-neighbor distance of
150 $\mu$m. The simulations account for the non-negligible spatial extent of
the deposition of the energy of the incident photon, and include detailed
modeling of the spreading of the free charge carriers as they move toward the
detector electrodes. We discuss methods to reconstruct the energies of the
incident photons, and the locations where the photons hit the detector. We show
that the charge recorded in the brightest pixel and six adjacent pixels
suffices to obtain excellent energy and spatial resolutions. The simulation
results are being used to guide the design of a hybrid application-specific
integrated circuit (ASIC)-CZT detector package.
