  The Nuclear Spectroscopic Telescope Array (NuSTAR) will be the first space
mission to focus in the hard X-ray (5-80 keV) band. The NuSTAR instrument
carries two co-aligned grazing incidence hard X-ray telescopes. Each NuSTAR
focal plane consists of four 2 mm CdZnTe hybrid pixel detectors, each with an
active collecting area of 2 cm x 2 cm. Each hybrid consists of a 32 x 32 array
of 605 micron pixels, read out with the Caltech custom low-noise NuCIT ASIC. In
order to characterize the spectral response of each pixel to the degree
required to meet the science calibration requirements, we have developed a
model based on Geant4 together with the Shockley-Ramo theorem customized to the
NuSTAR hybrid design. This model combines a Monte Carlo of the X-ray
interactions with subsequent charge transport within the detector. The
combination of this model and calibration data taken using radioactive sources
of Co-57, Eu-155 and Am-241 enables us to determine electron and hole
mobility-lifetime products for each pixel, and to compare actual to ideal
performance expected for defect-free material.
