  We report here on the optimization of 0.5 cm thick pixelated Orbotech CZT
detectors with regards to the best contacting materials and the use of steering
grids. We evaluated the performance of different contacting materials. Our
study differs from earlier ones in that we investigated the performance of
different anode and cathode materials separately. We obtain the best
performance with Au cathodes. For different anode materials Ti and In give the
best energy resolutions. The detector (2.0 x 2.0 x 0.5 cm, 8 x 8 pixels) shows
excellent 59 keV, 122 keV and 662 keV energy resolutions of 1.4 keV, 1.9 keV,
and 7.4 keV, respectively. Furthermore, we report on using steering grids to
improve on the performance of the pixelated detectors. Previously, the benefit
of steering grids had been limited by additional electronic noise associated
with currents between the negatively biased steering grids and the anode
pixels. We are currently exploring the possibility to isolate the steering grid
from the CZT substrates by a thin layer of Al$_2$O$_3$. We performed a series
of measurements to determine by how much the isolation layer reduces the
grid-pixel currents. Comparing the currents between two Au contacts before and
after isolating one of the two contacts from the CZT with a 700 nm thick layer
of Al2O3, we measure that the isolation layer reduces the currents by a factor
of about 10 at 500 V. We present some results from a detector before and after
deposition of an isolated steering grid. The grid indeed improves on the
detectors energy resolution and detection efficiency. We show that simulations
can be used to model the anode to cathode charge correlation in excellent
agreement with the experimental results.
