  We report results of an experiment conducted in May 1997 to measure CdZnTe
background and background reduction schemes in space flight conditions similar
to those of proposed hard X-ray astrophysics missions. A 1 cm^2 CdZnTe detector
was placed adjacent to a thick BGO anticoincidence shield and flown piggybacked
onto the EXITE2 scientific balloon payload. The planar shield was designed to
veto background countsproduced by local gamma-ray production in passive
material and neutron interactions in the detector. The CdZnTe and BGO were
partially surrounded by a Pb-Sn-Cu shield to approximate the grammage of an
X-ray collimator, although the field of view was still ~2 pi sr. At an altitude
of 127000 feet we find a reduction in background by a factor of 6 at 100 keV.
The non-vetoed background is 9 X 10^{-4} cts /cm^2-sec-keV at 100 keV, about a
factor of 2 higher than that of the collimated (4.5 deg FWHM) EXITE2 phoswich
detector. We compare our recorded spectrum with that expected from simulations
using GEANT and find agreement within a factor of 2 between 30 and 300 keV. We
also compare our results with those of previous experiments using passive lead
and active NaI shields, and discuss possible active shielding schemes in future
astronomy missions employing large arrays of CdZnTe detectors.
