  Background rates in several pixel array detectors are investigated with an
eye toward using them with hard, >6 keV, x-rays in very low-rate experiments -
e.g. at signal rates <0.1/s/cm^2. Measured background event rates for a
detector with an unshielded 0.75 mm thick CdTe sensor on the experimental floor
at SPring-8 varied from 0.4/s/cm^2 with a 6 keV lower-level discriminator (LLD)
threshold to 0.2/s/cm^2 with a 75 keV LLD threshold. The background for a
detector with a 1 mm thick silicon sensor was smaller, ~0.08/s/cm^2 for a 3keV
threshold dropping to ~0.07/s/cm^2 at a 17 keV threshold. These rates are
dominated by terrestrial sources, such as gamma rays emitted from trace
impurities in concrete, with only a small contribution, <0.01/s/cm2, from
direct detection of cosmic ray muons (CRMs). 15 mm of Pb shielding reduces the
measured rates to < 0.05/s/cm2 in CdTe and to <0.02/s/cm^2 in 1 mm silicon.
Additional processing, "time slicing", as may be used in low-rate experiments
where backgrounds may be most problematic, is shown to reduce the background
rates further, to < 0.004/s/cm^2 with the silicon sensor, and to between 0.002
and 0.02/s/cm^2 for CdTe, where the exact value for CdTe depends sensitively on
the detector threshold, and the use of the retriggering and/or the use of dual
discriminator thresholds. Background rates are presented as functions of
discriminator threshold, shielding, and processing. We also discuss the
magnitude of, and the correction for, the limitation of the detector dynamic
range that can be introduced by the time slicing. Finally, we present one
example where time slicing was used.
