  Active shielding is an effective technique to reduce background signals in
hard X-ray detectors and to enable observing darker sources with high
sensitivity in space. Usually the main detector is covered with some shield
detectors made of scintillator crystals such as BGO (Bi$_4$Ge$_3$O$_{12}$), and
the background signals are filtered out using anti-coincidence among them.
Japanese X-ray observing satellites "Suzaku" and "ASTRO-H" employed this
technique in their hard X-ray instruments observing at > 10 keV.
  In the next generation X-ray satellites, such as the NGHXT proposal, a single
hybrid detector is expected to cover both soft (1-10 keV) and hard (> 10 keV)
X-rays for effectiveness. However, present active shielding is not optimized
for the soft X-ray band, 1-10 keV. For example, Bi and Ge, which are contained
in BGO, have their fluorescence emission lines around 10 keV. These lines
appear in the background spectra obtained by ASTRO-H Hard X-ray Imager, which
are non-negligible in its observation energy band of 5-80 keV.
  We are now optimizing the design of active shields for both soft and hard
X-rays at the same time. As a first step, we utilized a BGO crystal as a
default material, and measured the L lines of Bi and K lines of Ge from it
using the X-ray SOIPIX, "XRPIX".
