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 scintillato
r 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.
