  A new detection system for X-/Gamma-ray broad energy passband detectors for
astronomy has been developed. This system is based on Silicon Drift Detectors
(SDDs) coupled with scintillator bars; the SDDs act as a direct detector of
soft (<30 keV) X-ray photons, while hard X-/Gamma-rays are stopped by the
scintillator bars and the scintillation light is collected by the SDDs. With
this configuration, it is possible to build compact, position sensitive
detectors with unprecedented energy passband (2 keV - 10/20 MeV). The X and
Gamma-ray Imaging Spectrometer (XGIS) on board the THESEUS mission, selected
for Phase 0 study for M7, exploits this innovative detection system. The Wide
Field Monitor - Imager and Spectrometer (WFM-IS) of the ASTENA (Advanced
Surveyor of Transient Events and Nuclear Astrophysics) mission concept consists
of 12 independent detection units, also based on this new technology. For the
WFM-IS, a coded mask provides imaging capabilities up to 150 keV, while above
this limit the instrument will act as a full sky spectrometer. However, it is
possible to extend imaging capabilities above this limit by alternatively
exploiting the Compton kinematics reconstruction or by using the information
from the relative fluxes measured by the different cameras. In this work, we
present the instrument design and results from MEGAlib simulations aimed at
evaluating the effective area and the imaging performances of the WFM-IS above
150 keV.
