Hard X-ray Imager
The Hard X-ray Imager (HXI) is mounted beneath the WFI. It will extend the energy coverage up to 40 keV and enable simultaneous observations with both instruments. HXI should have an energy resolution better than 1 keV (FWHM) at 40 keV and cover a FOV of 12 × 12 arcmin².
The HXI detector is made up of three layers, 2-Silicon (Si) and 1 Cadmium Telluride (CdTe) detector, sitting back from the WFI plane by about 20, 25 and 30 mm, respectively. The 30 mm offset of the CdTe detector from the focal plane causes a PSF broadening of about 15 arcsec, assuming a mirror effective diameter of 1 m at 30 keV.
A consequence of the co-location of the two instruments is that they share an optical axis, and the same baffle structure, with heavy metal sheets added for hard X-ray coverage. A thermal shield will be required between the WFI and HXI since they operate at different temperatures. The HXI cooling design, mechanical structure and electrical interfaces are all independent from the WFI, so that both instruments can be operated separately.
The high mass absorption coefficient of Cadmium Telluride (CdTe) allows for the efficient detection of hard X-ray photons. In order to obtain high spatial resolution that matches the angular resolution of IXO telescope, and to cover the wide FOV, the detectors will consist of Double-sided Strip CdTe detector (DS-CdTe detector). A detector with a thickness of 0.5 mm provides nearly 100 per cent detection efficiency for 40 keV hard X-ray photons. To suppress background, an active BGO anticoincidence shield is deployed around the imager. In addition, two layers of Double-sided Si Strip Detector (DSSD), with dimensions the same to those of the CdTe detector (i.e. 7×7 cm² wide and with a thickness of 0.5 mm) will be mounted above the CdTe and serve as particle background detectors as well as detectors of X-ray signals in the 10-30 keV band.
The HXI is based on proven technology – a similar instrument has been developed for the ASTRO-H spacecraft. One aspect that will require investigation is the design of an optimum engineering solution for mounting the HXI behind the WFI.