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The IXO science case calls for a large field of view, moderate energy resolution imager with high quantum efficiency across a broad energy range from low (~0.1 eV) to high (~40 keV) energies. See IXO Science Performance Requirements.
The Wide Field Imager (WFI) is an imaging X-ray spectrometer with a large field of view (18 arcmin × 18 arcmin). It obtains images and spectra in the 0.1–15 keV band, with nearly Fano-limited energy resolution (50 eV at 300 eV; < 150 eV at 5.9 keV). A 100 µm × 100 µm pixel size, corresponding to 1 arcsec, oversamples the beam minimizing pile up.
The Wide Field Imager and Hard X-ray Imager (HXI) are two detectors incorporated into a single instrument, with the HXI mounted directly behind the WFI. The HXI extends IXO´s energy coverage to 40 keV, observing simultaneously with the WFI.
WFI/HXI Layout: The WFI soft X-ray Active Pixel Sensor (APS) is in front of the HXI CdTe detector. Click the image for a larger view
4 × 4 pixel detail of the tentative WFI layout. Click the image for a larger view
Comprising roughly one million pixels on a sensitive area of 10 cm × 10 cm, the WFI will be the world's largest monolithic detector for X-ray imaging and spectroscopy.
The WFI´s key component is the Depleted P-channel Field Effect Transistor (DEPFET) Active Pixel Sensor (APS). Each APS pixel has an integrated amplifier allowing the charge produced by an incident X-ray photon to be read directly from each pixel. This allows on-demand pixel readout, reduces readout noise, and offers radiation hardness against charge transfer inefficiency.
Since the observatory´s large collecting area necessitates that the imager supports a high frame rate, using the monolithic DEPFET active-pixel-array employed by WFI offers many advantages over the use of more conventional CCD imagers. These include: nearly Fano-limited energy resolution, greater flexibility in data modes arising from the ability to control individual pixels, higher readout speed, and larger sizes for monolithic detectors. The energy range is extended to 40 keV by the CdTe detector of the HXI, with an additional Si Strip detector to improve overlap between HXI and WFI.
The WFI detector draws from the heritage of the X-ray CCD detectors that have flown on ASCA, Chandra, XMM-Newton, and Suzaku. Technologically, the closest match is the XMM-Newton EPIC PN. The DEPFET technology is well tested with detectors developed for several different missions, including MIXS that will fly on BepiColombo (~2013).
Schematic of the DEPFET sensor for the WFI. Click the image for a larger view
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