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X-ray Grating Spectrometer

The IXO X-ray Grating Spectrometer (XGS) is a wavelength-dispersive spectrometer that will provide high spectral resolution in the 0.3–1.0 keV band. XGS key performance requirements include spectral resolving power (λ/Δλ) of 3000 (FWHM) and effective area larger than 1000 square cm across its operating band. This spectrum in the soft x-ray band can be used to determine the properties of the warm-hot-intergalactic medium, outflows from active galactic nuclei, and plasma emissions from stellar coronae. See IXO Science Performance Requirements.

The XGS will intercept and disperse a fraction of the beam from the mirror onto a charge-coupled device (CCD) camera. It will operate simultaneously with the observing MIP instrument and collect instrumental background data which can occur when an instrument is not in the focal position. See also the XGS on the Instrument Module.

To achieve high-resolution spectroscopy, the XGS will consist of an array of wavelength-dispersive diffraction gratings with a fine pitch of 5000 lines/mm or more. This will intercept the converging X-ray beam exiting by the Flight Mirror Assembly (FMA) and disperse it to a series of CCD detectors. An array of CCDs (9–32, depending on the grating technology) used in photon-counting mode will be employed to image and read out the dispersed spectra. The CCD detectors provide better than the 80 eV resolution required for separation of the multiple diffraction orders produced by both kinds of gratings.

Grating technologies

Currently, there are two technologies under consideration for the XGS – critical-angle transmission gratings (CAT gratings) and "off-plane" reflection gratings:

1. Critical-angle transmission gratings draw heritage from the Chandra High Energy Transmission Grating Spectrometer, but provide substantially higher efficiency. See also X-ray Grating Spectrometer Milestone (MIT Team);
2. The "off-plane" mount at grazing incidence brings light onto the grating at a low graze angle, quasi-parallel to the direction of the grooves. The light is then diffracted through an arc, forming a cone. Light comes into the grating at an azimuthal angle of α along a cone with half angle γ. It is then diffracted along the same cone of half-angle γ, but now with an azimuthal angle of β. If γ is kept small (< 3º), then the arc of diffraction stays close to the plane of the grating.

 

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See also

• XGS Overview [.pdf]
• Critical-angle transmission gratings overview [.pdf]
• "Off-plane" reflection gratings overview [.pdf]

Selected References

• Development of a critical-angle transmission grating spectrometer for the International X-Ray Observatory, Ralf K. Heilmann et al., Proc. of SPIE Vol. 7437 74370G-8, 2009
• Fabrication and performance of blazed transmission grating…, Ralf K. Heilmann et al., SPIE, 2008
• Off-plane reflection gratings for Con-X, Randall L. McEntaffer et al., SPIE, 2008
• Thin substrate grating array for sounding rocket and satellite payloads, Ann Shipley et al., SPIE, 2008
• High-throughput high-spectral-resolution grating x-ray spectrometer for XEUS, Willingale, R., Proceedings of the SPIE, Volume 5900, pp. 66-72 (2005)