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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 keV band. Its key performance requirements are spectral resolution (l/Dl) of 3000 (FWHM) and effective area larger than 1000 cm² 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 (WHIM), outflows from active galactic nuclei (AGN), and plasma emissions from stellar coronae. See Requirements.

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, which will intercept the converging X-ray beam exiting by the Flight Mirror Assembly (FMA) mirrors and disperse it to a series of charge-coupled device (CCD) detectors. The CCD energy resolution will then separate the spatially overlapping spectral orders. See also the XGS on the Instrument Module.

Currently, there are two technologies under consideration for the XGS – Critical Angle Transmission 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 alpha along a cone with half angle gamma. It is then diffracted along the same cone of half-angle gamma, but now with an azimuthal angle of beta . If gamma is kept small (< 3°), then the arc of diffraction stays close to the plane of the grating.

Diagram showing the geometry of the two concepts for the XGS: a transmission grating (left) and an 'off-plane' reflection grating (right). In both images, red and blue lines indicate the light path for the grating instrument. Black lines indicate the direct light path through the instrument (light that is not intercepted by the gratings). This direct light path leads to the focal plane instruments, such as the X-ray Microcalorimeter Spectrometer and Wide Field Imager.

(Click images for larger view.)

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See also

• XGS Overview [.pdf]
• Critical Angle Transmission gratings overview [.pdf]
• “Off-plane” reflection gratings overview [.pdf]

Selected References

• Fabrication and performance of blazed transmission grating…, Ralph Hellmann 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)