X-RAY OPTICS: Multilayer mirrors image high-energy x-rays

An x-ray mirror technology under development at NASA Goddard Space Flight Center (Greenbelt, MD) and the University of Nagoya (Japan) allows images to be made from hard x-rays with energies as high as 35 keV, which corresponds to a wavelength of 0.035 nm. The mirrors allow small detectors to be used, which provide both imaging benefits and improved sensitivity because less background noise is collected.

The mirrors are being developed for the International Focusing Optics Collaboration for Micro-Crab Sensitivity (InFOCuS), a balloon-borne instrument proposed for launch in 1999. A micro-Crab is one-millionth of the x-ray emission from the Crab Nebula.

Hard x-rays are useful for x-ray astronomy because they are emitted by both hot objects, such as regions around black holes and active galactic nuclei, and neutron stars and other objects that can accelerate particles in high magnetic fields. Telescopes in this spectral region can observe x-ray emission from pulsars and evidence of production of heavy elements in supernova explosions.

Mirrors for soft x-rays, that is, those with energies of less than about 10 keV, focus the radiation using concentric grazing-incidence optics. Hard x-ray systems, however, have not provided imaging before now.

Foiled again

The hard x-ray mirrors use nested metal-foil reflectors with a multilayer coating. The NASA group is using foil rather than a quartz substrate, because "at very oblique incidence angles, a thick substrate blocks [too much of] the aperture," says developer Peter Serlemitsos.

The foil is also lighter and less-expensive than quartz optics. For hard x-ray optics, the surfaces must have a roughness of less than 0.3 nm. Foil optics developed by Serlemitsos are already being used in x-ray telescopes aboard the Japanese Advanced Satellite for Cosmology and Astrophyics.

Four mirrors are planned for the imaging instrument. Each mirror will cover a successive range of the hard x-ray spectrum, and each will incorporate as many as 2000 foil reflectors.

An x-ray enters the assembly from the top, grazes a foil mirror at a very shallow angle, is reflected by a second set of mirrors, and hits the detector. Multilayer coatings on the curved foil surfaces improve the frequency response of the mirrors. The coatings of platinum and carbon allow the foil mirrors to reflect hard x-rays with energies of up to 40 keV.

Serlemitsos says, "People were predicting that the foils would curl, but none of these predictions came true." Both NASA Goddard and the University of Nagoya have the necessary coating facilities for making the InFOCuS mirrors. The final four-mirror telescope will weigh about 22 lb and measure about 15 in. across by 9 in. high. The detector designed for use with the mirrors is a cadmium zinc telluride strip array developed by Jack Tueller at NASA Goddard.