The High Energy Focusing Telescope (HEFT)

HEFT Instrument Overview

Why Focusing?

Previous instruments operating in the hard X-rays today (20–100 keV) fall into two categories: coded aperture telescopes and collimated experiments. Detector background, resulting from particle interactions in the detector is the dominant factor in determining the minimum detectable X-ray flux. For non-focusing experiments, the detector area is approximately equal to the collecting area. For focusing experiments, however, a mirror concentrates the incoming flux onto a small spot on the detector, greatly reducing background—making much fainter astronomical sources detectable.

How to focus at high energies: Present day soft X-ray focusing telescopes (Chandra, ASCA, XMM-Newton, SXG) rely on total external reflection to focus the X-rays. X-rays are reflected from surfaces at very small incidence angles. The critical graze angle (the maximum incidence angle at which reflection can occur), is inversely proportional to the photon energy, so that reflecting high-energy X-rays requires very small incidence angles. This makes designing practical mirrors very difficult, unless some method is used to increase the typical reflection angles.

To reflect high energy photons at angles greater than the critical graze angle, HEFT uses multilayer coatings. A multilayer coating is a thin film coating consisting of alternating layers of high and low density materials. Constructive interference among small reflections at adjacent interfaces produces high reflectivity at angles above the critical graze angle.


Detectors

HEFT requires position-sensitive hard X-ray detectors with < 1 mm spatial resolution, and good energy resolution. We are developing room-temperature solid-state cadmium zinc telluride (CdZnTe) pixel detectors with a custom low-noise ASIC readout. In these detectors, the CdZnTe sensor has one monolithic contact, with the other (anode) segmented into pixels. Each pixel contact is bump bonded to a custom readout chip, with one complete pulse processing chain for each pixel. The photo shows the HEFT 8 × 8 pixel prototype detector.


Mirrors

HEFT is employing the conical-approximation to the Wolter-I geometry used on the ASCA and ASTRO-E experiments. We have developed new thermally-formed glass substrates for the HEFT mirrors. The glass was developed for flat panel displays, is available in thin sheets, and is smooth on the relevent length scales. The mirrors shells are made in quadrant section. The photo shows HEFT glass mirror substrates before and after coating with depth-graded multilayers.


Multilayers

The HEFT optics are coated with depth-graded multilayers. Multilayers coatings consist of alternating layers of high and low index of refraction materials, and operate on the principle of Bragg reflection. Constructive interference between reflections occurring at the interfaces between materials provides reflectance at angles larger than can be achieved with single metal coatings. The HEFT coatings have typically 500 layer pairs, which vary in thickness between 20 Å for a pair, and 200 Å per pair. The range in layer pair thickness in the coatings provides reflectivity over a wide range of X-ray energies. HEFT uses two different sets of material pairs to achieve response over the broadest possible energy band. The multilayers are coated using magnetron sputtering, in coating chambers specifically designed for the purpose.



http://www.srl.caltech.edu/HEFT/instrument.html