Alumina Ceramic “Dog Bone” Helps Chandra Detect High-Energy Events

Fine-grained alumina ceramic charge detector
Quakertown, PA

The Chandra X-Ray Observatory was launched into high elliptical orbit in 1998. The 39-foot-long, 10,000-pound observatory is designed to study high-energy events such as supernovae, black holes, quasars, and stellar coronae. At its core are several extremely precise instruments, including the high-resolution camera spectroscope (HRC-S). The spectroscopic detector consists of three major assemblies: a UV/ION shield, a pair of micro-channel plates, and a cross-grid charge detector (CGCD) made from a 99.98%-pure alumina ceramic made by Astro Met. The CGCD is referred to as the “dog bone” because of its shape.

altUsually such detectors consist of two separate layers of finely spaced gold wires wrapped in orthogonal directions around an insulating substrate such as alumina. The long and thin ceramic “dog bone” measures about 400 × 33 mm and has slight facets machined on its top face. Wire could not be wound along the length because it would vary in height above the surface due to the facets. Engineers deposited an array of 7-ml-wide gold traces just 0.7 mls apart on the substrate. Since the original alumina material they’d specified had too coarse a grain, this caused shorts or breaks in the traces.

The solution came from Insaco’s machining capabilities, which offered 1- to 3-micron grain size as opposed to the original 17 microns. The Astro Met AMALOX 87 fine-grained alumina ceramic was specified for its stability over wide temperature extremes, as well as resistance to chemicals, oxidation, and wear. The ceramic part had multiple precision features machined to a tolerance of 0.001", and several mounting holes and undercut features.

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