| The design and fabrication of a "dual-mode" nuclear radiation detector using planar silicon technology is described. The device operates at 0.3 K and detects simultaneously the ionization and the phonons produced by nuclear radiation interacting in the substrate. The intended purpose of the device is to detect atomic silicon recoils from the scattering of massive neutral particles that are hypothesized to compose the dark matter halo of our galaxy. The "dual mode" functionality was designed to permit atomic recoils to be distinguished event-by-event from the background due to unavoidable low-level radioactivity in the detector and its surroundings. The device consists of a back contact biased negatively relative to a comb-shaped electrode structure on the opposite face of a high purity wafer. The spacing of the comb teeth is less than the wafer thickness, providing a uniform charge collection electric field throughout most of the wafer volume. Between the teeth of the comb are superconducting transition edge devices consisting of serpentines of 400 A thick, 2 ;Investigations of ionization-detecting metal-on-silicon contacts at low temperatures are described, including diffused junctions, Au on Si, Au on oxidized Si, and Ti on Si. Diode characteristics continue to change qualitatively at temperatures below 4 K. The destruction of superconductivity in Ti by Au contamination during microfabrication procedures is also reported. |
