| Our group at Stanford is developing Silicon Crystal Acoustic Detectors (SiCADs), which operate at cryogenic temperatures and sense phonons generated when an incident particle scatters off a nucleus or electron in the crystalline Si wafer substrate. If the crystal is sufficiently pure and cold ({dollar}<{dollar}1 K), the phonons can propagate ballistically (i.e. without scattering) to the surface(s) of the crystal, where they are detected by sensitive phonon sensors. In the present design, the phonon sensors consist of a thin-film labyrinth pattern of superconducting titanium, which is dc current biased at a temperature just below its superconducting-to-normal transition ({dollar}approx{dollar}430 mK). Due to the anisotropy of silicon, the ballistic phonons get "focused" as they travel through the crystal, producing distinctive "phonon focusing" patterns on the surface of the SiCAD. The enhancement of the phonon signal due to focusing effects improves the detector energy and spatial resolutions, and provides a straightforward means to veto background events.; Our motivation for developing SiCADs includes their many direct applications to neutrino physics (e.g. to perform neutrino oscillation experiments), particle astrophysics (e.g. to measure the solar neutrino spectrum or search for the hypothetical dark matter in the universe) and solid state physics (e.g. to study phonon dynamics and focusing effects).; We have fabricated and characterized SiCADs with phonon sensors instrumented on both sides of a Si wafer substrate, and have used these devices to detect radioactive sources of gamma and x-rays, alpha particles and neutrons with incident energies of {dollar}<{dollar}6 keV to 10 MeV. Here, we present data from a series of time-coincidence experiments using multi-channel, 1 mm thick SiCADs and an {dollar}sp{lcub}241{rcub}{dollar}Am alpha particle source, and discuss the results in terms of ballistic and diffusive phonon propagation. From these experiments with alpha particles, we have observed ballistic phonon focusing effects in (100) silicon, and have determined that approximately {dollar}1over3{dollar} of the phonons which strike the back side of our 1 mm thick detector within {dollar}approx{dollar}1 {dollar}mu{dollar}sec arrive ballistically. Next, the data from various gamma and x-ray experiments are presented, and qualitatively discussed in terms of ballistic phonon propagation.; We also discuss current efforts to substantially improve SiCAD response, for example, by using an intrinsically more sensitive superconductor such as tungsten for the phonon sensors, or going to narrower line widths. Finally, we summarize our results, and conclude that silicon crystal acoustic detectors with titanium transition edge sensors operated at {dollar}approx{dollar}0.4 K have sufficient energy resolution for detecting particles with energy deposition above several keV, which makes them good candidates for use in neutrino and dark matter experiments. |
