We are preparing the CRESST experiment to search for dark matter WIMPs using cryogenic detectors with superconducting phase transition thermometers. In the first stage we plan to use four 250 g sapphire detectors with thresholds of 0.5 keV and resolutions of 0.2 keV at 1 keV. This will provide sensitivity to WIMP masses below 10 GeV, and is thus complementary to other dark matter searches.

We are developing a dark matter detector for Weakly Interacting Massive Particles (WIMPs) using Series Arrays of Superconducting Tunnel Junctions (SASTJs) to read out phonon signals from absorber crystals. We discuss the factors which have determined the choice of our detector scheme. As an improvement over previously produced SASTJs using a shadow mask technique, a new fabrication process of high-quality Al/Al-oxide/Al junctions and SASTJs is being developed using photolithography to define the structures. The processing and the electrical characterisation of the first devices produced in this way are described

We report on the experimental observation of high-frequency Rayleigh phonons on Si(0 0 1) surfaces. By use of the phonon-imaging technique we can measure the spatial phonon flux distribution of Rayleigh phonons in the Si(0 0 1) surface. Since the mean free path of Rayleigh phonons is limited by surface-roughness-induced scattering mainly into bulk phonons, a micro-imaging experiment was performed. As a suitable detector we use ion implanted Si: P bolometers of 18 μm in diameter and 200 nm in depth, operated at 1 K. To obtain as clean surfaces as possible, the technique of laser annealing in situ was applied. Experimental results showing propagation distances of several hundred micrometers for Rayleigh phonons are presented.

We discuss the design of a cryogenic detector for a WIMP dark matter search based on single crystal absorbers and using Series Arrays of Superconducting Tunnel Junctions (SASTJs).The distribution of recoil vectors of target nuclei from WIMP interactions are affected by the motion of the laboratory through the dark matter halo. The angular distribution of recoil directions is skewed due to the motion of the solar system around the galaxy and is modulated by the diurnal and annual rotation of the earth. We discuss the kinematics of the recoil events and how a directional signal might be identified in our cryogenic detectors using the fast response of SASTJs to the ballistic phonons arising in the absorber from WIMP interactions. We consider how the anisotropy of a dark matter recoil distribution can be used to place statistical limits on its component relative to the isotropic background signal. We also consider how the dark matter limit is altered if only the axis of the nuclear recoil, rather than the full recoil direction is available.We also briefly consider the effect of phonon focusing within single crystal absorbers. Focusing will modulate strongly the signal detected by the SASTJs, on the crystal surface, as the position of the interaction within the crystal varies. A comparison is made between the behaviour of phonons in strongly focusing crystals, such as Nb, Si and LiF, and their near isotropic propagation in BaF2.

More than fifty years after its discovery, the Kapitza resistance is still under debate. New information can be obtained from experiments with angular resolution of the phonons emitted into the solid.

The transmission of phonons with frequencies above 140 GHz across a solid-solid interface is examined by phonon imaging. Heat pulses transmitted through varying thicknesses of MgO films on Ge clearly display total internal reflection beyond a critical angle and frustrated total internal refraction when the film thickness is comparable to the phonon wavelength. These manifestations of Snell's law demonstrate the specular (i.e., not diffuse) refraction expected from acoustic mismatch theory, in contrast to most earlier measurements.