B. Neuhauser

Phonon-mediated detection of X-rays in silicon crystals using superconducting transition edge phonon sensors

The authors present data on the operation of thin-film superconducting strips of titanium as phonon sensors on the surface of silicon crystals. The superconducting films are biased at the foot of the resistive transition in temperature and below the critical latching current (the current above which a normal region in the film grows from self-heating). The interaction of an incident X-ray in the Si crystal generates a phonon source which propagates to the surface at the speed of sound. Such an event produces a several-microsecond-long self-terminating voltage pulse which is proportional to the amount of the sensor area driven normal. It is shown that these Ti superconducting transition edge sensors operated at 0.3 K have sufficient resolution for detecting particles with energy deposition above several keV, which makes them good candidates for use in neutrino (and other) experiments. >

Phonon-Mediated Detection of Alpha Particles with Aluminum Transition Edge Sensors

We report on particle detection experiments using aluminum superconducting transition edge sensors deposited on silicon wafers. The detectors consist of a 40 nm thick aluminum film patterned as a labyrinth with 299 lines. These films have Tc~1.35 K and Ic~8 µA at 1.30 K. New data have been obtained using a thin encapsulation 241Am alpha source (~1% FWHM source linewidth for 5.5 MeV) which allows a detailed study of these sensors. With a bias current of 1.2 µA at 1.33 K, 0.4 mV pulses 1.5 µs long are obtained for alpha particles incident directly on the aluminum film. The pulse height distributions are strongly peaked (~7% FWHM) and temperature dependent. In a second series of experiments, we have used the same alpha source to bombard the back of the silicon wafer (275 µm thick). With an alpha range of 25 µm, only the phonons generated by the alpha can produce the observed pulses in the aluminum film, demonstrating phonon mediated particle detection.

Phonon-mediated particle detection

When an incident particle collides with an electron or nucleus in an insulating crystal, the recoil kinetic energy is converted rapidly into a burst of low-energy phonons. If the crystal is very pure and free of defects, and if it is very cold (T approximately 0.1 K), the phonons will propagate ballistically for distances of several centimeters. The authors report on experiments with two types of superconducting phonon sensors being considered for use on a new kind of particle detector, which is based on this effect. The device, which is called a Silicon Crystal Acoustic Detector (SiCAD), reads out phonons generated by particle scattering events. >

Thermal Relaxation in Super fluid Helium-3

The temperature at the top of a column of liquid helium-3 has been measured as a function of time following incremental step changes in the temperature of the CMN paramagnetic salt refrigerant to which the column is coupled. This thermal time constant data has been taken in the super fluid A, super fluid B, and normal phases at pressures between 11 and 29 bar. For pressures remote from the polycritical point (PCP) the abrupt increase in relaxation time at the normal-to-super fluid transition temperature Tc is consistent with the increase in heat capacity. However, data acquired near the PCP reveals a striking difference in the nature of hydrodynamic heat flow in the A and B phases. This disparity can be used to identify the polycritical point on the phase diagram. The abrupt change in response time at Tc for pressures above the polycritical point suggests that caution must be exercised when measuring temperatures near the normal-to-A phase transition if the thermometer is remote from the experimental region.

Status of CDMS search for dark matter WIMPs

We report on the latest results from the CDMS (cryogenic dark matter search) experiment. The experiment uses superconducting particle detectors, operated below 100 mK, to search for dark matter in the form of weakly interacting massive elementary particles or WIMPs. These detectors are either Si or Ge crystals, where the electron-hole production and the phonon production are measured for each event, allowing the discrimination of electron recoils (most backgrounds due to gammas and betas) from nuclear recoils (due to WIMPs and neutrons). We have recently reported new limits from the Stanford shallow site experiment (CDMS-I) which explore supersymmetric models where the lightest supersymmetric particle is often an excellent WIMP candidate. We will also report on the Soudan deep site facility for the CDMS-II experiment which is under construction, and on the status of the CDMS-II detector fabrication.

Results of the Cryogenic Dark Matter Search

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for weakly interacting massive particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data, accounting for the neutron background, give limits on the spin-independent WIMP-nucleon elastic-scattering cross section that exclude unexplored parameter space above 10 GeV c-2 WIMP mass and. at > 75% CL. the entire 3σ allowed region for the WIMP signal reported by the DAMA experiment.

Recent Results from the Cryogenic Dark Matter Search for Weakly Interacting Massive Particles

In September 1999, the Cryogenic Dark Matter Search (CDMS) completed a nine month run with the first generation detectors (0.5 kg of germanium BLIPs sensing thermal phonons and ionization). The data acquired in this run significantly decrease the upper limits on WIMP interaction in germanium by a factor 2.5 to 10 depending on the mass region and begin to probe supersymmetry. The results are however strongly inconsistent with the claims of the DAMA experiment.