Brian Dougherty

Neutron background for a dark matter experiment at a shallow depth site

Abstract The neutron flux in the Stanford Underground Facility, a shallow depth site with an overburden of 17 ± 1 m.w.e., has been measured using a moderated BF 3 proportional counter. This counter has also been used to measure the neutron flux inside various shielding configurations required for a dark matter search at this site. These measurements demonstrate that lead shielding, used for photon attenuation, acts as a strong neutron source, with the neutrons being produced primarily by cosmic ray muon interactions within the lead. We have determined the production rate of cosmic ray induced neutrons in lead at this depth. Fortunately, these measurements also demonstrate that it is possible to moderate and capture these neutrons so that the resulting neutron flux is no longer sensitive to the presence of the lead shielding. For a dark matter search, however, care still has to be taken to minimize the inactive mass near the detector.

Detection of elementary particles using silicon crystal acoustic detectors with titanium transition edge phonon sensors

We are developing silicon crystal acoustic detectors (SiCADs), which operate at cryogenic temperatures and use thin-films of superconducting titanium (Tc = 435 mK) to sense phonons generated when an incident particle scatters off a nucleus or electron in pure and cold (< 1 K) silicon. 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 multi-channel 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 < 6 keV to 10 MeV. We discuss our results in terms of ballistic and quasi-diffusive phonon propagation, and show evidence for ballistic phonon focusing effects in [100] silicon.

Prompt phonon signals from particle interactions in Si crystals

We discuss the prompt (ballistic and quasidiffuse) phonon physics associated with elementary particle interactions within silicon crystals at temperatures below 1 K, and the differences in the ballistic and quasidiffuse phonon production from primary electron recoils versus primary nuclear recoils within these crystals. We then summarize the results from a growing body of direct experimental evidence on prompt phonon signals from particle detectors bombarded with alphas, x-rays, and neutrons.

Results and status of the Cryogenic Dark Matter Search (CDMS)

Abstract The Cryogenic Dark Matter Search experiment uses cooled germanium and silicon detectors for a direct search for weakly interacting massive particles in our Galaxy. The novel detectors allow a high degree of background rejection by discriminating between electron and nuclear recoils through the simultaneous measurement of the energy deposited in phonons and ionization. Exposures on the order of one kilogram-day from initial runs of our experiment yield (preliminary) upper limits on the WIMP-nucleon cross section that are comparable to much longer runs of other experiments. Current and future runs promise significant improvement, primarily due to improved detectors and reduced surface-electron backgrounds.

Charge collection and trapping in low‐temperature silicon detectors

Charge collection efficiency measurements in silicon detectors at low temperature (T<0.5 K) and low applied electric field (E=0.1–100 V/cm) were performed using a variety of high‐purity, p‐type silicon samples with room‐temperature resistivity in the range 2–40 kΩ cm. Good charge collection under these conditions of low temperature and low electric field is necessary for background suppression, through the simultaneous measurement of phonons and ionization, in a very low event rate dark matter search or neutrino physics experiment. Charge loss due to trapping during drift is present in some samples, but the data suggest that another charge–loss mechanism is also important. We present results which indicate that, for 60 keV energy depositions, a significant fraction of the total charge loss by trapping occurs in the initial electron‐hole cloud near the event location which may briefly act as a shielded, field‐free region. In addition, measurements of the lateral size, transverse to the applied electric fie...

Simultaneous Measurement of Phonons and Ionization using SiCAD s

Our group at Stanford has configured a double-sided Silicon Crystal Acoustic Detector (SiCAD) for simultaneous measurement of both phonons and ionization. This detector operates at ∼370mK and consists of Ti Transition Edge Sensors (TES), having microsecond resolution times, patterned on both sides of a 300µm thick Si wafer. Distinguishing an electron-recoil event from a nuclear-recoil event is possible due to the different partitioning of energy into phonons vs. ionization for the two types of event. This is a powerful background rejection technique for neutrino physics experiments and dark matter searches where the events of interest are nuclear-recoils. In addition, the phonon sensor is position sensitive. In particular, it determines the depth of an event and can be used to reject events occurring near the surface. This detector also allows us to refine our recent measurement of the fraction of phonon energy propagating ballistically from nuclear-recoil events1.

The first cryogenic dark matter experiment

An experimental search for dark matter particle candidates using cryogenic detectors requires a low radioactive background environment. We discuss the status of a cryogenic dark matter experiment to be performed in the Stanford Underground Facility. The detectors will be cooled in a specially designed cryostat connected to a modified side access Oxford 400 dilution refrigerator. Details of the cryostat design and its operating performance are presented. The effectiveness of the multi-level shield surrounding the cryostat, as well as the background levels we expect to achieve in the pilot experiment are discussed. Finally, we examine the limits which can be set on dark matter candidates with such an experiment.

Charge-carrier collection by superconducting transition-edge sensors deposited on silicon

Abstract Superconducting transition-edge sensors deposited on high-purity silicon have been found to operate in two distinct “modes”, distinguished by different intrinsic gains. We propose that this gain-shift is due to a change in the prompt collection of energy carried by electrons and holes.

Charge trapping effects in cryogenic particle detectors made using single-crystal semiconductor substrates

We explore charge-trapping effects in cryogenic particle detectors composed of single-crystal silicon substrates with both titanium transition-edge sensors (TES) and charge-collection electrodes deposited upon them. These effects include transients on various time scales which follow the evolution of different kinds of space charge, intrinsic gain and linearity shifts in signals characteristic of changes in the absorption of energy carried by electrons and holes, variations in charge-collection efficiency and ionization resolution, etc., The physics involved, relevant for many other cryogenic, semiconductor-based devices, includes a variety of charge trapping and transport mechanisms.

Preliminary limits on the WIMP-nucleon cross section from the cryogenic dark matter search (CDMS)

Abstract We are conducting an experiment to search for WIMPs, or weakly-interacting massive particles, in the galactic halo using terrestrial detectors. This generic class of hypothetical particles, whose properties are similar to those predicted by extensions of the standard model of particle physics, could comprise the cold component of nonbaryonic dark matter. We described our experiment, which is based on cooled germanium and silicon detectors in a shielded low-background cryostat. The detectors achieve a high degree of background rejection through the simultaneous measurement of the energy in phonons and ionization. Using exposures on the order of one kilogram-day from initial runs of our experiment, we have achieved (preliminary) upper limits on the WIMP-nucleon cross section that are comparable to much longer runs of other experiments.