B. Chugg

A quasiparticle-trap-assisted transition-edge sensor for phonon-mediated particle detection

We have demonstrated the operation of composite superconducting tungsten and aluminum transition‐edge sensors which take advantage of quasiparticle trapping and electrothermal feedback. We call these devices W/Al QETs (quasiparticle‐trap‐assisted electrothermal feedback transition‐edge sensors). The quasiparticle trapping mechanism makes it possible to instrument large surface areas without increasing sensor heat capacity, thus allowing larger absorbers and reducing phonon collection times. The sensor consists of a 30‐nm‐thick superconducting tungsten thin film with Tc∼80 mK deposited on a high‐purity silicon substrate. The W film is patterned into 200 parallel lines segments, each 2 μm wide and 800 μm long. Eight superconducting aluminum thin film pads are electrically connected to each segment, and cover a much larger surface area than the W. When phonons from particle interactions in the silicon crystal impinge on an aluminum pad, Cooper pairs are broken, forming quasiparticles which diffuse to the tun...

A self-biasing cryogenic particle detector utilizing electrothermal feedback and a SQUID readout

We are developing and testing a new type of superconducting transition edge sensor for phonon mediated particle detection. This sensor consists of a superconducting tungsten thin film deposited on a silicon substrate. The temperature of the film is held constant within the superconducting transition (T/sub c//spl ap/70 mK) by an electrothermal feedback process, while the substrate temperature is well below the film temperature. Phonon energy deposited in the film is removed by a reduction in feedback Joule heating, which is measured using a series array of DC SQUIDs. The resulting signals show improvements in linearity and signal to noise ratio over our previous transition edge sensors.<<ETX>>

Installation of the cryogenic dark matter search (CDMS)

Abstract We discuss the status of a cryogenic dark matter search beginning operation 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. We discuss two detector designs and performance, the cryostat construction and operation, and the multi-level shield surrounding the cryostat. Finally, we will examine the limits which we will be able to set on WIMP dark matter with this experiment.

SQUID based WAl quasiparticle trapping assisted transition edge sensor

Abstract We have demonstrated a new type of phonon sensor for cryogenic particle detectors with a high-bandwidth SQUID readout. Our Quasiparticle trapping assisted Electrothermal feedback Transition edge sensor (QET) utilizes aluminum quasiparticle traps attached to a tungsten superconducting transition edge sensor patterned on a silicon substrate. The tungsten lines are voltage biased and self-regulate in the transition region. We have tested three versions of these detectors. One detector consisted of four QET sensors patterned on the surface of a 1 cm × 1 cm × 1 mm silicon substrate. With this detector, we have shown an energy resolution of ∼400 eV FWHM and position sensitivity of ∼0.3 mm for 6 keV X-rays from an 55 Fe source. We tested a second detector identical to the first but with the addition of an ionization sensor. With information from the ionization channel, we were able to distinguish electron-recoil events from nuclear-recoil events. Most recently, we have tested a 2 cm × 2 cm × 2 mm detector with four QET sensors.

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.

Technique for fabricating tungsten thin film sensors with T/sub c/ /spl les/100 mK on germanium and silicon substrates [dark matter detectors]

Until recently, our work on superconducting thin film phonon sensors for cryogenic detector applications was limited to silicon substrates only. We have now successfully extended low T/sub c/ (/spl les/100 mK) tungsten sensor technology and sensor fabrication capability to include high purity germanium substrates as well. Here, we describe a technique for fabricating low T/sub c/ superconducting tungsten films on germanium, and we present first results from cryogenic characterization experiments with these films. We also summarize our work on the development of a process to independently etch aluminum and tungsten films deposited on the same germanium substrate. The capability to selectively etch aluminum and tungsten films is critical for the fabrication of our silicon and soon also germanium detectors which utilize overlapping thin films of superconducting tungsten and aluminum for the phonon sensors. Due to the nature of their operation, we refer to these sensors as W/Al Quasiparticle trap assisted-Electrothermal feedback-Transition edge (QET) Sensors.

Advances in Stanford phonon-mediated elementary particle detectors

Abstract We have demonstrated a new phonon sensor design based on aluminum phonon collection pads connected to tungsten transition-edge sensors (TES). The device is patterned onto a silicon crystal and phonons produced by events in the Si are absorbed into the aluminum films where about half of the energy is converted into long-lived quasiparticle excitations. These diffuse until they encounter the lower gap W regions where they deposit their potential energy to the electron system in the W. With the Si crystal at ∼ 40 mK, the W is kept near the center of its resistive transition ( ∼ 90 mK) using voltage bias and Joule self-heating. Current changes induced by particle events are measured with a high-bandwidth SQUID amplifier readout. We have demonstrated an energy resolution of ∼ 360 eV FWHM and a position sensitivity of ∼ 0.2 mm for 55 Fe X-rays. We have also completed a new analysis of the nuclear recoil versus electron recoil ballistic phonon production experiments which used our earlier generation Ti TES on Si crystals. The new results set an upper limit on the distinction between the two phonon spectra and provide a better understanding of the phonon sensor response.

Low temperature detectors for dark matter searches

We present an overview of the low temperature detectors being developed for dark matter searches. The important background discrimination techniques are discussed. In addition, we describe recent advances in the athermal phonon detectors being developed by our group at Stanford.

Development of 100 g Si and 250 g Ge Detectors for a Dark Matter Search

Over the last two years we have proposed and implemented a new phonon sensing scheme for Cryogenic elementary particle detectors based upon Transition Edge Sensors (TES) operated in the (negative) Electrothermal-feedback (ETF) mode, and utilizing large Al collection pads for the initial phonon absorption. We have also implemented an ionization electrode, in addition to the phonon sensors, to allow the simultaneous measurement of ionization and phonon signals in Si and Ge absorbers. Our progress to date include successfully discriminating between electron and nuclear recoils down to a threshold of 4 keV recoil energy for a 4 g Si detector. Our first 100 g Si detectors have been fabricated, and initial work on Ge detectors indicates that our phonon sensing scheme will also work on large mass Ge absorbers.

Design of Kilogram Mass Scale TES for the Cryogenic Dark Matter Search

Recently there has been much interest in the direct detection of the dark matter candidates known as WIMPs. We are developing very sensitive detectors based on phonon detection with transition edge sensors on silicon substrates. These detectors will be deploy ed as part of the Cryogenic Dark Matter Search in collaboration with the Center for Particle Astrophysics. As we extend this technology to practical WIMP searches we will need much higher mass scale detectors. We have demonstrated detectors on 500 µm substrates. To reach the kilogram mass scales we need to pattern wafers that are an order of magnitude thicker with a detector that is at least two orders of magnitude more sensitive. Progress is reported on both these areas and a detector design is discussed.