M. Pyle

Search for Low-Mass WIMPs with SuperCDMS

We report a first search for weakly interacting massive particles (WIMPs) using the background rejection capabilities of SuperCDMS. An exposure of 577 kg-days was analyzed for WIMPs with mass < 30 GeV/c2, with the signal region blinded. Eleven events were observed after unblinding. We set an upper limit on the spin-independent WIMP-nucleon cross section of 1.2e-42 cm2 at 8 GeV/c2. This result is in tension with WIMP interpretations of recent experiments and probes new parameter space for WIMP-nucleon scattering for WIMP masses < 6 GeV/c2.

Projected sensitivity of the SuperCDMS SNOLAB experiment

SuperCDMS SNOLAB will be a next-generation experiment aimed at directly detecting low-mass particles (with masses ≤ 10 GeV/c^2) that may constitute dark matter by using cryogenic detectors of two types (HV and iZIP) and two target materials (germanium and silicon). The experiment is being designed with an initial sensitivity to nuclear recoil cross sections ∼ 1×10^(−43) cm^2 for a dark matter particle mass of 1 GeV/c^2, and with capacity to continue exploration to both smaller masses and better sensitivities. The phonon sensitivity of the HV detectors will be sufficient to detect nuclear recoils from sub-GeV dark matter. A detailed calibration of the detector response to low-energy recoils will be needed to optimize running conditions of the HV detectors and to interpret their data for dark matter searches. Low-activity shielding, and the depth of SNOLAB, will reduce most backgrounds, but cosmogenically produced ^3H and naturally occurring ^(32)Si will be present in the detectors at some level. Even if these backgrounds are 10 times higher than expected, the science reach of the HV detectors would be over 3 orders of magnitude beyond current results for a dark matter mass of 1 GeV/c^2. The iZIP detectors are relatively insensitive to variations in detector response and backgrounds, and will provide better sensitivity for dark matter particles with masses ≳ 5 GeV/c^2. The mix of detector types (HV and iZIP), and targets (germanium and silicon), planned for the experiment, as well as flexibility in how the detectors are operated, will allow us to maximize the low-mass reach, and understand the backgrounds that the experiment will encounter. Upgrades to the experiment, perhaps with a variety of ultra-low-background cryogenic detectors, will extend dark matter sensitivity down to the “neutrino floor,” where coherent scatters of solar neutrinos become a limiting background.

Surface Electron Rejection from Ge Detector with Interleaved Charge and Phonon Channels

To achieve the surface electron rejection required for ton scale dark matter searches with germanium, we have demonstrated an advanced interleaved charge and phonon detector. This iZIP design has both an outer charge and outer phonon sensor guard ring. There is 3D event position reconstruction from two inner phonon channels on one side that give x‐axis location, two on the other side that give y‐axis location, and timing and energy differences between the two sides for z‐axis location. Biasing with +2V on one side charge lines and −2V on opposite side charge lines with all interleaved phonon channels at ground gives us less than 1:3,000 leakage of surface electrons from 109Cd into the nuclear recoil band with 69% WIMP search volume efficiency. 1:1,000 rejection of surface electrons has also been demonstrated using charge asymmetry discrimination alone for an additional 10% volume loss. The double‐sided phonon channels themselves provide excellent surface electron rejection as well through both timing info...

SuperCDMS Detector Fabrication Advances

For its dark matter search the SuperCDMS collaboration has developed new Ge detectors using the same athermal phonon sensors and ionization measurement technology of CDMS II but with larger mass, superior sensor performance and increased fabrication efficiency. The improvements in fabrication are described, a comparison of CDMS II and SuperCDMS detector production yield is reported, and future scalability addressed.

Simulations of noise in phase-separated transition-edge sensors for superCDMS

We briefly review a simple model of superconducting-normal phase-separation in transition-edge sensors (TESs) in the SuperCDMS experiment. After discussing some design considerations relevant to the TESs in the experiment, we study noise sources in both the phase-separated and phase-uniform cases. Such simulations will be valuable for optimizing the critical temperature and TES length of future SuperCDMS detectors.

A Measurement of Electron and Hole Drift Velocities in a Germanium CDMS Detector, at a Temperature of 31 milliKelvin

The Cryogenic Dark Matter Search (CDMS) uses ultrapure germanium detectors at a temperature of approximately 40 mK. These detectors are maintained in a state where shallow impurities are well neutralized. Free electrons and holes, generated by particle events, are “hot” and remain out of equilibrium in the bulk. In this system, carrier drift and diffusion occur simultaneously with the generation of free carriers, and with recombination to localized bulk and surface states. These processes jointly affect the space charge distribution in the bulk and at the surfaces of our detectors. The impact of space charge on the electric field likely accounts for much of the phenomenology found in our detectors. Any ability to predict the evolution of the space charge distribution requires a fundamental understanding of carrier transport processes under our operating conditions. To this end, we have measured carrier drift velocities as a function of electric field in an high‐purity CDMS detector of germanium at a...

Phase Separation in Tungsten Transition Edge Sensors

To optimize the signal efficiency in detectors utilizing Transition Edge Sensor (TES) technology we have fabricated and characterized test devices which approximate the electrical and thermal properties of the tungsten TES parallel arrays used for the Cryogenic Dark Matter Search (CDMS) phonon sensors. We measure the equilibrium power as a function of bias voltage by sweeping the bias current through the TES array and measuring the resulting current through the sensor. Our results are in agreement with previous estimates of the critical length for a TES to separate into superconducting and normal phases. However, we found that the presence of the tungsten sections, which connect the TES to the aluminum fins, significantly shortens the critical length for the onset of phase separation, and indicate that many CDMS phonon sensors have operated with phase separated TESs. We have also improved the determination of the electron‐phonon coupling in our tungsten films to be (0.32±0.02)×109W/m3K5. Finally, we also found that the thermal conductance between the tungsten electron and phonon systems does not scale linearly with added fin connector volume, instead ∼75% of added volume contributes.To optimize the signal efficiency in detectors utilizing Transition Edge Sensor (TES) technology we have fabricated and characterized test devices which approximate the electrical and thermal properties of the tungsten TES parallel arrays used for the Cryogenic Dark Matter Search (CDMS) phonon sensors. We measure the equilibrium power as a function of bias voltage by sweeping the bias current through the TES array and measuring the resulting current through the sensor. Our results are in agreement with previous estimates of the critical length for a TES to separate into superconducting and normal phases. However, we found that the presence of the tungsten sections, which connect the TES to the aluminum fins, significantly shortens the critical length for the onset of phase separation, and indicate that many CDMS phonon sensors have operated with phase separated TESs. We have also improved the determination of the electron‐phonon coupling in our tungsten films to be (0.32±0.02)×109W/m3K5. Finally, we also ...

Bulk and Surface Charge Collection: CDMS Detector Performance and Design Implications

The Cryogenic Dark Matter Search (CDMS) searches for Weakly Interacting Massive Particles (WIMPs) with cryogenic germanium particle detectors. These detectors discriminate between nuclear‐recoil candidate and electron‐recoil background events by collecting both phonon and ionization energy from interactions in the crystal. Incomplete ionization collection results in the largest background in the CDMS detectors as this causes electron‐recoil background interactions to appear as false candidate events. Two primary causes of incomplete ionization collection are suface and bulk charge trapping. Recent work has been focused on reducing surface trapping through the modification of fabrication methods for future detectors. Analyzing data taken with test devices shows that hydrogen passivation of the amorphous silicon blocking layer does not reduce the effects of surface trapping. Other data shows that the iron‐ion implantation used to lower the critical temperature of the tungsten transition‐edge sensors increases surface trapping, causing a degradation of the ionization collection. Using selective implantation on future detectors may improve ionization collection for events near the phonon side detector surface. Bulk trapping is minimized by neutralizing ionized lattice impurities. Detector investigations at testing facilities and at the experimental site in Soudan, MN have provided methods to optimize the neutralization process and monitor running conditions to maintain maximal ionization collection.

Characterization of SUPERCDMS 1-inch Ge detectors

The newly commissioned SuperCDMS Soudan experiment aims to search for WIMP dark matter with a sensitivity to cross sections of 5×10^(−45)cm^2 and larger (90% CL upper limit). This goal is facilitated by a new set of germanium detectors, 2.5 times more massive than the ones used in the CDMS-II experiment, and with a different athermal phonon sensor layout that eliminates radial degeneracy in position reconstruction of high radius events. We present characterization data on these detectors, as well as improved techniques for correcting position-dependent variations in pulse shape across the detector. These improvements provide surface-event discrimination sufficient for a reach of 5×10^(−45)cm^2.

SuperCDMS Detector Readout Cryogenic Hardware

SuperCDMS employs 1‐inch thick germanium crystals operated below 50mK in a dilution cryostat. Each detector produces ionization and phonon signals. Ionization signals are amplified by JFETs operating at 150K within an assembly mounted on the 4K cryostat stage. These high impedance signals are carried to the FETs by superconducting “vacuum coaxes” which minimize thermal conductivity, stray capacitance, and microphonics. Transition edge sensors produce low‐impedance phonon signals, amplified by SQUID arrays mounted on a 600mK stage. Detectors are mounted in a six‐sided wiring configuration called a “tower”, which carries signals from 40mK to 4K. A flex circuit 3 meters in length carries amplified signals for each detector from 4K to a vacuum bulkhead. We describe the methods used to support the detectors, wiring and amplifier elements at various thermal stages, minimizing electrical noise and thermal loads.