Cryogenic Dark Matter Search Detectors Research Articles

Confocal sputtering of conformal α-β phase W films on etched Al features

The authors report on thin-film processing improvements in the fabrication of superconducting quasiparticle-trap-assisted electrothermal-feedback transition-edge sensors used in the design of cryogenic dark matter search detectors. The work was performed as part of a detector upgrade project that included optimization of a new confocal sputtering system and development of etch recipes compatible with patterning 40 nm-thick, α-β mixed-phase W films deposited on 300–600 nm-thick, patterned Al. The authors found that their standard exothermic Al wet etch recipes provided inadequate W/Al interfaces and led to poor device performance. The authors developed a modified Al wet-etch recipe that effectively mitigates geometrical step-coverage limitations while maintaining their existing device design. Data presented here include scanning electron microscope and focused ion beam images of films and device interfaces obtained with the new Al etch method. The authors also introduce a method for quantitatively measuring the energy collection efficiency through these interfaces.

Neganov–Luke Phonon Amplification in P-type Point Contact Detectors

The Cryogenic Dark Matter Search (CDMS) detectors measure ionization and athermal phonons in high purity germanium crystals to discriminate between nuclear recoils from dark matter candidates and radioactive backgrounds. In order to reach lower energy detection thresholds, the CDMSlite experiment operates the CDMS detectors with a larger voltage bias to increase the signal-to-noise ratio using the Neganov–Luke effect. Breakdown in those detectors was observed at fields of order 30 V/cm, but the reason for the breakdown is unknown. It is unclear if the breakdowns are due to surface leakage current, impact ionization in the bulk of the crystals, or some other effect due to the very low operating temperatures of the detectors. Germanium detectors used in gamma spectroscopy at 77 K are regularly operated with fields in excess of 1,000 V/cm. In order to understand the origin of breakdown in the CDMS detectors, a P-type Point Contact detector was equipped with transition edge phonon thermistors and operated at a base temperature of \(\sim \)30 mK. The linearity of the Neganov–Luke phonon amplification was studied and no sign of breakdown for biases up to 400 V was observed. This excludes impact ionization on neutral impurity states as the primary cause of the breakdown observed in the CDMSLite detectors. This demonstrates that the Neganov–Luke phonon amplification is a viable method for lowering the energy threshold in germanium detectors of masses of order 1 kg.

Invited Review Article: Physics and Monte Carlo techniques as relevant to cryogenic, phonon, and ionization readout of Cryogenic Dark Matter Search radiation detectors

This review discusses detector physics and Monte Carlo techniques for cryogenic, radiation detectors that utilize combined phonon and ionization readout. A general review of cryogenic phonon and charge transport is provided along with specific details of the Cryogenic Dark Matter Search detector instrumentation. In particular, this review covers quasidiffusive phonon transport, which includes phonon focusing, anharmonic decay, and isotope scattering. The interaction of phonons in the detector surface is discussed along with the downconversion of phonons in superconducting films. The charge transport physics include a mass tensor which results from the crystal band structure and is modeled with a Herring-Vogt transformation. Charge scattering processes involve the creation of Neganov-Luke phonons. Transition-edge-sensor (TES) simulations include a full electric circuit description and all thermal processes including Joule heating, cooling to the substrate, and thermal diffusion within the TES, the latter of which is necessary to model normal-superconducting phase separation. Relevant numerical constants are provided for these physical processes in germanium, silicon, aluminum, and tungsten. Random number sampling methods including inverse cumulative distribution function (CDF) and rejection techniques are reviewed. To improve the efficiency of charge transport modeling, an additional second order inverse CDF method is developed here along with an efficient barycentric coordinate sampling method of electric fields. Results are provided in a manner that is convenient for use in Monte Carlo and references are provided for validation of these models.

Time Evolution of Electric Fields in CDMS Detectors

The Cryogenic Dark Matter Search (CDMS) utilizes large mass, 3″ diameter×1″ thick target masses as particle detectors. The target is instrumented with both phonon and ionization sensors, the later providing a ∼1 V cm−1 electric field in the detector bulk. Cumulative radiation exposure which creates ∼200×106 electron-hole pairs could be sufficient to produce a comparable reverse field in the detector thereby degrading the ionization channel performance, if it was not shielded by image charges on the electrodes. To study this, the existing CDMS detector Monte Carlo has been modified to allow for an event by event evolution of the bulk electric field, in three spatial dimensions. Surprisingly, this simple model is not sufficient to explain the degradation of detector performance. Our most recent results and interpretation are discussed.

Monte Carlo comparisons to a cryogenic dark matter search detector with low transition-edge-sensor transition temperature

We present results on phonon quasidiffusion and Transition Edge Sensor (TES) studies in a large, 3 inch diameter, 1 inch thick [100] high purity germanium crystal, cooled to 50 mK in the vacuum of a dilution refrigerator, and exposed with 59.5 keV gamma-rays from an Am-241 calibration source. We compare calibration data with results from a Monte Carlo which includes phonon quasidiffusion and the generation of phonons created by charge carriers as they are drifted across the detector by ionization readout channels. The phonon energy is then parsed into TES based phonon readout channels and input into a TES simulator.