Optimizing the design and analysis of cryogenic semiconductor dark matter detectors for maximum sensitivity

Abstract

For the past 15 years, the Cryogenic Dark Matter Search or CDMS has searched for Weekly Interacting Massive Particle dark matter (WIMPs) using Ge and Si semiconductor crystals instrumented with both ionization and athermal phonon sensors so that the much more common electron recoil leakage caused by photons and βs from naturally present radioactive elements can be easily distinguished from elastic WIMP nucleon interactions by looking at the fraction of total recoil energy which ends up as potential energy of e − /h + pairs .Due to electronic carrier trapping at the surface of our semiconductor crystals, electron recoils which occur near the surface have suppressed ionization measurements and can not be distinguished from WIMP induced nuclear recoils and thus sensitivity to the WIMP nucleon interaction cross section was driven in CDMS II by our ability to define a full 3D fiducial volume in which all events had full collection. To remain background free and maximally sensitive to the WIMP-nucleus interaction cross section, we must improve our 3D fiducial volume definition at the same rate as we scale the mass of the detector, and thus proposed next generation experiments with an order of magnitude increase in active mass were unfortunately not possible with our previous CDMS II detector design, and a new design with significantly improved fiducialization performance is required.In this thesis, we illustrate how the complex E-field geometry produced by interdigitated electrodes at alternating voltage biases naturally encodes 3D fiducial volume information into the charge and phonon signals and thus is a natural geometry for our next generation dark matter detectors. Secondly, we will study in depth the physics of import to our devices including transition edge sensor dynamics, quasiparticle dynamics in our Al collection fins, and phonon physics in the crystal itself so that we can both understand the performance of our previous CDMS II device as well as optimize the design of our future devices. Of interest to the broader physics community is the derivation of the ideal athermal phonon detector resolution and it's T 3 c scaling behavior which suggests that the athermal phonon detector technology developed by CDMS could also be used to discover coherent neutrino scattering and iv search for non-standard neutrino interaction and sterile neutrinos. These proposed resolution optimized devices can also be used in searches for exotic MeV-GeV dark matter as well as novel background free searches for 8GeV light WIMPs.Initial performance studies of our first two next generation iZIP detectors at the University of California Berkeley CDMS test facility indicate that electron recoil surface event misidentification is < 2x10 −5 ± 2.5x10 −5 2x10 −5 (90%CL) for a recoil energy range of 8keV r -60keV r strongly indicating that z fiducial volume performance will not limit our WIMP sensitivity in next generation experiments. Furthermore, phonon only fiducial volume selections were created for nuclear recoil energies >2keV ...

Detecting superlight dark matter with Fermi-degenerate materials

“…Then, as long as the produced athermal phonon has E > 2∆, it will break an additional Cooper pair, and so on. At the end of the cascade process, the total kinetic energy of the recoil has been converted into ∼ 60% quasiparticle potential and kinetic energy, with the remaining energy in athermal phonons with a distribution that is strongly near the 2∆ cutoff in the superconductor [32].…”

“…between the TES sensor and the excitation collection time, which degrades the energy sensitivity of current SuperCDMS [52] and CRESST detectors [53], is naturally suppressed.…”

A Search for Light Weakly-Interacting Massive Particles with SuperCDMS and Applications to Neutrino Physics

“…In this section, we describe the rudiments of the detector physics and then how the iZIP design combines with the phonon and charge physics to enable background discrimination. A more detailed discussion of the design principles of the iZIP can be found in the theses of Matt Pyle [49] and Scott Hertel [50].…”

CDMS Detector Fabrication Improvements and Low Energy Nuclear Recoil Measurements in Germanium

“…A new circuit design has been implemented to combat this. The design is called the iZIP (interleaved Z-sensitive Ionization and Phonon detector) [22][23] [24], and as the name suggests, it utilizes interleaved electrodes on each surface. The interleaved electrodes alternate from ground to +2V on one face and from ground to -2V on the other (see Figure 3.11).…”