Cryogenic Ge Detectors with Interleaved Electrodes: Design and Modeling

Detectors fitted with interleaved collection electrodes offer a promising solution to the problem of surface event rejection in cryogenic Ge detectors for dark matter search. Detector design and modeling are discussed, based on computer simulations of the collection field and on pulse-shape analysis of the ionization signals. Test experiments with a prototype detector are presented in a companion paper by X. Defay et al. in this conference.      

Detectors of the Cryogenic Dark Matter Search: Charge Transport and Phonon Emission in Ge 〈100〉 Crystals at 40 mK

The Cryogenic Dark Matter Search (CDMS) measures both the ionized charge and the energy in athermal phonons created by particle interactions in ultrapure germanium and silicon crystals at a temperature of 40 mK. Charge collection potentials must remain at only a few volts, else emitted phonons from drifted carriers will dominate the phonons of the original interaction. At these drift fields, there are practically no thermal phonons and carrier transport is determined by phonon emission. We present results for drift-limited carrier dynamics and rates of phonon emission in 〈100〉 germanium. As these transport conditions represent an extreme limit, a Monte Carlo technique was used to bypass analytical assumptions commonly used in solving the Boltzmann transport equation. This work will assist us in understanding phenomena found in our detectors.      

Surface Event Rejection of the EDELWEISS Cryogenic Germanium Detectors Based on NbSi Thin Film Sensors

Near-surface-events are a major limitation to the performance of cryogenic massive germanium heat and ionisation detectors for dark matter search, due to their incomplete charge collection. We present here a powerful method of surface event identification based on the transient heat signal of a Ge bolometer, equipped with two NbSi high impedance thin film sensors. Calibration runs using electrons and low energy gamma particles from a ¹⁰⁹Cd source show highly effective surface event rejection down to the heat threshold energy. Neutron and gamma source calibrations were realised to get information on the fiducial volume of the Ge absorber. First results from low background data taking are discussed.     

Surface trapping and detector degradation in Ge bolometers for the EDELWEISS Dark Matter search: experiment and simulation

Surface charging effects are investigated in a germanium ionization-heat detector operated at cryogenic temperatures. Under properly defined biasing conditions, carriers of one type only (either electrons or holes) are driven in a controlled manner into the free surfaces of the detector where they become trapped. Computer simulations allow to determine the surface densities of trapped carriers, and to reconstruct the scatter plot of γ-events in the degraded state. They allow one in particular to define several event populations with respect to detector degradation, attributed to different areas of energy deposition.     

Kinetic Inductance Phonon Sensors for the Cryogenic Dark Matter Search Experiment

An important challenge faced by phonon-mediated detectors for the next generation of dark matter detectors (>100 kg) is to instrument large target mass at low cost, while maintaining the large background suppression offered by the combination of phonons and ionization (or scintillation) measurement. Kinetic inductance phonon sensors, operating far below the superconducting transition temperature, offer an interesting solution to this scaling problem. They do not critically depend on the uniformity of T _c and their resonant-cavity readout is easy to multiplex. We are studying a microstrip (two parallel planes) transmission line architecture that may offer the additional advantage of a separation of functions: the main detector is just covered by an unpatterned aluminum film and the number of quasi-particles created in it by athermal phonons are sensed by a second film, which has been independently patterned and is mounted a few microns away from the detector. We present current results on the responsivity and noise of large area (∼33 mm²) microstrip kinetic inductance phonon sensors.      

A WIMP Dark Matter Detector Using MKIDs

We are pursuing the development of a phonon- and ionization-mediated WIMP dark matter detector employing microwave kinetic inductance detectors (MKIDs) in the phonon-sensing channel. Prospective advantages over existing detectors include: improved reconstruction of the phonon signal and event position; simplified readout wiring and cold electronics; and simplified and more reliable fabrication. We have modeled a simple design using available MKID sensitivity data and anticipate energy resolution as good as existing phonon-mediated detectors and improved position reconstruction. We are doing preparatory experimental work by fabricating strip absorber architectures. Measurements of diffusion length, trapping efficiency, and MKID sensitivity with these devices will enable us to design a 1 cm²×2 mm prototype device to demonstrate phonon energy resolution and position reconstruction.      

Status of the Cryogenic Dark Matter Search?Experiment

The Cryogenic Dark Matter Search experiment (CDMS) is using Phonon+ Ionization detectors to search for Dark Matter in the form of Weakly Interactive Massive Particles (WIMPs). We report the current status of the experiment and its perspective to achieve the sensitivity goal of the cross section: σ _WIMP-nucleon∼1×10⁻⁴⁴ cm² (Spin independent). N. Mirabolfathi for the CDMS collaboration.     

Hot on the Tail of the Elusive WIMP: Cryogenic Dark Matter Searches in the 21st Century

It is now well established and accepted that the universe has a total density equal to the critical density (Ω=1) and that roughly 25% of that amount is accounted for by non-relativistic particles. That these particles, referred to as Dark Matter, have remained a mystery has served to motivate physicists to design more ingenious and far reaching experiments in an attempt to identify and understand them. This paper will review various ongoing and proposed Dark Matter searches which employ cryogenic techniques to both detect the rare Dark Matter interactions as well as reject the vast number of background events from cosmic ray and radioactive backgrounds. Such experiments are already sensitive to and are able to reject certain models of supersymmetry, and with the increases in sensitivity projected over the next few years may even be able to detect these elusive particles.      

New TeO2/NbSi Detectors for Rare Event Search

The present limitation for experiments looking for rare events, such as neutrinoless double beta decay (0ν β β) and direct WIMPs detection, is the radioactive background and in particular its discrimination. When the search is based on the bolometric technique, near-surface events constitute a serious problem, due to full volume sensitivity of the detector; in fact for a 0ν β β experiment they may cause signals close to the Q-value while for a dark matter (DM) experiment, which exploits ionization signal for particle identification, they may originate an incomplete charge collection simulating a nuclear recoil. We will describe a technique for the active suppression of the surface background which uses NbSi thin film thermometers acting as out-of-equilibrium phonon sensor. After very promising results obtained on germanium and sapphire bolometers, here we present some results on a small TeO₂ one equipped with two NbSi films; in fact TeO₂ is an interesting material for the search for 0ν β β of ¹³⁰Te. In particular we show that we are able to identify α surface events on TeO₂ by pulse shape analysis with a good separation between bulk and surface events.      

The SciCryo Project and Cryogenic Scintillation of Al2O3 for Dark Matter

We discuss cryogenic scintillation of Al₂O₃. Room-temperature measurements with α particles are first carried out to study effect of Ti concentration on response. Measurements under X-rays between room temperature and 10 K confirm a doubling of light output. The integration of a scintillation-phonon detector into an ionization-phonon dark matter search is underway, and the quenching factor for neutrons has been verified.      

Practical Design for Improving the Sensitivity to Search for Dark Matter Axions with Rydberg Atoms

A microwave single-photon detector was developed with highly-excited alkaline Rydberg-atoms in a cooled resonant cavity to search for dark matter axions. This detector belongs to a microwave single-photon counter, thus being free from the standard quantum limit (SQL). High sensitivity of the present detector system was demonstrated by measuring the thermal blackbody radiations in the cavity at temperatures as low as 70 mK where the sensitivity is below the SQL. The detection sensitivity of the present system is mainly limited by stray electric fields present in the detection region. Practical design of a new experimental scheme with a guiding electric field through the atomic-beam trajectory is here presented and discussed to avoid the effect of stray electric field and thus to improve the detection sensitivity.      

Search for Dark Matter with Fermi Large Area Telescope: The Galactic Center

Dark Matter (DM) as a weakly interacting massive particle could annihilate or decay and give rise to high energy gamma-rays. Then an indirect search for Dark Matter is possible by means of the Large Area Telescope on board the Fermi satellite. A relatively large signal is expected from the regions where the Dark Matter is expected to have the greatest density, such as the central region of the Milky Way. This region also hosts many high-energy gamma ray sources, of many different classes. Furthermore diffuse gamma-ray emission due to cosmic-ray interactions with interstellar gas and radiation is detected from the same direction. A greatly improved understanding of the gamma ray emission from the Galactic Center region is going to be obtained with the Fermi LAT first-year data. The data along with refined modelling of the diffuse emission and a careful evaluation of the discrete sources will improve our ability to disentangle a potential dark matter signal from the astrophysical background and to place new limits on the mass and annihilation rate (or lifetime) of Dark Matter particles     

Optimization of Cryogenic Ge Detector Equipped with NbSi Thin Film Thermometers: Fiducial Volume and Energy Resolution

Ionisation-heat detectors are very useful in direct detection dark matter experiments because they lead to an active rejection of the background. Their rejection performances are presently limited by near electrode surface events leading to incomplete charge collection. We propose an identification of near NbSi electrodes events based on pulse shape analysis of the phonon signal delivered by the thermometric layer. We discuss recent progress in the near surface event identification accuracy together with the remaining fiducial volume and the heat channel resolution.      

\hbox {H}^{-} -Like Centers and Space-Charge Effects in Cryogenic Germanium Detectors for Dark Matter Search

Space-charge build-up phenomena are investigated in germanium detectors for dark matter search, associated with the thermal emission of carriers by the H \(^{-}\) -like, dopant-related A \(^{+ }\) and D \(^{-}\) shallow trap centers. Evidence for such processes follows from a combined study of the time evolution of the pattern of charge sharing between the different collection electrodes of the device, and of the kinetics of carrier emission by the A \(^{+ }\) and D \(^{- }\) centers. Implications for detector physics are discussed.     

Cresst-II: dark matter search with scintillating absorbers

In the CRESST-II experiment, scintillating CaWO₄ crystals are used as absorbers for direct weakly interacting massive particles (WIMP) detection. Nuclear recoils can be discriminated against electron recoils by measuring phonons and scintillation light simultaneously. The absorber crystal and the silicon light detector are read out by tungsten superconducting phase transition thermometers. Results on the sensitivity of the phonon and the light channel, radiopurity, the scintillation properties of CaWO₄, and on the WIMP sensitivity are presented.     

Cryogenic composite detectors for the dark matter experiments CRESST and EURECA

Weakly interacting massive particles (WIMPs) are candidates for non-baryonic dark matter. WIMPs are supposed to interact with baryonic matter via scattering off nuclei producing a nuclear recoil with energies up to a few 10 keV with a very low interaction rate of 10⁻⁶ events per kg of target material and day in the energy region of interest. The dark matter experiment cryogenic rare event search with superconducting thermometers (CRESST) and the European underground rare event calorimeter array (EURECA) project are aimed at the direct detection of WIMPs with the help of very sensitive modularised cryogenic detectors that basically consist of a transition edge sensor (TES) in combination with a massive absorber crystal. In the CRESST experiment the search for coherent WIMP-nucleon scattering events is validated by the detection of two processes. In the scintillating absorber single crystal, CaWO₄, heat (phonons) and scintillation light are produced and detected with two independent cryogenic detectors: a phonon channel and a separate light channel.The development of such cryogenic detectors and the potential ton-scale production are investigated in this paper. To decouple the TES production from the choice of the target material in order to avoid heating cycles of the absorber crystal and to allow pretesting of the TESs, a composite detector design (CDD) for the detector production has been developed and studied. An existing thermal detector model has been extended to the CDD, in order to investigate, understand, and optimize the performance of composite detectors. This extended model, which has been worked out in detail, can be expected to provide a considerable help when tailoring composite detectors to the requirements of various experiments.     

Modelling of the Surface-Event Identification Mechanism in Ge Detectors Equipped with NbSi Thin Films

A new generation of germanium composite bolometers, equipped with NbSi thin films, has been developed in the framework of the EDELWEISS experiment, presenting impressive surface event identification capabilities and a substantial improvement in the background rejection of heat and ionization detectors. In this work we present a simple thermal model which explain the surface-event identification mechanism via NbSi thin films sensors.      

Identification of near surface events using athermal phonon signals in low temperature Ge bolometers for the EDELWEISS experiment

We present a study of a 100 g low temperature Ge detector, allowing identification of surface events down to the energy threshold. The bolometer is fitted with segmented electrodes and two NbSi Anderson insulator thermometric layers. Analysis of the athermal signals amplitudes allows us to identify and reject all events occurring in the first millimeter under the electrodes.