Results of Quenching Factor Measurements of CaWO_4 at mK Temperatures for the Direct Dark Matter Search Experiment CRESST

The CRESST experiment aims at a direct detection of WIMP dark matter (DM) using scintillating CaWO \(_4\) crystals operated as phonon detectors at mK temperatures. An important feature of the experiment is the active background discrimination technique exploiting the different light outputs depending on the kind of particle interaction. The reduced light yield of nuclear recoils compared to electron recoils is quantified by quenching factors (QFs). The precise measurement of the QFs and thus the identification of the individual recoiling nucleus in the multi-target material CaWO \(_4\) is crucial for neutron background discrimination and assuming a positive DM signal would allow to a certain extent WIMP-mass spectroscopy. At the Munich tandem accelerator a dedicated neutron scattering facility has been set up to measure the QFs of CaWO \(_4\) , in particular that of tungsten, at mK temperatures. Monoenergetic neutrons (11 MeV) produced by the accelerator are scattered off a CRESST-like detector module that is operated in a dilution refrigerator. In this setup, the recoiling nucleus (O, Ca and W) is identified by time-of-flight measurement in liquid-scintillator detectors placed at fixed scattering angles. The QF of W could be determined with unprecedented accuracy at mK temperatures and under realistic measurement conditions: \(QF_W=0.0196\pm 0.0022\) (preliminary value).     

Recent Performance of Scintillating Bolometers Developed for Dark Matter Searches

Scintillating bolometers of sapphire, BGO and LiF have been recently tested both at the Institut d’Astrophysique Spatiale (IAS) and the Laboratorio Subterráneo de Canfranc (LSC) in the frame of the ROSEBUD collaboration. We report on their response to different particles (α, β, γ and nuclear recoils) both in the heat and light channel. Signal output parameters for the different particles have been compared. We have focused on the study of the nuclear recoils discrimination against β/γ background events at low energy. The β/γ and nuclear recoil spectra obtained in the different runs are shown and analyzed. We also discuss the sensitivity of the LiF bolometer for measuring thermal neutron flux at LSC.      

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.      

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.     

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.     

Results and Prospects for the Cryogenic Dark Matter Search (CDMS) Experiment

The Cryogenic Dark Matter Search (CDMS) experiment, located at the Soudan underground mine, operated 30 low-temperature Ge and Si detectors for several years to search for Weakly Interacting Massive dark matter Particles (WIMPs). Due to their excellent background discrimination power and low energy threshold, our detectors have provided world-leading sensitivity for WIMP-nucleon interactions for most of the past decade over a large WIMP mass range. The final exposure of our CDMS?II detectors yielded two candidate events, with an expected background of 0.9 ± 0.2 events, and this result was published in March 2010 in Science. A reanalysis of 8 Ge detectors with a lowered, 2 keV recoil energy threshold, provided increased sensitivity to interactions from WIMPs with masses below ?10 GeV/c² and excluded possible low-mass WIMP signals from the DAMA/LIBRA and CoGeNT experiments. The CDMS collaboration is moving forward with the SuperCDMS experiments at Soudan and SNOLAB with the goal of probing the zeptobarn scale and beyond over the next decade. To achieve this goal, a new generation of larger mass detectors with interleaved geometry for the phonon and ionization readout has been developed.     

Scintillating Bolometers for Double Beta Decay Search

In the field of Double Beta Decay searches the possibility to have high resolution detectors in which a very large part of the natural background can be discriminated with respect to the tiny expected signal, results very appealing. This very interesting possibility can be fulfilled in the case of a scintillating crystal bolometer containing a DBD emitter whose transition energy exceeds the one of the natural 2615 keV gamma line of ²⁰⁸Tl. We present the results achieved in the development of bolometric light detectors for double beta searches. The detectors are 1 mm thick germanium disk coated with a layer of SiO₂ in order to increase the light collection. The adopted temperature sensors are NTD Ge thermistors optimized to work at temperatures between 9 and 13 mK. A light detector with a considerable large area (35 cm²) was constructed and run in a test measurement. A 140 g CdWO₄ crystal (¹¹⁶Cd has a DBD transition energy of 2802 keV) was operated as bolometer and the scintillation light was read by the light detector. The excellent results combined with extreme easy light detector assembly represent the first tangible proof demonstrating the feasibility of this kind of technique.      

Fiducial Volume of the Edelweiss Germanium Bolometers for Dark Matter Search Using Cosmogenic Activation

In this work, we use 10.36 and 8.98 keV X-rays originating from the decay of ⁶⁸Ge and ⁶⁵Zn which are homogeneously produced in a Ge crystal in order to evaluate the fiducial volume of a 400 g coplanar grid HP-Ge bolometers in operation in the Edelweiss-II experiment. Different energy estimators are tested on the experimental data and a fiducial fraction of (39.9±5)% is found. The results are compared to simulations taking into account the anisotropy of the electron transport in Ge.     

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     

Cryogenic Ge Detectors for Dark Matter Search: Surface Event Rejection with Ionization Signals

A new design of an ionization-heat germanium detector for dark matter search is presented, based on the use of interleaved electrodes for charge collection. This device allows active rejection of surface events by performing the appropriate cuts in the charge measurement data. Test experiments were performed with a prototype device, fitted with annular collection electrodes and with a neutron transmutation-doped Ge thermometer. Calibration runs with gamma and neutron sources demonstrate the remarkable possibilities offered by this type of detector in terms of surface event discrimination.      

Dark Matter direct searches

Different techniques are currently competing in the direct search of Weakly Interacting Massive Particles (WIMP) dark matter, which consists in observing in the laboratory nuclear recoils due to impact of WIMPs from our galactic halo. As an introduction to this Dark Matter session, the experimental context, the key challenges and the different techniques will be briefly summarized. The performance achieved by low-temperature detectors confers to them a leading role in this domain.     

Effects of a Weak Magnetic Field on Carrier Transport in a Cryogenic Germanium Detector for Dark Matter Search

A magnetic field of a few gauss produces sizeable effects on carrier trans-port and charge collection in a germanium dark matter detector operated at millikelvin temperatures. The magnitude of the effects is explained by the large values of the velocities imparted to the carriers, even under the low electric field conditions typical for charge collection in these devices (a few 10⁶?cm/s at ～1?V/cm). Using a suitable experimental setup, effects of the magnetic field on electron and hole transport were investigated separately. A?dependence of these effects on the orientation of the field relative to the detector axis is demonstrated, arising in part from magnetic flux conservation through the superconducting (Al) annular electrodes on these devices.     

Direct detection of Dark Matter particles

Arguments on the investigation of the Dark Matter particles in the galactic halo are addressed. Recent results obtained by the DAMA/LIBRA set-up—exploiting the model independent annual modulation signature for Dark Matter (DM) particles—are shortly summarized. In fact, the DAMA project is an observatory for rare processes and it is operative deep underground at the Gran Sasso National Laboratory of the INFN. Its main apparatus is at present the DAMA/LIBRA set-up, whose sensitive volume is made of highly radiopure NaI(Tl) detectors. The first DAMA/LIBRA results confirm the evidence for the presence of DM particles in the galactic halo as pointed out by the former DAMA/NaI set-up; cumulatively the data support such evidence at 8.2σ CL and satisfy all the many peculiarities of the DM annual modulation signature. Future perspectives are briefly addressed.     

An Improved ZnMoO4 Scintillating Bolometer for the Search for Neutrinoless Double Beta Decay of 100Mo

We present a prototype scintillating bolometer for the search for neutrinoless double β decay of ¹⁰⁰Mo, consisting of a single ≈5?g ZnMoO₄ crystal operated aboveground in the 20–30?mK temperature range. The scintillation light is read out by two thin Ge bolometers. The phonon signals are collected by NTD Ge thermistors. The ZnMoO₄ crystal was grown with an advanced method (low-thermal-gradient Czochralski technique) and after purification of molybdenum. The results are very encouraging: the intrinsic energy resolution of the heat channel is ≈800?eV FWHM, the α/β rejection factor (crucial for background suppression) is better than 99.9% in the region of interest for double β decay (≈3?MeV), and the radiopurity of ZnMoO₄ looks substantially improved with respect to previous devices.     

Light detector development for CRESST II

CRESST-II detector modules rely on the ability to actively discriminate electron recoils from nuclear recoils via simultaneous measurement of phonons and scintillation light. The scintillation light produced in each target crystal is detected via an associated calorimeter consisting of a thin silicon wafer read out by a tungsten phase transition thermometer deposited on its surface. About 1% of the energy deposited in CaWO₄ is detected as scintillation light; therefore, the sensitivity of the light detector is crucial for the discrimination of electron recoils from nuclear recoils at energies relevant for WIMP searches. We report the detector performance obtained using a thermometer geometry characterized by phonon collectors and a thin film thermal coupling to the heat sink (Fig. 1). This concept allows a high sensitivity by decoupling the area required for the collection of non-thermal phonons and the heat capacity of the sensor. With a 30×30×0.45 mm³ light detector, energy thresholds below 5 keV referred to energy deposition in CaWO₄ have been obtained. Results achieved will be presented and an overview on further possibilities of development will be given.     

Cryogenic Wide-Area Light Detectors for Neutrino and Dark Matter Searches

Large-mass arrays of bolometers proved to be good detectors for neutrinoless double beta decay (0 \(\nu \) DBD) and dark matter searches. CUORE and LUCIFER are bolometric 0 \(\nu \) DBD experiments that will start to take data in 2015 at Laboratori Nazionali del Gran Sasso in Italy. The sensitivity of CUORE could be increased by removing the background due to \(\alpha \) particles, by detecting the small amount of ?erenkov light ( \(\sim \) 100 eV) emitted by the \(\beta \) s’ signal and not by \(\alpha \) s. LUCIFER could be extended to detect also dark matter, provided that the background from \(\beta \) / \(\gamma \) particles ( \(\sim \) 100 eV of scintillation light) is discriminated from nuclear recoils of about 10 keV energy (no light). We have recently started to develop light detectors for CUORE, LUCIFER and similar bolometric experiments. The aim is to obtain detectors with an active area of \(5\times 5~\mathrm{cm}^2\) (the face of bolometric crystals), operating at 10 mK, and with an energy resolution at the baseline below 20 eV RMS. We have chosen to develop phonon-mediated detectors with KID sensors. We are currently testing the first prototypes.     

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.