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.      

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.      

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.     

Ultra Low Background Cryogenic Test Facility for Far-Infrared Radiation Detectors

The next generation of far infrared radiation detectors is aimed to reach photon noise limited performance in space based observatories such as SPICA and BLISS. These detectors operate at loading powers of the astronomical signal of a few Attowatt (10^?18 W) or less, corresponding to a sensitivity expressed in noise equivalent power as low as $\mathrm{NEP} = 2\times10^{-20}\ \mbox{W}/\sqrt{\mathrm{Hz}}$ . We have developed a cryogenic test setup for microwave Kinetic Inductance Detectors (MKIDs) that aims to reach these ultra-low background levels. Stray light is stopped by using a box in a box design with a sample holder inside another closed box. Microwave signals for the MKID readout enters the outer box through custom made coax cable filters. The stray light loading per pixel is estimated to be less than 60×10^?18 W during nominal operation, a number limited by the intrinsic sensitivity of the MKIDs used to validate the system.     

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.      

\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.     

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.      

Search for Time Reversal Violating Effects: R-Correlation Measurement in Neutron Decay

An experiment aiming at the simultaneous determination of both transversal polarization components of electrons emitted in the decay of free neutrons begins data taking using the polarized cold neutron beam (FUNSPIN) from the Swiss Neutron Spallation Source (SINQ) at the Paul-Scherrer Institute, Villigen. A non-zero value of R due to the e⁻ polarization component, which is perpendicular to the plane spanned by the spin of the decaying neutron and the electron momentum, would signal a violation of time reversal symmetry and thus physics beyond the Standard Model. Present status of the project and the results from analysis of the first data sample will be discussed.     

A Testing Approach for MOS Circuit Using Single-Photon Detectors Under High Magnetic Fields

The MOS circuits must be tested thoroughly for insuring the reliability. A new testing approach for MOS circuits is presented in this paper, which makes use of single-photon detectors and high magnetic fields. If there are faults in the circuit under test, the photon emission from the circuit components is detected by a single-photon detector, the faults are located by the amount of the emitted photons. The following two techniques are proposed in this paper. First, the high magnetic field is applied to the circuit under test, i.e., the circuit is put in high magnetic field environment. To some extent this technique can solve the problem that some faults have poor strengths of emitted photons under general environment. Second, the special circuit input vectors are designed by using binary decision diagrams. The input vectors can make the positions of circuit components to produce signal transitions or switching behaviors, therefore the photon emission strengths of circuit components are enhanced. A lot of experimental results show that the faults in MOS circuits can be tested accurately by the approach proposed in this paper.     

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 Shrinkage Approach for Failure Rate Estimation of Rare Events

Systems have become more and more reliable due to technological advancement. For highly reliable systems, there are usually very few or even no failures during the testing and operation. On the other hand, given a short operating or testing time, the failure of the system is also rare even the failure rate is relatively high. The classical maximum likelihood estimation approach results in degenerated estimates zero when no failure occurs and hence meaningless. To overcome this problem, we investigate a shrinkage approach for the estimation of the failure rate of rare events from multiple heterogeneous systems provided that some of them have failures. The shrinkage estimator shrinks the MLE toward a predetermined data‐dependent point in the parameter space and could be expressed as a weighted average of MLE and the data‐dependent point. Examples are shown how the procedure can be implemented. Simulation studies show that our approach performs better than the other approaches in most cases. Copyright © 2014 John Wiley & Sons, Ltd.     

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.