Cryogenic Germanium Detectors Research Articles

Phonon and Charge Signals from IR and X Excitation in the SELENDIS Ge Cryogenic Detector

The aim of the SELENDIS project within the EDELWEISS collaboration is to observe single $e^- h^+$ pairs in lightweight (3.3 g) cryogenic germanium bolometers with charge and phonon readout at biases up to $\sim 100$ V. These devices are ideal to characterize in detail the mechanism of charge creation and collection in cryogenic germanium detectors. Electron-hole pairs are produced in the bulk of the detector either by the injection of pulsed IR laser or by neutron activation of germanium inducing the K, L and M lines from $^{71}$Ge electron capture decays. Low-energy laser pulses are also used to probe the single $e^- h^+$ pair sensitivity of Ge bolometers. Preliminary results are used to compare these two modes of charge creation, an important step toward a detailed characterization of Ge bolometers for their use in sub-MeV Dark Matter (DM) searches.

Constraints on dark photons and axionlike particles from the SuperCDMS Soudan experiment

We present an analysis of electron recoils in cryogenic germanium detectors operated during the SuperCDMS Soudan experiment. The data are used to set new constraints on the axioelectric coupling of axion-like particles and the kinetic mixing parameter of dark photons, assuming the respective species constitutes all of the galactic dark matter. This study covers the mass range from 40 eV/$c^2$ to 500 eV/$c^2$ for both candidates, excluding previously untested parameter space for masses below ~1 keV/$c^2$. For the kinetic mixing of dark photons, values below $10^{-15}$ are reached for particle masses around 100 eV/$c^2$; for the axioelectric coupling of axion-like particles, values below $10^{-12}$ are reached for particles with masses in the range of a few-hundred eV/$c^2$.

Recent results from EDELWEISS Dark Matter searches

The EDELWEISS collaboration is performing direct searches for light Dark Matter particles using cryogenic germanium detectors equipped with a charge and thermal signal readout. This versatile and highly performing technology opens new possibilities for searches for signals involving either electrons or nuclear recoils. This is attested to by results on Axion- Like Particles in the keV range, and by the attainment of the first sub-GeV spin-independent dark matter limit based on a germanium target. The search has been extended to Strongly Interacting Particles (SIMP) down to 45 MeV by exploiting the Migdal effect. New results on SIMPs with spin-dependent interactions will also be presented.

Cosmogenic Activation of Germanium Detectors in EDELWEISS III

Activation of germanium crystals due to cosmic rays becomes a serious limitation for experiments searching for rare events with germanium detectors. Cosmic ray induced activation of the detector components and, even more importantly, of the germanium itself during production, transportation and storage at the Earth’s surface, might result in the production of radioactive isotopes with long half-lives, with a possible impact on the expected background. We present a measurement of the cosmogenic activation in the cryogenic germanium detectors of the EDELWEISS III direct dark matter search experiment. The decay rates measured in detectors with different exposures to cosmic rays above ground are converted into production rates of different isotopes. They are compared to model predictions present in literature and to estimates calculated with the ACTIVIA code.

Low-mass WIMP searches with the EDELWEISS experiment

The EDELWEISS experiment is a direct search for Weakly Interacting Massive Particles (WIMP) dark matter using an array of twenty-four 860 g cryogenic germanium detectors equipped with a full charge and thermal signal readout. The experiment is located in the ultra-low radioactivity environment of the Modane underground laboratory in the Frejus tunnel. WIMP limits, background rejection factors and measurements of cosmogenic activation recorded in long exposures are used to assess the performance of the third generation of EDELWEISS detectors in view of the search for WIMPs in the mass range from 1 to 20 GeV/c2. The developments in progress to pursue this goal in the coming years are also presented.

Low-Noise HEMTs for Coherent Elastic Neutrino Scattering and Low-Mass Dark Matter Cryogenic Semiconductor Detectors

We present the noise performance of High Electron Mobility Transistors (HEMT) developed by CNRS-C2N laboratory. Various HEMT's gate geometries with 2 pF to 230 pF input capacitance have been studied at 4 K. A model for both voltage and current noises has been developed with frequency dependence up to 1 MHz. These HEMTs exhibit low dissipation, excellent noise performance and can advantageously replace traditional Si-JFETs for the readout of high impedance thermal sensor and semiconductor ionization cryogenic detectors. Our model predicts that cryogenic germanium detectors of 30 g with 10 eV heat and 20 eVee baseline resolution are feasible if read out by HEMT based amplifiers. Such resolution allows for high discrimination between nuclear and electron recoils at low threshold. This capability is of major interest for Coherent Elastic Neutrino Scattering and low-mass dark matter experiments such as Ricochet and EDELWEISS.

Status and Prospects of the EDELWEISS-III Direct WIMP Search Experiment

The EDELWEISS collaboration is performing a direct search for WIMP dark matter using an array of up to twenty-four 820–890 g cryogenic germanium detectors equipped with a full charge and thermal signal readout. The experiment is located in the ultra-low-radioactivity background environment of the Modane underground laboratory, in the French–Italian Frejus tunnel. We present the analysis of data obtained in extended data taking periods. WIMP limits, background rejection factors and measurements of cosmogenic activation are used to assess the performance of the third generation of EDELWEISS detectors in view of the search for WIMPs in the mass range from 1 to $$20~\hbox {GeV}/\hbox {c}^{2}$$ . The developments in progress to pursue this goal in the coming years are also presented.

Performance of the EDELWEISS-III experiment for direct dark matter searches

We present the results of measurements demonstrating the efficiency of the EDELWEISS-III array of cryogenic germanium detectors for direct dark matter searches. The experimental setup and the FID (Fully Inter-Digitized) detector array is described, as well as the efficiency of the double measurement of heat and ionization signals in background rejection. For the whole set of 24 FID detectors used for coincidence studies, the baseline resolutions for the fiducial ionization energy are mainly below 0.7 keVee (FHWM) whereas the baseline resolutions for heat energies are mainly below 1.5 keVee (FWHM). The response to nuclear recoils as well as the very good discrimination capability of the FID design has been measured with an AmBe source. The surface β- and α-decay rejection power of Rsurf < 4 × 10−5 per α at 90% C.L. has been determined with a 210Pb source, the rejection of bulk γ-ray events has been demonstrated using γ-calibrations with 133Ba sources leading to a value of Rγ −mis−fid < 2.5 × 10−6 at 90% C.L.. The current levels of natural radioactivity measured in the detector array are shown as the rate of single γ background. The fiducial volume fraction of the FID detectors has been measured to a weighted average value of (74.6 ± 0.4)% using the cosmogenic activation of the 65Zn and 68,71Ge isotopes. The stability and uniformity of the detector response is also discussed. The achieved resolutions, thresholds and background levels of the upgraded EDELWEISS-III detectors in their setup are thus well suited to the direct search of WIMP dark matter over a large mass range.

Measurement of the cosmogenic activation of germanium detectors in EDELWEISS-III

We present a measurement of the cosmogenic activation in the cryogenic germanium detectors of the EDELWEISS III direct dark matter search experiment. The decay rates measured in detectors with different exposures to cosmic rays above ground are converted into production rates of different isotopes. The measured production rates in units of nuclei/kg/day are 82 ± 21 for 3H, 2.8 ± 0.6 for 49V, 4.6 ± 0.7 for 55Fe, and 106 ± 13 for 65Zn. These results are the most accurate for these isotopes. A 90% C.L. lower limit on the production rate of 68Ge of 71 nuclei/kg/day is also presented. They are compared to model predictions present in literature and to estimates calculated with the ACTIVIA code.

Background studies for the MINER Coherent Neutrino Scattering reactor experiment

The proposed Mitchell Institute Neutrino Experiment at Reactor (MINER) experiment at the Nuclear Science Center at Texas A&M University will search for coherent elastic neutrino-nucleus scattering within close proximity (about 2m) of a 1MW TRIGA nuclear reactor core using low threshold, cryogenic germanium and silicon detectors. Given the Standard Model cross section of the scattering process and the proposed experimental proximity to the reactor, as many as 5–20events/kg/day are expected. We discuss the status of preliminary measurements to characterize the main backgrounds for the proposed experiment. Both in situ measurements at the experimental site and simulations using the MCNP and GEANT4 codes are described. A strategy for monitoring backgrounds during data taking is briefly discussed.

Evaluation of neutron background in cryogenic Germanium target for WIMP direct detection when using reactor neutrino detector as neutron veto

Abstract A direct WIMP (Weakly Interacting Massive Particle) detector with a neutron veto system is designed to better reject neutrons. An experimental configuration is studied in the present paper: 984 Ge modules are placed inside a reactor neutrino detector. In order to discriminate between nuclear and electron recoil, both ionization and heat signatures are measured using cryogenic germanium detectors in this detection. The neutrino detector is used as a neutron veto device. The neutron background for the experimental design has been estimated using the Geant4 simulation. The results show that the neutron background can decrease to O(0.01) events per year per tonne of high purity Germanium. We calculate the sensitivity to spin-independent WIMP–nucleon elastic scattering. An exposure of one tonne × year could reach a cross-section of about 2×10−11 pb.

New Results from the Search for Low-Mass Weakly Interacting Massive Particles with the CDMS Low Ionization Threshold Experiment.

The CDMS low ionization threshold experiment (CDMSlite) uses cryogenic germanium detectors operated at a relatively high bias voltage to amplify the phonon signal in the search for weakly interacting massive particles (WIMPs). Results are presented from the second CDMSlite run with an exposure of 70kg day, which reached an energy threshold for electron recoils as low as 56eV. A fiducialization cut reduces backgrounds below those previously reported by CDMSlite. New parameter space for the WIMP-nucleon spin-independent cross section is excluded for WIMP masses between 1.6 and 5.5 GeV/c^{2}.

Dark matter direct detection with SuperCDMS Soudan

The Cryogenic Dark Matter Search Experiment (CDMS) and its successor, SuperCDMS, have had a long history of establishing world-leading upper limits on the interaction of Weakly Interacting Massive Particles (WIMPs) with standard model nucleons. SuperCDMS uses arrays of cryogenic germanium detectors to achieve excellent discrimination between the nuclear recoils expected for WIMP interactions and radioactively produced electron recoils through the collection of ionization and athermal phonons. Recent analyses of the data collected from the current installation of Interleaved Z-Sensitive Ionization and Phonon (iZIP) detectors have probed new regions of the WIMP parameter space, particularly in the region of low-mass WIMP interactions. This paper will discuss the current work of the SuperCDMS collaboration.

Signals Induced by Charge Carrier Trapping

Cryogenic germanium detectors used in dark matter searches operate at ranges of low temperature \(<\)100 \(\mathrm {mK}\) and electric field \(\sim \)1 \(\mathrm {V/cm}\) such that charge trapping in the bulk of the detector plays a significant role in the net charge collected following a particle interaction. It is shown that in EDELWEISS FID800 detectors, these trapped charges induce residual charge signals that perturb the measurement of the total charge, significantly degrading the energy resolution at high energy. It is also shown that, by reading out the signal on all the electrodes, it is possible to clearly identify these effects and correct the total charge measurement accordingly, resulting in a \(\sim \)30 % improvement of the resolution at high energy. This effect is demonstrated with data. A simple analytical model based on Shockley-Ramo’s theorem is presented, whereby this effect is due to the position dependence of signals induced by trapped charges.

The EDELWEISS-III Project and the Rejection Performance of Its Cryogenic Germanium Detectors

The EDELWEISS-III experiment searches for Dark matter using cryogenic germanium detectors in the low radioactivity environment of the Laboratoire Souterrain de Modane. The experiment and the FID detector design, whereby all surfaces are covered with interleaved electrodes, are described. The performance for gamma-ray and surface event rejection of these detectors is discussed. In particular, recent calibrations with a \(^{210}\)Pb source reveals that the excellent surface event rejection already demonstrated with planar interleaved electrodes also extends to the entire surface of the large FID detectors, even on their cylindrical outer surfaces, where the electric field has a more complicated geometry. Expected results with the full exposure of the EDELWEISS-III experiment are discussed.

Hot Carrier Trapping in High-Purity and Doped Germanium Crystals at Millikelvin Temperatures

A new set of experimental data is presented for the mean drift lengths and the drift velocities of hot electrons and holes as a function of the electric field in ultra-pure and in lightly doped (n- and p-type) germanium single crystals at mK temperatures. Measurements are made in the field range between $$\sim $$ 0.1 and 15 V/cm, typical for the operation of cryogenic germanium detectors for dark matter search. The analysis of the experimental data strongly suggests that the dominant trapping centers are the dopant species in the neutral state.

Phonon-Based Position Determination in SuperCDMS iZIP Detectors

SuperCDMS is currently operating a 10-kg array of cryogenic germanium detectors in the Soudan underground laboratory to search for weakly interacting massive particles, a leading dark matter candidate. These detectors, known as iZIPs, measure ionization and athermal phonons from particle interactions with sensors on both sides of a Ge crystal. The ionization signal can be used to efficiently tag events at high radius and near the top and bottoms surfaces, where diminished charge collection can cause events to mimic WIMP-induced nuclear recoils. Using calibration data taken with a 210Pb source underground at Soudan, we demonstrate rejection of surface events of (4.5 +/- 0.9) x 10^-4 with 46% acceptance of nuclear recoils using the phonon signal only. We also show with 133Ba calibration data underground that the phonon channels can efficiently identify events near the sidewall. This phonon-based approach can also be extended to lower energies than the ionization-based position reconstruction.

Demonstration of surface electron rejection with interleaved germanium detectors for dark matter searches

The SuperCDMS experiment in the Soudan Underground Laboratory searches for dark matter with a 9-kg array of cryogenic germanium detectors. Symmetric sensors on opposite sides measure both charge and phonons from each particle interaction, providing excellent discrimination between electron and nuclear recoils, and between surface and interior events. Surface event rejection capabilities were tested with two 210Pb sources producing ∼130 beta decays/hr. In ∼800 live hours, no events leaked into the 8–115 keV signal region, giving upper limit leakage fraction 1.7 × 10−5 at 90% C.L., corresponding to &amp;lt; 0.6 surface event background in the future 200-kg SuperCDMS SNOLAB experiment.

Background discrimination in new iZIP detectors at SuperCDMS

SuperCDMS is a direct dark matter search experiment in the Soudan Underground Laboratory (Minnesota, USA). Cryogenic germanium detectors are used to identify rare nuclear recoils induced by elastic scattering of WIMPs from our Galactic halo. The detectors measure phonon and ionization signals simultaneously, allowing an event-by-event discrimination between the electronic recoils, tracers of electromagnetic background, and nuclear recoils originated by neutrons and WIMPs. To further increase the background discrimination power, novel detectors with a special interleaved electrode scheme have been tested and installed. I will present the background rejection performances achieved, giving the resulting improvement in the sensitivity of the experiment for spin-independent WIMPs interactions.

Searching for WIMPs with EDELWEISS

The EDELWEISS-II experiment, operated in the Frejus laboratory in a low-background environment, uses cryogenic germanium detectors to look for WIMPs. We present the results of a WIMP search carried out recently with ten so-called InterDigit detectors. This technology enables a high level of gamma radioactivity rejection within a controlled fiducial volume. A cross-section of 4.4 × 10−8 pb could be excluded for a WIMP mass of 85 GeV. We also present the status of the EDELWEISS-III project, which will operate 40 newly-designed FID detectors in an upgraded installation to improve significantly the sensitivity to low WIMP scattering cross-sections.