COBRA Experiment Research Articles

Commissioning of the COBRA extended demonstrator at the LNGS

The COBRA experiment aims to search for the neutrinoless double beta decay by using CdZnTe (CZT) semiconductor detectors. A demonstrator setup, comprising 64 CZT detectors with a volume of 1 cm3 each, was operated at the underground Laboratori Nazionali del Gran Sasso (LNGS). The COBRA collaboration has recently upgraded the demonstrator to the extended demonstrator (XDEM). An array of nine large volume 2 × 2 × 1.5 cm3 CZT room temperature semiconductor detectors with a total mass of 315.1 g was installed to increase the half-life sensitivity. The innovative detector design and improved detector handling procedures during commissioning allow for an improved energy resolution of the detectors while the most prominent backgrounds that were identified during the demonstrator operation are expected to be reduced significantly. This article gives an overview of the new hardware components and the background present in the setup. Furthermore, the energy and position reconstruction is described in detail, including the correction of electron trapping. All functional detectors demonstrated a good stability in the long-term operation. The analysis of the commissioning data confirms the excellent energy resolution of the detectors, as well as an improvement of the background level by more than one order of magnitude compared to the previous demonstrator setup.

Quenching of gA deduced from the β-spectrum shape of 113Cd measured with the COBRA experiment

A dedicated study of the quenching of the weak axial-vector coupling strength gA in nuclear processes has been performed by the COBRA collaboration. This investigation is driven by nuclear model calculations which show that the β-spectrum shape of the fourfold forbidden non-unique decay of 113Cd strongly depends on the effective value of gA. Using an array of CdZnTe semiconductor detectors, 45 independent 113Cd spectra were obtained and interpreted in the context of three nuclear models. The resulting effective mean values are g‾A(ISM)=0.915±0.007, g‾A(MQPM)=0.911±0.013 and g‾A(IBFM-2)=0.955±0.022. These values agree well within the determined uncertainties and deviate significantly from the free value of gA. This can be seen as a first step towards answering the long-standing question regarding quenching effects related to gA in low-energy nuclear processes.

Suppression of alpha-induced lateral surface events in the COBRA experiment using CdZnTe detectors with an instrumented guard-ring electrode

The COBRA collaboration searches for neutrinoless double beta-decay (0νββ-decay) using CdZnTe semiconductor detectors with a coplanar-grid readout and a surrounding guard-ring structure. The operation of the COBRA demonstrator at the Gran Sasso underground laboratory (LNGS) indicates that alpha-induced lateral surface events are the dominant source of background events. By instrumenting the guard-ring electrode it is possible to suppress this type of background. In laboratory measurements this method achieved a suppression factor of alpha-induced lateral surface events of 5300+2660−1380, while retaining (85.3 ±0.1%) of gamma events occurring in the entire detector volume. This suppression is superior to the pulse-shape analysis methods used so far in COBRA by three orders of magnitude.

Pulse-shape discrimination techniques for the COBRA double beta-decay experiment at LNGS

In modern elementary particle physics several questions arise from the fact that neutrino oscillation experiments have found neutrinos to be massive. Among them is the so far unknown nature of neutrinos: either they act as so-called Majorana particles, where one cannot distinguish between particle and antiparticle, or they are Dirac particles like all the other fermions in the Standard Model. The study of neutrinoless double beta-decay (0νββ-decay), where the lepton number conservation is violated by two units, could answer the question regarding the underlying nature of neutrinos and might also shed light on the mechanism responsible for the mass generation. So far there is no experimental evidence for the existence of 0νββ-decay, hence, existing experiments have to be improved and novel techniques should be explored. One of the next-generation experiments dedicated to the search for this ultra-rare decay is the COBRA experiment. This article gives an overview of techniques to identify and reject background based on pulse-shape discrimination.

Investigation of the Depth Reconstruction and Search for Local Performance Variations With a Large Coplanar-Quad-Grid CdZnTe Detector

CdZnTe coplanar-grid (CPG) detector is used in the COBRA experiment to search for neutrinoless double beta decay. In the next phase of the experiment, significantly larger detectors with a volume of $(20 \times 20 \times 15)~\text {mm}^{3}$ and a coplanar-quad-grid will be used. This paper presents the results of a spatially resolved investigation of the performance of one detector of this type, focusing on its usability in a double-beta experiment. With these data, two modifications of the interaction depth formula, which is of great importance for background reduction in COBRA, are investigated for the first time with such a detector, and the results are compared with a detector with a conventional CPG. Furthermore, we report the mobility-lifetime product for electrons for each sector of the detector.

Status and Perspectives of the COBRA Experiment

COBRA is a neutrinoless double beta decay (0νββ) experiment using an array of Cadmium-Zinc-Telluride semiconductor detectors, the isotope of interest being 116Cd with a Q-value of 2814 keV. To investigate the experimental challenges of operating CdZnTe detectors in low background mode and to identify potential background components, a demonstrator setup is operated at the Gran Sasso underground laboratory (LNGS) in Italy, while additional studies are proceeding in surface laboratories. The experiment consists of monolithic, calorimetric detectors of coplanar grid design (CPG de- tectors). These detectors have a size of 1×1×1cm3 and are arranged in four layers of 4×4 detectors. An overview of the current status and of future perspectives is given. Results of pulse shape analyses are presented as well as background estimates using the data collected so far.

Pulse-shape discrimination of surface events in CdZnTe detectors for the COBRA experiment

Events near the cathode and anode surfaces of a coplanar grid CdZnTe detector are identifiable by means of the interaction depth information encoded in the signal amplitudes. However, the amplitudes cannot be used to identify events near the lateral surfaces. In this paper a method is described to identify lateral surface events by means of their pulse shapes. Such identification allows for discrimination of surface alpha particle interactions from more penetrating forms of radiation, which is particularly important for rare event searches. The effectiveness of the presented technique in suppressing backgrounds due to alpha contamination in the search for neutrinoless double beta decay with the COBRA experiment is demonstrated.

Status and perspectives of COBRA

The COBRA experiment is a search for neutrinoless double-beta decay using CdZnTe semiconductor detectors. Currently COBRA is collecting data from 32 coplanar grid (CPG) detectors installed in the Gran Sasso National Laboratory. This serves as a prototype for a proposed design of over 100 kg target mass in order to achieve the sensitivity necessary to probe new parameter space for 0νββ. Pixelated detector allowing tracking and thus particle identification are also explored as a unique option in double beta searches.

Current Status and Future Perspectives of the COBRA Experiment

The aim of the COBRA experiment is to prove the existence of neutrinoless double-beta-decay (0νββ-decay) and to measure its half-life. For this purpose a detector array made of cadmium-zinc-telluride (CdZnTe) semiconductor detectors is operated at the Gran Sasso Underground Laboratory (LNGS) in Italy. This setup is used to investigate the experimental issues of operating CdZnTe detectors in low-background mode and to identify potential background components, whilst additional studies are proceeding in surface laboratories. The experiment currently consists of monolithic, calorimetric detectors of coplanar grid design (CPG detectors). These detectors are 1 × 1 × 1 cm3and are arranged in 4 × 4 detector layers. Ultimately four layers will be installed by the end of 2013, of which two are currently operating. To date 82.3 kg·days of data have been collected. In the region of interest for116Cd around 2.8 MeV, the median energy resolution is 1.5% FWHM, and a background level near 1 counts/keV/kg/y has been reached. This paper gives an overview of the current status of the experiment and future perspectives.

Nuclear responses of64,66Zn isotopes to supernova neutrinos

Nuclear responses to energy spectra of supernova (SN) neutrinos are studied by folding original $\ensuremath{\nu}$-nucleus cross sections calculated in the context of the quasiparticle random-phase approximation using realistic two-body forces. To this purpose, we employ two-parameter neutrino-energy distributions of Fermi-Dirac and power-law shapes for various SN neutrino scenarios. As concrete examples of promising neutrino detectors we have chosen the ${}^{64,66}$Zn isotopes, contents of (i) the semiconductor CdZnTe (COBRA experiment) and (ii) the crystal scintillators ZnMoO${}_{4}$ and ZnWO${}_{4}$. These materials are quite advantageous for measuring double-$\ensuremath{\beta}$-decay events and for potential use in studying current neutrino physics issues. We concentrate on the evaluation of folded differential cross sections, ${[d\ensuremath{\sigma}(\ensuremath{\omega})/d\ensuremath{\omega}]}_{\mathrm{fold}}$, and flux-averaged cumulative cross sections, ${[d\ensuremath{\sigma}(\ensuremath{\omega})/d\ensuremath{\omega}]}_{\mathrm{fold}}^{\mathrm{cum}}$, as well as total cross sections, $\ensuremath{\langle}{\ensuremath{\sigma}}_{\mathrm{tot}}\ensuremath{\rangle}$, of the above Zn isotopes by adopting several realistic SN neutrino models parametrized by the neutrino temperature or the mean neutrino energy and the width of the aforementioned distributions.

Particle fluxes and condensational uptake over sea ice during COBRA

Particle fluxes were measured over sea ice at Hudson Bay, Canada, during the COBRA experiment in February and March, 2008. Eddy covariance particle fluxes were measured using a condensation particle counter and an ultrasonic anemometer on a 2.5 m mast on the sea ice. After applying appropriate corrections and filtering, the mean net deposition velocity was 0.12 ± 0.11 mm s−1 for particles measured with a CPC 3776 (lower size threshold, Dp50 = 2.5 nm) and was at the detection limit of the measurement system. No evidence of nucleation events was seen. Two optical particle counters (at heights 0.2 and 1.35 m on the mast) allowed size segregated fluxes of particles in the accumulation and coarse mode diameter range 0.3–20 μm to be derived using the aerodynamic flux gradient method. Strong net emission fluxes were observed around midday, 3rd March, when winds increased to around 10 m s−1, suggesting ice particle resuspension. The fluxes during this period had a significant influence on the derived condensational loss rate to the available particle surfaces, kt. Number fluxes were greatest in the smallest size channels, while the largest sizes dominated the mass flux. Number fluxes also increased with wind speed, and this relationship was strongest for the smaller sizes. Particle mass size distributions showed an enhanced mode around 400 nm (dry size). Values of kt were well approximated by the molecular regime and were found to be much smaller and less variable than values derived for marine air.

The COBRA double beta decay experiment

The COBRA experiment is searching for double beta decay using CdZnTe semiconductor detectors. The main focus is on the isotope 116Cd. In addition to pure energy measurements, pixelisation allows also for tracking capabilities. This kind of semiconductor tracker is unique in the field. The current status of the experiment is shown including the latest half-life limits.

Recent progress of the COBRA experiment

The COBRA experiment uses CdZnTe semiconductor detectors to search for neutrinoless double beta decays. The main focus is on the isotope 116Cd, with a decay energy of 2813.5 keV well above the highest naturally occurring gamma lines. Also 130Te and 106Cd, a double β+ emitter, are under investigation.An overview of the recent improvements of the COBRA low-background set-up at the LNGS underground laboratory is given, including first results obtained with new FADC readout electronics which allows background reduction by pulse-shape analysis. Furthermore studies on detector characterization, the use of liquid scintillator for background suppression and Monte-Carlo simulations of the shielding are presented. Also pixelated detectors and their capabilities of background reduction are discussed.

Double Beta Decay Measurement with COBRA

The COBRA experiment aims to use a large array of CdZnTe semiconductor detectors to search for neutrinoless double beta decay. Extensive simulation studies and data collected with a small proto-type experiment have been used to address the major design specifications for a large scale experiment sensitive to 116 Cd half-lives in excess of 10 26 years. The current and future prospects of the COBRA experiment are presented.

Exploration of Pixelated detectors for double beta decay searches within the COBRA experiment

Abstract The aim of the COBRA experiment is the search for neutrinoless double beta decay events in Cadmium Zinc Telluride (CdZnTe) room temperature semiconductor detectors. The development of pixelated detectors provides the potential for clear event identification and thus major background reduction. The tracking option of a semiconductor is a unique approach in this field. For initial studies, several possible detector systems are considered with a special regard for low background applications: the large volume system Polaris with a pixelated CdZnTe sensor, Timepix detectors with Si and enriched CdTe sensor material and a CdZnTe pixel system developed at the Washington University in St. Louis, USA. For all detector systems first experimental background measurements taken at underground laboratories (Gran Sasso Underground Laboratory in Italy, LNGS and the Niederniveau Messlabor Felsenkeller in Dresden, Germany) and additionally for the Timepix detectors simulation results are presented.

The status of the COBRA double-beta-decay experiment

The current status of the COBRA experiment is described. Results on the fourfold-forbidden beta decay of 113Cd and a variety of double-beta-decay limits for Cd, Zn and Te isotopes are presented, based on 18 kg× days of exposure with an array of sixteen CdZnTe semiconductor detectors. A short description of the activities with pixelated detectors for tracking is given.

Status of the COBRA double beta decay experiment

The current status of the COBRA experiment is described. Results on the 4-fold forbidden beta decay of 113Cd and a variety of double beta decay limits of Cd, Zn and Te isotopes are presented based on 18 kg × days of exposure with an array of sixteen CdZnTe semiconductor detectors. A short description on the activities with pixelated detectors for tracking is given.

An investigation on cooling of CZT co-planar grid detectors

The effect of moderate cooling on CdZnTe semiconductor detectors has been studied for the COBRA experiment. Improvements in energy resolution and low energy threshold were observed and quantified as a function of temperature. Leakage currents are found to contribute typically ∼ 5 keV to the widths of photopeaks.

A torquing shearing interferometer for cylindrical wire array experiments

In standard shearing interferometry, a single probing beam passes through a perturbing medium and is then split into two beams. A linear shift results in an overlap, an interference, and a fringe pattern yielding the perturbing medium density profile. The probing beam usually needs to be larger than the perturbing medium so that part of it passes through a well separated low density region. During early time axial (end-on) views of imploding cylindrical wire arrays low density regions lie in between the high density regions that are near the initial wire positions. In addition, for end-on viewing, the probing beam diameter is limited by electrodes and is comparable to the array diameter. In this case a linear translation will not work but the overlap can be accomplished by an azimuthal rotation of one beam with respect to the other. Such a torquing shearing interferometer has been set up on the COBRA experiment to give time resolved, radial, and azimuthal electron density profiles during early time cylindrical wire array implosions.

The COBRA experiment

The COBRA experiment aims to use a large array of CdZnTe semiconductor detectors to search for neutrinoless double beta decay. The COBRA collaboration are currently operating a small proto-type array of crystals in a low-background environment. This paper presents the current status of the experiment, results from current and previous proto-types and future prospects for COBRA.