Dark Matter Research Research Articles

Measurement of photocathode spectral response at cryogenic temperature

Noble-gas liquids, such as xenon and argon, have been recently proposed as light scintillators in some experiments dedicated to neutrino physics and dark matter research. These experiments need the use of photomultiplier tubes directly immersed in the liquid phase and operating at cryogenic temperatures. We carried out an investigation about the spectral response and its variation from room to cryogenic temperature for two different kind of cryogenic photocathodes manufactured by Electron Tubes Ltd. and Hamamatsu Photonics K.K. Measurements were carried out by means of a xenon continuous lamp and a UV–VIS monocromator which allows the analysis from 300 to 700 nm with 5 nm resolution. Cryogenic tests were made at the liquid nitrogen temperature (77 K).

Cold Dark Soft Matter Research and Atmosphere in the Theatre

Recent research has experimentally confirmed the existence of cold, dark, soft matter (which I abbreviate CDSM). The paper provides a brief survey of the characteristics of CDSM and proposes how CDSM may explain specific experiences and phenomena of the theatre.

Performance and behaviour of photomultiplier tubes at cryogenic temperature

Noble-gas liquids, such as xenon and argon, have been recently proposed as scintillators in some experiments dedicated to neutrino physics and dark matter research. These experiments need to use large-area high-sensitivity light detectors directly immersed in the liquid phase and operating at cryogenic temperature. We carried out a detailed investigation on the use of conventional and dedicated photomultiplier tubes in collaboration with two manufacturers: Electron Tubes Ltd. and Hamamatsu Photonics K.K. Once verified the capability to withstand thermal shocks from room to cryogenic temperature, we studied the device characteristics in different temperature conditions. Good quantum efficiencies can be achieved with multialkali photocathodes or bialkali photocathodes on platinum under-coatings. Gain losses and an increase of the dark count rate at low temperature are also observed.

Characterization of ETL 9357FLA photomultiplier tubes for cryogenic temperature applications

We carried out a careful evaluation of the performance of the large cathode area ETL 9357FLA photomultiplier tube operating at cryogenic temperature. The measurements were focused on evaluating the parameters which mainly characterize the operating performances of the device down to 77 K and the spread of the distinctive features over 54 samples assembled in the ICARUS apparatus. The results that we obtained demonstrate that the photomultiplier is suited for light detection in such unconventional operating conditions, certifying this device for the direct measurement of scintillation light coming from noble-gas liquids in detectors dedicated to neutrino physics and dark matter research.

Dark matter search at Boulby mine

The Boulby Dark Matter Collaboration (BDMC) is running a WIMP Dark Matter research programme in the underground laboratory of Boulby Mine. The two axes of the programme are based on (1) liquid Xenon (LXe) as the WIMP target and (2) directional detection in low pressure gas detectors. ZEPLIN-1 is a 3.1kg of LXe scintillation detector with a background discrimination based on Pulse Shape Analysis. A new limit from a preliminary analysis is presented. Further generation setups with improved background discrimination tools (as the ionisation is also recorded) are in construction. ZEPLIN-2 and ZEPLIN-3, are predicted to be sensitive to rate of 0.1-0.01 events/kg/day within 2 years of data, with fiducial masses of 30kg and 6kg, respectively. These are important steps towards the design of a tonne-scale LXe Dark Matter detector array. The DRIFT programme relies on discrimination by tracking and directionnal detection. This enables the measurement of any sidereal modulation, as the earth's rotation changes the flux angle of Galactic WIMPs. A low pressure 1m$^3$ chamber (DRIFT-1) is currently taking data underground. A setup for a bigger volume with increased spatial resolution (enabling the pressure, and hence the target mass to be increase) is in the designing process.

A simulation code for the ionization and heat signals in low-temperature germanium detectors for Dark Matter research

A computation code is presented, allowing numerical simulation and pulse-shape analysis of the ionization and heat signals in low-temperature massive Ge detectors for Dark Matter research. The effects on charge collection are pointed out, arising from the Coulomb repulsion of free carriers within the charge clouds. Test experiments are described, aimed at event localization for improved background rejection.

The ROSEBUD experiment at Canfranc : 2001 report

The ROSEBUD experiment for Direct Dark Matter detection settled in 1999 in the Canfranc Underground Laboratory. The first phase of the experiment was dedicated to the understanding and reduction of the radioactive background following successive removals of the radioimpure materials. Sapphire (25g, 50g) and germanium (67g) absorbers were used. Thresholds respectively lower than 1keV and 450 eV were achieved on these detectors. The second phase of the experiment plans to use scintillating bolometers to discriminate between recoiling nuclei and electrons. Prototypes using commercial CaWO 4 (54g ) and BGO (46g) were designed for this purpose. While internal contamination was found and identified in both targets, neutron calibrations revealed their high discrimination power. A 6 keV threshold on the heat channel of the BGO bolometer points out the interest of such a novel material, for Dark Matter research on neutralinos having spin-dependent or spin-independent interactions.

Ionization measurement at very low temperature for nuclear and electron recoils discrimination by ionization-heat simultaneous measurement for dark matter research

To achieve a high level of discrimination between nuclear recoils and electron recoils for dark matter research, we realized and studied ionization-heat detectors working at very low temperature (10–20mK). To understand the mechanisms underlying the ionization measurement at this temperature range, we made systematic studies of detectors performances (time stability, energy resolution, etc.) in X- and γ-rays detection for Ge and Si detectors. Results are presented and discussed. We found that the time stability is governed by the progressive space charge build-up due to impurities ionization by far-infrared radiation. Moreover if the energy resolution of HPGe-pin detectors is limited by the noise, it appears to be intrinsically limited by carrier trapping on neutral impurities in Si detectors.

Alpha/gamma discrimination with a CaF 2(Eu) target bolometer optically coupled to a composite infrared bolometer

On account of its qualities for dark matter research, a 300 mg luminescent CaF 2(Eu) bolometer light-coupled to an infrared sapphire bolometer has been successfully investigated. At a working temperature of about 130 mK, a discrimination between alpha particles and gamma irradiation has been achieved. The rejection power as a function of energy is given. We finally discuss an extrapolation of our results to CaF 2(Eu) targets of several grams and lower working temperatures.

A proposal for solar axion detection via Bragg scattering

The paper suggests using Bragg scattering to search for solar axions or other bosons. The enhancement factor from constructive interference can be as large as ∼ 10 4 over the whole solar axion spectrum. The angular correlation of the axion-to-photon cross section can be utilized to focus the converted celestial axions by using a large surface antenna covered with a thin film (∼ μm) of single crystals. A few cases are discussed, and the potential importance of this interaction-mode in other crystalline radiation detectors is mentioned. Alternatively, existing experimental data with semiconductor or other thermal (cryogenic) detectors — not only in dark matter research — should be reevaluated. Experiments based on this proposal could measure a coupling constant at the level g aγγ =10 −8 GeV −1 or lower for an axion mass up to a few keV.