G. Dambier

First results of the ROSEBUD dark matter experiment

Abstract ROSEBUD (Rare Objects SEarch with Bolometers UndergrounD) is an experiment which attempts to detect low mass weak interacting massive particles through their elastic scattering off Al and O nuclei. It consists of three small sapphire bolometers (of a total mass of 100 g) with NTD-Ge sensors in a dilution refrigerator operating at 20 mK in the Canfranc Underground Laboratory. We report in this paper the results of several runs (of about 10 days each) with successively improved energy thresholds, and the progressive background reduction obtained by improvement of the radiopurity of the components and subsequent modifications in the experimental assembly, including the addition of old lead shields. Mid-term plans and perspectives of the experiment are also presented.

Performances and prospects of the “ROSEBUD” dark matter search experiment

Abstract A cryogenic search for WIMP Dark Matter with small sapphire bolometers through the WIMP scattering off Al2O3 nuclei, the ROSEBUD (Rare Objects SEarch with Bolometers UndergrounD) experiment, is being installed in the Canfranc Underground Laboratory (Spain) at 2450 m water equivalent. The performances of the bolometers, the radioactive background expected from the measurement of the radiopurity of the ROSEBUD components and the estimated sensitivity of the experiment for low mass WIMP detection are presented.

Light yield of undoped sapphire at low temperature under particle excitation

In this letter, we report on the performance of scintillating sapphire bolometers developed in the framework of the ROSEBUD (Rare Objects Search with Bolometers UnDerground) Collaboration devoted to dark matter searches. The simultaneous measurement of heat and light allows the discrimination of the type of particle increasing the sensitivity for these searches. A heat versus light negative correlation for gamma events has been observed. Its interpretation, in a simple framework, allows the estimation of the light yield of undoped sapphire at a temperature of about 20 mK under gamma, alpha, and neutron excitation.

Performance and applications of cooled composite bolometers in the field of ionizing radiation metrology

Two major limitations met in the field of ionizing radiation metrology, i.e. energy resolution and detection energy threshold, may be improved by one or two orders of magnitude by using composite bolometers cooled below 300 mK and operating in the pulse mode. The major advantages are: possibility of choosing the absorber (from diamond to BGO: Al2O3, Ge, Si, BaF2, LiF, etc), the absence of statistical noise (phonon quanta are in the 10−4 eV range) and ultimate resolution in the 10−1 eV range for small bolometers. The theoretical and experimental limits are analyzed and solutions for extraneous noise limitations in a complete system are described. The most recent results in alpha-particle and gamma-ray spectrometry using a new design, the composite-composite bolometer, are given. In these, a resolution of 10.5 keV was obtained for 6 MeV alpha particles and < 2.5 keV for241Am 18 keV X rays using a 24 sapphire bolometer. The ideal bolometer concept is also discussed. In this, the simultaneous detection of quanta (due to ionization, fluorescence, ballistic phonons) and thermal energies will make it possible to have lower thresholds by amplification in the target. It should thus be possible to determine the characteristics of the particles (their precise arrival time and the positions of the impacts) and provide improvements in the field of integral counting for the absolute measurement of radioactivity.

Highly sensitive large-area bolometers for scintillation studies below 100 mK

We present the design, performance, and tests of a new generation of cooled large-area (5 cm 2 ) optical composite bolometers with a pure germanium absorber, permitting a good efficiency from near-IR to x-rays. With a sensitivity often better than photomultipliers or semiconductor diodes, they allow fluorescence measurements of cold targets with no window, no infrared background, good optical couplings, and a flat response on a large spectral band. Performance obtained at 25 mK is very promising: noise equivalent power as low as 4×10 –17 W/Hz ½ in the continuous mode, energy threshold about 50 eV in the pulse detection mode, and time constant τ~3 ms. These detectors of low mass (0.25 g) have been successfully used for detecting the fluorescence emitted by much more massive bolometers, having, for example, a BGO (92 g) or a CaWO 4 (54 g) target. The simultaneous detection of heat and light in these double bolometers permits the identification of each event in the massive target (α decay, γ, or cosmic ray interaction, neutron recoil...). Thanks to the consecutive excellent subtraction of the radioactive and cosmic ray background, it is a powerful tool developed by several groups for fundamental research: study of very rare decays of atoms, measurement of internal very low radioactivity content in single crystals, direct detection of dark matter recoils in massive fluorescence targets, and detection of solar neutrino fluorescence events in liquid 4 He. Recently obtained results are reviewed: the first detection of the rare alpha decay of 209 Bi, and new scintillation data on Al 2 O 3 (sapphire), LiF, or TeO 2 at 20 mK. By cooling at 10 mK, sensitivity can yet be increased by more than one order of magnitude.

Recent developments on scintillating bolometers for WIMP searches: ROSEBUD status

The particle discrimination capability of various scintillating bolometers has been tested, proving their suitability for dark matter searches. In particular, BGO and undoped sapphire have shown low particle discrimination energy threshold (down to around 20 and 10 keV, respectively). We report on the present status of the ROSEBUD (Rare Objects SEarch with Bolometers UnDerground) Experiment and its prospects.

Towards measurement of recoils below 4 keV with the ⪡ROSEBUD⪢ experiment

Abstract An experiment to look for light WIMPs with sapphire bolometers is to be installed in the Canfranc Underground Laboratory (at 2450 m.w.e) in the Spanish Pyrenees. In the first stage two small 25g bolometers with NTD-Ge sensors, operating at 25mK, with an energy threshold of 300eV and FWHM energy resolution of 120eV at 1.5keV will perform a background test experiment. The features of the bolometers, the radioactivity measurements of the components and the set-up and status of the experiment are reported.

Scintillation of sapphire under particle excitation at low temperature

The scintillation properties of undoped sapphire at very low temperature have been studied in the framework of the ROSEBUD (Rare Objects SEarch with Bolometers UnDerground) Collaboration devoted to dark matter searches [1]. We present an estimation of its light yield under gamma, alpha and neutron excitation.

Status of the ROSEBUD dark matter experiment in 1999

Abstract The ROSEBUD (Rare Objects SEarch with Bolometers UndergrounD) experiment consists of three small sapphire bolometers with NTD-Ge sensors operating at 20 mK in the Canfranc Underground Laboratory in an attempt to directly detect low mass Weakly Interacting Massive Particles (WIMPs) through their scattering off Al and O nuclei. We report in this paper the results of three runs (about 10 days each) performed along 1999 and the progressive background reduction obtained essentially from modifications in the cryostat. A new run is in progress to complete the first phase of the experiment.

THERMAL SPECTROMETRY OF PARTICLES AND γ-RAYS WITH COOLED COMPOSITE BOLOMETERS OF MASS UP TO 25 GRAMS

The composite bolometer with monolithic doped germanium thermistor is a good approach towards the ideal thermal spectrometer. We have measured the thermal parameters of heavily doped germanium that are required to optimize the thermal coupling between substrate and sensor. Sensors in the megohm range are obtained by high precision resistivity tests and pre-selection at 300K. Heat capacity corresponding to 2 eV resolution has been measured at 40 mK. We have tested with success several bolometers with mass from 1 mg to 25 g. Best FWHM resolutions obtained are 29 KeV on αs, 2 KeV on e − s and 300 eV on γs. These results are obtained by filtering extraneous sources of energy like microphonics. We have reached a resolution of 16 KeV on a γ line with a 25 gram sapphire composite bolometer at 0.1K which is the largest and most massive bolometer ever successfully tested; it confirms the promising possibilities of bolometers for dark matter particles detection.