Samuel Henry

Abstract:

The CRESST experiment aims for a detection of dark matter in the form of WIMPs. These particles are expected to scatter elastically off the nuclei of a target material, thereby depositing energy on the recoiling nucleus. CRESST uses scintillating CaW04 crystals as such a target. The energy deposited by an interacting particle is primarily converted to phonons which are detected by transition edge sensors. In addition, a small fraction of the interaction energy is emitted from the crystals in the form of scintillation light which is measured in coincidence with the phonon signal by a separate cryogenic light detector for each target crystal. The ratio of light to phonon energy permits the discrimination between the nuclear recoils expected from WIMPs and events from radioactive backgrounds which primarily lead to electron recoils. CRESST has shown the success of this method in a commissioning run in 2007 and, since then, further investigated possibilities for an even better suppression of backgrounds. Here, we report on a new class of background events observed in the course of this work. The consequences of this observation are discussed and we present the current status of the experiment. © 2009 American Institute of Physics.

Abstract:

Magnesium tungstate (MgWO4) crystals of ~1 cm³ volume were obtained for the first time using a flux growth technique. The crystal was subjected to comprehensive characterisation that included room-temperature measurements of the transmittance, X-ray luminescence spectra, afterglow under X-ray excitation, relative photoelectron output, energy resolution, non-proportionality of scintillation response to γ-quanta, response to α-particles, and pulse shape for γ-quanta and α-particles. The light output and decay kinetics of MgWO4 were studied over the temperature range 7-305 K. Under X-ray excitation the crystal exhibits an intense luminescence band peaking at a wavelength of 470 nm; the intensity of afterglow after 20 ms is 0.035%. An energy resolution of 9.1% for 662 keV γ-quanta of ¹³⁷Cs was measured with a small (≈0.9 g) sample of the MgWO4 crystal. The photoelectron output of the MgWO4 crystal scintillator is 35% that of CdWO4 and 27% that of NaI(Tl). The detector showed pulse-shape discrimination ability in measurements with α-particles and γ-quanta, which enabled us to assess the radioactive contamination of the scintillator. The results of these studies demonstrate the prospect of this material for a variety of scintillation applications, including rare event searches. © 2008 Elsevier B.V.

Abstract:

The CRESST-II direct Dark Matter search is located in the Gran Sasso underground laboratories, Italy. CaWO4 crystals are used as scintillating targets for WIMP (weakly interacting massive particle) interactions. They are operated as cryogenic calorimeters in combination with a second cryogenic detector used to measure the scintillation light produced in the target crystal. For each particle interaction, the combination of phonon and light signals provides an event by event discrimination which allows to distinguish known particles (alphas, betas, gammas, neutrons) from the expected signal of WIMPs. A major upgrade of the setup comprises modifications of the shielding, installation of a muon-veto, and new read out electronics, as well as a new detector-support structure to accommodate up to 33 detector modules, i.e. 10 kg of target mass. The experiment was thereafter successfully commissioned in 2007. Data obtained during this commissioning phase from 2 detector modules are presented here. Combining the data collected with these two detector modules with data from one single module obtained during the CRESST-I phase, the experiment could already place a limit of ∼6 × 10^-7 pb for the spin independent WIMP-nucleon scattering cross section at a WIMP mass of ∼60 GeV/c². © BAS, BDP Sciences 2009.

Abstract:

Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO4, ZnMoO4, PbWO4, PbMoO4, and MgWO4) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter and/or neutrinoless double beta decay. © 2006 IEEE.

Abstract:

The CRESST cryogenic direct dark matter search at Gran Sasso, searching for WIMPs via nuclear recoil, has been upgraded to CRESST-II by several changes and improvements. The upgrade includes a new detector support structure capable of accommodating 33 modules, the associated multichannel readout with 66 SQUID channels, a neutron shield, a calibration source lift, and the installation of a muon veto. We present the results of a commissioning run carried out in 2007. The basic element of CRESST-II is a detector module consisting of a large (∼ 300 g) CaWO4 crystal and a very sensitive smaller (∼ 2 g) light detector to detect the scintillation light from the CaWO4. The large crystal gives an accurate total energy measurement. The light detector permits a determination of the light yield for an event, allowing an effective separation of nuclear recoils from electron-photon backgrounds. Furthermore, information from light-quenching factor studies allows the definition of a region of the energy-light yield plane which corresponds to tungsten recoils. A neutron test is reported which supports the principle of using the light yield to identify the recoiling nucleus. Data obtained with two detector modules for a total exposure of 48 kg-days are presented. Judging by the rate of events in the "all nuclear recoils" acceptance region the apparatus shows a factor ∼10 improvement with respect to previous results, which we attribute principally to the presence of the neutron shield. In the "tungsten recoils" acceptance region three events are found, corresponding to a rate of 0.063 per kg-day. Standard assumptions on the dark matter flux, coherent or spin independent interactions, then yield a limit for WIMP-nucleon scattering of 4.8 × 10^{- 7} pb, at MWIMP ∼ 50 GeV. © 2009 Elsevier B.V. All rights reserved.