Welcome

• Philippe Di Stefano (Institut de Physique Nucléaire de Lyon)

Room: Amphitéatre

Scintillators for cryogenic application: status of research and future directions

• Vitalii Mikhailik (Univ of Oxford)

Room: Amphitéatre The simultaneous detection of a phonon and a scintillation signal, as a result of energy deposition in the target material, by particles or high-energy quanta, is a promising technique. This detection method allows efficient discrimination against electron recoils caused by radioactive background. It is believed that phonon-scintillation detectors can achieve the sensitivity levels required by future experiments searching for rare events, such as interactions of Weakly Interactive Massive Particles (WIMP) and double beta decay. Therefore, identification, characterisation and optimisation of potential scintillation materials for low-temperature application is an important task driven by the demands of cryogenic experiments searching for such rare events. In this talk we will discuss recent progress achieved in the research and development of scintillation materials for these applications, both in the material development and the understanding of scintillation mechanisms, as well as the underlying physics. The results of the studies of optical (transmission, luminescence) and scintillation characteristics (light yield and decay time) of the materials so far selected for rare event searches will be presented. Given that the ultimate sensitivity of the detector is defined by the intrinsic radioactivity of the material, this issue will also be addressed through research effort aimed at scintillator development. Finally, the performance characteristics and the potential of different scintillators will be discussed from the standpoint of the requirements imposed on the materials.

Scintillating bolometers for Double Beta Decay search

• Luca Gironi (Università di Milano-Bicocca)

Room: Amphitéatre In the field of Double Beta Decay searches the possibility to have high resolution detectors in which background can be discriminated results very appealing. This very interesting possibility can be fulfilled in the case of a scintillating bolometers containing a Double Beta Decay emitter whose transition energy exceeds the one of the natural gamma line of 208Tl. We present the latest results obtained in the development of such kind of scintillating bolometers. For the first time an array of five CdWO4 (116Cd has a Double Beta Decay transition energy of 2805 keV) crystals is tested. The array consists of a plane of four 3x3x3 cm^3 crystals and a second plane consisting of a single 3x3x6 cm^3 crystal. This setup is mounted in hall C of National Laboratory of Gran Sasso inside a lead shielding in order to further decrease the environmental background. The aim of this test are finally demonstrate the technical feasibility of this technique through an array of detectors and perform a long background measurement in the best conditions in order to prove the achievable background in the 0nDBD region. In the same run a new, large, ZnSe crystal is tested. ZnSe crystal works very well as thermal bolometer and further study are on going about scintillation light.

Improvement of the CRESST Phonon/Light Detectors

• Michael Kiefer (Max-Planck-Institut für Physik München)

Room: Amphitéatre The goal of the CRESST-Collaboration is to directly detect WIMPs by nuclear recoils in a low-temperature-calorimeter. The origin of detector events can be discriminated by comparing the phonon- and light signals generated in a scintillating CaWO4 crystal. During production of the phonon sensor, the scintillation properties of the crystal are being degraded. In order to circumvent this, there are investigations wheter it is feasible to produce the phonon sensor separate from the scintillator crystal and then glue the two parts together. A proof-of-principle experiment with a promising outcome performed in 2007 has lead to a set of prototype detectors which are to be included in the 2008 run.

Development of superconducting absorbers for CRESST light detectors

• Antonio Bento (Max-Planck-Institut Fuer Physik Muenchen)

Room: Amphitéatre An important aspect of Dark Matter search experiments is the active background reduction by identification of the type particle interacting in the detector. In CRESST this is achieved by a simultaneous detection of heat and light produced by an interaction in a scintillating absorber. The overall light collection efficiency is a crucial parameter in order to achieve enough sensitivity to measure the small fraction of the deposited energy that is emitted as scintillation light. With this purpose a thin superconducting lead film deposited on sapphire substrates has been tested as an alternative light absorber to a standard silicon absorber. The first results already show a better light absorption of the lead film. Other superconducting films are also being analyzed.

Neutron Scattering Facility for Characterization of CRESST and EURECA Detectors at mK Temperatures

• Jean-Côme Lanfranchi Lanfranchi (TU München)

Room: Amphitéatre CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) is an experiment aimed at the direct detection of dark matter particles (WIMPs = Weakly Interacting Massive Particles). The CRESST detectors, based on CaWO4 scintillating crystals, are able to discriminate gamma and beta background by simultaneously measuring the light and phonon signal produced by particle interactions. The discrimination of the background is possible because of the different light output (quenching factor) for nuclear and electron recoils. At the Technische Universitaet Muenchen in Garching an experimental set-up as well as a dedicated detector for high count rates have been installed in Hall 2 of the Maier-Leibnitz-Accelerator-Laboratory. This neutron scattering facility (NSF) will allow to measure the quenching factors of the different nuclei (O, Ca, W) of the detector crystal at mK temperatures. With a view to next generation experiments like EURECA (European Underground Rare Event Calorimeter Array) the NSF can then also be used for the characterization and calibaration of other target materials like ZnWO4, PbWO4 etc.

Photoluminescence characterization of GaN scintillator for radioluminescence-based dosimetry

• Patrick Pittet (Institut des Nanotechnologies de Lyon)

Room: Amphitéatre Via photoluminescence (PL) measurements, we have investigated GaN scintillator for developing dosimetric systems for external beam radiotherapy. GaN is a wide direct-gap semiconductor with interesting features of near-band-edge emission: high intensity, narrow-band, not being directly related to defects, fast and linear response. We have used Si-doped n-type GaN substrates in this work. The samples have common dominant band-edge emission at room temperature (RT), with a strongly doping-dependence on their PL spectra. GaN with concentrations up to 1018cm-3 exhibits a non-negligible yellow luminescence (YL) or red luminescence (RL) broad band contribution, which is probably due to Ga-vacancy native defect. In contrast, heavily Si-doped n-type GaN ( 1.5x1019cm-3) has much more intense band-edge emission, with no significant contributions of lower-energy bands. Evidently, heavily-doped GaN provides much more free majority carriers to increase the band-to-band recombination rate, which consequently reduces other competing indirect recombination rates, including nonradiative ones. Therefore heavily-doped GaN is preferred to be used as scintillator for radioluminescence (RL)-based dosimetry. It gives enhanced narrow-band emission intensity allowing improvements on sensitivity of RL conversion and signal-to-noise ratio by spectral background rejection. Moreover, heavily doped GaN has reduced self-absorption effects on bulk RL emission. At low temperature (up to 200K), the PL intensity of the used heavily doped GaN is dominated by a DBE (donor-bound exciton) line (10K D0XA at 3.467eV). When further increasing the temperature, band-to-band recombination becomes dominant with red-shift in connection with gap narrowing. The shift in peak wavelength from 10 to 300K is about 6nm, while the corresponding PL integrated intensity is decreased only by 30%. Two samples of heavily-doped GaN are irradiated to receive a 200Gy dose with 6MV photon and 6MeV electron beams respectively. RT PL measurements on irradiated and non-irradiated samples show no noticeable spectral variations. At 10k, slight increases of dominant PL peak on the irradiated samples are observed. An in vivo dosimetric probe employing heavily doped GaN has been designed and tested in radio-therapy conditions. The obtained results confirm that GaN scintillator is suitable for such applications.

Thermal Detector Model for Cryogenic Composite Detectors for the Dark Matter Experiments CRESST and EURECA

• Sabine Roth (TU München, Physik Department E15)

Room: Amphitéatre Weakly Interacting Massive Particles (WIMPs) are candidates for non-baryonic Dark Matter. WIMPs are supposed to interact with baryonic matter via scattering off nuclei producing a nuclear recoil with energies of a few 10keV with a very low interaction rate of ~10^(-6) events per kg of target material and day in the energy region of interest. The Dark Matter experiments CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) and the EURECA (European Underground Rare Event Calorimeter Array) project are aimed at the direct detection of WIMPs with the help of very sensitive modularised cryogenic detectors that basically consist of a transition edge sensor (TES) in combination with a massive absorber crystal. The development of these cryogenic detectors and the potential ton scale production are investigated in the Garching group of the CRESST collaboration. To decouple the TES production from the choice of the target material in order to avoid heating cycles of the absorber crystal and to allow pretesting of the TESs, a composite detector design (CDD) for the detector production is developed and studied. On the basis of an existing thermal detector model for cryogenic detectors, an extension of this model, including the CDD, has been developed to further investigate, understand and optimize the performance of composite detectors. This extended model can be expected to provide an enormous help when tailoring composite detectors to the requirements of various experiments.

Development of phonon detectors based on ZnWO4 and CaMoO4 crystals for the CRESST Dark Matter Search

• Irina Bavykina (Max Planck Institute for Physics (Munich))

Room: Amphitéatre The goal of the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) Dark Matter search experiment is the direct detection of Weakly Interacting Massive Particles (WIMP) via their elastic scattering off the nuclei in the target absorber. The low temperature detectors employed by CRESST can discriminate between electron recoil events (caused by beta- and gamma-particles) and nuclear recoil events (caused by WIMPs and neutrons) by simultaneous measurement of phonon and scintillation light signals. A high scintillation yield in the mK temperature range is a key criterion in the choice of a scintillating absorber. This work reports on the development of the first prototype phonon detectors based on calcium molybdate and zinc tungstate crystals. The characterization tests of the scintillation light yields at room temperature are presented. The influences of annealing and W-film deposition are discussed.

The SciCryo optical cryostat and BGO scintillation temperature dependance

• Marc-Antoine Verdier (IPNL)

Room: Amphitéatre The optical cryostat "SciCryo" has been built in aim to study scintillation of cristals at low temperature down to 4K. Its geometry is specially optimized to get a maximal light collection. Our firsts experiments were to sudy the increase of the decay time and the light yield of a BGO cristal with the temperature.

Scintillation properties of Bi4Ge3O12 (BGO) down to a temperature of 20mK

• Johann GIRONNET (Institut d'Astrophysique Spatiale / Paul Scherrer Institut)

Room: Amphitéatre In the framework of rare events detection, especially for the Eureca collaboration, investigations are made to improve the sensibility of the dedicated detectors. In this way, the scintillation properties of the BGO crystal were studied at Oxford (Department of Physics) down to 6K. The light response and the decay time of the BGO crystal are measured as a function of the temperature using a multi-photon counting technique. To complete this work, the time profile of the scintillation pulse obtained at 6K is compared with the one deduced from BGO data obtained previously at 20mK at Orsay (Institut d'Astrophysique Spatiale).

BGO scintillating bolometer as dark matter detector prototype

• Ysrael Richard Ortigoza Paredes (Universidad de Zaragoza)

Room: Amphitéatre Properties of a 46 g BGO scintillating bolometer has been tested at 20 mK as a prototype for dark matter direct detection in the frame of the ROSEBUD (Rare Objects Search with Bolometers UndergrounD) collaboration. The bolometer has been operated in an ultralow background environment at the Canfranc Underground Laboratory (2450 m.w.e.). We analyze its response both in heat and light and its particle discrimination capability focusing on the discrimination of nuclear recoils (like those produced by hypothetical WIMPs) against electron recoils (produced by beta/gamma radioactive background).

CHARACTERISATION OF SCINTILLATION CRYSTALS FOR CRYOGENIC EXPERIMENTAL SEARCH FOR RARE EVENTS

• Valentyna Mokina (Institute for Nuclear Research, MSP 03680 Kyiv, Ukraine)

Room: Amphitéatre During the last decade there has been a steady increase in the research activity towards the developments of new scintillation materials for their use in the cryogenic detectors. These detectors play important role in the experiments searching for rare events, such as Dark Matter, and neutrinoless double beta decay. There is need in variety of scintillation targets and therefore the measurement of the scintillation characteristics at low temperatures is an important objective allowing to assess the suitability of the material for cryogenic applications. Relative intensity and scintillation decay kinetics were studied in CaWO4, CaMoO4, ZnWO4, ZnMoO4, PbWO4, PbMoO4, MgWO4, ZnSe, and LiF(W) crystal scintillators over the temperature range 7 – 310 K. Samples of the crystals 5*5*1 mm of size were placed into an optical cryostat and excited with an 241Am alpha-source. The measurements where carried out using the multiple photon counting technique and a green sensitive photomultiplier as a detector. The following values of relative intensity were obtained at the temperature T=7 K (prior spectral correction): CaWO4 (100%), ZnWO4 (77%), ZnSe (61%), CaMoO4 (46%), PbWO4 (24%), PbMoO4 (21%), MgWO4 (15%), LiF(W) (<5%), and ZnMoO4 (<5%).

Workshop summary

• Maurice CHAPELLIER (CSNSM/CNRS)

Room: Amphitéatre

Discussion: future of CryoScint Room: Amphitéatre