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 lightquenching 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 ∼ten 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 M WIMP ∼ 50 GeV.

The luminescence spectra of CaWO4, CaMoO4, and ZnWO4 scintillating crystals were investigated in the temperature range 8–400K. The excitation photon energy was varied from the ultraviolet (4.5eV) to the hard x-ray region (35keV). It is found that as the excitation energy decreases the relative intensity of the low-energy luminescence band, attributed to the extrinsic emission of defect centers in CaWO4 and CaMoO4 crystals, increases. This observation is interpreted in terms of the total absorption of incident radiation, i.e., the variation of the mean penetration depth of the photons with their energy. It indicates that the centers responsible for the extrinsic emission in the crystals with scheelite structure are mainly localized in a thin (∼100nm) surface layer. On the other hand no noticeable changes with the excitation energy were found in the emission spectra of ZnWO4 crystals with wolframite structure. The possible implication of this finding is discussed. The light yield of the crystals is compared at low temperature using monochromatic x-ray excitation and it is shown that ZnWO4 has ∼10% higher light yield than CaWO4, while this parameter has a factor of 4 lower in CaMoO4.

An increasing number of applications of scintillators at low temperatures, particularly in cryogenic experiments searching for rare events, has motivated the investigation of scintillation properties of materials over a wide temperature range. This paper provides an overview of the latest results on the study of luminescence, absorption and scintillation properties of materials selected for rare event searches so far. These include CaWO4, ZnWO4, CdWO4, MgWO4, CaMoO4, CdMoO4, Bi4Ge3O12, CaF2, MgF2, ZnSe and AL2O3-Ti. We discuss the progress achieved in research and development of these scintillators, both in material preparation and in the understanding of scintillation mechanisms, as well as the underlying physics. To understand the origin of the performance limitation of self-activated scintillators we employed a semi-empirical model of conversion of high energy radiation into light and made appropriate provision for effects of temperature and energy transfer. We conclude that the low-temperature value of the light yield of some modern scintillators, namely CaWO4, CdWO4 and Bi4Ge3O12, is close to the theoretical limit. Finally, we discuss the advantages and limitations of different materials with emphasis on their application as cryogenic phonon-scintillation detectors (CPSD) in rare event search experiments

The ribosome, the largest RNA-containing macromolecular machinery in cells, requires metal ions not only to maintain its three-dimensional fold but also to perform protein synthesis. Despite the vast biochemical data regarding the importance of metal ions for efficient protein synthesis and the increasing number of ribosome structures solved by X-ray crystallography or cryo-electron microscopy, the assignment of metal ions within the ribosome remains elusive due to methodological limitations. Here we present extensive experimental data on the potassium composition and environment in two structures of functional ribosome complexes obtained by measurement of the potassium anomalous signal at the K-edge, derived from long-wavelength X-ray diffraction data. We elucidate the role of potassium ions in protein synthesis at the three-dimensional level, most notably, in the environment of the ribosome functional decoding and peptidyl transferase centers. Our data expand the fundamental knowledge of the mechanism of ribosome function and structural integrity.

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The search for rare events is an important topic on the agenda of the astro-particle physics community. Finding neutrinoless double-beta decay and the detection of weakly interacting massive particles (WIMP) possibliy constituting the galactic Dark Matter requires the capability of discriminating between rare interactions with nuclei and radioactive backgrounds with electrons. Research and development are underway for the implementation of new instrumentation in low-background particle physics experiments. Event-by-event discrimination through measuring a combination of phonon and scintillation response from cryogenic phonon-scintillation detectors (CPSD) is considered to be a particularly promising technique for this purpose because of its ability to achieve active background rejection for a range of different materials [1]. These detectors are likely to play an important role in experiments searching for rare events, such as interactions with WIMPs [2], double beta decay [3] and radioactive decay of very long-living isotopes [4].There is strong interest in additional materials capable of meeting the stringent requirements of low-temperature and low-background experiments. A high light yield at low temperatures is a key criterion for CPSDs and tungstates are an obvious scintillator choice [1,2,4]. Interest is now turning to materials in the molybdate family. As for the optical properties, to the best of our knowledge only UVexcited luminescence and reflectivity of CaMoO 4 have been studied [5,6]. Therefore investigation of low-temperature luminescence properties of these materials is an important task.We carried out a series of optical measurements of three crystals, MgMoO 4 CaMoO 4 and CdMoO 4 . In this paper we present the results of measurements on reflection, luminescence and luminescence excitation spectra of the crystals, made over a wide temperature range (8 -295 K), using monochromatic VUV synchrotron radiation in the energy range 4 -25 eV.Single crystals have been grown by the Czochralski technique. The sample of MgMoO 4 was cleaved from a larger ingot, while CaMoO 4 and CdMoO 4 were polished to the optical quality. The absorption edges of the crystals at 300 K are estimated to be 3.8, 3.7 and 3.25 eV respectively.The luminescence characterisation of the crystals was carried out using the spectroscopic instrumentation of the SUPERLUMI station at HASYLAB. The 2-m primary vacuum monochromator equipped with an Al-coated gratReflection, emission and luminescence spectra of Czochralski grown molybdate crystals, i.e. MgMoO 4 , CaMoO 4 and CdMoO 4 have been investigated over a 8-295 K temperature range using VUV synchrotron radiation. Preliminary interpretation of the spectroscopic properties has been carried out on the basis of present knowledge of the electronic structure and emission properties of these materials. The results of this study support the conclusion that molybdate crystals have good prospect for application in the search for rare events as cryogenic phonon-scintillation detectors.

Inorganic scintillators are important elements of a new type of cryogenic phonon scintillation detector (CPSD) being developed for single particle detection. These detectors, exhibiting superior energy resolution and the ability to identify the type of interaction in an event, are considered to be the next generation of instrumentation in the search for extremely rare events. This paper presents the latest results of our research on cryogenic scintillators for CPSD applications in the search for dark matter. The paper gives a description of the concept of direct dark matter detection and the operation principles of CPSD, discusses the major material requirements and summarizes the results of investigations over a wide temperature range of the luminescence and scintillation properties of tungstates (CaWO 4 and ZnWO 4 ), molybdates (CaMoO 4 , MgMoO 4 and CdMoO 4 ) and Ti-doped Al 2 O 3 .