A. Ulrich

Technische Universität München, München, Bavaria, Germany

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Publications (106)177.96 Total impact

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    ABSTRACT: The purpose of this document is to describe the upgrade of the CRESST dark matter search at LNGS. The proposed strategy will allow to explore a region of the parameter space for spin-independent WIMP-nucleon elastic scattering corresponding to WIMP masses below 10GeV/c$^\text{2}$, that has not been covered by other experiments. These results can be achieved only with outstanding detector performances in terms of threshold and background. This proposal shows how CRESST can match these performance requirements, adding a unique piece of information to the dark matter puzzle. The results of this program will fix a new state-of-the-art in the low mass WIMP exploration, opening new perspectives of understanding the dark matter scenario.
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    ABSTRACT: The CRESST experiment (Cryogenic Rare Event Search with Superconducting Thermometers) searches for dark matter via the phonon and light signal of elastic scattering processes in scintillating crystals. The discrimination between a possible dark matter signal and background requires good energy resolution of the light detector, therefore a high light yield is important. In this article, we present a method for understanding the light yield measured with entire detector modules in terms of the efficiencies of light production and detection. Based on data taken during a dark matter search phase, it considers the entire process of conversion of deposited energy into scintillation light as well as transport and collection of the light that occur in a detector module. We can confirm the results by using a cross-check method with different systematic uncertainties. We found that with the detectors operated in CRESST-II phase 1, about 20% of the produced scintillation light is detected. A part of the light loss is likely caused by light absorption creating meta-stable excitations in the scintillating crystals. We also found that, consistent with the relatively low detection efficiency, an additional light detector increases the amount of detected light within an otherwise unmodified detector module.
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    ABSTRACT: The CRESST experiment (Cryogenic Rare Event Search with Superconducting Thermometers) searches for dark matter via the phonon and light signal of elastic scattering processes in scintillating crystals. The discrimination between a possible dark matter signal and background requires good energy resolution of the light detector, therefore a high light yield is important. In this article, we present a method for understanding the light yield measured with entire detector modules in terms of the efficiencies of light production and detection. Based on data taken during a dark matter search phase, it considers the entire process of conversion of deposited energy into scintillation light as well as transport and collection of the light that occur in a detector module. We can confirm the results by using a cross-check method with different systematic uncertainties. We found that with the detectors operated in CRESST-II phase 1, about 20% of the produced scintillation light is detected. A part of the light loss is likely caused by light absorption creating meta-stable excitations in the scintillating crystals. We also found that, consistent with the relatively low detection efficiency, an additional light detector increases the amount of detected light within an otherwise unmodified detector module.
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    ABSTRACT: Scintillators are employed for particle detection and identification using light-pulse shapes and light quenching factors. We developed a comprehensive model describing the light generation and quenching in CaWO$_4$ single crystals used for direct dark matter search. All observed particle-dependent light-emission characteristics can be explained quantitatively, light-quenching factors and light-pulse shapes are calculated on a microscopic basis. This model can be extended to other scintillators such as inorganic crystal scintillators, liquid noble gases or organic liquid scintillators.
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    ABSTRACT: Vacuum ultraviolet light emission from xenon-doped liquid argon is described in the context of liquid noble gas particle detectors. Xenon concentrations in liquid argon from 0.1 ppm to 1000 ppm were studied. The energy transfer from the second excimer continuum of argon (similar to 127 nm) to the second excimer continuum of xenon (similar to 174 nm) is observed by recording optical emission spectra. The transfer almost saturates at a xenon concentration of similar to 10 ppm for which, in addition, an intense emission in the infrared at a peak wavelength of 1.17 mu m with 13000 +/- 4000 photons per MeV deposited by electrons had been found. The corresponding value for the VUV emission at a peak wavelength of 174 nm (second excimer continuum of xenon) is determined to be 20000 +/- 6000 photons per MeV electron energy deposited. Under these excitation conditions pure liquid argon emits 22000 +/- 3000 photons per MeV electron energy deposited at a peak wavelength of 127 nm. An electron-beam-induced emission spectrum for the 10 ppm Ar-Xe liquid mixture ranging from 115 nm to 3.5 mu m is presented. VUV emission spectra from xenon-doped liquid argon with exponentially varied xenon concentrations from 0.1 ppm to 1000 ppm are also shown. Time structure measurements of the light emissions at well-defined wavelength positions in the vacuum ultraviolet as well as in the near-infrared are presented. Copyright (C) EPLA, 2015
    EPL (Europhysics Letters) 01/2015; 109(1):12001. DOI:10.1209/0295-5075/109/12001 · 2.27 Impact Factor
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    ABSTRACT: The experiment CRESST-II aims at the detection of dark matter with scintillating CaWO$_4$ crystals operated as cryogenic detectors. Recent results on spin-independent WIMP-nucleon scattering from the CRESST-II Phase 2 allowed to probe a new region of parameter space for WIMP masses below 3$\,$GeV/c$^2$. This sensitivity was achieved after background levels were reduced significantly. We present extensive background studies of a CaWO$_4$ crystal, called TUM40, grown at the Technische Universit\"at M\"unchen. The average beta/gamma rate of 3.44/[kg$\,$keV$\,$day] (1-40$\,$keV) and the total intrinsic alpha activity from natural decay chains of $3.08\pm0.04\,$mBq/kg are the lowest reported for CaWO$_4$ detectors. Contributions of gamma lines resulting from cosmogenic activation, external X-rays and intrinsic beta emitters are investigated in detail.
    Journal of Cosmology and Astroparticle Physics 10/2014; 2015(06). DOI:10.1088/1475-7516/2015/06/030 · 5.88 Impact Factor
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    ABSTRACT: The cryogenic dark matter experiment CRESST-II aims at the direct detection of WIMPs via elastic scattering off nuclei in scintillating CaWO$_4$ crystals. We present a new, highly improved, detector design installed in the current run of CRESST-II Phase 2 with an efficient active rejection of surface-alpha backgrounds. Using CaWO$_4$ sticks to hold the target crystal a detector housing with fully-scintillating inner surface could be realized. The presented detector (TUM40) provides an excellent threshold of ${\sim}\,0.60\,$keV and a resolution of $\sigma\,{\approx}\,0.090\,$keV (at 2.60$\,$keV). With significantly reduced background levels, TUM40 sets stringent limits on the spin-independent WIMP-nucleon scattering cross section and probes a new region of parameter space for WIMP masses below 3$\,$GeV/c$^2$. In this paper, we discuss the novel detector design and the surface-alpha event rejection in detail.
    European Physical Journal C 10/2014; 75(8). DOI:10.1140/epjc/s10052-015-3572-9 · 5.44 Impact Factor
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    ABSTRACT: A novel kind of electron beam sustained discharge is presented in which a 12 keV electron beam is combined with a 2.45 GHz microwave power to excite argon gas at atmospheric pressure in a continuous mode of operation. Optical emission spectroscopy is performed over a wide wavelength range from the vacuum ultraviolet (VUV) to the near infrared (NIR). Several effects which modify the emission spectra compared to sole electron beam excitation are observed and interpreted by the changing plasma parameters such as electron density, electron temperature and gas temperature.
    The European Physical Journal D 09/2014; 68(9). DOI:10.1140/epjd/e2014-50334-9 · 1.40 Impact Factor
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    ABSTRACT: Atmospheric and solar neutrinos scattering coherently off target nuclei could be a serious background source for the next generation of direct dark matter searches. We present our studies on the maximal sensitivity on the elastic spin-independent WIMP-nucleon cross section which can be achieved by a background-free experiment based on calcium tungstate as target material. An experiment achieves this maximal sensitivity when one neutrino event is expected for the experiment's energy threshold and exposure. Thus, a first detection of coherent neutrino nucleus scattering (CNNS) could also be in reach of such an experiment, if neutron-like backgrounds are small enough ($\lesssim 0.1$ events for the respective exposures). Due to the small energies of solar neutrinos, calcium tungstate with its light nuclei oxygen and calcium seems to be well suited for a detection of CNNS. We show that for a counting experiment using only the integral above an energy threshold as well as a Bayesian analysis taking into account spectral shapes a detection of CNNS on a 3$\sigma$ confidence level is possible for exposures between 50 and 300 kg-years.
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    ABSTRACT: The CRESST-II cryogenic dark matter search aims for the detection of WIMPs via elastic scattering off nuclei in CaWO$_4$ crystals. We present results from a low-threshold analysis of a single upgraded detector module. This module efficiently vetoes low energy backgrounds induced by $\alpha$-decays on inner surfaces of the detector. With an exposure of 29.35 kg live days collected in 2013 we set a limit on spin-independent WIMP-nucleon scattering which probes a new region of parameter space for WIMP masses below 3 GeV/c$^2$, previously not covered in direct detection searches. A possible excess over background discussed for the previous run (from 2009 to 2011) is not confirmed.
    European Physical Journal C 07/2014; DOI:10.1140/epjc/s10052-014-3184-9 · 5.44 Impact Factor
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    ABSTRACT: A table top setup for studying plasma chemical reactions is described. Low energy (15 keV) electrons are used to induce the chemical reactions. The experiments are performed with pure gases and gas mixtures at atmospheric pressure. The radiolysis of CO2 is studied as an example and a G-value of 3 was achieved for a gas flow of 14 ml/min. The experiments were performed with a total beam power of 450 mW sent into a gas cell with a volume of 40 mm3. Mass spectrometry and a spectroscopic study were used for diagnostics. A catalytic effect of xenon for the splitting of CO2 molecules was observed and supported by comparative measurements with Ar-CO2 and Ar-Xe-CO2 mixtures. The power deposition profiles in the target cell are discussed on the basis of a numerical model.
    The European Physical Journal D 06/2014; 68(6). DOI:10.1140/epjd/e2014-50126-3 · 1.40 Impact Factor
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    ABSTRACT: Intense infrared (IR) light emission from liquid Ar-Xe mixtures has been observed using 12 keV electron-beam excitation. The emission peaks at a wavelength of and the half-width of the emission band is . Maximum intensity has been found for a 10 ppm xenon admixture in liquid argon. The conversion efficiency of electron-beam power to IR light is about 1% (10000 photons per MeV electron energy deposited). A possible application of this intense IR emission for a new particle discrimination concept in liquid noble gas detectors is discussed. No light emission was found for perfectly purified liquid argon in the wavelength range from 0.5 to on the current level of sensitivity.
    EPL (Europhysics Letters) 05/2014; 106(3):32001. DOI:10.1209/0295-5075/106/32001 · 2.27 Impact Factor
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    ABSTRACT: The EURECA (European Underground Rare Event Calorimeter Array) project is aimed at searching for dark matter particles using cryogenic bolometers. The proponents of the present project have decided to pool their strengths and expertise to build a facility to house up to 1000 kg of detectors, EURECA, consisting in the first instance of germanium and CaWO4 crystals. The shielding will be provided through a large water tank in which the cryostat with detectors will be immersed. The EURECA infrastructure will be an essential tool for the community interested in using cryogenic detectors for dark matter searches. Beyond European detectors, it will be designed to host other types of similar cryogenic detectors, requiring millikelvin operating temperatures. In particular, this includes the germanium detectors currently in use by the SuperCDMS team, following the current collaborative work performed by the EURECA and SuperCDMS collaborations. EURECA will have two stages. The first phase aims at a sensitivity of 3 . 10(-10) pb and will involve building the infrastructure, cryostat and shielding, and operating 150 kg of detectors. The second phase will be completed with 850 kg of additional detectors, the relative weight between the different detectors being decided by the collaboration according to the physics reach. A sensitivity of 2 . 10(-11) pb is aimed for at the second stage. EURECA will ideally benefit from the planned extension of the deepest underground laboratory in Europe - LSM. With a site-independent design, it can also be hosted in other locations at similar or deeper sites such as SNOLAB. (C) 2014 The Authors. Published by Elsevier B.V.
    Physics of the Dark Universe 04/2014; 3:41. DOI:10.1016/j.dark.2014.03.004
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    ABSTRACT: The direct dark matter search experiment CRESST uses scintillating CaWO$_4$ single crystals as targets for possible WIMP scatterings. An intrinsic radioactive contamination of the crystals as low as possible is crucial for the sensitivity of the detectors. In the past CaWO$_4$ crystals operated in CRESST were produced by institutes in Russia and the Ukraine. Since 2011 CaWO$_4$ crystals have also been grown at the crystal laboratory of the Technische Universit\"at M\"unchen (TUM) to better meet the requirements of CRESST and of the future tonne-scale multi-material experiment EURECA. The radiopurity of the raw materials and of first TUM-grown crystals was measured by ultra-low background $\gamma$-spectrometry. Two TUM-grown crystals were also operated as low-temperature detectors at a test setup in the Gran Sasso underground laboratory. These measurements were used to determine the crystals' intrinsic $\alpha$-activities which were compared to those of crystals produced at other institutes. The total $\alpha$-activities of TUM-grown crystals as low as 1.21 $\pm$ 0.11 mBq/kg were found to be significantly smaller than the activities of crystals grown at other institutes typically ranging between ~15 mBq/kg and ~35 mBq/kg.
    Journal of Cosmology and Astroparticle Physics 03/2014; 2014(05). DOI:10.1088/1475-7516/2014/05/018 · 5.88 Impact Factor
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    ABSTRACT: Scintillating CaWO$_4$ single crystals are a promising multi-element target for rare-event searches and are currently used in the direct Dark Matter experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers). The relative light output of different particle interactions in CaWO$_4$ is quantified by Quenching Factors (QFs). These are essential for an active background discrimination and the identification of a possible signal induced by weakly interacting massive particles (WIMPs). We present the first precise measurements of the QFs of O, Ca and W at mK temperatures by irradiating a cryogenic detector with a fast neutron beam. A clear energy dependence of the QFs and a variation between different CaWO$_4$ single crystals were observed for the first time. For typical CRESST detectors the QFs in the region-of-interest (10-40$\,$keV) are $QF_O^{ROI}=(11.2\pm0.5)\,$%, $QF_{Ca}^{ROI}=(5.94\pm0.49)\,$% and $QF_W^{ROI}=(1.72\pm0.21)\,$%. The latest CRESST data (run32) is reanalyzed using these fundamentally new results on light quenching in CaWO$_4$ having moderate influence on the WIMP analysis. Their relevance for future CRESST runs and for the clarification of previously published results of direct Dark Matter experiments is emphasized.
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    ABSTRACT: Light emission from a ~100 mbar Ar-Kr mixture excited by a pulsed discharge is described. The discharge was arranged to form a homogeneous cathode layer and spatial filtering was used to measure time-dependent spectra emitted from a region where electrons accelerated in the cathode sheath induce the light emission. Novel excimer bands were observed around a wavelength of 315 nm in addition to the better known so called third excimer continua of Ar and Kr. A tentative assignment for these bands to charge-transfer transitions: ArKr2+ → Kr+ + Ar+ + hν is provided and discussed in the context of earlier works on heteronuclear ionic excimer molecules. Predictions for the wavelength positions of similar emission bands are provided for other combinations of noble gases. The rate constants for the formation of heteronuclear ArKr2+ excimers in three-body reactions and two-body collisional quenching of Kr2+∗ ions by Ar atoms have been determined from the time dependence of the ArKr2+ emission.
    The European Physical Journal D 02/2013; 67(2). DOI:10.1140/epjd/e2012-30642-x · 1.40 Impact Factor
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    ABSTRACT: A measurement of the absolute fluorescence yield of the 337 nm nitrogen band, relevant to ultra-high energy cosmic ray (UHECR) detectors, is reported. Two independent calibrations of the fluorescence emission induced by a 120 GeV proton beam were employed: Cherenkov light from the beam particle and calibrated light from a nitrogen laser. The fluorescence yield in air at a pressure of 1013 hPa and temperature of 293 K was found to be Y337 ¼ 5:61 � 0:06stat � 0:22syst photons/MeV. When compared to the fluorescence yield currently used by UHECR experiments, this measurement improves the uncertainty by a factor of three, and has a significant impact on the determination of the energy scale of the cosmic ray spectrum.
    Astroparticle Physics 01/2013; 424:90–102. · 4.45 Impact Factor
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    ABSTRACT: A measurement of the absolute fluorescence yield of the 337 nm nitrogen band, relevant to ultra-high energy cosmic ray (UHECR) detectors, is reported. Two independent calibrations of the fluorescence emission induced by a 120 GeV proton beam were employed: Cherenkov light from the beam particle and calibrated light from a nitrogen laser. The fluorescence yield in air at a pressure of 1013 hPa and temperature of 293 K was found to be $Y_{337} = 5.61\pm 0.06_{stat} \pm 0.21_{syst}$ photons/MeV. When compared to the fluorescence yield currently used by UHECR experiments, this measurement improves the uncertainty by a factor of three, and has a significant impact on the determination of the energy scale of the cosmic ray spectrum.
    Astroparticle Physics 10/2012; 42. DOI:10.1016/j.astropartphys.2012.12.006 · 4.45 Impact Factor
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    ABSTRACT: Extensive air showers initiate the fluorescence emissions from nitrogen molecules in air. The UV-light is emitted isotropically and can be used for observing the longitudinal development of extensive air showers in the atmosphere over tenth of kilometers. This measurement technique is well-established since it is exploited for many decades by several cosmic ray experiments. However, a fundamental aspect of the air shower analyses is the description of the fluorescence emission in dependence on varying atmospheric conditions. Different fluorescence yields affect directly the energy scaling of air shower reconstruction. In order to explore the various details of the nitrogen fluorescence emission in air, a few experimental groups have been performing dedicated measurements over the last decade. Most of the measurements are now finished. These experimental groups have been discussing their techniques and results in a series of Air Fluorescence Workshops commenced in 2002. At the 8$^{\rm{th}}$ Air Fluorescence Workshop 2011, it was suggested to develop a common way of describing the nitrogen fluorescence for application to air shower observations. Here, first analyses for a common treatment of the major dependences of the emission procedure are presented. Aspects like the contributions at different wavelengths, the dependence on pressure as it is decreasing with increasing altitude in the atmosphere, the temperature dependence, in particular that of the collisional cross sections between molecules involved, and the collisional de-excitation by water vapor are discussed.
    The European Physical Journal Conferences 10/2012; DOI:10.1051/epjconf/20135301010