708 research outputs found
Photodiode read-out of the ALICE photon spectrometer crystals
Proposal of abstract for LEB99, Snowmass, Colorado, 20-24 September 1999The PHOton Spectrometer of the ALICE experiment is an electromagnetic calorimeter of high granularity consisting of 17280 lead-tungstate (PWO) crystals of dimensions 22x22x180 mm3, read out by large-area PIN-diodes with very low-noise front-end electronics. The crystal assembly is operated at -25C to increase the PWO light yield. A 16.1x17.1 mm2 photodiode, optimized for the PWO emissio spectrum at 400-500 nm, has been developed. The 20x20 mm2 preamplifier PCB is attached to the back side of the diode ceramic frame. The charge sensitive preamplifier is built in discrete logic with two input JFETs for optimum matching with the ~150pF PIN-diode. A prototype shaper has been designed and built in discrete logic. For a detector matrix of 64 units the measured ENCs are between 450-550e at -25C. Beam tests demonstrate that the required energy resolution is reached.Summary:The PHOton Spectrometer of the ALICE experiment is an electromagnetic calorimeter of high granularity consisting of 17280 lead-tungstate (PWO) crystals of dimensions 22x22x180 mm3, coupled to large-area PIN-diodes with matching low-noise preamplifiers. PHOS is optimized for measuring photons, pi0s and eta mesons in the momentum ranges 0.5-10, 1-10 and 2-10 GeV/c, respectively, and is designed for the expected large number of particles that will be produced in central Pb-Pb collisions. Lead tungstate (PWO) is a fast scintillating crystal with a rather complex emission spectrum, consisting of two components: a blue component peaking at 420 nm and a green component peaking at 480-520 nm. The light yield of PWO at room temperature is low compared with other heavy scintillating crystals, for instance BGO. However, the yield depends strongly on the temperature with a coefficient of ~-2 degree. At the selected operating temperature of -25C the yield is about a factor of 3 higher compared to room temperature. Still, in order to reach the required energy resolution for a PHOS channel, an ENC noise of less than 600e for the PIN-diode-preamplifier-shaper stage is required. This is a very low value taking into account the high capacitance of 150-200 pF of the large area PIN-diodes. In collaboration with the PHOS project, the company AME (Horten, Norway) has designed and produced a PIN-photodiode optimized for the cross-section and spectral responsivity of the PHOS PWO crystal. The photodiode has an active area of 17.1x16.1 mm2 and is fabricated on n-type silicon material of thickness 280 um. The wafer specific resistivity is between 3000 and 6000 ohm-cm, which corresponds to a depletion voltage of 70V. The photodiode response is optimized for the spectral region 400-500 nm in order to match the PWO emission spectrum. The PIN-diode is mounted on a ceramic substrate 0.65 mm thick. On this substrate the diode is surrounded by a ceramic frame. The preamplifier PCB of dimension 20x20 mm2 is attached to the back side of the frame. The PIN-diode and bondings to ground and preamplifier input are protected by an optically transparent epoxy layer. The front side of the PIN-diode is glued onto the endface of the PWO crystal with optically transparent glue (Melt-Mount Quick-Stick, Cargille Laboratories, USA). Each crystal is wrapped in White Tyvek to ensure maximum light collection efficiency and optical insulation between the crystals. The PHOS detector consists of four independent modules, each with 4320 channels. The crystal assembly with the photo detectors are operated at -25 +/- 0.3C. The power dissipation per module is ~1 kW. The charge sensitive preamplifier is an operational amplifier built in discrete logic and with two input JFETs (BF861A). Using two JFETs in parallel gives the lowest noise for detector capacitance >100 pF. A prototype shaper, comprising three amplification stages, has been designed and built in discrete logic. For a PIN-diode with capacitance ~150 pF and a leakage current <1 nA under cooling, calculations give optimum time differentiation and integration constants around 3 microsec. For a detector matrix of 64 units the measured ENCs are between 450-550 e at -25C. Beam tests of this matrix show that the required energy resolution for the PHOS is reached
Two-pion Bose-Einstein correlations in central Pb-Pb collisions at = 2.76 TeV
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb
collisions at TeV at the Large Hadron Collider is
presented. We observe a growing trend with energy now not only for the
longitudinal and the outward but also for the sideward pion source radius. The
pion homogeneity volume and the decoupling time are significantly larger than
those measured at RHIC.Comment: 17 pages, 5 captioned figures, 1 table, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/388
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
We report the first measurement of charged particle elliptic flow in Pb-Pb
collisions at 2.76 TeV with the ALICE detector at the CERN Large Hadron
Collider. The measurement is performed in the central pseudorapidity region
(||<0.8) and transverse momentum range 0.2< < 5.0 GeV/. The
elliptic flow signal v, measured using the 4-particle correlation method,
averaged over transverse momentum and pseudorapidity is 0.087 0.002
(stat) 0.004 (syst) in the 40-50% centrality class. The differential
elliptic flow v reaches a maximum of 0.2 near = 3
GeV/. Compared to RHIC Au-Au collisions at 200 GeV, the elliptic flow
increases by about 30%. Some hydrodynamic model predictions which include
viscous corrections are in agreement with the observed increase.Comment: 10 pages, 4 captioned figures, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/389
The small angle tile calorimeter in the DELPHI experiment
The {\bf S}mall angle {\bf TI}le {\bf C}alorimeter ({\bf STIC}) provides calorimetric coverage in the very forward region of the DELPHI experiment at the CERN LEP collider. The structure of the calorimeters, built with a so-called ``shashlik'' technique, gives a perfectly hermetic calorimeter and still allows for the insertion of tracking detectors within the sampling structure to measure the direction of the showering particle. A charged-particle veto system, composed of two scintillator layers, makes it possible to trigger on single photon events and provides e- separat ion. Results are presented from the extensive studies of these detectors in the CERN testbeams prior to installation and of the detector performance at LEP
Accelerometer-based physical activity in a large observational cohort - study protocol and design of the activity and function of the elderly in Ulm (ActiFE Ulm) study
<p>Abstract</p> <p>Background</p> <p>A large number of studies have demonstrated a positive effect of increased physical activity (PA) on various health outcomes. In all large geriatric studies, however, PA has only been assessed by interview-based instruments which are all subject to substantial bias. This may represent one reason why associations of PA with geriatric syndromes such as falls show controversial results. The general aim of the Active-Ulm study was to determine the association of accelerometer-based physical activity with different health-related parameters, and to study the influence of this standardized objective measure of physical activity on health- and disability-related parameters in a longitudinal setting.</p> <p>Methods</p> <p>We have set up an observational cohort study in 1500 community dwelling older persons (65 to 90 years) stratified by age and sex. Addresses have been obtained from the local residents registration offices. The study is carried out jointly with the IMCA - Respiratory Health Survey in the Elderly implemented in the context of the European project IMCA II. The study has a cross-sectional part (1) which focuses on PA and disability and two longitudinal parts (2) and (3). The primary information for part (2) is a prospective 1 year falls calendar including assessment of medication change. Part (3) will be performed about 36 months following baseline. Primary variables of interest include disability, PA, falls and cognitive function. Baseline recruitment has started in March 2009 and will be finished in April 2010.</p> <p>All participants are visited three times within one week, either at home or in the study center. Assessments included interviews on quality of life, diagnosed diseases, common risk factors as well as novel cognitive tests and established tests of physical functioning. PA is measured using an accelerometer-based sensor device, carried continuously over a one week period and accompanied by a prospective activity diary.</p> <p>Discussion</p> <p>The assessment of PA using a high standard accelerometer-based device is feasible in a large population-based study. The results obtained from cross-sectional and longitudinal analyses will shed light on important associations between PA and various outcomes and may provide information for specific interventions in older people.</p
Alignment of the ALICE Inner Tracking System with cosmic-ray tracks
37 pages, 15 figures, revised version, accepted by JINSTALICE (A Large Ion Collider Experiment) is the LHC (Large Hadron Collider) experiment devoted to investigating the strongly interacting matter created in nucleus-nucleus collisions at the LHC energies. The ALICE ITS, Inner Tracking System, consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two layers of pixel detectors, two layers each of drift, and strip detectors. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 micron in some cases (pixels). The sources of alignment information include survey measurements, and the reconstructed tracks from cosmic rays and from proton-proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10^5 charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnet switched off.Peer reviewe
Search for Neutral Heavy Leptons Produced in Z Decays
Weak isosinglet Neutral Heavy Leptons () have been searched for using data collected by the DELPHI detector corresponding to hadronic~Z decays at LEP1. Four separate searches have been performed, for short-lived production giving monojet or acollinear jet topologies, and for long-lived giving detectable secondary vertices or calorimeter clusters. No indication of the existence of these particles has been found, leading to an upper limit for the branching ratio Z of about at 95\% confidence level for masses between 3.5 and 50 GeV/. Outside this range the limit weakens rapidly with the mass. %Special emphasis has been given to the search for monojet--like topologies. One event %has passed the selection, in agreement with the expectation from the reaction: %. The results are also interpreted in terms of limits for the single production of excited neutrinos
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