2,613 research outputs found
Investigation of a direction sensitive sapphire detector stack at the 5 GeV electron beam at DESY-II
Extremely radiation hard sensors are needed in particle physics experiments
to instrument the region near the beam pipe. Examples are beam halo and beam
loss monitors at the Large Hadron Collider, FLASH or XFEL. Currently artificial
diamond sensors are widely used. In this paper single crystal sapphire sensors
are considered as a promising alternative. Industrially grown sapphire wafers
are available in large sizes, are of low cost and, like diamond sensors, can be
operated without cooling. Here we present results of an irradiation study done
with sapphire sensors in a high intensity low energy electron beam. Then, a
multichannel direction-sensitive sapphire detector stack is described. It
comprises 8 sapphire plates of 1 cm^2 size and 525 micro m thickness,
metallized on both sides, and apposed to form a stack. Each second metal layer
is supplied with a bias voltage, and the layers in between are connected to
charge-sensitive preamplifiers. The performance of the detector was studied in
a 5 GeV electron beam. The charge collection efficiency measured as a function
of the bias voltage rises with the voltage, reaching about 10 % at 950 V. The
signal size obtained from electrons crossing the stack at this voltage is about
22000 e, where e is the unit charge.
The signal size is measured as a function of the hit position, showing
variations of up to 20 % in the direction perpendicular to the beam and to the
electric field. The measurement of the signal size as a function of the
coordinate parallel to the electric field confirms the prediction that mainly
electrons contribute to the signal. Also evidence for the presence of a
polarisation field was observed.Comment: 13 pages, 7 figures, 3 table
Solomonoff Induction Violates Nicod's Criterion
Nicod's criterion states that observing a black raven is evidence for the
hypothesis H that all ravens are black. We show that Solomonoff induction does
not satisfy Nicod's criterion: there are time steps in which observing black
ravens decreases the belief in H. Moreover, while observing any computable
infinite string compatible with H, the belief in H decreases infinitely often
when using the unnormalized Solomonoff prior, but only finitely often when
using the normalized Solomonoff prior. We argue that the fault is not with
Solomonoff induction; instead we should reject Nicod's criterion.Comment: ALT 201
Signals of the QCD phase transition in core-collapse supernovae
We explore the implications of the QCD phase transition during the postbounce
evolution of core-collapse supernovae. Using the MIT bag model for the
description of quark matter and assuming small bag constants, we find that the
phase transition occurs during the early postbounce accretion phase. This stage
of the evolution can be simulated with general relativistic three-flavor
Boltzmann neutrino transport. The phase transition produces a second shock wave
that triggers a delayed supernova explosion. If such a phase transition happens
in a future galactic supernova, its existence and properties should become
observable as a second peak in the neutrino signal that is accompanied by
significant changes in the energy of the emitted neutrinos. In contrast to the
first neutronization burst, this second neutrino burst is dominated by the
emission of anti-neutrinos because the electron-degeneracy is lifted when the
second shock passes through the previously neutronized matter.Comment: 5 pages, 3 figures, 1 table, submitted to PR
The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
Photometry with HST's ACS reveals that the subgiant branch (SGB) of the
globular cluster NGC 1851 splits into two well-defined branches. If the split
is due only to an age effect, the two SGBs would imply two star formation
episodes separated by 1 Gyr. We discuss other anomalies in NGC 1851
which could be interpreted in terms of a double stellar population. Finally, we
compare the case of NGC 1851 with the other two globulars known to host
multiple stellar populations, and show that all three clusters differ in
several important respects.Comment: 22 pages, 7 figures, accepted for pubblication on Ap
Evidence of d-wave Superconductivity in K_(1-x)Na_xFe_2As_2 (x = 0, 0.1) Single Crystals from Low-Temperature Specific Heat Measurements
From the measurement and analysis of the specific heat of high-quality
K_(1-x)Na_xFe_2As_2 single crystals we establish the presence of large T^2
contributions with coefficients alpha_sc ~ 30 mJ/mol K^3 at low-T for both x=0
and 0.1. Together with the observed square root field behavior of the specific
heat in the superconducting state both findings evidence d-wave
superconductivity on almost all Fermi surface sheets with an average gap
amplitude of Delta_0 in the range of 0.4 - 0.8 meV. The derived Delta_0 and the
observed T_c agree well with the values calculated within the Eliashberg
theory, adopting a spin-fluctuation mediated pairing in the intermediate
coupling regime.Comment: 8 pages, 5 figures, field dependence of the specific heat added,
slightly changed title, changed sequence of authors, one author added,
accepted by Phys. Rev. B Rapid Communication
A multi-technique study of altered granitic rock from the Krunkelbach Valley uranium deposit, Southern Germany
Herein, a multi-technique study was performed to reveal the elemental speciation and microphase composition in altered granitic rock collected from the Krunkelbach Valley uranium (U) deposit area near an abandoned U mine, Black Forest, Southern Germany. The former Krunkelbach U mine with 1â2 km surrounding area represents a unique natural analogue site with the rich accumulation of secondary U minerals suitable for radionuclide migration studies from a spent nuclear fuel (SNF) repository. Based on a micro-technique analysis using several synchrotron-based techniques such as X-ray fluorescence analysis, X-ray absorption spectroscopy, powder X-ray diffraction and laboratory-based scanning electron microscopy and Raman spectroscopy, the complex mineral assemblage was identified. While on the surface of granite, heavily altered metazeuneriteâmetatorbernite (Cu(UO)(AsO)(PO)·8HO) microcrystals were found together with diluted coatings similar to cuprosklodowskite (Cu(UO)(SiOOH)·6HO), in the cavities of the rock predominantly well-preserved microcrystals close to metatorbernite (Cu(UO)(PO)·8HO) were identified. The Cu(UO)(AsO)(PO)·8HO species exhibit uneven morphology and varies in its elemental composition, depending on the microcrystal part ranging from well-preserved to heavily altered on a scale of âŒ200 ÎŒm. The microcrystal phase alteration could be presumably attributed to the microcrystal morphology, variations in chemical composition, and geochemical conditions at the site. The occurrence of uranyl-arsenate-phosphate and uranyl-silicate mineralisation on the surface of the same rock indicates the signatures of different geochemical conditions that took place after the oxidative weathering of the primary U- and arsenic (As)-bearing ores. The relevance of uranyl minerals to SNF storage and the potential role of uranyl-arsenate mineral species in the mobilization of U and As into the environment is discussed
Massive open star clusters using the VVV survey III: A young massive cluster at the far edge of the Galactic bar
Context: Young massive clusters are key to map the Milky Way's structure, and
near-IR large area sky surveys have contributed strongly to the discovery of
new obscured massive stellar clusters.
Aims: We present the third article in a series of papers focused on young and
massive clusters discovered in the VVV survey. This article is dedicated to the
physical characterization of VVV CL086, using part of its OB-stellar
population.
Methods: We physically characterized the cluster using near-infrared
photometry from ESO public survey VVV images, using the VVV-SkZ pipeline, and
near-infrared -band spectroscopy, following the methodology presented in the
first article of the series.
Results: Individual distances for two observed stars indicate that the
cluster is located at the far edge of the Galactic bar. These stars, which are
probable cluster members from the statistically field-star decontaminated CMD,
have spectral types between O9 and B0V. According to our analysis, this young
cluster ( Myr age Myr) is located at a distance of
kpc, and we estimate a lower limit for the cluster total mass of
. It is likely that the cluster
contains even earlier and more massive stars.Comment: Accepted for publication as a Letter in A&
Facile Bulk Synthesis of pi Cubic SnS
The cubic modification of binary tin sulfide SnS has gained significant interest as an earth abundant, low toxicity solar absorber material with a band gap close to the optimal value for the conversion of sunlight. We herein report a simple synthesis for the metastable material, which will allow more elaborate characterization methods to be used on this material, and present a full powder refinement of the material along with some preliminary results on the optical and thermal stability propertie
Strange quark matter in explosive astrophysical systems
Explosive astrophysical systems, such as supernovae or compact star binary
mergers, provide conditions where strange quark matter can appear. The high
degree of isospin asymmetry and temperatures of several MeV in such systems may
cause a transition to the quark phase already around saturation density.
Observable signals from the appearance of quark matter can be predicted and
studied in astrophysical simulations. As input in such simulations, an equation
of state with an integrated quark matter phase transition for a large
temperature, density and proton fraction range is required. Additionally,
restrictions from heavy ion data and pulsar observation must be considered. In
this work we present such an approach. We implement a quark matter phase
transition in a hadronic equation of state widely used for astrophysical
simulations and discuss its compatibility with heavy ion collisions and pulsar
data. Furthermore, we review the recently studied implications of the QCD phase
transition during the early post-bounce evolution of core-collapse supernovae
and introduce the effects from strong interactions to increase the maximum mass
of hybrid stars. In the MIT bag model, together with the strange quark mass and
the bag constant, the strong coupling constant provides a parameter
to set the beginning and extension of the quark phase and with this the mass
and radius of hybrid stars.Comment: 6 pages, 5 figures, talk given at the International Conference on
Strangeness in Quark Matter (SQM2009), Buzios, Brasil, September 28 - October
2, 2009, to be published in Journal Phys.
The Emergence of the Infrared transient VVV-WIT-06
We report the discovery of an enigmatic large-amplitude (ÎKs> 10.5 mag) transient event in near-IR data obtained by the VISTA Variables in the Via Lactea (VVV) ESO Public Survey. The object (designated VVV-WIT-06) is located at R.A. = 17:07:18.917, decl. = -39:06:26.45 (J2000), corresponding to Galactic coordinates l = 347.14539, b = 0.88522. It exhibits a clear eruption, peaking at Ks = 9 mag during 2013 July and fading to Ks ~ 16.5 in 2017. Our late near-IR spectra show post-outburst emission lines, including some broad emission lines (upward of {FWHM} ~ 3000 k/s). We estimate a total extinction of A_V=10--15 mag in the surrounding field, and no progenitor was observed in ZYJHKs images obtained during 2010-2012 (down to Ks> 18.5 mag). Subsequent deep near-IR imaging and spectroscopy, in concert with the available multiband photometry, indicate that VVV-WIT-06 may be either: (I) the closest Type I SN observed in about 400 years, (II) an exotic high-amplitude nova that would extend the known realm of such objects, or (III) a stellar merger. In all of these cases, VVV-WIT-06 is a fascinating and curious astrophysical target under any of the scenarios considered.Peer reviewe
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