6,216 research outputs found
Status of the Silicon Strip Detector at CMS
The CMS Tracker is the world's largest silicon detector. It has only recently been moved underground and installed in the 4T solenoid. Prior to this there has been an intensive testing on the surface, which confirms that the detector system fully meets the design specifications. Irradiation studies with the sensor material shows that the system will survive for at least 10 years in the harsh radiation environment prevailing within the Tracker volume. The planning phase for SLHC as the successor of LHC, with a ten times higher luminosity at the same energy has already begun. First R\&D studies for more robust detector materials and a new Tracker layout have started
Geant4 Simulation of a filtered X-ray Source for Radiation Damage Studies
Geant4 low energy extensions have been used to simulate the X-ray spectra of
industrial X-ray tubes with filters for removing the uncertain low energy part
of the spectrum in a controlled way. The results are compared with precisely
measured X-ray spectra using a silicon drift detector. Furthermore, this paper
shows how the different dose rates in silicon and silicon dioxide layers of an
electronic device can be deduced from the simulations
Low-energy electric dipole response in 120Sn
The electric dipole strength in 120Sn has been extracted from proton
inelastic scattering experiments at E_p = 295 MeV and at forward angles
including 0 degree. Below neutron threshoild it differs from the results of a
120Sn(gamma,gamma') experiment and peaks at an excitation energy of 8.3 MeV.
The total strength corresponds to 2.3(2)% of the energy-weighted sum rule and
is more than three times larger than what is observed with the (gamma,gamma')
reaction. This implies a strong fragmentation of the E1 strength and/or small
ground state branching ratios of the excited 1- states.Comment: 7 pages, 6 figure
Cones, pringles, and grain boundary landscapes in graphene topology
A polycrystalline graphene consists of perfect domains tilted at angle
{\alpha} to each other and separated by the grain boundaries (GB). These nearly
one-dimensional regions consist in turn of elementary topological defects,
5-pentagons and 7-heptagons, often paired up into 5-7 dislocations. Energy
G({\alpha}) of GB computed for all range 0<={\alpha}<=Pi/3, shows a slightly
asymmetric behavior, reaching ~5 eV/nm in the middle, where the 5's and 7's
qualitatively reorganize in transition from nearly armchair to zigzag
interfaces. Analysis shows that 2-dimensional nature permits the off-plane
relaxation, unavailable in 3-dimensional materials, qualitatively reducing the
energy of defects on one hand while forming stable 3D-landsapes on the other.
Interestingly, while the GB display small off-plane elevation, the random
distributions of 5's and 7's create roughness which scales inversely with
defect concentration, h ~ n^(-1/2)Comment: 9 pages, 4 figure
Optimal General Matchings
Given a graph and for each vertex a subset of the
set , where denotes the degree of vertex
in the graph , a -factor of is any set such that
for each vertex , where denotes the number of
edges of incident to . The general factor problem asks the existence of
a -factor in a given graph. A set is said to have a {\em gap of
length} if there exists a natural number such that and . Without any restrictions the
general factor problem is NP-complete. However, if no set contains a gap
of length greater than , then the problem can be solved in polynomial time
and Cornuejols \cite{Cor} presented an algorithm for finding a -factor, if
it exists. In this paper we consider a weighted version of the general factor
problem, in which each edge has a nonnegative weight and we are interested in
finding a -factor of maximum (or minimum) weight. In particular, this
version comprises the minimum/maximum cardinality variant of the general factor
problem, where we want to find a -factor having a minimum/maximum number of
edges.
We present an algorithm for the maximum/minimum weight -factor for the
case when no set contains a gap of length greater than . This also
yields the first polynomial time algorithm for the maximum/minimum cardinality
-factor for this case
Dipole polarizability of 120Sn and nuclear energy density functionals
The electric dipole strength distribution in 120Sn between 5 and 22 MeV has
been determined at RCNP Osaka from a polarization transfer analysis of proton
inelastic scattering at E_0 = 295 MeV and forward angles including 0{\deg}.
Combined with photoabsorption data an electric dipole polarizability
\alpha_D(120Sn) = 8.93(36) fm^3 is extracted. The dipole polarizability as
isovector observable par excellence carries direct information on the nuclear
symmetry energy and its density dependence. The correlation of the new value
with the well established \alpha_D(208Pb) serves as a test of its prediction by
nuclear energy density functionals (EDFs). Models based on modern Skyrme
interactions describe the data fairly well while most calculations based on
relativistic Hamiltonians cannot.Comment: 6 pages, 4 figure
Breakdown of the Isobaric Multiplet Mass Equation for the A = 20 and 21 Multiplets
Using the Penning trap mass spectrometer TITAN, we performed the first direct
mass measurements of 20,21Mg, isotopes that are the most proton-rich members of
the A = 20 and A = 21 isospin multiplets. These measurements were possible
through the use of a unique ion-guide laser ion source, a development that
suppressed isobaric contamination by six orders of magnitude. Compared to the
latest atomic mass evaluation, we find that the mass of 21Mg is in good
agreement but that the mass of 20Mg deviates by 3{\sigma}. These measurements
reduce the uncertainties in the masses of 20,21Mg by 15 and 22 times,
respectively, resulting in a significant departure from the expected behavior
of the isobaric multiplet mass equation in both the A = 20 and A = 21
multiplets. This presents a challenge to shell model calculations using either
the isospin non-conserving USDA/B Hamiltonians or isospin non-conserving
interactions based on chiral two- and three-nucleon forces.Comment: 5 pages, 2 figure
Two-dimensional readout of GEM detectors
The recently introduced Gas Electron Multiplier (GEM) permits the amplification of electrons released by ionizing radiation in a gas by factors approaching ten thousand, larger gains can be obtained combining two GEMs in cascade. We describe methods for implementing two- and three-dimensional projective localization of radiation, with sub-millimeter accuracy, making use of specially manufactured and patterned pick-up electrodes. Easy to implement and flexible in the choice of the readout geometry, the technology has the distinctive advantage of allowing all pick-up electrodes to be kept at ground potential, thus substantially improving the system simplicity and reliability. Preliminary results demonstrating the two-dimensional imaging capability of the devices are provided and discussed, as well as future perspectives of development
Kapazitive pH-Sensoren auf der Basis von makroporösem Silizium mit Doppelisolatorschicht aus thermisch oxidiertem SiO2 und LPCVD-Si3N4
Halbleitersensoren für den Ionennachweis in wässrigen Lösungen lassen sich einfach und kostengünstig als kapazitive Feldeffektstrukturen in Form von sogenannten EIS- (Elektrolyt-Isolator-Silizium) Sensoren realisieren. Allerdings sind solche Sensoren begrenzt miniaturisierbar, da ihre geometrische Fläche direkt proportional in das Meßsignal, die Kapazitätsänderung, eingeht. Um diesen Nachteil zu umgehen, haben wir auf dem ersten BioSensorSymposium in München (1999) einen neuartigen Lösungsansatz vorgeschlagen, bei dem makroporöses Silizium als Basismaterial für verschiedene sensoraktive Substanzen, wie z.B. pH-sensitive Schichten und Enzyme eingesetzt werden kann. Bei der Verwendung von makroporösem Silizium als Transducermaterial hat die durch den Herstellungsprozeß bedingte Vergrößerung der sensoraktiven Oberfläche nämlich eine Zunahme der Meßkapazität zur Folge. Aufgrund der Ätzanordnung zur Herstellung von porösem Silizium war es bisher allerdings nur möglich, Niedertemperaturprozesse, wie das PECVD (Plasma-Enhanced-Chemical-Vapour-Deposition)-Verfahren, zur Abscheidung von SiO2 als Isolatorschicht und Si3N4 als pH-sensitiver Schicht zu verwenden. Solche Sensoren besitzen allerdings keine hohe Langzeitstabilität im Meßbetrieb (ca. 2 Monate), da die dielektrischen Schichten unzureichende Korrosionseigenschaften aufweisen.
Zur Verbesserung der Langzeitstabilität von Sensoren aus porösem Silizium bietet sich die Verwendung von thermisch oxidiertem Silizium als Isolatorschicht und das Abscheiden von Siliziumnitrid als pH-sensitive Schicht mittels LPCVD (Low-Pressure- Chemical-Vapour-Deposition)-Verfahren an. Vorangegangene Arbeiten aus unserer Arbeitsgruppe hatten gezeigt, daß planare Sensoren mit LPCVD-Nitrid als Transducermaterial über einen Zeitraum von sieben Monaten konstant hohe Sensitivitäten nahe dem Nernst-Idealwert aufweisen
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
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