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 $G=(V,E)$ and for each vertex $v \in V$ a subset $B(v)$ of the set $\{0,1,\ldots, d_G(v)\}$, where $d_G(v)$ denotes the degree of vertex $v$ in the graph $G$, a $B$-factor of $G$ is any set $F \subseteq E$ such that $d_F(v) \in B(v)$ for each vertex $v$, where $d_F(v)$ denotes the number of edges of $F$ incident to $v$. The general factor problem asks the existence of a $B$-factor in a given graph. A set $B(v)$ is said to have a {\em gap of length} $p$ if there exists a natural number $k \in B(v)$ such that $k+1, \ldots, k+p \notin B(v)$ and $k+p+1 \in B(v)$. Without any restrictions the general factor problem is NP-complete. However, if no set $B(v)$ contains a gap of length greater than $1$, then the problem can be solved in polynomial time and Cornuejols \cite{Cor} presented an algorithm for finding a $B$-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 $B$-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 $B$-factor having a minimum/maximum number of edges. We present an algorithm for the maximum/minimum weight $B$-factor for the case when no set $B(v)$ contains a gap of length greater than $1$. This also yields the first polynomial time algorithm for the maximum/minimum cardinality $B$-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(0001ˉ\bar{1}) Substrate

Grain boundaries in epitaxial graphene on the SiC(000$\bar{1}$) 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 $\theta_c = 19 \pm~2^\circ$. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed $\theta = 33\pm2^\circ$ 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