The top 500 results for ""R Weiss" " are:

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We report on the detection of a new megamaser, the 3_{1, 3}-2_{2, 0} H₂O line (ν₀ = 183.310 GHz) in Arp 220, using the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. The line is about 350 km s^-1 wide with a total luminosity of ~2.5 × 10⁸ K km s^-1 pc². Although OH megamasers were first discovered in this source, no emission is seen in the 6_{1, 6}-5_{2, 3} H₂O transition (ν₀ = 22.235 GHz), a line otherwise detected as a megamaser in about 50 sources to date. This fact puts interesting constraints on the physical conditions of the central region of Arp 220 that are further strengthened by the HCN and HNC J = 3-2 and J = 1-0 luminosities [in the range (1.5-10) × 10⁸ K km s^-1 pc²]. A scenario with ~10⁶ star-forming cores similar to those found in Sgr B2 in the central kiloparsec of Arp 220 would be compatible with these data and would explain the lack of 22 GHz H₂O emission. This result opens up the possibility of using the 183 GHz H₂O line as an additional tool to explore the physical conditions in luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs, respectively) and their starburst or active galactic nucleus (AGN) nature, with a potential interest for high angular resolution observations with the Atacama Large Millimeter Array (ALMA).

As described in the paper by John Mather and Tom Kelsall, the COBE (Cosmic Background Explorer) project is designed to make a substantial improvement in our knowledge of the condition in the universe at large red shifts. The focus of the mission is to study attributes of the primeval cosmic background radiation and to set limits on the universal radiation energy density at shorter wavelengths - radiation emanating from distant sources but at later times than the primeval fireball. In introducing the accompanying paper, I thought it might be worthwhile to describe the scientific strategy driving the COBE mission.

The COBE mission has been under study since 1974 by a team of scientists consisting of S. Gulkis (Jet Propulsion Laboratory), M. Hauser (NASA Goddard Space Flight Center), J. Mather (NASA), G. Smoot (University of California, Berkeley), R. Weiss (MIT), and D. Wilkinson (Princeton). The team members have been involved in ground based, balloon borne and airborne experiments to measure the background radiation and are keenly aware of limitations of these platforms and the arguments in favor of a space borne experiment. It is worth repeating these arguments:

(1) Freedom from atmospheric emission and fluctuations in the emission.

(2) Full sky coverage with a single instrument.

(3) A benign and controlled thermal environment to reduce systematic errors.

(4) The ability to perform absolute primary calibration in flight without the necessity of windows to avoid condensation of the atmosphere on calibrators and instruments.

(5) Sufficient time both to perform tests for systematic errors and to gain the increase in sensitivity permitted by extended observation time.

In planning the COBE mission, the team came to know the peculiar difficulties of carrying out a space mission. Aside from the ever present problem of maintaining the project within an assigned budget, the mission had to be designed so that it would still be the "right thing to do" in the field after the 5 to 10 years it takes between initial planning and execution. One had to anticipate the progress that could still be made by using other platforms and blend this with the fact that in the space mission one would be dealing with technology that was 3 to 5 years behind the state of the art (to allow time for space quahfication).

In an interim report to NASA in 1977, the conclusion of the team was that the technology was sufficiently advanced and the instrument systematic noise sources were well enough understood or controllable so that the major limitation in a space mission to perform precision measurements of the background would be the "noise" produced by the local astrophysical environment. The complement of instruments chosen for the mission, as well as the need for full sky coverage and extended observation time, are based primarily on the hope that the local astrophysical "noise" can be discriminated from the cosmic background by its peculiar set of spectra and anisotropic angular distribution. In the COBE mission, the data from one instrument truly serves to enhance the value of that from another. Examples of this are discussed in the accompanying paper.

We report the experimental realization of double quantum dots in single-walled carbon nanotubes. The device consists of a nanotube with source and drain contact, and three additional top-gate electrodes in between. We show that, by energizing these top gates, it is possible to locally gate a nanotube, to create a barrier, or to tune the chemical potential of a part of the nanotube. At low temperatures, we find (for three different devices) that in certain ranges of top-gate voltages our device acts as a double quantum dot, evidenced by the typical honeycomb charge stability pattern.

A correction was made to the author list on 19 October 2006. The corrected electronic version is identical to the print version.

An advanced variable energy positron beam (~2 eV to 20 keV) has been designed, tested and utilized for coincidence Doppler broadening (CDB) measurements at the University of Texas at Arlington (UTA). A high efficiency solidified rare gas (Neon) moderator was used for the generation of a slow positron beam. The gamma rays produced as a result of the annihilation of positrons with the sample electrons are measured using a high purity Germanium (HPGe) detector in coincidence with a NaI(Tl) detector. Modifications to the system, currently underway, permits simultaneous measurements utilizing Positron annihilation induced Auger Electron Spectroscopy (PAES) and CDB. The tendency of positrons to become trapped in an image potential well at the surface will allow the new system to be used in measurements of the chemical structure of surfaces, internal or external and interfaces. The system will utilize a time of flight (TOF) technique for electron energy measurements. A 3m flight path from the sample to a micro-channel plate (MCP) in the new system will give it superior energy resolution at higher electron energies as compared to previous TOF systems utilizing shorter flight paths.

In this paper, we present results of numerical modelling of the University of Texas at Arlington's time of flight positron annihilation induced Auger electron spectrometer (UTA TOF-PAES) using SIMION® 8.1 Ion and Electron Optics Simulator. The time of flight (TOF) spectrometer measures the energy of electrons emitted from the surface of a sample as a result of the interaction of low energy positrons with the sample surface. We have used SIMION® 8.1 to calculate the times of flight spectra of electrons leaving the sample surface with energies and angles dispersed according to distribution functions chosen to model the positron induced electron emission process and have thus obtained an estimate of the true electron energy distribution. The simulated TOF distribution was convolved with a Gaussian timing resolution function and compared to the experimental distribution. The broadening observed in the simulated TOF spectra was found to be consistent with that observed in the experimental secondary electron spectra of Cu generated as a result of positrons incident with energy 1.5 eV to 901 eV, when a timing resolution of 2.3 ns was assumed.

Low energy (~2 eV) positrons were deposited onto the surface of highly oriented pyrolytic graphite (HOPG) using a positron beam equipped with a time of flight (TOF) spectrometer. The energy of the electrons emitted as a result of various secondary processes due to positron annihilation was measured using the University of Texas at Arlington's (UTA) TOF spectrometer. The positron annihilation-induced electron spectra show the presence of a carbon KLL Auger peak at ~263 eV. The use of a very low energy beam allowed us to observe a new feature not previously seen: a broad peak which reached to a maximum intensity at ~4 eV and extended up to a maximum energy of ~15 eV. The low energy nature of the peak was confirmed by the finding that the peak was eliminated when a tube in front of the sample was biased at -15 V. The determination that the electrons in the peak are leaving the surface with energies up to 7 times the incoming positron energy indicates that the electrons under the broad peak were emitted as a result of a positron annihilation related process.

One of the main problems in neutron imaging is the scattered radiation that accompanies the direct neutrons that reach the imaging detectors and affect the image quality. We have developed a dedicated collimator for 14.2 MeV fast neutrons. The collimator optimizes the amount of scattered radiation to primary neutrons that arrive at the imaging plane. We have used different materials within the collimator in order to lower the scattered radiation that arrives at the scanned object. The image quality and the signal to noise ratios that are measured show that a mixture of BORAX (Na₂B₄O₇⋅10H₂O) and water in the experimental beam collimator give the best results. We have used GEANT4 to simulate the collimator performance, the simulations predict the optimized material looking on the ratios of the scattered to primary neutrons that contribute in the detector. We present our experimental setup, report the results of the experimental and related simulation studies with neutrons beam generated by a 14.2 MeV D-T neutron generator.

Carbon Fibre Reinforced Metal Hybrid (CFRMH) materials that combine a sheet metal substrate with a reinforcing carbon fibre patch represent a promising solution to reduce weight while increasing the structural and crash performance of future automotive vehicles. CFRMHs cannot be formed with conventional stamping processes and at high volumes. This currently reduces their widespread application. Roll forming is increasingly used in the automobile industry for the forming of lightweight and high-strength metal structural components. The major deformation mode in roll forming is simple bending and this reduces interlaminar shear and compressive stresses that lead to fibre failure and delamination issues when stamping CFRMH sheet materials. This work analyses the potential of applying the conventional roll forming process for the manufacture of a simple top hat section shape from CFRMH sheet. For this, first, the material is produced in a hot press and then formed to shape in a laboratory roll forming facility at room temperature. Different layup sequences are tested, and component quality is analysed after roll forming by visual inspection. The results suggest that roll forming presents a promising manufacturing method for the production of automotive components from CFRMH sheets. The roll formed open shells are joined to produce crash box structure and tested in 3-point bending. The results show that depending on the fibre orientation a significant increase in weight specific strength is achieved.

Conventional roll forming is limited to components with uniform cross-section; the recently developed flexible roll forming (FRF) process can be used to form components which vary in both width and depth. It has been suggested that this process can be used to manufacture automotive components from Ultra High Strength Steel (UHSS) which has limited tensile elongation. In the flexible roll forming process, the pre-cut blank is fed through a set of rolls; some rolls are computer-numerically controlled (CNC) to follow the 3D contours of the part and hence parts with a variable cross-section can be produced. This paper introduces a new flexible roll forming technique which can be used to form a complex shape with the minimum tooling requirements. In this method, the pre-cut blank is held between two dies and the whole system moves back and forth past CNC forming rolls. The forming roll changes its angle and position in each pass to incrementally form the part. In this work, the process is simulated using the commercial software package Copra FEA. The distribution of total strain and final part quality are investigated as well as related shape defects observed in the process. Different tooling concepts are used to improve the strain distribution and hence the part quality.