387 research outputs found
Study of solid state photomultiplier
Available solid state photomultiplier (SSPM) detectors were tested under low-background, low temperature conditions to determine the conditions producing optimal sensitivity in a space-based astronomy system such as a liquid cooled helium telescope in orbit. Detector temperatures varied between 6 and 9 K, with background flux ranging from 10 to the 13th power to less than 10 to the 6th power photons/square cm-s. Measured parameters included quantum efficiency, noise, dark current, and spectral response. Experimental data were reduced, analyzed, and combined with existing data to build the SSPM data base included herein. The results were compared to analytical models of SSPM performance where appropriate models existed. Analytical models presented here were developed to be as consistent with the data base as practicable. Significant differences between the theory and data are described. Some models were developed or updated as a result of this study
Solid state photomultiplier for astronomy, phase 2
Epitaxial layers with varying donor concentration profiles were grown on silicon substrate wafers using chemical vapor deposition (CVD) techniques, and solid state photomultiplier (SSPM) devices were fabricated from the wafers. Representative detectors were tested in a low background photon flux, low temperature environment to determine the device characteristics for comparison to NASA goals for astronomical applications. The SSPM temperatures varied between 6 and 11 K with background fluxes in the range from less than 5 x 10 to the 6th power to 10 to the 13th power photons/square cm per second at wavelengths of 3.2 and 20 cm. Measured parameters included quantum efficiency, dark count rate and bias current. Temperature for optimal performance is 10 K, the highest ever obtained for SSPMs. The devices exhibit a combination of the lowest dark current and highest quantum efficiency yet achieved. Experimental data were reduced, analyzed and used to generate recommendations for future studies. The background and present status of the microscopic theory of SSPM operation were reviewed and summarized. Present emphasis is on modeling of the avalanche process which is the basis for SSPM operation. Approaches to the solution of the Boltzmann transport equation are described and the treatment of electron scattering mechanisms is presented. The microscopic single-electron transport theory is ready to be implemented for large-scale computations
The solid state photomultiplier: Status of photon counting beyond the near-infrared
Rockwell International's Solid State Photomultiplier (SSPM) is an impurity-band avalanche device which can count individual photons with wavelengths between 0.4 and 28 micrometers. Its response to a photon is a pulse of between 10(exp 4) and 10(exp 5) conduction electrons, making it an important device for use in phenomenology. The characteristics of the SSPM make it a potentially important device for use in astronomical applications. Contract NAS2-12400 was initiated in June 1986 to conduct modeling and characterization studies of the SSPM to provide a basis for assessing its use in astronomical systems. Some SSPM models and results of measurements which characterize the group of SSPMs recently fabricated on this contract are discussed
Analytic computation of the Instantaneous Normal Modes spectrum in low density liquids
We analytically compute the spectrum of the Hessian of the Hamiltonian for a
system of N particles interacting via a purely repulsive potential in one
dimension. Our approach is valid in the low density regime, where we compute
the exact spectrum also in the localized sector. We finally perform a numerical
analysis of the localization properties of the eigenfunctions.Comment: 4 RevTeX pages, 4 EPS figures. Revised version to appear on Phys.
Rev. Let
Heterogeneities in Supercooled liquids: A Density Functional Study
A metastable state, characterized by a low degree of mass localization is
identified using Density Functional Theory. This free energy minimum, located
through the proper evaluation of the competing terms in the free energy
functional, is independent of the specific form of the DFT used. Computer
simulation results on particle motion indicate that this heterogeneous state
corresponds to the supercooled state.Comment: 10 pages, 6 figure
Potential Energy Landscape Equation of State
Depth, number, and shape of the basins of the potential energy landscape are
the key ingredients of the inherent structure thermodynamic formalism
introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys.
Rev. A 25, 978 (1982)]. Within this formalism, an equation of state based only
on the volume dependence of these landscape properties is derived. Vibrational
and configurational contributions to pressure are sorted out in a transparent
way. Predictions are successfully compared with data from extensive molecular
dynamics simulations of a simple model for the fragile liquid orthoterphenyl.Comment: RevTeX4, 4 pages, 5 figure
Windsurfing : an extreme form of material and embodied interaction?
This paper makes reference to the development of water based board sports in the world of adventure or action games. With a specific focus on windsurfing, we use Parlebas (1999) and Warnier's (2001) theoretical interests in the praxaeology of physical learning as well as Mauss' (1935) work on techniques of the body. We also consider the implications of Csikzentimihalyi's notion of flow (1975). We argue that windsurfing equipment should not merely be seen as protection but rather as status objects through which extreme lifestyles are embodied and embodying
Tolerating the Community Detection Resolution Limit with Edge Weighting
Communities of vertices within a giant network such as the World-Wide Web are
likely to be vastly smaller than the network itself. However, Fortunato and
Barth\'{e}lemy have proved that modularity maximization algorithms for
community detection may fail to resolve communities with fewer than
edges, where is the number of edges in the entire network.
This resolution limit leads modularity maximization algorithms to have
notoriously poor accuracy on many real networks. Fortunato and Barth\'{e}lemy's
argument can be extended to networks with weighted edges as well, and we derive
this corollary argument. We conclude that weighted modularity algorithms may
fail to resolve communities with fewer than total edge
weight, where is the total edge weight in the network and is the
maximum weight of an inter-community edge. If is small, then small
communities can be resolved.
Given a weighted or unweighted network, we describe how to derive new edge
weights in order to achieve a low , we modify the ``CNM'' community
detection algorithm to maximize weighted modularity, and show that the
resulting algorithm has greatly improved accuracy. In experiments with an
emerging community standard benchmark, we find that our simple CNM variant is
competitive with the most accurate community detection methods yet proposed.Comment: revision with 8 pages 3 figures 2 table
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