25,321 research outputs found
Scaling of data communications for an advanced supercomputer network
The goal of NASA's Numerical Aerodynamic Simulation (NAS) Program is to provide a powerful computational environment for advanced research and development in aeronautics and related disciplines. The present NAS system consists of a Cray 2 supercomputer connected by a data network to a large mass storage system, to sophisticated local graphics workstations and by remote communication to researchers throughout the United States. The program plan is to continue acquiring the most powerful supercomputers as they become available. The implications of a projected 20-fold increase in processing power on the data communications requirements are described
Interfacial Tension of Electrolyte Solutions
A theory is presented to account for the increase in surface tension of water
in the presence of electrolyte. Unlike the original ``grand-canonical''
calculation of Onsager and Samaras, which relied on the Gibbs adsorption
isotherm and lead to a result which could only be expressed as an infinite
series, our approach is ``canonical'' and produces an analytic formula for the
excess surface tension. For small concentrations of electrolyte, our result
reduces to the Onsager-Samaras limiting law.Comment: contains two figures. Journal of Chemical Physics, in pres
Elastic scattering and the proton form factor
We compute the differential and the total cross sections for scattering
using the QCD pomeron model proposed by Landshoff and Nachtmann. This model is
quite dependent on the experimental electromagnetic form factor, and it is not
totally clear why this form factor gives good results even at moderate
transferred momentum. We exchange the eletromagnetic form factor by the
asymptotic QCD proton form factor determined by Brodsky and Lepage (BL) plus a
prescription for its low energy behavior dictated by the existence of a
dynamically generated gluon mass. We fit the data with this QCD inspired form
factor and a value for the dynamical gluon mass consistent with the ones
determined in the literature. Our results also provide a new determination of
the proton wave function at the origin, which appears in the BL form factor.Comment: 10 pages, 2 figures. Submitted to Physics Letters B. Submitted to
Phys. Lett.
Testing saturation with diffractive jet production in deep inelastic scattering
We analyse the dissociation of a photon in diffractive deep inelastic
scattering in the kinematic regime where the diffractive mass is much bigger
than the photon virtuality. We consider the dominant q\bar{q}g component
keeping track of the transverse momentum of the gluon which can be measured as
a final-state jet. We show that the diffractive gluon-jet production
cross-section is strongly sensitive to unitarity constraints. In particular, in
a model with parton saturation, this cross-section is sensitive to the scale at
which unitarity effects become important, the saturation scale. We argue that
the measurement of diffractive jets at HERA in the limit of high diffractive
mass can provide useful information on the saturation regime of QCD.Comment: 12 pages, 5 figures, misprints corrected, published versio
A single-photon sampling architecture for solid-state imaging
Advances in solid-state technology have enabled the development of silicon
photomultiplier sensor arrays capable of sensing individual photons. Combined
with high-frequency time-to-digital converters (TDCs), this technology opens up
the prospect of sensors capable of recording with high accuracy both the time
and location of each detected photon. Such a capability could lead to
significant improvements in imaging accuracy, especially for applications
operating with low photon fluxes such as LiDAR and positron emission
tomography.
The demands placed on on-chip readout circuitry imposes stringent trade-offs
between fill factor and spatio-temporal resolution, causing many contemporary
designs to severely underutilize the technology's full potential. Concentrating
on the low photon flux setting, this paper leverages results from group testing
and proposes an architecture for a highly efficient readout of pixels using
only a small number of TDCs, thereby also reducing both cost and power
consumption. The design relies on a multiplexing technique based on binary
interconnection matrices. We provide optimized instances of these matrices for
various sensor parameters and give explicit upper and lower bounds on the
number of TDCs required to uniquely decode a given maximum number of
simultaneous photon arrivals.
To illustrate the strength of the proposed architecture, we note a typical
digitization result of a 120x120 photodiode sensor on a 30um x 30um pitch with
a 40ps time resolution and an estimated fill factor of approximately 70%, using
only 161 TDCs. The design guarantees registration and unique recovery of up to
4 simultaneous photon arrivals using a fast decoding algorithm. In a series of
realistic simulations of scintillation events in clinical positron emission
tomography the design was able to recover the spatio-temporal location of 98.6%
of all photons that caused pixel firings.Comment: 24 pages, 3 figures, 5 table
InGaAsP/InP laser development for single-mode, high-data-rate communications
Materials studies as well as general and specific device development were carried out in the InGaAsP system. A comparison was made of three standard methods of evaluating substrate quality by means of dislocation studies. A cause of reduced yield of good wafers, the pullover of melt from one bin to the next, has been analyzed. Difficulties with reproducible zinc acceptor doping have been traced to segregation of zinc in the In/Zn alloy used for the doping source. Using EBIC measurments, the pn junction was shown to drift in location depending on factors not always under control. An analysis of contact structures by SIMS showed that the depth to which the sintered Au/Zn contact penetrates into the structure is typically 0.13 microns, or well within the cap layer and out of the p-type cladding and thus not deleterious to laser prformance. The problem of single-mode laser development was investigated and it was shown to be related to the growth habit over four different possible substrate configurations. The fabrication of constricted double heterojunctions, mesa stripe buried heterostructures, and buried heterostructures was discussed, and measurements were presented on the device properties of single-mode buried heterostructure lasers. Results include single spectral line emission at 3 mW and a threshold current of 60 mA
Charge reversal of colloidal particles
A theory is presented for the effective charge of colloidal particles in
suspensions containing multivalent counterions. It is shown that if colloids
are sufficiently strongly charged, the number of condensed multivalent
counterion can exceed the bare colloidal charge leading to charge reversal.
Charge renormalization in suspensions with multivalent counterions depends on a
subtle interplay between the solvation energies of the multivalent counterions
in the bulk and near the colloidal surface. We find that the effective charge
is {\it not} a monotonically decreasing function of the multivalent salt
concentration. Furthermore, contrary to the previous theories, it is found that
except at very low concentrations, monovalent salt hinders the charge reversal.
This conclusion is in agreement with the recent experiments and simulations
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