571 research outputs found
Recombination of H3+ Ions in the Afterglow of a He-Ar-H2 Plasma
Recombination of H3+ with electrons was studied in a low temperature plasma
in helium. The plasma recombination rate is driven by two body, H3+ + e, and
three-body, H3+ + e + He, processes with the rate coefficients 7.5x10^{-8}cm3/s
and 2.8x10^{-25}cm6/s correspondingly at 260K. The two-body rate coefficient is
in excellent agreement with results from storage ring experiments and
theoretical calculations. We suggest that the three-body recombination involves
formation of highly excited Rydberg neutral H3 followed by an l- or m- changing
collision with He. Plasma electron spectroscopy indicates the presence of H3.Comment: 4 figure
Temperature dependence of binary and ternary recombination of H3+ ions with electron
We study binary and the recently discovered process of ternary He-assisted
recombination of H3+ ions with electrons in a low temperature afterglow plasma.
The experiments are carried out over a broad range of pressures and
temperatures of an afterglow plasma in a helium buffer gas. Binary and
He-assisted ternary recombination are observed and the corresponding
recombination rate coefficients are extracted for temperatures from 77 K to 330
K. We describe the observed ternary recombination as a two-step mechanism:
First, a rotationally-excited long-lived neutral molecule H3* is formed in
electron-H3+ collisions. Second, the H3* molecule collides with a helium atom
that leads to the formation of a very long-lived Rydberg state with high
orbital momentum. We present calculations of the lifetimes of H3* and of the
ternary recombination rate coefficients for para and ortho-H3+. The
calculations show a large difference between the ternary recombination rate
coefficients of ortho- and para-H3+ at temperatures below 300 K. The measured
binary and ternary rate coefficients are in reasonable agreement with the
calculated values.Comment: 15 page
Multilevel Contracts for Trusted Components
This article contributes to the design and the verification of trusted
components and services. The contracts are declined at several levels to cover
then different facets, such as component consistency, compatibility or
correctness. The article introduces multilevel contracts and a
design+verification process for handling and analysing these contracts in
component models. The approach is implemented with the COSTO platform that
supports the Kmelia component model. A case study illustrates the overall
approach.Comment: In Proceedings WCSI 2010, arXiv:1010.233
Azimuthal Correlations in the Target Fragmentation Region of High Energy Nuclear Collisions
Results on the target mass dependence of proton and pion pseudorapidity
distributions and of their azimuthal correlations in the target rapidity range
are presented. The data have been taken with the
Plastic-Ball detector set-up for 4.9 GeV p + Au collisions at the Berkeley
BEVALAC and for 200 GeV/ p-, O-, and S-induced reactions on
different nuclei at the CERN-SPS. The yield of protons at backward rapidities
is found to be proportional to the target mass. Although protons show a typical
``back-to-back'' correlations, a ``side-by-side'' correlation is observed for
positive pions, which increases both with target mass and with impact parameter
of a collision. The data can consistently be described by assuming strong
rescattering phenomena including pion absorption effects in the entire excited
target nucleus.Comment: 7 pages, figures included, complete postscript available at
ftp://qgp.uni-muenster.de/pub/paper/azi-correlations.ps submitted to Phys.
Lett.
Fission Barrier of Thallium-201
A new method involving the detection of fission fragments in mica has been applied to the measurement of the fission cross section of the compound nucleus Tl201 produced by bombardments of Au197 with helium ions. These data have been interpreted in terms of an expression for the ratio of fission to neutron-emission probabilities similar to those used conventionally, but modified to include the effect of quantum-mechanical barrier penetrability. In this way a height of 22.5±1.5 MeV was found for the fission barrier of Tl201 and a lower limit on the width could be established. The above value of the barrier, when interpreted on the basis of the liquid-drop theory, leads to an accurate determination of the ratio of the electrostatic to the surface energy of nuclei. This serves to establish the constant of proportionality between the "fissionability parameter" x and the value of Z2/A as follows: x=(Z2/A)/(48.4±0.5). This measured barrier height, when added to the ground-state mass of Tl201, gives a saddle-point mass of this nucleus equal to 200.9949±0.0015 mass units (carbon scale)
Interferometry of Direct Photons in Central 280Pb+208Pb Collisions at 158A GeV
Two-particle correlations of direct photons were measured in central
208Pb+208Pb collisions at 158 AGeV. The invariant interferometric radii were
extracted for 100<K_T<300 MeV/c and compared to radii extracted from charged
pion correlations. The yield of soft direct photons, K_T<300 MeV/c, was
extracted from the correlation strength and compared to theoretical
calculations.Comment: 5 pages, 4 figure
Xcd - Modular, Realizable Software Architectures
Connector-Centric Design (Xcd) is centred around a new formal architectural description language, focusing mainly on complex connectors. Inspired by Wright and BIP, Xcd aims to cleanly separate in a modular manner the high-level functional, interaction, and control system behaviours. This can aid in both increasing the understandability of architectural specifications and the reusability of components and connectors themselves. Through the independent specification of control behaviours, Xcd allows designers to experiment more easily with different design decisions early on, without having to modify the functional behaviour specifications (components) or the interaction ones(connectors).
At the same time Xcd attempts to ease the architectural specification by following (and extending) a Design-by-Contract approach, which is more familiar to software developers than process algebras like CSP or languages like BIP that are closer to synchronous/hardware specification languages. Xcd extends Design-by-Contract (i) by separating component contracts into functional and interaction sub-contracts, and (ii) by allowing service consumers to specify their own contractual clauses. Xcd connector specifications are completely decentralized, foregoing Wright’s connector glue, to ensure their realizability by construction
Event-by-Event Fluctuations in Particle Multiplicities and Transverse Energy Produced in 158.A GeV Pb+Pb collisions
Event-by-event fluctuations in the multiplicities of charged particles and
photons, and the total transverse energy in 158 GeV Pb+Pb collisions
are studied for a wide range of centralities. For narrow centrality bins the
multiplicity and transverse energy distributions are found to be near perfect
Gaussians. The effect of detector acceptance on the multiplicity fluctuations
has been studied and demonstrated to follow statistical considerations. The
centrality dependence of the charged particle multiplicity fluctuations in the
measured data has been found to agree reasonably well with those obtained from
a participant model. However for photons the multiplicity fluctuations has been
found to be lower compared to those obtained from a participant model. The
multiplicity and transverse energy fluctuations have also been compared to
those obtained from the VENUS event generator.Comment: To appear in Physical Review C; changes : more detailed discussion on
errors and few figures modifie
Systematics of Inclusive Photon Production in 158 AGeV Pb Induced Reactions on Ni, Nb, and Pb Targets
The multiplicity of inclusive photons has been measured on an event-by-event
basis for 158 AGeV Pb induced reactions on Ni, Nb, and Pb targets. The
systematics of the pseudorapidity densities at midrapidity (rho_max) and the
width of the pseudorapidity distributions have been studied for varying
centralities for these collisions. A power law fit to the photon yield as a
function of the number of participating nucleons gives a value of 1.13+-0.03
for the exponent. The mean transverse momentum, , of photons determined
from the ratio of the measured electromagnetic transverse energy and photon
multiplicity, remains almost constant with increasing rho_max. Results are
compared with model predictions.Comment: 16 pages including 4 figure
Scaling of Particle and Transverse Energy Production in 208Pb+208Pb collisions at 158 A GeV
Transverse energy, charged particle pseudorapidity distributions and photon
transverse momentum spectra have been studied as a function of the number of
participants (N_{part}) and the number of binary nucleon-nucleon collisions
(N_{coll}) in 158 A GeV Pb+Pb collisions over a wide impact parameter range. A
scaling of the transverse energy pseudorapidity density at midrapidity as
N_{part}^{1.08 \pm 0.06} and N_{coll}^{0.83 \pm 0.05} is observed. For the
charged particle pseudorapidity density at midrapidity we find a scaling as
N_{part}^{1.07 \pm 0.04} and N_{coll}^{0.82 \pm 0.03}. This faster than linear
scaling with N_{part} indicates a violation of the naive Wounded Nucleon Model.Comment: 13 pages, 16 figures, submitted to European Physical Journal C
(revised results for scaling exponents
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