559 research outputs found
Approaching Utopia: Strong Truthfulness and Externality-Resistant Mechanisms
We introduce and study strongly truthful mechanisms and their applications.
We use strongly truthful mechanisms as a tool for implementation in undominated
strategies for several problems,including the design of externality resistant
auctions and a variant of multi-dimensional scheduling
Experiments on Quantum and Thermal Desorption from ^4He Films
Desorption of He atoms from thin films may be resolved experimentally into quantum and thermal components. We show that quantum desorption becomes the dominant part of the signal in submonolayer films. We also show that, when all effects of collisions between desorbed atoms are eliminated, quantum desorption is not focused normal to the surface of optically polished sapphire crystals
Molecular Hydrogen Formation on Low Temperature Surfaces in Temperature Programmed Desorption Experiments
The study of the formation of molecular hydrogen on low temperature surfaces
is of interest both because it allows to explore elementary steps in the
heterogeneous catalysis of a simple molecule and because of the applications in
astrochemistry. Here we report results of experiments of molecular hydrogen
formation on amorphous silicate surfaces using temperature-programmed
desorption (TPD). In these experiments beams of H and D atoms are irradiated on
the surface of an amorphous silicate sample. The desorption rate of HD
molecules is monitored using a mass spectrometer during a subsequent TPD run.
The results are analyzed using rate equations and the activation energies of
the processes leading to molecular hydrogen formation are obtained from the TPD
data. We show that a model based on a single isotope provides the correct
results for the activation energies for diffusion and desorption of H atoms.
These results can thus be used to evaluate the formation rate of H_2 on dust
grains under the actual conditions present in interstellar clouds.Comment: 30 pages, 1 table, 6 figures. Published versio
Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling
Molecular hydrogen has an important role in the early stages of star
formation as well as in the production of many other molecules that have been
detected in the interstellar medium. In this review we show that it is now
possible to study the formation of molecular hydrogen in simulated
astrophysical environments. Since the formation of molecular hydrogen is
believed to take place on dust grains, we show that surface science techniques
such as thermal desorption and time-of-flight can be used to measure the
recombination efficiency, the kinetics of reaction and the dynamics of
desorption. The analysis of the experimental results using rate equations gives
useful insight on the mechanisms of reaction and yields values of parameters
that are used in theoretical models of interstellar cloud chemistry.Comment: 23 pages, 7 figs. Published in the J. Phys.: Conf. Se
Buffer gas cooling and trapping of atoms with small magnetic moments
Buffer gas cooling was extended to trap atoms with small magnetic moment
(mu). For mu greater than or equal to 3mu_B, 1e12 atoms were buffer gas cooled,
trapped, and thermally isolated in ultra high vacuum with roughly unit
efficiency. For mu < 3mu_B, the fraction of atoms remaining after full thermal
isolation was limited by two processes: wind from the rapid removal of the
buffer gas and desorbing helium films. In our current apparatus we trap atoms
with mu greater than or equal to 1.1mu_B, and thermally isolate atoms with mu
greater than or equal to 2mu_B. Extrapolation of our results combined with
simulations of the loss processes indicate that it is possible to trap and
evaporatively cool mu = 1mu_B atoms using buffer gas cooling.Comment: 17 pages, 4 figure
High-precision prompt-γ-ray spectral data from the reaction Pu 241 (nth, f)
In this paper we present results from the first high-precision prompt-γ-ray spectral measurements from the reaction Pu241(nth, f). Apart from one recent experiment, no data are reported in the literature for this fissioning system, which motivated a new dedicated experiment. We have measured prompt-fission γ rays with three cerium-doped LaBr3 (two 5.08cm×5.08 cm and one 7.62cm×7.62 cm) and one CeBr3 (5.08cm×5.08 cm) scintillation detectors, which all exhibit excellent timing and good energy resolution. The average γ-ray multiplicity was determined to be ν̄γ=(8.21±0.09) per fission, the average energy to be εγ=(0.78±0.01) MeV, and the total energy to be Eγ,tot=(6.41±0.06) MeV as the weighted average from all detectors. Since the results from all detectors are in excellent agreement, and the total released γ energy is modestly higher than the one in the present evaluated nuclear data files, we suspect that the underestimation of the prompt-γ heating in nuclear reactors is due to fast-neutron-induced fission on U238 or rather from fission induced by γ rays from neutron capture in the construction material.European Commision NeutAndalus FP7-PEOPLE-2012-CIGPrograma ERINDA 26949
Scaling of Island Growth in Pb Overlayers on Cu(001)
The growth and ordering of a Pb layer deposited on Cu(001) at 150 K has been
studied using atom beam scattering. At low coverage, ordered Pb islands with a
large square unit cell and nearly hexagonal internal structure are formed. This
is a high order commensurate phase with 30 atoms in the unit cell. From the
measurement of the island diffraction peak profiles we find a power law for the
mean island - size versus coverage with an exponent . A
scaling behavior of growth is confirmed and a simple model describing island
growth is presented. Due to the high degeneracy of the monolayer phase,
different islands do not diffract coherently. Therefore, when islands merge
they still diffract as separate islands and coalescence effects are thus
negligible. From the result for we conclude that the island density is
approximately a constant in the coverage range where the
ordered islands are observed. We thus conclude that most islands nucleate at
and then grow in an approximately self similar fashion as
increases.Comment: 23 pages, 10 Figures (available upon request). SU-PHYS-93-443-375
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