670 research outputs found
Neutron-induced Fission Cross Section of240,242Pu
A sensitivity analysis for the new generation of fast reactors [Salvatores (2008)] has shown the importance
of improved cross section data for several actinides. Among them, the240,242Pu(n,f) cross sections require an
accuracy improvement to 1-3% and 3-5%, respectively, from the current level of 6% and 20%. At the Van de Graaff
facility of the Institute for Reference Materials and Measurements (JRC-IRMM) the fission cross section of the two
isotopes was measured relative to two secondary standard reactions,237Np(n,f) and238U(n,f), using a twin Frisch-grid
ionization chamber. The secondary standard reactions were benchmarked through measurements against the primary
standard reaction235U(n,f) in the same geometry. Sample masses were determined by means of low-geometry alpha
counting or/and a 2p Frisch-grid ionization chamber, with an uncertainty lower than 2%. The neutron flux and the
impact of scattering from material between source and target was examined, the largest effect having been found
in cross section ratio measurements between a fissile and a fertile isotope. Our240,242Pu(n,f) cross sections are in
agreement with previous experimental results and slightly lower than present evaluations. In case of the242Pu(n,f)
reaction no evidence for a resonance at En=1.1 MeV was found.Postprint (published version
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
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
Truthfulness on a Budget: Trading Money for Approximation through Monitoring
Albeit a pervasive desideratum when computing a novel paradigm wherein agents’ declarations can be partially checked against their actual costs. in the presence of selfish agents, truthfulness typically imposes severe limitations to what can be implemented. The price of these limitations is typically paid either economically , in terms of the financial resources needed to enforce truthfulness, or algorithmically , in terms of restricting the set of implementable objective functions, which often leads to renouncing optimality and resorting to approximate allocations. In this paper, with regards to utilitarian problems, we ask two fundamental questions: (i) what is the a minimum sufficient budget needed by optimal truthful mechanisms, and (ii) whether it is possible to sacrifice optimality in order to achieve truthfulness with a lower budget. To answer these questions, we connect two streams of work on mechanism design and look at monitoring – a paradigm wherein agents’ actual costs are bound to their declarations. In this setting, we prove that the social cost is always a sufficient budget, even for collusion-resistant mechanisms, and, under mild conditions, also a necessary budget for a large class of utilitarian problems that encompass set system problems. Furthermore, for facility location, a well-studied problem outside of this class, we draw a novel picture about the relationship between approximation and frugality
Probing the metal-nonmetal transition in thin metal overlayers using resonant photoemission
We have studied one and two monolayers of barium on Ni(111) and of mercury on Cu(100). Using resonant photoemission, we have found core excited electrons become delocalized with increasing barium coverage. Similarly, upon formation of the mercury bilayer (as determined by low-energy electron diffraction and by atom-beam scattering), there is a substantial increase in the screening of the photohole. A transition of the electronic structure akin to a metal-nonmetal (metal-insulator) transition is apparent in these final-state effects. The band structure for Hg is similar to the band structure expected for a free-standing film with a free-electron sd band. The delocalization of the core excited electrons resembles the exciton unbinding that occurs at the metal-nonmetal Mott transition
Design and Synthesis of Novel Hyperforin Analogues - Fascinating skeletal rearrangements of polycyclic polyprenylated acylphloroglucinols core
Submonolayer Epitaxy Without A Critical Nucleus
The nucleation and growth of two--dimensional islands is studied with Monte
Carlo simulations of a pair--bond solid--on--solid model of epitaxial growth.
The conventional description of this problem in terms of a well--defined
critical island size fails because no islands are absolutely stable against
single atom detachment by thermal bond breaking. When two--bond scission is
negligible, we find that the ratio of the dimer dissociation rate to the rate
of adatom capture by dimers uniquely indexes both the island size distribution
scaling function and the dependence of the island density on the flux and the
substrate temperature. Effective pair-bond model parameters are found that
yield excellent quantitative agreement with scaling functions measured for
Fe/Fe(001).Comment: 8 pages, Postscript files (the paper and Figs. 1-3), uuencoded,
compressed and tarred. Surface Science Letters, in press
An Unusual Michael-Induced Skeletal Rearrangement of a Bicyclo[3.3.1]nonane Framework of Phloroglucinols to a Novel Bioactive Bicyclo[3.3.0]octane
A novel skeletal rearrangement of bicyclo[3.3.1]nonane-2,4,9-trione (16) to an unprecedented highly functionalized bicyclo[3.3.0]octane system (17), induced by an intramolecular Michael addition, is presented. This novel framework was found to be similarly active to hyperforin (1), against PC-3 cell lines. A mechanistic study was examined in detail, proposing a number of cascade transformations. Also, reactivity of the Δ7,10-double bond was examined under several conditions to explain the above results
- …