1,161 research outputs found
The Multifragmentation Freeze--Out Volume in Heavy Ion Collisions
The reduced velocity correlation function for fragments from the reaction Fe
+ Au at 100 A~MeV bombarding energy is investigated using the
dynamical--statistical approach QMD+SMM and compared to experimental data to
extract the Freeze--Out volume assuming simultaneous multifragmentation.Comment: 8 pages; 3 uuencoded figures available with figures command, LateX,
UCRL-J-1157
Generation of Intrinsic Vibrational Gap Modes in Three-Dimensional Ionic Crystals
The existence of anharmonic localization of lattice vibrations in a perfect
3-D diatomic ionic crystal is established for the rigid-ion model by molecular
dynamics simulations. For a realistic set of NaI potential parameters, an
intrinsic localized gap mode vibrating in the [111] direction is observed for
fcc and zinc blende lattices. An axial elastic distortion is an integral
feature of this mode which forms more readily for the zinc blende than for the
fcc structure. Molecular dynamics simulations verify that in each structure
this localized mode may be stable for at least 200 cycles.Comment: 5 pages, 4 figures, RevTeX, using epsf.sty. To be published in Phys.
Rev. B. Also available at http://www.msc.cornell.edu/~kiselev
The Human Health Implications of Antibiotic Resistance in Environmental Isolates from Two Nebraska Watersheds
Fine-tuning the balance between peptide thioester cyclization and racemization
Ring-closure towards seven-membered bislactams containing an -amino acid and -alanine is problematic. Such difficult lactamizations are accompanied by side-reactions such as hydrolysis, oligomerization and racemization. By starting from linear peptide 4-methoxythiophenol esters, dilution to 1 mM in combination with phosphate buffer (pH 6.8) intermolecular reactions and racemization could be largely suppressed. Thioesters with the chiral residue at the C-terminus gave the bislactams in yields up to 60% and enantiomeric excess up to 99%. Enantiopure bislactams were obtained exclusively from the reversed sequence bearing -alanine at the C-terminus
quasiharmonic equations of state for dynamically-stabilized soft-mode materials
We introduce a method for treating soft modes within the analytical framework
of the quasiharmonic equation of state. The corresponding double-well
energy-displacement relation is fitted to a functional form that is harmonic in
both the low- and high-energy limits. Using density-functional calculations and
statistical physics, we apply the quasiharmonic methodology to solid periclase.
We predict the existence of a B1--B2 phase transition at high pressures and
temperatures
Optimization of the design of OMNIS, the observatory of multiflavor neutrinos from supernovae
A Monte Carlo code has been developed to simulate the operation of the
planned detectors in OMNIS, a supernova neutrino observatory. OMNIS will detect
neutrinos originating from a core collapse supernova by the detection of
spalled neutrons from Pb- or Fe-nuclei. This might be accomplished using
Gd-loaded liquid scintillator. Results for the optimum configuration for such
modules with respect to both neutron detection efficiency and cost efficiency
are presented. Careful consideration has been given to the expected levels of
radioactive backgrounds and their effects. The results show that the amount of
data to be processed by a software trigger can be reduced to the <10kHz region
and a neutron, once produced in the detector, can be detected and identified
with an efficiency of >30%.Comment: Elsevier preprint; 29 pages, 23 figure
The importance of initial-final state correlations for the formation of fragments in heavy ion collisions
Using quantum molecular dynamics simulations, we investigate the formation of
fragments in symmetric reactions between beam energies of E=30AMeV and 600AMeV.
After a comparison with existing data we investigate some observables relevant
to tackle equilibration: dsigma/dErat, the double differential cross section
dsigma/pt.dpz.dpt,... Apart maybe from very energetic E>400AMeV and very
central reactions, none of our simulations gives evidence that the system
passes through a state of equilibrium. Later, we address the production
mechanisms and find that, whatever the energy, nucleons finally entrained in a
fragment exhibit strong initial-final state correlations, in coordinate as well
as in momentum space. At high energy those correlations resemble the ones
obtained in the participant-spectator model. At low energy the correlations are
equally strong, but more complicated; they are a consequence of the Pauli
blocking of the nucleon-nucleon collisions, the geometry, and the excitation
energy. Studying a second set of time-dependent variables (radii,
densities,...), we investigate in details how those correlations survive the
reaction especially in central reactions where the nucleons have to pass
through the whole system. It appears that some fragments are made of nucleons
which were initially correlated, whereas others are formed by nucleons
scattered during the reaction into the vicinity of a group of previously
correlated nucleons.Comment: 45 pages text + 20 postscript figures Accepted for publication in
Physical Review
Measurement of the electron electric dipole moment using YbF molecules
The most sensitive measurements of the electron electric dipole moment d_e
have previously been made using heavy atoms. Heavy polar molecules offer a
greater sensitivity to d_e because the interaction energy to be measured is
typically 10^3 times larger than in a heavy atom. We report the first
measurement of this kind, for which we have used the molecule YbF. Together,
the large interaction energy and the strong tensor polarizability of the
molecule make our experiment essentially free of the systematic errors that
currently limit d_e measurements in atoms. Our first result d_e = (- 0.2 \pm
3.2) x 10^-26 e.cm is less sensitive than the best atom measurement, but is
limited only by counting statistics and demonstrates the power of the method.Comment: 4 pages, 4 figures. v2. Minor corrections and clarifications made in
response to referee comment
Measurements of Ion Stopping Around the Bragg Peak in High-Energy-Density Plasmas
For the first time, quantitative measurements of ion stopping at energies around the Bragg peak (or peak ion stopping, which occurs at an ion velocity comparable to the average thermal electron velocity), and its dependence on electron temperature (T[subcontract e]) and electron number density (n[subcontract e]) in the range of 0.5–4.0 keV and 3 × 10[superscript 22] to 3 × 10[superscript 23] cm[superscript −3] have been conducted, respectively. It is experimentally demonstrated that the position and amplitude of the Bragg peak varies strongly with T[subscript e] with n[subscript e]. The importance of including quantum diffraction is also demonstrated in the stopping-power modeling of high-energy-density plasmas.United States. Dept. of Energy (Grant DE-FG03-03SF22691)Lawrence Livermore National Laboratory (Subcontract Grant B504974)University of Rochester. Laboratory for Laser Energetics (Subcontract Grant 412160-001G
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