703 research outputs found
Switched Control of Electron Nuclear Spin Systems
In this article, we study control of electron-nuclear spin dynamics at
magnetic field strengths where the Larmor frequency of the nucleus is
comparable to the hyperfine coupling strength. The quantization axis for the
nuclear spin differs from the static B_0 field direction and depends on the
state of the electron spin. The quantization axis can be switched by flipping
the state of electron spin, allowing for universal control on nuclear spin
states. We show that by performing a sequence of flips (each followed by a
suitable delay), we can perform any desired rotation on the nuclear spins,
which can also be conditioned on the state of the electron spin. These
operations, combined with electron spin rotations can be used to synthesize any
unitary transformation on the coupled electron-nuclear spin system. We discuss
how these methods can be used for design of experiments for transfer of
polarization from the electron to the nuclear spins
Quantitative analysis of pedestrian counterflow in a cellular automaton model
Pedestrian dynamics exhibits various collective phenomena. Here we study
bidirectional pedestrian flow in a floor field cellular automaton model. Under
certain conditions, lane formation is observed. Although it has often been
studied qualitatively, e.g., as a test for the realism of a model, there are
almost no quantitative results, neither empirically nor theoretically. As basis
for a quantitative analysis we introduce an order parameter which is adopted
from the analysis of colloidal suspensions. This allows to determine a phase
diagram for the system where four different states (free flow, disorder, lanes,
gridlock) can be distinguished. Although the number of lanes formed is
fluctuating, lanes are characterized by a typical density. It is found that the
basic floor field model overestimates the tendency towards a gridlock compared
to experimental bounds. Therefore an anticipation mechanism is introduced which
reduces the jamming probability.Comment: 11 pages, 12 figures, accepted for publication in Phys. Rev.
Astrophysical S_{17}(0) factor from a measurement of d(7Be,8B)n reaction at E_{c.m.} = 4.5 MeV
Angular distribution measurements of H(Be,Be)H and
H(Be,B) reactions at ~4.5 MeV were performed to
extract the astrophysical factor using the asymptotic normalization
coefficient (ANC) method. For this purpose a pure, low emittance Be beam
was separated from the primary Li beam by a recoil mass spectrometer
operated in a novel mode. A beam stopper at 0 allowed the use of a
higher Be beam intensity. Measurement of the elastic scattering in the
entrance channel using kinematic coincidence, facilitated the determination of
the optical model parameters needed for the analysis of the transfer data. The
present measurement significantly reduces errors in the extracted
Be(p,) cross section using the ANC method. We get
~(0)~=~20.7~~2.4 eV~b.Comment: 15 pages including 3 eps figures, one figure removed and discussions
updated. Version to appear in Physical Review
New pathway to bypass the 15O waiting point
We propose the sequential reaction process
O(,)O as a new pathway to bypass of the
O waiting point. This exotic reaction is found to have a surprisingly
high cross section, approximately 10 times higher than the
O(,)O. These cross sections were calculated after
precise measurements of energies and widths of the proton-unbound F low
lying states, obtained using the H(O,p)O reaction. The large
cross section can be understood to arise from the more
efficient feeding of the low energy wing of the ground state resonance by the
gamma decay. The implications of the new reaction in novae explosions and X-ray
bursts are discussed.Comment: submitte
Elastic scattering and breakup of 17^F at 10 MeV/nucleon
Angular distributions of fluorine and oxygen produced from 170 MeV 17^F
incident on 208^Pb were measured. The elastic scattering data are in good
agreement with optical model calculations using a double-folding potential and
parameters similar to those obtained from 16^O+208^Pb. A large yield of oxygen
was observed near \theta_lab=36 deg. It is reproduced fairly well by a
calculation of the (17^F,16^O) breakup, which is dominated by one-proton
stripping reactions. The discrepancy between our previous coincidence
measurement and theoretical predictions was resolved by including core
absorption in the present calculation.Comment: 9 pages, 5 figure
Initial State Dependence of the Breakup of Weakly Bound Carbon Isotopes
The one-neutron nuclear breakup from the Carbon isotopes C and
C, is calculated as an example of application of the theory of transfer
to the continuum reactions in the formulation which includes spin coupling.
The effect of the energy sharing between the parallel and transverse neutron
momentum distributions is taken into account thus resulting in a theory which
is more general than sudden eikonal approaches. Both effects are necessary to
understand properly the breakup from not too weakly bound orbitals.
Breakup which leaves the core into an excited state below particle threshold is
also considered. The core-target interaction is treated in the smooth cut-off
approximation. By comparing to presently available experimental data we show
how to make some hypothesis on the quantum numbers and occupancy of the neutron
initial state. Possible ambiguities in the interpretation of inclusive cross
sections are discussed.Comment: 22 RevTeX pages,3 ps figures. Phys. Rev. C, accepte
Enhanced empirical data for the fundamental diagram and the flow through bottlenecks
In recent years, several approaches for modelling pedestrian dynamics have
been proposed and applied e.g. for design of egress routes. However, so far not
much attention has been paid to their 'quantitative' validation. This
unsatisfactory situation belongs amongst others on the uncertain and
contradictory experimental data base. The fundamental diagram, i.e. the
density-dependence of the flow or velocity, is probably the most important
relation as it connects the basic parameter to describe the dynamic of crowds.
But specifications in different handbooks as well as experimental measurements
differ considerably. The same is true for the bottleneck flow. After a
comprehensive review of the experimental data base we give an survey of a
research project, including experiments with up to 250 persons performed under
well controlled laboratory conditions. The trajectories of each person are
measured in high precision to analyze the fundamental diagram and the flow
through bottlenecks. The trajectories allow to study how the way of measurement
influences the resulting relations. Surprisingly we found large deviation
amongst the methods. These may be responsible for the deviation in the
literature mentioned above. The results are of particular importance for the
comparison of experimental data gained in different contexts and for the
validation of models.Comment: A contribution to: Pedestrian and Evacuation Dynamics 2008 (Springer)
12 pages, 7 figure
Computational Cancer Biology: An Evolutionary Perspective
ISSN:1553-734XISSN:1553-735
Nuclear structure and reaction studies at SPIRAL
The SPIRAL facility at GANIL, operational since 2001, is described briefly.
The diverse physics program using the re-accelerated (1.2 to 25 MeV/u) beams
ranging from He to Kr and the instrumentation specially developed for their
exploitation are presented. Results of these studies, using both direct and
compound processes, addressing various questions related to the existence of
exotic states of nuclear matter, evolution of new "magic numbers", tunnelling
of exotic nuclei, neutron correlations, exotic pathways in astrophysical sites
and characterization of the continuum are discussed. The future prospects for
the facility and the path towards SPIRAL2, a next generation ISOL facility, are
also briefly presented.Comment: 48 pages, 27 figures. Accepted for publication in Journal of Physics
alpha-particle production in the scattering of 6He by 208Pb at energies around the Coulomb barrier
New experimental data from the scattering of 6He+208Pb at energies around and
below the Coulomb barrier are presented. The yield of breakup products coming
from projectile fragmentation is dominated by a strong group of
particles.
The energy and angular distributions of this group have been analyzed and
compared with theoretical calculations. This analysis indicates that the
particles emitted at backward angles in this reaction are mainly due
to two-neutron transfer to weakly bound states of the final nucleus.Comment: 20 pages, 5 figures. Nuclear Physics A792 (2007) 2-1
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