18,865 research outputs found
Fragmentation of spherical radioactive heavy nuclei as a novel probe of transient effects in fission
Peripheral collisions with radioactive heavy-ion beams at relativistic
energies are discussed as an innovative approach for probing the transient
regime experienced by fissile systems evolving towards quasi-equilibrium. A
dedicated experiment using the advanced technical installations of GSI,
Darmstadt, permitted to realize ideal conditions for the investigation of
relaxation effects in the meta-stable well. Combined with a highly sensitive
experimental signature, it provides a measure of the transient effects with
respect to the flux over the fission barrier. Within a two-step reaction
process, 45 proton-rich unstable spherical isotopes produced by
projectile-fragmentation of a stable 238U beam have been used as secondary
projectiles. The fragmentation of the radioactive projectiles on lead results
in nearly spherical compound nuclei which span a wide range in excitation
energy and fissility. The decay of these excited systems by fission is studied
with a dedicated set-up which permits the detection of both fission products in
coincidence and the determination of their atomic numbers with high resolution.
The width of the fission-fragment nuclear charge distribution is shown to be
specifically sensitive to pre-saddle transient effects and is used to establish
a clock for the passage of the saddle point. The comparison of the experimental
results with model calculations points to a fission delay of (3.3+/-0.7).10-21s
for initially spherical compound nuclei, independent of excitation energy and
fissility. This value suggests a nuclear dissipation strength at small
deformation of (4.5+/-0.5).1021s-1. The very specific combination of the
physics and technical equipment exploited in this work sheds light on previous
controversial conclusions.Comment: 38 pages, 15 figure
Phase transitions during fruiting body formation in Myxococcus xanthus
The formation of a collectively moving group benefits individuals within a
population in a variety of ways such as ultra-sensitivity to perturbation,
collective modes of feeding, and protection from environmental stress. While
some collective groups use a single organizing principle, others can
dynamically shift the behavior of the group by modifying the interaction rules
at the individual level. The surface-dwelling bacterium Myxococcus xanthus
forms dynamic collective groups both to feed on prey and to aggregate during
times of starvation. The latter behavior, termed fruiting-body formation,
involves a complex, coordinated series of density changes that ultimately lead
to three-dimensional aggregates comprising hundreds of thousands of cells and
spores. This multi-step developmental process most likely involves several
different single-celled behaviors as the population condenses from a loose,
two-dimensional sheet to a three-dimensional mound. Here, we use
high-resolution microscopy and computer vision software to spatiotemporally
track the motion of thousands of individuals during the initial stages of
fruiting body formation. We find that a combination of cell-contact-mediated
alignment and internal timing mechanisms drive a phase transition from
exploratory flocking, in which cell groups move rapidly and coherently over
long distances, to a reversal-mediated localization into streams, which act as
slow-spreading, quasi-one-dimensional nematic fluids. These observations lead
us to an active liquid crystal description of the myxobacterial development
cycle.Comment: 16 pages, 5 figure
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