55,573 research outputs found
Induced fission of 240Pu
We study the fission dynamics of 240Pu within an implementation of the
Density Functional Theory (DFT) extended to superfluid systems and real-time
dynamics. We demonstrate the critical role played by the pairing correlations.
The evolution is found to be much slower than previously expected in this fully
non-adiabatic treatment of nuclear dynamics, where there are no symmetry
restrictions and all collective degrees of freedom (CDOF) are allowed to
participate in the dynamics.Comment: 8 pages, 4 figures, talk given at The 6th International Conference on
Fission and Properties of Neutron-Rich Nuclei, Sanibel Island, Florida,
November 6-2 (2016
Global Models of Planet Formation and Evolution
Despite the increase in observational data on exoplanets, the processes that
lead to the formation of planets are still not well understood. But thanks to
the high number of known exoplanets, it is now possible to look at them as a
population that puts statistical constraints on theoretical models. A method
that uses these constraints is planetary population synthesis. Its key element
is a global model of planet formation and evolution that directly predicts
observable planetary properties based on properties of the natal protoplanetary
disk. To do so, global models build on many specialized models that address one
specific physical process. We thoroughly review the physics of the sub-models
included in global formation models. The sub-models can be classified as models
describing the protoplanetary disk (gas and solids), the (proto)planet (solid
core, gaseous envelope, and atmosphere), and finally the interactions
(migration and N-body interaction). We compare the approaches in different
global models and identify physical processes that require improved
descriptions in future. We then address important results of population
synthesis like the planetary mass function or the mass-radius relation. In
these results, the global effects of physical mechanisms occurring during
planet formation and evolution become apparent, and specialized models
describing them can be put to the observational test. Due to their nature as
meta models, global models depend on the development of the field of planet
formation theory as a whole. Because there are important uncertainties in this
theory, it is likely that global models will in future undergo significant
modifications. Despite this, they can already now yield many testable
predictions. With future global models addressing the geophysical
characteristics, it should eventually become possible to make predictions about
the habitability of planets.Comment: 30 pages, 16 figures. Accepted for publication in the International
Journal of Astrobiology (Cambridge University Press
Nuclear Fission: from more phenomenology and adjusted parameters to more fundamental theory and increased predictive power
Two major recent developments in theory and computational resources created
the favorable conditions for achieving a microscopic description of nuclear
fission almost eighty years after its discovery in 1939 by Hahn and Strassmann
(1930). The first major development was in theory, the extension of the
Time-Dependent Density Functional Theory (TDDFT) to superfluid fermion systems.
The second development was in computing, the emergence of powerful enough
supercomputers capable of solving the complex systems of equations describing
the time evolution in three dimensions without any restrictions of hundreds of
strongly interacting nucleons. Even though the available nuclear energy density
functionals (NEDFs) are phenomenological still, their accuracy is improving
steadily and the prospects of being able to perform calculations of the nuclear
fission dynamics and to predict many properties of the fission fragments,
otherwise not possible to extract from experiments, are within reach, all
without making recourse anymore to uncontrollable assumptions and simplified
phenomenological models.Comment: 6 pages, account of invited talk given at FUSION17, Hobart, Tasmania,
February 20-24, 201
p-wave Feshbach molecules
We have produced and detected molecules using a p-wave Feshbach resonance
between 40K atoms. We have measured the binding energy and lifetime for these
molecules and we find that the binding energy scales approximately linearly
with magnetic field near the resonance. The lifetime of bound p-wave molecules
is measured to be 1.0 +/- 0.1 ms and 2.3 +/- 0.2 ms for the m_l = +/- 1 and m_l
= 0 angular momentum projections, respectively. At magnetic fields above the
resonance, we detect quasi-bound molecules whose lifetime is set by the
tunneling rate through the centrifugal barrier
The Signal Estimator Limit Setting Method
A new method of background subtraction is presented which uses the concept of a signal estimator to construct a confidence level which is always conservative and which is never better than e^-s. The new method yields stronger exclusions than the Bayesian method with a flat prior distribution
Higher-order Continuum Approximation for Rarefied Gases
The Hilbert-Chapman-Enskog expansion of the kinetic equations in mean flight
times is believed to be asymptotic rather than convergent. It is therefore
inadvisable to use lower order results to simplify the current approximation as
is done in the traditional Chapman-Enskog procedure, since that is an iterative
method. By avoiding such recycling of lower order results, one obtains
macroscopic equations that are asymptotically equivalent to the ones found in
the Chapman-Enskog approach. The new equations contain higher order terms that
are discarded in the Chapman-Enskog method. These make a significant impact on
the results for such problems as ultrasound propagation. In this paper, it is
shown that these results turn out well with relatively little complication when
the expansions are carried to second order in the mean free time, for the
example of the relaxation or BGK model of kinetic theory.Comment: 20 pages, 2 figures, RevTeX 4 macro
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