96 research outputs found
A Global Model of -Decay Half-Lives Using Neural Networks
Statistical modeling of nuclear data using artificial neural networks (ANNs)
and, more recently, support vector machines (SVMs), is providing novel
approaches to systematics that are complementary to phenomenological and
semi-microscopic theories. We present a global model of -decay
halflives of the class of nuclei that decay 100% by mode in their
ground states. A fully-connected multilayered feed forward network has been
trained using the Levenberg-Marquardt algorithm, Bayesian regularization, and
cross-validation. The halflife estimates generated by the model are discussed
and compared with the available experimental data, with previous results
obtained with neural networks, and with estimates coming from traditional
global nuclear models. Predictions of the new neural-network model are given
for nuclei far from stability, with particular attention to those involved in
r-process nucleosynthesis. This study demonstrates that in the framework of the
-decay problem considered here, global models based on ANNs can at
least match the predictive performance of the best conventional global models
rooted in nuclear theory. Accordingly, such statistical models can provide a
valuable tool for further mapping of the nuclidic chart.Comment: Proceedings of the 16th Panhellenic Symposium of the Hellenic Nuclear
Physics Societ
Coupled Cluster Method Calculations Of Quantum Magnets With Spins Of General Spin Quantum Number
We present a new high-order coupled cluster method (CCM) formalism for the
ground states of lattice quantum spin systems for general spin quantum number,
. This new ``general-'' formalism is found to be highly suitable for a
computational implementation, and the technical details of this implementation
are given. To illustrate our new formalism we perform high-order CCM
calculations for the one-dimensional spin-half and spin-one antiferromagnetic
{\it XXZ} models and for the one-dimensional spin-half/spin-one ferrimagnetic
{\it XXZ} model. The results for the ground-state properties of the isotropic
points of these systems are seen to be in excellent quantitative agreement with
exact results for the special case of the spin-half antiferromagnet and results
of density matrix renormalisation group (DMRG) calculations for the other
systems. Extrapolated CCM results for the sublattice magnetisation of the
spin-half antiferromagnet closely follow the exact Bethe Ansatz solution, which
contains an infinite-order phase transition at . By contrast,
extrapolated CCM results for the sublattice magnetisation of the spin-one
antiferromagnet using this same scheme are seen to go to zero at , which is in excellent agreement with the value for the onset of
the Haldane phase for this model. Results for sublattice magnetisations of the
ferrimagnet for both the spin-half and spin-one spins are non-zero and finite
across a wide range of , up to and including the Heisenberg point at
.Comment: 5 Figures. J. Stat. Phys. 108, p. 401 (2002
Nuclear mass systematics by complementing the Finite Range Droplet Model with neural networks
A neural-network model is developed to reproduce the differences between
experimental nuclear mass-excess values and the theoretical values given by the
Finite Range Droplet Model. The results point to the existence of subtle
regularities of nuclear structure not yet contained in the best
microscopic/phenomenological models of atomic masses. Combining the FRDM and
the neural-network model, we create a hybrid model with improved predictive
performance on nuclear-mass systematics and related quantities.Comment: Proceedings for the 15th Hellenic Symposium on Nuclear Physic
Statistical Global Modeling of Beta-Decay Halflives Systematics Using Multilayer Feedforward Neural Networks and Support Vector Machines
In this work, the beta-decay halflives problem is dealt as a nonlinear
optimization problem, which is resolved in the statistical framework of Machine
Learning (LM). Continuing past similar approaches, we have constructed
sophisticated Artificial Neural Networks (ANNs) and Support Vector Regression
Machines (SVMs) for each class with even-odd character in Z and N to global
model the systematics of nuclei that decay 100% by the beta-minus-mode in their
ground states. The arising large-scale lifetime calculations generated by both
types of machines are discussed and compared with each other, with the
available experimental data, with previous results obtained with neural
networks, as well as with estimates coming from traditional global nuclear
models. Particular attention is paid on the estimates for exotic and halo
nuclei and we focus to those nuclides that are involved in the r-process
nucleosynthesis. It is found that statistical models based on LM can at least
match or even surpass the predictive performance of the best conventional
models of beta-decay systematics and can complement the latter.Comment: 8 pages, 1 fiqure, Proceedings of the 17th HNPS Symposiu
The Physics of Liquid Para-Hydrogen
Macroscopic systems of hydrogen molecules exhibit a rich thermodynamic phase
behavior. Due to the simplicity of the molecular constituents a detailed
exploration of the thermal properties of these boson systems at low
temperatures is of fundamental interest. Here,we report theoretical and
experimental results on various spatial correlation functions and corresponding
distributions in momentum space of liquid para-hydrogen close to the triple
point. They characterize the structure of the correlated liquid and provide
information on quantum effects present in this Bose fluid. Numerical
calculations employ Correlated Density-Matrix(CDM)theory and Path-Integral
Monte-Carlo(PIMC)simulations. A comparison of these theoretical results
demonstrates the accuracy of CDM theory. This algorithm therefore permits a
fast and efficient quantitative analysis of the normal phase of liquid
para-hydrogen.We compare and discuss the theoretical results with available
experimental data.Comment: 14 pages, 7 figure
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