293 research outputs found
Exotic nuclei far from the stability line
The recent availability of radioactive beams has opened up a new era in
nuclear physics. The interactions and structure of exotic nuclei close to the
drip lines have been studied extensively world wide, and it has been revealed
that unstable nuclei, having weakly bound nucleons, exhibit characteristic
features such as a halo structure and a soft dipole excitation. We here review
the developments of the physics of unstable nuclei in the past few decades. The
topics discussed in this Chapter include the halo and skin structures, the
Coulomb breakup, the dineutron correlation, the pair transfer reactions, the
two-nucleon radioactivity, the appearance of new magic numbers, and the pygmy
dipole resonances.Comment: 43 pages, 30 eps figures. chapter in "100 years of subatomic
physics", edited by E.M. Henley and S. Elli
Coupled-channels analysis of the O+Pb fusion barrier distribution
Analyses using simplified coupled-channels models have been unable to
describe the shape of the previously measured fusion barrier distribution for
the doubly magic O+Pb system. This problem was investigated by
re-measuring the fission excitation function for O+Pb with
improved accuracy and performing more exact coupled-channels calculations,
avoiding the constant-coupling and first-order coupling approximations often
used in simplified analyses. Couplings to the single- and 2-phonon states of
Pb, correctly taking into account the excitation energy and the phonon
character of these states, particle transfers, and the effects of varying the
diffuseness of the nuclear potential, were all explored. However, in contrast
to other recent analyses of precise fusion data, no satisfactory simultaneous
description of the shape of the experimental barrier distribution and the
fusion cross-sections for O+Pb was obtained.Comment: RevTex, 29 pages, 7 postscript figures, to appear in PR
Two-neutron halo nuclei in one dimension: dineutron correlation and breakup reaction
We propose a simple schematic model for two-neutron halo nuclei. In this
model, the two valence neutrons move in a one-dimensional mean field,
interacting with each other via a density-dependent contact interaction. We
first investigate the ground state properties, and demonstrate that the
dineutron correlation can be realized with this simple model due to the
admixture of even- and odd-parity single-particle states. We then solve the
time-dependent two-particle Schr\"odinger equation under the influence of a
time-dependent one-body external field, in order to discuss the effect of
dineutron correlation on nuclear breakup processes. The time evolution of
two-particle density shows that the dineutron correlation enhances the total
breakup probability, especially for the two-neutron breakup process, in which
both the valence neutrons are promoted to continuum scattering states. We find
that the interaction between the two particles definitely favours a spatial
correlation of the two outgoing particles, which are mainly emitted in the same
direction.Comment: 17 pages, 11 figure
Time-dependent approach to many-particle tunneling in one-dimension
Employing the time-dependent approach, we investigate a quantum tunneling
decay of many-particle systems. We apply it to a one-dimensional three-body
problem with a heavy core nucleus and two valence protons. We calculate the
decay width for two-proton emission from the survival probability, which well
obeys the exponential decay-law after a sufficient time. The effect of the
correlation between the two emitted protons is also studied by observing the
time evolution of the two-particle density distribution. It is shown that the
pairing correlation significantly enhances the probability for the simultaneous
diproton decay.Comment: 9 pages, 10 eps figure
Dynamic study on fusion reactions for Ca+Zr around Coulomb barrier
By using the updated improved Quantum Molecular Dynamics model in which a
surface-symmetry potential term has been introduced for the first time, the
excitation functions for fusion reactions of Ca+Zr at
energies around the Coulomb barrier have been studied. The experimental data of
the fusion cross sections for Ca+Zr have been reproduced
remarkably well without introducing any new parameters. The fusion cross
sections for the neutron-rich fusion reactions of Ca+Zr around
the Coulomb barrier are predicted to be enhanced compared with a
non-neutron-rich fusion reaction. In order to clarify the mechanism of the
enhancement of the fusion cross sections for neutron-rich nuclear fusions, we
pay a great attention to study the dynamic lowering of the Coulomb barrier
during a neck formation. The isospin effect on the barrier lowering is
investigated. It is interesting that the effect of the projectile and target
nuclear structure on fusion dynamics can be revealed to a certain extent in our
approach. The time evolution of the N/Z ratio at the neck region has been
firstly illustrated. A large enhancement of the N/Z ratio at neck region for
neutron-rich nuclear fusion reactions is found.Comment: 21 pages, 7 figures,3 table
Current helicity of active regions as a tracer of large-scale solar magnetic helicity
We demonstrate that the current helicity observed in solar active regions
traces the magnetic helicity of the large-scale dynamo generated field. We use
an advanced 2D mean-field dynamo model with dynamo saturation based on the
evolution of the magnetic helicity and algebraic quenching. For comparison, we
also studied a more basic 2D mean-field dynamo model with simple algebraic
alpha quenching only. Using these numerical models we obtained butterfly
diagrams both for the small-scale current helicity and also for the large-scale
magnetic helicity, and compared them with the butterfly diagram for the current
helicity in active regions obtained from observations. This comparison shows
that the current helicity of active regions, as estimated by
evaluated at the depth from which the active region arises, resembles the
observational data much better than the small-scale current helicity calculated
directly from the helicity evolution equation. Here and are
respectively the dynamo generated mean magnetic field and its vector potential.
A theoretical interpretation of these results is given.Comment: 11 pages, 5 figures, revised versio
Scalar ground-state observables in the random phase approximation
We calculate the ground-state expectation value of scalar observables in the
matrix formulation of the random phase approximation (RPA). Our expression,
derived using the quasiboson approximation, is a straightforward generalization
of the RPA correlation energy. We test the reliability of our expression by
comparing against full diagonalization in 0 h-bar omega shell-model spaces. In
general the RPA values are an improvement over mean-field (Hartree-Fock)
results, but are not always consistent with shell-model results. We also
consider exact symmetries broken in the mean-field state and whether or not
they are restored in RPA.Comment: 7 pages, 3 figure
Effects of MDMA on Extracellular Dopamine and Serotonin Levels in Mice Lacking Dopamine and/or Serotonin Transporters
3,4-Methylendioxymethamphetamine (MDMA) has both stimulatory and hallucinogenic properties which make its psychoactive effects unique and different from those of typical psychostimulant and hallucinogenic agents. The present study investigated the effects of MDMA on extracellular dopamine (DAex) and serotonin (5-HTex) levels in the striatum and prefrontal cortex (PFC) using in vivo microdialysis techniques in mice lacking DA transporters (DAT) and/or 5-HT transporters (SERT). subcutaneous injection of MDMA (3, 10 mg/kg) significantly increased striatal DAex in wild-type mice, SERT knockout mice, and DAT knockout mice, but not in DAT/SERT double-knockout mice. The MDMA-induced increase in striatal DAex in SERT knockout mice was significantly less than in wildtype mice. In the PFC, MDMA dose-dependently increased DAex levels in wildtype, DAT knockout, SERT knockout and DAT/SERT double-knockout mice to a similar extent. In contrast, MDMA markedly increased 5-HTex in wildtype and DAT knockout mice and slightly increased 5-HTex in SERT-KO and DAT/SERT double-knockout mice. The results confirm that MDMA acts at both DAT and SERT and increases DAex and 5-HTex
Sub- and above barrier fusion of loosely bound Li with Si
Fusion excitation functions are measured for the system Li+Si
using the characteristic -ray method, encompassing both the sub-barrier
and above barrier regions, viz., = 7-24 MeV. Two separate experiments
were performed, one for the above barrier region (= 11-24 MeV) and
another for the below barrier region (= 7-10 MeV). The results were
compared with our previously measured fusion cross section for the
Li+Si system. We observed enhancement of fusion cross section at
sub-barrier regions for both Li and Li, but yield was substantially
larger for Li. However, for well above barrier regions, similar type of
suppression was identified for both the systems.Comment: 8 pages, 6 figures, as accepted for publication in Eur.Phys.J.
MOP Reduction During Long-Term Methamphetamine Withdrawal was Restored by Chronic Post-Treatment with Fluoxetine
Previously, we found fluoxetine reduces methamphetamine preference in mice. However, effects of fluoxetine on developed methamphetamine preference and on methamphetamine induced gene expression changes have been largely unknown. The present study investigates effects of post-treatment with fluoxetine on methamphetamine dependence and on gene expressions after long-term withdrawal in mice. First, we examined whether chronic post-treatment with fluoxetine attenuated methamphetamine-conditioned place preference. Next, we examined the changes in gene expression levels after long-term withdrawal (with saline or fluoxetine treatment) following chronic methamphetamine treatment. Using mRNA from the pooled frontal cortices of 10 mice per group, gene expression analyses were performed using a custom-developed cDNA array and a real-time quantitative reverse transcription-PCR. Chronic post-treatments with fluoxetine abolished the conditioned place preference developed by methamphetamine administrations. Even after long-term withdrawal from repeated methamphetamine administration, µ-opioid receptor (MOP) gene expression was significantly reduced in the frontal cortex. The reduced MOP gene expression in the frontal cortex was restored by chronic administration with fluoxetine. These changes were confirmed by Western blot analyses. These findings suggest that the chronic post-treatments with fluoxetine might be effective for restoring the reduction of MOP levels in the frontal cortex following long-term abstinence from methamphetamine
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