65 research outputs found
Pairing dynamics in particle transport
We analyze the effect of pairing on particle transport in time-dependent
theories based on the Hartree-Fock-Bogoliubov (HFB) or BCS approximations. The
equations of motion for the HFB density matrices are unique and the theory
respects the usual conservation laws defined by commutators of the conserved
quantity with the Hamiltonian. In contrast, the theories based on the BCS
approximation are more problematic. In the usual formulation of TDHF+BCS, the
equation of continuity is violated and one sees unphysical oscillations in
particle densities. This can be ameliorated by freezing the occupation numbers
during the evolution in TDHF+BCS, but there are other problems with the BCS
that make it doubtful for reaction dynamics. We also compare different
numerical implementations of the time-dependent HFB equations. The equations of
motion for the and Bogoliubov transformations are not unique, but it
appears that the usual formulation is also the most efficient. Finally, we
compare the time-dependent HFB solutions with numerically exact solutions of
the two-particle Schrodinger equation. Depending on the treatment of the
initial state, the HFB dynamics produces a particle emission rate at short
times similar to that of the Schrodinger equation. At long times, the total
particle emission can be quite different, due to inherent mean-field
approximation of the HFB theory.Comment: 11 pages, 9 figure
Molecular weight effects on chain pull-out fracture of reinforced polymeric interfaces
Using Brownian dynamics, we simulate the fracture of polymer interfaces
reinforced by diblock connector chains. We find that for short chains the
interface fracture toughness depends linearly on the degree of polymerization
of the connector chains, while for longer chains the dependence becomes
. Based on the geometry of initial chain configuration, we propose a
scaling argument that accounts for both short and long chain limits and
crossover between them.Comment: 5 pages, 3 figure
Growth, microstructure, and failure of crazes in glassy polymers
We report on an extensive study of craze formation in glassy polymers.
Molecular dynamics simulations of a coarse-grained bead-spring model were
employed to investigate the molecular level processes during craze nucleation,
widening, and breakdown for a wide range of temperature, polymer chain length
, entanglement length and strength of adhesive interactions between
polymer chains. Craze widening proceeds via a fibril-drawing process at
constant drawing stress. The extension ratio is determined by the entanglement
length, and the characteristic length of stretched chain segments in the
polymer craze is . In the craze, tension is mostly carried by the
covalent backbone bonds, and the force distribution develops an exponential
tail at large tensile forces. The failure mode of crazes changes from
disentanglement to scission for , and breakdown through scission
is governed by large stress fluctuations. The simulations also reveal
inconsistencies with previous theoretical models of craze widening that were
based on continuum level hydrodynamics
Extraction of astatine isotopes for development of radiopharmaceuticals using a 211Rn–211At generator
In-gas-cell laser spectroscopy of the magnetic dipole moment of the N≈126 isotope Pt199
The magnetic dipole moment and mean-square charge radius of 199gPt (Iπ= 5/2−,t1/2=30.8 min) ground state and 199mPt (Eex=424 keV, Iπ= (13/2)+,t1/2=13.6 s) isomeric state are evaluated for the first time from investigations of the hyperfine splitting of the λ1=248.792 nm transition by in-gas-cell laser ionization spectroscopy. Ground and isomeric states of neutron-rich 199Pt nucleus were produced by a multinucleon transfer reaction at the KEK Isotope Separation System (KISS), designed for the study of nuclear spectroscopy in the vicinity of N=126. The measured magnetic dipole moments +0.75(8)μN and −0.57(5)μN are consistent with the systematics of those of nuclei with Iπ= 5/2− and Iπ= 13/2+, respectively
BDNF/TrkB Signaling as a Potential Novel Target in Pediatric Brain Tumors: Anticancer Activity of Selective TrkB Inhibition in Medulloblastoma Cells
Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Deregulation of brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) signaling has been associated with increased proliferative capabilities, invasiveness, and chemoresistance in several types of cancer. However, the relevance of this pathway in MB remains unknown. Here, we show that the selective TrkB inhibitor N-[2-[[(hexahydro-2-oxo-1H-azepin-3-yl)amino]carbonyl]phenyl]-benzo[b]thiophene-2-carboxamide (ANA-12) markedly reduced the viability and survival of human cell lines representative of different MB molecular subgroups. These findings provide the first evidence supporting further investigation of TrkB inhibition as a potential novel strategy for MB treatment
Fracture Toughness of Polymer Interface Reinforced With Diblock Copolymer: Effect of Homopolymer Molecular Weight
Cavernous hemangioma of the pineal region
Cavernous hemangiomas may involve any sites of the central nervous system, but be very rarely encountered in the pineal region. Only nine cases of cavernous hemangioma in the pineal region have been reported previously. We report a case of pineal region cavernous hemangioma correctly diagnosed by CT and MR images.Link_to_subscribed_fulltex
- …