324 research outputs found
Binding between endohedral Na atoms in Si clathrate I; a first principles study
We investigate the binding nature of the endohedral sodium atoms with the
ensity functional theory methods, presuming that the clathrate I consists of a
sheaf of one-dimensional connections of Na@Si cages interleaved in three
perpendicular directions. Each sodium atom loses 30% of the 3s charge to
the frame, forming an ionic bond with the cage atoms; the rest of the electron
contributes to the covalent bond between the nearest Na atoms. The presumption
is proved to be valid; the configuration of the two Na atoms in the nearest
Si cages is more stable by 0.189 eV than that in the Si and
Si cages. The energy of the beads of the two distorted Na atoms is more
stable by 0.104 eV than that of the two infinitely separated Na atoms. The
covalent bond explains both the preferential occupancies in the Si cages
and the low anisotropic displacement parameters of the endohedral atoms in the
Si cages in the [100] directions of the clathrate I.Comment: First page: Affiliation added to PDF and PS versio
Behavioral-Independent Features of Complex Heartbeat Dynamics
We test whether the complexity of cardiac interbeat interval time series is
simply a consequence of the wide range of scales characterizing human behavior,
especially physical activity, by analyzing data taken from healthy adult
subjects under three conditions with controls: (i) a ``constant routine''
protocol where physical activity and postural changes are kept to a minimum,
(ii) sympathetic blockade, and (iii) parasympathetic blockade. We find that
when fluctuations in physical activity and other behavioral modifiers are
minimized, a remarkable level of complexity of heartbeat dynamics remains,
while for neuroautonomic blockade the multifractal complexity decreases.Comment: 4 pages with 6 eps figures. Latex file. For more details or for
downloading the PDF file of the published article see
http://polymer.bu.edu/~amaral/Heart.html and
http://polymer.bu.edu/~amaral/Multifractal.htm
correlations from the stopped reaction on He
We have investigated correlations of coincident pairs from the
stopped reaction on He, and clearly observed and
branches of the two-nucleon absorption process in the
invariant mass spectra. In addition, non-mesonic reaction channels, which
indicate possible exotic signals for the formation of strange multibaryon
states, have been identified.Comment: 5 pages, 3 figures, submitted to Physical Review Letter
An Electron-Tracking Compton Telescope for a Survey of the Deep Universe by MeV gamma-rays
Photon imaging for MeV gammas has serious difficulties due to huge
backgrounds and unclearness in images, which are originated from incompleteness
in determining the physical parameters of Compton scattering in detection,
e.g., lack of the directional information of the recoil electrons. The recent
major mission/instrument in the MeV band, Compton Gamma Ray
Observatory/COMPTEL, which was Compton Camera (CC), detected mere
persistent sources. It is in stark contrast with 2000 sources in the GeV
band. Here we report the performance of an Electron-Tracking Compton Camera
(ETCC), and prove that it has a good potential to break through this stagnation
in MeV gamma-ray astronomy. The ETCC provides all the parameters of
Compton-scattering by measuring 3-D recoil electron tracks; then the Scatter
Plane Deviation (SPD) lost in CCs is recovered. The energy loss rate (dE/dx),
which CCs cannot measure, is also obtained, and is found to be indeed helpful
to reduce the background under conditions similar to space. Accordingly the
significance in gamma detection is improved severalfold. On the other hand, SPD
is essential to determine the point-spread function (PSF) quantitatively. The
SPD resolution is improved close to the theoretical limit for multiple
scattering of recoil electrons. With such a well-determined PSF, we demonstrate
for the first time that it is possible to provide reliable sensitivity in
Compton imaging without utilizing an optimization algorithm. As such, this
study highlights the fundamental weak-points of CCs. In contrast we demonstrate
the possibility of ETCC reaching the sensitivity below erg
cm s at 1 MeV.Comment: 19 pages, 12 figures, Accepted to the Astrophysical Journa
A search for deeply bound kaonic nuclear states
We have measured proton and neutron energy spectra by stopping negative kaons
on liquid helium4. Two distinct peak structures were found on both spectra,
which were assigned to the formation of new kinds of strange stribaryons. In
this paper, we summarize both results.Comment: 7 pages, 3 figures, HYP2003 conference proceeding
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