Нейтронная голография /// Рассеяние позитрония /// Сверхпроводимость л

 

au

   
★★
NEUTRON HOLOGRAPHY with atomic-scale resolution has been performed,
for the
first time, with an "inside-detector" approach. Holography
generally
includes a source of illuminating waves, an object to be imaged, and a
detector or film in which waves direct from the source interfere with
waves
scattered from parts of the object. The interference pattern, stored
in the
detector medium, is later read out (and a 3D image of the object
viewed) by
sending waves into the detector. Holograms with visible light are
common
enough: they adorn most credit cards. Holograms using electrons
(considered
in their "wave" manifestation, not as particles) provide
sharp pictures, but
because the electrons cannot penetrate far into a solid sample, the
imaging
process is usually restricted to surface regions. Holograms using x
rays go
can penetrate much farther, but their limitation consists of the fact
that
the penetration depth improves as the square of the atomic number.
Therefore x-holography is not very good for materials with light
elements.
Holograms with neutrons are different; rather than scattering from the
electrons in the atoms of the sample, neutrons scatter only from
nuclei,
which are 100,000 times smaller than the atoms in which they reside.
This
is important when it comes time to reconstruct an image of the
interior of a
crystal lattice. In an experiment carried out with a beam of
neutrons from
a reactor at the Institute Laue-Langevin in Grenoble, a group of
scientists
has produced, for the first time, an atomic-scale map of a crystal, in
particular a sample of lead atoms, using a technique in which the
"detector," a trace amount of atoms (cadmium-113) whose
nuclei readily
absorb neutrons, are embedded inside the sample itself. The
holographic
process unfolds as follows: neutron waves can strike a Cd nucleus
directly
(reference beam) or by first scattering from a Pb nucleus. In either
case,
the absorption of a neutron stimulates a Cd nucleus to emit a high
energy
photon observable in a nearby detector. The overall interference
pattern
for these two processes (absorbing scattered or direct neutron waves)
is
monitored as the profile of the sample to the beam is stepped through
various angles.
The result: a crisp picture of a unit cell of 12 lead atoms (see
figure
at http://www.aip.org/mgr/png/2002/165.htm ).
This process should be great
for spotting foreign atoms in a solid (dopants if the atoms are
desired,
impurties if they're not). Since the neutron has a magnetic moment,
n-holography might also to contribute to an understanding of the
magnetic
nature of the sample atoms, in addition to imaging their whereabouts.
(Cser
et al., Physical Review Letters, 21 October 2002; contact Laszlo Cser,
Central Research Institute for Physics, Budapest, csersunserv.kfhi.hu,

36-1-392-2222 extension 1526.)

FIRST DETAILED POSITRONIUM SCATTERING EXPERIMENT. The lightest atom
made
of an electron and a positively charged mate is not hydrogen but
positronium
(abbreviated Ps), a bound electron-positron pair. The lifetime for
these
no-nucleus atoms is hardly more than about 100 nanoseconds but, if
things
are expedited, this is long enough for doing an experiment. (The
brief
lifespan comes not from the intrinsic instability of the Ps
"atom" but from
the fact that the constituents will, left to themselves, annihilate
each
other.) In recent years physicists have been able to gather Ps
beams, made
by sending a beam of positrons through a neutralizing gas, and have
measured
the total cross section (likelihood of scattering) for Ps scattering
from
various targets. Now a team of scientists at University College
London
reports the first experiment in which a specific type of inelastic
scattering takes place. In particular, the London researchers found
that in
many encounters with helium atoms, the Ps will split apart but that
the
fragmented partners continue to be highly correlated, moving through
the lab
with roughly the same velocities. Learning more about this
fragmentation
process will aid proposed schemes for using Ps beams for studying
material
surfaces. Furthermore, Ps is unusual in that its centers of mass and
charge
coincide. This allows for interactions between the electron in the
Ps and
electrons in target atoms to be more potent than if the electron were
yoked
to a much heavier proton, as in a hydrogen atom. (Armitage et al.,
Physical
Review Letters, 21 October 2002; contact Gaetana Laricchia,
44-20-7679-3470,
g.laricchiaucl.ac.uk)


SUPERCONDUCTIVITY IN LITHIUM now has the highest demonstrated
transition
temperature of any element, 20 K. Great pressure, 48 GPa, was needed
to
achieve superconductivity. According to the physicists at the
University of
Tokyo and Osaka University who performed the experiment on lithium
(the
sample and its electrical leads are squeezed in a diamond anvil
press),
their result bears out an expectation that lighter elements should
possess
higher transition temperatures. Extrapolating this principle
further, they
argue, might produce room temperature superconductivity in hydrogen,
but
only at crushing pressures above 400 GPa. (Shimizu et al., Nature, 10
October 2002.)






Neutron hologram of a lead crystal. The spots represent the positions of 12 lead atoms forming the first neighbors of a cadmium nucleus, as displayed on a sphere of radius .35 nm.

Neutron hologram of a lead crystal. The spots represent the positions of 12 lead atoms forming the first neighbors of a cadmium nucleus, as displayed on a sphere of radius .35 nm.

Neutron hologram of a lead crystal. The spots represent the positions of 12 lead atoms forming the first neighbors of a cadmium nucleus, as displayed on a sphere of radius .35 nm.
 
RU Адрон #18.10.2002 10:47
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Адрон

втянувшийся
au>SUPERCONDUCTIVITY IN LITHIUM now has the highest demonstrated
au>transition
au>temperature of any element, 20 K. Great pressure, 48 GPa, was needed
au>to
au>achieve superconductivity

Yes! Вспоминаем ранее описанный нейтринный движок на поляризованных ядрах лития-8. Выше упомянутый эффект - первый шаг к получению поляризованного лития для практической его реализации. Напомню, что нескомпенсированный импульс от 1 кг полностью поляризованного лития -8 составляет 0,33*1,6e-19J/ev*6,5e6 eV*6e23 *1000 / (8 *3e8)=85800 кг*м/с
 

yuu2

опытный

au>NEUTRON HOLOGRAPHY with atomic-scale resolution has been performed,
au>for the
au>first time,

PR да и только! В Дубне уже давно атомарную текстуру вируса СПИДА нейтронами срисовали и теперь смотрят нейтронами же на атомарном уровне как какие лекарства на него воздействую.
 

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