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Charge radius of helium-6
1 November 2004
The charge radius of the 6He nucleus has been measured at the
Argonne National Laboratory by laser spectroscopy of
atoms. According to theoretical predictions, the difference
in the transition frequencies (or isotope shift) between levels
23S1-33P2 as one goes from 4He to
6He depends on the
difference of the squares of the nuclear charge radii. Using the
measured isotope shift and the charge radius of the 4He
nucleus known from previous experiments, the charge radius
of the 6He nucleus is found to be 2.054+-0.014fm. 6He atoms
were produced at the ATLAS accelerator by bombarding a
graphite target with a beam of 6Li nuclei. Exposed to radio
frequency radiation, the 6He atoms were transferred to the
metastable 23S1 level and then captured in a magneto-optical
trap. The researchers used a 0.389 micron laser to induce
transitions between energy levels required for spectroscopic
measurements. The nucleus of the 6He isotope resembles the
4He nucleus surrounded by an extended halo of two neutrons.
Although the 4He and 6He nuclei possess the same charge, the
charge radius of the 6He nucleus turned out to be larger by
0.4fm, due to the inner core of the nucleus expanding because
of the strong interaction the halo neutrons exert on it.
Source:
Physics News Update, Number 702
Clock synchronization using quantum-correlated photons
1 November 2004
The precision of today's atomic clocks has reached a very high
level, but their potential is limited by the drawbacks of
synchronizing distant clocks with usual radio and light
pulses. Now, an experimental study at the University of
Maryland, Baltimore County has demonstrated for the first
time that photons in entangled states can, in principle, be used
for clock synchronization purposes. Quantum-correlated
photon pairs were produced by splitting single photons in a
nonlinear crystal. The photons were picked up by two
photodetectors attached to clocks and located a distance of
3km apart. This was followed by a similar experiment in
which the two photons at the exit from the crystal are
interchanged. After classical information on photon detec-
tion times is exchanged, clock synchronization correction can
be calculated. It was shown that clocks can be synchronized to
within a picosecond in this way.
Source:
Physics News Update, Number 701
Laser frequency conversion
1 November 2004
Many practical applications require a simple way for the light
of a monochromatic laser to be converted to radiation of a
different wavelength. An often used approach boils down to
taking advantage of the molecular Raman effect, where
conversion occurs due to the rotational and vibrational
modes being excited by light passing through a gas. This
requires a highly intense input beam, however, because an
ensemble of many molecules scatters almost isotropically. A
team of researchers led by P Russell of the University of
Bath in England greatly improved the method using a long
7-mm-diameter hollow-core optical fiber with 50-mm-thick
walls made of a photonic crystal. The photonic crystal
consists of interwoven glass tubes and only reflects a light in
a limited range of frequencies. For the inner channel, which
transmitted the laser light, molecular hydrogen was used as a
filling material. Raman conversion on the inner channel
occurred with high efficiency due to the fact that, whereas
light travelled in only one direction, each photon could be
scattered several times by the molecules. According to the
measurements, about 92% of the laser beam photons
generated Raman-shifted photons. This method, compared
to the conventional one, requires an input beam power several
orders of magnitude lower to produce the desired output
intensity.
Source: Phys. Rev. Lett. 93 123903 (2004)
Solidification on heating
1 November 2004
Unlike the usual situation, some liquids solidify whilst being
heated. However, in all known cases of this kind irreversible
reactions - such as polymerization - accompany the
solidification process. Now M.Plazanet and her colleagues
at the Universite Joseph Fourier and the Institut Laue-Langevin
(both in Grenoble, France) have for the first time
obtained a material which solidifies on heating and reversibly
melts on cooling. The material, a water solution of two
organic compounds, cyclodextrine and 4-methylpyridine,
can change from a transparent liquid solution to a white
colored solid when it is heated. Neutron-scattering studies
showed that heating creates additional electronic coupling
(hydrogen bonds) between the molecules of the two organic
components and that cooling breaks these bonds. The same
picture is confirmed by molecular dynamics simulations.
Source: J. Chem. Phys. 121 5031 (2004)
Shock wave behind a moving pulsar
1 November 2004
Detailed NASA's Chandra observations have been made of
G359.23-0.82, an extended X-ray and radio source 5kpc
away, called the `Mouse' because of its shape. A bright object
at its `head' is the pulsar J1747-2958 which was discovered
earlier with the 64-m Parkes radio telescope and is moving
through space at a supersonic speed of about 600 km s-1 . The
ultrarelativistic particles it emits collide with interstellar gas
and in so doing form a shock wave, behind which powerful
X-ray radiation is generated by the synchrotron mechanism.
An unusual morphological feature of the `Mouse' is a long,
narrow tail which radiates in the radio and X-ray portions of
the spectrum and is likely to have been produced by the inner
shock wave. Because a fast moving pulsar - unlike those in
supernova remnants - interacts with unperturbed interstellar
gas, both the theory of pulsars and research into the
properties of the interstellar medium will benefit from the
observation of G359.23-0.82.
Sources: astro-ph/0312362;
http://chandra.harvard.edu/
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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources.
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