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Testing mass-energy equivalence directly
1 February 2006
Einstein's formula E=mc2 that relates the mass of a body to the energy
its contains has been given a new direct test at the Laue Langevin
Institute in Grenoble, France. The test involved the study of
reactions in which Si and S nuclei capture a neutron to form the
respective 29Si and 33S isotopes. The masses of the isotopes before and
after the reaction were measured from the cyclotron frequencies of
their ions in a Penning trap. The isotope mass ratio was measured
with a precision two to three orders of magnitude better than
previous experiments. After capturing a neutron the nuclei
deexcited to their ground states with emission of photons, whose
frequency and energy were determined by measuring the photon
diffraction angles from the crystal. From the series of experiments
performed, 10-7, which is 55 times the precision of similar results
obtained five years ago from the annihilation of electron-positron
pairs. Source: Nature 438 1096 (2005)
Electron bubbles in superfluid helium
1 February 2006
An electron placed in a superfluid helium environment repels the
neighboring helium atoms, making a spherical cavity around itself.
The size of this `electron bubble' is such as to minimize the sum
of the quantized kinetic energy of the electron, the surface
tension energy, and the work of the pressure forces acting in the
liquid. In a number of previous experiments, bubble radii of 19A
have been observed. Now A Ghost and H Maris of Brown University in
the US have discovered a new, larger type of an electron bubble in
liquid helium. In their experiments, electron emission from a beta-
radioactive source or a metal tip was directed into a container of
helium in which, in addition to that, a controlled-amplitude
ultrasound wave was generated and focused to the center. The
ultrasound caused the electron bubbles to rapidly grow in size and
then collapse, leading to cavitation effects which were detected by
laser light scattering. The threshold ultrasound amplitude for the
onset of cavitation determines the pressure in the wave and hence
the size of the bubbles. In the phase diagram of temperature vs
threshold pressure, three regions corresponding to different types
of electron bubbles were observed. The first type are the ordinary
electron bubbles - those that have been observed before. The
second, newly-discovered region is interpreted in terms of electron
bubbles attached to vortex filaments in the superfluid helium. The
properties of bubbles near vortices have been the subject of
theoretical studies, and it is known that they can grow to large
sizes due to the fact that the pressure near vortices is less than
the average because of the Bernoulli effect. The third type of
bubbles form at low temperature and have the largest threshold
pressure. Their nature is not yet clear. It is hypothesized that
these bubbles interact simultaneously with two vertices or,
alternatively, with vortex filaments each having an angular
momentum 2h.
Source: Phys. Rev. Lett. 95 265301 (2005)
Direct observation of non-Poissonian atom statistics in adegenerate gas
1 February 2006
Until recently, the quantum statistics of degenerate Fermi and Bose
gases have only been examined by studying the macroscopic
properties of the gases, but now an experiment by C.S.Chuu and his
colleagues at the University of Texas has for the first time
revealed the deviation from the classical statistics by counting
atoms directly. The team measured fluctuations in the number of
Bose condensed 87Rb atoms held in a trap. The system under study was
in the Thomas-Fermi limit, meaning that the number of atoms and the
trap potential depth are related. The counting techniques used
relied on either laser light absorption by atoms or the
fluorescence of excited atoms, depending on whether the number of
atoms exceeded or was less than a thousand, respectively. The
marked instrument noise reduction the team achieved compared with
previous experiments provided a sufficient counting accuracy to
directly observe deviations from the Poisson fluctuation law for
the number of atoms, N<500. As expected theoretically, the magnitude of
the fluctuations exceeds N1/2 in correspondence with Bose-Einstein
statistics. Source: Phys. Rev. Lett. 95 260403 (2005)
A new source of coherent radiation
1 February 2006
A new type of coherent radiation source, involving a mechanism
fundamentally different from that of the laser, can be created
according to theoretical calculations and numerical simulations
performed at Livermore and MIT. It is established that as a
mechanical shock wave propagates through a dielectric crystal, its
front makes the atoms at the crystal sites to vibrate in phase,
leading to the radiation of coherent light waves in the frequency
range from one to a hundred terahertz (1terahertz=1012Hz).
Radiation generated in this way may have properties which cannot be
obtained using conventional lasers. The experimental realization of
this theoretical prediction is an important and interesting problem
to be solved in the near future.
Source: Phys. Rev. Lett. 96 013904 (2005)
A giant gas cloud above the Galaxy's disk plane
1 February 2006
A giant cloud of gas, 10,000 light-years above the Galaxy's plane
and 23,000 light years from Earth, has been discovered by
Y.Pidopryhora (National Radio Astronomy Observatory (NRAO) and Ohio
University), J.Lockman (NRAO), and J.Shields (Ohio University)
using the GBT telescope. The cloud is a million times as massive as
the Sun, and its outflow from the disk is about a hundred times as
energetic as a supernova explosion. Whereas the upper layers of the
cloud are primarily comprised of neutral hydrogen, which is seen at
a wavelength of 21 cm, it interior is likely to be filled with
ionized hydrogen. The hydrogen is strongly disturbed in that there
are many smaller outflows closer to the plane of the disk. The
astronomers believe that the gas was initially in a cluster of
young stars in one of the Galaxy's arms and that it was blown from
there either by supernova explosions or by stellar winds. In the
latter case, the age of the cloud should be between 10 and 30
million years. Such gas outflows may be crucial for the evolution
of galaxies because they transfer heavy chemical elements and
control the star formation rate, among other things.
Source: http://www.nrao.edu/pr/2006/plume/
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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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