Acceleration of neutrons in solids
1 February 2008
Altarev and coworkers in Germany and Switzerland measured the spectrum of a beam
of neutrons that passed through a specimen of solid 2H2; the neutrons in
the beam were scattered by phonons (thermal vibrations of crystal lattice).
Neutrons from a nuclear reactor of the University of Mainz (Germany) passed
through a converter of solid 2H2 and were recorded with gas counters.
The energy of neutrons was measured by recording the deviation of their trajectory
in the gravitational spectrometer. Neutrons are accelerated as they cross the
interface, which results in a transformation of the initial energy spectrum of
the beam. By studying the form of the spectrum it is possible to get better data
on the shape of the interaction potential of neutrons; at very low energies this
potential is theoretically described by the so-called optical model of
scattering which takes into account the refraction of neutrons' wave functions.
According to calculations, the minimum kinetic energy of neutrons is Ec = 99 ± 7
neV, which agrees with the theoretical value Ec = 106 Ýř┬.
Sources: Phys. Rev. Lett. 100 014801 (2008)
Nuclear charge radius of the nucleus 8He
1 February 2008
The nucleus 8He holds a record among known stable isotopes for the relative
number of neutrons. P. Mueller and coworkers in the USA, France and Canada
measured the charge radius of this nucleus. Nuclei of 8He were created when a
hot graphite target was bombarded with a beam of 13C ions with energy 1 GeV
and were captured into a magnetooptic trap. Nuclei of 6He created under the
same conditions were studied at the same time. The charge radii (the mean-square
radius of charge distribution) were obtained by measuring the isotopic shift of
atomic spectra. The charge radius of the nucleus of 6He is 2.068 fm and agrees
with previous measurements while the charge radius of 8He, measured for the
first time, is 1.93 fm. According to theoretical estimates, two neutrons in the
nucleus of 6He are paired (are spatially correlated) and form a more extended
ôhaloö around the main nucleus 4He, while the halo in the nucleus of
consists of two such pairs of neutrons.The additional pair of neutrons makes the
halo of 8He more symmetrical and consequently the oscillations of the two
protons at the halo center have a smaller amplitude relative to the center of
mass of the nucleus, which results in the lower value of the charge radius of
the nucleus 8He in comparison with that of 6He.
Sources: Phys. Rev. Lett. 99 252501 (2007)
Superfluidity of the Bose – Einstein condensate
1 February 2008
The phenomenon of superfluidity is often connected with the Bose – Einstein
condensation of particles (or their Cooper pairs in the case of superconductivity). Conversely, it was
assumed that the Bose – Einstein condensates of atomic gases possess
superfluidity. However, the only indication of their superfluidity until very recently was the
observation of short-lived quantized vortices. A new and more straightforward evidence of
superfluidity – a persistent flow of the Bose – Einstein condensate
– was obtained by ┬. Phillips, ╩. Kalmerson and their colleagues at the National Institute of Standards and Technology (NIST). A gas of sodium atoms in a toroidal trap was converted to the Bose – Einstein condensate state by chilling it and then the condensate cloud
received angular momentum by scattering laser light sent into it, and then the condensate revolved
in the trap without internal friction for about 10 seconds, which indicated that it was
superfluid. Rotation was damped only because the trap was not ideal. The researchers
believe that a superfluid analog of superconducting tunneling contacts (SQUIDs) could be
created on the basis of the superfluid Bose – Einstein condensate.
Sources: Phys. Rev. Lett. 99 260401 (2007)
Spin Hall effect for photons
1 February 2008
The spin Hall effect was recently observed for electrons in thin semiconducting films (see
Physics Uspekhi 111 1240 (2007)). This phenomenon manifests itself through
an excess of electrons with one orientation of spin on facets of a specimen, even in the absence of
external magnetic field. Researchers at the University of California ╬. Hosten Ŕ P. Kwait
discovered a similar spin Hall effect for photons. The effect consists in splitting of
linearly polarized light beam into two beams with circular polarization when the laser
beam is incident on glass from the air. The splitting is a consequence of different phase
shifts for Fourier components of light when light intersects the air-glass interface.
This method may prove useful in building ultra-precise optical tools for measuring
Sources: Science 319 787 (2008)
Double Einstein ring
1 February 2008
An international team of astronomers lead by R. Gavazzi and Ď. Treu (University
of California) observed for the first time a double Einstein ring. The discovery
was made in the framework of the SLACS program on the Hubble space telescope.
The double ring is a concentric image of two galaxies created owing to
gravitational lensing of their light by a lens galaxy. The lens galaxy has a red
shift z = 0.222 while the sources have z = 0.609 and
z = 3.1(+2.0)(-1.0).
All three galaxies are accidentally projected onto a single line of sight and
the probability of observing such a configuration is approximately 1/10000.
Astronomers were able to separate the light of the source galaxies from the 1000
times more intense radiation from the lens galaxy. The gravitational field of
the intermediate galaxy also contributes to the lensing of the farthermost
galaxy. The study of gravitational lensing is important for reconstructing the
distribution of dark matter in galaxies. Moreover, the observation of
approximately 50 double Einstein rings by future telescopes would be sufficient
to measure to 10 percent precision the equation of state of the dark energy
whose dynamics determines the lensing geometry via the law of light propagation in the
Positron cloud at the center of out Galaxy
1 February 2008
A map of emission of 511 keV radiation from the center of the Galaxy that is
produced when positrons annihilate was synthesized using the data of the space
gamma observatory. It was unexpectedly discovered that the distribution of
positrons is not spherical: the positron cloud is squashed by a factor of two.
This shape of the cloud does not comply with the earlier hypothesis of its
origin as a result of annihilation of particles of dark matter because the
distribution of dark matter is assumed to be spherically symmetric relative to
the center of the Galaxy. At the same time, the shape of the positron cloud
roughly coincides with the form of distribution of low-mass x-ray binary systems
that are composed of an ordinary star and an accreting compact object (a neutron
star or a black hole). The close similarity in the shapes of these distributions
indicates that less than one half of all positrons at the Galactic center
originate in x-ray binaries. Positrons could have been created in the intense
field of radiation generated in the course of accretion. However, it is still
unknown why the distribution of x-ray binaries is nonspherical and what the
mechanism is that makes positrons leave the compact object and reach interstellar
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.