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Effect of the chemical environment on nuclear decay
1 October 2004
T.Ohtsuki and his colleagues from Tohoku University and
Yokohama National University in Japan have measured the
decay half-life of the nuclei of the isotope 7Be encapsulated in
spherical fullerene C60 molecules to find an electron-capture
decay rate 0.83% higher than for similar nuclei in the bulk of
metallic beryllium. The 7Be â 7Li decay proceeds by the
electron capture mechanism . that is, a nuclear proton
absorbing an electron from the atomic outer shell becomes a
neutron and emits a neutrino. Therefore, the decay probability
of this event is sensitive to the configuration of the
electron wave functions, which depends on the chemical
environment of the radioactive atom. The observed result
for 7Be nuclei inside the C60 cage sets a record for how
chemical environment can speed up nuclear decay, the
previous values of a decay half-life shift never exceeding
0.15% (see also papers by B.A.Mamyrin and Yu.A.Akulov
Phys. Usp. 46 1153 (2003)
and Phys. Usp. 47 729 (2004)).
The 7Be nuclei were created by bombarding a
lithium target with a 17 MeV-proton beam and had enough
recoil energy to penetrate fullerene C60 molecules. The decay
events were detected through the gamma emission generated
by 7Li nuclei making the transition from the first excited state
to the ground state. All in all, 330 measurements were made
during the 170 days of the experiment.
Source: Phys. Rev. Lett. 93 112501 (2004)
Anomalous properties of microfiber superconductors
1 October 2004
Superconductivity in MgCNi3 was discovered two years ago.
Even though this compound is not a high-temperature
superconductor, it has a crystal lattice similar to that of
cuprate high-temperature superconductors and is therefore
of great interest for researchers. Due to Mg volatility,
MgCNi3 has previously only been obtainable in polycrystalline
powder form. Now, D.P.Young, M.Moldovan, and
P.W.Adams of Louisiana State University have for the first
time synthesized it in the form of a 80-nm thick layer
deposited onto a carbon fiber 7 mm in diameter. In the
process of the synthesis, a carbon fiber coated with a nickel
was held for 20 to 30min in excess magnesium vapor at 700oC
in a vacuum sealed quartz tube. The fibers prepared in this
way exhibited unusual superconducting properties. The
temperature dependence Jc(T) of the critical current differs from the theore-
tically predicted. In addition, the extrapolated
value Jc(0)=4x107A/cm2 is an order of magnitude higher
than in powders. The magnetic field dependence follows
exponential law over the
entire H-field range from 0 to 9T. While the unconventional
functional dependence Jc(T,H) has not yet been fully
explained theoretically, the anomalous temperature dependence
of the London penetration depth l may be the answer.
Source: Phys. Rev. B 70 064508 (2004)
Magnetorotational instability
1 October 2004
An experiment focusing on the study of the instability of a
rotating conducting liquid in a magnetic field was performed
at the University of Maryland using liquid sodium placed
between differentially rotating concentric spheres. The inner
copper sphere 5 cm in diameter rotated between 2.5 to
50 revolutions per second. The stationary outer sphere of
diameter 10 cm was made of a nonmagnetic material.
Magnetic fields in the liquid were measured by magnetometers
located in the external space, and the velocity
distribution was determined using the Doppler effect. The
system was placed in a magnetic field which was parallel to the
axis of rotation and whose magnitude was varied from 0 to
0.2T. In the absence of an external magnetic field, the
differentially rotating liquid exhibits turbulence at a level of
10-20%. Increasing the field above a certain threshold value
led to a rapid increase in instability with a concomitant
generation of oscillating magnetic fields and noncircular
flows. The exact type of instability depended on several
dimensionless parameters of the given magnetohydrodynamic
system. Because of the great effect of turbulence, the
numerical simulationmethod is of no use for such studies. It is
assumed that the magnetorotational instability plays an
important role in the interiors of planets and stars, and is
also of much relevance to astrophysical disks around black
holes and neutron stars. This type of instability causes
angular momentum to be transferred from the center to the
periphery of the disk and forces matter to fall onto the central
object.
Source: Phys. Rev. Lett. 93 114502 (2004)
Electric field in a laser pulse
1 October 2004
F Krausz and his colleagues at the University of Vienna, the
University of Bielefeld, and the Max Planck Institute for
Quantum Optics have for the first time measured the
oscillation of the electric field in a laser pulse. As a starting
point, a gas of neon atoms was ionized by short (with a
duration of 250x10-18s) extreme-UV laser pulses. This
prepared a medium through which the longer femtosecond
laser pulse to be studied (consisting of only a few cycles of the
electromagnetic field) was passed. The electric field of the
longer pulse imparted an additional acceleration to plasma
electrons. Using a spectrometer, the dependence of electron
energy on the time delay between two laser pulses was
constructed. From these data, the dynamic evolution and the
exact value of the electric field in the femtosecond light pulse
was restored.
Source: http://physicsweb.org
X-ray observations of Cassiopeia A
1 October 2004
New detailed X-ray observations of the Cassiopeia A . the
remnant of a supernova explosion having occurred 340 years
ago . were performed using NASA's Chandra X-ray
Observatory. From the center of the explosion, where the
neutron star is located, two large, opposed jet-like structures,
rich in silicon atoms but relatively poor in iron, are seen to
stretch out. The discovery of these bipolar jets in CassiopeiaA
suggests that supernova explosions produce jets more frequently
than previously believed. The collimation of the jets is
produced by an intense magnetic field. Gas clouds are seen at
the ends of the jets, which are elongated perpendicular to the
jets and, unlike the jets, are rich in iron. These clouds were
presumably ejected from the central region of the explosion
before the formation of the jets. By and large, Cassiopeia A
looks like a scaled down model of a `hypernova' explosion, a
concept which was proposed to explain the origin of gamma-ray
bursts.
Source: http://www.nasa.gov
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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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