Text 850, 71 rader
Skriven 2006-08-23 19:55:01 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 789
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 789  August 22, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu

SMALLEST PYRAMIDS IN THE UNIVERSE.  French physicists believe they
can solve the mystery behind dozens of nuclear experiments conducted
years ago.  The experiments, conducted with a variety of detectors,
energies, and colliding nuclear species, left puzzling results, so
puzzling and hard to interpret that many of the experimenters turned
their attention to the study of highly spinning nuclei, a quite
fashionable subject at the time.  Now, Jerzy Dudek of the Universite
Louis Pasteur (Strasbourg) and his colleagues at Warsaw University
and the Universidad Autonoma de Madrid claim that the old results
can be explained by arguing that some nuclei, made in the
tempestuous conditions of a sufficiently high-energy collision, can
exist in the form of a tetrahedron or a octahedron.  Like a
pyramid-shaped  methane (CH4) molecule held together by the
electromagnetic force, a pyramidal nucleus would consist of protons
and neutrons held together by the strong nuclear force.  Such a
nuclear molecule--in effect the smallest pyramid in the
universe--would be only a few fermis (10^-15 m) on a side and
millions of times smaller in volume than methane molecules.  Just as
there are so-called "magic" nuclei with just the right number of
neutrons and protons that readily form stable spherical nuclei, so
there are expected to be such magic numbers for forming pyramid
nuclei too.  Stable, in this case, means that the state persists for
10^12 to 10^14 times longer than the typical timescale for nuclear
reactions, namely 10^-21 seconds.
Dudek says that gadolinium-156 and ytterbium-160 are nuclei very
conducive to residing in a stable pyramid configuration.  Nuclei
might exist also in stable octahedral (diamond) forms also.  These
nuclei would all possess a quantum property not seen before in
nuclei: in the process of filling out an energy-level diagram for
the nucleus, four nucleons of the same kind (neutrons or protons)
could share a single energy level instead of the customary one or
two permitted nucleons.  This rule-of-four would inhibit the
normally observed decay patterns by which non-spherical nuclei throw
off energy, usually by emitting gamma rays.  In fact, in the case of
nuclear pyramids it is expected to result in new and unprecedented
decay rules.  This inhibition would explain the puzzling results of
earlier experiments.  Dudek (Jerzy.Dudek@IReS.in2p3.fr,
33-388-10-6498) and his colleagues plan to test these ideas in
upcoming experiments.  (Dudek et al., Physical Review Letters, 18
August 2006)

HIGH-FLUX, SHORT-BURST GAMMA SOURCE.  Bursts of gamma rays (the most energetic
form of light) can be made by scattering laser light from an electron beam. 
Present examples--including those at the SPring 8 machine in Japan and at
Brookhaven in the US--deliver relatively long gamma pulses (greater than 100
picoseconds in duration) with relatively low brightness (yielding about a
million gamma per second).  A new proposal shows how a gamma source with pulses
as short as 100 femtoseconds and fluxes as high as a billion per second
could be built.  One of the most important things you can do with a
bright stream of gammas is to pass them through a thin target where
the gammas can generate electron-positron pairs.  From this process,
the positrons can be skimmed and, owing to their ability to probe
certain processes within materials that cannot be probed by x rays,
be used to study such things as defects in bulk metals.  Basically,
the positrons render valuable clues about a material sample
(structural and magnetic) by taking up positions throughout the
sample, where the positrons meet and annihilate with electrons,
creating telltale radiation.  One of the researchers, Yuelin Lin
(ylli@aps.anl.gov ), says that because the positron bursts are so
short (as short as a trillionth of second long), they can be used to
make slow-motion movies of ephemeral and hard-to-watch activities
like metals melting at high temperatures.  (Li et al., Applied
Physics Letters, 10 July 2006)

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 * Origin: Big Bang (1:106/2000.7)