From: LARRY KLAES (ljk4@msn.com)
Date: Thu May 02 2002 - 20:56:20 PDT
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Sent: Thursday, May 02, 2002 11:00 PM
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 587 April 30, 2002 by Phillip F. Schewe, Ben Stein, and
James Riordon
POINT SOURCES FOR EXTREME COSMIC RAYS? The
1930s and 40s was a golden age for cosmic rays; positrons, pions,
and muons were first discovered in those years not at accelerators
but in emulsions exposed to incoming cosmic ray showers. It
could be argued that we now live in another cosmic ray golden
age: fields of detectors spread out over many square miles on the
Earth's surface have piled up an impressive bank of events with
reconstructed shower energies stretching above 10^20 eV. But
how does the great accelerator in the sky work? Are the highest
energy cosmic rays coming from inside or outside the Milky Way?
Can we pinpoint specific sources or is the generation of the rays
amorphous? Why are the energies so high? A session at last
week's AAS/APS meeting in Albuquerque addressed these issues.
Masahiro Teshima of the University of Tokyo reported on a
study of 59 events with energies greater than 4 x 10^19 eV, as
recorded by the Akeno Giant Air Shower Array (AGASA) in
Japan. Teshima pointed to a modest but unmistakable clustering
in the form of five doublet and even one triplet alignment. That is,
in five cases pairs of energetic events had come from the same
place in the sky, while in once case three different events
originated at the same place. No identification has yet been made
of specific celestial objects with those coordinates. Teshima said
that the number of doublets or triplets might be even higher if one
could properly model the Milky Way's magnetic field (the galactic
equivalent of Earth's magnetosphere) and thus take into account
how the trajectories of cosmic rays were distorted on their way
toward Earth.
HYDROGEN AT EXTREMELY HIGH PRESSURES, upwards
of a million times that on the Earth's surface, can now be produced
in physics laboratories. Understanding hydrogen's behavior under
such extreme conditions answers questions about the interior of
Jupiter, provides coveted information on designing optimal fuel
pellets for fusion energy, and yields information on aging nuclear
weapons without having to test them. Reporting at the
Albuquerque meeting, two national labs are producing seemingly
contradictory high-pressure data on the universe's most abundant
element. Using Sandia's Z machine, which runs tremendous
amounts of electric current to generate very high magnetic fields,
researchers (Marcus Knudson, 505-845-7796,
mdknuds@sandia.gov) launch a metal plate that travels at high
speeds (up to 28 km/s, making it the fastest gun in the world)
towards a target containing low-temperature deuterium molecules
(D2). The impact of the plate launches a shock wave that
compresses D2 to up to megabars of pressure. Deuterium, a
neutron-containing isotope of hydrogen, is used because its higher
density enables it to be compressed to much higher pressures than
ordinary hydrogen. The Livermore experiments, on the other
hand, used the high-power (and recently decommissioned) Nova
laser to shock compress liquid D2.
The Livermore researchers (Robert Cauble, 925-422-1174,
cauble@llnl.gov) find D2 to be much more compressible than do
the Sandia researchers. At a million atmospheres, for example,
Livermore finds the D2 to be compressed by a factor of 6 while
Sandia sees a compression of a factor of 4. If the Livermore
results are correct, then there is more metallic hydrogen in Jupiter's
interior than previously thought and it is easier than expected to
trigger self-sustaining nuclear fusion in deuterium fuel pellets,
since they would be more compressible. If the Sandia results are
right, then more traditional assumptions hold. But it's also
possible, Cauble says, that both results are right (each group's
compression occurs in slightly different time scales). As a final
possibility, Cauble and Knudson admit, both results could be
wrong (they are both relatively new techniques). These
possibilities are being carefully explored in conjunction with
computer simulations of high-pressure hydrogen, which require the
fastest available computers in the world.
The question is likely to be settled with further experimental
research, including more data from Sandia and future laser
experiments, possibly occurring at Rochester's Omega facility.
The ultimate goal of these experiments is to determine hydrogen's
equation of state, the interrelationship between such properties as
its pressure and temperature, at these high-pressure conditions.
Such information can provide information on such things as the
intriguing possibility that gas-giant Jupiter has a solid-rock core.
A NEW OPTICAL TECHNIQUE FOR IDENTIFYING DNA
BASES has been demonstrated by Marquette University
researchers (Troy Alexander and Chieu Tran, chieu.tran@mu.edu,
414-288-5428), offering prospects for reducing errors in DNA
sequencing techniques. To determine a sequence of genetic code,
workers traditionally break a DNA strand into smaller fragments,
and attach a different dye to each of the four bases (adenine,
guanine, cytosine, guanine), the four letters of genetic code. When
exposed to light, each kind of dye fluoresces a different range of
colors, or wavelengths. However, dye molecules by their very
nature are broadband. Since molecules can rotate and vibrate in
many different ways, they emit a wide range of colors that often
overlap with those of other dyes. In turn, the overlap means that
one DNA base will sometimes be mistaken for another when DNA
sequences are read. To address this problem, the Marquette
researchers designed a special pair of dyes whose colors do not
overlap. One of the dyes exhibits two-photon fluorescence: it must
be excited by two photons before it releases a photon. The other
exhibits ordinary one-photon fluorescence, which emits a much
different, higher-wavelength set of colors than the other dye. Tran
believes it will be possible to design four dyes for DNA
sequencing, in which two dyes exhibit one-photon fluorescence,
and the other two exhibit two-photon fluorescence. Generating
two-photon fluorescence traditionally requires expensive high-
powered lasers that deliver continuous bursts of light, but Tran
says that a new generation of pulsed lasers delivers peak powers
that would produce two-photon fluorescence less expensively. In
other efforts to bring this technique closer to real-world sequencing
applications, the Marquette team is also simplifying their system
for detecting one- and two-photon fluorescence signals
simultaneously. (Alexander and Tran, Applied Optics, April 20)
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