Text 241, 83 rader
Skriven 2004-11-24 17:57:29 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 710
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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 710 November 24, 2004
by Phillip F. Schewe, Ben Stein
        
MERCATOR OF THE NUCLEAR WORLD.  The medieval alchemists tried in vain to create
new elements in their crucible-based experiments out of just a few ingredients
such as lead and mercury and some common acids.  In the 20th century nuclear
physicists not only finally succeeded in transmuting one element into another
but were able to create new elements.  A new experiment at the Gesellschaft fur
Schwerionenforschung (GSI) in Darmstadt does not create new elements (although
in previous experiments GSI discovered 6 elements: 107-112) but it has created
and analyzed the largest number of elements (from nitrogen up to uranium) and
the largest number of subsidiary isotopes (1400) ever seen in a single nuclear
research effort.  The only ingredients: uranium and hydrogen.  The crucible in
which the elements were warmed up: a particle accelerator.  The GSI physicists
did not, as you might guess, smash a beam of protons (bare hydrogen nuclei)
into a stationary uranium target but rather the other way around.  The reason
for slamming energetic U-238 nuclei into a stationary liquid-hydrogen target is
that fragment nuclei of all sizes, flying away from the collision point, don't
glom together (as they might if emerging from a uranium target) and,
furthermore, can be more accurately identified since they are free of bound
electrons whose electrical charge might confuse the task of measuring the
number of protons in the detected particle. What comes out of this meticulous
and comprehensive of nuclear experiment is a set of cross sections---each a
measure of the likelihood for creating that particular nuclide (that is, each
stable  element and its complement of isotopes, variations on the same nucleus
but containing differing numbers of neutrons).  The GSI work, in other words,
not only enumerates a chart of the nuclides (the sort of thing on the wall of
every nuclear lab in the world) but produces a chart of cross sections for
producing those nuclides in a collision (see figure at
http://www.aip.org/png/2004/228.htm).  This information is valuable for a
number of reasons: for planning a future accelerator of rare isotopes, for
studying how to break down nuclear waste in sub-critical reactors, and for
studying fundamental aspects of nuclear fission and nuclear viscosity.
(Armbruster et al., Physical Review Letters, 19 Nov 2004; lab website at
www-w2k.gsi.de/charms/; contact Karl-Heinz  Schmidt, k.h.schmidt@gsi.de)
                                        
DETECTING MEGASONIC BUBBLES ON COMPUTER CHIPS. In the multibillion-dollar
semiconductor industry, there has been no reliable way to monitor silicon
wafers as they undergo dozens of crucial "megasonic" cleaning steps, in which
the wafer is immersed in a liquid and blasted with very-high-frequency
(megahertz) sound waves.  By generating scrubbing bubbles in the liquid,
megasonic cleaning does an excellent job of removing impurities such as very
small particles.  However, the process (possibly through the action of
overzealous "killer bubbles") can inadvertently damage circuit components and
thereby reduce yields of computer chips.  Collateral damage from megasonic
cleaning only stands to worsen in the future as new processors shrink further:
for example, the new Apple Power Mac G5 has 90-nm features.  At last week's
meeting of the Acoustical Society of America in San Diego, Gary W. Ferrell
(gferrell@us.sez.com) of SEZ America, Inc., a Silicon Valley office of an
Austrian electronics firm, described a new optical probe for monitoring--and
potentially reducing--the side effects of megasonic cleaning.  Ferrell and
coworkers take advantage of the fact that megasonic cleaning generates
"multibubble sonoluminescence" (MBSL), the emission of light from multiple
bubbles as they collapse in the liquid. Therefore, the team has developed
"sonoluminescence imaging" which maps the location of the collapsing bubbles.
By comparing the location of the collapsed bubbles with optical images of
removed particles, they can currently monitor the removal of 100-nm-and-larger
objects in the chip. Already, they have used sonoluminescence imaging to
increase the efficiency of megasonic cleaning. With their new tool, the
researchers also aim to make megasonic cleaning more uniform throughout the
chip. Their optical probe is possibly the first practical application of
sonoluminescence, which up to now has resided primarily in the realm of basic
science. (Paper 2pPA6 at meeting; abstract at asa.aip.org/asasearch.html).

NOVEL QUASICRYSTAL FRICTION PROPERTIES.  Quasicrystals, solid materials
possessing an odd five-fold or ten-fold symmetry (making the ten-fold solid
partly periodic and partly aperiodic) and which form dodecahedral grains, seem
to present less friction than do many other materials.  For the past ten years
no explanation for this has been found; does it arise from some macroscopic
cause---hardness or surface chemistry, say---or from some fundamental property
related to the exotic quasicrystal structure.  J.Y. Park and his colleagues at
LBL and Ames Lab have looked at this issue by dragging a probe microscope
across a sample. At last week's AVS Science & Technology meeting in Anaheim,
Park reported finding was a highly anisotropic friction for his Al-Ni-Co
quasicrystal: low friction when sliding the probe in the aperiodic direction
and high friction when sliding along the periodic direction (jypark@lbl.gov,
see website at stm.lbl.gov/research/Quasicrystal/Quasicrystal.html).  (Paper
NS-WeA9, http://www2.avs.org/symposium/anaheim/pressroom/park.pdf )

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