PHYSICS NEWS UPDATE -- Number 710 November 24, 2004 by Phillip F. Schewe, Ben Stein

From: Sam Wormley (swormley1_at_mchsi.com)
Date: 11/24/04 

Date: Wed, 24 Nov 2004 16:17:07 GMT

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 http://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
http://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
http://stm.lbl.gov/research/Quasicrystal/Quasicrystal.html). (Paper
NS-WeA9, http://www2.avs.org/symposium/anaheim/pressroom/park.pdf )

***********
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