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{at}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{at}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{at}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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