Number 710, November 24, 2004
by Phil Schewe and 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 http://asa.aip.org/web2/asa/abstracts/search.nov04/asa283.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.
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