Number 597, July 9, 2002
by Phil Schewe, James Riordon, and Ben Stein
Nanotube Diagnostic X-Rays
The design of the x-ray sources used in many medical and dental diagnostics
hasn't changed much in a hundred years. A cathode heats up to 1500 C
and then emits lots of electrons, which are pulled across a vacuum tube
toward a target where the ensuing impact kicks up a beam of x rays.
Now a team of physicists and doctors at the University of North Carolina
and the nearby firm of Applied Nanotechnologies, Inc. have created,
for the first time, an x-ray source using a room-temperature array of
carbon nanotubes to create the electrons and deliver a sufficient x-ray
flux for doing practical medical imaging (see figure).
The device is much smaller and cooler and the old models, and the resultant
x-ray pulse is more focused. Also response time is sharper and the pulse
shape can be programmed, which helps in the tracking of moving objects.
(Yue et al.,
Applied Physics Letters, 8 July 2002; contact Otto Zhou, 919-962-3297,
zhou@physics.unc.edu.)
Element 118 Retraction
In 1999, physicists at Lawrence Berkeley National Lab reported observing
three events amid high energy collisions in which it appeared that a
nucleus corresponding to element 118 had been produced; in each case
the nucleus had quickly decayed into daughter nuclei (Update 432).
Two years later these same researchers came to believe that their analysis
of the events, and therefore their claim for discovery of the element,
was doubtful. The (brief) official retraction appears
in the 15 July issue of Physical Review Letters.
Artificial Leaves
Artificial leaves, made from semiconductors, might one day help to
remove excess airborne carbon dioxide and maybe even turn it into fuel.
Real leaves, the green ones deployed by plants, perform many valuable
tasks, not the least being the removal of CO2 from air and
its replacement with breathable O2. Artificial CO2
fixation needs several ingredients: light, a catalyst (such as CdS),
and organic molecules. A new study by a Oak Ridge-Vanderbilt team of
physicists suggests how this process can be made more efficient, a necessary
step if artificial fixation is ever to practical on a large scale. Contrary
to previous ideas, the study shows, fixation does not take place directly
on the catalyst surface. Rather it's a two step process: ionization
of the CO2 occurs at the surface, creating a highly reactive
radical which can later combine with other CO2 molecules
or organic molecules in the vicinity. Stephen Pennycook (pennycooksj@ornl.gov,
865-574-5504) says that his study looks at the role of catalyst surface
roughness (flat planes of CdSe don't work as photocatalysts, but nanocrystals
of the same material do) and at the possibility that nanocrystal doping
might obviate the need for light, which would allow some fixation to
take place in dark smokestacks. (Wang et al., Physical Review
Letters, upcoming.)
Gamma Knife
Gamma knife is the name for a machine in which high energy gamma-rays
are used to irradiate intracranial tumor cells difficult to treat with
other methods. Acoustic neuroma, a tumor lodged in the vestibular nerve,
is an example. In the Boston Gamma Knife Center of Jen-San Tsai, Ph.D.,
at Tufts New England Medical Center of Boston an array of 201 gamma-emitting
cobalt-60 sources is laid out in such a way that the rays converge on
the target tumor, whose coordinates are carefully determined by CT and
MRI scans. The resultant noninvasive procedure, called stereotactic
radiosurgery, is in use at 66 facilities in North America, and 154 facilities
installed worldwide. At the meeting of the American Association of Physicists
in Medicine (AAPM) in Montreal this week, Tsai (jtsai@lifespan.org,
617-636-1681) reports
on new methods for coordinating MRI and CT scans to obtain the best
possible tumor location to insure proper dosages. (See AAPM
meeting site.)
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