Number 554, August 30, 2001
by Phil Schewe, James Riordon, and Ben Stein
Evidence for the Onset of Quark Effects
Evidence for the onset of quark effects in a nuclear reaction has been
observed for the first time. When a particle strikes a nucleus at low
energies, one can effectively describe the resulting behavior of the
nucleus in terms of its constituent nucleons (neutrons and protons)
and the mesons which hold them together. At low energies, one does not
have to worry about the fact that each nucleon is itself made of three
quarks held together by gluons.
When a particle strikes a nucleus at high energies, however, it penetrates
the nucleus so deeply that this "effective theory" breaks
down, and one must describe the nuclear action in terms of only quarks
and gluons. There is a middle ground, alas, where neither descriptive
picture can do the job completely.
Just as urbanologists strive to locate where a city truly ends and
its suburbs begin, physicists wish to find the boundary at which nucleon-based
descriptions give way to quark-based ones. Towards this end, researchers
study the behavior of the deuteron, the simplest nucleus, made of a
proton and a neutron bound together.
In experiments at Jefferson Lab in Virginia, a multi-institutional
collaboration (Elaine Schulte, Argonne National Lab, 630-252-4032, Schulte@mep.phy.anl.gov)
fired a high-energy electron beam at a copper target, which decelerated
the electrons, creating high-energy photons as a result. In a process
known as "photodisintegration," the photons impinged upon
a deuterium target, and broke apart deuterons into their constituent
protons and neutrons.
The researchers then studied the properties of protons emitted at various
angles from the collision. When the emitted proton has at least 1 GeV/c
of momentum perpendicular (transverse) to the incoming beam, the data
were best described by quark-counting rules, which take into account
the behavior of individual quarks.
The transverse momentum translates to an interaction with the nucleus
at a distance scale of 0.1 fermi (10-16 m), about a tenth
of the width of a proton. In this situation, an individual quark, rather
than the entire nucleon, absorbs the momentum of the collision. This
was surprising, since the 0.1-fermi distance scale is larger than many
current theoretical expectations for the onset of quark-counting-rule
behavior. (E.C.
Schulte et al., Physical Review Letters, 3 September
2001.)
Bronze Age Artifacts
Bronze age artifacts, physical links between us and people alive 3000
years ago, have long been closely examined with physics-based instruments
such as x-ray crystallography and mass spectrometry.
Now scrutiny of microchemical surface properties of such ancient bronze
in some respects surpasses the diagnostic information gained by previous
bulk-phase studies. Ernesto Paparazzo of the Instituto di Stuttura della
Materia in Rome (paparazzo@ism.rm.cnr.it, 39-06-4993-4153), and his
colleagues at the Pacific Northwest National Lab and Oxford, have looked
at an early-first-millennium BCE belt from Syria with scanning auger
microscopy (SAM), a process in which specific elements in a material
can be identified when electrons with characteristic energies are knocked
out of atoms by an incoming electron beam.
Bronze is an alloy of copper and tin---generally a mixture of about
85% copper and 10% tin, with minor amounts of other metals being also
possible (e.g., zinc, lead, etc.). Metals put back into the earth naturally
rust but unequally, leading in the case of bronze to "decuprification,"
that is, the disproportionate detachment of copper atoms from the bronze.
With their SAM device, the researchers have studied this process, and
have characterized the microchemistry of the bronze at a level of spatial
resolution as good as 15 nm, the best yet achieved for the analytical
study of an archeomaterial. They can inventory the invasion of silicates
into the alloy from surrounding soil during the burial phase and even
spot alloy inhomogeneities introduced by the smith during the manufacturing
phase. (Paparazzo et
al., Journal of Vacuum Science and Technology A, July 2001.)
Spiraling in on Nanosprings
Wrap a nanowire into a helix and what do you get? A nanospring of course.
Although wires tens of nanometers in diameter are not actually wrapped
to make springs, they are grown that way through a process known as
vapor-liquid-solid (VLS) growth mode. VLS growth occurs when a catalyst
droplet resting on a surface absorbs wire-building material from a surrounding
vapor. Once the concentration of the building material reaches super-saturation
in the droplet, a portion of the material is secreted out the droplet
base, and a wire gradually forms. Under some circumstances, the material
deposition is asymmetrical and the wire develops into a helical nanospring
(see image).
Until recently, the mechanism that leads to this asymmetry has been
unclear, but now researchers at the University of Idaho have proposed
a model that sheds new light on nanowire formation (Dave McIlroy, 208-885-6809,
dmcilroy@uidaho.edu). It seems that a small catalyst droplet, roughly
the same diameter as a growing nanowire, remains centered on top of
the wire, and the resulting growth is linear. If the droplet exceeds
the wire diameter, however, its balance atop the structure is precarious
and a small perturbation can bump the droplet to one side, abruptly
jolting the growth pattern from straight to helical. The researchers
confirmed the model by accurately predicting the growth conditions of
the world's first boron carbide nanosprings, which they produced in
their laboratory.
Nanosprings may someday make highly sensitive magnetic field detectors,
perhaps finding application in hard drive read heads. Alternatively,
nanosprings could serve as positioners, or even as tiny conventional
springs, for nanomachines of the future. (D.
N. McIlroy; D. Zhang; Y. Kranov; M. Grant Norton, Applied Physics
Letters, 3 September 2001.)
Enlarge text
Shrink text
Print
Digg this
Del.icio.us
Furl