Number 647, July 23, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
The Proton has a Different Size in Different
Nuclei
The electron, which is mostly impervious to the nuclear forces, can
penetrate deep inside a nucleus. Therefore, scattering high energy electrons
from a nucleus is an excellent way of exploring the electric and magnetic
properties of the nucleus as a whole and of its constituent protons
and neutrons, especially when the electron transfers some of its spin
to a proton in a telltale way. For example, recent results from such
an experiment, conducted at the Jefferson Lab, gave evidence that the
proton is not necessarily spherical. Now a new experiment at Jlab, comparing
electrons scattering from single protons (a hydrogen nucleus) with electron
scattering from helium nuclei, suggests that each nucleus "kneads"
its protons in a different way (see
figure). The kneading allows the constituent quarks inside the proton
to spread out a bit at time, perhaps into a peanut shape, even though
its average shape is round. (Strauch et al., Physical
Review Letters, upcoming article)
NMR Without the Magnet or RF Coils
To image an object's interior with nuclear magnetic resonance (NMR)
a magnetic field of several tesla (1 T =10,000 gauss) is usually required
to polarize protons in the sample and then radio waves are used to tip
the protons and to detect a weak signal as they upright themselves again.
The strength of the signal depends on the size of the magnetic field
and the degree of polarization, which is often only one part in 105,
and somewhat limits the use of NMR (including its medical application,
MRI) because of the need for a bulky, expensive magnet. One way of improving
things is to use laser light to produce a polarization as high as 10%
in a gas of xenon atoms. The Xe atoms can then be injected into an empty
space, such as lungs, and used to image their interior, which couldn't
be done using conventional NMR (see Update
398). Another NMR advance has been the use of ultrasensitive SQUID
detectors for picking up the magnetic fields produced by protons, greatly
reducing the need for large magnets (see Update
528) but at the expense of weak signals, with a proton polarization
of only one part in 108.
Now, Princeton physicist Michael Romalis and co-workers, while studying
whether the Xe nucleus is slightly nonspherical (equivalent to saying
that the nucleus possesses a nonzero electric dipole moment, which would
imply the existence of "new physics" beyond the Standard Model),
have worked out a way to combine different techniques to obtain a strong
NMR signal in a very weak 1 micro-tesla magnetic field. They transfer
polarization from laser-polarized Xe to protons in an organic liquid
and then use SQUID detectors to measure the magnetic field produced
by the polarized protons. Romalis
(609-258-5586) expects that this low-field NMR technique would work
for any sample---whether liquid, surface, or biological tissue---with
good solubility for xenon. (Heckman et al., Physical
Review Letters, upcoming article; see also Princeton website)
Milling Diamond Films
Milling diamond films can be performed with gallium beams. Diamond
films, created by first installing tiny diamonds in a pitted silicon
surface and then laying down subsequent atoms to form a near-planar
diamond surface, have many of the electrical properties of semiconductors,
but can operate at much higher temperatures, voltages, and power. Because
of its resistance to hostile environments and its bio-compatibility,
diamond films are also expected to be act as handy protective coatings
in microfluidic research Because of its hardness, however, diamond films
are difficult to sculpt through micromachining, during which stresses
on the sample can crack the film. Now scientists at the Nanyang Technological
University in Singapore have devised a versatile way of making possible
micro-optical elements out of diamond films by wielding a carefully
focused gallium ion beam. Optical tests of the resultant structures
show that such properties as transmission and index of refraction were
not distorted by the milling process. By the way, this research was
undertaken as part of the Singapore-MIT Alliance, an innovative engineering
education and research collaboration established in 1998 among three
top engineering research universities: National University of Singapore
(NUS), Nanyang Technological University (NTU), and Massachusetts Institute
of Technology (MIT). (Fu
et al., Review of Scientific Instruments, August 2003)
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