Number 269, May 6, 1996 by Phillip F. Schewe and Ben Stein
THE LIQUID SCINTILLATOR NEUTRINO DETECTOR (LSND) collaboration at Los
Alamos looks for neutrino oscillations in the following way. First a beam
of pi mesons is created by smashing protons into a water target. The mesons
eventually decay into a variety of neutrinos, electrons, and muons. Contriving
to exclude all electron antineutrinos from the vicinity, the LSND researchers
surmise that most electron antineutrinos that actually turn up in their
detector must be coming from the transformation (oscillation) of a muon
antineutrino. The LSND team has now analyzed twice as much data as was
contained in their publication of a year ago. If neutrino oscillations
were not occurring, one would expect to see 5 or 6 electron antineutrino
scattering events, heralded by the creation of a positron and a neutron.
According to Fred Federspiel of Los Alamos, who reported new results at
the American Physical Society meeting last week in Indianapolis, the researchers
record 22 events (compared to 9 events last year). Federspiel asserted
that this new larger excess of events above background constituted "strong
evidence for neutrino oscillation."
THE WORLD'S RAREST ELEMENT has successfully been trapped, setting the
stage for high-precision tabletop measurements on how the weak nuclear
force manifests itself at the atomic level. Francium is the least stable
of the first 103 elements on the periodic table; less than an ounce of
it exists on the Earth at any one time. Creating francium artificially
has not been a problem; however, it has been a major challenge to trap
francium atoms and study them. In work described at last week's APS Meeting,
researchers at SUNY-Stony Brook (Luis Orozco, lorozco@ccmail.sunysb.edu)
produce a million ions per second of francium-210 (half-life=3 minutes)
at their accelerator. After converting the ions into neutral atoms and
slowing them down considerably, they send the francium into a magneto-optical
trap, a device employing six laser beams and a nonuniform magnetic field.
Inside the traps, the atoms bounce back and forth between specially coated
glass walls, slowing down some atoms enough to be caught at the center
of the trap. With their setup, the researchers can confine approximately
10,000 francium atoms at a time. Researchers at Stony Brook, Berkeley,
and elsewhere have previously used magneto-optic traps to collect radioactive
atoms, but a challenge with francium has been to figure out how to tune
the trapping lasers since there are no known stable isotopes of francium
to use as a reference. Future studies of the 7S-8S francium energy transition,
forbidden by the electromagnetic interaction because it violates parity
but permitted by the parity-violating weak interaction, could then be used
to gain precise information on the weak force. The effects of parity violation
are at least 18 times more pronounced in francium than in cesium, another
atom in which parity violation has been studied. (See also J.E. Simsarian
et al, Physical Review Letters, 6 May 1996.)
HOLOGRAM TEMPLATE FOR ATOMS . Physicists at NEC (Japan) and the University
of Tokyo have invented a rudimentary form of lithography using atoms instead
of light waves to produce an image. The researchers reconstruct a desired
pattern at a detector by using a computer-generated hologram (essentially
the Fourier transform of the pattern recorded in a silicon nitride membrane)
to manipulate a beam of cold neon atoms. Cold enough to act as waves (with
a quantum wavelength of 7.1 nm), the atoms were diffracted at the hologram
and deposited onto a fluorescent plate. (J. Fujita et al., Nature, 25 April
1996.)
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