Physics News Update
The American Institute of Physics Bulletin of Physics News
Number 311, March 13, 1997 by Phillip F. Schewe and Ben Stein
EXPLOSIONS OF ATOM CLUSTERS YIELD HIGH ENERGIES . Femtosecond lasers
can be used to convert chemical energy into kinetic energy with great pyrotechnic
effect. For example, they can blow up molecules, imparting a kinetic energy
of 100 eV to individual outgoing ions. Aiming fsec pulses at a solid can
produce ions with keV energies. Now scientists at Imperial College (London)
have observed much higher energy ions (up to 1 MeV) flying away from the
miniature fireball caused by shooting ultrashort (150 fsec), high-intensity
(2 x 1016W/cm2) laser pulses at clusters of xenon
atoms. It is not yet understood why clusters explode so much more violently
than molecules. The researchers look on their explosions as a novel and
modest way of achieving high-temperature plasmas in a gas of clusters.
They point to the possibility of tabletop fusion experiments. (T. Ditmire
et al., Nature, 6 March 1997.)
ARE STANDARD SOLAR MODELS RELIABLE?Yes, says John Bahcall of the Institute
for Advanced Study. Fewer than expected solar neutrinos register in terrestrial
detectors. This solar neutrino problem is usually attributed to the hypothetical
transmutation of neutrinos from one type into another en route from sun
to earth. One alternative is to propose that helium-3 from the cooler outer
layer of the sun sinks to the lower, warmer depths, thus moderating neutrino
production (Update
295). This modification in the model of the sun is undesirable, Bahcall
believes (Bahcall et al.,Physical
Review Letters, 13 Jan. 1997). He cites recent helioseismic measurements
of the velocity of sound waves inside the sun; the velocity in turn depends
on the ratio of the temperature to the mean molecular weight at any particular
depth inside the sun. Bahcall's analysis finds a very good agreement (for
all depths in the sun) between the measured values of sound velocity and
the values predicted using the standard solar model, and a much poorer
agreement for models including helium-mixing. (Physics
Today, March 1997.)
DNA CHIPS are silicon- or glass-based surfaces which are split up into
regions onto which have been deposited (by, for example, lithographic or
ink-jet-printing techniques) known sequences of DNA nucleotides (adenine,
cytosine, guanine, and thymine) corresponding to those in genes of specific
interest, such as the mutation-prone gene p53. The aim is that this DNA
probe sequence should combine with a complementary sequence (an A nucleotide
always binding with T, C always binding with G, etc.) of single strands
of DNA from a sample injected onto the chip. These strands can be tagged
with fluorescent chemicals so that they light up after being combined with
the appropriate probe sequence positioned on the test bed. Thus, like a
forensic comparison of fingerprints, the optical matchup of known and unknown
genetic sequences can be performed in a methodical way and can rapidly
detect such things as gene mutations in a sample of cells. And from this
one can study genetically-based diseases. Researchers at Affymetrix, a
company in California, have made chips with up to a million different chemical
sequences. Scientists foresee the possibility of using several, or even
single, DNA chips to detect mutations in perhaps all 100,000 human genes.
For simpler organisms such as yeast, a DNA chip coded for the entire genome
(6000 in this case) will be available soon. (Science
News, March 8, 1997; San
Jose Mercury News, November
26, 1996.)
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