Number 178, May 10, 1994 by Phillip F. Schewe and Ben Stein
THE LATTICE SPACING IN SILICON (220) has been measured to be 192015.569
fm (1 fm is 10**-15 m) with a relative uncertainty of 3 x 10**-8. Because
silicon technology can provide the largest pure crystals with well-characterized
dimensions (the <220> notation refers to the particular direction
through which the crystal is viewed) a careful measurement of the spacing
between atoms can facilitate a count of the atoms in the crystal. This
in turn can be used to calculate a better value for the Avogadro constant,
equal to the number of atoms in a mole. Scientists at the University of
Torino in Italy, employing a combination of x-ray and optical interferometry
to measure the lattice spacing, believe their work will also lead to a
better determination of the wavelengths for thermal neutrons and for x
rays and gamma rays. (G. Basile et al., Physical Review Letters, 16 May
1994.)
HALTING AND REVERSING THE DESTRUCTION OF OZONE in the stratoshpere is
a problem of great importance: the less protective ozone there is, the
more harmful ultraviolet radiation reaches the Earth. Alfred Wong of UCLA
studies this problem using a 2.4-m ozone- filled chamber. When CFCl-3 molecules
are introduced and when an artificial source of sunlight starts to liberate
free chlorine, the density of ozone in the chamber begins to fall, mimicking
the catalytic destruction of ozone in the atmosphere. Wong's effort to
counteract this effect is to inject a current of negatively-charged oxygen
atoms and molecules. These attach themselves to the chlorine atoms which
can then be collected on a positively-charged surface. Thereafter the ambient
uncharged oxygen can reform as ozone now that it is spared the destructive
presence of chlorine. Wong believes that much more research is necessary
before a field test of this method can be attempted. He points out that
the charge-induced approach is quite different from proposed chemical means
for the regeneration of ozone since the negative charges are all recovered,
leaving behind no greenhouse-inducing substances. (A.Y. Wong et al., Phys.
Rev. Lett., 9 May.)
NEUTRAL ATOMS HAVE BEEN TRAPPED WITH MICROWAVES , paving the way for
new attempts to achieve Bose-Einstein condensation, a hypothesized state
of matter in which cold atoms are so densely packed together that they
all collapse into a single quantum state. A Harvard-NIST team (contact
Isaac Silvera, 617-495-9075) has trapped cesium atoms with microwaves,
and is ready to move on to atomic hydrogen, a highly promising candidate
for Bose-Einstein condensation. In their design, the magnetic component
of a microwave field traps atoms in their lowest-energy spin state. In
earlier magnetic traps for atomic hydrogen, the atoms were confined in
a high-energy spin state but could escape easily by dropping into their
lower-energy state, depleting the large concentration of atoms needed for
Bose-Einstein condensation to occur. (R.J.C. Spreeuw et al., Phys. Rev.
Lett., 16 May.)
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