Number 125, April 22, 1993 by Phillip F. Schewe and Ben Stein
NEW MEASUREMENTS OF ATOMIC MASSES with 20 to 1000 times the precision
of previous values have been made by an MIT team. Masses of hydrogen, deuterium,
oxygen, neon, and argon have been measured at precision levels of around
100 parts per trillion. The researchers made their measurements using Penning
trap, a device in which an isolated ion's cyclotron motion in a magnetic
field is compared to that of a reference ion. The ratio of the cyclotron
frequencies determines their relative masses. These are then converted
to a scale based on the carbon-12 mass. David Pritchard (617-253-6812)
of MIT announced his values for the atomic masses at the APS April meeting,
where he also mentioned that he hoped to "weigh" the binding
energies of chemical bonds; but this requires a tenfold improvement in
precision. Pritchard believes an improvement of this magnitude entails
the development of a technique in which the two ions occupy the Penning
trap simultaneously. Robert Van Dyck (206-685-1097) of the University of
Washington reported that Penning trap measurements of the mass difference
between helium-3 and tritium gives a value of 18,590.1 eV with an uncertainty
of 1.7 eV. This measurement provides a systematic check of tritium beta
decay experiments investigating the possibility of a nonzero neutrino mass.
ASTROPHYSICAL GAMMA RAY BURSTS COME IN TWO FORMS , according to Don Lamb
of the University of Chicago who examined 200 of the 600 or so bursts recorded
so far by the Gamma Ray Observatory. In one class (25-33% of the sample)
the bursts are relatively short (less than 10 seconds), faint, rapidly
varying in intensity, have relatively "hard" spectra (much of
their energy is above 300 keV), and are uniformly distributed in space.
For the other class, the bursts are faint and bright, long (between .3
and 1000 seconds in duration), softer, less variable in intensity, and
not uniform in distribution. Lamb said that he expected his new classification
scheme to help in the effort to determine the origin of the mysterious
bursts, none of which has yet been correlated with a known object in the
sky. (APS meeting.)
NEUTRAL CURRENTS IN NUCLEI have been detected by the KARMEN collaboration
using neutrinos from the ISIS neutron source in the UK. At the APS April
Meeting, researchers announced that they had observed the excitation of
the carbon-12 nucleus through inelastic neutrino scattering. Such scattering
events, occurring as a result of the weak force, involve the exchange of
neutral Z bosons, which constitute a "neutral current." Previously
detected only in subatomic particle interactions, neutral-current reactions
in nuclei may have very practical applications in neutrino detection schemes;
the carbon-12 reaction, for example, could be triggered by muon as well
as electron neutrinos arriving from a supernova. Neutral-current detection
schemes based on other nuclei could be used to measure the total neutrino
flux from the sun. (For additional information, contact Dr. Guido Drexlin
of the KARMEN Collaboration at 011-49-7247-82-3534.)
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