Number 287, September 20, 1996 by Phillip F. Schewe and Ben Stein
THE QUEST FOR ANTIHYDROGEN continues. Last year in a sort of shotgun
wedding the first antihydrogen atoms made in a lab (at CERN) came about
when a few antiprotons were hitched to positrons at the collision point
between a jet of xenon gas and an antiproton beam. However, there was no
chance to study the private lives of the newlyweds because they quickly
annihilated against a nearby wall. To create, trap, and examine anti-H
at leisure will take patience and finesse. The anti-ingredients for making
anti-H are of course anti-protons and positrons. These species have separately
been cooled and trapped. To herd both species simultaneously is more difficult,
however, since traps designed for both positively and negatively charged
particles are usually leaky. As a prelude to making anti-H atoms, Harvard
physicists Gerald Gabrielse (gabrielse@hussle.harvard.edu) and David Hall
have spent some time working with protons and electrons, seeing if they
can get some of these humdrum particles to combine into ordinary H atoms
(which they refer to as "anti- anti-hydrogen"). Recently they
succeeded in trapping both electrons and protons in a single device which
consists of one Penning trap (which uses a combination of electric and
magnetic fields) for the negative particles nested inside another for the
positive particles. Outnumbering the protons by a factor of thousands to
one, the electrons help to cool the protons to a common temperature of
only 4 K. Why so cold? Estimates have shown that the likelihood of one
important reaction---in which two positrons and an antiproton form an anti-H
plus an extra positron to carry off surplus energy--- improves by 8 orders
of magnitude if the temperature is reduced from 300 to 4 K. Besides cooling
protons, did Gabrielse make any hydrogen atoms as a dry run for making
anti-H atoms later on? Detecting the neutral H atoms, Gabrielse says, is
quite difficult. Knowing you have anti-H atoms in the trap will actually
be much easier since some of them can be allowed to escape, whereupon they
spectacularly annihilate with any ordinary matter in their path. The next
step is to try his new trap with positrons, which he obtains from radioactive
decays, and antiprotons from the Low Energy Antiproton Ring at CERN. The
Harvard trap may be the final resting place of the very last antiprotons
from LEAR, which will permanently shut down at the end of the year. (D.S
Hall and G. Gabrielse, Physical Review Letters, 2 September 1996.)
DO MANY PHYSICS POSTDOCS CONSIDER THEMSELVES "UNDEREMPLOYED"?
No, unless their appointments extend beyond three years, according to a
new report from the AIP Education and Employment Statistics Division. Within
6 months of graduation, 63 percent of all U.S. physics PhDs in 1994 held
postdoctoral appointments, which are temporary positions that provide continued
education and training in research. Approximately 30 percent of fourth-year
postdocs classified themselves as "underemployed"; the figure
is less than 5 percent for first-year postdocs. (The 213 postdocs who responded
to the AIP survey were left to define the term "underemployment"
for themselves). But postdocs at all stages responded that their physics
education is being put to good use: over 95 percent of first year postdocs
and 80 percent of the fourth-year postdocs responded that they consider
their current jobs "professionally challenging." (For more information,
and a free copy of the report, contact Raymond Chu of AIP, rchu@aip.acp.org,
301-209-3069.)
|
