Science > Physics > PHYSICS NEWS UPDATE -- Number 796 October 11, 2006 by Phillip F.Schewe, Ben Stein and Davide Castelvecchi
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PHYSICS NEWS UPDATE -- Number 796 October 11, 2006 by Phillip F.Schewe, Ben Stein and Davide Castelvecchi |
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 796 October 11, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu
FIRST ANTIMATTER CHEMISTRY. The Athena collaboration, an
experimental group working at the CERN lab in Geneva, has measured
chemical reactions involving antiprotonic hydrogen, a bound object
consisting of a negatively charged antiproton paired with a
positively charged proton. This composite object, which can also be
called protonium, eventually annihilates itself, creating an even
number of telltale charged pions. Normally the annihilation comes
about in a trillionth of a second, but in the Athena apparatus (and
its very thorough vacuum conditions) the duration is a whopping
millionth of a second. The protonium comes about in the following
way. First, antiprotons are created in CERN’s proton synchrotron by
smashing protons into a thin target. The resultant antiprotons then
undergo the deceleration, from 97% down to 10% the speed of light.
Several more stages of cooling, including immersion in a bath of
slow electrons, brings the antiprotons to a point where they can be
caught in Athena’s electrostatic trap. This allows the researchers
to study then, for the first time, a chemical reaction between the
simplest antimatter ion---the antiproton---and the simplest matter
molecular ion, namely H2+ (two H atoms with one electron missing).
Joining these two ions results in the protonium plus a neutral
hydrogen atom (see figure at http://www.aip.org/png/2006/269.htm ).
This represents the first antimatter-matter chemistry, if you don’t
count the interaction of positrons (anti-electrons) with ordinary
matter. (Previously antiprotons have been inserted into helium atoms
but this did not really constitute “chemistry” since the antiprotons
merely replaced an electron in the helium atom.)
According to Nicola Zurlo of the Universita’ di Brescia
(zurlo@bs.infn.it) and her colleagues, the experimental output from
the eventual protonium annihilation (see depiction at
www.aip.org/png) allowed the Athena scientists to deduce that the
principal quantum number (denoted by the letter n) of the protonium
had an average value of 70 rather than the expected value of 30.
Furthermore, the angular momentum of the protonium was typically
much lower than expected---perhaps because of the low relative
velocity at which the matter and antimatter ions approached each
other before reaction. The Athena scientists hope to perform more
detailed spectroscopy on their proton-antiproton “atom”in addition
to the already scheduled spectroscopy of trapped anti-hydrogen
atoms, which consist of antiprotons wedded to positrons. (Zurlo et
al., Physical Review Letters, 13 October 2006 lab website at
http://athena.web.cern.ch/athena/ )
URANIUM BEAM-PUMPED UV LASER. Lasers consist of an active medium of
excitable atoms, a pumping mechanism for exciting those atoms, and a
cavity for building up a pulse of coherent radiation. At the GSI
lab in Darmstadt, Germany, scientists have succeeded for the first
time in using a beam of uranium ions as the pump for producing UV
laser light. It works like this: the U beam ionizes argon atoms,
which ionize krypton atoms, which in turn form excited molecules
with fluorine. The KrF molecules are the excited entities which
emit coherent light at a wavelength of 248 nm. A laser that uses
this rare gas-halide mixture is called an excimer (excited dimer)
laser. This is not the shortest laser wavelength ever achieved and
the uranium pumping scheme is not all that energy efficient. So why
then use this approach to producing laser light, especially when
electrically pumped commercial KrF lasers are available? Because
this was a test run for producing laser light in excimers that can’t
be electrically pumped. According to Andreas Ulrich of the
Technische Universitat Munchen (andreas.ulrich@ph.tum.de), the goal
is to excite excimers of pure rare gases for producing radiation in
the VUV (vacuum ultraviolet) and soft x-ray region of the spectrum.
Only now have uranium beams at GSI been powerful enough to provide
the pumping power for lasers in this wavelength region. Being so
heavy, uranium atoms deposit their energy into a gas much more
efficiently that lighter particles such as electrons. (Ulrich et
al., Physical Review Letters, 13 October 2006)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
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