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The Athena collaboration, an
experimental group working at the CERN laboratory 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 percent down to 10 percent 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 hydrogen atoms with one electron missing).
Joining these two ions results in the protonium plus a neutral
hydrogen atom (see figure).
 Antimatter chemistry takes place inside a Penning trap, an electrostatic trap whose electrodes (pink cylinders) hold charged particles nearly in place. Antiprotons, created in the CERN proton synchrotron and then slowed down, are further cooled when they enter the trap by sending them into a swarm of positrons. Then the antiprotons chemically interact with molecular hydrogen ions (H2+) to form neutral hydrogen atoms and protonium (which consists of a proton and antiproton bound together briefly). Later the antiprotons annihilate with the protons, before reaching the trap surface. Pic: aip.org.
According to Nicola Zurlo of the Università di Brescia and his colleagues, the experimental output from
the eventual protonium annihilation 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.
Source: AIP
Related Links:
Zurlo et al.,
Physical Review Letters, 13 October 2006
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