First Antimatter Chemistry

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 at Physics News Graphics).

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 Università di Brescia (zurlo@bs.infn.it) and his colleagues, the experimental output from the eventual protonium annihilation (see the Physics News Graphics depiction) 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
Image at Physics News Graphics
Contact Nicola Zurlo
Università di Brescia
zurlo@bs.infn.it
Lab Web site