Phys. Rev. Lett. 106, 023401 (2011)

Positronium is a short-lived exotic atom that contains an electron and its antiparticle, a positron, bound together. It has a lifetime of about 140 ns and decays into two gamma rays when the electron and positron annihilate each other. One way to make positronium is to direct a beam of positrons at a porous material: the positrons capture electrons from the material and the resulting positronium atoms diffuse into the pores, where they remain trapped until they decay. Physicists are interested in improved methods for the production of positronium because it can be used in a wide range of fundamental experiments, and also as a stepping stone for the production of antihydrogen beams. However, measurements on the positronium can also tell us more about the porous material used to make these exotic atoms in the first place.

David Cassidy and colleagues at the University of California Riverside and San Diego State University have now used laser spectroscopy to measure the 1s–2p transition in positronium atoms trapped in nanoporous silicon. They find that trapping the atoms reduces the linewidth of this transition by a factor of about two compared with measurements made in vacuum, and also increases the energy separation of the 1s and 2p levels. The results are consistent with a pore size of about 5 nm in the silica. The AEGIS collaboration at CERN has also explored positronium in a variety of other porous materials including Vycor, metal organic frameworks and aerogels (J. Phys. Conf. Ser. 199, 012009; 2010).