First Detailed Positronium Scattering Experiment

The lightest atom made of an electron and a positively charged mate is not hydrogen but positronium (abbreviated Ps), a bound electron-positron pair.

The lifetime for these no-nucleus atoms is hardly more than about 100 nanoseconds but, if things are expedited, this is long enough for doing an experiment. (The brief lifespan comes not from the intrinsic instability of the Ps "atom" but from the fact that the constituents will, left to themselves, annihilate each other.)

In recent years physicists have been able to gather Ps beams, made by sending a beam of positrons through a neutralizing gas, and have measured the total cross section (likelihood of scattering) for Ps scattering from various targets.

Now a team of scientists at University College London reports the first experiment in which a specific type of inelastic scattering takes place. In particular, the London researchers found that in many encounters with helium atoms, the Ps will split apart but that the fragmented partners continue to be highly correlated, moving through the lab with roughly the same velocities.

Learning more about this fragmentation process will aid proposed schemes for using Ps beams for studying material surfaces. Furthermore, Ps is unusual in that its centers of mass and charge coincide. This allows for interactions between the electron in the Ps and electrons in target atoms to be more potent than if the electron were yoked to a much heavier proton, as in a hydrogen atom. (Armitage et al., Physical Review Letters, 21 October 2002; contact Gaetana Laricchia, 44-20-7679-3470, g.laricchia@ucl.ac.uk)