Positronium reflection

A high intensity positron beam has been used to create a beam of monoenergetic positronium (Ps) atoms. The Ps beam was directed at surfaces to measure the reflected fraction of Ps. The positron beam was based on 63Cu irradiated with thermal neutrons in the High Flux Beam Reactor at Brookhaven National Laboratory to obtain β+ emitting isotope 64Cu. Prepared as a single crystal the copper source also and guided magnetically to a differentially pumped gas cell filled with 10-3torr Ar. Positrons pick up electrons from the gas atoms to form a beam of Ps. The differential cross section is strongly forward directed. With the additional help of collimators a monoenergetic beam of Ps emanates from the cell. The Ps beam was scattered from a single crystal of LiF(100) and the reflected Ps fraction was observed as a function of the scattering angle as well as the Ps beam energy. A strong specularly reflected peak was observed. Below 8eV as much as 29% of the incident beam is reflected when the sample is heated to about 160C. At higher energies up to 56eV between 1% and 3% are reflected specularly. This thesis represents the first measurement of Ps reflection from a surface and is a major stepping stone towards the investigation of low energy Ps diffraction from ordered solid surfaces. Important new insights about surface structures can be expected from such a new probe