The Mono-Energetic Positronium Beam and the Positronium Scattering from Surfaces.

Using the mono-energetic high intensity positron beam at Brookhaven National Laboratory as a variable energy positron source, a mono-energetic positronium (Ps) beam of variable (0-80 eV) energy and an angular width of 5 ^circ has been successfully constructed and used to conduct the Ps surface reflection and scattering measurements. The Ps atoms were produced by the positrons' charge-exchange collisions with Ar atoms, when a high intensity positron beam was magnetically guided into a differentially pumped gas cell of 10^ {-3} torr Ar. The Ps surface reflection and scattering measurements were performed on a cleaved LiF(100) surface and a cleaned Cu(100) surface. The Ps emission from both surfaces bombarded by low energy ( <=q150 eV) positrons was also measured to help us understand the Ps surface interaction and explore new applications of Ps emission in surface studies. Strong Ps specular reflection of 30% at energy below 7 eV and 1 ~ 3% up to 50 eV was observed from the LiF(100) surface with an angular width about the same as that of the Ps beam. The Ps scattering of a few percent was observed from the Cu(100) surface. Our measurements also showed Ps surface scattering was very sensitive to the surface condition. A theoretical model considering the surface potential reflection and the ion core scattering was used to understand our measurements. Our data analysis showed that there was relatively high intensity of Ps scattering by the ion cores on both surfaces, strongly suggesting the positronium surface diffraction can be observed in the future with a proper experimental set up.