A new electrostatic positron beam for surface studies

A new electrostatic positron beam has been constructed and tested. The energy range spans more than three decades, from less than 50 eV to more than 50 KeV. The machine can operate in a low-energy mode (50 eV to 2 keV) with a beam diameter (FWHM) from 2.9 to 1.7 mm or in a high-energy mode (1 keV to 50 keV) with a FWHM from 1.4 to 1.0 mm. The transverse spot position on the target is very stable, with wandering being much smaller than the beam diameter. The apparatus transmission function is very flat, with an intensity change of only 30% when going from 10 eV to 50 keV. The product of moderation and optical transmission efficiency is 1.7 × 10^-4, corresponding to 800-1000 e⁺ s^-1 with a 1 mCi ²²Na source. The apparatus shows low noise in S-parameter measurements. The background of the high-energy wing of the annihilation line is also one of the best reported. Optical and constructional details are given.