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 m ore than 50 keV. The machine can operate in a low-energy mode (50 eV t o 2 keV) with a beam diameter (FWHM) from 2.9 to 1.7 mm or in a high-e nergy mode (1 keV to 50 keV) with a FWHM from 1.4 to 1.0 mm. The trans verse spot position on the target is very stable, with wandering being much smaller than the beam diameter. The apparatus transmission funct ion 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 e fficiency is 1.7 x 10(-4), corresponding to 800-1000 e(+) s(-1) with a 1 mCi Na-22 source. The apparatus shows low noise in S-parameter meas urements. The background of the high-energy wing of the annihilation l ine is also one of the best reported. Optical and constructional detai ls are given.