POSITRONIUM SPECTROSCOPY AT A LINAC-BASED SLOW POSITRON SOURCE

The slow positron facility TEPOS at the Giessen electron LINAC (36 MeV , 120 muA) has been used to produce an intense beam of moderated posit rons which is magnetically guided over a distance of 9 m. At a transpo rtation energy of 100 eV about 10(6) slow e+/s could be extracted out of the magnetic field (0.01 T) and have been electrostatically focusse d inside a microwave guide. A small fraction of the positrons form pos itronium in the excited state n = 2. The spontaneous emission of Lyman -alpha photons (lambda = 243 nm) from the 2P-states is observed by a p hotomultiplier. Microwave induced fine-structure transitions 2S-3(1) - -> 2P-3(2,1,0) have been observed at 8617(2), 13010(3) and 18494(2) MH z by an increase of the Lyman-alpha counting rate. The present errors take into consideration only statistical contributions; systematic err ors in the same order of magnitude may originate from frequency depend ent variations of the microwave power. The observed linewidth exceeds the natural linewidth of 50 MHz by Doppler-effect and power broadening . Values around 100 MHz could be reached at the lowest applied power l evels.