A continuous time-tagged positron beam and its application to materials research

Abstract

A time-tagged relativistic positron beam with very low time jitter (≅ 80 ps with the present setup) has been installed at the Stuttgart pelletron accelerator. A position-sensitive ß+ detector with good angular resolution and small e+ directional straggling supplies the start signal for positron lifetime measurements. The stop signal of the positron lifetime spectrometer based on this “positron clock” is obtained in the conventional way from one of the annihilation γ quanta. Compared with the conventional γγ lifetime-measurement technique, this ß+γ lifetime spectrometer does not only give a substantially better time resolution but also, owing to the ß+ detector efficiency unity, a much higher coincidence count rate. This will permit routine age-momentum-correlation (AMOC) measurements, using the second annihilation photon. The positron clock can handle very high positron beam fluxes such as they will become available in e+ factories. In the Stuttgart setup we hope to reach, by using solid rare-gas moderators in the accelerator terminal, positron fluxes of about 106e+/s. A comparison between the moderation efficiencies of tungsten and rare-gas moderators under the vacuum conditions of the accelerator terminal is presented. AMOC measurements allow us to obtain time-resolved information on the evolution of the positron states (e.g., trapping of positrons at defects or positronium formation). As a surprising effect, “giant” Doppler broadening of the annihilation quanta of “young” positrons in diamond has been observed by means of a beam-based AMOC technique.