Development of a pulsed slow-positron beam using time-varying pulsing-bias

In order to study the near-surface characterization of polymers using posit ron-annihilation lifetime spectroscopy (PALS), a pulsed slow-positron beam system is under development. Positrons emitted hom Na-22 were injected peri odically into a target by adjusting the rime of flight of positrons between the moderator and the target using a time-varying electric field. By incre asing the slope of the pulsing bias, the time resolution of this system, a full width at half maximum (FWHM) of 0.82 ns has been achieved. The lifetim es of ortho-positronium (o-Ps) in high-density polyethylene (HDPE) and poly tetrafluoroethylene (PTFE) were obtained as 2.5 and 4.1 ns, respectively, b y PALS using our new system. These values agree with those obtained by the conventional PALS. (C) 1999 Elsevier Science B.V. All rights reserved.