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

In order to study the near-surface characterization of polymers using positron-annihilation lifetime spectroscopy (PALS), a pulsed slow-positron beam system is under development. Positrons emitted hom Na-22 were injected periodically into a target by adjusting the rime of flight of positrons between the moderator and the target using a time-varying electric field. By increasing 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 lifetimes of ortho-positronium (o-Ps) in high-density polyethylene (HDPE) and polytetrafluoroethylene (PTFE) were obtained as 2.5 and 4.1 ns, respectively, by PALS using our new system. These values agree with those obtained by the conventional PALS. (C) 1999 Elsevier Science B.V. All rights reserved.