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 from 22Na were injected periodically into a target by adjusting the time 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 polytetrafluoroethy- lene 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. q1999 Elsevier Science B.V. All rights reserved