Design of a high efficiency pulsed slow positron beam for measurements of porous silicon and polymer films

Abstract

This document presents the design of a pulsed slow positron beam to study porous silicon and polymer films. Slow positrons are produced with a conven- tional technique using a ^22Na source and a tungsten film moderator. The goal of the proposed design is to reach a pulse duration of 0.3 ns FWHM at the target. The scheme of the beam includes two choppers and two bunchers. The distinctive features of the proposed design lie in the high compression ratio of positron pulses from an initial duration of 120 ns to a final width of 0.3 ns and in the versatility of the operation mode, which offers the possibility of re- tuning the operation parameters in order to adjust the pulse width and the interval between pulses. A high compression ratio (400) is achieved thanks to an optimization of the voltage pulse shapes applied to the buncher drift tubes. This pulse shape is generated by a two-channel arbitrary waveform generator followed by a wideband amplifier. This design eliminates nonlinear aberrations in the bunching process and allows a compression ratio limited only by the in- trinsic energy spread of the initial positron beam. The design of the apparatus allows to tune the time interval between pulses from 220 ns to infinity allowing to measure positron annihilation lifetime spectra, e.g. from porous silicon.