Probing the positron moderation process using high-intensity, highly polarized slow-positron beams

Abstract

We have generated a highly polarized ($P=0.48\ifmmode\pm\else\textpm\fi{}0.02$) intense (5\ifmmode\times\else\texttimes\fi{}${10}^{5}$/sec) beam of "slow" ($\ensuremath{\Delta}E\ensuremath{\sim}2$ ev) positrons (${e}^{+}$) and shown that it is possible to achieve polarization as high as $P=0.69\ifmmode\pm\else\textpm\fi{}0.04$ with reduced intensity. The measured polarization of the slow ${e}^{+}$ emitted by five different positron moderators showed no dependence on the moderator atomic number (2s$Z$). This allows us to conclude that only source positrons with final kinetic energy below 17 keV contribute to the slow-${e}^{+}$ beam, in disagreement with recent yield functions derived from low-energy measurements. Measurements of polarization and yield with absorbers of different $Z$ between the source and moderator show the effects of the energy and angular distributions of the source positrons on $P$. The depolarization of fast positrons transmitted through high-$Z$ absorbers has been measured. Applications of polarized slow- ${e}^{+}$ beams are discussed.