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

We have generated a highly polarized ($P=0.48\mathrm{}\pm 0.02$) intense (5×${10}^5$/sec) beam of "slow" ($\Delta E\sim 2$ ev) positrons ($e^{+}$) and shown that it is possible to achieve polarization as high as $P=0.69\mathrm{}\pm 0.04$ 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.