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

We have generated a highly polarized (P=0.48±0.02) intense (5×{}10⁵/sec) beam of "slow" (ΔE~2 ev) positrons (e⁺) and shown that it is possible to achieve polarization as high as P=0.69±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 (2sZ). 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.