A Laser-cooled Positron Plasma

Abstract

We present results on trapping and cooling of positrons in a Penning trap. Positrons from a 2 mCi 22 Na source travel along the axis of a 6 T magnet and through the trap after which they strike a Cu reflec-tion moderator crystal. Up to a few thousand positrons are trapped and lose energy through Coulomb collisions (sympathetic cooling) with laser-cooled 9 Be + . By imaging the 9 Be + laser-induced fluorescence, we observe centrifugal separation of the 9 Be + ions and positrons, with the positrons coalescing into a column along the trap axis. This indicates the positrons have the same rotation frequency and comparable den-sity (4 × 10 9 cm −3) as the 9 Be + ions, and places an upper limit of approximately 5 K on the positron temperature of motion parallel to the magnetic field. We estimate the number of trapped positrons from the volume of this column and from the annihilation radiation when the positrons are ejected from the trap. The measured positron lifetime is > 8 days in our room temperature vacuum of 10 −8 Pa.