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 reflection 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 density ( 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.

Weakly bound positron–electron pairs in a strong magnetic field

“…As pointed out in section 1, Jelenković et al [8] find yields around three orders of magnitude lower, and much closer to the figure found here. Specifically, their maximum quoted yield was 32 trapped positrons per hour for an incident flux of about 1.4 × 10 7 high energy positrons per second [22], resulting in a yield per fast positron of just over 6 × 10 −10 . Using the fast-slow positron conversion quoted above of 10 −3 results in a projected yield of 6 × 10 −7 per incident slow positron, which is close to our value.…”

“…The positron beamline used for the present investigation is, in essential detail, identical to that described previously elsewhere [19]. A (∼18 mCi; 0.67 GBq) 22 Na source and a solid neon moderator (e.g. [20]) provide a quasi-monoenergetic positron beam of ∼1 × 10 6 e + s −1 at a kinetic energy of ∼100 eV.…”

“…Simplifying somewhat, Estrada et al [7] found that the efficiency of magnetized positronium production was around 10 −6 in their system, assuming irradiation of the surface of their target with positrons direct from a 22 Na source. By contrast, Jelenković et al [8] found a result closer to 10 −9 .…”