Buffer-gas trap

The BGT is used for a variety of research applications, particularly those that benefit from specially tailored positron gases, plasmas and/or pulsed beams.

Such traps are renowned for their good confinement properties for particles (such as positrons) of a single sign of charge.

Molecular nitrogen is used because it is unique in having an electronic energy level below the threshold for Ps formation; hence it is the trapping gas of choice.

[5] Similarly, carbon tetrafluoride (CF4) and sulfur hexafluoride (SF6) have very large vibrational excitation cross sections, and so these gases are used for cooling to the ambient temperature (typically ~ 300 K).

[4] The moderator of choice for the BGT is solid neon (~ 1% conversion efficiency [7]), frozen on a cold metal surface.

If larger particle numbers are desired, the positrons are transferred to an ultra-high vacuum (UHV) Penning–Malmberg trap in a several Tesla magnetic field.

[19] BGTs are also expected to play similarly important roles in efforts to create and study positronium atom Bose–Einstein condensates (BEC)[20] and a classical electron-positron "pair" plasmas.

Fig. 1. Three-stage BGT to accumulate positrons: (above) electrode structure, and (below) electrical potential along the direction of the 0.15 T magnetic field. Positrons incident from the left are trapped and cooled, first by electronic excitation of N 2 molecules (A, B, C), and then by inelastic vibrational (rotational) collisions with CF 4 (N 2 ).
Fig. 2. Electrode structure (gold plated, 1.7 m in length) for a three-stage BGT circa 1996.