buffer-gas trap

The buffer-gas trap (BGT) is a device used to accumulate positrons (the antiparticles of electrons) efficiently while minimizing positron loss due to annihilation, which occurs when an electron and positron collide and the energy is converted to gamma rays. The BGT is used for a variety of research applications, particularly those that benefit from specially tailored positron gases, plasmas and/or pulsed beams. Examples include use of the BGT to create antihydrogen and the positronium molecule. Design and operation The schematic design of a BGT is illustrated in Fig. 1. It consists of a specially designed (Penning or Penning–Malmberg) type electromagnetic trap. Positrons are confined in a vacuum inside an electrode structure consisting of a stack of hollow, cylindrical metal electrodes such as that shown in Fig. 2. A uniform axial magnetic field inhibits positron motion radially, and voltages imposed on end electrodes prevent axial loss. Such traps are renowned for their good confinement properties for particles (such as positrons) of a single sign of charge. Given a trap designed for good confinement, a remaining challenge is to efficiently fill the device. In the BGT, this is accomplished using a series of inelastic collisions with a molecular gas. In a positron-molecule collision, annihilation is much less probable than energy loss due to electronic or vibrational excitation. The BGT has a stepped potential well (Fig. 1) with regions at successively lower gas pressure. Electronic excitation of molecular nitrogen (N2) in the highest-pressure region is used to trap the positrons. This process is repeated until the particles are in a sufficiently low-pressure environment and the annihilation time is acceptably long. The particles cool to the ambient gas temperature due to inelastic vibrational and rotational collisions.