Magnetized Rydberg positronium forms when an energetic positron slows within a tungsten crystal and picks up an electron as it emerges in a strong magnetic field.  The signature is equal numbers of positrons and electrons when a weak electric field is applied, either of which can be accumulated and counted. The new positron accumulation technique is simple, robust and much more efficient than any other demonstrated to be compatible with a cryogenic vacuum.  Possible applications include the study of cold single component plasmas of positrons and the formation of cold antihydrogen. This is the most efficient mechanism yet demonstrated for accumulating 4.2 K positrons in the cyrogenic vacuum desired for the production of cold antihydrogen.

(a) A detected electrical signal reveals how many positrons have been accumulated into the trap. (b) The number of positrons accumulated in our trap increases with the time, climbing over one million.