Efficient linac-based production of slow positrons

Two methods are under study for bunching 15 ns pulses of slow positrons produced by the Oak Ridge electron linear accelerator (ORELA). The first method involves the application of a time-dependent potential to a remoderating foil during the time positrons are being emitted from it. Lifetime spectra of polymer films have been collected with a resolution of 500 ps. The positron flux into the specimen during the pulse was about 3.9 × 1011 /s. A molybdenum moderator foil was used with an ORELA power of 3 kW. The quadratic potential bunching technique is also being considered. In this method a voltage is applied to a string of lenses, such that the potential experienced by positrons contained within the lenses varies as the square of the distance from a point that is defined at ground potential. Calculations indicate that a time resolution of about 200 ps can be achieved with this method.