PRODUCTION OF SLOW POSITRON BUNCHES AT ORELA AND THEIR APPLICATIONS

Two methods are under study for bunching 15 ns pulses of slow positron s produced by the Oak Ridge electron linear accelerator (ORELA). The f irst method involves the application of a time-dependent potential to a remoderating foil during the time positrons an being emitted from it . Lifetime spectra of polymer films have been collected with a resolut ion of 500 ps. The positron flux into the specimen during the pulse wa s about 3.9 x 10(11)/s. A molybdenum moderator foil was used with an O RELA 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 wit hin the lenses varies as the square of the distance from a point that is defined at ground potential. Calculations indicate that a time reso lution of about 200 ps can be achieved with this method.