The positron microprobe at LLNL

The electron linac-based positron source at Lawrence Livermore National Lab oratory (LLNL) provides the world's highest current beam of keV positrons. We are building a positron microprobe that will produce a pulsed, focused p ositron beam for 3-dimensional scans of defect size and concentration with sub-micron resolution. The widely spaced and intense positron packets from the tungsten moderator at the end of the 100 MeV LLNL linac are captured an d trapped in a magnetic bottle. The positrons are then released in 1 ns bun ches at a 20 MHz repetition rate. With a three-stage re-moderation we will compress the cm-sized original beam to a 1 mu m diameter final spot on the target. The buncher will compress the arrival time of positrons on the targ et to less than 100 ps. A detector array with up to 60 BaF2 crystals in pai red coincidence will measure the annihilation radiation with high efficienc y and low background. The energy of the positrons can be varied from less t han 1 keV up to 50 keV. The target can be instrumented to allow in situ ann ealing, cooling and stress. (C) 1999 Elsevier Science B.V. All rights reser ved.