Design of the ghent pulsed variable energy slow positron beam

In order to be able to investigate surface and near surface properties of materials with positron annihilation lifetime spectroscopy, the construction was started of a pulsed variable energy slow positron beam. The beam runs at 50 MHz and consists of a chopper-prebuncher-buncher-accelerator mechanism in a guiding magnetic field. A novel feature in this design compared to the existing pulsed beams is that the chopping and prebunching of the moderated positrons is achieved through one device close to the moderator in front of the Na-22 source. A resonator buncher compresses the time window of the beam further to a resolution suitable for timing experiments. Some details of the timing elements and first results of the timing measurements are given.