Construction of the Helsinki University of Technology HUT/ pulsed positron beam

We are constructing a pulsed positron beam facility for lifetime measurements in thin surface layers. Our beam system comprises a 22Na positron source and a tungsten foil moderator followed by a prebuncher and a chopper. A double-drift buncher will compress the beam into 120-ps pulses at the target. The end energy of the positron beam can be adjusted between 3 keV and 30 keV by changing the potential of the source end of the beam. The bunching electronics and most of the beam guiding magnets are also floating at the high voltage. The sample is at ground potential to facilitate variable temperature measurements. With a test source of 6 mCi 22Na we get a prebunched beam intensity of 4=103 positrons per second in 1.5-ns wide pulses the bunching frequency is 33.33 MHz.. We are currently testing the chopper and the following buncher stages and building the final acceleratorrdecelerator system. q1999 Elsevier Science B.V. All rights reserved.