Construction of the Helsinki University of Technology (HUT) pulsed positron beam

We are constructing a pulsed positron beam facility for lifetime measuremen ts in thin surface layers. Our beam system comprises a Na-22 positron sourc e and a tungsten foil moderator followed by a prebuncher and a chopper. A d ouble-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 e lectronics and most of the beam guiding magnets are also floating at the hi gh voltage. The sample is at ground potential to facilitate variable temper ature measurements. With a test source of 6 mCi Na-22 we get a prebunched b eam intensity of 4 x 10(3) 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 accelerator/decelerator system. (C) 1999 Elsevier Science B.V. All rights reserved.