AN OVERVIEW OF THE SLOW-POSITRON BEAM FACILITY AT THE PHOTON FACTORY,KEK

The KEK slow-positron source is in the final stage of construction. Th e beam line comprises a 31 m long vacuum duct within an axial magnetic field and a following electrostatic guided section. In order to vary the energy of a positron beam dedicated to depth-profile measurements, a high voltage station capable of applying 60 kV has been installed i n the beam transport system. The target assembly (a water-cooled tanta lum rod of 5 radiation lengths and a moderator with multiple tungsten vanes) and the following straight section (8 m; used for positron stor age) are under high voltage. The beam duct located downstream is at gr ound potential. Positron beams passing through this region have a high kinetic energy. A focusing triplet quadrupole lens and a moderator on the retarding electrode are located at the end of the magnetic transp ort. This beam line has 9 right-angle-curved ducts, comprising a radiu s of curvature of 40 cm. Positrons with a maximum energy of 60 keV are guided by bending magnets attached to the beam-transport ducts. A tra nsport system to switch from magnetically guided to electrostatically guided has been installed. The design of the brightness-enhancement st age of the positron beam for positron re-emission microscopy is in pro gress. In a preliminary experiments at 2.0 GeV with a 2 kW primary bea m, 4 X 10(6) e(+)/s of slow positrons were observed by detecting annih ilation gamma-rays at the end of the magnetic beam-transport line. Fur ther improvements are expected by careful surface and thermal treatmen ts of the moderator.