High performance hybrid slow positron beam and its user application spectrum

Abstract

A true hybrid beam is operational at PSI. From a β + radioisotope source and a premoderation of the positrons to medium energies in a high magnetic field confinement of Tesla order, a 3 keV wide spectrum of positrons is extracted and adiabatically guided to low fields of less than 100 G allowing extraction in a highly efficient and quasi-divergency-free way into field free space by increasing the positron's transverse energy spread by less than 20 eV. Double electrostatic focussing followed each time by W-remoderators (air lock anneal is possible) results in a beam diameter of μm dimensions. The microbeam is then transported through a cylindrical mirror beam switch device to five different user ports (one at a time, within minutes). The first target station is equipped with a retractable beam gun and a high precision sample manipulator with six movements for low (−200°C) and high (+950°C) temperatures (sample introduction by an air lock system). At the target chamber a UHV of 3.5 × 10 −11 mbar has been reached and the chamber can be equipped for surface physics experiments, for example to perform PAES. The sample is always at ground potential and the beam energy can be tuned between a few eV and 60 keV. The facility, initially equipped with a source of 500 mCi 58Co, produces a monoenergetic beam intensity (energy spread < 2 eV) at the target of 3 × 10 6 e +/s with an average transverse phase space corresponding to 100 μm × 50 mrad. In one of the user ports an optional 7 mm diameter beam of intensity 10 7 e +/s will be available. Initial beam experiments and possible applications will be discussed and the full potential of beam performance up-grade (intensity > 10 10 e +/ s, pulsing, repolarization) will be outlined.