Positron-beam-brightness enhancement: Low-energy positron diffraction and other applications

Abstract

The implementation and application of the first brightness-enhanced slow positron beam is described. The general concept of brightness enhancement by positron remoderation and the importance of such a technique for improving the phase-space parameters (beam diameter D and angular divergence \ensuremath{\theta}) of positron beams is reviewed. A theoretical brightness gain per remoderation stage of 180 is derived, corresponding to a reduction in D by a factor of 26. Fundamental difficulties in achieving these gains such as those due to lens aberrations and limitations inherent in our particular ``backscattering'' remoderation technique are described. Details of the construction and performance of a brightness-enhanced electrostatically focused beam are given. This beam achieves a diameter reduction of a factor of 10 per stage. With the use of two stages of remoderation it produces a beam on target with D and \ensuremath{\theta} values of approximately 1 mm and 1\ifmmode^\circ\else\textdegree\fi{}, and an energy width of 0.07 eV at a beam energy of 100 eV. The beam energy is tunable over the range 20--500 eV. These parameters are consistent with those found in standard low-energy electron diffraction beams. Using this new positron beam the first multiple-spot, low-energy positron diffraction pattern has been obtained. A W(110) crystal was used and an electron diffraction pattern was also acquired under identical conditions for comparison. A discussion of the potential uses of brightness-enhanced beams in diffraction studies and a variety of other solid-state and atomic physics measurements is given. Finally, future prospects for brightness-enhanced positron beams themselves including timing techniques, spin polarization, and microprobe development are considered.