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 θ) 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 θ values of approximately 1 mm and 1°, 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.