The High Brightness Beam at Brandeis

Abstract

At the first workshop in this series, York University — 1981, the “brightness enhancement” proposal¹ which was about a year old at the time was referred to as an “ambitious method” of increasing the brightnessper-volt R_V of a slow positron beam.² Today, however, there are several groups actively pursuing or actually using brightness enhancement in their research. This talk will deal mainly with the brightness enhanced beam at Brandeis since it is a particularly high performance beam and uses well documented optics. The latter feature is important since the lack of well documented electrostatic optics for slow positron beams, in particular the immersion lens gun, has often inhibited the use of electrostatic transport in favor of the more intuitively simple, but generally more restrictive, magnetically guided beam. Equally important have been the improvements due to single crystal metal moderators in the planar back-scattering geometry³ over the large transverse energy spreads of parallel vane moderators.⁴ A large transverse energy component E_T (perpendicular to the initial beam direction) can be as injurious to the focussability of the beam as it is to the predictability of the positron trajectories through the system. In addition to briefly reviewing the design and operation of the Brandeis beam, some results will be presented in the context of the current diffraction and microbeam experiments for which the beam is presently being used.