Slow positron beam design notes

Recent methods of producing high flux. (105–106) s−1 slow positron beams are briefly reviewed. Currently, slow positron beams are produced most efficiently using a single crystal Cu(111) + S backscatter geometry moderator for ultra-high vacuum (UHV) conditions and an annealed W-vane converter for non-UHV conditions. The respective fast positron to slow positron conversion efficiencies for the Cu(111) + S and W-vane converters are (9 ± 3) × 10−4 and (1.2 ± 0.2) × 10−4. A new figure of merit, the "normalized brightness-per-volt", for converters is introduced which takes into account the transverse energy spread as well as the conversion efficiency. The importance of the normalized-brightness-per-volt in beam design and future methods to improve this figure of merit are discussed.