Premoderation of positrons. Experiments on positron trapping and slowing-down

For a recently proposed new beam concept we have measured the confinement and the extraction efficiencies, and we thus showed that the premoderation in the sense of a trapping-slowing-down mechanism acting on the β+ spectrum works in the way as predicted. Premoderation efficiencies have been determined experimentally to be as high as 68%, and, because very good agreement between the theoretical model calculations and experimental results has been achieved, can reliably be extrapolated to similar 81%. Premoderation down to energies of a few keV is a decisive step, before implantation of the low energy positrons into the final solid neon moderator takes place. In this arrangement an overall conversion efficiency for obtaining slow positrons of up to 41% can be deduced. The conclusion is that in 2nd generation slow positron beams an overall improvement of total conversion efficiencies for producing usable slow positrons by a factor of over 200 as compared to conventional source-moderator schemes can be achieved. ((orig.))