Extraction of radioactive positive ions across the surface of superfluid helium:A new method to produce cold radioactive nuclear beams

Alpha-decay recoils Rn-219 were stopped in superfluid helium and positive ions were extracted by electric field into the vapour phase. This first quantitative observation of extraction was successfully conducted using highly sensitive radioactivity detection. The efficiency for extraction across the liquid surface was 23 +/- 4% at 1.60 K, the release time was 90 +/- 10 ms at 1.50 K and the barrier for positive ions through a free superfluid-helium surface was 19.4 +/- 4.5 K. The pulsed second sound proved to be effective in enhancing the extraction.