High positronium yield and emission into the vacuum from oxidized tunable nanochannels in silicon

A positron-positronium converter composed of regular nanochannels with size in the 5-100 nm range and length about 2 mu m has been produced in silicon by electrochemical etching. After controlled oxidation of the inner surface of the channels, a positronium yield up to 45% of the positrons implanted at 1 keV energy was observed. At 1 keV positron implantation energy about 42% of positronium is estimated to be emitted into the vacuum. At 10 keV positron implantation energy, corresponding to a depth of similar to 800 nm, the positronium fraction emitted into the vacuum is still 10% in a sample with channels of 4-7 nm and up to 23-25 % in samples with larger channels. The positronium diffusion length and the fast positronium fraction emitted into the vacuum have been investigated as a function of the channel size tuned in the 5-100 nm range. The dependence of the positronium cooling from the size of the nanochannel is discussed and an evaluation of the expected fraction of thermalized orthopositronium in vacuum is given.