Condensation of isotopic impurities on the negative ion in He II

Experimental evidence is reported that the rate ν at which a negative ion nucleates vortex rings in He II is strongly influenced by 3He isotopic impurity concentrations close to that of the natural abundance. It is suggested that the anomalous temperature dependence of ν observed near 0.3K in earlier experiments in natural 4He is a consequence of 3He condensation on the ions, and it is deduced from the data that the presence of as few as five condennsed atoms is sufficient to completely eliminate the bare ion signal in a typical experimental geometry. (author)