In a steady-state experiment, a stream of ${\mathrm{He}}^4$ containing an extremely small concentration of ${\mathrm{He}}^6$ has been cooled below the lambda point and passed through a narrow slit as superfluid. If the ${\mathrm{He}}^6$ atoms had passed through the slit, most of them would have been detected by their radioactive decay in the gas stream leaving the cryostat. The fraction actually found was 0.04±0.07 (standard deviation) of those expected if there had been no separation of the isotopes. This result is consistent with the Bose-Einstein condensation model for the superfluidity of ${\mathrm{He}}^4$. A new type of generator for ${\mathrm{He}}^6$ is described.

• Received 27 July 1953

DOI:https://doi.org/10.1103/PhysRev.92.547